National Aeronautics and

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IS02

George C. Marshall Space Flight Center

Huntsville, Alabama 35812
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i

NASA/TM—2016–218229

Natural Environments Definition for Design

H.L. Justh (Ed.), K.M. Altino, R.K. Decker, H.M. Koehler, F.B. Leahy, J.I. Minow,

B.C. Roberts, R.M. Suggs, R.J. Suggs, and P.W. White

Marshall Space Flight Center, Huntsville, Alabama

R.E. Barbre, Jr., J.W. Howard, and L.N. Parker

Jacobs ESSSA Group, Huntsville, Alabama

L. Burns

Raytheon/Jacobs, ESSSA Group, Huntsville, Alabama

R.D. Leach

Dynetics, Huntsville, Alabama

S.K. Noble

NASA Headquarters, Washington, DC

W.M. Farrell

Goddard Space Flight Center, Greenbelt, Maryland

J.B. Plescia

Applied Physics Laboratory, Johns Hopkins University,

Laurel, Maryland

October 2016

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA STI Information Desk

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681–2199, USA

757–864–9658

This report is also available in electronic form at

<http://www.sti.nasa.gov>
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

1.1 Purpose ........................................................................................................................ 1

1.2 Scope ............................................................................................................................ 1

1.3 Format and Use of This Technical Memorandum ........................................................ 2

2. TERRESTRIAL GROUND ENVIRONMENTS .............................................................. 4

2.1 Ground Winds .............................................................................................................. 5

2.1.1 Eastern Range .................................................................................................... 5

 2.1.1.1 Peak Wind Profile .................................................................................. 6

 2.1.1.2 Ground Wind Gust Factors .................................................................. 8

 2.1.1.3 Spectral Gust Environment ................................................................... 10

 2.1.1.4 Discrete Gust Model ............................................................................. 11

2.1.2 Edwards Air Force Base ..................................................................................... 12

2.2 Solar Radiation ............................................................................................................ 15

2.2.1 Eastern Range .................................................................................................... 17

2.2.2 Edwards Air Force Base ..................................................................................... 20

2.2.3 White Sands Missile Range ................................................................................ 23

2.3 Temperature ................................................................................................................. 26

2.3.1 Eastern Range .................................................................................................... 26

2.3.2 Edwards Air Force Base ..................................................................................... 29

2.4 Pressure ........................................................................................................................ 31

2.4.1 Eastern Range .................................................................................................... 32

2.4.2 Edwards Air Force Base ..................................................................................... 34

2.5 Humidity ...................................................................................................................... 36

2.5.1 Eastern Range .................................................................................................... 36

2.5.2 Edwards Air Force Base ..................................................................................... 37

2.5.3 Fog ..................................................................................................................... 38

 2.5.3.1 Radiation Fog ....................................................................................... 39

 2.5.3.2 Advection Fog ....................................................................................... 39

 2.5.3.3 Up-Slope Fog ........................................................................................ 40

 2.5.3.4 Frontal Fog ........................................................................................... 40

 2.5.3.5 Steam Fog ............................................................................................. 40

 2.5.3.6 Ice Fog .................................................................................................. 40

 2.5.3.7 Eastern Range Probability of Fog ......................................................... 41

 2.5.3.8 Eastern Range Fog Climatology ............................................................ 41

2.6 Surface Aerosols ........................................................................................................... 42

2.6.1 Eastern Range .................................................................................................... 43

2.6.2 Edwards Air Force Base ..................................................................................... 45
iv

TABLE OF CONTENTS (Continued)

2.7 Precipitation ............................................................................................................... 45

2.7.1 Eastern Range ................................................................................................. 45

2.7.2 Edwards Air Force Base .................................................................................. 47

2.8 Lightning .................................................................................................................... 48

 2.8.1 Eastern Range ................................................................................................. 48

2.8.2 Edwards Air Force Base .................................................................................. 53

2.8.3 Lightning Design and Test Criteria ................................................................. 56

 2.8.3.1 External Lightning Design ................................................................ 56

 2.8.3.1.1 Voltage Waveforms ............................................................. 56

 2.8.3.1.2 Current Waveforms ............................................................ 58

 2.8.3.1.3 Direct Effects ...................................................................... 58

 2.8.3.1.4 Indirect Effects ................................................................... 59

 2.8.3.2 Internal Lightning Design Environment ............................................ 63

2.9 Tropical Cyclones ........................................................................................................ 65

2.9.1 Eastern Range ................................................................................................. 65

2.9.2 Edwards Air Force Base .................................................................................. 66

2.10 Sea State ..................................................................................................................... 66

2.10.1 Sea State Data.................................................................................................. 66

 2.10.1.1 C-ERA 40 Hindcast Global Database ............................................... 66

 2.10.1.2 Buoy Networks .................................................................................. 67

2.10.2 Significant Wave Height and Extreme Wave Height Values ............................. 67

2.10.3 Wave Height and Wind Speed Durations and Intervals ................................... 69

2.10.4 Wave Period .................................................................................................... 69

2.10.5 Water Surface Slope ........................................................................................ 72

2.10.6 Ocean Temperature and Salinity ..................................................................... 78

 2.10.6.1 Sea Surface Temperature Data .......................................................... 79

2.10.7 Ocean Surface Currents ................................................................................... 80

2.11 Geologic Hazards ....................................................................................................... 84

2.11.1 Earthquakes .................................................................................................... 84

2.11.2 Tsunamis and Seiches ...................................................................................... 85

2.11.3 Volcanic Hazards ............................................................................................. 86

 2.11.3.1 Hazards Near Volcanic Activity ........................................................ 86

 2.11.3.2 Hazards Distant From Volcanic Activity ........................................... 86

2.11.4 Geology and Geologic Hazards at the Eastern Range ..................................... 87

 2.11.4.1 Geology ............................................................................................. 87

 2.11.4.2 Earthquakes ...................................................................................... 87

 2.11.4.3 Tsunamis and Seiches ........................................................................ 87

 2.11.4.4 Volcanic Hazards ............................................................................... 87

 2.11.4.5 Summary ........................................................................................... 87

2.11.5 Geology and Geologic Hazards at Edwards Air Force Base ............................ 88

 2.11.5.1 Geology ............................................................................................. 88

 2.11.5.2 Earthquakes ...................................................................................... 88
v

TABLE OF CONTENTS (Continued)

 2.11.5.3 Volcanic Hazards ............................................................................... 89

 2.11.5.4 Conclusions ....................................................................................... 89

2.12 Terrestrial Ionizing Radiation Environments and Effects ............................................ 89

2.12.1 Terrestrial Ionizing Radiation Environments ................................................... 90

2.12.2 Ground Environments ..................................................................................... 94

2.12.3 Terrestrial Ionizing Radiation Effects .............................................................. 94

2.12.4 Ground-Based Effects ..................................................................................... 95

3. TERRESTRIAL ALOFT ENVIRONMENTS .................................................................. 96

3.1 Winds Aloft ................................................................................................................ 96

3.1.1 Eastern Range .................................................................................................. 97

 3.1.1.1 Monthly Vector Wind Profile Model (0 to 27 km 

 (0 to 88.6×103 ft)) ............................................................................. 98

 3.1.1.2 Jimsphere Wind Profile Database 

 (0 to 20 km (0 to 65.6×103 ft)) ........................................................... 100

 3.1.1.3 Kennedy Space Center 50-MHz Doppler Radar Wind Profiler

 Database (2.7 to 18.6 km (8.9 ×103 to 61×103 ft)) ............................. 100

 3.1.1.4 High-Altitude Ascent Wind Model (up to 90 km (295.3 ×103 ft)) ...... 100

3.1.2 Edwards Air Force Base .................................................................................. 101

 3.1.2.1 Monthly Vector Wind Profile Model (0 to 25 km (0 to 82×103 ft)) .... 101

 3.1.2.2 High-Altitude Ascent Wind Model (up to 90 km (295.3×103 ft)) ...... 101

 3.1.2.3 Low-Level Wind Shear, Eastern Range ............................................. 101

 3.1.2.4 Low-Level Wind Shear, Edwards Air Force Base ............................... 102

3.2 Discrete Gust Model ................................................................................................... 102

3.2.1 NASA Discrete Gust Model ............................................................................ 103

3.2.2 Discrete Gust Summary .................................................................................. 110

3.3 Solar Radiation ........................................................................................................... 112

3.4 Temperature ................................................................................................................ 112

3.4.1 Eastern Range ................................................................................................. 113

3.4.2 Edwards Air Force Base .................................................................................. 115

3.5 Pressure ....................................................................................................................... 117

3.5.1 Eastern Range ................................................................................................. 117

3.5.2 Edwards Air Force Base .................................................................................. 119

3.6 Density ........................................................................................................................ 121

3.6.1 Eastern Range ................................................................................................. 121

3.6.2 Edwards Air Force Base .................................................................................. 123

3.7 Humidity .................................................................................................................... 125

3.7.1 Eastern Range ................................................................................................. 126

3.7.2 Edwards Air Force Base ................................................................................... 128
vi

TABLE OF CONTENTS (Continued)

3.8 Aloft Aerosols ............................................................................................................. 129

3.8.1 Gaseous Constituents ..................................................................................... 129

3.8.2 Upper Atmospheric Aerosols ......................................................................... 129

3.9 Earth-Global Reference Atmospheric Model 2010....................................................... 132

3.9.1 National Centers for Environmental Prediction Database 

 (Surface to 10 mb (0.15 lb/in2)) ........................................................................ 133

3.9.2 Middle Atmosphere Program (20 to 120 km (65.6 to 393.7×103 ft)) ................ 133

3.9.3 Marshall Engineering Thermosphere (Above 90 km (295.3×103 ft)) ............... 134

3.9.4 Range Reference Atmosphere (0 to 70 km (0 to 229.7×103 ft)) ........................ 134

3.9.5 Atmospheric Species Concentration Data ....................................................... 134

3.9.6 Available Special Output .................................................................................. 135

3.10 Stratospheric and Mesospheric Clouds ....................................................................... 135

3.11 Aloft (150 m to 90 km) Ionizing Radiation Environments and Effects ........................ 136

3.11.1 Aloft Ionizing Radiation Environments ........................................................... 136

3.11.2 Galactic Cosmic Ray Flux in the Atmosphere ................................................. 136

3.11.3 Aloft Ionizing Radiation Effects ...................................................................... 139

3.11.4 Aloft-Based Effects .......................................................................................... 139

3.12 Lightning Aloft ........................................................................................................... 139

4. NEAR EARTH SPACE ENVIRONMENTS ..................................................................... 140

5. LOW EARTH ORBIT SPACE ENVIRONMENTS (90 to 2,000 km) ................................ 141

5.1 Thermosphere ............................................................................................................. 141

5.1.1 Thermospheric Process ...................................................................................... 141

5.1.2 Variations .......................................................................................................... 143

 5.1.2.1 Variations in Solar Activity .................................................................. 143

 5.1.2.2 Variations During Periods of Increased Solar Activity (Flares,

 Coronal Mass Ejections, and Coronal Holes) ...................................... 144

 5.1.2.3 The Diurnal Variation ......................................................................... 144

 5.1.2.4 Semiannual Variation .......................................................................... 145

 5.1.2.5 Seasonal-Latitudinal Variations of the Lower Thermosphere

 Density ................................................................................................ 145

 5.1.2.6 Seasonal-Latitudinal Variations of Helium ......................................... 145

 5.1.2.7 Thermospheric Waves .......................................................................... 146

5.1.3 Thermospheric Winds ....................................................................................... 146

 5.1.3.1 Austral (South Polar Region) Thermospheric Winds ........................... 146

 5.1.3.2 Thermospheric Tide ............................................................................ 146

 5.1.3.3 Solar Storm-Induced Effects in the Thermosphere .............................. 149
vii

TABLE OF CONTENTS (Continued)

 5.1.3.4 Ionospheric Storm Effects ................................................................... 150

 5.1.3.4.1 Positive Storm Effects ........................................................... 150

 5.1.3.4.2 Negative Storm Effects ......................................................... 150

5.1.4 Solar and Geomagnetic Indices .......................................................................... 151

5.1.5 Orbital and Suborbital Neutral Atmosphere Model .......................................... 160

 5.1.5.1 Marshall Engineering Thermosphere Model ......................................... 160

 5.1.5.2 Statistical Analysis Mode ...................................................................... 162

 5.1.5.3 Earth Global Reference Atmospheric Model ........................................ 168

5.1.6 Spacecraft Effects ............................................................................................... 168

5.1.7 Atomic Oxygen .................................................................................................. 170

5.2 Thermal Environment .................................................................................................. 170

5.2.1 Fundamental Thermal Environment Parameters ............................................... 171

 5.2.1.1 Solar Constant ...................................................................................... 171

 5.2.1.2 Solar Irradiance .................................................................................... 172

 5.2.1.3 Albedo .................................................................................................. 172

 5.2.1.4 Outgoing Long-Wave Radiation ............................................................ 174

 5.2.1.5 Geometric Factors ................................................................................. 174

 5.2.1.5.1 Orbital Altitude and ‘Top of Atmosphere’ ............................. 174

 5.2.1.5.2 Orbital Inclination, Beta Angle, and Solar Zenith Angle ........ 175

5.2.2 Technical Background ........................................................................................ 175

 5.2.2.1 Earth Radiation Budget Experiment ..................................................... 175

 5.2.2.2 Details of Application ........................................................................... 176

 5.2.2.2.1 Albedo Correction for Solar Zenith Angle ............................. 176

 5.2.2.2.2 Temporal and Orbital Variations ........................................... 178

 5.2.2.3 Engineering Values for Critical Applications ......................................... 179

 5.2.2.4 Engineering Margin for Critical Systems ............................................... 186

 5.2.2.5 Design Conditions for Noncritical Systems ........................................... 187

 5.2.2.6 Applications and the Simple Thermal Environment Model ................... 189

5.3 Geomagnetic Field ....................................................................................................... 191

5.3.1 Models ............................................................................................................... 192

 5.3.1.1 International Geomagnetic Reference Field .......................................... 193

5.3.2 Geomagnetic Field Effects ................................................................................. 193

5.4 Plasma Environment .................................................................................................... 195

5.4.1 Plasma Ionosphere ............................................................................................. 196

5.4.2 Plasma Characteristics ....................................................................................... 196

5.4.3 Low Inclination Ionosphere ............................................................................... 198

 5.4.3.1 Ionospheric Density .............................................................................. 198

 5.4.3.2 Ionospheric Temperature ...................................................................... 201

 5.4.3.3 Spacecraft Charging in Ionosphere ....................................................... 203

5.4.4 Auroral Region .................................................................................................. 203

 5.4.4.1 Auroral Morphology .............................................................................. 204

 5.4.4.1.1 Quiet Magnetosphere State .................................................... 210
viii

TABLE OF CONTENTS (Continued)

 5.4.4.1.2 Active Polar Cap State ........................................................... 210

 5.4.4.1.3 Active Auroral Oval State ...................................................... 210

 5.4.4.2 Electrodynamics and Convection ........................................................... 211

 5.4.4.3 Transient Fluxes in Polar Low Earth Orbit ............................................ 212

 5.4.4.4 Spacecraft Charging in Auroral Region ................................................. 213

5.4.5 Plasmas in Polar Cap .......................................................................................... 214

 5.4.5.1 Ionosphere and Polar Wind ................................................................... 215

5.4.6 Plasma Effects for Design Consideration ............................................................ 216

 5.4.6.1 Power System/Solar Array ..................................................................... 216

 5.4.6.2 Contamination ....................................................................................... 216

 5.4.6.3 Plasma Emissions .................................................................................. 216

 5.4.6.4 Ionospheric Scintillation ........................................................................ 217

 5.4.6.5 Secondary Plasma Effects ...................................................................... 217

 5.4.6.6 Electromagnetic Force ........................................................................... 217

5.4.7 Spacecraft Charging ............................................................................................ 217

 5.4.7.1 Types of Spacecraft Charging ................................................................ 218

 5.4.7.2 Spacecraft Charging Mitigation ............................................................. 220

5.4.8 Plasma Environments and Spacecraft Charging Models ..................................... 221

 5.4.8.1 Plasma Environment Models ................................................................. 221

 5.4.8.1.1 International Reference Ionosphere......................................... 221

 5.4.8.1.2 GeoSpace ................................................................................ 222

 5.4.8.2 Space Charging Analysis Models ........................................................... 222

 5.4.8.2.1 NASCAP-2k............................................................................ 222

5.5 Low Earth Orbit (90 to 2,000 km) Ionizing Radiation Environments and Effects ......... 223

5.5.1 LEO Ionizing Radiation Environments ............................................................... 223

5.5.2 LEO Orbit Environments .................................................................................... 223

5.5.3 LEO Ionizing Radiation Effects .......................................................................... 226

 5.5.3.1 Total Ionizing Dose ............................................................................... 226

 5.5.3.1.1 Displacement Effects............................................................... 229

5.5.4 Low Earth Orbit-Based Effects ........................................................................... 230

5.6 Meteoroid and Orbital Debris Environments ............................................................... 231

5.6.1 Meteoroid Environment ...................................................................................... 231

 5.6.1.1 Meteor Showers ..................................................................................... 232

5.6.2 Orbital Debris Environment ................................................................................ 233

5.6.3 Models/Analysis .................................................................................................. 234

 5.6.3.1 Meteoroid Environment Models and Analysis ....................................... 234

 5.6.3.1.1 Flux ........................................................................................ 234

 5.6.3.1.2 Directionality .......................................................................... 235

 5.6.3.1.3 Velocity ................................................................................... 237

 5.6.3.1.4 Gravitational Focusing and Planetary Shielding ..................... 238

 5.6.3.1.5 Meteoroid Densities ................................................................ 240

 5.6.3.2 Orbital Debris Environment Model and Analysis .................................. 242
ix

TABLE OF CONTENTS (Continued)

5.6.4 Meteoroid and Orbital Debris Protection ........................................................... 244

 5.6.4.1 Collision Avoidance ............................................................................... 245

 5.6.4.2 Robust Design ....................................................................................... 245

 5.6.4.3 Shielding ................................................................................................ 245

 5.6.4.4 Orientation ............................................................................................ 245

 5.6.4.5 Crew Escape .......................................................................................... 245

 5.6.4.6 Operational Protection—Meteoroids ..................................................... 246

 5.6.4.7 Pressurized Tanks and Volumes ............................................................. 246

 5.6.4.8 Damage Detection and Fault Isolation .................................................. 247

 5.6.4.9 Wire Routing and Shielding ................................................................... 247

 5.6.4.10 Radiator Design ..................................................................................... 247

 5.6.4.11 Window and Surface Degradation ......................................................... 247

 5.6.4.12 Plasma Effects ....................................................................................... 248

5.7 Gravitational Field ....................................................................................................... 248

5.7.1 Spacecraft Effects ................................................................................................ 250

5.7.2 Gravity Recovery and Climate Experiment Earth Gravity Model GGM02C ...... 250

6. MEDIUM AND HIGH EARTH ORBITS (2,000 km to 10 Re) ......................................... 251

6.1 Thermosphere .............................................................................................................. 251

6.2 Thermal Environment .................................................................................................. 251

6.3 Geomagnetic Field ....................................................................................................... 251

6.3.1 Geomagnetic Field Model ................................................................................... 252

 6.3.1.1 Tsyganenko Model ................................................................................. 252

6.3.2 Geomagnetic Field Effects .................................................................................. 252

6.4 Plasma Environment .................................................................................................... 254

6.4.1 Plasma Regimes in Near Earth Space ................................................................. 254

 6.4.1.1 Plasmasphere ......................................................................................... 256

 6.4.1.2 Plasma Sheet .......................................................................................... 256

 6.4.1.3 Ring Current ......................................................................................... 256

 6.4.1.4 Magnetotail ........................................................................................... 257

 6.4.1.5 Radiation Belt ........................................................................................ 257

6.4.2 Solar Wind .......................................................................................................... 257

6.4.3 Plasma Characteristics......................................................................................... 258

 6.4.3.1 Geosynchronous Altitude Plasma .......................................................... 259

6.4.4 Spacecraft Charging Environment ...................................................................... 260

 6.4.4.1 Geostationary Transit and Geostationary Orbits and Internal

 Charging ................................................................................................ 261

6.4.5 Charging Consideration for Spacecraft Design ................................................... 261

6.4.6 Other Plasma Effects for Design Consideration .................................................. 262

6.4.7 Spacecraft Charging Models ............................................................................... 262
x

TABLE OF CONTENTS (Continued)

6.5 Ionizing Radiation Environment .................................................................................. 263

6.6 Meteoroid and Orbital Debris Environments ............................................................... 263

6.6.1 Meteoroid Environment ...................................................................................... 263

6.6.2 Orbital Debris Environment ................................................................................ 264

6.7 Gravitational Field ....................................................................................................... 264

7. CISLUNAR SPACE ENVIRONMENTS ........................................................................... 265

7.1 Lunar Exosphere (Lunar Atmosphere).......................................................................... 265

7.2 Solar Wind (Interplanetary Fields)................................................................................ 265

7.3 Plasma Environments.................................................................................................... 266

7.3.1 Solar Wind ......................................................................................................... 266

7.3.2 Magnetosheath and Magnetotail ........................................................................ 268

 7.3.2.1 Extreme Electron Flux Environment for Internal (Bulk)

 Charging Evaluations ........................................................................... 268

7.4 Ionizing Radiation Environment .................................................................................. 276

7.5 Meteoroid Environment ............................................................................................... 276

8. LUNAR SPACE ENVIRONMENTS ................................................................................. 277

8.1 Thermal Environments ................................................................................................. 277

8.1.1 Solar Constant ................................................................................................... 277

8.1.2 Lunar Long-Wave Radiance ............................................................................... 278

8.2 Plasma Environments ................................................................................................... 279

8.2.1 Lunar Plasma (Solar Wind, Magnetosheath, and Magnetotail) 

 Environments...................................................................................................... 279

8.2.2 Lunar Wake Plasma ........................................................................................... 281

8.2.3 WIND Observations of the Distant Wake .......................................................... 282

8.2.4 Plasma Interactions and Effects ......................................................................... 286

 8.2.4.1 Lunar Prospector Observations of the Inner Wake ............................... 287

 8.2.4.2 Simulations ........................................................................................... 289

8.2.5 Plasma Wake Interactions and Spacecraft Effects .............................................. 292

8.2.6 Magnetosphere Plasma Interactions and Spacecraft Effects ............................... 292

8.3 Ionizing Radiation Environment .................................................................................. 293

8.4 Lunar Gravity ............................................................................................................... 293

8.4.1 Lunar Gravity Design Model ............................................................................. 293

8.5 Lunar Eclipse ............................................................................................................... 295

9. LUNAR SURFACE ENVIRONMENTS ........................................................................... 297

9.1 Principal Surface Features—Lunar Highlands ............................................................. 298
xi

TABLE OF CONTENTS (Continued)

9.2 Surface Feature Properties ............................................................................................ 299

9.2.1 Crater Size Distributions .................................................................................... 299

9.2.2 Rock Size Distributions ...................................................................................... 302

9.2.3 Slope Distributions ............................................................................................. 315

9.3 Regolith Composition and Characteristics ................................................................... 316

9.3.1 Lunar Regolith Properties .................................................................................. 316

 9.3.1.1 Size-Frequency Distributions ............................................................... 317

 9.3.1.2 Particle Shape ....................................................................................... 319

9.3.2 Physical Properties .............................................................................................. 320

 9.3.2.1 Derived Physical Properties .................................................................... 328

9.3.3 Regolith Composition ......................................................................................... 330

 9.3.3.1 Chemistry ............................................................................................... 330

 9.3.3.2 Mineralogy/Lithology ............................................................................ 333

9.3.4 Lunar Regolith Electrical Properties ................................................................... 335

 9.3.4.1 Introduction ........................................................................................... 335

 9.3.4.2 Lunar Dust Charging Processes ............................................................. 336

 9.3.4.3 Electrical Properties for Charging Analysis ............................................ 336

 9.3.4.3.1 Measured Bulk (Dust) Electrical Properties ............................ 336

 9.3.4.3.2 Bulk Conductivity of Lunar Regolith ..................................... 337

 9.3.4.3.3 Bulk Conductivity (Cryogenic Temperature) ........................... 337

 9.3.4.3.4 Dielectric Constant ................................................................. 338

 9.3.4.4 Lunar Surface Potential ......................................................................... 340

 9.3.4.4.1 Surface and Near-Surface Electrical Environments ................. 340

 9.3.4.4.1.1 Dayside region ..................................................... 341

 9.3.4.4.1.2 Nightside region .................................................. 341

 9.3.4.4.1.3 Terminator/polar region ...................................... 343

 9.3.4.4.2 Validation of Model Results—Past Data Sets ....................... 343

 9.3.4.4.3 Temporal Effects: Solar Storms and Passages

 Into the Magnetosphere ........................................................ 343

 9.3.4.4.4 Effects on Landed Operations: Roving, Power Systems,

 and Dust ................................................................................ 344

 9.3.4.4.4.1 Roving and charge dissipation ............................. 344

 9.3.4.4.4.2 C×P power system .............................................. 344

 9.3.4.4.4.3 Lunar dust ........................................................... 345

 9.3.4.5 Photoelectric Emission of Lunar Fines .................................................. 345

 9.3.4.6 Secondary Electron Emission—Bulk Material ....................................... 346

 9.3.4.7 Triboelectric Charging ............................................................................ 346

 9.3.5 Contamination ................................................................................................... 347

 9.3.5.1 Lunar Regolith as a Contaminant .......................................................... 347

 9.3.5.2 Micrometeorids ...................................................................................... 348

 9.3.5.3 Condensed Gas ...................................................................................... 348

 9.3.5.4 Implanted Nuclei ................................................................................... 348

 9.3.6 Transient Lunar Phenomena ............................................................................... 349
xii

TABLE OF CONTENTS (Continued)

9.3.7 Lunar Interior and Moonquakes ........................................................................ 349

 9.3.7.1 Introduction ........................................................................................... 349

 9.3.7.2 Seismometer ........................................................................................... 350

 9.3.7.3 Seismic Events ........................................................................................ 350

 9.3.7.3.1 Deep Moonquakes .................................................................. 351

 9.3.7.3.2 Shallow Moonquakes .............................................................. 351

 9.3.7.3.3 Thermal Moonquakes ............................................................. 351

 9.3.7.3.4 Natural Impact Events ............................................................ 351

 9.3.7.3.5 Artificial Impact Events .......................................................... 351

 9.3.7.3.6 Unclassified Events ................................................................. 352

 9.3.7.4 Seismic Risk ........................................................................................... 352

 9.3.8 Lunar Volcanism ............................................................................................... 352

9.4 Lunar Poles .................................................................................................................. 352

 9.4.1 Solar Illumination Conditions ............................................................................ 353

 9.4.2 Volatile Traps ...................................................................................................... 354

9.5 Lunar Dust ................................................................................................................... 354

9.6 Thermal Environment .................................................................................................. 355

 9.6.1 Surface Temperature .......................................................................................... 355

 9.6.2 Subsurface Temperature ..................................................................................... 358

9.7 Ionizing Radiation Environment .................................................................................. 358

 9.7.1 Lunar Neutron Environment .............................................................................. 359

9.8 Meteoroid Environment ............................................................................................... 360

 9.8.1 Flux .................................................................................................................... 360

 9.8.2 Lunar Shielding .................................................................................................. 363

 9.8.3 Density ............................................................................................................... 363

 9.8.4 Speed .................................................................................................................. 363

 9.8.5 Meteor Showers .................................................................................................. 364

 9.8.6 Lunar Secondaries .............................................................................................. 364

REFERENCES ....................................................................................................................... 366
xiii

LIST OF FIGURES

 1. Peak wind speed profile envelopes for a 5% risk of exceeding 10 m (32.8 ft) 

peak wind speeds for various exposure periods for the ER ...................................... 7

 2. 95th percentile value of direct incident solar radiation on a plane normal 

to the Sun for the ER .............................................................................................. 19

 3. 95th percentile value for direct incident solar radiation on a plane normal 

to the Sun for EAFB ............................................................................................... 23

 4. 95th percentile value of direct incident solar radiation on a plane normal 

to the Sun for WSMR ............................................................................................. 26

 5. Monthly maximum, mean, and minimum temperatures for the ER ......................... 27

 6. Cumulative probability for 3-, 6-, and 12-hr temperature change at the ER 

(based on hourly surface observations, 1957–2002) ................................................. 29

 7. Monthly maximum, mean, and minimum temperatures for EAFB ......................... 30

 8. Cumulative probability for 3-, 6-, and 12-hr temperature change at EAFB 

(based on hourly surface observations, 1971–2001) ................................................. 31

 9. Maximum, mean, and minimum sea level air pressure at the ER (based 

on hourly surface observations, 1957–2002) ............................................................ 32

10. Cumulative probability of sea level pressure at the ER (based on hourly 

surface observations, 1957–2002) ............................................................................. 34

11. Maximum, mean, and minimum sea level air pressure at EAFB (based 

on hourly surface observations, 1971–2001) ............................................................ 34

12. Cumulative probability of sea level pressure at EAFB (based on hourly 

surface observations, 1971–2001) ............................................................................. 36

13. Surface psychrometric data for the ER (based on hourly surface observations, 

1957–2002) .............................................................................................................. 37

14. Surface psychrometric data for EAFB (based on hourly surface observations, 

1971–2001) .............................................................................................................. 38
xiv

LIST OF FIGURES (Continued)

15. Challenger (STS-6) rollout to pad 39a in fog, December 8, 1982 ............................. 39

16. Probability of precipitation or fog with visibility ≤0.8 km (≤0.5 mi) at KSC............. 41

17. Sea salt concentration at two altitudes as a function of wind speed near 

the surface of the sea: 600–800 m altitude (Woodcock) and 15 m altitude 

(Lovett). (The number of observations averaged for each data point is given, 

as well as standard deviation.) ................................................................................. 44

18. Diurnal/monthly variation of yearly average lightning flashes within a 50 km 

(26.9 nmi) radius of the ER, 1988–2001 .................................................................. 49

19. Mean annual CG lightning flash (flashes/km2/yr) for the ER area ........................... 52

20. Diurnal/monthly variation of yearly average lightning flashes within a 50 km 

(26.9 nmi) radius of EAFB, 1988–2001 ................................................................... 53

21. Mean annual CG lightning flash density (flashes/km2/yr) for EAFB ....................... 55

22. Voltage waveform A is characterized by a rise at the rate of 1,000 kV/μs 

(±50%) until its increase is interrupted by puncture or flashover .............................. 56

23. Voltage waveform B is a 1.2×50 μs waveform that crests at 1.2 μs (±20%) 

and decays to half of crest amplitude 50 μs (±20%) ................................................. 57

24. Voltage waveform C ramps up to breakdown voltage in 2 μs (±50%) ...................... 57

25. Voltage D slow-fronted waveform with a rise time between 50 and 250 μs ............... 58

26. Lightning environment direct effects current components A through D .................. 59

27. Lightning environment indirect effects current component A .................................. 60

28. Lightning environment indirect effects multiple stroke waveform set ....................... 60

29. Lightning environment indirect effects current component D .................................. 61

30. Lightning environment indirect effects multiple burst waveform set ........................ 62

31. Rayleigh probability density and cumulative probability distribution 

(x=α corresponds to the mode) .............................................................................. 68
xv

LIST OF FIGURES (Continued)

32. Examples of regular and irregular waves: (a) Regular wave (temporal),

(b) regular wave (spatial), (c) irregular wave, and (d) random wave .......................... 70

33. Cumulative distributions of water surface slope for 2 m (6.6 ft) SWH ..................... 77

34. Cumulative distributions of water surface slope for 4 m (13.1 ft) SWH ................... 77

35. Major ocean surface currents of the world .............................................................. 81

36. Satellite image of the Gulf Stream that shows the temperature differential 

of the ocean water off the coast of Florida ............................................................. 82

37. Peak acceleration with 10% probability of exceedance in 50 years ........................... 85

38. Cartoon depiction of a nuclear cascade event initiated by a high-energy 

heavy ion and yielding a distribution of atmospheric neutrons ................................ 91

39. Plot of the atmospheric neutron population as a function of altitude.

The peak environment is at an altitude of approximately 18.3 km ........................... 93

40. Plot of the atmospheric neutron population as a function of latitude. 

The peak environment is at high latitudes where the incident heavy ions 

have less geomagnetic screening ............................................................................... 94

41. Twelve vector wind model profiles, the ER, February, reference height=9 km 

(29.5×103 ft), wind components (m/s), flight azimuth=90°: (a) In-plane (x), 

profiles 1–6, (b) in-plane (x), profiles 7–12, and (c) out-of-plane (y), profiles 1–12 ... 99

42. STS-1 pitch and yaw aerodynamic load indicators, Qα and Qβ (lb/ft2 deg), 

Mach = 1.05, 150 April Jimsphere profiles, ER (addition of gusts to synthetic

wind profiles yields an overconservative result compared to the measured rates) ..... 100

43. Twelve vector wind model profiles, the EAFB, February, reference height=9 km 

(29.5×103 ft), wind components (m/s), flight azimuth=90°: (a) In-plane (x), 

profiles 1–6, (b) in-plane (x), profiles 7–12, and (c) out-of-plane (y), profiles 1–12.... 101

44. Discrete gust model (1-cosine) ................................................................................. 103

45. Nondimensional discrete gust magnitude Vm/σ as a function 

of nondimensional gust half-width, dm/L, longitudinal component ........................ 104

46. MIL-SPEC discrete gust at 8 km (26.2×103 ft) for severe turbulence 

(solid) and the NASA classical (dashed) discrete gust for half-widths 

of 60 and 150 m (196.9 and 492.1 ft) ....................................................................... 111
xvi

LIST OF FIGURES (Continued)

47. Annual mean and ±2σ of atmospheric temperature as a function 

of altitude at the ER ................................................................................................ 113 

48. Annual mean and ±2σ range of atmospheric temperature as a function 

of altitude at EAFB ................................................................................................. 115

49. Annual mean atmospheric pressure as a function of altitude at the ER ................... 117

50. Annual mean atmospheric pressure as a function of altitude at EAFB ................... 119

51. Annual mean atmospheric density as a function of altitude at the ER .................... 121

52. Annual mean atmospheric density as a function of altitude at EAFB ..................... 123

53. Reference profile of middle atmosphere number concentration mean, 

variability, and accuracy; representative of Northern Hemisphere, 

mid-latitude, springtime conditions ......................................................................... 125

54. Annual mean and ±2σ range of mixing ratio versus altitude about the annual 

mean for the ER ...................................................................................................... 126

55. Annual mean and ±2σ range of atmospheric mixing ratio versus altitude 

for EAFB ................................................................................................................ 128

56. Graphical summary of the atmospheric regions in Earth-GRAM 2010 

and sources for the model and the data from which the mean monthly 

values are based ....................................................................................................... 133

57. Plot of the atmospheric neutron flux as a function of altitude 

for >10 MeV neutrons ............................................................................................. 137

58. Plot of the proton flux as a function of energy for an altitude >200 km .................. 138

59. Plot of the heavy ion flux as a function of LET for an altitude >200 km ................. 138

60. Number density of atmospheric constituents versus altitude ................................... 143

61. Typical neutral densities for low and high solar activities (the atmospheric 

density increases toward the left on the abscissa) ..................................................... 144

62. Schematic of the zonal mean meridional circulation in the thermosphere 

at equinox for various levels of magnetic (auroral) activity (maximum 

velocities are typically hundreds of meters per second): (a) Quiet,

(b) average, and (c) storm ........................................................................................ 147
xvii

LIST OF FIGURES (Continued)

63. Schematic of the zonal mean meridional circulation in the thermosphere 

at solstice for various levels of magnetic (auroral) activity (maximum 

velocities are typically hundreds of meters per second): (a) Quiet,

(b) average, and (c) storm ........................................................................................ 148

64. 13-month smoothed values of solar flux (F10.7) over the mean solar cycle 

from 2002–2028 ....................................................................................................... 160

65. 13-month smoothed values of geomagnetic activity index (Ap) over the mean 

solar cycle from 2002–2028 ..................................................................................... 160

66. Probability of meeting or exceeding a given time interval without exceeding 

the (a) 95th percentile density or (b) 99th percentile density (bins represent 

a range of F10.7: (1) 66–102, (2) 102–138, (3) 138–174, (4) 174–210, 

and (5) 210–246) ...................................................................................................... 169

67. Albedo correction term, C(SZA), as a function of SZA .......................................... 177

68. OLR distributions for low inclination (≤30°) and medium inclination 

(30° to ≤60°) orbits, 128-s averaged paired and unpaired data ................................. 179

69. Albedo-OLR correlation for low inclination orbits, 128-s averaged data 

(contour intervals indicate relative frequency of occurrence) ................................... 181

70. Albedo-OLR correlation for medium inclination orbits, 128-s averaged data

(contour intervals indicate relative frequency of occurrence) ................................... 182

71. Albedo-OLR correlations for high inclination orbits, 128-s averaged data 

(contour intervals indicate relative frequency of occurrence) ................................... 182

72. Hot side tail of the 128-s average OLR distribution for low inclination orbits ......... 186

73. Example low inclination orbit, beta of 40o: (a) Albedo profile and (b) OLR

profile ...................................................................................................................... 190

74. IGRF prediction of the Earth’s magnetic field at sea level ....................................... 194

75. IGRF prediction of the Earth’s magnetic field at 650 km altitude ........................... 194

76. Plasma density (m–3) at 400 km ............................................................................... 199

77. Noon-midnight cross section of plasma density (m–3) as a function of altitude 

from 150 to 700 km for June 21 and solar minimum conditions

(demonstration only) ............................................................................................... 200
xviii

LIST OF FIGURES (Continued)

78. Electron temperatures (eV) at 400 km ...................................................................... 202

79. Noon-midnight cross section of plasma temperature (eV) as a function 

of altitude from 150 to 700 km ................................................................................ 203

80. Ionospheric electron density in an aurora (×104 ne/cm3) .......................................... 205

81. Average integral precipitating electron energy flux: (a) Kp=0 and (b) Kp=2 

(polar plots are in corrected geomagnetic coordinates, electron flux is in units 

of KeV/cm2-s-sr—demonstration only) ................................................................... 206

82. Average integral precipitating electron energy flux: (a) Kp=4 and (b) Kp=6) 

(the polar plots are in corrected geomagnetic coordinates, electron flux 

is in units of KeV/cm2-s-sr—demonstration only) ................................................... 207

83. Average integral energy (KeV) of precipitating electrons: (a) Kp=0 

and (b) Kp=2) (polar plots are in corrected geomagnetic coordinates

—demonstration only) ............................................................................................. 208

84. Average integral energy (KeV) of precipitating electrons (Kp=4, Kp=6) 

(polar plots are in corrected geomagnetic coordinates—demonstration only) ......... 209

85. Auroral electron intensity (center of arc) ................................................................ 212

86. NASA worst case spacecraft charging environment for internal charging issues ...... 220

87. Depiction of all the radiation types that a spacecraft can experience, 

including the inner and outer trapped radiation belts, solar particle event, 

and galactic cosmic radiation .................................................................................. 224

88. Two-dimensional artist’s depiction of the trapped radiation environment ............... 224

89. Three-dimensional views of the trapped radiation environment: (a) The 

true toroidal shape of the radiation belts, with transparent green representing 

the outer electron belt and solid green the proton belts (data from a particularly 

strong storm event where a second proton belt was formed), and (b) edge-on 

view shows cross-sectional data for the belt regions with low to high density 

represented by blue to red ........................................................................................ 225

90. Pictogram showing regions of space where radiation types are significant. 

Regions are plotted as a function of the equatorial radius, in Earth radii 

(~6,370 km), because the belts vary with the magnetic field that changes 

with increasing inclination off the equator .............................................................. 226
xix

LIST OF FIGURES (Continued)

91. Contour plot of the trapped proton environment overlaying a map

of the Earth. Proton density increases from blue to green to yellow 

to red. Contours are highlighting the SAA where the radiation particle 

population is high due to the offset nature of the Earth’s magnetic field .................. 227

92. Plot of trapped electron flux as a function of relative orbital time for a 900 km 

polar orbit. The graph shows the regions of the radiation belts 

and the noncontinuous exposure (horn identifies regions of space (at high 

latitudes) where the outer electron belt is seen at lower altitudes) ............................ 227

93. Plot of the total ionizing dose as a function of effective aluminum 

shield thickness ........................................................................................................ 229

94. Radiant distribution of meteor sky according to radar (the coordinate frame 

places the Earth’s apex of motion at the center, the solar (helion) direction 

at a longtitude of –90°, and the north ecliptic pole at latitude 90°) .......................... 231

95. Sporadic meteoroid flux as a function of mass ........................................................ 235

96. Flux on spacecraft surfaces for limiting mass, 10–6 g at a 1 AU orbit 

(no gravitational focusing or planetary shielding effects are considered) .................. 236

97. Average meteoroid speed distribution at 1 AU (no gravitational focusing 

or planetary shielding effects are considered) .......................................................... 237

98. Flux intensity plots of the sporadic meteoroid directionality relative to local 

spacecraft coordinates moving in circular orbits at (a) 1 AU from the Sun 

and (b) 400 km above the Earth, respectively ........................................................... 239

99. Bulk density histogram of very high frequency head echoes observed

by Advanced Research Project Agency long-range tracking and identification 

radar calculated by (a) the third spherical model and (b) the overdense model ........ 241

100. Orbital debris flux prediction using ORDEM2000 for an ISS orbit 

for the year 2015 ...................................................................................................... 243

101. Orbital debris flux prediction using ORDEM2000 for the 28.5° inclination 

and 277.8 km circular orbit for the year 2015 .......................................................... 243

102. Design measures to shield STS thermal radiators from M/OD penetrations:

(a) Before M/OD and (b) after M/OD ..................................................................... 248
xx

LIST OF FIGURES (Continued)

103. Earth’s magnetic field from Tsyganenko model during Kp=2 

for different seasons: (a) Winter, (b) spring, (c) summer, and (d) fall ....................... 253

104. Near-Earth magnetosphere ..................................................................................... 255

105. A composite of several typical plasmapause crossings representing different 

levels of magnetic activity (plots of H+ versus L represent all of the outbound 

passes in the local time region from midnight to 04:00.74) ...................................... 257

106. Demonstration of plasma density and temperature variation during 

the GEOTAIL mission from February 25, 2005, through March 1, 2005 

(updates to the plasma environments are planned when new data are available) ...... 259

107. NASA worst case environment for internal charging ............................................... 262

108. Statistical solar wind flux: (a) Electrons and (b) ions are represented 

Maxwellian environments at low energies with nonthermal tails extending 

megaelectronvolt energies ........................................................................................ 267

109. Survey of magnetotail and magnetosheath plasma characteristics (data are

from the University of Iowa/comprehensive plasma instrument (CPI) 

onboard the GEOTAIL satellite for the first 180 days in 1993. Values for all 

GSE y and z distances are plotted on the same axis ................................................. 269

110. Maximum measured electron flux by the CPME instrument from 1973 

to 2001 corresponding to durations equal to the number of days indicated. 

CPME instrument measurements are plotted along with curve fit (see table 93) ...... 271

111. Flux values obtained from the CPME instrument data associated with 

the number of 1-day events the given flux was greater than or equal to that 

indicated by the number of events curve. CPME instrument measurements 

are plotted along with curve fit (see table 94) ........................................................... 272

112. Flux values obtained from the CPME instrument data associated with 

the number of 2-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 95) .................................... 273

113. Flux values obtained from the CPME instrument data associated with 

the number of 7-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 96) .................................... 274
xxi

LIST OF FIGURES (Continued) 

114. Flux values obtained from the CPME instrument data associated with 

the number of 14-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 97) .................................... 275

115. Lunar 10-year, free field plasma fluence: (a) Electron and (b) ion fluence 

environment for sunward-facing surfaces of the lunar surface ................................. 280

116. Schematic view of the solar wind-Moon interaction as deduced from 

the WIND/3DP results on December 27, 1994 ........................................................ 282

117. Plots of WIND/3DP plasma data during the lunar wake crossing 

in December 27, 1994: (a) Np, (b) Vx, Vy, Vz, (c) Ne, and (d) Vx, 

Vy, Vz ...................................................................................................................... 283

118. Plasma and magnetic field parameters for lunar wake crossing ................................ 284

119. WIND spacecraft data from the moon wake tail encounter on November 13, 

1996 (spacecraft distance from the Moon center was approximately 25 RL) ............ 285

120. Normalized LP electron density measurements from altitudes of 20 to 115 km, 

binned by wake longitude and latitude (white circle shows the solar wind flow 

terminator) .............................................................................................................. 288

121. Normalized LP electron temperature measurements from altitudes of 20 

to 115 km, binned by wake longitude and latitude (white circle shows the 

solar wind flow terminator) ..................................................................................... 288

122. Electrostatic potential measurements from altitudes of 20 to 115 km, 

binned by wake longitude and latitude (white circle shows the solar wind 

flow terminator) ...................................................................................................... 289

123. Kinetic simulation of the lunar wake showing the formation of the void 

region and associated electrostatic fields (development of strong ambipolar 

E-fields appear along the wake flank near the Moon): (a) Ion wake

density and (b) wake electric field ............................................................................ 290

124. Comparison of wind density and simulation density along with wind 

ion beams and simulation ion distributions for the December 24, 1994, 

wake encounter near 7 RL (development of two separate beams appears 

in both measured and modeled sets) ........................................................................ 291
xxii

LIST OF FIGURES (Continued)

125. Indication in the growth in truncation error relative to orbit propagation 

with the 100×100 model (30-day propagation of circular polar orbits 

beginning at 0° longitude and 50, 100, and 400 km altitude. The graph shows average 

error in eccentricity over the last 7 days of the simulation) ...................................... 295

126. Near side of the Moon—the bright rayed crater in the southern highlands 

is Tycho (fig. 127) .................................................................................................... 297

127. Central southern highlands—the fresh crater at center left with the central 

peak and terraced rim is Tycho ................................................................................ 298

128. A portion of Oceanus Procellarum, including the craters Schiaparellic 

and Schiaparellie—a typical mare surface with relatively few impact craters ........... 299

129. Crater size frequency distributions for several areas. The red line denotes 

the steady state or equilibrium condition. At diameters <200–300 m, the 

surface is in approximate equilibrium ...................................................................... 300

130. Mare surface in northwest mare imbrium under very low illumination; Sun 

is illuminating from the right. The cratered nature of mare surface is evident.

Due to the low Sun, even the older shallow craters are apparent. ‘A’ is an old 

1,500 m crater. Creep of the regolith down the interior slopes 

has removed small diameter craters and produced the observed texture

(LROC M116465524RE).......................................................................................... 301

131. West Crater—fresh impact crater east of the Apollo landing site with rocks 

scattered around the rim. A11: Apollo 11 LM descent stage, LWC: Little West 

Crater (visited by Armstrong), and West Crater (M117338434R) ............................ 302

132. Map of rock locations around lunar craters showing the range of spatial 

distributions. Red circle denotes the crater rim (three from Bart and Melosh) ......... 303

133. Impact crater with rocky inner wall; note the presence of rocks along the inner 

wall compared with the exterior where rocks are rare to absent (LROC image

M119449091l) .......................................................................................................... 304

134. Cumulative size-frequency distributions for rocks near the Surveyor spacecraft 

derived from surface images .................................................................................... 305

135. Maps of the rock distributions at the Apollo 16 landing site. Figures are from 

the Apollo 16 Preliminary Science Report: (a) Station 4, (b) station 5, 

and (c) station 6 ....................................................................................................... 306
xxiii

LIST OF FIGURES (Continued)

136. Cumulative size-frequency distributions for rocks at stations 4 –6 

at the Apollo 16 landing site .................................................................................... 307

137. Cumulative size-frequency distribution for rocks around the Surveyor 7 

landing site based on lunar orbiter data ................................................................... 308

138. Western rim of North Ray Crater at the Apollo 16 landing site: A, lobes 

of blocky ejecta extending from the rim; B, areas between rocky lobes with 

fewer blocks; and C, partially buried blocks (LROC image: M117392541l) ............. 309

139. NRC at the Apollo 16 landing site. Counting area is the area 

to the west of the crater rim (LROC image M117392541l) ...................................... 309

140. Cumulative size frequency distribution for rocks on the western rim of NRC 

(area shown in fig. 138) ............................................................................................ 310

141. Concentric crater with diameter of 130 m. Area outlined in red indicates 

the total area counted; area outlined in blue is subarea with higher rock 

frequency (LROC image M119449091l, crater at –11.8721 °S./339.432 °E. .............. 310

142. Size frequency distribution of ejected rocks for the crater illustrated

in figure 143. Subarea is outlined in blue; total area in red in figure 141 .................. 311

143. Counting area is the northwest quadrant of West Crater (LROC image 

M117338434R) ........................................................................................................ 311

144. Cumulative size-frequency distribution for the northwest quadrant 

of the ejecta of West Crater ..................................................................................... 312

145. Area surrounding the Surveyor VII landing site (LROC image M157668488RE) .... 312

146. Cumulative size-frequency for rocks in the area outlined in figure 143. 

Counts exclude rocks within craters ......................................................................... 313

147. Area surrounding the Apollo 16 landing site; LM descent stage

(LROC image M117392541L) ................................................................................. 313

148. Cumulative size-frequency distribution of rocks observed around 

the Apollo 16 landing site ........................................................................................ 314

149. Apollo 16 landing sites: (a) Station 5 and (b) station 6 (LROC image: 

M122108795L) ........................................................................................................ 314
xxiv

LIST OF FIGURES (Continued)

150. Cumulative size-frequency distributions of rocks at stations 5 and 6 

of the Apollo 16 landing site ................................................................................... 315

151. Cumulative size-frequency distribution of regolith grain size from a variety 

of samples. Note the very coarse material from the Apollo 12 double core 

and the Apollo 14 trench ......................................................................................... 317

152. In situ bulk density of the regolith based on drill core samples from 

the Apollo 15, 16, and 17 missions. The abrupt increases and decreases result 

from different layers at different depths ................................................................... 322

153. Relative density as a function of depth .................................................................... 323

154. Void ratio and porosity as a function of relative density for lunar samples. 

Also shown is a function for ideal, variably packed uniform spheres. There is 

considerable scatter among the lunar samples ......................................................... 324

155. Normal shear stress versus shear strength. The slope of the relation defines 

the friction angle (inset) ........................................................................................... 326

156. Compressibility measurements on Apollo 12 samples. Inset shows a diagram 

of a 1D oedometer for measuring compressibility ................................................... 328

157. Change in void ratio as a function of applied vertical stress. The arrows 

indicate the changes in void ratio as the sample is loaded and then unloaded .......... 329

158. Grain composition in regolith samples. x axis lists the component 

composition, y axis lists the different samples (data from table 107) ........................ 335

159. Temperature dependence of lunar soil samples (Apollo 15) and lunar rock 

samples (Apollo 16) ................................................................................................. 338

160. Lunar near-surface electrical environment ............................................................... 340

161. A model of the near-surface E-field, associated surface potential, ambient 

electron plasma density and temperature above the sheath, and surface 

electric field in the terminator .................................................................................. 342

162. Number of surface-ejected electron beams with energies >500 eV 

as a function of time (white bars at the bottom indicate times of solar storm 

events (solar energetic particle events with solar storms); gray bars indicate 

passages through the geomagnetic tail) .................................................................... 344
xxv

LIST OF FIGURES (Continued)

163. Location of ALSEP seismometers. Lunar image with landers

courtesy of NASA GSFC Visualization Studio ....................................................... 349

164. Passive seismic experiment at the Apollo 16 site ...................................................... 350

165. North Pole mosaic from Lunar Reconnaissance Orbiter (courtesy

of NASA/GSFC/Arizona State University) ............................................................. 353

166. South Pole mosaic from Clementine (courtesy of NASA) ....................................... 354

167. Generalized temperature as a function of time and latitude .................................... 355

168. Surface temperature as a function of time; when the thermal properties 

of the regolith are considered the lowest, nighttime temperature can vary.

(Circles represent lunar sample values; the lower solid line represents values 

for a mature regolith; and the upper solid line represents values for 

a theoretical rock surface. The blue area illustrates the difference 

in temperature between immature and mature regolith; the yellow area 

illustrates the difference between a regolith-covered and rock-covered surface) ....... 357

169. Temperature variation as a function of depth at the Apollo 15 and 17 landing 

sites (hatched areas show day-to-night temperature variations). Below 

approximately 50 cm, there is essentially no temperature variation due to 

the day-night cycle ................................................................................................... 358

170. Lunar neutron spectrum in terms of neutron differential flux (n/cm2/s/MeV) 

as a function of energy) MeV and lunar soil composition ....................................... 359

171. Effects of directionality for an oriented cube-like spacecraft in low lunar orbit, 

with focusing and shielding ..................................................................................... 360

172. Meteoroid surface area flux near the Moon as a function of particle size 

(no correction has been made for lunar shielding, and the particle sizes 

were computed assuming a density of 1 g/cm3) ....................................................... 361

173. Example of the directionality of the meteoroid environment for stationary

observer at (a) lunar Equator and (b) lunar North Pole ........................................... 362

174. Average impacting meteoroid speed distribution in lunar space, 

no gravitational focusing no lunar shielding ............................................................ 363

175. Comparison of meteoroid speed distributions at the lunar surface and in lunar 

space at 1 AU .......................................................................................................... 364
xxvi

LIST OF TABLES

 1. Summary of key design factors ............................................................................... 4

 2. Peak wind speed profile envelopes for a 5% risk of exceeding 10 m (32.8 ft) 

for various exposure periods for the ER .................................................................. 8

 3. Ten-minute gust factors for the ER .......................................................................... 10

 4. Gust factor profile for τ=10 min and u18.3=9.27 m/s (30.4 ft/s) .............................. 10

 5. Nondimensional constraints for the longitudinal, lateral, and vertical 

components of turbulence for the vehicle on-pad, transportation, 

and launch operation phases at the ER ................................................................... 11

 6. EAFB 5% risk, peak wind speeds versus exposure period at 4 m (13.1 ft) 

ANG reference height ............................................................................................. 12

 7. EAFB peak wind speed profile for 5% risk, 1 hr exposure, 4 m (13.1 ft) 

ANG reference height ............................................................................................. 13

 8. EAFB steady state wind speed profile for 5% risk, 1 hr exposure, 4 m 

(13.1 ft) ANG reference height ................................................................................ 14

 9. Dryden turbulence spectra parameters for altitudes below the 295 m 

(967.8 ft) ANG level ................................................................................................ 15

10. Extreme high values of solar radiation for the ER ................................................... 17

11. Extreme low values of solar radiation for the ER .................................................... 18

12. Extreme high values of solar radiation for EAFB ................................................... 21

13. Extreme low values of solar radiation for EAFB ..................................................... 22

14. Extreme high values of solar radiation for WSMR ................................................. 24

15. Extreme low values of solar radiation for WSMR ................................................... 25

16. Surface air and sky radiation temperature extremes for the ER ............................... 27

17. Monthly mean and extreme (maximum and minimum) temperatures 

for the ER ................................................................................................................ 28
xxvii

LIST OF TABLES (Continued)

18. Cumulative probability for 3-, 6-, and 12-hr temperature change at the ER ............. 28

19. Surface air and sky radiation temperature extremes for EAFB ................................ 29

20. Monthly mean and extreme temperatures for EAFB ............................................... 30

21. Cumulative probability for 3-, 6-, and 12-hr temperature change at EAFB .............. 31

22. Monthly sea level pressure at the ER (based on hourly surface observations,

1957–2002) .............................................................................................................. 33

23. Cumulative probability of sea level pressure at the ER (based on hourly surface 

observations, 1957–2002) ........................................................................................ 33

24. Monthly sea level pressure at EAFB (based on hourly surface observations,

1971–2001) .............................................................................................................. 35

25. Cumulative probability of sea level pressure at EAFB (based on hourly surface 

observations, 1971–2001) ........................................................................................ 35

26. Maximum vapor concentration for the ER .............................................................. 36

27. Surface psychrometric data, dewpoint versus temperature envelope for the ER ...... 37

28. Surface psychrometric data, dewpoint versus temperature envelope for EAFB ....... 38

29. Normally expected fog days at KSC ........................................................................ 42

30. KSC fog onset and dissipation time: range and peak hour ..................................... 42

31. Mean sea salt particle concentrations in maritime air masses 

and corresponding altitudes .................................................................................... 44

32. Highest rainfall rate versus duration for various probabilities, given a day 

with rain for the highest rain month at the ER ........................................................ 46

33. Design rainfall at the ER based on yearly largest rate for stated periods 

(use average rate of fall for raindrops of 6.5 m/s (21.3 ft/s) for all times) .................. 46

34. Estimated hail characteristics for the ER (reference height=61 m (200.1 ft)) ........... 46

35. EAFB design ground-level rainfall based on yearly largest rate 

for stated durations ................................................................................................. 47
xxviii

LIST OF TABLES (Continued)

36. Estimated hail characteristics for EAFB (reference height=18 m (59.1 ft)) ............. 47

37. Diurnal variation of lightning flashes for the ER .................................................... 50

38. Lightning statistics for the ER, 1988–2001 .............................................................. 51

39. Diurnal variation of lightning flashes for EAFB ..................................................... 54

40. Lightning statistics for EAFB, 1988–2001 ............................................................... 55

41. Application of external lightning environment to vehicle zones 

(includes direct and indirect effects) ......................................................................... 62

42. Maximum induced environment for electromagnetic regions ................................... 63

43. Percentage of wave period observations in 1-s intervals that are within 

a specified SWH range (0.5 m) from January–December 1971–2001 ....................... 71

44. Minimum average wave period corresponding to given significant wave height ....... 72

45. Energy spectrum for 2 m (6.6 ft) SWH .................................................................... 74

46. Energy spectrum for 4 m (13.1 ft) SWH .................................................................. 75

47. Ocean temperatures (°C) in the ER booster recovery areas ...................................... 79

48. Ocean temperatures (°F) in the ER booster recovery areas ...................................... 79

49. Summary of key design factors ............................................................................... 96

50. Mean horizontal (longitudinal) and vertical turbulence standard deviation 

(σh and σw) and length scale (Lh and Lw), as a function of altitude ......................... 105

51. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (km) and gust half-width, dm (m), 

for severe turbulence ................................................................................................ 107

52. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (ft) and gust half-width, dm (ft), for severe turbulence ....... 108

53. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (km) and gust half-width, dm (m), 

for moderate turbulence .......................................................................................... 109
xxix

LIST OF TABLES (Continued)

54. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (ft) and gust half-width, dm (ft), for moderate 

turbulence ................................................................................................................ 110

55. Annual mean and ±2s range of atmospheric temperature as a function

of altitude at the ER ................................................................................................ 114

56. Annual mean and ±2s range of atmospheric temperature as a function 

of altitude at EAFB ................................................................................................. 116

57. Annual mean and ±2s range of atmospheric pressure as a function 

of altitude at the ER ................................................................................................ 118

58. Annual mean and ±2s range of atmospheric pressure as a function 

of altitude at EAFB ................................................................................................. 120

59. Annual mean and ±2s range of atmospheric density as a function 

of altitude at the ER ................................................................................................ 122

60. Annual mean and ±2s range of atmospheric density as a function 

of altitude at EAFB ................................................................................................. 124

61. Annual mean and ±2s range of atmospheric mixing ratio versus altitude

at the ER ................................................................................................................. 127

62. Annual mean and ±2s range of atmospheric mixing ratio versus altitude

at EAFB .................................................................................................................. 128

63. Average concentrations (standard atmosphere values) of various gaseous 

constituents from the Earth’s surface up to 90 km (295.3×103 ft) altitude ............... 130

64. Characteristics of stratospheric aerosols ................................................................. 132

65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic activity 

index over the mean solar cycle, 2002–2028 ............................................................. 151

66. Pressure parameters in Earth orbit (ambient pressure in pascal; torr 

in parentheses) ......................................................................................................... 161

67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range ..................................................................... 163
xxx

LIST OF TABLES (Continued)

68. Probabilities of achieving a time interval without encountering thermospheric 

density level above a given percentile value .............................................................. 168

69. Summary of key design factors ............................................................................... 171

70. Solar spectral irradiance-standard curve, abridged version ...................................... 173

71. Values of orbital average albedo correction term, <c> (add this correction

to the SZA=0 albedo value) .................................................................................... 177

72. Albedo, OLR pairs for critical systems in low inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re + 30 km) ............ 183

73. Albedo, OLR pairs for critical systems in medium inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re+30 km) ............. 184

74. Albedo, OLR pairs for critical systems in high inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re+30 km) ............. 185

75. Albedo-OLR pairs for noncritical applications ....................................................... 188

76. Pulse-averaged SZA correction terms for albedo assuming a 5,400-s orbit .............. 188

77. Summary of key design factors (geometric field) ..................................................... 192

78. Summary of key design factors (ambient/energetic plasma condition) .................... 196

79. Charging environments in low Earth polar orbits .................................................... 213

80. Ambient plasma at 350–600 km altitude in three latitude zones (0°–30°, 30°–50°,

and 50°–70° inclination) .......................................................................................... 214

81. Plasma environment at 50°–70° inclination and 350–600 km altitude ...................... 214

82. Meteor streams known to produce enhanced or storm level activity ........................ 232

83. The six background meteor radiants ....................................................................... 236

84. Meteoroid average speeds at 1 AU, without Earth focusing or shielding 

included ................................................................................................................... 238

85. Flux and average speed values for ram/wake surfaces of spacecraft in 

circular orbits at 1 AU from the Sun and 400 km above the Earth, respectively ....... 240
xxxi

LIST OF TABLES (Continued)

86. Parameters of the lognormal distribution ................................................................ 242

87. Summary of key design factors ............................................................................... 254

88. Solar wind parameters near the Earth ..................................................................... 258

89. Geoplasma parameters ............................................................................................ 260

90. Solar wind parameters near the Earth ..................................................................... 266

91. Average solar wind properties .................................................................................. 266

92. Magnetosheath and magnetotail plasma parameters ............................................... 270

93. Equation and coefficients for the maximum electron flux events 

(refer to fig. 110) ...................................................................................................... 271

94. Equation and coefficients for average daily electron flux bounds 

(refer to fig. 111) ...................................................................................................... 272

95. Equation and coefficients for 2-day duration electron flux events 

(refer to fig. 112) ...................................................................................................... 273

96. Equation and coefficients for 7-day duration electron flux events 

(refer to fig. 113) ...................................................................................................... 274

97. Equation and coefficients for 14-day duration electron flux events 

(refer to fig. 114) ...................................................................................................... 275

98. Moments for lunar plasma environment .................................................................. 280

99. Projected worst case minimum solar flux during lunar eclipse, 

June 26, 2029 ........................................................................................................... 296

100. Rock frequency data ................................................................................................ 315

101. Particle size nomenclature ....................................................................................... 316

102. Particle size frequency distribution for the dust component as illustrated 

in figure 151 ............................................................................................................. 318

103. Summary of grain-specific properties (<1 mm size fraction) ................................... 319
xxxii

LIST OF TABLES (Continued)

104. Summary of bulk regolith properties ....................................................................... 321

105. Bulk element compositions of Apollo samples ........................................................ 331

106. Trace element compositions of Apollo samples ....................................................... 332

107. Modal volume abundance of particles in the 1,000–90 μ size range 

for different sites ...................................................................................................... 334

108. Direct current electrical conductivity of lunar rocks ................................................ 337

109. Contamination of the lunar surface ........................................................................ 347

110. Seismic events on the Moon detected by the Apollo 16 PSE .................................... 351

111. Frequency and energy of moonquakes and earthquakes ......................................... 352

112. Thermal properties of the lunar regolith ................................................................. 357

113. Lunar surface temperatures ..................................................................................... 357

114. Interplanetary meteoroid flux at Earth’s distance (1 AU) as a function 

of particle size ......................................................................................................... 362

115. Cumulative ejecta flux versus velocity ...................................................................... 365
xxxiii

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS

AFRL (United States) Air Force Research Laboratory

Alb albedo

Al2O3 aluminum oxide

ALSEP Apollo lunar surface experiments package

Amb. ambient

ANG above natural grade

AO atomic oxygen

Ar argon

AU astronomical unit

Au gold

Ba barium

Ca calcium

CaO calcium oxide

CC cloud-to-cloud (lightning)

CCAFS Cape Canaveral Air Force Station

CDIP Coastal Data Information Program

CD–ROM compact disk-read only memory

Ce cerium

C-ERA40 40-year corrected-European Centre for medium-range weather forecasts 

reanalysis

CEV crew exploration vehicle
xxxiv

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

CG cloud-to-ground (lightning)

CGLSS Cloud-to-Ground Lightning Surveillance System

CH4 methane

CME coronal mass ejection

CMOS complementary metal oxide semiconductor

CO carbon monoxide

Co cobalt

CO2 carbon dioxide

Comb combined

COS carbonyl sulfide

CPI comprehensive plasma instrument

CPME Charged Particle Measurement Experiment

Cr chromium

Cr2O3 chromium(III) oxide

Cs cesium

CS2 carbon disulfide cation

DC direct current

DD displacement damage

DMSP Defense Meteorological Satellite Program

DOL day of launch

DRWP Doppler radar wind profiler

Dy dysprosium
xxxv

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

E3 electromagnetic environmental effects

EAFB Edwards Air Force Base

Earth-GRAM Earth Global Reference Atmospheric Model

ECMWF European Centre for Medium Range Weather Forecasts

ELDRS enhanced low dose rate sensitivity

EMF electromagnetic force

EOM end of mission

ER Eastern Range; electron reflectometer

Er erbium

ERBE Earth Radiation Budget Experiment

ERBS Earth Radiation Budget Satellite

ESD Exploration Systems Development; electrostatic discharge

Eu europium

EUV extreme ultraviolet

EVA extravehicular activity

Fe iron

FeO ferrous oxide

Ga giga years ago

GCR galactic cosmic rays/radiation

Gd gadolinium

GEO geosynchronous orbit

GN&C guidance, navigation, and control
xxxvi

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

GPS Global Positioning System

GRACE Gravity Recovery and Climate Experiment

GRAM Global Reference Atmospheric Model

GRC Glenn Research Center

GRS gamma ray spectrometer

GSE ground support equipment

H hydrogen

H+ hydrogen ion

H2O water

H2S hydrogen sulfide

H2SO4 sulfuric acid

HCl hydrogen chloride

He helium

He+ helium ion

HEO high Earth orbit

Hf hafnium

Hg mercury

HNO3 nitric acid

Ho holmium

HRMP Harvard Radio Meteor Project

HWM Harmonic Wind Model

IC intracloud (lightning)
xxxvii

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

IGRF International Geomagnetic Reference Field

IMF interplanetary magnetic field

IMP-8 Interplanetary Monitoring Platform-8 (satellite)

IR infrared radiation

Ir iridium

IRI International Reference Ionosphere

Is ratio of the intensity in arbitrary units of the ferromagnetic resonance 

to the iron oxide conecentration

ISEE International Sun-Earth Explorer (satellite)

ISS International Space Station

K potassium

K2O potassium oxide

Kr krypton

KSC Kennedy Space Center

La lanthanum

LCC Launch Commit Criteria

LDAR Lightning Detection and Ranging (system)

LDEF Long Duration Exposure Facility

LEAM Lunar Ejecta and Meteorite (experiment)

LEO low Earth orbit

LET linear energy transfer

Li lithium

LM lunar module
xxxviii

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

LP Lunar Prospector (satellite)

LPLWS Launch Pad Lightning Warning System

LRO Lunar Reconnaissance Orbiter

LROC Lunar Reconaissance Orbiter Camera

LST local standard time

Lu lutetium

M/OD meteoroid and orbital debris

MAP Middle Atmosphere Program

MARECS Maritime European Communications Satellite

Max. maximum

MEEC Mars Experiment on Electrostatic Charging

MEM Meteoroid Engineering Model

MEO Meteoroid Environment Office; middle Earth orbit

MET Marshall Engineering Thermosphere (model)

Mg magnesium

MgO magnesium oxide

mho/m measurement of conductivity, replaced by Siemens

MIL-SPEC Military Specification

MIL-STD Military Standard

Min. minimum

MLT magnetic local time

Mn manganese
xxxix

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

MnO manganese (II) oxide

M/OD meteoroid and orbital debris

MSFC Marshall Space Flight Center

MSIS Mass Spectrometer Incoherent Scatter (model)

MSL mean sea level

MVWP monthly vector wind profile (model)

N2 nitrogen

N2

+ nitrogen ion

N2O nitrous oxide

Na sodium

Na2O sodium oxide

NAC nacreous cloud

NASCAP NASA/Air Force Spacecraft Charging Analyzer program

NASCAP-2k NASA/Air Force Spacecraft Charging Analyzer Program-2000

Nb niobium

NCEP National Centers for Environmental Prediction

Nd neodymium

NDBC National Data Buoy Center

NH3 ammonia

NH4

+ ammonium ion

Ni nickel

NLC noctilucent cloud

NLDN National Lightning Detection Network
xl

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

NO nitric oxide

NO+ nitrous oxide ion

NO2 nitrogen dioxide 

NO3 nitrogen trioxide

NOAA National Oceanic and Atmospheric Administration

NOZOMI Martian orbiter spacecraft (Japanese for “HOPE”)

NS neutron spectrometer

O+ atomic oxygen ion

O2 molecular oxygen

O2

+ molecular oxygen cation

O2

– molecular oxygen anion

O3 ozone

OLR outgoing long-wave radiation

ORDEM2000 Orbital Debris Engineering Model 2000

P phosphorus

P2O5 phosphorus pentoxide

POLAR Potentials of Large Spacecraft in Auroral Regions (satellite)

POR period of record

Pr praseodymium

PSC polar stratospheric cloud

PSE Passive Seismic Experiment

RADAR radio detection and ranging
xli

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

Rb rubidium

RMP Re for the magnetopause

RRA Range Reference Atmosphere

S sulfur

SAA South Atlantic Anomaly

SAE Society of Automotive Engineers

SAM statistical analysis mode

Sc scandium

SCATHA spacecraft charging at high altitudes

SEE single event effect

SEU single event upset

Si silicon

SIDE Superthermal Ion Detector Experiment

SiO2 silcon dioxide

Sm samarium

SO2 sulfur dioxide

SO4

2– sulfate ion

SPE solar particle event

Sr strontium

SSA specific surface area

SSP Space Shuttle Program; Space Station Program

SST sea surface temperature
xlii

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

STEM Simple Thermal Environment Model

STS Space Transportation System

SWH significant wave height

SZA solar zenith angle

Ta tantalum

Tb terbium

Th thorium

Ti titanium

Ti

O2 titanium dioxide

TID traveling ionospheric disturbance; total ionizing dose

TLP transient lunar phenomena

TM Technical Memorandum

TOA time of arrival

TPS Thermal Protection System

U uranium

UN United Nations

U.S. United States

USAF United States Air Force

UTC universal coordinated time

UV ultraviolet

V vanadium

VAGSC Vehicle Ascent Guidance Steering Commands 
xliii

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

W tungsten

WSMR White Sands Missile Range

WTS westward traveling surge

Y yttrium

Zn zinc

Zr zirconium

Yb ytterbium
xliv

NOMENCLATURE

A orbital altitude; fitting parameter; cross-sectional area normal to fluid flow 

direction (m2); constant; ratio of electron density to ion density (ne/ni

)

[Ai(μi)]2 energy spectrum (m2/Hz)

Ap daily planetary geomagnetic index

A(μ) directional hemispherical albedo (reflectivity)

a average lunar normal bolometric albedo = 0.15

ac reference radius of the Moon

ap planetary geomagnetic activity index

B magnetic field strength; constant; square foot of the ratio of electron to ion 

temperatures (Te 

/Ti)1/2

By component 

Bz component 

C heat capacity; constant; square root of the ratio of ion-to-electron mass 

(mi 

/me)1/2; constant (7.4)

Cc compression index

Cnm harmonic coefficient

Cp heat capacity (J/kg K)

Cr recompression index

c parameter; cohesion (kPa)

D0–λ percentage of the solar constant (1,366.1 W•m–2) associated with 

wavelengths shorter than λ

Da diameter (km)
xlv

NOMENCLATURE

DR relative density

d distance; particle diameter

dm gust half-width

E electron energy (MeV); activation energies; energy of incident electron

Emax energy at which the maximum yield occurs

Eλ solar spectral irradiance averaged over small bandwidth centered at λ 

(W•m–2•m–1)

e void ratio of regolith

emax maximum void ratio at which the regolith can be placed

emin minimum void ratio at which the regolith can be placed

erfc complementary error function

F flux (No./cm2-s-ster-MeV)

!

F vector force acting on a particle (N)

F10.7 solar radio noise flux 

F10.7B centered solar radio noise flux averaged over six rotations 

Fa error

f frequency

fmax maximum frequency (Hz)

G gust factor (function of peak wind speed); specific gravity

g gravity (m/s2)

!

g accelerator vector

gm gravity (m/s2)
xlvi

NOMENCLATURE

H wave height; reference altitude

H1/10 average height of the highest one-tenth of the waves

H1/100 average height of the highest one-hundredth of the waves

Hmax maximum wave height 

Hp highest desired fraction of the wave

H p

average height of the highest desired fraction of the waves

Hrms root mean square wave height 

h height (m ANG) 

I inertia (J/(m2 s1/2 K)

I* omnidirectional slope variance

IDH direct horizontal solar radiation

IDI direct incident solar radiation

IDN(0) intensity of solar radiation normal to surface at the Earth’s surface 

assuming clear skies

IDN(Z) intensity of solar radiation normal to surface at required height Z

IDβ direct incident solar radiation

ITH total horizontal diffuse radiation

IdH horizontal diffuse sky radiation

i solar incidence angle; hydraulic gradient

K absolute permeability (m2); thermal conductivity (J/(smK))

K0 coefficient of lateral stress

k thermal conductivity; modulus of subgrade reaction parameter 

Boltzmann’s constant 
xlvii

NOMENCLATURE

kp logarithm of ap, the planetary geomagnetic activity index

L integral scales of turbulence; smallest wavelength (m); turbulence length 

scale; size of spacecraft; vector along the length of the conductor

M percent FeO + TiO2

Mp mass of the particle (g)

Mx mean of the Rayleigh random variable, x (m)

m moment of wave spectrum; mass of particle (kg)

m0 zeroth moment of wave spectrum

mi mass, ith series

N number of recorded wave cycles

n number of standard deviations away from the mean; total number 

of frequency bins; porosity

ne electron number 

ni number density

Pnm associated Legendre functions of the first kind of degree n and order m

p nondimensional parameter; highest fraction of interest

p(x) density function

Q flow rate (m3/s)

Qin radiative heat input into the surface

q nondimensional parameter; aerodynamic pressure

R autocorrelation

Ra depth (km), relative to external surface
xlviii

NOMENCLATURE

Re Earth radii

RL lunar radii 

r distance between particle and center of Earth (m)

rˆ unit vector pointing from the center of the Earth to the particle

rc unit variance Gaussian random variable

rud zero-mean variable

S solar constant (W/m2)

S(f) spectral density function

Snm harmonic coefficient

So average solar constant at 1 AU

T temperature (K)

T,t time

Ti temperature, ith series

Tm mean zero up-crossing wave period

Tp peak spectral wave period

Ts lunar surface temperature at the subsolar point

Tv virtual temperature

U4 peak wind speed at the 4-m reference height 

Uh steady state wind at height h (m/s)

Uh peak wind at height h (m/s)

Uz steady state wind speed (m/s)
xlix

NOMENCLATURE

u longitudinal component of turbulence; wind speed

ui bulk flow velocity, ith species; instantaneous wind speed

V random gust; spacecraft velocity vector; gravitational potential

V1 initial gust

V2 random gust

Vm gust magnitude

v lateral component; particle velocity

w vertical component; annual mean and +–2σ mixing ratio

Xχ compression index

x random variable; desired frequency (Hz)

xm mean of x in the distribution

xN normalized variate

y resulting energy (m2/Hz)

z height

α Sun’s azimuth (measured from the south direction toward positive west); 

angle of attack

β tilt angle; angle of sideslip 

γ fitting parameter

Δ size of particle (cm)

Δe change in void ratio

Δlogσv log change of applied vertical stress
l

NOMENCLATURE

ΔQ amount of heat

ΔT temperature change

Δμi energy spectrum (m2/Hz)

δ number of secondary electrons emitted per incident electron

δmax maximum number of secondary electrons emitted per incident electron

ε long-wave emissivity of the surface (assumed to equal 1)

θ solar zenith angle; angle relative to surface normal

κ thermal conductivity (W/mK)

λ wavelength (μm); longitude

μ total water surface slope (deg); fluid viscosity (Ns/m2) 

μc crosswind water surface slope component (rad) 

μE gravitational constant for the Earth 

μud upwind-downwind water surface slope component (rad) 

μV mean of random gusts

ρ density of meteoroid; bulk density; density of the fluid (kg/m3); 

density (g/m3)

ρS atmospheric density at sea level (kg/m3)

ρw density of water

ρZ atmospheric density at required height (kg/m3)

σ standard deviation; Stefan-Boltzmann constant (5.67×10–8 W/m2K4); 

normal stress (kPa); conductivity

σ2 variance
li

NOMENCLATURE

σ2

c crosswind slope variance

σ2

ud upwind-downwind slope variance

σh horizontal stress

σV standard deviation of random gusts; vertical stress

σx standard deviation

τ average period

φ geocentric declination; latitude; friction angle (o)

φ(Ω) gust spectral density function (m2/s2 (cGy/m)–1

Ω wave number
lii
1

TECHNICAL MEMORADUM

NATURAL ENVIRONMENTS DEFINITION FOR DESIGN

1. INTRODUCTION

Planning for future National Aeronautics and Space Administration (NASA) missions will 

encompass a variety of operational and engineering activities that involve a multitude of issues, 

constraints, and influences derived from the natural environment. This Technical Memorandum 

(TM) presents a definition of the natural environment, i.e., a description in engineering handbook 

format of models and data specifically selected to support the architecture development, engineering design, and technology development for NASA’s Exploration Systems Development (ESD) 

initiatives.

1.1 Purpose

This TM provides a uniform description of the natural environment to serve as a support 

framework for missions under ESD initiatives. It is intended to support engineering and analysis, 

requirements development, and verification involved in the development of exploration concepts 

and architectures, flight hardware, and new technologies. Presenting a single benchmark definition 

of natural environment parameters provides an easily accessible and uniform baseline for competitive studies, independent analyses, and concept studies.

Since the definitions specified herein are, for the most part, definitions developed and used 

on prior programs, they are well understood by NASA and significant portions of the contractor 

community. Thus, use of this TM enables a better understanding of program technical risk than 

would be possible without a baselined definition document.

1.2 Scope

Natural environment, as the term is used here, is intended to include all environmental factors that are independent, i.e., outside the influence, of the program. Orbital debris and some other 

manmade environments are included because they are beyond program control. All induced environments, contamination, and aeroheating, for example, are excluded because they are dependent 

on system design. Likewise, ‘environmental impact,’ the effects of a program on the environment, 

is not within this scope.

This TM provides all natural environments needed to support aerospace vehicle design 

and development activities. The material is divided as follows:
2

• Introduction (sec. 1).

• Terrestrial ground and aloft environments (secs. 2 and 3). The terrestrial environments sections include the natural environments from the Earth’s surface to the edge of the thermosphere, 

approximately 90 km (295.3×103 ft) altitude. These environments will be experienced at ground 

facilities at the primary launch and landing sites, and during ground operations and processing, 

launch, and landing. Examples include surface and upper level winds, severe weather, lightning, 

and thermodynamic parameters.

• Near and low Earth space environments, and medium and high Earth orbits (HEOs) (secs. 4–6). 

The near Earth space environments sections describe the Earth orbital environments, including 

thermosphere density, ionizing radiation, plasma, solar activity, geomagnetic fields, orbital debris, 

and meteoroids, etc.

• Cislunar and lunar space and surface environments (secs. 7–9). The cislunar and lunar environments sections focus on the lunar orbital and surface environments.

This TM is a handbook and source document for the environment models and data needed 

to support a program throughout the development phase. It is not a science text.

The Natural Environments Branch at Marshall Space Flight Center (MSFC), Huntsville, 

Alabama, should be notified when environment information is needed that has not been provided 

in this TM. They will work with appropriate NASA programs to provide the information and 

update this TM as appropriate. Clarification and interpretation of the information contained 

herein is also the responsibility of the Natural Environments Branch.

The application of the models and data provided herein, and documentation of the applications, is the responsibility of the user. The intent of this TM is to support design and development 

phases.

1.3 Format and Use of This Technical Memorandum

Each section within this TM contains an explanation and description of the natural environment phenomenon to which it is devoted.The physics of the phenomenon and its dynamics 

are briefly described. Key data describing the environment are presented in handbook style format. 

Any analysis and models that are applicable and available for providing the key data are listed. 

Graphs and diagrams, unless otherwise noted, are for illustration purposes only and should not 

be used for specifications. Finally, lessons learned concerning the potential impacts of the environmental factors are discussed and mitigation techniques are suggested for consideration.

In recent years it has become popular to express requirements in terms of probabilities, e.g., 

probability of loss of crew or loss of vehicle, probability of launch delay, etc. For this reason, this 

TM defines the probability distribution functions for the various natural environment parameters 

wherever possible, i.e., the intensity as a function of probability of occurrence over some period 

of record (POR). This is intended to allow architecture developers to trade robust design against 
3

operational mitigation and/or acceptable risks wherever the environmental parameters affect architecture performance against one of the key requirements. Unfortunately, the format and detail 

of the probability distribution varies from parameter to parameter, depending on the quality and 

amount of data available for the particular environment. In some cases, a complete distribution 

function is available either directly or within a model. For other cases, only a few points can be 

provided (estimates of a 95th or 96th percentile, for example). In some cases, there is inadequate 

information available about the environment to define the distribution, e.g., orbital debris. In any 

case, the manner and limitations of data presentation are not intended to define the design points.
4

2. TERRESTRIAL GROUND ENVIRONMENTS

This section provides a baseline description of the natural terrestrial environment for the 

ground surface. Definitions and descriptions of terrestrial environmental phenomena for the surface, zero to 150 m (zero to 492.1 ft), are provided in this section. Definitions and descriptions for 

aloft environments, 150 m to 90 km (492.1 to 295.3×103 ft), are provided in section 3. Environment 

definitions are given for the following surface locations: Eastern Range (ER) (28.90 °N., 80.55 °W.) 

and Edwards Air Force Base (EAFB) (34°54′ N., 117°52′ W.).

Introductions to various statistical models and their databases that are pertinent to the 

vehicle design are presented. It is important to remember that the interdependence of the various 

parameters in this section is critical when evaluating launch/landing probability. A summary of key 

design factors for ground environments is displayed in table 1.

Table 1. Summary of key design factors.

Parameter Source Select Based On

Winds Sections 2.1, 2.1.1, 2.1.1.1– 2.1.1.4, 

and 2.1.2

Location, altitude, steady state winds, 

peak wind profile, wind gusts

Solar radiation Section 2.2; tables 10 and 11 Location, season, time of day, altitude, 

cloudiness

Temperature means/extremes Figures 5 and 7 Location, month

Pressure means/extremes Figures 9 and 11 Location, month

Humidity Figures 13 and 14 Location, temperature, dew point 

temperature

Aerosols Section 2.6; table 31 Location, humidity, wind speed, altitude

Precipitation: rain Tables 33 and 35 Location, altitude

Precipitation: hail Tables 34 and 36 Location, altitude

Atmospheric electricity/lightning Tables 38, 40, and 42 Location, month

Tropical cyclones Sections 2.9.1 and 2.9.2 Location, season

Sea state statistics Tables 43–46 Location, month, wind speed, and direction

Ocean temperatures Tables 47 and 48 Location, season, depth in ocean

Earthquakes Sections 2.11.1, 2.11.4.2, and 2.11.5.2 Location

Tsunamis and seiches Sections 2.11.2 and 2.11.4.3 Location

Volcanic hazards Sections 2.11.3, 2.11.3.1, 2.11.3.2, 

2.11.4.4, and 2.11.5.3

Location

Ionizing radiation: Secondary 

neutron environment

Sections 2.12.1, 2.12.2, and 2.12.3 Critical device LET threshold
5

2.1 Ground Winds

Ground winds are defined in this TM as the winds from the surface up to 150 m (492.1 ft). 

This layer is referred to as the boundary layer. The winds within the boundary layer are characterized by very complicated three-dimensional (3D) flow patterns with turbulent accelerations in both 

space and time. Turbulent circulations generated by surface heating during the daytime influence 

the surface winds by transferring momentum vertically throughout the depth of the boundary 

layer. Turbulence is small during nighttime conditions as the wind speeds are typically light within 

the statically stable boundary layer. Winds in the boundary layer will affect not only the vehicle, 

but also the ground support equipment (GSE) and facilities. Natural environmental factors influencing the magnitude and direction of the winds range from synoptic, large-scale (>1,000 km 

(539.9 nmi)), steady state wind to localized weather phenomena, such as thunderstorm downdrafts 

and thermally induced circulations, which tend to induce peak winds and/or turbulent gusts.

Note: Standard meteorology convention is adopted for wind direction, i.e., north is designated as 0°, east as 90°, etc. An east wind is therefore directed from east to west.

2.1.1 Eastern Range

The ER as defined in this TM is the region that comprises Cape Canaveral Air Force Station 

(CCAFS), which includes NASA’s Kennedy Space Center (KSC) and Patrick Air Force Base. All 

areas of the region are situated along the eastern shore in central Florida.

The weather station elevation at the ER is approximately 5 m (16.4 ft) above mean sea level 

(MSL).1 Some measured atmospheric variables, such as pressure and density, are a function 

of station elevation. For standardization purposes, these values are reduced to the value at MSL.

Note that since the ER elevation is only 5 m (16.4 ft) MSL, all data reported from the ER 

are in terms of MSL values, except the 18.3-m- (60-ft-) tall pad light pole that is reported above 

natural grade (ANG).

The ER is exposed to a variety of synoptic-scale weather patterns during a year. In the 

winter, cold front passages can result in steady state northerly winds at speeds from 5 to 10 m/s 

(16.4 to 32.8 ft/s) for consecutive days. In the summer, an expansive high-pressure system typically 

develops in the central Atlantic, resulting in 5 to 10 m/s (16.4 to 32.8 ft/s) easterly flow at the ER. 

During the autumn months, northeasterly winds of approximately 10 to 15 m/s (32.8 to 49.2 ft/s) 

typically occur and are associated with a continental high-pressure system over the central United 

States. These northeasterly winds introduce a crosswind component on the runways and could 

affect vehicle landings. The proximity of the ER to the Atlantic Ocean also makes it susceptible 

to tropical storms and hurricanes, which produce strong winds over long durations. The hurricane 

season extends from June through November. Further discussion of tropical storms and hurricanes 

is presented in section 2.9.

Locally, the ER proximity to the Atlantic Ocean and tropical latitudes results in a large 

dispersion of winds in the boundary layer. Thermally induced circulations develop due to differential heating of land versus sea. These circulations cause boundary layer winds to blow from the sea 
6

during the day and from the land at night, and are known as sea breezes and land breezes, respectively. The circulations that develop act to either accelerate or reduce the prevailing wind speed and 

can change the direction of the wind. When acting against the prevailing wind, convergent boundaries can form at the boundary between the thermally induced circulation and the prevailing wind. 

These convergent zones can act as a trigger mechanism for thunderstorm development that can 

produce downdrafts with vertical velocities up to 15 m/s (49.2 ft/s) and horizontal velocity components of approximately 25 m/s (82 ft/s). Thunderstorm initiation from sea breeze circulations tends 

to occur over the central Florida peninsula and move eastward with the mean wind; whereas, land 

breeze-initiated thunderstorms develop off the Florida Atlantic coast in the early morning hours, 

03:00 to 06:00 local standard time (LST), and move inland.

From the brief description of the natural wind environment for the ER, engineers need to 

account for steady state winds in addition to the peak winds with the associated turbulent spectra 

and discrete gusts when designing an aerospace vehicle, GSE, and facilities. Peak wind speed is 

the fundamental measure of wind instead of steady state wind speed since the loads placed on the 

vehicle are greater for peak winds versus steady state winds.1

Exposure duration to the natural environment is also important in the design of the vehicle, 

GSE, and facilities. For example, the aerospace vehicle’s exposure time will likely be less than the 

GSE and facilities. Therefore, the design requirements for withstanding a single-occurrence peak 

ground wind will differ between the vehicle and the GSE or facilities.

Due to these natural environment conditions, NASA has developed empirical formulas for 

peak wind and steady state wind profiles based on extensive observations of winds measured at 

NASA’s ground wind tower at the ER. From these formulas, wind speed envelopes are calculated 

at a specified risk level, based on probability distributions for either peak or steady state wind 

values, during a specified exposure period (e.g., 1 hr, 30 day, 3 mo, or 1 yr) at the ER. In order to 

perform loading and response calculations resulting from steady state and random turbulence drag 

loads and von Karman vortex shedding loads, the engineer requires information about the vertical 

variation of the mean wind and the structure and turbulence in the atmospheric boundary layer. 

The philosophy is to extrapolate the peak wind statistics up in height via a peak wind profile model 

or empirical formula. The associated steady state or mean wind profile is obtained by applying 

a gust factor that is a function of wind speed and height.

2.1.1.1 Peak Wind Profile. The peak wind profile empirical formula was developed at 

discrete heights, 18 m (59.1 ft) , 30 m (98.4 ft), 60 m (196.9 ft), 90 m (295.3 ft), 120 m (393.7 ft), 

and 150 m (492.1 ft), and is defined with a power law relationship as:1

u(z) = u18.3

z

18.3

!

"

# $

%

&

k

, (1)

where u(z) is the peak wind speed at height z in meters ANG, u18.3 is a known peak wind speed 

at 18.3 m (60 ft), and for u18.3 >2 m/s (6.6 ft/s), k is defined as:

k = c u( ) 18.3

!3/4 , (2)
7

where parameter c is normally distributed with a mean of 0.52 (m/s)3/4 and standard deviation (σ) 

of 0.36 (m/s)3/4; thus, c=0.52+0.36n (m/s)3/4, where n is the number of standard deviations 

away from the mean. For example, when n=3 (3σ), c=1.6 (m/s)3/4, corresponding to a risk level 

of 0.135%.2 The peak wind is referenced to the 18.3-m (60-ft) level, which is the base height 

of the launch pad at the ER.

Referencing a standard level in the discussion of ground winds avoids confusion in the 

interpretation of risk statements and structural load calculations. For a given σ level, the peak wind 

speed profile will envelope the appropriate percentage of wind speed profiles for a given peak wind 

at the reference level. The reference level wind and the peak wind speed profile can have different 

levels of risk. For example, a reference level wind can have a 5% risk of being exceeded, but one 

can build a 3σ (n=3) profile that will envelope 99.865% of wind speeds given that particular reference level wind. This formula for peak wind provides an envelope of the peak winds in the boundary layer associated with a reference peak wind at 18.3 m (60 ft) at a specified standard deviation. 

The formula does not provide instantaneous wind profiles.

Figure 1, along with the raw data in table 2, is an example of peak wind speed profile envelopes (n=3) for the 5% risk of exceeding the 10 m (32.8 ft) peak wind speed for various exposure 

periods.1

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30 35 40 45

Wind Speed (m/s)

F1

Height (m)

1 Day

10 Days

30 Days

90 Days

365 Days

Figure 1. Peak wind speed profile envelopes for a 5% risk of exceeding 10 m (32.8 ft) 

peak wind speeds for various exposure periods for the ER.
8

Table 2. Peak wind speed profile envelopes for a 5% risk of exceeding 10 m (32.8 ft) 

for various exposure periods for the ER.

Height

Exposure Days

1 10 30 90 365

(m) (ft) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s)

10 33 17.3 56.8 22.6 74.1 26.4 86.6 29 95.1 32.1 105.3

18.3 60 19.3 63.3 24.7 81 28.5 93.5 31.2 102.4 34.3 112.5

30.5 100 21 68.9 26.5 86.9 30.5 100.1 33.2 108.9 36.4 119.4

61 200 23.7 77.8 29.3 96.1 33.3 109.3 36.2 118.8 39.3 128.9

91.4 300 25.5 83.7 31.1 102 35.1 115.2 38 124.7 41.2 135.2

121.9 400 26.8 87.9 32.4 106.3 36.5 119.8 39.3 128.9 42.5 139.4

152.4 500 27.8 91.2 33.5 109.9 37.5 123 40.4 132.5 43.6 143

From figure 1, as the exposure time increases, so does the peak wind speed throughout 

the depth of the boundary layer. The steady state wind profiles obtained from the peak wind profile 

for design purposes are based on the following:2

U(z) = u(z) 1+

18.3

z

!

"

#

$

0.283%0.435e

%0.2u18.3 ( )

1.98 %1.887e

%0.2u18.3

!

"

&

&

&

&

&

#

$

'

'

'

'

'

!

"

&

&

&

&

&

#

$

'

'

'

'

'

, (3)

where u(z) is the peak wind found in equation (1), z is height in meters ANG, and u18.3 is the 

known peak wind speed at 18.3 m (60 ft).

2.1.1.2 Ground Wind Gust Factors. The gust factor (G) is defined as:

G = u

u , (4)

where u is the maximum wind speed at height z within an average period (t), and u is the mean 

wind speed associated with τ, given by:

u = 1

!

ui(t)dt , 0

!

! (5)

where ui(t) is the instantaneous wind speed at time (t), and t is the time reckoned from the beginning of the averaging period.

If t=0, then u =u according to equation (5), and it follows from equation (4) that G=1. 

As t increases, u departs from u, u ≤ u, and G > 1. Also, as τ increases, the probability of finding amaximum wind of a given magnitude increases. In other words, the maximum wind speed 
9

increases as τ increases. In the case of u → 0 and u≥0 ( u =0 might correspond to windless 

free convection), G ∞→ . As u or u increases, G tends to decrease for fixed t > 0, while for very 

high wind speeds, G tends to approach a constant value for given values of z and t. Finally, as z

increases, G decreases. Thus, the gust factor is a function of the averaging time (t) over which the 

mean wind speed is calculated, the height (z), and wind speed (mean or maximum).

Investigations of gust factor data have revealed that the vertical variation of the gust factor 

can be described with the following relationship:3

G = 1+

1

g0

18.3

z

!

"

# $

%

&

p

, (6)

where z is the height in meters ANG. The parameter (p), a function of the 18.3 m (60 ft) peak wind 

speed in m/s, is given by:

p = 0.283! 0.435e !0.2u18.3 ( ) . (7)

The parameter g0 depends on the averaging time and the 18.3 m (60 ft) peak wind speed 

and is given by:

g0 = 0.085 ln !

10

!

"

# $

%

&

2

' 0.329 ln !

10

!

"

# $

%

& +1.98 '1.887e

'0.2u18.3 , (8)

where τ is given in minutes and u18.3 is given in m/s.

These relationships are valid for u18.3 ≥ 4 m/s (13.1 ft/s) and τ ≤ 10 min. In the interval 

10 min ≤ τ ≤ 60 min, G is a slowly increasing monotonic function of τ, and for all engineering purposes, the 10-min gust factor (τ=10 min) can be used as an estimate of the gust factors associated 

with averaging times >10 min and <60 min.

The calculated 10-min gust factors for values of u18.3 in the interval 4.63 m/s (15.2 ft/s) 

≤ u18.3 ≤ ∞ are presented in table 3 in the metric and U.S. customary units for u18.3 and z. As an 

example, the gust factor profile for τ=10 min and u18.3 = 9.3 m/s (30.5 ft/s) is given in table 4. Since 

the basic wind statistics are given in terms of hourly peak wind, the τ=10-min gust factors are used 

to convert the peak winds to mean winds by dividing by G. All gust factors in these sections are 

expected values for any particular set of values for u, τ, and z.
10

Table 3. Ten-minute gust factors for the ER.

Reference Height 

18.3 m (60 ft) Height ANG in Meters (ft)

10

(32.8)

18.3

(60)

30.5

(100)

61

(200.1)

91.4

(299.8)

121.9

(399.9)

152.4

(500)

Peak Wind

(m/s) (ft/s)

4.63 15.2 1.868 1.812 1.767 1.71 1.679 1.658 1.642

5.15 16.9 1.828 1.766 1.718 1.657 1.624 1.602 1.585

5.66 18.6 1.795 1.729 1.678 1.614 1.58 1.556 1.539

6.18 20.3 1.768 1.699 1.645 1.579 1.544 1.52 1.502

6.69 21.9 1.746 1.674 1.618 1.552 1.514 1.489 1.471

7.21 23.7 1.727 1.652 1.595 1.525 1.488 1.464 1.446

7.72 25.3 1.712 1.634 1.576 1.505 1.467 1.442 1.424

8.24 27 1.698 1.619 1.559 1.487 1.449 1.424 1.409

8.75 28.7 1.686 1.606 1.545 1.472 1.424 1.409 1.39

9.27 30.4 1.676 1.594 1.532 1.459 1.421 1.395 1.377

9.78 32.1 1.668 1.584 1.522 1.447 1.409 1.384 1.365

10.3 33.8 1.66 1.575 1.512 1.437 1.399 1.374 1.355

12.9 42.3 1.634 1.545 1.48 1.403 1.365 1.339 1.321

15.4 50.5 1.619 1.528 1.462 1.385 1.346 1.321 1.302

∞ ∞ 1.599 1.505 1.437 1.359 1.32 1.295 1.277

Table 4. Gust factor profile for τ=10 min and u18.3=9.27 m/s (30.4 ft/s).

Height

(m) (ft) Gust Factors

10 32.8 1.676

18.3 60 1.594

30.5 100 1.532

61 200.1 1.459

91.4 299.8 1.421

121.9 399.9 1.395

152.4 500 1.377

2.1.1.3 Spectral Gust Environment. At any moment in time, in a fully developed turbulent 

flow, there will exist a departure in the instantaneous wind vector from the quasi-steady state wind 

vector. This departure will have two horizontal turbulent components, longitudinal and lateral, 

which are, respectively, parallel and perpendicular to the quasi-steady state wind vector. The 

vertical turbulence component is assumed to equate to the lateral component. Because of these 

departures from the quasi-steady state wind vector, design of the launch vehicle must take into consideration the vehicle’s ability to withstand either the spectral gust environment or the discrete gust 

environment from any azimuth induced by the turbulent flow.2
11

The longitudinal, lateral, and vertical turbulent components at height z ANG are given by:

S( f ) =

A z

18.3

!

"

#

$

a

z

U18.3

2

Uz

!

"

%

#

$

&

1+ B 18.3

z

!

"

#

$

C2 fz

Uz

!

"

%

#

$

&

!

"

%

#

$

&

! C4

"

%

%

#

$

&

&

b , (9)

where the values for the nondimensional constants A, B, a, b, C2, and C4, given in table 5, are 

a function of the turbulent components, z is height in meters ANG, Uz is the steady state wind 

speed (m/s) at height z, and U18.3 is the steady state wind speed (m/s) at 18.3 m (60 ft).The quantity S(f) is a spectral density function of frequency f. S(f) is defined such that integration over the 

frequency domain, 0 ≤ f ≤ ∞, yields the variance of the component of turbulence of concern. The 

units of S(f) are m2/s2 (cycle/s)–1 and the units of f are cycles/s.

Table 5. Nondimensional constraints for the longitudinal, lateral, and vertical 

components of turbulence for the vehicle on-pad, transportation, 

and launch operation phases at the ER.

Component A B a b C2 C4

Longitudinal 1.35 29.035 –1.1 1.972 1 0.845

Lateral and vertical 0.258 9.059 –0.93 2.134 0.58 0.781

2.1.1.4 Discrete Gust Model. The discrete gust model consists of longitudinal, lateral, 

and vertical discrete gusts given by:

V =

Vm

2

1! cos !

d

dm

!

"

#

$

%

&

!

"

#

$

%

&, 0 ' d ' 2dm , (10)

where Vm is the gust magnitude, dm is the gust half-width, and d is distance. The discrete gusts 

are applied individually, and the resulting loads and responses are combined with the loads 

and responses resulting from the steady state wind profile with appropriate statistical procedures 

to determine design loads and responses.2 Refer to section 3.2 for a more complete description 

and application of the discrete gust model.

The Natural Environments Branch at MSFC has an extensive database consisting of surface 

observations from meteorological towers measuring wind speed and direction from the surface up 

to 150 m (492.1 ft) that can be utilized in support of vehicle development. These observations can 

be used to determine peak/steady state wind speed profile envelopes for a specified risk level, reference period, and spectral/discrete gust moments for input into load simulations.
12

2.1.2 Edwards Air Force Base

EAFB is located on the western edge of the Mojave Desert approximately 145 km (78.3 nmi) 

northeast of Los Angeles, CA, at 34°54 ́ N., 117°52 ́ W. The field elevation is 701.6 m (2,302 ft).

EAFB lies on a desert plateau, averaging 731.5 m (2,400 ft) above sea level. The plateau is generally 

flat and sandy with typical cacti and scrub vegetation, and no open water. The Rogers and Rosamond dry lakes on base are the only truly flat surfaces in the area. Sudden moderate to heavy rainfall can cover the dry lakes with water up to 0.3 m (1 ft) deep, which can be a source of fog after 

frontal passages. EAFB is surrounded southeast through northwest by mountain ranges. The low 

Garlock Range lies north of the base a few kilometers, but rises only approximately 152.4 m (500 ft).

More significant are the Tehachapi Mountains, 55.6 km (30 nmi) to the west and northwest, with 

peaks to 2,438.4 m (8,000 ft). Also, the San Gabriel Mountains lie 48 km (25.9 nmi) to the south 

with peaks to 3,048 m (10,000 ft). The topography changes little in character to the east, but several 

large mountains of the Sierra Nevadas, oriented north-south, lie 48 km (25.9 nmi) to the north of 

the base.4

Wind becomes a complex problem at EAFB as speeds and directions may vary greatly in 

time and space. Speeds to 20.6 m/s (67.6 ft/s) are not uncommon with frontal passage but are usually of short duration. Topography also affects the local wind. During the late spring and summer, a daily temperature and pressure differential develops between the high desert and the coastal 

basins, which causes a heating wind to occur during the late afternoon and evening. The direction 

is southwesterly and speed can be up to 20.6 m/s (67.6 ft/s). Because of the blocking effect of the 

Tehachapi Mountains, EAFB rarely experiences northwest winds. Northeast winds are expected 

tobe funneled through the Owens Valley.4

Wind data and risk levels are based upon the concept of providing comprehensive coverage 

and minimizing risk and cost, while maximizing operational capability. For deorbit, descent, and 

landing/post-landing phases, where EAFB wind conditions can be monitored, the design risk is 

generally 5% for the worst month. This provides for adequate operational capability and scheduling 

flexibility and, combined with the wind monitoring, results in very low-risk operations. For those 

flight regimes where the atmospheric conditions cannot be monitored, the design risk must necessarily be significantly lower and is set at 1%.

Table 6 shows the 5% risk peak wind speeds for various exposure periods at EAFB at the 

4 m (13.1 ft) ANG reference height.5

Table 6. EAFB 5% risk, peak wind speeds versus exposure period 

at 4 m (13.1 ft) ANG reference height.

Exposure

Period

(hr)

5% Risk Wind Speed

(m/s) (ft/s)

1 18 59.1

2 19 62.3

3 20 65.6

24 23 75.5
13

Equation (11) can be used to calculate peak wind speeds above the 4 m (13.1 ft) reference 

height:5

Uh =U4

h

4

!

"

# $

%

&

0.085

, (11)

where Uh is in m/s, U4 is the peak wind speed at the 4 m (13.1 ft) reference height, and h is height 

in m ANG. The EAFB peak wind speed profile, as calculated with equation (11), is shown in 

table 7.

Table 7. EAFB peak wind speed profile for 5% risk, 1 hr exposure,

4 m (13.1 ft) ANG reference height.

Height ANG Uh Peak Wind Speed

(m) (ft) (m/s) (ft/s)

4 13.1 18 59.1

30.5 100.1 21 68.9

61 200.1 23 75.5

91.4 299.8 24 78.7

121.9 399.9 24 78.7

152.4 500 25 82

295 967.8 26 85.3

Equation (12) can be used to calculate the steady state wind profile that is associated with 

the peak wind profile given by equation (11):5

Uh = Uh

G U( ) h

, (12)

where Uh is the steady state wind at height h, Uh is the peak wind at height h, and G is the gust 

factor which is a function of peak wind speed. The gust factor (G) is calculated by equation (13):5

G = 0.9936 + 0.895e

(!0.05UhC )

+10.09(UhC)

(!1.19) , (13)

where Uh is in m/s and C is 1.94254 s/m.

Table 8 contains selected values of the steady state wind profile at various heights ANG 

for the worst month.5
14

Table 8. EAFB steady state wind speed profile for 5% risk,

1 hr exposure, 4 m (13.1 ft) ANG reference height.

Height ANG Wind Speed

(m) (ft) (m/s) (ft/s)

4 13.1 14 45.9

30.5 100.1 17 55.8

61 200.1 19 62.3

91.4 299.8 20 65.6

121.9 399.9 20 65.6

152.4 500 21 68.9

295 967.8 22 72.2

Wind shear below the 295 m (967.8 ft) level can be computed first by selecting a point (shear 

computation point) on the steady state wind profile and then linearly interpolating (not to exceed 

0.25/s shear rate) to a zero wind (or higher if constrained by 0.25/s limit) condition at the surface 

of the Earth. The mean (steady state) wind profile will then be used above the shear computation 

point with the linear extension below. A necessary number of shear computation points must be 

used up to the 295 m (967.8 ft) level for a sufficient number of design steady state wind envelopes 

to ensure an adequate vehicle control system design study.1

The 5% risk vector wind shear in the worst month for heights below approximately 762 m 

(2,500 ft) ANG is 15 m/s (49.2 ft/s).5 This wind shear should be representative of low-level jet 

conditions since the shear calculations were made from 04:00 LST observations, a time when that 

phenomenon is most likely to be present.

The Dryden Spectral gust environment with longitudinal (u), lateral (v), and vertical (w) 

spectral density functions is given by:

• Longitudinal: !u(!) = "2 2L

#

1

1+ (L!)

2 (14)

and

• Lateral vertical: !v,w(!) = "2 L

#

1+ 3(L!)

2

1+ (L!)

2 "

# $%

2 , (15)

where φ(Ω) is the gust spectral density function with units of m2/s2 (cGy/m)–1 at wave number Ω

with units of cGy/m.

The quantities σ and L denote the standard deviation and integral scales of turbulence.

The longitudinal component of turbulence (u) is defined as the component of turbulence parallel 

to the horizontal projection of the flight path of the vehicle. The lateral component (v) lies in the 

horizontal plane and is perpendicular to the longitudinal component, and the vertical component 
15

(w) is perpendicular to the other two components and is assumed to equal v. The spectral density 

functions are defined such that integration over the domain 0 ≤ Ω ≤ ∞ yields the variance. In order 

to define the design input forcing function for spectral response calculations and the simulation 

of forcing function time histories along the flight path of the vehicle, the turbulent velocity vector 

must be added to the steady state wind vector to define the instantaneous wind vector. To generate atmospheric turbulence time sequences, apply equations (14) and (15) in conjunction with the 

procedures and equations given in sections 2.3.14 and 2.3.14.1 of Johnson.1

Atmospheric turbulence below the 295 m (967.8 ft) ANG level is anisotropic so that the 

statistical properties of turbulence will vary from component to component, i.e., the standard 

deviations and integral scales of turbulence vary from component to component. The longitudinal, 

lateral, and vertical components of turbulence are as defined above. The vertical profiles of the 

standard deviations and the integral scales of turbulence requirements for the flight phases below 

the 295 m (967.8 ft) ANG level are given by:

! h = ! 4

h

4

!

"

# $

%

&

p

(16)

and

Lh = L4

h

4

!

"

# $

%

&

q

, (17)

where h denotes height ANG in m, and σ4 and L4 denote the 4 m (13.1 ft) level values of σ and L, 

and p and q are nondimensional parameters.

The design values of these parameters (σ, L, p, and q) for each component of turbulence, 

which are used to establish design spectral gust requirements for the vehicle descent and landing, 

below the 295 m (967.8 ft) ANG level, are given in table 9.2

Table 9. Dryden turbulence spectra parameters for altitudes below the 295 m 

(967.8 ft) ANG level.

Component

σ4

p

L4

(m/s) (ft/s) (m) (ft) q

Longitudinal 2 6.6 0.16 11.7 38.39 0.65

Lateral 1.33 4.4 0.25 5.2 17.06 0.83

Vertical 0.83 2.7 0.36 2 6.56 1.05

2.2 Solar Radiation

The total (global) solar radiation measured on a horizontal plane at the surface is a function of the intensities of direct solar radiation from the Sun and diffuse radiation from the total 

sky atmosphere. The total amount of direct solar radiation striking the surface is dependent on the 
16

solar zenith angle (SZA) (θ). Maximum intensity of direct solar radiation to a horizontal plane on 

a daily basis will occur when the SZA is the smallest. Seasonally, the maximum intensities of direct 

solar radiation are a function of the Earth’s distance to the Sun. For example, the maximum intensity of direct solar radiation occurs in December, near perihelion, in the early afternoon hours, 

when θ is the smallest.

A diffuse radiation incident at the surface depends upon the humidity and the amount of 

scattering particles present in the atmosphere. When the atmosphere is clear and dry, diffuse radiation is minimized and increases with increasing humidity and/or scattering particles.

For design studies, when radiation data are used, direct solar radiation is assumed to emanate from a single point source (the Sun) and travel in parallel rays. Since absorbed solar radiation 

is converted to heat, differential heating of surfaces occurs (due to differential absorption, e.g., 

direct sun versus shade, or differences in surface absorptivity of different system components).

Unequal heating may result in stresses or deformations to the vehicle or GSE.

Computations of total (direct plus diffuse) horizontal solar radiation (ITH) require measurements of direct horizontal radiation (IDH) and diffuse sky radiation (IdH) on a horizontal surface.1

The relationship is as follows:

ITH = IDH + IdH . (18)

After subtracting out the horizontal diffuse radiation (IdH) from the total horizontal solar radiation (ITH), the resultant direct horizontal solar radiation (IDH) can be used to compute the direct 

incient solar radiation (IDI) by the following:

IDI = IDH

cos! , (19)

where IDI is the direct incident solar radiation, IDH is the direct horizontal solar radiation, 

and θ is the SZA.

To determine the amount of solar radiation on a south-facing surface, with tilt angle β, 

the following equations may be used:

ID! = IDH (cos! + tan" cos# sin!) , (20)

where IDβ = intensity of direct solar radiation on a (more or less) south-facing surface with tilt 

angle β, IDH the direct horizontal solar radiation=ITH – IdH, α the Sun’s azimuth measured from 

the south direction (toward west positive), θ , the SZA (angle between local zenith and direction 

to Sun), and β the tilt angle of surface (angle between local zenith and normal to plane of surface).

Solar radiation data for the locations in this section were obtained from the National Solar 

Radiation Database, 1991–2005.6 The database consists of modeled values and, when available, 

measured data of total (global) horizontal solar radiation (ITH), direct incident solar radiation to 
17

a plane normal to the Sun (IDI), and diffuse horizontal radiation (IdH). Extreme values of direct 

incident solar radiation (IDI) were first calculated, and the corresponding values of total horizontal 

(ITH) and diffuse (IdH) associated with the extreme direct values were then determined.

2.2.1 Eastern Range

Table 10 shows the frequency distributions for the maximum and the 95th percentile values 

for the different types of solar radiation as a function of LST for the ER in June and December. 

Table 11 shows the same, but for the minimum and 5th percentile values.

Table 10. Extreme high values of solar radiation for the ER.

Time of Day 

(LST)

Total (Direct+Diffuse) 

Solar Radiation to a 

Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Max 95% Max 95% Max 95%

June

05:00 – – – – – –

06:00 11 2 8 1 56 49

07:00 144 157 51 97 440 280

08:00 341 296 82 73 613 528

09:00 535 536 106 144 701 637

10:00 777 713 130 152 833 722

11:00 901 852 161 166 823 766

12:00 965 939 146 178 844 782

13:00 995 954 169 168 833 793

14:00 1,007 921 203 167 835 784

15:00 836 822 125 160 819 758

16:00 686 660 123 133 766 718

17:00 516 463 91 97 753 639

18:00 287 265 62 79 621 515

19:00 105 90 41 46 376 268

20:00 3 3 3 3 20 3

21:00 – – – – – –

December

07:00 – – – – – –

08:00 53 11 19 4 373 185

09:00 236 255 39 77 731 631

10:00 422 399 53 73 864 768

11:00 536 535 56 82 927 833

12:00 651 615 70 85 952 862

13:00 667 628 71 84 957 875

14:00 614 583 67 82 942 859

15:00 497 476 59 82 899 814

16:00 327 314 46 57 808 723

17:00 161 144 34 40 630 538

18:00 4 12 2 8 122 85

19:00 – – – – – –

 * Associated with corresponding direct incident solar radiation values.
18

Table 11. Extreme low values of solar radiation for the ER.

Time of Day 

(LST)

Total (Direct+Diffuse) 

Solar Radiation to a 

Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Min 5% Min 5% Min 5%

June

05:00 – – – – – –

06:00 – – – – – –

07:00 19 52 19 52 – –

08:00 50 115 50 115 – –

09:00 86 191 86 191 – –

10:00 108 265 108 265 – –

11:00 131 257 131 257 – –

12:00 140 349 140 349 – –

13:00 150 363 150 363 – –

14:00 142 276 142 276 – –

15:00 127 139 127 139 – –

16:00 107 108 107 108 – –

17:00 71 76 71 76 – –

18:00 40 56 40 56 – –

19:00 11 22 11 22 – –

20:00 – – – – – –

21:00 – – – – – –

December

07:00 – – – – – –

08:00 – – – – – –

09:00 32 42 32 42 – –

10:00 61 77 61 77 – –

11:00 77 102 77 102 – –

12:00 92 117 92 117 – –

13:00 96 108 96 108 – –

14:00 93 115 93 115 – –

15:00 73 95 73 95 – –

16:00 47 68 47 68 – –

17:00 21 37 21 37 – –

18:00 – – – – – –

19:00 – – – – – –

* Associated with corresponding direct incident solar radiation values.
19

These 2 months were selected because they best represent the periods of year with the 

longest (June) and shortest (December) durations of daylight.1 Maximum intensity of direct normal incident solar radiation in the early afternoon hours is greater in December, even though the 

SZA is larger than in June. However, the length of exposure to solar radiation for the vehicle and/

or GSE in June is longer than in December. The actual radiation absorbed by a surface is a function of the surface optical properties and the surface geometry relative to the Sun vector. Figure 2 

shows the 95th percentile value of the direct incident solar radiation to a plane normal to the Sun 

for June and December.

0

100

200

300

400

500

600

700

800

900

1,000

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Time (LST)

F2

Direct Incident Solar Radiation (W/m2)

June

December

Figure 2. 95th percentile value of direct incident solar radiation on a plane normal 

to the Sun for the ER.
20

2.2.2 Edwards Air Force Base

Table 12 shows the frequency distributions for the maximum and the 95th percentile values 

for the different types of solar radiation as a function of LST for EAFB in June and December.1 

Table 13 shows the same information, but for the minimum and 5th percentile values. The actual 

radiation absorbed by a surface is a function of the surface optical properties and the surface 

geometry relative to the Sun vector. Figure 3 shows the 95th percentile value of the direct incident 

solar radiation to a plane normal to the Sun for June and December.
21

Table 12. Extreme high values of solar radiation for EAFB.

Time of Day

(LST)

Total (Direct+Diffuse) 

Solar Radiation to 

a Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Max 95% Max 95% Max 95%

June

04:00 0 0 0 0 0 0

05:00 4 4 2 2 85 59

06:00 109 102 24 24 551 499

07:00 313 307 41 47 774 738

08:00 528 522 56 64 875 847

09:00 724 720 68 81 929 905

10:00 884 867 80 88 961 938

11:00 995 981 86 88 979 961

12:00 1,049 1,023 89 82 986 966

13:00 1,039 1,036 86 102 985 964

14:00 974 964 81 95 980 952

15:00 848 841 74 87 956 930

16:00 681 664 65 77 925 895

17:00 477 500 52 89 859 829

18:00 261 274 35 57 741 705

19:00 71 67 20 26 462 369

20:00 – – – – 30 14

21:00 – – – – – –

December

06:00 – – – – – –

07:00 4 2 1 2 98 30

08:00 104 81 21 18 605 513

09:00 274 236 31 36 828 781

10:00 427 407 41 44 917 895

11:00 549 496 58 50 959 935

12:00 596 590 68 70 979 961

13:00 589 541 79 51 978 958

14:00 509 483 59 62 962 930

15:00 398 349 63 38 918 899

16:00 248 214 61 46 842 806

17:00 24 14 6 3 341 263

18:00 – – – – – –

* Associated with corresponding direct incident solar radiation values.
22

Table 13. Extreme low values of solar radiation for EAFB.

Time of Day

(LST)

Total (Direct+Diffuse) 

Solar Radiation to

a Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Min 5% Min 5% Min 5%

June

04:00 – – – – – –

05:00 – – – – – –

06:00 34 48 34 40 – 51

07:00 75 211 75 146 – 185

08:00 137 364 137 220 – 266

09:00 195 512 195 304 – 294

10:00 242 650 242 356 – 351

11:00 343 808 343 435 – 402

12:00 362 788 362 361 – 438

13:00 359 604 359 260 – 355

14:00 339 680 339 394 – 314

15:00 147 579 147 364 – 269

16:00 122 364 122 227 – 205

17:00 151 206 151 151 – 116

18:00 54 108 54 83 – 88

19:00 – – – – – –

20:00 – – – – – –

21:00 – – – – – –

December

06:00 – – – – – –

07:00 – – – – – –

08:00 10 28 10 28 – –

09:00 33 84 33 84 – –

10:00 61 149 61 149 – –

11:00 84 179 84 177 – 6

12:00 90 191 90 187 – 6

13:00 89 171 89 171 – –

14:00 77 165 77 165 – –

15:00 58 123 58 123 – –

16:00 30 69 30 65 – 21

17:00 1 15 1 15 – –

18:00 – – – – – –

* Associated with corresponding direct incident solar radiation values.
23

0

100

200

300

400

500

600

700

800

900

1,000

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Time (LST)

F3

Direct Incident Solar Radiation (W/m2)

June

December

Figure 3. 95th percentile value for direct incident solar radiation on a plane normal 

to the Sun for EAFB.

2.2.3 White Sands Missile Range

Table 14 shows the frequency distributions for the maximum and the 95th percentile values for the different types of solar radiation as a function of LST for White Sands Missile Range 

(WSMR) in June and December.1 Table 15 shows the same information, but for the minimum and 

5th percentile values. The actual radiation absorbed by a surface is a function of the surface optical 

properties and the surface geometry relative to the Sun vector. Figure 4 shows the 95th percentile 

value of the direct incident solar radiation to a plane normal to the Sun for June and December.
24

Table 14. Extreme high values of solar radiation for WSMR.

Time of Day

(LST)

Total (Direct+Diffuse) 

Solar Radiation to

a Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Max 95% Max 95% Max 95%

June

05:00 – – – – – –

06:00 54 53 18 25 392 309

07:00 254 237 37 47 744 658

08:00 482 460 54 62 876 814

09:00 696 667 68 78 942 884

10:00 874 847 78 96 980 923

11:00 1,003 969 85 103 1,000 943

12:00 1,059 1,046 91 120 992 950

13:00 1,067 1,051 92 114 993 953

14:00 1,013 992 89 112 986 945

15:00 898 866 83 84 969 924

16:00 732 714 73 94 936 883

17:00 528 502 60 66 879 821

18:00 305 278 43 50 768 699

19:00 95 88 24 29 513 425

20:00 2 1 1 1 48 20

21:00 – – – – – –

December

06:00 – – – – – –

07:00 1 – – – 56 7

08:00 138 123 54 43 712 676

09:00 307 257 53 57 875 835

10:00 472 430 60 58 955 921

11:00 575 543 61 49 990 961

12:00 646 597 66 54 1,019 982

13:00 620 606 52 54 1,007 985

14:00 571 552 59 60 987 969

15:00 447 416 46 49 940 907

16:00 282 254 38 41 843 811

17:00 80 66 17 18 571 499

18:00 1 – – – 49 11

19:00 – – – – – –

* Associated with corresponding direct incident solar radiation values.
25

Table 15. Extreme low values of solar radiation for WSMR.

Time of Day

(LST)

Total (Direct+Diffuse) 

Solar Radiation to a 

Horizontal Surface*

(W/m2)

Diffuse Radiation 

to a Horizontal 

Surface*

(W/m2)

Direct Incident

Solar Radiation 

Normal to the Sun

(W/m2)

Min 5% Min 5% Min 5%

June

05:00 – – – – – –

06:00 – – – – – –

07:00 40 96 40 93 – 12

08:00 76 80 76 76 – 8

09:00 104 358 104 321 – 55

10:00 132 310 132 252 – 71

11:00 154 538 154 439 – 109

12:00 164 636 164 524 – 115

13:00 293 730 293 647 – 85

14:00 159 338 159 242 – 103

15:00 138 386 138 335 – 61

16:00 112 271 112 236 – 49

17:00 75 194 75 188 – 12

18:00 44 69 44 69 – –

19:00 10 26 10 26 – –

20:00 – – – – – –

21:00 – – – – – –

December

06:00 – – – – – –

07:00 – – – – – –

08:00 – – – – – –

09:00 38 60 38 60 – –

10:00 67 116 67 116 – –

11:00 83 146 83 146 – –

12:00 98 175 98 175 – –

13:00 95 172 95 172 – –

14:00 86 162 86 162 – –

15:00 65 119 65 119 – –

16:00 39 74 39 74 – –

17:00 9 23 9 23 – –

18:00 – – – – – –

19:00 – – – – – –

* Associated with corresponding direct incident solar radiation values.
26

0

100

200

300

400

500

600

700

800

900

1,000

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Time (LST)

F4

Direct Incident Solar Radiation (W/m2)

June

December

Figure 4. 95th percentile value of direct incident solar radiation on a plane normal 

to the Sun for WSMR.

2.3 Temperature

The normal thermodynamic definition of temperature, the derivative of energy with respect 

to entropy, applies to the atmospheric environment. The surface temperatures are primarily the 

result of a balance between incoming and outgoing radiative energy along with convection effects. 

As a black-body radiator, the clear sky is considered equivalent to a cold surface. The radiation 

temperature of the clear sky is the same during the day and night. It is the clear sky acting as 

a cold sink, without the incoming solar radiation heating of the surface, which causes near-surface 

air temperatures to be lower at night than during the day. At night, clouds act as a barrier to the 

outgoing radiation. Clouds absorb outgoing infrared radiation and emit radiation at a lower temperature, making the effective atmospheric temperatures warmer than the clear sky. Thus, the air 

near the ground will not cool off to as low a temperature on a cloud-covered night. Therefore, 

the greatest cooling of the Earth’s surface occurs with calm winds (no mixing with warmer air) 

and clear skies.

2.3.1 Eastern Range

The intensity of incoming radiative energy has a seasonal dependence, hence the annual 

variability in surface temperature at the ER. The coldest temperatures observed at the ER have 

occurred in the early morning hours and were associated with calm winds and clear skies after the 

passage of a cold front. Maximum air temperatures observed occur in the summer months during 

the hours of 14:00 to 16:00 LST after prolonged exposure and absorption of radiative energy at the 

surface. Expected extreme surface temperatures and the sky radiation values for the ER are shown 

in table 16.1
27

Table 16. Surface air and sky radiation temperature extremes for the ER.

Area

Surface Air Temperature Extremes* Sky Radiation

Maximum Minimum

Extreme Minimum

Equivalent

Temperature

Equivalent

Radiation

(W/m2 Extreme 95%** Extreme 95%** )

KSC,

Florida

°C 38 35 –6 0.6 –15 252

°F 100.4 95 21.2 33.1 5

 * The extreme maximum and minimum temperatures will be encountered during periods of wind speeds less than 

or approximately 1 m/s (3.3 ft/s).

** Based on daily extreme (maximum or minimum) observations for worst month. 

The monthly mean temperature and monthly temperature extremes for the ER are shown 

in figure 5 and table 17. The data are based on hourly surface observations at the ER from 1957 

through 2002. It is possible that the true extreme temperature was slightly different from that 

shown in figure 5 and table 17 because the observations were taken on the hour, not necessarily 

at the time of extreme occurrence.

40

35

30

25

20

15

10

5

0

–5

–10

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

F5

Temperature (°C)

Maximum

Mean

Minimum

Figure. 5. Monthly maximum, mean, and minimum temperatures for the ER.
28

Table 17. Monthly mean and extreme (maximum and minimum) temperatures for the ER.

Month

Mean

Temperature *,

 **

Maximum

Temperature *, **

Minimum

Temperature *, **

(°C) (°F) (°C) (°F) (°C) (°F)

January 15.6 60.1 30 86 –6 21.2

February 16.2 61.2 31 87.8 –3 26.6

March 18.6 65.5 32 89.6 –1 30.2

April 21.3 70.3 34 93.2 1 33.8

May 23.9 75 36 96.8 7 44.6

June 25.9 78.6 38 100.4 12 53.6

July 26.7 80.1 38 100.4 13 55.4

August 26.6 79.9 37 98.6 18 64.4

September 26.2 79.1 35 95 14 57.2

October 23.7 74.7 34 93.2 4 39.2

November 20.1 68.2 32 89.6 – 32

December 16.7 62.1 31 87.8 –5 23

 * Data from hourly surface observations at the ER.

** POR: 1957–2002.

Table 18 and figure 6 show the probability of a given maximum 3-, 6-, or 12-hr temperature 

change occurring at the ER. For example, from table 18, there is a 90% chance that the maximum 

3-hr temperature change will be ≤4.1 °C (≤7.4 °F). The data in table 18 and figure 6 were obtained 

from hourly surface observations from 1957 through 2002.

Table 18. Cumulative probability for 3-, 6-, and 12-hr temperature change at the ER.

Percentile

Temperature Change Over Specified Period*

3-hr 6-hr 12-hr

(°C) (°F) (°C) (°F) (°C) (°F)

25 0.1 0.2 0.8 1.4 1.8 3.2

50 1 1.8 2.3 4.1 3.8 6.8

75 2.4 4.3 4.4 7.9 6.1 11

90 4.1 7.4 6.6 11.9 8.3 14.9

95 5.4 9.7 8.1 14.6 9.7 17.5

99 8.1 14.6 11.4 20.5 12.4 22.3

* Based on hourly surface observations, 1957–2002.
29

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20

Temperature Change (°C)

F6

Cumulative Probability

3-hour

6-hour

12-hour

Figure 6. Cumulative probability for 3-, 6-, and 12-hr temperature change at the ER 

(based on hourly surface observations, 1957–2002).

2.3.2 Edwards Air Force Base

Expected extreme surface temperatures and the sky radiation values for EAFB are shown 

in table 19.1

Table 19. Surface air and sky radiation temperature extremes for EAFB.

Area

Surface Air Temperature Extremes*

Sky Radiation

Maximum Minimum Extreme Minimum

Equivalent

Temperature

Equivalent

Radiation

(W/m2 Extreme 95%** Extreme 95%** )

EAFB,

California

°C 45 41.7 –15.6 –7.8 –30 198

°F 113 107.1 3.9 17.9 –22

 * The extreme maximum and minimum temperatures will be encountered during periods of wind speeds less than 

 or approximately 1 m/s (3.3 ft/s).

 ** Based on daily extreme (maximum or minimum) observations for worst month. 

The monthly mean temperature and monthly temperature extremes for the EAFB are 

shown in figure 7 and table 20. These data are based on hourly surface observations at EAFB from 

1971 through 2001. It is possible that the true extreme temperature was slightly different from that 

shown in figure 7 and table 20 because the observations were taken on the hour, not necessarily at 

the time of extreme occurrence.
30

–20

–10

0

10

20

30

40

50

F7

Temperature (°C)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

Month

Maximum

Mean

Minimum

Figure 7. Monthly maximum, mean, and minimum temperatures for EAFB.

Table 20. Monthly mean and extreme temperatures for EAFB.

Month

Mean

Temperature *,

 **

Maximum

Temperature *, **

Minimum

Temperature *, **

(°C) (°F) (°C) (°F) (°C) (°F)

January 7.1 44.8 28 82.4 –12 10.4

February 9.5 49.1 28 82.4 –10 14

March 12.3 54.1 32 89.6 –7 19.4

April 15.8 60.4 38 100.4 –3 26.6

May 20.4 68.8 40 104 2 35.6

June 25.5 77.9 44 111.2 6 42.8

July 28.8 83.8 45 113 8 46.4

August 28.2 82.7 44 111.2 8 46.4

September 24.2 75.5 42 107.6 3 37.4

October 17.7 63.9 38 100.4 –7 19.4

November 11.1 51.9 30 86 –8 17.6

December 6.7 44.1 26 78.8 –15 5

 * Data from hourly surface observations at EAFB.

** POR: 1971–2001.
31

Table 21 and figure 8 show the probability of a given maximum 3-, 6-, or 12-hr temperature 

change occurring at EAFB. For example, from table 21, there is a 90% chance that the maximum 

3-hr temperature change will be ≤8 °C (≤14.4 °F). The data in table 21 and figure 8 were obtained 

from hourly surface observations from 1971 through 2001.

Table 21. Cumulative probability for 3-, 6-, and 12-hr temperature change at EAFB.

Percentile

Temperature Change Over Specified Period*

3-hr 6-hr 12-hr

(°C) (°F) (°C) (°F) (°C) (°F)

25 1.4 2.5 3 5.4 3.5 6.3

50 3.3 5.9 6.1 11.0 8 14.4

75 5.7 10.3 9.9 17.8 12.9 23.2

90 8 14.4 13.4 24.1 16.7 30.1

95 9.3 16.7 15.5 27.9 18.5 33.3

99 11.5 20.7 18.7 33.7 21 37.8

* Based on hourly surface observations, 1971–2001.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25

Temperature Change (°C)

F8

Cumulative Probability

3-hour

6-hour

12-hour

Figure 8. Cumulative probability for 3-, 6-, and 12-hr temperature change at EAFB 

(based on hourly surface observations, 1971–2001).

2.4 Pressure

Atmospheric pressure (also called barometric pressure) is the force exerted, resulting from 

gravitational attraction, by the mass of the column of air of unit cross section lying directly above 

the area in question. It is expressed as force per unit area and is typically defined in the unit of 
32

hectopascals (hPa (lbf/in2)). Because variations in surface elevation, and hence the volume of the 

air column over the area, surface pressures recorded are reduced to a standardized sea level pressure value for continuity purposes.

The total variation of pressure from day to day is relatively small. For example, a gradual 

rise or fall in pressure of 3 hPa (0.04 lbf/in2) and then a return to original pressure can be expected 

within a 24-hr period. Diurnal, semidiurnal, and terdiurnal tidal variations can all affect the normal surface atmospheric pressure pattern. Rapid and slightly greater variations of pressure occur as 

the result of the passage of synoptic systems, and the passage of a hurricane can cause somewhat 

larger changes in the atmospheric pressure environment. Typically, a maximum pressure change of 

6 hPa (0.09 lbf/in2) (rise or fall) can be expected within a 1-hr period at all localities. Only the pressure drop in a tornado is significant and can exceed 20% of the ambient pressure during the few 

seconds of its passage.

2.4.1 Eastern Range

Pressure variations at the ER are representative of the descriptions given in the previous 

paragraph. However, due to the ER’s susceptibility to tropical storm passages, extreme sea level 

pressure minimums observed are 1%–2% lower than extreme sea level pressure minimums at interior continental locations.1 Figure 9 represents hourly sea level pressure mean and extreme values 

from the hourly measurements during each month observed at the ER for a period from 1957 to 

2002. The extreme minimum in September was due to the passage of a tropical storm near the ER. 

Further discussion of hurricanes is presented in section 2.9. Values plotted in figure 9 are provided 

in table 22.

1,050

1,040

1,030

1,020

1,010

1,000

990

980

970

960

F9

Pressure (hPa)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Maximum

Mean

Associated With Tropical Cyclone

Minimum

Figure 9. Maximum, mean, and minimum sea level air pressure at the ER 

(based on hourly surface observations, 1957–2002).
33

Table 22. Monthly sea level pressure at the ER (based on hourly surface observations,

1957–2002).

Sea Level

Pressure 

Extreme Jan Feb Mar Apr May Jun

Min. (hPa) 999.3 993.9 991.4 1,000.7 1,001.7 996.6

(lbf/in2) 14.494 14.416 14.379 14.514 14.529 14.455

Mean (hPa) 1,020 1,018.9 1,017.7 1,017.8 1,016.5 1,016.4

(lbf/in2) 14.794 14.778 14.761 14.762 14.743 14.742

Max. (hPa) 1,037.4 1,035.9 1,032.5 1,030.8 1,026.4 1,026.4

(lbf/in2) 15.046 15.025 14.975 14.951 14.887 14.887

Sea Level

Pressure 

Extreme Jul Aug Sep Oct Nov Dec

Min. (hPa) 1,006.8 1,000 973.9 992.9 999.3 1,000.7

(lbf/in2) 14.603 14.504 14.125 14.401 14.494 14.514

Mean (hPa) 1,018 1,017.1 1,015.8 1,016.6 1,018.6 1,020.1

(lbf/in2) 14.765 14.752 14.733 14.745 14.774 14.796

Max. (hPa) 1,025.4 1,024.5 1,025.1 1,032.9 1,031.1 1,035.9

(lbf/in2) 14.872 14.859 14.868 14.981 14.955 15.025

Table 23 and figure 10 show the cumulative probability of sea level pressure at the ER.

Table 23. Cumulative probability of sea level pressure at the ER

(based on hourly surface observations, 1957–2002).

Percentile

Sea Level Pressure

(hPa) (lbf/in2)

1 1,007.1 14.607

5 1,010.8 14.661

10 1,012.5 14.685

25 1,015.2 14.724

50 1,017.9 14.764

75 1,020.3 14.798

90 1,023 14.838

95 1,024.7 14.862

99 1,027.8 14.907
34

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

970 980 990 1,000 1,010 1,020 1,030 1,040

Pressure (hPa)

F10

Cumulative Probability

Figure 10. Cumulative probability of sea level pressure at the ER (based on hourly 

surface observations, 1957–2002).

2.4.2 Edwards Air Force Base

Figure 11 represents hourly sea level pressure mean and extreme values from the hourly 

measurements during each month observed at EAFB for a period from 1971 to 2001. Values 

plotted in figure 11 are provided in table 24.

1,050

1,040

1,030

1,020

1,010

1,000

990

980

970

960

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

F11

Pressure (hPa)

Maximum

Mean

Minimum

Figure 11. Maximum, mean, and minimum sea level air pressure at EAFB 

(based on hourly surface observations, 1971–2001).
35

Table 24. Monthly sea level pressure at EAFB (based

on hourly surface observations, 1971–2001).

Sea Level 

Pressure

Extreme Jan Feb Mar Apr May Jun

Min. (hPa) 993.4 992.1 993 1,001.4 1,000.4 998.3

(lbf/in2) 14.408 14.389 14.402 14.524 14.510 14.479

Mean (hPa) 1,019.6 1,017.6 1,015 1,013.6 1,011.2 1,010

(lbf/in2) 14.788 14.759 14.722 14.701 14.666 14.649

Max. (hPa) 1,035.5 1,035.9 1,034.7 1,026.1 1,024.7 1,020

(lbf/in2) 15.019 15.025 15.007 14.882 14.862 14.794

Sea Level 

Pressure

Extreme Jul Aug Sep Oct Nov Dec

Min. (hPa) 1,001.3 1,000.1 999.2 998.7 995.4 1,000.9

(lbf/in2) 14.523 14.505 14.492 14.485 14.437 14.517

Mean (hPa) 1,010.7 1,011 1,011.2 1,014.4 1,017.7 1,020.1

(lbf/in2) 14.659 14.664 14.666 14.713 14.761 14.796

Max. (hPa) 1,019.4 1,019.6 1,021.9 1,031.1 1,034.2 1,038.7

(lbf/in2) 14.785 14.788 14.822 14.955 15 15.065

Table 25 and figure 12 show the cumulative probability of sea level pressure at EAFB.

Table 25. Cumulative probability of sea level pressure at EAFB

(based on hourly surface observations, 1971–2001).

Percentile

Sea Level Pressure

(hPa) (lbf/in2)

1 1,004.3 14.566

5 1,007 14.606

10 1,008.2 14.623

25 1,010.4 14.655

50 1,013.3 14.697

75 1,017.5 14.758

90 1,022.1 14.824

95 1,024.7 14.862

99 1,029 14.925
36

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

980 990 1,000 1,010 1,020 1,030 1,040

Pressure (hPa)

F12

Cumulative Probability

Figure 12. Cumulative probability of sea level pressure at EAFB (based on hourly 

surface observations, 1971–2001).

2.5 Humidity

The amount of water vapor the atmosphere can hold is a function of the air temperature 

and total atmospheric pressure (partial pressure of dry and moist air). Periods of high water vapor 

content in the atmosphere can have adverse effects on the vehicle and GSE components. Temperatures above 20 °C (68 °F) and relative humidity above 75% are conducive to high growth rates of 

fungi and bacteria.

2.5.1 Eastern Range

The Atlantic Ocean provides an unlimited source of water for evaporation into the atmosphere near the ER. Table 26 provides maximum vapor concentration for the ER.1

Table 26. Maximum vapor concentration for the ER.

Geometric Altitude

Maximum Vapor 

Concentration

Temperature Associated 

With Maximum Vapor 

Concentration

(km) (ft) (g/m3) (lb/ft3) °C °F

0.005 (SFC) 16.4 27 0.00169 30.6 87.1
37

Surface psychrometric data for the ER is presented in table 27 and figure 13. The surface 

psychrometric data are based on hourly surface observations from 1957 to 2002.

Table 27. Surface psychrometric data, dewpoint versus

temperature envelope for the ER.

Temperature Dewpoint

(C°) (F°) (C°) (F°)

–4 24.8 –14 6.8

–4 24.8 –4 24.8

28 82.4 28 82.4

36 96.8 28 82.4

40 104 15 59

11 51.8 –14 6.8

40

30

20

10

0

–10

–20

–30

–20 –10 0 10 20 30 40 50

Temperature (°C)

F13

Dewpoint (°C)

Figure 13. Surface psychrometric data for the ER (based on hourly surface 

observations, 1957–2002).

2.5.2 Edwards Air Force Base

Surface psychrometric data for EAFB is presented in table 28 and figure 14. The surface 

psychrometric data are based on hourly surface observations from 1971 to 2001.
38

Table 28. Surface psychrometric data, dewpoint versus 

temperature envelope for EAFB.

Temperature Dewpoint

(C°) (F°) (C°) (F°)

–7 19.4 –26 –14.8

–16 3.2 –16 3.2

24 75.2 –24 75.2

44 111.2 13 55.4

44 111.2 –12 10.4

40

30

20

10

0

–10

–20

–30

–20 –10 0 10 20 30 40 50

Temperature (°C)

F14

Dewpoint (°C)

Figure 14. Surface psychrometric data for EAFB (based on hourly surface observations, 

1971–2001).

2.5.3 Fog

Fog is a visible aggregate of minute water droplets suspended in the atmosphere near 

the Earth’s surface or a cloud in contact with the Earth’s surface. Fog is responsible for reducing visibility to <1 km (<0.6 mi). Several types of fog can be classified according to formation, 

which include advection fog, frontal fog, up-slope fog, and radiation fog.7 Ground fog is usually 

a radiation fog with a depth of <6 m (19.7 ft), whereas a shallow ground fog has a depth of <2 m 

(<6.6 ft). Fogs are classified as either warm or supercooled fog based on whether the ambient 

temperature is above or below 0 °C (32 °F).

Figure 15 shows a photo of the Challenger (Space Transportation System-6 (STS)-6) rollout 

to pad 39A in fog, December 8, 1982.
39

F15

Figure 15. Challenger (STS-6) rollout to pad 39a in fog, 

December 8, 1982 (courtesy of NASA).

The conditions most favorable for the formation of fog are high relative humidity, light 

surface winds, ground temperature colder than that of the air, no cloud overcast, and an abundance 

of condensation nuclei. Fog occurs more frequently in coastal areas than in inland areas because 

of an abundance of water vapor.

Fog is formed either by cooling the air until the water vapor condenses or by the evaporation of additional water vapor into the air. Common types of fog are: (a) radiation fog, (b) advection fog, (c) up-slope fog, (d) frontal fog, and (e) steam fog.

2.5.3.1 Radiation Fog. Radiation fog forms over land on clear nights when the Earth loses 

heat very rapidly to the atmosphere and the air temperature falls to or below its dewpoint. When 

humidity is high and cooling takes place rapidly, condensation occurs. If there are no winds, the 

fog will be very shallow or will be reduced to a dew or frost deposit. If winds are present (~0.5 m/s 

(1.6 ft/s)), then the fog will thicken and deepen. These fogs do not occur at sea since the sea surface 

does not cool as the land does.

2.5.3.2 Advection Fog. Advection fog is caused by the movement (advection) of mild, 

humid air over a colder surface, and the consequent cooling of that air to below its initial dewpoint. These fogs occur in coastal areas because the moist, warm air moves inland by breezes over 

the colder land in the winter. In summer, the warm, moist air may be carried out to sea, where it 

forms a fog over the cool water (sea fog) and then the sea breeze advects the fog inland. These summer fogs are common along the coast of California.
40

2.5.3.3 Up-Slope Fog. Up-slope fog forms when stable, moist air moves up sloping terrain 

and is adiabatically cooled by expansion to or below its original dewpoint. This cooling produces 

condensation, and fog forms. An up-slope wind is necessary for the formation and maintenance 

of this type of fog. Usually these fogs produce low stratus-type clouds.

2.5.3.4 Frontal Fog. Frontal fog forms in the cold air mass of the frontal system. The 

precipitation from the warm air mass, overrunning the cold air mass, evaporates as it falls through 

and saturates the cold air, thus producing the frontal-type fog. These fogs form rapidly, cover large 

areas, occur frequently in winter, and are associated with slow-moving or stationary fronts.

2.5.3.5 Steam Fog. Steam fog forms by the movement of cold air over a warmer water surface. Steam fog rises from the surface of lakes, rivers, and oceans and has the appearance of rising 

streamers.

2.5.3.6 Ice Fog. Although not classified as a common type fog, ice fog is a fog type referred 

to as ice (crystal) fog that is of interest. This fog normally occurs at high latitudes under calm 

conditions when the air temperature is approximately less than –30 °C (–22 °F), and as water vapor 

from the exhaust of aircraft engines, automobiles, etc., is produced, the vapor changes directly 

to ice particles (crystals) instead of condensing directly to liquid drops. The suspension of the ice 

crystals (10 to 100 μm (3.9×10–4 to 3.9×10–3 in) in diameter) in the atmosphere produces the ice 

fog. Ice fog can persist from a few minutes to several days and is quite a problem in arctic or polar 

regions.

Some typical microphysical characteristics of radiation and advection types of fogs are 

as follows:

• Radiation fog (inland):

– Diameter of drops 

 (average): 10 μm (3.9×10–4 in)

– Typical drop size: 5 to 35 μm (2×10–4 to 1.4×10–3 in)

– Liquid water content: 110 mg/m3 (6.9×10–6 lb/ft3)

– Droplet concentration: 200/cm3 (3,278.7/in3)

– Vertical depth:

• Typical: 100 m (328 ft)

• Severe: 300 m (984.3 ft)

– Horizontal visibility: 100 m (328 ft)

• Advection fog (coastal):

– Diameter of drops (average): 20 μm (7.9×10–4 in)

– Typical drop size: 7 to 65 μm (2.7×10–4 to 2.6×10–3 in)

– Liquid water content: 170 mg/m3 (1.1×10–5 lb/ft3)

– Droplet concentration: 40/cm3 (655.7/in3)

– Vertical depth:

• Typical: 200 m (656.1 ft)

• Severe: 600 m (1,968.5 ft)

– Horizontal visibility: 300 m (984.3 ft)
41

2.5.3.7 Eastern Range Probability of Fog. Figure 16, showing the percentage frequency of 

precipitation or fog with visibility ≤0.8 km (≤0.5 mi) at KSC was developed from historical records 

of hourly observations. Unfavorable climatic conditions occur mainly during summer afternoons 

due to the high frequency of fog from summer afternoon showers in central Florida.

J

F

M

A

M

J

J

A

S

O

D

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23

N

<2 >10

2%

5% 5%

5%

10%

5%5%

>5

5%

Hour (LST)

F16

Month

Figure 16. Probability of precipitation or fog with visibility ≤0.8 km (≤0.5 mi) at KSC.

For climatological studies useful in operational design data for spacecraft and aircraft 

operations, the Department of Transportation, Federal Aviation Administration has produced 

a tabulation of ceilings, visibilities, wind, and weather data by various periods of the day and by 

various temperature and wind categories for 41 airports.8

2.5.3.8 Eastern Range Fog Climatology. A 5-year (1986–1990) study was conducted at KSC 

that was to improve the 90-min forecasts made in support of the Space Shuttle landing at KSC. 

This concerns rapid fog development that would affect the <7 statute mile visibility flight rule 

(4-64) for end-of-mission shuttle landings at KSC. Space Shuttle Program (SSP) flight rule 4-64 

also stated that <5 statute mile can be used as the GO/NO GO decision visibility constraint under 

certain other measurement conditions. Therefore, fog statistics for <7 statute miles and <5 statute 

miles were developed for these two visibility criteria. Over the 5-year study period, 335 fog events 

occurred at KSC for the <7 mile criteria, and 267 fog events occurred for the <5 mile criteria.

The KSC fog season is normally defined as October 1 through April 30, although fog occurs 

there during other months of the year (table 29). The fog climatology for the KSC area is complicated due to the Cape Canaveral topography which includes numerous water bodies (rivers, ocean), 

its land mass, and land/sea breeze effects. Of all the fog cases, only 36 fog events were completely 

analyzed, resulting in KSC fog events falling into three categories: (1) Advection fog (21/36=58%), 

(2) prefrontal fog (13/36=36%), and (3) radiation fog (2/36=6%).
42

Table 29. Normally expected fog days at KSC.9

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

13 10 9 7 7 6 4 6 4 7 8 12 93 Days

However, using all the fog event cases, the time of fog onset and dissipation are given 

in table 30.

Table 30. KSC fog onset and dissipation time: range and peak hour.10

Parameter

Number of KSC Fog Cases

335 267

Visibility criteria <7 statute miles <5 statute miles

Fog onset time: Range of hours 05:00 – 14:59 UTC 05:00 – 14:59 UTC

Fog onset time: Peak hour

(peak hour percent frequency)

09:00 – 09:59 UTC

95/335 = 28.4%

10:00 – 10:59 UTC

46/267 = 17.2%

Fog dissipation time: Range hours 10:00 – >20:00 UTC 10:00 – >20:00 UTC

Fog dissipation time: Peak hour 

(peak hour percent frequency)

13:00 – 13:59 UTC

75/335 = 22.4%

12:00 – 12:59 UTC

62/267 = 23.2%

2.6 Surface Aerosols

The distribution of aerosols is regionally dependent. However, particles may have a ‘local’ 

distribution as well as a ‘regional’ distribution. The local aerosol occurs in the area surrounding 

factories, wild fires, oceans, arid/semiarid regions, and terrestrial biota. Factors such as wind speed, 

distance from source, altitude, and particle size play an important part in determining the makeup 

and concentration of aerosols. On a regional scale, number concentrations of particles in the 

atmosphere increase rapidly with decreasing particle size to sizes smaller than 0.1 μm (3.9×10–6 in) 

radius.

Dust and sand particles are transported through the air by wind blowing across a disturbed 

soil area. Strong winds are required to uplift the submicron sand and dust particles because strong 

adhesive forces exist between the particles and the ground. Dense vegetation and ground cover 

also provide considerable protection from strong winds. The concentration of these particles is 

highly dependent on wind speed (the higher the speed, the greater the volume of sand and dust), 

the nature of the soil, and the amount of moisture in the soil and in the air. Threshold air velocities 

for the input of soil particles into the air increase with different types of soil surfaces in the following order: disturbed soils (except disturbed heavy clay soils), sand dunes, alluvial and Aeolian sand 

deposits, disturbed playa (dry lake) soils, skirts of playa centers, and desert pavements (alluvial 

deposits).1

The larger, more abrasive particles in dust and sand storms are mostly in the lower 2 km 

(1.2 mi) of the atmosphere, although fine dust can reach great heights and travel great distances.
43

Measurements of aerosols in the central Atlantic has shown aerosols of continental origin up to 

at least 1,500 km (809.9 nmi) from land. California has two general regions of dust activity. One 

region extends into southwestern Arizona and covers all of southeastern California with a maximum aerosol measurement occurring north of the Salton Sea and the western Mojave Desert. 

The second region is situated in central California.

Salt particles, whether from the ocean or from areas where salt occurs in nature may be 

detrimental to space vehicles and associated systems because of their corrosive actions and their 

ability to coat transparent areas until they become opaque. Salt attacks many metals, and the 

corrosion is especially rapid at high humidity and high temperature. Salt solutions also provide 

a conductive path that can alter or short electrical circuits.

2.6.1 Eastern Range

In the region around the ER, sea salt particles are the most common aerosol component.1 

The Corrosion Technology Laboratory Web site <http://corrosion.ksc.nasa.gov/index.htm>11 

includes two documents that contain data on the salt contents of the atmosphere at the ER: Local 

Atmosphere Salt Profiles, July 1974, <http://corrosion.ksc.nasa.gov/lasp.htm>12 and the corrosion rate at various locations at ER including the launch pad area <http://corrosion.ksc.nasa.gov/

pubs/099-74.pdf>.13

Most airborne sea salt droplets are formed by the breaking of myriads of air bubbles 

at the surface of the sea. Sea salt particle concentration is also dependent on the wind speed, as 

stronger winds will increase the particle concentration as shown in figure 17.1 The suspended droplets are also sensitive to the atmospheric relative humidity; droplets evaporate when the humidity 

falls below 75%. If humidity measurements above 75% occur, the sea salt particles become droplets 

again.

Atmospheric temperature inversions over the oceans, such as the tropical inversion, tend 

to keep sea salt particles below a few kilometers in altitude. Above such inversions, the particles 

are largely of continental origin, except near clouds or near the residues from dissipated clouds. 

Table 31 lists three average concentrations of sea salt with respect to altitude.1
44

Wind Speed (ms–1)

Sea Salt (

μg–3)

0

1,000

100

10

1

0.1

10 20 30

F17

3

3

3

3

12 4

600–800 m

15 m

Figure 17. Sea salt concentration at two altitudes as a function of wind speed near 

the surface of the sea: 600–800 m altitude (Woodcock)14 and 15 m altitude

(Lovett).15 (The number of observations averaged for each data point is

given, as well as standard deviation.)

Table 31. Mean sea salt particle concentrations in maritime air masses 

and corresponding altitudes.

Altitude Concentration

(m) (ft) (particles/cm3) (particles/in3)

Sea level Sea level 200–300 3,277–4,916

600–800 1,968.5–2,624.6 10–20 164–328

1,200 3,937 2–4 33–66
45

2.6.2 Edwards Air Force Base

The terrain around EAFB causes some unique phenomena. The range to the south blocks 

much of the pollution from the Los Angeles basin and virtually all the marine effects prevalent 

there. However, with moderate to strong southwesterly winds, the pollutants can be seen coming 

through the passes and at times, over the mountains. The advection of pollutants from the Los 

Angeles basin is the primary cause of degraded visual ranges at EAFB.4

2.7 Precipitation

Precipitation is defined as all forms of hydrometeors, liquid or solid, which are free in the 

atmosphere and reach the ground. Because precipitation does occur in discrete events, statistical 

representation may be misleading.

Hail is precipitation in the form of balls or irregular lumps of ice and is always produced by 

convective clouds. By definition, hail has a diameter of 5 mm (0.2 in) or more. Hail falls are smallscale area phenomena, with a relatively infrequent occurrence at any given geographical point. The 

resulting time and space variability of hail is its prime characteristic. A generally accepted value for 

the density of hail at all locations is 0.89 g/cm3 (55.56 lb/ft3).1

Hail characteristics estimated for use in evaluating hail protection needs and requirements 

are:

• Hailstone size

• Terminal velocity

• Number of hailstones per hail fall

• Horizontal velocity of hailstones

• Density of hailstones.

These estimated characteristics are provided for the ER and EAFB.1

2.7.1 Eastern Range

Rainfall rates of various durations for the 50th, 95th, and 99th percentile, on a given day 

with rain in the month that has the highest average of rain, also referred to as the highest rain 

month, are given in table 32 for the ER.1 The precipitation amounts should not be interpreted that 

the rain fell uniformly for the entire reference periods. As an example, the 99th percentile total of 

49 mm (1.93 in) for a 1-hr duration as shown in table 32 could have occurred as follows: 25 mm 

(0.98 in) could have fallen during a 5-min period within a particular hour with an additional 

24 mm (0.95 in) of rainfall for another 5-min period, making a total of 49 mm (1.93 in) for a total 

of ~10 min. Subsequently, no rain would have fallen for 50 min of the hypothetical 1-hr period. 

The 99th percentile rainfall data are referenced in that such extremes are important to consider in 

vehicle and facility design studies. Extremes in rainfall totals can also be associated with tropical 

storm passages. More information on tropical storms is given in section 2.9. Table 33 has rainfall rates listed as well as total accumulation, raindrop size, etc. for various periods for the ER.1 

Table 34 shows the estimated hail characteristics for the ER.1
46

Table 32. Highest rainfall rate versus duration for various probabilities, given a day 

with rain for the highest rain month at the ER.

Duration

Percentile

50th 95th 99th

(mm) (in) (mm/hr) (in/hr) (mm) (in) (mm/hr) (in/hr) (mm) (in) (mm/hr) (in/hr)

5 min 5.6 0.22 66 2.6 18 0.72 221 8.7 25 1 305 12

15 min 5.8 0.23 24 0.93 22 0.88 89 3.5 33 1.3 132 5.2

1 hr 6.4 0.25 6.4 0.25 30 1.17 30 1.17 49 1.93 49 1.93

6 hr 7.1 0.28 1.3 0.05 39 1.55 6.6 0.26 81 3.18 13 0.53

24 hr 10.9 0.43 0.5 0.02 67 2.62 2.8 0.11 127 5 5.3 0.21

Table 33. Design rainfall at the ER based on yearly largest rate for stated periods.

(Use average rate of fall for raindrops of 6.5 m/s (21.3 ft/s) for all times.)

Time 

Period

Rainfall Rate

Rainfall

Total Accumulation

Raindrop Diameter

Average Largest

(mm/hr) (in/hr) (mm) (in) (mm) (mm)

1 min 492 19.4 8 0.3 2 6

5 min 220 8.7 18 0.7 2 5.8

15 min 127 5 32 1.3 2 5.7

1 hr 64 2.5 64 2.5 2 5

6 hr 26 1 156 6.1 1.8 5

12 hr 18 0.7 220 8.7 1.6 4.5

24 hr 13 0.5 311 12.2 1.5 4.5

Table 34. Estimated hail characteristics for the ER

(reference height=61 m (200.1 ft)).

Estimated Hail Characteristics

Exposure time risk (%)

 Worst month reference period 1

 Worst 6 months reference period 7

Mean number of hailstorm days per year 0.1

Average point of duration of hail fall 5 min

Average number of hailstones per 929 cm2 (1 ft2) 24

Density of hailstones 0.9 g/cm3 (56.1 lb/ft3)

Diameter and terminal velocity

 Representative diameter (50% risk) 0.9 cm (0.4 in)

 Terminal velocity 11 m/s (36 ft/s)

 Large diameter (5% risk) 2.2 cm (0.9 in)

 Terminal velocity 17 m/s (55.8 ft/s)

Horizontal velocity—all directions

Mean speed 9 m/s (29.5 ft/s)

5% risk speed 15 m/s (49.2 ft/s)

Month of maximum frequency May

POR—years 22
47

2.7.2. Edwards Air Force Base

The major effect of the topography around EAFB is on precipitation. The orographic uplift 

and subsequent dynamic descent over the mountains, which happens to storms moving towards 

EAFB from the northwest, tends to dry out storms. Systems moving from the west and southwest are also affected in the same manner but with a lesser influence. With only 96 km (51.8 nmi) 

of mountains between EAFB and the coast, there is usually enough moisture left to provide significant rainfall. Occasional summer air mass thunderstorms develop almost exclusively over the 

higher terrain in the area, even over the relatively small ranges scattered throughout the Mojave 

Desert. A few cells may advect over EAFB, primarily in the late afternoon. Table 35 contains the 

EAFB design rain rates, drop size, and rate of fall values versus time period.1 Table 36 shows estimated hail characteristics for EAFB.1

Table 35. EAFB design ground-level rainfall based on yearly largest rate 

for stated durations.

Time

Period

Rainfall Rate

Rainfall Total

Accumulation

Raindrop Diameter Average Rate

Average Largest of Fall*

(mm/hr) (in/hr) (mm) (in) (mm) (in) (mm) (in) (m/s) (ft/s)

1 min 197 7.7 3 0.1 2 0.08 5.6 0.22 6.5 21.3

5 min 88 3.5 7 0.3 2 0.08 5.3 0.21 6.5 21.3

15 min 51 2 13 0.5 2 0.08 5 0.2 6.5 21.3

1 hr 25 1 25 1 1.8 0.07 5 0.2 6.5 21.3

6 hr 10 0.4 62 2.4 1.5 0.06 4.6 0.18 6 19.7

12 hr 7 0.3 88 3.5 1.3 0.05 4.3 0.17 5.8 19

24 hr 5 0.2 124 4.9 1.3 0.05 4 0.16 5.5 18

*Average size raindrops.

Table 36. Estimated hail characteristics for EAFB

(EAFB reference height=18 m (59.1 ft)).

Estimated Hail Characteristics

Exposure time risk (%)

 Worst month reference period 5

 Worst 6 months reference period 25

Mean number of hailstorm days per year 0.6

Average point of duration of hail fall 5 min

Average number of hailstones per 929 cm2 (1 ft2) 24

Density of hailstones 0.9 g/cm3 (56 lb/ft3)

Diameter and terminal velocity

 Representative diameter (50% risk) 0.9 cm (0.4 in)

 Terminal velocity 11 m/s (36.1 ft/s)

 Large diameter (5% risk) 2.2 cm (0.9 in)

 Terminal velocity 17 m/s (55.8 ft/s)

Horizontal velocity—all directions

 Mean speed 13.m/s (42.7 ft/s)

 5% risk speed 22 m/s (72.2 ft/s)

Months of maximum frequency February to April

POR—years 28
48

2.8 Lightning

Atmospheric electricity must be considered in the design, transportation, and operation of 

aerospace vehicles. Inadequately protected aerospace vehicles can be upset, damaged, or destroyed 

by a direct lightning stroke to the vehicle or launch support equipment before or after launch. 

Damage can also result from current induced in the vehicle from changing electric fields produced 

by a nearby lightning stroke.16 Direct (physical) and indirect (electromagnetic) effects of lightning 

on vehicle systems, along with mitigation techniques, are described in MPCV 70080, Cross Program Electromagnetic Environmental Effects (E3) Requirements Document and SLS-RQMT-040, 

Space Launch System Program Electromagnetic Environmental Effects (E3) Requirements.

All clouds are electrified to some degree. In some convective clouds, updrafts can lead to 

sufficient electrical charge separation to give rise to thunderstorms. The processes involved in this 

charge separation are yet to be fully understood. Several electrification mechanisms have been 

theorized, including charge transfer from colliding polarized cloud particles, thermoelectric effects 

where positive ions migrate toward colder cloud particles, and the gathering of charge from riming 

graupel and ice crystals. The result of these electrification mechanisms is a thunderstorm with a net 

positive charge in the upper regions of the cloud (~20 C) and a net negative charge (~20 C) in the 

lower regions.

As electrical charges become separated in a cloud, the potential gradient between the cloud 

and the ground, and between various regions of the cloud, increases and eventually exceeds the 

critical breakdown potential for air. The resulting dielectric breakdown becomes a lightning flash. 

A typical ground flash consists of a first return stroke followed by several subsequent strokes 

separated in time by ~50 ms. The subsequent ground strokes are the cause of the ‘flicker’ typically 

seen in ground flashes. The peak current in the first return stroke is usually twice as large as the 

current in subsequent strokes. Each stroke can eliminate the 20 C of charge in the lower region 

of the thundercloud. The charge-generating mechanisms within the cloud must then replenish 

the charge before another stroke can occur. Lightning flashes also occur within thunderstorms 

neutralizing the main positive and negative charge centers. Instead of consisting of several discrete 

strokes, an intracloud discharge generally consists of a single stroke. Studies have shown that the 

ratio of intracloud (IC) to cloud-to-ground (CG) (IC:CG) lightning is approximately 2 to 1 for the 

ER area.17

2.8.1 Eastern Range

The formation of cumulonimbus clouds is strongly dependent upon the time of day and 

season in the ER region. During the summer months, convergence zones develop within the boundary layer between the sea breeze winds and the large-scale flow. These zones are focal points for 

thunderstorm formation as updrafts are intensified. Also, the presence of light wind shears aloft 

allows the thunderstorms to grow vertically, which intensifies the charge gradient within the cloud. 

Studies have shown that these warm season convergence zones develop more frequently in the ER 

area when there is a southwest flow in the low-level wind pattern.18 For the other months, frontal 

passages are the primary cause of thunderstorm formation and do not depend on time of day.
49

Figure 18 and table 37 provide insight into the diurnal variation of lightning flashes that 

occur near the ER. The data displayed are from the National Lightning Detection Network 

(NLDN), which has a detection efficiency of 90% for ER versus the Cloud-to-Ground Lightning 

Surveillance System (CGLSS) detection efficiency of 98%; NLDN and CGLSS only record the first 

CG return stroke from a lightning flash. Lighting flashes often have multiple return strokes. It is 

estimated (Personal Communication, Dr. V.A. Rakov, University of Florida, Gainesville) that the 

actual numbers could be increased by 70%.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sept

Oct

Nov

Dec

F18

Month

0 6 12 18 24

Time of Day (LST)

25

25

25

25

225

25

25

100

100

100

400

100

625

400

900

1,225

1,600

225

Figure 18. Diurnal/monthly variation of yearly average lightning flashes within a 50 km 

(26.9 nmi) radius of the ER, 1988–2001.
50

Table 37. Diurnal variation of lightning flashes for the ER.*

Hour

(LST) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

0 3 16 22 4 12 111 51 147 116 20 11 – 513

1 35 33 31 3 15 59 70 122 72 11 11 – 462

2 29 26 56 10 29 41 51 52 65 8 14 – 381

3 7 17 27 7 8 20 82 29 62 8 9 – 276

4 6 9 12 2 3 13 34 31 45 5 3 1 164

5 3 3 4 2 4 8 17 41 40 6 2 2 132

6 8 5 8 1 6 21 26 30 38 13 3 1 160

7 4 2 5 2 1 28 22 28 22 19 3 2 138

8 7 – 3 2 1 20 40 37 31 7 2 1 151

9 12 – 7 6 2 52 41 52 50 9 2 7 240

10 3 – 11 37 28 103 78 76 82 8 2 5 433

11 9 2 13 59 56 435 145 163 158 13 4 9 1,066

12 9 5 67 37 97 925 448 537 371 13 12 3 2,524

13 8 22 93 78 132 1,336 1,346 1,203 612 20 9 2 4,861

14 10 33 139 127 270 1,679 2,221 1,749 776 92 4 8 7,108

15 8 36 93 86 365 1,617 2,108 2,081 597 277 8 4 7,280

16 7 11 63 101 375 1,058 1,904 1,733 549 351 27 2 6,181

17 8 7 123 73 522 757 1,060 1,137 350 199 20 4 4,260

18 6 9 106 68 582 589 398 682 354 84 12 – 2,890

19 8 16 71 64 386 361 282 697 418 45 9 1 2,358

20 18 25 152 49 336 223 145 495 371 74 5 2 1,895

21 30 35 70 42 114 118 61 251 257 71 6 2 1,057

22 9 23 60 23 85 179 38 162 257 30 7 5 878

23 1 43 35 6 26 156 42 168 234 23 9 – 743

Totals 248 378 1,271 889 3,455 9,909 10,710 11,703 5,927 1,406 194 61 46,151

* Numbers represent the 14-yr (1988–2001) average numbers of flashes, separated by month and hour of the day, occurring within 50 km 

(26.9 nmi) of the ER. The bottom row indicates the average number of flashes by month, independent of time of day. The far right 

column indicates the average number of flashes by time of day, independent of month.

The Air Force’s 45th Weather Squadron uses several systems to monitor lightning activity 

at the ER. The Launch Pad Lightning Warning System (LPLWS) is a network of ~30 groundbased electric field mills that measure the electric field at the surface and detect electric discharges 

near the ER. Data from the LPLWS are used for evaluation of lightning Launch Commit Criteria 

(LCC) electric field rules. The effective range of the LPLWS is approximately 20 km (10.8 nmi).

The CGLSS is a network of six ground-based magnetic direction finders (MDFs).The MDFs use 

the magnetic field from a CG lightning flash to determine the azimuth to the CG strike point. The 

CGLSS determines location by direction finder and time of arrival (TOA) techniques. The effective range of the CGLSS is approximately 100 km (53.9 nmi). The Lightning Detection and Ranging (LDAR) system is a network of seven sensors that detect cloud-to-cloud (CC), IC, and CG 

lightning pulses. The LDAR uses a TOA technique to determine the location of lightning pulses. 

With this method, locations are established by taking the difference between the arrival times of the 

same lightning pulse signal to different LDAR sensors. The effective range of the LDAR system is 

approximately 100 km (53.9 nmi).19
51

The NLDN is a commercially owned network of more than 100 ground-based sensors 

distributed across the continental United States. The network consists of a mixture of magnetic 

direction finder and TOA sensors used to detect CG flashes. Along with lightning ground strike 

locations, the NLDN provides data on the number of strokes (first return and subsequent) and the 

peak current (in units of kiloamps) of the first return stroke for each CG lightning flash.

Table 38 shows the monthly and annual 50th and 99th percentile and maximum values for 

first return stroke peak current obtained from the NLDN for the years 1988–2001. Also given is the 

risk (%) of exceeding a 200 kA lightning flash. The CG lightning test waveform typically used for 

vehicle design has a first return stroke peak current of 200 kA. Lightning test waveforms for design 

are described in section 2.8.4.

Table 38. Lightning statistics for the ER, 1988–2001.*

Parameter Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

50th percentile 

peak current (kA)

50th percentile 

peak current (kA) 23.2 19.5 19.9 23.8 22.2 24.9 25.3 26.6 26.4 26.9 27.1 26.9

99th percentile 

peak current (kA)

99th percentile 

peak current (kA) 113.9 87.7 90.3 91.2 74.8 84.7 81.5 89.2 110 97.2 128.8 144.2

Maximum peak 

current (kA)

Maximum peak 

current (kA) 187.8 200.1 226 170.9 191.3 236.8 738.3 260.1 250.3 385.7 178.8 208.7

Risk (%) of peak 

currents >200 kA

Risk (%) of peak 

currents >200 kA – 0.018 0.023 – – 0.006 0.032 0.013 0.019 0.02 – 0.118

* Only flashes within 50 km (26.9 nmi) of the ER are considered. Peak current values are the absolute value of the first return stroke only. Values are determined from NLDN.

The average lightning flash density, i.e., the number of lightning ground flashes per square 

kilometer per year is an important phenomenological parameter that aids in the design of lightning protection systems. This parameter is critical in almost all lightning protection designs (such 

as the lightning over voltage protection of a utility power line), since the number of power outages 

or related failures is directly proportional to the number of CG discharges per unit area per year.

Figure 19 shows the mean annual CG lightning flash density (in units of flashes/km2/yr) for 

the ER and KSC area. Data were obtained from the NLDN for the years 1988–2001. Standards for 

lightning protection of ground equipment and facilities at KSC are presented in a NASA Standards document.20 The data displayed are from the NLDN, which has a detection efficiency of 90% 

for the ER versus CGLSS detection efficiency of 98%. Both NLDN and CGLSS only record the 

first CG return stroke from a lightning flash. Lighting flashes often have multiple return strokes. It 

is estimated (by Dr. Rakov, University of Florida) that the actual numbers could be increased by 

70%. Figure 19 shows a local maximum in flash density (approximately 12 flashes/km2/yr) just west 

of Cape Canaveral due to the sea breeze convergence zone.
52

–81.6 –81.2

KSC

–80.8 –80.4 –80

50 km

–79.6

–81.6

Latitude (deg)

Longitude (deg)

8

6

6

4

8

8 8

8 8

8

8

10

10

–81.2 –80.8 –80.4 –80 –79.6

27.6 28 28.4 28.8 29.2 29.6

29.6 29.2 28.8 28.4 28 27.6

F19

Figure 19. Mean annual CG lightning flash (flashes/km2/yr) for the ER area.

Triggered lightning is one threat to aerospace vehicles after launch, and is caused when the 

vehicle and its electrically conductive exhaust plume passes through a sufficiently strong preexisting 

ambient electric field and spans a potential voltage difference across that field. As the vehicle passes 

through such a high electric field, electrical charges are induced on the vehicle’s extremities, which 

produce a locally enhanced electric field sufficient to exceed the local breakdown strength of the air 

surrounding the vehicle. This local enhancement in the high preexisting ambient field leads to the 

growth of electric streamers at the vehicle extremities which support the initiation of conductive 

leaders and the resultant triggered lightning event.21

The danger of rocket-triggered lightning was first realized moments after the launch of 

Apollo 12 in 1969 at KSC. The rocket triggered two lightning strikes during a nonlightning producing weather situation. The lightning strikes produced major system upsets but only minor permanent damage. The vehicle and crew survived, and the crewmembers were able to complete their 

mission. This event led to the implementation of a set of lightning LCC, which are rules designed 

to avoid lightning threats to launch vehicles.

The next major event was the 1987 Atlas/Centaur-67 accident at KSC. The rocket caused 

a triggered lightning strike in nonlightning weather similar to the Apollo 12 incident that disrupted 
53

the vehicle guidance electronics. As a result, the rocket yawed unexpectedly and Range Safety was 

forced to destroy the vehicle. In subsequent years, several studies and working groups produced 

many lightning LCC revisions and recommendations.21 The most recent lightning LCC revision 

occurred in 2005 by the Lightning Advisory Panel, which advises the United States Air Force 

(USAF) and NASA on LCC issues.22 These criteria consist of 11 rules used to protect against the 

natural and triggered lightning threat to in-flight vehicles. The lightning LCC include rules for 

cloud thickness, attached and detached electrified anvil clouds, and surface electric fields. Electrified

anvil clouds can be particularly troublesome during the warm season due to the abundance of sea 

and land breeze-induced thunderstorms.23 A total of 4.7% of the launches from October 1, 1988 to 

September 1, 1997 were scrubbed and 35% were delayed due to the lightning LCC.21

2.8.2 Edwards Air Force Base

Figure 20 and table 39 provide insight into the diurnal variation of lightning flashes that 

occur near EAFB. Figure 21 shows the mean annual CG lightning flash density (in units of flashes/

km2/yr) for EAFB. Data were obtained from the NLDN for the years 1988–2001.

Table 40 shows the monthly and annual 50th and 99th percentile and maximum values for 

first return stroke peak current obtained from the NLDN for the years 1988–2001. Also given is 

the risk (%) of exceeding a 20 kA lightning flash.The CG lightning test waveform typically used 

for vehicle design has a first return stroke peak current of 200 kA. The lightning test waveform for 

design is described in section 2.8.4.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sept

Oct

Nov

Dec

F20

Month

0 6 12 18 24

Time of Day (LST)

3

3

3

3

48 75

3

1227

Figure 20. Diurnal/monthly variation of yearly average lightning flashes within a 50 km 

(26.9 nmi) radius of EAFB, 1988–2001.
54

Table 39. Diurnal variation of lightning flashes for EAFB.*

Hour

(LST) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

0 – – – – – – 7 – 2 – – – 9

1 – – – – 6 – – 1 – – – – 7

2 – – – – 7 – 1 1 – – – – 9

3 – – – – 1 – 2 – 1 – – – 4

4 – – – – 2 – – 2 2 1 – – 7

5 – – – – 4 – – 2 3 – – – 9

6 – – – – – – 1 1 1 1 – – 4

7 – – – – – – 1 – 2 – – – 3

8 – – – – – – – – – – – – –

9 – – – – – – – – – – – – –

10 – – – – – – 1 – – – – – 1

11 – – – – 3 – 3 5 – – – – 11

12 – – 1 2 1 2 6 21 3 – – – 36

13 – 1 3 5 2 3 30 72 10 – – – 126

14 – 1 1 3 6 4 58 126 17 4 – – 220

15 – 1 3 1 11 8 88 133 35 2 – – 282

16 – 1 5 1 17 5 72 106 18 3 – – 228

17 – 3 2 – 15 3 33 60 10 5 – – 131

18 – 2 1 – 4 1 14 26 20 2 – – 70

19 – – – – 2 1 9 35 11 3 – – 61

20 – – – – 1 – 9 14 3 5 – – 32

21 – – – – 1 – 6 1 1 2 – – 11

22 – – – – – – 8 – 2 1 1 – 12

23 – – – – – – 12 – 2 – – – 14

Total – 9 16 12 83 27 361 606 143 29 1 – 1,287

* Numbers represent the 14 yr (1988–2001) average numbers of flashes, separated by month and hour of the day, occurring within 50 km (26.9 nmi) of the EAFB. 

The bottom row indicates the average number of flashes by month, independent of time of day. The far right column indicates the average number of flashes by

time of day, independent of month.
55

EAFB

50 km

–119.2

Latitude (deg)

Longitude (deg)

0.1

0.1

0.1

0.2

0.2

0.3

0.2

0.3

0.3

0.2

0.3

0.3

0.3

0.1

0.2

0.4

0.5

0.4 0.3 0.2

0.1

8

–118.8 –118.4 –118 –117.6 –117.2 –116.8

–119.2 –118.8 –118.4 –118 –117.6 –117.2 –116.8

35.6 35.2 34.8 34.4 34

35.6 35.2 34.8 34.4 34

F21 Figure 21. Mean annual CG lightning flash density (flashes/km2/yr) for EAFB.

Table 40. Lightning statistics for EAFB, 1988–2001.*

Parameter Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

50th percentile 

peak current (kA) 65.9 32.2 24.8 22.4 20.3 20.3 18 18.5 17.9 24.6 29 33.1 18.6

99th percentile 

peak current (kA) 119.2 241.3 197.8 223.3 96.7 79.8 52.8 49.1 55 91.5 68.3 73.3 60.4

Maximum peak 

current (kA) 119.2 241.3 212.2 223.3 297.4 171.2 94.8 177.3 115.6 156 68.3 73.3 297.4

Risk (%) of peak 

currents >200 kA – 2.632 0.495 0.599 0.084 – – – – – – – 0.033

* Only flashes within 50 km (26.9 nmi) of EAFB are considered. Peak current values are the absolute value of the first return stroke only. Values were determined from 

NLDN data.
56

2.8.3 Lightning Design and Test Criteria

The lightning environment at or external to the vehicle is characterized by composites of 

several voltage and current waveform components in accordance to the specified zone. The lightning environment within the vehicle is characterized by voltage and current waveforms, induced by 

the external lightning environment, in accordance with levels or electromagnetic regions specified 

within the vehicle.

Vehicle lightning strike zones are defined and described in Society of Automotive Engineers 

(SAE) ARP5414, Aircraft Lightning Zoning.24

2.8.3.1 External Lightning Design.

2.8.3.1.1 Voltage Waveforms. Voltage waveforms used to characterize the (external) lightning environment simulate electric fields, which increase until breakdown occurs either by puncture 

or flashover. They are intended for evaluation of possible vehicle lightning attachment locations 

and dielectric breakdown paths through nonconducting surfaces and structures.25 Voltage waveforms A–D are displayed in figures 22–25.

Volts dV/dt=1,000 kV/μs±50%

Breakdown

Time

Figure 22. Voltage waveform A is characterized by a rise at the rate of 1,000 kV/μs F22

(±50%) until its increase is interrupted by puncture or flashover.
57

T1 T2

Volts

Peak

50%

Time

T1=1.2 μs±20%

T2=50 μs±20%

Figure 23. Voltage waveform B is a 1.2 × F23 50 μs waveform that crests at 1.2 μs (±20%) 

and decays to half of crest amplitude 50 μs (±20%).

2 μs

Volts

Breakdown

Time

F24 Figure 24. Voltage waveform C ramps up to breakdown voltage in 2 μs (±50%).
58

T1=50–250 μs

T2=~2,000 μs

T1 T2

Volts

Peak

50%

Breakdown

Time

F25 Figure 25. Voltage D slow-fronted waveform with a rise time

between 50 and 250 μs.

2.8.3.1.2 Current Waveforms. The external lightning environment comprises current components A, Ah, B, C, D, and H, and the multiple stroke and multiple burst waveform sets. The 

multiple stroke and multiple burst comprise current components D (and D/2) and H, respectively.

Current components A–D are compositely used to evaluate direct effects; whereas, current 

component A and waveform sets multiple stroke (current component D and multiple D/2 current 

components) and multiple burst (multiple H current components) are applicable for evaluating 

indirect effects.

2.8.3.1.3 Direct Effects. Lightning environment direct effects for current components A–D 

(displayed in fig. 26):

• Current component A (first return stroke)

Peak amplitude: 200 kA (±10%) 

Action integral: 2×106A2s (±20%) (in 500 μs) 

Time duration: ≤500 μs

• Current component B (intermediate current)

Maximum charge transfer: 10 C (±10%) 

Average amplitude: 2 kA (±20%) 

Time duration: ≤5 ms
59

Current

(not to scale)

A

B C

D

F26

≤500μs ≤5ms 0.25s ≤ t ≤ 1s ≤500μs

Figure 26. Lightning environment direct effects current components A through D.

• Current component C (continuing current)

Amplitude: 200–800 A

Charge transfer: 200 C (±20%) 

Time duration: 0.25–1 s

• Current component D (subsequent return stroke)

Peak amplitude: 100 kA (±10%)

Action integral: 0.25×106 (A)2s (±20%) (in 500 μs)

Time duration: ≤500 μs

2.8.3.1.4 Indirect Effects. Lightning environment indirect effects for current component A 

(displayed in fig. 27).

• Current component A (first return stroke)

Peak amplitude: 200 kA

10% rise time: 0.15 μs

90% rise time: 3 μs

Peak crest: 6.4 μs

50% fall time: 69 μs
60

T1=6.4 μs

T2=69 μs

Action Integral=2×106 A2 s

Within 500 μs

T1 T2

I Peak

200 kA

50%

Time

F27

Time to 90%=3 μs

di/dt=1×1011 A/s @ 0.5 μs

Time to 10%=0.15 μs

Peak di/dt=1.4×1011 A/s @ t=0

Figure 27. Lightning environment indirect effects current component A.

Lightning environment indirect effects for multiple stroke waveform set (displayed in fig. 28):

• Multiple stroke waveform set (current component D followed by 13 component D/2s)

Duration (total): ≤1.5 s

Time between components: 10–200 ms

10 ms ≤ ∆t ≤ 200 ms

1 2

D

D/2 D/2 D/2 D/2

3

1.5 s

One current component D followed by 13 current component D/2s distributed over 

a period of up to 1.5 s

100 kA

50 kA

13 14

F28

Figure 28. Lightning environment indirect effects multiple stroke waveform set.
61

Lightning environment indirect effects for current component D (displayed in fig. 29):

• Current component D (subsequent return stroke)

Peak amplitude: 100 kA

10% rise time: 0.08 μs

90% rise time: 1.5 μs

Peak crest: 3.18 μs

50% fall time: 34.5 μs

T1=3.18 μs

T2=34.5 μs

Action Integral=0.25×106 A2s

Within 500 μs Time to 90%=1.5 μs

50%

Time

di/dt =1011 A/s @ 0.25 μs

Time to 10%=0.08 μs

Peak di/dt =1.4×1011 A/s @ t=0

T1 T2

100 kA

F29

Figure 29. Lightning environment indirect effects current component D.

Lightning environment indirect effects for multiple burst waveform set (displayed in fig. 30).

• Multiple burst waveform set (repetitive current component H waveforms in three bursts 

of 20 pulses each)

Duration (total): ≤0.62 s

Time between bursts: 30–300 ms

Time between pulses: 50–1,000 μs
62

30 ms ≤ ∆t ≤ 300 ms

20 Pulses

One Burst is Composed of 20 Pulses

t

t

10 kA

10 kA

1

H H H H

2 3 20

F30

50 μs ≤ ∆ t ≤ 1,000 μs

H

Figure 30. Lightning environment indirect effects multiple burst waveform set.

Table 41 displays the application of external lightning environment to vehicle zones.24

Table 41. Application of external lightning environment to vehicle zones

(includes direct and indirect effects).

Vehicle Zone

Voltage Waveform 

Components

Current Waveform 

Components

1A A, B, D A, B, C, H

1B A, B, D A, B, C, D, H

1C A Ah, B, C, D, H

2A A D, B, C, H

2B – D, B, C, H

3 – A, B, C, D, H

Lightning strike 

model tests

C –
63

2.8.3.2 Internal Lightning Design Environment. High amplitudes and high rates of change 

of the external lightning environment current components A, D, and H induce major transients 

internal to the vehicle.

Magnetic fields penetrating through vehicle apertures induce currents and voltages inside 

the vehicle with waveforms similar to the current component A waveform (waveform 1), and similar to the derivative of the current component A waveform (waveform 2), respectively. Internal 

lightning environment amplitudes are not as severe as those of the external environment, varying 

with the electromagnetic regions designated within the vehicle and summarized in table 42.

Table 42. Maximum induced environment for electromagnetic regions.

Electromagnetic 

Region

Waveform 1 

(A)

Waveform 2

(V)

Waveform 3 

(V/A)

Waveform 4 

(V)

Waveform 5 

(A)

Level 1 100 50 100/20 50 150

Level 2 250 125 250/50 125 400

Level 3 600 300 600/120 300 1,000

Level 4 1,500 750 1,500/300 750 2,000

Level 5 3,200 1,600 3,200/640 1,600 5,000

Descriptions and conditions for the application of lightning environment waveforms 

are detailed in the following documents:

• SAE ARP5412, Aircraft Lightning Environment and Related Test Waveforms.25

• Radio Technical Commission for Aeronautics DO-160E, Environmental Conditions and Test 

Procedures for Airborne Equipment, section 22, Lightning Induced Transient Susceptibility, 

and section 23, Lightning Direct Effects.26

Electric and magnetic fields penetrating vehicle apertures create resonance, in the form 

of damped sinusoidal voltages and currents (waveform 3), on cables internal to the vehicle.

Structural IR voltage and diffusion flux coupling produce loop voltages, which have the 

shape of current component A (waveform 4). If the vehicle structure is highly resistive, the current 

waveform is lengthened (longer rise and fall times), but still similar to current component A 

(waveform 5).

All the induced transient waveform responses (waveforms 1–5) can occur during multiple 

stroke mode.
64

The multiple burst waveform set induces a voltage waveform 3 response or a current waveform 6 response, which has the same shape as the external environment component H:

• Waveform 1 (current waveform):

Peak amplitude: (varies according to electromagnetic region) 

10% rise time: 0.15 μs 

90% rise time: 3 μs 

Peak crest: 6.4 μs (±20%)

50% fall time: 69 μs (±20%)

• Waveform 2 (derived voltage waveform):

Peak amplitude: (varies according to electromagnetic region) 

Peak crest: 100 ns

Zero crossover: 6.4 μs (±20%)

• Waveform 3 (voltage or current waveform):

Peak amplitude: (varies according to electromagnetic region) 

Range of resonance: 1–50 MHz

• Waveform 4 (voltage waveform):

Peak amplitude: (varies according to electromagnetic region) 

10% rise time: 0.15 μs 

90% rise time: 3 μs 

Peak crest: 6.4 μs (±20%)

50% fall time: 69 μs (±20%)

• Waveform 5A (current waveform):

Peak amplitude: (varies according to electromagnetic region) 

10% rise time: 1.44 μs 

90% rise time: 23.7 μs 

Peak crest: 40 μs (±20%)

50% fall time: 120 μs (±20%)

• Waveform 5B (current waveform):

Peak amplitude: (varies according to electromagnetic region) 

10% rise time: 1.21 μs 

90% rise time: 23.9 μs 

Peak crest: 50 μs (±20%)

50% fall time: 500 μs (±20%)

• Waveform 6 (current waveform):

Peak amplitude: (varies according to electromagnetic region) 

10% rise time: 5.3 ns

90% rise time: 0.11 μs 

Peak crest: 0.244 μs (±20%)

50% fall time: 4 μs (±20%)
65

2.9 Tropical Cyclones

2.9.1 Eastern Range

The occurrence of hurricanes and tropical storms near the ER increases the risk and potential damage to the vehicle by surface winds and the induced loads placed on the vehicle. Extreme 

maxima in rainfall rates and total rainfall along with extreme minima in MSL atmospheric pressure are also associated with tropical cyclones. For this TM, the term tropical cyclone includes 

hurricanes and tropical storms. By definition, a hurricane is a storm of tropical origin with maximum sustained (1-min mean) surface winds ≥33.5 m/s (≥109.9 ft/s). A tropical storm is a storm 

whose origin is in the tropics with sustained wind speeds ranging between a maximum of 33.5 m/s 

(109.9 ft/s) and a minimum of 18 m/s (59 ft/s).1 Tropical cyclone season in the Atlantic basin typically lasts from June 1 to November 30, but tropical cyclones have been reported as early as February and as late as December.15

Development of tropical cyclones typically occurs in two regions: the tropical western 

Atlantic or Gulf of Mexico/Caribbean Sea. In the months of July through September, tropical 

storms have a tendency to develop over the tropical western Atlantic; whereas, in the early season 

(May–June) and late season (October–December), cyclones tend to develop in the Gulf of Mexico/

Caribbean Sea region. A report by Turpin,27 for a POR from 1886 to 1997, showed that 92.2% of 

the storms that passed within 333 km (179.8 nmi) of the ER originated in the regions from the east 

down through the southwest of the ER.

The potential threat of a tropical cyclone to near the ER is presented from data contained 

in Johnson and Rawlins 28 for the tropical seasons from 1871 to 1999, a POR of 129 years. Over the 

POR, there have been 1,076 tropical cyclones in the Atlantic basin. Of those cyclones, 856 storms 

(79.6%) occurred during the 3 months of August–October with September accounting for 387 (36% 

of the total), the most of any single month during tropical cyclone season. Of the 1,076 tropical 

cyclones in the Atlantic Basin, 47% reached a radial distance of <928 km (<501.1 nmi) from the 

ER. The number decreased as the radial distance from the ER decreased. Only 9.1% were within 

a radial distance of 185 km (99.9 nmi). The conditional probability a tropical cyclone with a radial 

distance of 928 km (501.1 nmi) from the ER would track to within 185 km (99.9 nmi) of the ER 

was 19.3%. Specifically for September, the month with the greatest number of storms, the conditional probability of a storm initially within a radial distance of 928 km (501.1 nmi) moving to 

within a radial distance of 185 km (99.9 nmi) was only 16.2%.28

Maximum sustained winds within a tropical cyclone present a threat to the vehicle and 

facilities. Tropical cyclones, in addition to air mass thunderstorms, have produced the strongest 

winds measured at the ER.29 In general, tropical cyclones approaching the ER from the east produce higher winds than those approaching the ER from over the Florida peninsula.1 Over a POR 

of 55 years, 1945 to 1999, 12 tropical cyclones have produced sustained winds exceeding 17.5 m/s 

(57.4 ft/s) at the ER. From an annual perspective, this means that there is a 0.06% chance of 

encountering sustained winds that exceed 17.5 m/s (57.4 ft/s) annually. During tropical cyclone season, the probability increases to 0.10%, and during the months August to October, it is 0.24%.28
66

2.9.2 Edwards Air Force Base

The only impact to EAFB from tropical cyclones is from tropical cyclone remnants yielding 

large amounts of rainfall over the site. From 1900 to 1997, there have been 42 occurrences of tropical cyclones in the southwestern United States. Only one of those cyclones has ever made landfall 

at tropical storm strength. This storm occurred in September 1939 and made landfall at Long 

Beach, with sustained winds of 22.4 m/s (73.5 ft/s).29 The majority of systems track along the Baja 

California peninsula and make landfall in southwestern Arizona.

2.10 Sea State

Knowledge of sea state characteristics and probabilities are important to aerospace vehicle 

design and trade studies in the development of detailed design requirements and specifications, and 

for operational analyses. It is important to characterize the sea conditions for landing and recovery 

zones (including pad and ascent abort, normal, and off-nominal zones), launch availability, and the 

possible sea recovery and tow back of reusable components. Some of the site-specific information 

contained in this section is based on the work done by the MSFC Natural Environments Branch 

for the Orion Multi-Purpose Crew Vehicle. Though certain information is specific to the anticipated Orion configuration and splashdown zones, the general information provided within this 

section can be applied to other aerospace vehicles. Additional information can be provided in the 

future as needed.

2.10.1 Sea State Data

2.10.1.1 C-ERA 40 Hindcast Global Database. The 40-year Corrected-European Centre 

for Medium-Range Weather Forecasts Re-Analysis (C-ERA40)30 is developed from the European 

Centre for Medium-Range Weather Forecasts (ECMWF) over a 45-year POR (September 1957–

August 2002), making the C-ERA40 the longest and most complete global wave data set presently 

available.

The products provided by the C-ERA40 were constructed through the reanalysis of historical data using an atmospheric numerical model coupled with a wave hindcast numerical model and 

were validated with in situ and satellite measurements.30,31 The databases include six hourly (00, 

06, 12, and 18 coordinated universal time (UTC)) reports of wind speed at a 10 m (32.8 ft) height, 

significant wave height (SWH), and mean zero up-crossing wave period with a 1.5o×1.5o spatial 

resolution. Each individual SWH, wind speed, and wave period report represents an average of 

the last 20 min of the given 6-hr time period over the entire grid box centered on the corresponding latitude and longitude provided in the database. It has also been found that model errors in 

SWH and wind speed are consistent with, or better than, previous scientific and engineering studies 

which compared model and buoy data.31,32

Several limitations do exist with the database. The C-ERA40 reanalysis only applies to 

deep water. Tropical cyclones cannot be resolved properly; therefore, data in regions where tropical 

cyclones are prevalent may not fully represent the true environment.33
67

2.10.1.2 Buoy Networks. Several buoy networks exist which provide both real-time and historical observations. The National Data Buoy Center (NDBC) provides hourly meteorological and 

wave data such as wind speed, air temperature, and SWH. In addition, some buoys in the NDBC 

network have the capability to record spectral wave data. While the NDBC buoys range in size and 

shape (3 to 12 m (9.8 to 39.3 ft) discus hulls to a 6-m (19.7-ft) boat-shaped hull), the buoys owned 

by the Coastal Data Information Program (CDIP) are spherical and 0.9 m (2.9 ft) in diameter. 

The CDIP buoys not only record data on a finer temporal resolution than the NDBC buoys (every 

30 min), but the upper frequency limit that the wave spectra are reported to is also higher. Unfortunately, the CDIP buoys do not have the instrumentation to make meteorological observations, 

which only allows the buoys to record wave data. 

A limitation that both networks share is the greater concentration of buoys along coastlines 

versus the open ocean. While this is helpful for data needed for Orion pad aborts at KSC in Florida, or within the normal end-of-mission (EOM) landing zone near San Clemente Island, California, archived observations for ocean locations that could potentially see ascent aborts over open 

ocean or off-nominal EOM zones are sparse or do not exist.

An overview of the ECMWF can be found at <www.ecmwf.int>.34 Recent and archived 

observations from the NDBC and CDIP buoy networks can be accessed online by the public at 

<http://www.ndbc.noaa.gov/> and <http://cdip.ucsd.edu/>, respectively. 35,36

2.10.2 Significant Wave Height and Extreme Wave Height Values

A regular sea surface is considered nearly sinusoidal with constant wave height, period, 

and direction, but natural waves are often associated with an irregular sea surface where the 

observable waves move in different directions and the associated heights and periods can vary.37,38

This variability in wave height from wave to wave leads to a more statistical definition of the 

parameter, where wave height is the distance from the crest to trough. Wave height measurements 

tend to follow a Rayleigh distribution, as shown in figure 31.37 Given enough recorded wave cycles 

exist to create a distribution, the Rayleigh distribution may be used to determine the average height 

of the highest desired fraction of the waves ( H p ), or the wave height exceeded by the highest 

desired fraction of the waves (Hp):

Hp = ln 1

p

!

"

# $

%

& +

!

2 p !erfc ln 1

p

"

#

$ %

&

'

"

#

$

%

&

'

(

)

*

*

+

,

-

-

!Hrms (21)

and

Hp = ln 1

p

!

"

# $

%

& 'Hrms , (22)

where p equals the highest fraction of interest and erfc is the complementary error function. 

An estimation of various wave heights includes SWH, which corresponds to the average height 
68

Density

Cumulative Distribution

Random Variable, x

1/3 Highest

Mean

Density Function, p(x)

Cumulative Distribution, P(x)

1

0.5

1

0.5

x 2 1

x 1/3

P(x 1)

0 α α

Most Probable

Max = xm=3.717 α

Mx=1.2533 α

3α 4α

F31

Figure 31. Rayleigh probability density and cumulative probability distribution 

(x=α corresponds to the mode).37

of the highest one-third of the waves, and maximum wave height (Hmax) which depends on the 

number of observed waves and the SWH. While estimated wave heights from the middle of the 

Rayleigh distribution, like SWH and root-mean-square wave height (Hrms), are not dependent on 

the total waves observed, maximum wave height (Hmax) is dependent on the number of recorded 

wave cycles:39

SWH ! 4" m0 = 1.416"Hrms , (23)

H1/10 = 1.27 !SWH=1.8!Hrms = 5.091! m0 , (24)

H1/100 = 1.67 !SWH=2.36!Hrms = 6.672 ! m0 , (25)

and

Hmax = 1.86!SWH (for 1,000 wave cycles in the record) , (26)

where 

m0 = zeroth moment of the wave spectrum

H1/10 = average height of the highest one-tenth of the waves

H1/100 = average height of the highest one-hundredth of the waves.
69

If the average of the highest 1/N waves ( ) Hmax is desired instead:

Hmax = ln(N) +

!

2

N

!erfc( ) ln(N)

"

#

$

$

$

%

&

'

'

'

!Hrms , (27)

where N is the number of recorded wave cycles.

Although Rayleigh distributions are usually adequate for deep water, they tend to overestimate the number of larger wave heights in coastal waters. Currently, no other distribution exists 

that better fits the waves in shallow water, therefore the Rayleigh is still used with the assumption 

that waves will likely be overpredicted. 

2.10.3 Wave Height and Wind Speed Durations and Intervals 

The time an episode of SWH or wind speed persists above a given threshold is referred to as 

the duration of the event. The time between two episodes is termed an interval. The length of time 

an episode is likely to last once it has begun is important to such aerospace applications as launch 

operations, rescue, and recovery.

The United States (U.S.) Navy has compiled a historical data set in the form of a wind and 

wave climatology called the U.S. Navy Hindcast Spectral Ocean Wave Model Climatic Atlases for 

both the North Atlantic Ocean39 and the North Pacific Ocean.40 Duration and interval tables are 

provided for various grid points across the oceans at various months of the year. Information on 

the construction and use of these tables including examples of their applications are provided in 

the Navy Atlases.40,41A subset of the tables for the ER and a brief description of their use was also 

provided in the reference 42. Since these tables are rather voluminous and have not been updated 

since their inclusion in NASA/TM—2008–215633,41 they are not duplicated here. Note that as 

indicated in NASA/TM—2008–215633, the Spectral Ocean Wave Model underestimates wind 

speed and wave height near U.S. east coast areas. However, the Spectral Ocean Wave Model is the 

only known source for duration/interval statistics.

2.10.4 Wave Period

Wave period can have different definitions depending on which wave assumption is used 

to characterize the sea surface. In cases where the sea surface is assumed to have regular waves, or 

constant wave properties, the wave period is defined as the amount of time between two successive 

crests or troughs at a given point. Realistically, the sea contains irregular, or random, waves where 

the variability in wave height and period from wave to wave leads to more statistical definitions of 

the wave properties. In random seas, the more common definitions of wave period, like zero up- or 

down-crossing period, take into account the zero (or mean) water line which represents the mean 

level of the water surface. Figure 32 illustrates the differences between regular and irregular seas 

where the zero line is represented by the x-axis in each wave profile.37
70

η

ηα

ηα

η

η

η

(a)

(b)

(c)

(d) t

F32

T

L

T

t

t

x

Figure 32. Examples of regular and irregular waves: (a) Regular wave (temporal),

(b) regular wave (spatial), (c) irregular wave, and (d) random wave.37

While the zero up-crossing period is defined as the time it takes a point on a water surface 

to cross the zero line in an upward direction and continue along the wave profile to the next zero 

up-crossing, the zero down-crossing is described as the amount of time between successive downcrossings. Although both zero crossing methods tend to provide statistically similar mean values 

of wave period, the mean zero up-crossing wave period (Tm) is the quantity that is utilized in the 

C-ERA40 database and is given by:

Tm = m0

m2

, (28)

where m0 and m2 are the zeroth and second moments of the wave spectrum.37,41 Wave period can 

also be represented by the peak spectral wave period (Tp), which is defined as the period associated 

with the largest wave energy in a wave record, or the spectral peak frequency, fp=1/Tp.
71

Using the products from the C-ERA40 database, minimum wave period design specifications are determined by analyzing the distribution of Tm in 1-s time intervals within specified halfmeter SWH ranges. In each SWH and Tm range, the first value is included in the range, while the 

second value is not. According to Sorensen,42 as wave height increases, the wave period distribution narrows. This narrowing is visible in table 43, which shows the percentage of Tm reports in 1-s 

intervals that are within a specified SWH range. The sum of the Tm probabilities within each SWH 

range equals 100%. 

Table 43. Percentage of wave period observations in 1-s intervals that are within 

a specified SWH range (0.5 m) from January–December 1971–2001.

Significant Wave 

Height

(m) (ft) Mean Zero Up-Crossing Wave Period (s)

Significant Wave Weight (m)

≥7 ≥23 – – – – – – * 0.7 23.5 53.1 18.7 3.6 0.4 – –

6.5–7 21.3–23 – – – – – * 4.3 57.4 30.6 5.9 1.5 0.3 – –

6–6.5 19.7–21.3 – – – – – * 0.1 11.2 62.1 19.6 5.2 1.5 0.3 – –

5.5–6 18–19.7 – – – – – * 0.3 25.6 52.6 14.6 5.1 1.4 0.3 – –

5–5.5 16.4–18 – – – – – * 1.5 43.2 36.6 12.1 5 1.3 0.3 – –

4.5–5 14.8–16.4 – – – – – * 6.6 50.7 24.8 11.3 4.9 1.3 0.3 – –

4–4.5 13.1–14.8 – – – – * 0.2 19.7 43 19.5 11.4 4.6 1.3 0.2 – –

3.5–4 11.5–13.1 – – – * * 1.2 33 30.6 17.7 11.5 4.5 1.3 0.2 – –

3–3.5 9.8–11.5 – – * * * 6.1 35.6 23.7 17.4 11.5 4.6 0.9 0.1 – –

2.5–3 8.2–9.8 – – * * 0.3 14.7 29.6 20.7 18.3 12 3.8 0.6 0.1 – –

2–2.5 6.6–8.2 – * * * 2.2 19.2 22.9 20.9 21.3 10.8 2.3 0.3 – – –

1.5–2 4.9–6.6 – – * 0.2 7.2 17.2 21.2 27.2 20 5.4 1.2 0.2 – – –

1–1.5 3.3–4.9 – * * 3.8 12.6 17.6 26.9 26.2 9.1 2.8 0.8 0.2 – – –

0.5–1 1.6–3.3 – * 2.6 19.8 17.7 18.2 19.4 12.9 6.2 2.3 0.6 0.1 – – –

0–0.5 0–1.6 – 0.2 22.7 33.6 18.9 11.1 7 3.9 1.7 0.6 0.1 – – – –

0–1 1–2 2–3 3–4 4–5 5–6 6–7 7–8 8–9 9–10 10–11 11–12 12–13 13–14 ≥14

Note:

– Indicates no report existed for the specific Tm range within the SWH range.

* Indicates that reports existed, but the probability is <1/10 of a percent.

The minimum average wave periods indicated in table 44 are selected by determining which 

1-s wave period range contains the first percentile of wave periods observed for the given SWH 

range. Since the first percentile falls between two values within a wave period range, the lower of 

the two wave period values is used to be conservative. Therefore, the minimum average wave period 

is approximately the lowest mean zero up-crossing wave period associated with the specified SWH 

range and the dashed line in table 43 is used to indicate this boundary across all the given SWH 

ranges.
72

Table 44. Minimum average wave period corresponding to given significant wave height.

Significant Wave Height Minimum Average 

Wave Period

(m) (ft) (s)

0–0.5 0–1.6 2

0.5–1 1.6–3.3 2

1–1.5 3.3–4.9 3

1.5–2 4.9–6.6 4

2–2.5 6.6–8.2 4

2.5–3 8.2–9.8 5

3–3.5 9.8–11.5 5

3.5–4 11.5–13.1 5

4–4.5 13.1–14.8 6

4.5–5 14.8–16.4 6

5–5.5 16.4–18 6

5.5–6 18–19.7 7

6–6.5 19.7–21.3 7

6.5–7 21.3–22.9 7

≥ 7 22.9 8

Values of Tm associated with the Orion normal and off-nominal design maximum SWH 

(2 and 4 m (6.6 and 13.1 ft), respectively) are displayed in table 44.

2.10.5 Water Surface Slope

Several different methods for calculating wave slope for Orion were examined. These methods included: (1) Wave height divided by wave length, which provided a gross overestimate and 

assumed that the water surface was a straight line without any curve to it, (2) the Neumann spectrum, which was the ocean model used by the Apollo program but it lacked advancements in wave 

theory that more current spectra contained, (3) the Pierson-Moskowitz (PM) spectrum, which 

replaced analyses conducted with the Neumann spectrum but did not provide the steeper 3σ slopes 

seen by real ocean data. 

The approach that was eventually used to calculate wave slope is based on historical wave 

spectrum databases, which allows for the calculation of water surface slopes based on measured 

data.
73

The energy spectrum associated with the Orion normal and off-nominal design maximum 

SWH (2 and 4 m (6.6 and 13.1 ft), respectively) are provided in table 45 (for 2 m (6.6 ft) SWH) and 

table 46 (for 4 m (13.1 ft) SWH). Each energy spectrum may be truncated on the high-frequency 

end to limit the calculated water surface slope to the dimensions of interest to the engineering 

application. The limit of the high-frequency end of the energy spectrum can be determined using 

the following equation:

fmax = g

2 !! ! L

2

"

#

$

%

, (29)

where fmax is the maximum frequency (Hz), g is gravity (m/s2), and L is the smallest wavelength (m) 

of interest for the engineering application. The energy spectrum is used to derive the Orion normal 

and off-nominal water surface slope distributions using the following equations: 

I* = μi

2

g

!

"

#

$

%

& ' Ai μ( )i (

) *+

2

' ,μi

(

)

-

-

*

+

.

. i=1

n / , (30)

where I* is the omnidirectional slope variance, n represents the total number of frequency bins, g

is gravity, and [Ai (mi)]2 and Dmi are the energy spectrum (m2/Hz) and bandwidth (Hz), respectively, 

associated with the ith frequency bin.

The directional components of slope variance are determined by multiplying slope variance 

by the constants f (θ)=0.625 and g(θ)=0.375,43 which are representative values of how the slope 

variance is divided into its directional components: 

! ud

2 = I *!f (" ) (31)

and

! c

2 = I *!g(" ), (32)

where ! ud

2 is the upwind-downwind slope variance component and ! c

2

 is the crosswind slope variance component. Each directional slope standard deviation component is used to run a 10,000 case 

(typical) Monte Carlo Simulation, assuming the water surface slope fits a Gaussian distribution. 

The directional water surface slope components are determined using the following equations:

μud = arctan ! ud !r ( ) ud (33)

and

μc = arctan ! c !r ( ) c , (34)
74

Table 45. Energy spectrum for 2 m (6.6 ft) SWH.

Frequency

(Hz)

Energy

(m2/Hz)

Bandwidth

(Hz)

Frequency

(Hz)

Energy

(m2/Hz)

Bandwidth

(Hz)

0.025 0.0033 0.005 0.38 0.101 0.01

0.03 0.0069 0.005 0.39 0.135 0.01

0.035 0.0121 0.005 0.4 0.131 0.01

0.04 0.0113 0.005 0.41 0.1227 0.01

0.045 0.0289 0.005 0.42 0.0737 0.01

0.05 0.0946 0.005 0.43 0.0869 0.01

0.055 0.1462 0.005 0.44 0.0715 0.01

0.06 0.129 0.005 0.45 0.0865 0.01

0.065 0.2023 0.005 0.46 0.0583 0.01

0.07 0.3837 0.005 0.47 0.0477 0.01

0.075 0.3455 0.005 0.48 0.0489 0.01

0.08 0.3837 0.005 0.49 0.0517 0.01

0.085 0.4284 0.005 0.5 0.0543 0.01

0.09 0.614 0.005 0.51 0.0379 0.01

0.095 0.3799 0.005 0.52 0.0298 0.01

0.1013 0.2611 0.0075 0.53 0.0343 0.01

0.11 0.0961 0.01 0.54 0.0325 0.01

0.12 0.1051 0.01 0.55 0.0198 0.01

0.13 0.169 0.01 0.56 0.0183 0.01

0.14 0.4548 0.01 0.57 0.0259 0.01

0.15 0.9438 0.01 0.58 0.0203 0.01

0.16 1.0642 0.01 0.59 0.01829 0.01

0.17 1.1356 0.01 0.6 0.0167 0.01

0.18 1.3596 0.01 0.61 0.01524 0.01

0.19 1.2059 0.01 0.62 0.01392 0.01

0.2 0.9438 0.01 0.63 0.0127 0.01

0.21 1.1076 0.01 0.64 0.0116 0.01

0.22 0.5929 0.01 0.65 0.01058 0.01

0.23 0.5639 0.01 0.66 0.00966 0.01

0.24 0.7314 0.01 0.67 0.00882 0.01

0.25 0.7499 0.01 0.68 0.00805 0.01

0.26 0.464 0.01 0.69 0.00735 0.01

0.27 0.4241 0.01 0.7 0.00671 0.01

0.28 0.4199 0.01 0.71 0.00612 0.01

0.29 0.3668 0.01 0.72 0.00559 0.01

0.3 0.4458 0.01 0.73 0.0051 0.01

0.31 0.16 0.01 0.74 0.00466 0.01

0.32 0.2064 0.01 0.75 0.00425 0.01

0.33 0.2484 0.01 0.76 0.00388 0.01

0.34 0.1179 0.01 0.77 0.00354 0.01

0.35 0.1329 0.01 0.78 0.00323 0.01

0.36 0.1624 0.01 0.79 0.00295 0.01

0.37 0.0985 0.01 0.8 0.0027 0.01
75

Table 46. Energy spectrum for 4 m (13.1 ft) SWH.

Frequency

(Hz)

Energy

(m2/Hz)

Bandwidth

(Hz)

Frequency

(Hz)

Energy

(m2/Hz)

Bandwidth

(Hz)

0.025 0.0353 0.005 0.38 0.1336 0.01

0.03 0.0477 0.005 0.39 0.0951 0.01

0.035 0.1419 0.005 0.4 0.1046 0.01

0.04 0.1608 0.005 0.41 0.0951 0.01

0.045 0.8124 0.005 0.42 0.0856 0.01

0.05 4.9392 0.005 0.43 0.0985 0.01

0.055 5.9132 0.005 0.44 0.067 0.01

0.06 2.0693 0.005 0.45 0.0641 0.01

0.065 0.349 0.005 0.46 0.0398 0.01

0.07 0.8329 0.005 0.47 0.0341 0.01

0.075 1.3128 0.005 0.48 0.0546 0.01

0.08 2.2416 0.005 0.49 0.0525 0.01

0.085 3.041 0.005 0.5 0.0501 0.01

0.09 2.0693 0.005 0.51 0.0375 0.01

0.095 3.7516 0.005 0.52 0.0468 0.01

0.1013 4.6515 0.0075 0.53 0.0364 0.01

0.11 7.8632 0.01 0.54 0.0313 0.01

0.12 10.1472 0.01 0.55 0.0242 0.01

0.13 7.0794 0.01 0.56 0.0277 0.01

0.14 7.3683 0.01 0.57 0.0225 0.01

0.15 6.4378 0.01 0.58 0.024 0.01

0.16 4.9392 0.01 0.59 0.01906 0.01

0.17 3.1336 0.01 0.6 0.01741 0.01

0.18 2.9072 0.01 0.61 0.01591 0.01

0.19 2.1005 0.01 0.62 0.01453 0.01

0.2 1.9782 0.01 0.63 0.01327 0.01

0.21 1.6606 0.01 0.64 0.01212 0.01

0.22 1.3128 0.01a 0.65 0.01107 0.01

0.23 1.0857 0.01 0.66 0.01012 0.01

0.24 0.9922 0.01 0.67 0.00924 0.01

0.25 0.5639 0.01 0.68 0.00844 0.01

0.26 0.4481 0.01 0.69 0.00771 0.01

0.27 0.5958 0.01 0.7 0.00704 0.01

0.28 0.5668 0.01 0.71 0.00643 0.01

0.29 0.2637 0.01 0.72 0.00588 0.01

0.3 0.4014 0.01 0.73 0.00537 0.01

0.31 0.2843 0.01 0.74 0.0049 0.01

0.32 0.1552 0.01 0.75 0.00448 0.01

0.33 0.1648 0.01 0.76 0.00409 0.01

0.34 0.2192 0.01 0.77 0.00374 0.01

0.35 0.1716 0.01 0.78 0.00341 0.01

0.36 0.1934 0.01 0.79 0.00312 0.01

0.37 0.1227 0.01 0.8 0.00285 0.01
76

where mud is the upwind-downwind water surface slope component (rad), mc is the crosswind water 

surface slope component (rad), and rud and rc are zero-mean, unit variance Gaussian random 

variables. Each pair of directional water surface slope component values is applied to the following 

equation to create a water surface slope distribution: 

μ = arctan tan2 μud + tan2 μc !

180

!

"

#

$ %

&

' , (35)

where μ is the total water surface slope (degrees).

The normal (off-nominal) energy spectrum for the Orion landing zones in tables 45 and 46 

was developed using 5-m wind speed data from buoys 46047 and 46069, near San Nicolas Island, 

California, in the NDBC network along with spectral data from buoy 067 near San Nicolas Island 

in the CDIP buoy network. The spectral data ranges from 0.025 to 0.58 Hz and is reported every 

half hour, along with the corresponding SWH. These archived buoy data, when compared with 

the NDBC spectral data, provide spectral data at higher frequencies in the area of interest. The 

energy spectrum for Orion normal water landings is the enveloping spectrum during August over 

a 9-year POR (1999–2007), where SWH is ≤2 m (6.6 ft) and wind speed is ≤8.2 m/s (26.9 ft/s). The 

energy spectrum in table 45 represents a single spectrum from August 23, 2001, at 0349 UTC and 

is created using (1) the energy in each frequency bin from 0.025 to 0.58 Hz and (2) energy that is 

extrapolated from 0.58 to 0.8 Hz with an exponential curve (y=3.9708 · exp–9.1189x) that is fit to 

the energy found in each frequency bin from 0.3 to 0.58 Hz. The variable x is the desired frequency 

(Hz) and y is the resulting energy (m2/Hz). 

The energy spectrum for the Orion off-nominal landing zone is the enveloping spectrum 

during March over a 6-year POR (2000–2005), where SWH is ≤4 m (≤13.1 ft) and wind speed is 

≤13.9 m/s (≤45.6 ft/s). The energy spectrum in table 46 represents a single spectrum from March 29, 

2001, at 0013 UTC and is created using (1) the energy in each frequency bin from 0.025 to 0.58 Hz 

and (2) energy that is extrapolated from 0.58 to 0.80 Hz with an exponential curve (y=3.9773

· exp–9.052x) that is fit to the energy found in each frequency bin from 0.30 to 0.58 Hz.The variable x

is the desired frequency (Hz) and y is the resulting energy (m2/Hz).

A water surface slope distribution for the normal Orion landing zone was derived from 

the full energy spectrum and is illustrated in figure 33. A water surface slope distribution for the 

off-nominal Orion landing zone was derived from the full energy spectrum and is illustrated in 

figure 34. The figures show the probability that the water surface slope is below a given value. The 

normal and off-nominal slope distributions represent the extreme case of 5,422 and 5,934 energy 

spectra, respectively. The normal spectrum was selected based on the distributions seen in figure 33, 

and the off-nominal spectrum was selected based on the distributions seen in figure 34. Both spectra not only cover the higher probabilities of steeper water surface slopes, but they also cover the 

water surface slopes seen at higher frequencies when the energy spectrum is extrapolated.
77

110

100

90

80

70

60

50

40

30

20

10

0

F33

Probability (%)

1 4 7 10 13 16 19 22 25 28 31 34

Water Surface Slope (deg)

Nonextrapolated (0.58 Hz)

Extrapolated (0.8 Hz)

Figure 33. Cumulative distributions of water surface slope for 2 m (6.6 ft) SWH.

110

100

90

80

70

60

50

40

30

20

10

0

F34

Probability (%)

1 4 7 10 13 16 19 22 25 28 31 34

Water Surface Slope (deg)

Nonextrapolated (0.58 Hz)

Extrapolated (0.8 Hz)

Figure 34. Cumulative distributions of water surface slope for 4 m (13.1 ft) SWH.
78

2.10.6 Ocean Temperature and Salinity 

Physical properties of seawater are dependent primarily upon salinity and temperature. Differences in salinity and/or temperature throughout the ocean cause density differences that drive 

subsurface water masses. These subsurface water mass movements are called thermohaline 

currents.

Salinity is defined as the total amount of solid material in grams contained in 1 kg of seawater when carbonate has been converted to oxide, bromine, and iodine replaced by chlorine, and 

all organic material completely oxidized. While coastal waters can exhibit a wide range of salinity 

as aresult of freshwater runoff, most of the world’s ocean water lies in the narrow salinity range 

of 33.8 to 36.8 g of dissolved salts per kilogram of seawater. The North Atlantic is the most saline 

of the world’s oceans, having a mean salinity of 35.09 g/kg compared to the global average of 

34.72g/kg. At any given temperature and pressure, seawater is denser than freshwater.

Temperature in the ocean varies widely, both horizontally and with depth. Some of the 

warmest sea surface temperatures (SSTs) on Earth have been recorded at the surface in the Persian 

Gulf in summer. The lowest possible temperature values hover around –2 °C (28.4 °F), the usual 

minimum freezing point of seawater. Freshwater reaches its maximum density at 4 °C (39.2 °F) 

and then expands (becomes less dense) as it cools to 0 °C (32 °F) and freezes. This is why freshwater ponds in cold climates convectively turn over every season. Saltwater, on the other hand, 

continues to become denser as it cools to its freezing point. When saltwater freezes, the salt impurities (primarily composed of sodium chloride) are not easily incorporated into the ice crystal lattice 

structure and the newly formed ice is composed primarily of freshwater. The solution rejected when 

seawater freezes is even higher in salt content, and denser as well, and has an even lower freezing 

point than the original solution. Therefore, the greatest changes in density of seawater occur near 

the surface where density is decreased by either precipitation, water runoff from land surfaces, 

melting ice, or heating. Density is increased by the formation of sea ice, cooling, and by evaporation. The greatest increase in density occurs in the polar regions, where the water is cold and great 

quantities of ice form. This cold, dense polar water sinks and spreads to lower latitudes via thermohaline currents.

At depths more than a couple hundred meters (600 to 700 ft) below the surface of the 

ocean, the temperature and salinity of water normally change very slowly. When it does change 

rapidly, it is because a ‘new’ water mass (with new salinity and temperature values) advects into, 

and displaces, the original water from that location. In this case, both temperature and salinity 

are conservative properties of a fluid (i.e., changed only by advection) and, as a result, the movement of water mass can be traced by its temperature and salinity. Origin regions and relative depths 

characterize water masses. Four depths, beginning at the surface and extending to the ocean bottom, are used: surface, intermediate, deep, and bottom.
79

The greatest known depth of water in the oceans is 11,524 m (37,808.4 ft) in the Marianas 

Trench in the Pacific. The average ocean depth worldwide is 3,795 m (12,450.8 ft). By way of comparison, the highest land on Earth—Mount Everest—is 8,840 m (29,002.6 ft) tall and the average height of land is 840 m (2,755.9 ft) above sea level. About 23% of the ocean is shallower than 

3,000 m (9,842.5 ft); about 76% is between 3,000 and 6,000 m (9,842.5 and 19,685 ft); and a little 

more than 1% is deeper than 6,000 m (19,685 ft). In addition, about 77% of the world ocean volume is colder than 4 °C (39.2 °F), with salinities in the relatively narrow range 34.1 to 35.1 g of 

dissolved salts per kilogram of seawater.

Maximum, mean, and minimum water temperatures for 3-month periods from the surface 

to depths of 50 m (164 ft) for the waters off the coast of the KSC ER areas are given in tables 47 

and 48.

Table 47. Ocean temperatures (°C) in the ER booster recovery areas.

Months Jan to Mar Apr to Jun Jul to Sep Oct to Dec

Depth (m) Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min

0 26 23 16 29 26 21 31 29 27 29 26 19

10 26 23 16 29 26 20 30 29 26 29 26 19

20 26 23 16 29 26 19 30 28 23 29 26 20

30 26 23 17 28 26 17 29 28 21 29 28 21

50 26 23 17 28 25 17 29 27 19 28 26 22

Table 48. Ocean temperatures (°F) in the ER booster recovery areas.

Months Jan to Mar Apr to Jun Jul to Sep Oct to Dec

Depth (ft) Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min

0 78.8 73.4 60.8 84.2 78.8 69.8 87.8 84.2 80.6 84.2 78.8 66.2

33 78.8 73.4 60.8 84.2 78.8 68 86 84.2 78.8 84.2 78.8 66.2

66 78.8 73.4 60.8 84.2 78.8 66.2 86 82.4 73.4 84.2 78.8 68

98 78.8 73.4 62.6 82.4 78.8 62.6 84.2 82.4 69.8 84.2 78.8 69.8

164 78.8 73.4 62.6 82.4 78.8 62.6 84.2 80.6 66.2 84.2 78.8 71.6

2.10.6.1 Sea Surface Temperature Data. Although most NDBC and CDIP buoys have the 

instrumentation to record sea surface temperature (SST), the observations only cover a limited 

area of the globe. This makes it difficult to analyze certain ocean regions, especially those that are 

in open water since most buoys are located in coastal areas. Several databases exist which provide 

better global SST coverage through gridded reanalysis. The National Centers for Environmental 

Prediction (NCEP) optimum interpolation provides weekly mean SST observations on a 1o latitude×1o longitude global grid dating back to November 1981. This database was created using 

in situ and satellite SST observations, with the satellite data corrected for biasing, and simulated 

SST from sea ice cover.44 This database was used to look at the global water temperatures for the 

Orion project. Using a POR from November 1981 through December 2006, it was found that the 
80

warmest SSTs were seen in the Persian Gulf and Arabian Sea areas. The warmest temperature of 

approximately 36 oC (96.8 oF) occurred in the Persian Gulf during the summer months.

2.10.7 Ocean Surface Currents 

Ocean currents transport seawater from one location of the ocean to another. They also 

transport momentum, chemicals (such as salts, oxygen, and carbon dioxide), biological species 

(such as plankton and fish), and any objects purposely or inadvertently inserted into the ocean. 

Knowledge of ocean currents is important to marine navigation, to dispersal of pollutants, and 

toaerospace applications such as search and rescue at sea or recovery of space-borne objects. 

Ocean currents are composed of both surface and subsurface currents. Since subsurface currents 

are primarily driven by differences in the density of the sea water, they generally travel at much 

slower speeds (2 to 3 orders of magnitude slower) than surface flows and are typically of less interest to aerospace applications. Ocean surface currents are primarily driven by atmospheric wind 

above the surface waters. Frictional stress at the interface between the ocean and the wind causes 

the water to move in the direction of the wind. Steady winds cause the sea surface to slope as water 

is piled up in the direction the wind is blowing. The maximum difference in water surface height 

due to this effect is generally about 1 m per 100 km (3.3 ft per 54 nmi).

Water flows from the region of higher pressure to the region of lower pressure. The Coriolis 

force, which results from the Earth’s rotation, causes the current to move to the right of the pressure gradient path in the Northern Hemisphere and to the left in the Southern Hemisphere. On 

small scales, water flows from high to low pressure (the pressure gradient force) in the ocean, but 

for large-scale motion, the Earth’s rotation leads to flow along lines of constant pressure (isobars). 

Such water flows that are controlled by a balance between the pressure gradient force and the 

Coriolis deflection result in currents called geostrophic currents that flow along isobars. Largescale, mid-latitude ocean flow is an approximate geostrophic current. On a global scale, large ocean 

currents are constrained by the continents that border the three major oceanic basins. Continental 

borders cause these currents to develop large-scale, almost closed circular patterns called gyres. 

These gyres actually result from a combination of wind forcing, buoyancy forcing, and the Coriolis 

acceleration. Since Coriolis acceleration varies with latitude, gyre circulations are not symmetric 

and the ocean surface flow on western boundaries of a gyre is generally stronger. Figure 35 is asimplified illustration of the major ocean surface currents of the world.45
81

F35

60°

30°

0°

30°

60°

Warm Current Cold Current

f35

Figure 35. Major ocean surface currents of the world45 (courtesy of Scott Jones).

On either side of the equator, in all ocean basins, there are two west-flowing currents: the 

North and South Equatorial. These currents flow between 3 and 6 km (1.6 and 3.2 nmi) per day 

and usually extend 100 to 200 m (328.1 to 656.1 ft) in depth below the ocean surface. The Equatorial Counter Current, which flows towards the east, is a partial return of water carried westward 

by the North and South Equatorial currents. In El Niño years, this current intensifies in the Pacific 

Ocean.

Flowing from the equator to high latitudes are the western boundary currents. These warm 

water currents have specific names associated with their location: North Atlantic—Gulf Stream, 

North Pacific—Kuroshio, South Atlantic—Brazil, South Pacific—East Australia, and Indian 

Ocean—Agulhas. All of these currents are generally narrow, jet-like flows that travel at speeds 

between 40 and 120 km (21.6 and 64.8 nmi) per day. Western boundary currents are the deepest 

ocean surface flows, usually extending 1,000 m (3,280.8 ft) below the ocean surface. The largest 

and most prominent of the western boundary currents is the Gulf Stream in the North Atlantic 

Ocean (fig. 36).The warmer water (red thermal image) of the current is a good indicator of the path 

it takes on its journey north (J. Bosch, Coastal Ocean Observation Laboratory, Institute of Marine 

and Coastal Sciences, Rutgers University, Personal Communication). The component of the Gulf 

Stream that flows through the Florida Straits up to Cape Hatteras, North Carolina, is called the 

Florida Current. The near surface current speed off the coast of Florida is typically 2 m/s (6.6 ft/s). 

The Florida Current mean northward transport is approximately 30×106 m3/s (1,059.4×106 ft3/s). 

This is more than the total flow of all the rivers in the world combined. For comparison, the Mississippi River is approximately 0.02×106 m3/s (0.71×105 ft3/s). The width of the Florida Current 

is approximately 80 km (43.2 nmi) at 27 oN. and 120 km (64.8 nmi) at 29 oN. As the Florida Current 
82

Figure 36. Satellite image of the Gulf Stream that shows the temperature differential F36

of the ocean water off the coast of Florida (courtesy of Coastal Ocean 

Observation Laboratory, Institute of Marine and Coastal Sciences, 

Rutgers University).
83

is joined by other northward-flowing currents, the volume transport of the Gulf Stream increases 

to approximately 85×106 m3/s (3,001.7×106 ft3/s) near Cape Hatteras.

Flowing from high latitudes to the equator are the eastern boundary currents. These cold 

water currents also have specific names associated with their location: North Atlantic—Canary, 

North Pacific—California, South Atlantic—Benguela, South Pacific—Peru, and Indian Ocean—

West Australia. All of these currents are generally broad, shallow moving flows that travel at speeds 

between 3,000 and 7,000 m (9,842.5 to 22,965.9 ft) per day.

In the Northern Hemisphere, the east flowing North Pacific Current and North Atlantic 

Drift move the waters of western boundary currents to the starting points of the eastern boundary currents. The South Pacific Current, the South Indian Current, and the South Atlantic Current provide the same function in the Southern Hemisphere. These currents are associated with the 

Antarctic Circumpolar. Because of the absence of land masses at this latitude zone, the Antarctic 

Circumpolar flows in continuous fashion around Antarctica and only provides a partial return 

of water to the three southern hemispheric ocean basins.

In addition to water density gradients and atmospheric winds, another driving mechanism 

for ocean flow is the ocean tides generated by the Moon and Sun. For open ocean currents, the 

contribution to the flow from this source is generally considered noise. However, in coastal areas, 

tidal flow is usually the dominant flow component. Therefore, tidal forecasts are very important 

for many marine applications such as shipping, but less so for most aerospace applications.

General information on ocean currents is available from National Oceanic and Atmospheric 

Administration’s (NOAA’s) National Ocean Data Center at <http://nodc.noaa.gov>.46
84

2.11 Geologic Hazards

2.11.1 Earthquakes

Earthquakes are due to sudden releases of tectonic stresses that result in relative movement 

of rocks on opposite sides of a fault plane, as well as shaking of ground in areas near and sometimes far from the actual fault movement. Ground movement and shaking can trigger numerous 

other disasters, including landslides; liquefaction and sliding of unconsolidated sediments; destruction of buildings, dams, and roads; fires; tsunamis; seiches; changes in ground water level; and 

uplift or subsidence. They can also bring about far-reaching atmospheric pressure changes, sound 

waves, and oscillations of the ionosphere.47

Relative movement of different sections, plates, of the Earth’s crust causes stresses to build 

up near the boundaries between them. Movement along faults, which releases seismic waves, takes 

place when the effective stresses exceed either the strength of the solid rock or the frictional resistance between rocks on either side of a preexisting break or fault. This movement takes place along 

preexisting fault surfaces because they usually have lower strength than the surrounding rock.

Many microearthquakes take place along active faults, such as in parts of the San Andreas 

Fault, but a greater number do not correspond to any known surface fault. Many of the earthquakes that are not associated with surface faults occur under folds called anticlines, geologic 

structures formed when layered sediments are buckled upward in a broad arch. The presence of an 

anticline reflects crustal compression as two moving tectonic plates collide in the same way a carpet 

wrinkles when pushed across the floor. An unanswered question is whether these active folds conceal large faults that could provide the sites for large shocks.48

Earthquakes have proven to be one of the most disastrous and insurmountable of geologic 

hazards. Buildings constructed to withstand them have crumbled under their forces.47 Prediction 

of earthquake likelihood, intensity, and timing for a given location has not yet proved reliable. 

Experience has shown that the best protection against earthquakes is identification of high-risk 

areas and avoiding construction in those areas.

Definition of high-risk areas is a complicated process which includes mapping faults, dating 

movement on them to determine whether they are or might still be active, calculating theoretical 

maximum possible earthquake intensity for active faults, and predicting effects of possible earthquakes on sediments and rocks in the area. This information is then used to judge the safety of the 

area for construction.

Beginning in 1996, the U.S. Geological Survey prepared a new series of national seismic 

hazard maps. These maps show the severity of expected earthquake shaking for a particular level 

of probability. An example of such a map is shown in figure 37, which shows peak acceleration levels (% g, where g is the acceleration due to gravity) with a 10% chance of being exceeded 

in a 50-year period.49
85

F37

125 °W. 120 °W. 115 °W. 110 °W. 105 °W. 100 °W. 95 °W. 90 °W. 85 °W. 80 °W. 75 °W. 70 °W. 65 °W.

125 °W.

120 °W. 115 °W. 110 °W. 105 °W. 100 °W. 95 °W. 90 °W. 85 °W. 80 °W. 75 °W. 70 °W.

65 °W.

50 °N.

45 °N.

40 °N.

35 °N.

30 °N.

25 °N.

50 °N.

45 °N.

40 °N.

35 °N.

30 °N.

25 °N.

20

20

20

20

4

40

4

3

5

4

2

3

2

3

5

4

4

5

5

5

30

60

60

2

1

10

10

10

10

10

10

0

1

2

3

4

5

6

7

8

9

10

15

20

25

30

40

60

80

100

180

Figure 37. Peak acceleration with 10% probability of exceedance in 50 years 

(courtesy of the U.S. Geological Survey).

2.11.2 Tsunamis and Seiches

Tsunamis are seismic sea waves. Submarine earthquakes that suddenly elevate or lower portions of the sea floor, submarine landslides, or submarine volcanic eruptions can generate tsunamis. 

Tsunamis travel on the order of 500 km/hr (1,640.4×103 ft/hr) and can cross an ocean in <1 day. 

Their wavelengths are long: 100 to 200 km (53.9 to 107.9 nmi). Their amplitudes in deep water are 

low (<1 m (<3.3 ft)) but, as they approach a shoreline, their large volume of water piles up into 

sizable tidal waves. Configuration of the shoreline and tidal and wind conditions can help to form 

waves over 10 m (32.8 ft) high. In 1948, the U.S. Coast and Geodetic Survey established a seismic 

sea wave warning system for the Pacific Ocean. As a result of this warning system, the arrival of 

tsunamis from distant sources can now be anticipated by a few hours.

A seiche is a long surface wavelength standing wave occurring in an enclosed body of water.

Its period can vary from a few minutes to several hours and is very dependent on the dimensions of 

the basin, pond, lake, or enclosed bay. Commonly, seiches are low in amplitude and are not noticeable. Large-amplitude waves occur when a large-scale disturbance takes place and can continue to 

be reflected back and forth across the body of water for hours or days. Large seiches can be caused 

when tsunamis arrive in bays, or when earthquakes and large slope movements initiate them in an 

enclosed body of water. Seiches can also cause the piling up of water at one end of a lake or bay, 

given the proper steady wind conditions blowing over the lake. Near enclosed bodies of water, 

investigation of possible damaging seiche activity should be considered as a part of earthquake 

and slope movement studies.
86

2.11.3 Volcanic Hazards

Volcanic hazards fall into two categories: hazards near the volcanic activity and hazards 

distant from it.50,51

2.11.3.1 Hazards Near Volcanic Activity. Within a few tens of miles of a volcanic center, hazards include: lava flows, nuées ardentes (hot ash flows) and poisonous gases, ash falls and 

bombs, earthquakes, debris, and mud flows.

Some lava flows are much more dangerous to man than others. Lava flows vary a great 

deal in viscosity, depending on their chemistry and temperature. They can be up to 10 m (32.8 ft) 

thick, traveling 1 m (3.3 ft) per hour, or they can form a sheet as thin as 1 m (3.3 ft) that travels 

up to 50 km/hr (1.6×105 ft/hr). The latter type has been the most hazardous to man in the past. 

A trained geologist can predict, to some extent, the type of flow most likely to occur in a given volcanic area. If fast fluid flows are likely, guiding levees can be built to shunt them away from populous or otherwise valuable areas.

Nuées ardentes are heavier-than-air gasborne flows of incandescent volcanic ash released 

during explosive volcanic eruptions. Temperatures in the flows reach 800 °C (1,472 °F) and the 

gases that carry them may be poisonous. The gasborne flows are extremely dense. Their physical 

force is great enough to snap large trees and crumble strong buildings. It was a nuée ardent from 

Mt. Pelée that devastated St. Pierre, Martinique, in 1902, completely destroying the town and killing an estimated 40,000 people. Hot, dense, poisonous gases can also be emitted without ash.

Ash falls in the immediate vicinity of a volcano can be up to a few tens of meters deep and 

very hot. Near the eruption center, ash falls may contain sizable volcanic bombs of solid or solidifying rock, as well as pebble-sized fragments of pumice. They may give off gases for some time.

Earthquakes usually accompany volcanic activity and often trigger debris flows and mud 

flows. Debris flows and mud flows form from the unconsolidated material that makes up the flanks 

of active stratovolcanoes (composite volcanoes). The material becomes unstable because of doming 

of the volcano, rapid melting of snow by hot ash or lava, and/or percolation of hot volcanic gases 

through snow masses. Volcanic mud and debris flows have been known to travel 80 km (43.2 nmi) 

at speeds of several tens of kilometers per hour. Some flows from major volcanoes contain on the 

order of 2 to 4 km3 (0.5 to 0.9 mi3) of material. Dams in the paths of mud flow may break and 

contribute to the volume of flows that overtop them. In some places, where a mudslide hazard has 

been recognized, dams have been built and reservoirs kept empty to absorb them. In addition to 

downstream damage, volcano-caused landslides can cause instability at their point of origin. When 

a large volume of material is removed suddenly from the flank or summit of an active volcano, 

pressure is released and an eruption may be triggered (as in the May 18, 1980, eruption of Mount 

St. Helens).

2.11.3.2 Hazards Distant From Volcanic Activity. Far from volcanic centers, volcanic ash 

and tsunamis can still be serious hazards. An ash falls’ total volume depends on the size of the 

eruption that caused it. Its distribution depends on the elevation reached by the volcanic cloud 
87

and on wind conditions at the time of the eruption. A sizable ash fall can damage areas several 

hundred kilometers from the eruption site. Ash is detrimental to human health and damaging to 

mechanical equipment. It reduces visibility if there is wind or traffic and must be removed from 

buildings and pavement. Fine ash, if it reaches the stratosphere, may remain there for months or 

years, affecting climate by reducing insulation.

Tsunamis can result from submarine volcanic explosions and debris slides, which can travel 

thousands of kilometers from the volcanism that caused them. They endanger life and all coastal 

construction within 40 m (131.2 ft) of sea level.

When considering volcanic hazards, it is important to realize that, in any area, volcanism 

is sporadic. A volcanic area that has been inactive throughout historic times may reawaken to 

violent activity in a few days or weeks or it may remain inactive for centuries into the future. Earthquakes, almost always felt or recorded several days or weeks before activity commences, serve as 

a warning of impending danger. Once volcanism commences, the danger is greatest within a few 

tens of kilometers of the eruption. The effects of volcanism can easily be catastrophic, especially 

since volcanoes are virtually uncontrollable by man. Important construction should not be located 

in the immediate vicinity of active or dormant volcanoes or in areas likely to be affected by distant 

volcanism.

2.11.4 Geology and Geologic Hazards at the Eastern Range

2.11.4.1 Geology. The ER, on the eastern coast of the Florida peninsula, covers an expanse 

of barrier bars, swamps, and lagoons between the Atlantic Ocean and the mainland. The entire 

area of KSC lies within 80 m (262.5 ft) of sea level. Surficial deposits on the center of the ER are 

roughly 30 m (98.4 ft) of Miocene to recent shelly sand and clay and medium- to fine-grained sand 

and silt.52 These sediments overlie Eocene limestone and dolomite.

2.11.4.2 Earthquakes. Earthquakes are extremely unlikely in this area of the United States.

2.11.4.3 Tsunamis and Seiches. Tsunamis (sea waves) induced by earthquakes and/or volcanism elsewhere could be a hazard to the entire ER because of its low elevation. However, tsunamis 

are not common in the Atlantic Ocean and, although not impossible, are considered unlikely. The 

lagoons and rivers are also unlikely to develop seiches.

2.11.4.4 Volcanic Hazards. Volcanism near the ER is unknown in recent times. The only 

volcanic hazards to the ER are tsunamis caused by distant volcanism.

2.11.4.5 Summary. The ER is a low-risk area for geologic hazards. Only flooding due to 

hurricanes or seismically-induced waves is considered to be of possible importance. Dikes can protect crucial structures that would not survive high water.
88

2.11.5 Geology and Geologic Hazards at Edwards Air Force Base

2.11.5.1 Geology. Rock materials of three distinct age groups53 cover EAFB. The oldest 

rocks are pre-Tertiary (formed more than 65 million years ago) granite intrusive and metamorphic 

units. These rocks are similar in age and composition to the Sierra Nevada Batholith. They form 

most of the ridges and hills within the Air Force Base boundaries.

Minor amounts of Tertiary age rocks (2.6 to 65 million years old) are exposed at EAFB. 

Most of these rocks are dikes and sills of fine-grained rock. A few volcanic flows and pyroclastics, 

with interbedded sediments, crop out along the eastern boundary of the base. Some bentonite layers occur within the sedimentary units. Although the dikes and sills form stable slopes, some of the 

slopes covered by the pyroclastic and sedimentary interbeds are unstable.

Most of the terrain within the boundaries of EAFB is covered with thick units of Quaternary and Recent (less than 2.6 million years old) unconsolidated and weakly consolidated materials, which include alluvial sand and gravel, beach dunes and bars, playa clays, windblown sands, 

and older, partly consolidated gravels. These deposits generally occupy areas of low relief.

Alluvial sand and gravel, deposited by flowing water, form channel and fan deposits. Wavedeposited bars and wind-deposited dunes occur along the northern ‘shore’ of Rogers Lake. Minor 

clay balls occur in the wave-deposited bars. Windblown sand forms small dunes elsewhere within 

the base and also covers parts of the desert floor with a thick veneer of sand.

The playa clays are mudflat facies of the alluvium. They are hard when dry but become soft 

and sticky when wet. Studies by Droste54 found that playa clays from Rogers Lake consist of 40% 

to 50% montmorillonite and 40% to 50% illite. Clays from Rosamond Lake consist of 20% to 30% 

montmorillonite, 50% illite, and 20% to 30% chlorite. Although in the desert climate thorough wetting of the playas is rare, these high-montmorillonite clays are subject to severe swelling and shrinking, which should be considered when planning construction activities near the dry lake beds.

Several high-angle, northwest-trending faults have been mapped in the southern and eastern 

parts of the EAFB. They have small displacements and seem to edge granitic domal features. The 

faults are at present inactive.

2.11.5.2 Earthquakes. There were no recorded earthquakes with epicenter magnitude of 

4 or greater at EAFB or within 40 km (21.6 nmi) of the base between 1910 and 1974.55,56 The base 

is located on a relatively stable wedge between the San Andreas and Garlock Faults, both of which 

are less than 64 km (34.6 nmi) from the base. The proximity of these major active faults indicates 

regional tectonic instability.

The likelihood of surface fault rupture at the EAFB Dryden site is considered to be very 

remote. However, it cannot be dismissed completely because it is not presently known if any buried faults underlie the site which may belong to the group of Mojave block faults. Another risk, 

albeit a low one, is the possibility of sympathetic movement of these possible underlying faults in 

response to large motions from a great earthquake on the San Andreas Fault, including fault rupture extending to the ground surface.48
89

It is recommended that facilities that are to be constructed at EAFB be evaluated for their 

resistance to the two following types of earthquakes:57

(1) A magnitude 8.5 event on the nearest approach of the San Andreas Fault, approximately 47 km (25.4 nmi), would impose an acceleration of 0.4 g on the site with a bracketed duration of 40 s. It is suggested that a scaled trace of the N21E component of the Taft accelerogram 

of the 1952 Kern County earthquake is an adequate model.

(2) A near-field magnitude 4.5 event from a Mojave Block Fault would impose an acceleration of 0.2 g at the site with a short-bracketed duration of 6 s. It is suggested that the unscaled 

trace of the Lake Hughes No. 4 S69E component from the San Fernando Valley earthquake of 

1971 be used as an appropriate model.

2.11.5.3 Volcanic Hazards. No volcanic hazards are expected to affect this area.

2.11.5.4 Conclusions. EAFB, though mostly underlain by granite, is 65% covered by Pleistocene and recent unconsolidated sand, clay, and gravel. Despite the proximity of major active 

faults, seismic risk is low. Slopes are generally <10%; as a result, geologic hazards from slope 

processes are localized and most likely restricted to steep slopes consisting of weakly consolidated 

fanglomerate.

Approximately 30% of the Air Base is covered by unconsolidated clay-rich material. The 

clays include a high proportion of montmorillonite and are susceptible to expansion and shrinking. However, low precipitation of the Mojave Desert region greatly reduces the potential for such 

problems.

In summary, EAFB is located in a geologically low-risk area.

2.12 Terrestrial Ionizing Radiation Environments and Effects

In the course of performing any space mission, whether manned or unmanned, the electronics systems will be exposed to various forms of ionizing radiation. This exposure not only varies with the physical location of the spacecraft, but can also vary with time. So knowledge of the 

dynamic ionizing radiation environments, both terrestrial and in space, and their effect on electronic systems is critical to a successful mission.

There are various sources of ionizing radiation around the Earth; these are divided into 

trapped and transient environments. Surrounding the Earth is a magnetic field that traps electrons 

and protons, forming what are called the Van Allen Belts. Since these belts only exist in the space 

region surrounding the Earth, the discussion on this source of ionizing radiation will be covered 

in more detail in the appropriate sections later in this TM.

The transient ionizing radiation environments are ones that can impact the terrestrial environments. These sources are solar particle events (SPEs) and galactic cosmic radiation (GCR) 

and are sources of protons and heavier ions that can impact systems flying in space and also 

impinge on the upper atmosphere and generate the natural environment of ionizing radiation that 

is seen in the atmosphere and on the ground. 
90

In addition to the solar particle event (SPE), the Sun does produce another ionizing radiation environment called the solar wind. The solar wind consists of significantly lower energy electrons and protons than seen in SPE, GCR, and the trapped belts. While the solar wind environment 

is important for spacecraft charging issues, it is not energetically significant for electronic systems 

and does not yield any significant ionizing radiation environments in space or the atmosphere. 

The source for solar ionizing radiation environments pertinent to the terrestrial ionizing 

radiation environment are SPEs. When a magnetic disruption in the solar photosphere occurs, 

a variety of radiation types and energies will erupt into space from the Sun. SPEs can produce 

energetic protons and heavy ions that will produce effects in electronics. An important influence on 

solar particle radiation, and all transient environments, is the Earth’s magnetic field. The magnetic 

forces that cause charged particles to be trapped will also act on charged particles in an event from 

the Sun. For a given magnetic field strength, a particle of sufficient energy is required to penetrate 

that magnetic field. All lower energy particles are deflected along the magnetic field lines. This 

magnetic screening can offer significant protection for satellites in low inclination, low altitude 

Earth orbits. This screening is not seen at higher inclinations as the magnetic field lines terminate at 

the Earth’s poles and the low energy particles that are deflected along the field lines will eventually 

reach these regions.

The next source of ionizing radiation is GCR consisting of electrons, protons, and heavy 

ions (charged particles with atomic numbers >1) that are believed to have an origin outside the 

solar system and to be omnidirectional in all regions of space. This radiation source can be significant in the near-Earth environment. The GCR is affected in the same manner as solar particles due 

to the effects of the Earth’s magnetic field (i.e., low altitude equatorial orbits will receive significant 

screening, while higher inclination or altitude receive less).

Detailed descriptions of SPE and GCR environments can be found in references 58 and 59.

2.12.1 Terrestrial Ionizing Radiation Environments

As pointed out above, there are sources of high-energy protons and heavy ions that are constantly hitting the upper atmosphere of the Earth. These ions can undergo collisional interactions 

with nuclei of the atmosphere and generate a cascade of secondary particles that will continue to 

transit the atmosphere and possibly produce additional secondary and tertiary particles. A cartoon depiction of a typical cascade is shown in figure 38. Open circles in this figure show a location 

where an interaction occurs and the spray of particles that exit are shown by the labeled solid lines. 

An important take-away from figure 38 is that the production of neutrons can be significant.
91

F38

π+

π–

π

π±

γγ

μ ±

μ –

n

n

n

n

n

n

n

n n

n

n

n

n

n

n

n

n

n

n

e±

e+

e–

p

p

p

p

p

p

p

p

p p

p

p

Heavy Ion

Figure 38. Cartoon depiction of a nuclear cascade event initiated by a high-energy 

heavy ion and yielding a distribution of atmospheric neutrons.

A second point to take away is that there are a number of interaction locations that yield 

these ‘atmospheric’ neutrons; therefore, it is expected that there will be a distribution of the quantity of these neutrons as a function of altitude. If it is also considered that the quantity of the 

atmospheric neutrons is directly dependent on the quantity of incident particles producing the 

cascade, then there are two other parameters that drive the atmospheric neutron population. 

The first parameter that is considered in addition to altitude when evaluating the atmospheric neutron population is latitude. As discussed previously, the Earth’s geomagnetic field provides a shield for the Earth from the high-energy heavy ions that are incident on it. The magnetic 

field lines can be approximated as a dipole magnet with the filed lines terminating at the Earth’s 

north and south magnetic poles. Using this model, the magnetic field strength diminishes as the 

poles are approached which would yield less shielding. Therefore, the quantity of the incident ions 

would be larger as the magnetic poles of the Earth are approached. Or stated another way, the 

higher in latitude, the larger the incident ion population and therefore the larger the atmospheric 

neutron population.

In addition to altitude and latitude, the atmospheric neutron population is also driven by 

the solar activity. The solar activity impacts the incident ion population in two manners. The first, 

and most obvious, is when the Sun produces a solar particle event. In addition to the significant 
92

increase in the incident ion population, depending on the type and strength of the SPE, the geomagnetic field can be impacted, allowing for deeper penetration at lower latitudes. The second way 

the solar activity has an effect is through the Sun’s 11-year cycle of solar activity. The Sun has an 

11-year cycle that for 7 of the 11 years, the activity of the Sun is at a high level and is referred to as 

solar maximum conditions. This is the time period where SPE probability is at their highest levels. Also during this time, due to the high energy output of the Sun, the GCR population incident 

on the Earth is reduced due to an increased solar wind population and a stronger solar magnetic 

field. Similarly, during the solar minimum conditions, the low energy output of the Sun does not 

provide the same level of shielding for the Earth, and the GCR population incident on the Earth 

is increased.

A simplistic model60 was initially developed to predict the atmospheric neutron flux by 

assuming that the three factors affecting it (altitude, latitude, and solar activity) could be addressed 

independently.

A plot of how the neutron population varies as a function of altitude in this simplistic 

model is shown in figure 39. It can be seen from this figure that the neutron flux is a peaked function of altitude. This can be understood using the cartoon from figure 38. This figure shows that 

the significant portion of the neutron production occurs in the secondary and tertiary cascade 

events that occur lower in the atmosphere. This accounts for the rise in flux going from high to 

lower altitudes. The decrease in flux beyond the peak for lower altitudes is due simply to the range 

the incident ions have in the upper atmosphere. Only the highest energy incident ions can penetrate 

the atmosphere to sufficient depths to produce neutrons at low altitudes and on the Earth’s surface. The population of these ions is significantly smaller than lower energy particles and therefore 

would yield a lower neutron flux.
93

1.4

1.2

1

0.8

0.6

0.4

0.2

0

F39

Neutron Flux (n/cm2–s)

0 20 40 60 80 100

Altitude (1,000’s of feet)

Figure 39. Plot of the atmospheric neutron population as a function of altitude.

The peak environment is at an altitude of approximately 18.3 km.

The effect of the second parameter, latitude, is shown in figure 40. The effect shown here is 

what is expected based on the understanding geomagnetic field strength as a function of latitude. 

Up to about 30–40 deg of latitude, there is little increase in the neutron flux since the magnetic 

field strength is near maximum to this point. Also, above about 55 deg latitude, the magnetic field 

strength has diminished sufficiently to allow maximum neutron production, so little change is seen 

above that level. The middle region is just the transition between these two extremes.

The final parameter in the simplistic model is the solar activity. Since the three parameters are being considered independent, the effect of solar activity on the magnetic field strength 

is ignored. This allows the effect of solar activity to be nothing more than a scaling factor since 

the only effect is a higher incident ion rate.

This simplistic model gives a good qualitative understanding of the atmospheric neutron 

production. Since the three parameters are not really independent and the fact that the neutron 

flux is not just a single number but is a neutron energy spectrum at each location, a more detailed 

model is needed to properly predict the environment. Details of the measurement techniques and 

the modeled ground neutron energy spectra are contained in reference 61.
94

1.4

1.2

1

0.8

0.6

0.4

0.2

0

F40

Neutron Flux (n/cm2–s)

0 20 40 60 80 100

Latitude (deg)

Figure 40. Plot of the atmospheric neutron population as a function of latitude. 

The peak environment is at high latitudes where the incident heavy ions 

have less geomagnetic screening.

2.12.2 Ground Environments

The average flux at sea level for the latitude/longitude of New York City (Joint Electron 

Device Engineering Council reference location) for >10 MeV neutrons is approximately 13 n/cm2-hr.

Differential energy spectra for selected energy ranges for any location can be estimated from the 

spectra available for the reference location.61,62 SPEs generate an increase in neutron flux but do 

not generate direct ionization on the ground. However, measurements show increases of neutron 

flux as much as a factor of 30 from a large solar event in polar latitudes.63

2.12.3 Terrestrial Ionizing Radiation Effects

The effects of ionizing radiation on electronic systems can be divided into two categories: 

cumulative effects and transient effects. Cumulative effects are those that require exposure to the 

environments over time to produce an effect. Transient effects are those that can be induced at any 

time during a mission and can require nothing more than a single particle passing through an electronic device to produce the effect (that can either be temporary or permanent). Since the exposure 

time in the terrestrial environments is short and the ionizing radiation levels are small as compared 

to those in space, the cumulative effects for the terrestrial segments of a mission are negligible and 

will not be discussed here.
95

The only transient effect that is applicable to space systems is a single event effect (SEE). 

SEEs are effects in microelectronics that are induced by the passage of a single particle through 

thepart. This is the area where neutrons, protons (both from trapped environment and solar 

particle events), and galactic cosmic radiation are important. SEE is a generic term encompassing all possible effects. Over the years that effects of single particles have been investigated, many 

acronyms were devised to indicate the effects caused by these single particles. The most common 

of these are single event upset (SEU) and single event latchup. With the wide range of effects and 

associated acronyms, it is accepted in the radiation effects community to refer to SEE, then draw 

the distinction between destructive and nondestructive events. 

For protons and heavier ions the primary means of producing SEE is through interaction 

with the electrons of the material in the electronic component. This interaction occurs as the ion 

deposits energy in the material and the electron in the semiconductor material are raised sufficiently 

that they become available for conduction across the various junctions that are the structure of the 

electronic component.In short, the energy deposited produces stray currents within the device that 

may or may not produce an effect.

Neutrons and high-energy protons have another means of depositing energy in the devices, 

i.e., through direct nuclear interaction with the materials in the electronic component. In this case, 

the neutron or proton is captured into a nucleus where its energy is deposited. The most likely 

result of this interaction is that the nucleus expels the incident particle (at a lower energy) and 

recoils with the remaining energy. The recoiling nucleus does not have sufficient energy to produce 

a SEE but this type of interaction can be a significant contributor to a cumulative effect that will be 

discussed later. However, in approximately 1 in 104 nuclear interactions, the energy deposited in the 

nucleus will be sufficient to produce a spallation event, where the nucleus splits into larger atomic 

number fragments that can have sufficient energy to produce a SEE by the same ionization process 

described above for ions. More detailed descriptions of SEEs and the means by which they are produced can be found in reference 64.

Therefore, for the terrestrial environments that a space vehicle will encounter, the cumulative 

effects of ionizing radiation will not be significant. However, the electronic systems will be exposed 

to environments that can produce SEEs either through direct ionization interactions or through 

spallation events. Since this type of effect can occur at any time during a mission, and the effect 

may be nondestructive or destructive, it is imperative that these environments be considered for 

systems exposed during this segment of a mission.

2.12.4 Ground-Based Effects

While on the ground the only environment that a space vehicle will be exposed to is that 

population of atmospheric neutrons that reach the ground. While this is a relatively small environment, the electronics will be susceptible to these effects at any time they are powered. This can be 

a substantial amount of time during all the prelaunch operations. These types of events have been 

observed during the testing of the shuttle flight computers. Since current technology devices are 

much more susceptible to these types of effects than those used during the shuttle program, it is 

important that the electronic systems be analyzed for the effects of atmospheric neutrons while on 

the ground. Detailed treatment of atmospheric neutron effects on avionics can be found in 

reference 65.
96

3. TERRESTRIAL ALOFT ENVIRONMENTS

This section provides a baseline description of the natural terrestrial aloft environment 

(150 m to 90 km (492.1 to 295.3×103 ft)). A summary of key design factors for the aloft environment is displayed in table 49.

Table 49. Summary of key design factors.

Parameter Source Select Based On

Winds aloft: ascent/descent 

trajectories

Sections 3.1, 3.1.1, 3.1.1.1, and 

3.1.2.1; figures 41 and 42

Location, season, altitude

Turbulence/wind gusts: 

In-flight wind perturbations

Sections 3.2, 3.2.1, and 3.2.2 Altitude, length scale

Solar radiation Section 3.3 Altitude

Temperature Figures 48 and 49; tables 55 and 56 Location, altitude

Pressure Figures 50 and 51; tables 57 and 58 Location, altitude

Density Figures 52 and 53; tables 59 and 60 Location, altitude

Humidity Figures 55 and 56; tables 61 and 62 Location, altitude

Concentrations, altitude Section 3.8.1; table 63 Altitude

Particle type, characteristics Section 3.8.2; table 64 Altitude

Atmospheric models Sections 3.9, 3.9.1–3.9.6; EarthGRAM 2010: GUACA, MAP, MET, 

and RRA

Location, season, altitude

Clouds Section 3.10 Location, season, altitude

Ionizing radiation: atmospheric 

neutron environment

Section 3.11.3 Critical device neutron upset 

threshold, latitude, altitude

Ionizing radiation: atmospheric GCR 

environment

Section 3.11.2 Critical device upset or latch 

threshold, latitude, altitude

3.1 Winds Aloft

The winds aloft are a critical factor in the design and performance of vehicles. In-flight wind 

profiles are needed for all stages of the vehicle design process. A basic requirement is to define an 

annual design reference wind profile that will be applied (along with the annual design reference 

atmosphere) in the development of the design reference ascent and descent reference trajectories. 

The databases for this basic requirement are included in NASA/TM—2008–215633.1 When system 

component designs have been integrated to define a vehicle for the selected architecture, wind profile databases are required for application in design vehicle ascent trajectory simulations to establish 

the dispersion of trajectory variables relative to winter and summer month design reference trajectories. Developers of the vehicle’s guidance and control systems and structural elements either 

design the vehicle to withstand the trajectory dispersions, protect the vehicle from dispersions using 
97

operational constraints, or accept the risk of encountering the dispersions. If aerodynamic dispersion attributed to the wind environment cannot be safely accommodated with available technology, 

the vehicle design process will determine the need for operational constraints to achieve vehicle 

performance objectives. Likewise, the risk of encountering the wind environment can be accepted if 

neither vehicle design nor operational constraints mitigate the dispersion.

Analyses and models of in-flight winds are included in NASA/TM—2008–215633.1 Selected 

information in the handbook could be useful for special engineering studies other than vehicle trajectory simulations to establish the capability of vehicle systems to withstand certain characteristics 

of the wind profile. This extensive information, which includes historical and alternative methods 

for defining characteristics of ascent wind profiles, may not be universally applicable to methodologies used in the vehicle design process.

When trajectory simulations are based on a representative sample of wind profiles, all the 

characteristics of the wind profiles are included and need not be analyzed piecemeal, independent 

of the vehicle flight simulation. Computational speed, which was a serious impediment to multiple 

simulations of vehicle trajectories in the early history of spacecraft development, now permits 

atrajectory simulation in seconds on a desktop computer compared to ‘overnight’ during the 

NASA Saturn/Apollo program.

3.1.1 Eastern Range

Wind profiles for in-flight design studies are either modeled or measured. Both are available 

for the ER. The monthly vector wind profile (MVWP) model, outlined in section 3.1.1.1, has 

been developed for ascent from the ER (0 to 27 km (0 to 88.6×103 ft)) and EAFB (0 to 25 km 

(0 to 82×103 ft)), and could be developed for any proposed launch site that has a statistically 

representative sample of rawinsonde profiles. A sample of measured high-resolution Jimsphere 

wind profiles has been traditionally applied in design evaluation flight simulations to determine 

the operational capability of a launch system. An adequate sample of Jimsphere wind profiles for 

this application exists only for the ER. In addition, the MSFC Natural Environments Branch has 

developed an archive of measurements from the KSC 50-MHz and 915-MHz Doppler radar wind 

profiler (DRWP) network, which contains high temporal and spatial resolution measurements from 

0.3 to 18.6 km (656 to 61×103 ft). The utility of modeled or measured profiles, to be established 

by consultation with the design community, is related to (1) the complexity of wind representation 

required for the vehicle design application and (2) the flexibility for use in design trade studies.

Extensive measurements of winds aloft have been made at the ER through the use of rawinsondes, Jimspheres, and DRWPs to provide vertical profiles of winds. A sample wind profile contains mean wind, fine structures, and turbulence/gusts superimposed over a general trend. These 

measurements have provided insight into the temporal variability in winds aloft and aid in defining 

the wind environment the vehicle will be exposed to along a flight trajectory. Research has been 

performed to study the lifetime of wind features aloft measured by the instruments listed above 

for the purpose of determining how long before launch the measured winds accurately depict the 

environment through which the vehicle will travel.66,67 The length scale of features that are above 

a predetermined coherence scale increases as the time between measurements increases.
98

3.1.1.1 Monthly Vector Wind Profile Model (0 to 27 km (0 to 88.6×103 ft)). The basic 

application of the MVWP model is for evaluation of design vehicle aerodynamic load indicator 

dispersion attributable to wind profile dispersion during the ascent phase.66,67 The February and 

July MVWP model profile dispersion encompasses the wind profile dispersion for the entire year. 

These profiles are used in vehicle rigid body trajectory simulations for vehicle trade studies, development of vehicle guidance and control systems, and establishment of propellant requirements 

and reserves. For a selected reference altitude H, the model constructs 12 wind profiles that envelope the monthly wind vector dispersion at that altitude for a selected probability level. The construction process, based on quadravariate normal statistical probability functions, produces vectors 

at all the adjacent altitudes.

Note: For indicators, as used above, qα and qβ, where q is aerodynamic pressure, α is angle 

of attack, and β is angle of sideslip.

Given a wind vector at H, the wind vectors at each altitude above or below H are conditional bivariate normal distributed. The given wind vectors at H are defined at 30° increments on 

the 99th percentile monthly wind vector ellipse relative to the vector of the monthly means at H. 

The conditional wind vectors at all adjacent altitudes that intercept the 99th percentile conditional 

wind vector ellipse 180° from the direction of the given wind vector approximate the largest wind 

vector shear between H and each selected adjacent altitude. The 12 profiles (zero to 27 km (zero 

to 88.6×103 ft)) for the ER illustrated in figure 41 are the in-plane and out-of-plane wind components, x and y, for a 90° flight azimuth during February for H=9 km (29.5×103 ft). In-plane and 

out-of-plane components are defined in reference to the vehicle pitch plane. The wind components, 

x and y, are calculated from the u and v wind components. These profiles are constructed for the 

99th percentile wind vector dispersion at 9 km (29.5×103 ft) and from the 99th percentile conditional probability ellipses at all other adjacent altitudes. Similarly, but not illustrated herein, a total 

of 336 profiles are constructed from the ER rawinsonde statistical database, when all 28 of the 

available reference altitudes (zero to 27 km (zero to 88.6×103 ft)) are used.

The extreme profiles of the February and July mean in-plane and out-of-plane wind components can be applied in the development of the monthly Vehicle Ascent Guidance Steering Commands (VAGSC) used in the trajectory simulations for the corresponding monthly sets of MVWP 

model profiles. The trajectory simulation using either of these profiles of mean wind components 

produces the baseline aerodynamic load indicators at each altitude for the selected month. The 

aerodynamic load indicators (12 values of q, α, and β) derived from trajectory simulations using 

the 12 vector wind profiles for a selected reference altitude (for a particular month) represents the 

dispersion from the baseline at that altitude.

As evidenced by Space Shuttle and Department of Defense launch vehicle operational procedures, the aerodynamic load indicator dispersion can be greatly reduced by using a day of launch 

(DOL) wind profile for the VAGSC. The reduction of aerodynamic loads dispersion attributable to 

wind profile dispersion has allowed present vehicle programs to achieve performance and operability objectives.
99

F41

25

20

15

10

5

0

–80 –60 –40 –20 0 20 40

(a) In-Plane Wind Component (m/s) (b)

25

20

15

10

5

0

–80 –60 –40 –20 0 20 40

In-Plane Wind Component (m/s)

25

20

15

10

5

0

–40 –30 –20 –10 0 10 20 30 40 50

(c) Out-of-Plane Wind Component (m/s)

Height (km)Height (km)

Height (km)

Figure 41. Twelve vector wind model profiles, the ER, February, reference height=9 km 

(29.5×103 ft), wind components (m/s), flight azimuth=90°: (a) In-plane (x), 

profiles 1–6, (b) in-plane (x), profiles 7–12, and (c) out-of-plane (y), profiles 1–12.

The wind profiles generated with the MVWP model contain wavelengths as small as 2 km.

As illustrated in figure 42, it has been demonstrated, in an application of the original MVWP 

model during the Space Shuttle program,68 that adding smaller wavelength wind perturbations or 

gusts to the model profiles will result in excessive dispersion of rigid body aerodynamic load indicators. This was verified in an application of a later version of the MVWP model.69 Gusts are treated 

separately in elastic body dynamic loads analyses using discrete gust models or with simulated gust 

time series. Gust models are described in section 3.2.
100

F42

Ma = 1.05

1,000

–1,000

–2,000 –1,000 1,000 2,000

–2,000

qβ

qα 150 Measured Winds

95 Percentile Measured

95 Percentile VWM

Synthetic With Gust

–4,000

–3,000

Figure 42. STS-1 pitch and yaw aerodynamic load indicators, qα and

qβ (lb/ft2 deg), Mach = 1.05, 150 April Jimsphere profiles, 

ER (addition of gusts to the synthetic wind profiles yields 

an overconservative result compared to the measured rates).

3.1.1.2 Jimsphere Wind Profile Database (0 to 20 km (0 to 65.6×103 ft)). Profiles from the 

Jimsphere balloon measurement system have been archived for use in flight vehicle ascent loads 

and trajectory assessments. The database contains sets of individual profiles for each month and 

profile pairs for the winter, summer, and transition seasons. Individual profiles can be used to 

examine vehicle loading from both large- and small-scale wind features due to the high resolution 

of Jimsphere measurements. The Jimsphere pairs database contains wind profiles separated by 

either 2 or 3.5 hr, and can be used to characterize the effect of wind changes over these intervals 

to the vehicle during the ascent phase. The Jimsphere archive provides output from zero to 20 km 

(zero to 65.6×103 ft) every 25 m (82 ft).

3.1.1.3 Kennedy Space Center Doppler Radar Wind Profiler Database (0.2 to 18.6 km 

(656 to 61×103 ft)). The MSFC Natural Environments Branch has developed an extensive archive 

of wind profiles from the KSC 50-MHz and 915-MHz DRWP network.70 The archive contains 

wind profiles from 0.2 to 18.6 km (656 to 61×103 ft) every 100 m (328.1 ft), and serves the same 

purpose as the Jimsphere database described in section 3.1.1.2. However, the near-continuous 

measurement capability of the DRWP systems provides two major advantages to using the DRWP 

archive over the Jimsphere archive. First, the DRWP database contains a significantly greater number of profiles. Second, engineers can simulate the effects of wind changes to flight vehicles over 

any practical time interval.

3.1.1.4 High-Altitude Ascent Wind Model (up to 90 km (295.3×103 ft)). If required, the 

measured or MVWP model profiles can be appended with nominal or dispersed wind vectors 

derived from the Earth-Global Reference Atmosphleric Model (GRAM).71 For more details on 

Earth-GRAM, refer to section 3.9.
101

3.1.2 Edwards Air Force Base

3.1.2.1 Monthly Vector Wind Profile Model (0 to 25 km (0 to 82×103 ft)). An example 

MVWP model for EAFB is shown in figure 43. The 12 profiles illustrated in figure 43 are the inplane and out-of-plane wind components, x and y, for a 90° flight azimuth during February for 

H=9 km (29.5×103 ft). The 90° flight azimuth is shown for consistency with the other sites. Any 

desired flight azimuth can be produced from the MVWP model. The MVWP model is intended for 

use during the ascent phase only.

F43

(a) (b)

(c)

In-Plane Wind Component (m/s) In-Plane Wind Component (m/s)

Out-of-Plane Wind Component (m/s) Height (km)

Height (km)

25

20

15

10

5

0

25

20

15

10

5

0

Height (km)

–80 –60 –40 –20 0 20 40 –80 –60 –40 –20 0 20 40

25

20

15

10

5

0

–40 –30 –20 –10 0 10 20 30 40 50

Figure 43. Twelve vector wind model profiles, EAFB, February, reference height=9 km 

(29.5×103 ft), wind components (m/s), flight azimuth=90°: (a) In-plane (x), 

profiles 1–6, (b) in-plane (x), profiles 7–12, and (c) out-of-plane (y), profiles 1–12.68

3.1.2.2 High-Altitude Ascent Wind Model (up to 90 km (295.3×103 ft)). If required, the 

MVWP model profiles can be appended with nominal or dispersed wind vectors derived from 

Earth-GRAM.71 For more details on Earth-GRAM, refer to section 3.9.

3.1.2.3 Low-Level Wind Shear, Eastern Range. An abrupt wind change through a groundbased temperature (radiation) inversion occurs at the ER, especially in winter. Most commonly, 

strong winds exist at the top of the inversion with light and variable winds existing immediately 
102

below the top. These inversions develop at night, and endure until eroded by daytime surface heating, perhaps by 09:00 to 10:00 LST. 

3.1.2.4 Low-Level Wind Shear, Edwards Air Force Base. An abrupt wind change through 

a ground-based temperature (radiation) inversion occurs frequently at EAFB, especially in winter. Most commonly, strong winds exist at the top of the inversion with light and variable winds 

existing immediately below the top. These inversions develop at night, and endure until eroded by 

daytime surface heating, perhaps by 09:00 to 10:00 LST. 72

3.2 Discrete Gust Model

Idealized discrete gust models have been developed for vehicle design studies and other 

engineering applications in an attempt to represent, in a physically reasonable manner, characteristics of small-scale wind perturbations associated with vertical profiles of wind velocity. Evaluation of elastic body and buffeting response to in-flight wind perturbations or gusts is important in 

establishing vehicle design structural requirements and operational capability. In vehicle response 

analyses, the discrete gust is applied perpendicular to the launch vehicle longitudinal axis, and it is 

assumed that the vehicle is instantaneously immersed in the time-dependent gust profile.69 Simulation of the autopilot response to the gust produces the loads that are responses to the gust-induced 

deviations of the angle of attack and angle of sideslip from the prelaunch programmed vehicle 

guidance commands. Launch vehicles can have significant response to gusts that are not observable 

with wind profile measurement systems. Since it is not practical to perform elastic body loads analyses for flight vehicles on DOL because of time constraints and other considerations, the commit to 

launch decision is protected for gust uncertainty contributions to elastic body loads uncertainties.

This protection is in the form of an aerodynamic load increment (knockdown) that is developed 

in special engineering studies prior to the DOL. Development of the elastic body response to gust 

is also an important consideration during the vehicle design process, which is the initial intended 

application of material in this TM.

NASA has used two discrete gust models. The first model is the ‘classical-NASA’ 9 m/s 

(29.5 ft/s) quasi-square wave gust originally developed for Saturn/Apollo and certified for Shuttle 

program applications.2 The second model73 is an adaptation of the discrete gust model originally 

developed from Military Standard (MIL-STD-1797A) for the flying qualities of piloted aircraft,74

and was developed to improve representation of gusts with half-widths (dm) less than 100 m 

(328.1 ft) that had amplitudes that were overly conservative in the classical model. The classical 

NASA 9 m/s (29.5 ft/s) discrete gust model is considered to be the most conservative because of 

its quasi-square-wave form. In addition, no provision exists in the classical model for taking into 

account the expected variation of gust amplitude as a function of gust half-width and altitude. The 

MIL-STD model that includes such a variation yields smaller gust amplitudes than the classical 

model for gust half-widths less than 125 m (410.2 ft) at 8 km (26.2×103 ft) using severe turbulence 

criteria, and yields smaller gust amplitudes than the classical model for all gust half widths at all 

altitudes up to 90 km (295.3×103 ft) using moderate turbulence criteria.
103

3.2.1 NASA Discrete Gust Model

The basis for the derivation of gust magnitude as a function of gust half-width is given 

in MIL-STD-1797A74 and its background information and users guide.75 This MIL-STD is significant because it is based on the same aircraft turbulence data used in studies76–78 that led to the 

establishment of the NASA classical 9 m/s (29.5 ft/s) gust. The gust model has the ‘1-cosine’ shape 

(fig. 44) defined by MIL-STD-1797A:74

V = 0,d < 0 or d > 2dm, V =

Vm

2

1! cos

!d

dm

"

#

$

%

&

'

"

#

$

%

&

', and 0 ( d ( 2dm , (36)

where Vm is the gust magnitude, dm is the gust half-width, and d is distance.

Vm

dm 2dm

Distance

Gust Magnitude

newF44

Figure 44. Discrete gust model (1-cosine).

The MIL-STD relationship between nondimensional longitudinal gust magnitude, Vm /σ, and 

nondimensional gust half-width, dm /L is illustrated for the longitudinal gust component in figure 45,

where σ is the standard deviation of atmospheric turbulence and L is the scale length of atmospheric turbulence. The most recent compilation79 of these parameters (σ and L) as a function of 

altitude is presented in table 50 for moderate and severe turbulence. The relationship is derived by 

assuming gust perturbations are normally distributed about the steady state wind with:

f (V ) = 1

2!"V

2 exp ! 1

2

V ! μ ( ) V

2

!V

2

"

#

$

$

%

&

'

'

,

(37)
104

where V is the random gust and μV and σV are the mean and standard deviation of the random 

gusts.80,81 An initial gust (V1) will be related to a gust (V2) some distance (d) away by the conditional probability:

f V2 |V1 ( ) = f V1,V2 ( )

f V1 ( ) . (38)

3

2.5

2

1.5

1

0.5

0

F46

10–2 10–1 10 101

dm/L

Vm/σ

Figure 45. Nondimensional discrete gust magnitude Vm/σ as a function 

of nondimensional gust half-width, dm/L, longitudinal component.74
105

Table 50. Mean horizontal (longitudinal) and vertical turbulence standard deviation 

(σh and σw) and length scale (Lh and Lw), as a function of altitude.

Altitude MSL

Moderate Severe

Lh Lw σh σw σh σw

(km) (ft) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s) (m/s) (ft/s)

1 3.28 × 103 1.65 5.41 1.36 4.46 3.9 12.79 3.21 10.53 0.832 2.73 0.624 2.05

2 6.56 × 103 1.65 5.41 1.43 4.69 4.35 14.27 3.78 12.4 0.902 2.96 0.831 2.73

4 13.12 × 103 2.04 6.69 1.68 5.51 6.24 20.47 5.13 16.83 1.04 3.41 0.972 3.19

6 19.69 × 103 2.13 6.99 1.69 5.54 7.16 23.49 5.69 18.67 1.04 3.41 1.01 3.31

8 26.25 × 103 2.15 7.05 1.69 5.54 7.59 24.9 5.98 19.62 1.04 3.41 0.98 3.21

10 32.81 × 103 2.23 7.32 1.73 5.68 7.72 25.33 6 19.69 1.23 4.03 1.1 3.61

12 39.37 × 103 2.47 8.1 1.79 5.87 7.89 25.89 5.71 18.73 1.8 5.9 1.54 5.05

14 45.93 × 103 2.62 8.59 1.91 6.27 6.93 22.74 5.05 16.57 2.82 9.25 2.12 6.96

16 52.49 × 103 2.44 8.01 2.1 6.89 5 16.4 4.31 14.14 3.4 11.15 2.6 8.53

18 59.06 × 103 2.21 7.25 2.07 6.79 4.07 13.35 3.81 12.5 5 16.4 3.34 10.96

20 65.62 × 103 2.26 7.41 1.99 6.53 3.85 12.63 3.38 11.09 8.64 28.35 4.41 14.47

25 82.02 × 103 2.71 8.89 2.09 6.86 4.34 14.24 3.34 10.96 12 39.37 6.56 21.52

30 98.43 × 103 3.73 12.24 2.39 7.84 5.6 18.37 3.59 11.78 28.6 93.83 8.88 29.13

35 114.83 × 103 4.59 15.06 2.58 8.46 6.89 22.6 3.87 12.69 35.4 116.14 8.33 27.33

40 131.23 × 103 5.26 17.26 2.87 9.42 7.89 25.89 4.3 14.11 42.6 139.76 6.2 20.34

45 147.64 × 103 6.22 20.41 3.25 10.66 9.33 30.61 4.88 16.01 50.1 164.37 5.2 17.06

50 164.04 × 103 7.27 23.85 4.21 13.81 10.9 35.76 6.31 20.7 57.9 189.96 5.3 17.39

55 180.45 × 103 8.7 28.54 4.4 14.44 13.1 42.98 6.6 21.65 66 216.54 6 19.69

60 196.85 × 103 10.1 33.14 4.42 14.5 15.1 49.54 6.63 21.75 74.4 244.09 6.8 22.31

65 213.25 × 103 11.3 37.07 4.05 13.29 16.9 55.45 6 19.68 83.2 272.96 7.5 24.61

70 229.66 × 103 15.9 52.17 5.04 16.54 23.8 78.08 7.5 24.61 92.3 302.82 8.2 26.9

75 246.06 × 103 19.2 62.99 6.3 20.67 28.7 94.16 9.5 31.17 102 334.64 9 29.53

80 262.47 × 103 22.6 74.15 8.3 27.23 33.8 110.89 12.4 40.68 111 364.17 9.7 31.82

85 278.87 × 103 27.3 89.57 10.3 33.79 40.9 134.19 15.4 50.52 121 396.98 10.4 34.12

90 295.28 × 103 33.2 108.92 11.8 38.71 49.8 163.39 17.7 58.07 132 433.07 11.2 36.75

Assuming the gust perturbations have zero means, and that the initial gust (V1) begins at value 0, 

equation (38) becomes:

f V2 |V1 ( ) = 0 = 1

2!"2 1! #2 ( ) exp ! 1

2

1

1! !2

V2

2

"2

"

#

$

%

&

' , (39)

where ρ is the correlation between V1 and V2. Letting e2=σ 2(1–r2) and upon integration, 

equation (39) becomes:
106

F V2 ( ) = P x !V2 ( ) = 1

2!e

2 exp ! 1

2

V 2

e

2

"

#

$

%

&

' !(

V2

) dV . (40)

Since equation (40) cannot be integrated in closed form, computer routines are needed to compute 

the inverse of the normal cumulative distribution function. The inverse of equation (40) will provide the desired gust magnitude for a selected risk level. 

Typically, computer routines require the probability level, the mean, and the standard deviation as inputs. Since most routines calculate the one-sided probability, the proper value to input 

for a desired risk level would be P=1 –(risk/2), where ‘risk’ represents the probability of exceeding 

adesired gust magnitude. A value of zero should be used for the mean and e=√[σ2(1 – r 2)] should 

be used for the standard deviation. To determine e, the value for σ is selected from table 50 for the 

desired altitude. The correlation r is determined by the inverse Fourier transform of the Dryden 

spectral turbulence model. The equations for the Dryden model for the longitudinal, lateral, 

and vertical component of atmospheric turbulence are:

• Longitudinal !(!) = 2"2

L

#

1

1+ (L!)

2

"

#

$

%

&

' (41)

• Lateral, Vertical !(!) = "2

L

#

1+ 3(L!)

2

1+ (L!)

2 "

# $%

2 , (42)

where 0 ≤ Ω ≤ ∞, s 2 is the variance, and L is the turbulence length scale. Applying the inverse 

Fourier transform to equations (41) and (42) provides the autocorrelations:

• Longitudinal R(d ) = exp !d

L

"

#

$ %

&

' (43)

• Lateral, Vertical R(d ) = 1! d

2L

"

#

$ %

&

' exp !d

L

"

#

$ %

&

' , (44)

where R is the autocorrelation and d is the gust half-width (see eq. (36)). The value L is selected 

from table 50 for the desired altitude.

Values for Vm (m/s) at a selected altitude for a specified dm (m) are calculated using the 

values of s (m/s) and L (m) (sh and 103Lh, respectively) given in table 50 and the procedure 

described above. The values in table 50 are based on extensive published data gathered during the 

30-year period since the data were gathered for and became the basis of the NASA classical discrete gust model. The authors caution the reader that the caption at the top of the original table 

(ref. 1, table 2.79b), the source of the values listed in table 50, refers to ‘magnitudes (sh and sw),’ 

but should read ‘standard deviations (sh and sw).’ The principal author of the original report79 has 

verified this revision.
107

The derived gust magnitudes for the longitudinal gust component for severe and moderate turbulence as a function of altitude and gust half-width (dm) are listed in table 51 and table 52, 

respectively. The longitudinal component is defined as the horizontal gust in the direction of the 

mean wind. The longitudinal gust is superimposed with the steady state wind to excite vehicle 

structures in an elastic body simulation model.

Table 51. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (km) and gust half-width, dm (m), for severe 

turbulence.

Gust Magnitude (m/s)

Altitude

(km)

Gust Half-Width (m)

30 60 90 120 150 180 210 240 270 300

1 2.65 3.68 4.43 5.03 5.53 5.95 6.32 6.65 6.94 7.2

2 2.84 3.95 4.77 5.42 5.96 6.43 6.84 7.2 7.52 7.81

4 3.81 5.31 6.41 7.3 8.05 8.69 9.26 9.77 10.23 10.64

6 4.37 6.09 7.35 8.37 9.23 9.98 10.63 11.21 11.74 12.21

8 4.63 6.45 7.79 8.88 9.79 10.58 11.27 11.89 12.44 12.94

10 4.34 6.06 7.34 8.37 9.25 10.02 10.7 11.3 11.85 12.36

12 3.68 5.16 6.27 7.18 7.96 8.65 9.27 9.83 10.35 10.82

14 2.59 3.64 4.44 5.1 5.67 6.18 6.64 7.06 7.45 7.81

16 1.7 2.4 2.92 3.36 3.74 4.08 4.39 4.67 4.94 5.18

18 1.14 1.61 1.97 2.27 2.53 2.76 2.98 3.17 3.35 3.53

20 0.82 1.16 1.42 1.64 1.83 2 2.16 2.31 2.44 2.57

25 0.79 1.12 1.36 1.57 1.76 1.92 2.07 2.21 2.35 2.47

30 0.66 0.93 1.14 1.32 1.47 1.61 1.74 1.86 1.97 2.08

35 0.73 1.03 1.26 1.46 1.63 1.79 1.93 2.06 2.18 2.3

40 0.76 1.08 1.32 1.52 1.7 1.86 2.01 2.15 2.28 2.4

45 0.83 1.18 1.44 1.66 1.86 2.03 2.2 2.35 2.49 2.62

50 0.9 1.28 1.56 1.81 2.02 2.21 2.39 2.55 2.71 2.85

55 1.01 1.43 1.76 2.03 2.27 2.48 2.68 2.86 3.04 3.2

60 1.1 1.56 1.91 2.21 2.47 2.7 2.92 3.12 3.31 3.49

65 1.17 1.65 2.02 2.34 2.61 2.86 3.09 3.3 3.5 3.69

70 1.56 2.21 2.71 3.12 3.49 3.82 4.13 4.42 4.68 4.93

75 1.79 2.53 3.1 3.58 4.01 4.39 4.74 5.07 5.37 5.66

80 2.02 2.86 3.5 4.05 4.52 4.95 5.35 5.72 6.07 6.39

85 2.35 3.32 4.06 4.69 5.24 5.74 6.20 6.63 7.03 7.41

90 2.73 3.87 4.74 5.47 6.11 6.69 7.23 7.73 8.2 8.64
108

Table 52. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (ft) and gust half-width, dm (ft), for severe turbulence.

Gust Magnitude (ft/s)

Altitude

(km)

Gust Half-Width (ft)

98.4 196.9 295.3 393.7 492.1 590.6 688.9 787.4 885.8 984.3

3.3 8.69 12.07 14.53 16.5 18.14 19.50 20.73 21.82 22.77 23.62

6.6 9.32 12.96 15.65 17.78 19.55 21.09 22.44 23.62 24.67 25.62

13.1 12.5 17.42 21.03 23.95 26.41 28.51 30.38 32.05 33.56 34.91

19.7 14.34 19.98 24.11 27.46 30.28 32.74 34.88 36.78 38.52 40.06

26.2 15.19 21.16 25.56 29.13 32.12 34.71 36.98 39.01 40.81 42.45

32.8 14.24 19.88 24.08 27.46 30.35 32.87 35.1 37.07 38.88 40.55

39.4 12.07 16.93 20.57 23.56 26.12 28.38 30.41 32.25 33.96 35.5

45.9 8.49 11.94 14.57 16.73 18.6 20.28 21.78 23.16 24.44 25.62

52.5 5.58 7.87 9.58 11.02 12.27 13.39 14.4 15.32 16.21 16.99

59.1 3.74 5.28 6.46 7.45 8.3 9.06 9.78 10.4 10.99 11.58

65.6 2.69 3.81 4.66 5.38 6 6.56 7.09 7.58 8 8.43

82 2.59 3.67 4.46 5.15 5.77 6.29 6.79 7.25 7.71 8.1

98.4 2.17 3.05 3.74 4.33 4.82 5.28 5.71 6.1 6.46 6.82

114.8 2.39 3.38 4.13 4.79 5.35 5.87 6.33 6.76 7.15 7.55

131.2 2.49 3.54 4.33 4.99 5.58 6.1 6.59 7.05 7.48 7.87

147.6 2.72 3.87 4.72 5.45 6.1 6.66 7.22 7.71 8.17 8.6

164 2.95 4.19 5.12 5.94 6.63 7.25 7.84 8.37 8.89 9.35

180.4 3.31 4.69 5.77 6.66 7.45 8.14 8.79 9.38 9.97 10.5

196.9 3.61 5.12 6.27 7.25 8.1 8.86 9.58 10.24 10.86 11.45

213.3 3.84 5.41 6.63 7.68 8.56 9.38 10.14 10.83 11.48 12.11

229.7 5.12 7.25 8.89 10.24 11.45 12.53 13.55 14.5 15.35 16.17

246.1 5.87 8.3 10.17 11.75 13.16 14.4 15.55 16.63 17.62 18.57

262.5 6.63 9.38 11.48 13.29 14.83 16.24 17.55 18.77 19.91 20.96

278.9 7.71 10.89 13.32 15.39 17.19 18.83 20.34 21.75 23.06 24.31

295.3 8.96 12.69 15.55 17.95 20.05 21.95 25.36 25.36 26.9 28.35

As shown in tables 51 and 52, the smallest gust half-width (dm) associated with a 9 m/s 

(29.5 ft/s) gust assuming severe turbulence is approximately 130 m (426.5 ft) at 8 km (26.2×103 ft). 

This half-width is more than four times larger than the smallest half-width (30 m (98.4 ft)) of the 

NASA classical 9 m/s (29.5 ft/s) gust. This discussion emphasizes the conservatism of the NASA 

classical 9 m/s (29.5 ft/s) gust model for gust half-widths <130 m (426.5 ft). Application of the 

NASA discrete gust model for specification of gust magnitude requires that adequate protection 

exists for all vehicle response modes, including those for the gust half-widths greater than 130 m 

(426.5 ft) for which gust magnitudes can be larger than 9 m/s (29.5 ft/s) using severe turbulence.

Tables 53 and 54 show that moderate turbulence magnitudes range from 2.7 to 3.7 m/s (8.9 to 12.1 

ft/s) in the altitude and half-width regions where the severe turbulence magnitudes exceed 9 m/s 

(29.5 ft/s), which illustrates the conservatism of using severe turbulence relative to moderate 

turbulence.
109

Table 53. Discrete longitudinal gust magnitude at the 1% risk level of exceedance as a function 

of altitude (km) and gust half-width, dm (m), for moderate turbulence.

Gust Magnitude (m/s)

Altitude

(km)

Gust Half-Width (m)

30 60 90 120 150 180 210 240 270 300

1 1.12 1.56 1.87 2.13 2.34 2.52 2.68 2.81 2.94 3.05

2 1.08 1.5 1.81 2.05 2.26 2.44 2.59 2.73 2.85 2.96

4 1.24 1.73 2.09 2.39 2.63 2.84 3.03 3.19 3.34 3.48

6 1.3 1.81 2.19 2.49 2.75 2.97 3.16 3.34 3.49 3.63

8 1.31 1.83 2.21 2.51 2.77 3 3.19 3.37 3.52 3.67

10 1.25 1.75 2.12 2.42 2.67 2.89 3.09 3.27 3.42 3.57

12 1.15 1.62 1.96 2.25 2.49 2.71 2.9 3.08 3.24 3.39

14 0.98 1.38 1.68 1.93 2.14 2.34 2.51 2.67 2.82 2.95

16 0.83 1.17 1.43 1.64 1.83 1.99 2.14 2.28 2.41 2.53

18 0.62 0.88 1.07 1.23 1.37 1.5 1.62 1.72 1.82 1.91

20 0.48 0.68 0.84 0.96 1.08 1.18 1.27 1.35 1.43 1.51

25 0.49 0.7 0.85 0.98 1.1 1.2 1.29 1.38 1.46 1.54

30 0.44 0.62 0.76 0.88 0.98 1.07 1.16 1.24 1.31 1.38

35 0.49 0.69 0.84 0.97 1.09 1.19 1.28 1.37 1.45 1.53

40 0.51 0.72 0.88 1.02 1.13 1.24 1.34 1.43 1.52 1.6

45 0.55 0.78 0.96 1.11 1.24 1.36 1.46 1.56 1.66 1.75

50 0.6 0.85 1.04 1.2 1.35 1.47 1.59 1.7 1.8 1.9

55 0.68 0.96 1.17 1.35 1.51 1.65 1.78 1.91 2.02 2.13

60 0.74 1.04 1.28 1.48 1.65 1.81 1.95 2.09 2.21 2.33

65 0.78 1.11 1.35 1.56 1.75 1.91 2.07 2.21 2.34 2.47

70 1.04 1.48 1.81 2.09 2.33 2.56 2.76 2.95 3.13 3.3

75 1.20 1.7 2.08 2.4 2.68 2.94 3.17 3.39 3.59 3.79

80 1.35 1.91 2.34 2.71 3.02 3.31 3.58 3.82 4.06 4.27

85 1.57 2.21 2.71 3.13 3.5 3.83 4.14 4.42 4.69 4.95

90 1.82 2.58 3.16 3.64 4.07 4.46 4.82 5.15 5.46 5.76
110

Table 54. Discrete longitudinal gust magnitude at the 1% risk level of exceedance 

as a function of altitude (ft) and gust half-width, dm (ft), for moderate 

turbulence.

Gust Magnitude (ft/s)

Altitude

(km)

Gust Half-Width (ft)

98.4 196.9 295.3 393.7 492.1 590.6 688.9 787.4 885.8 984.3

3.3 3.67 5.12 6.14 6.99 7.68 8.27 8.79 9.22 9.65 10.01

6.6 3.54 4.92 5.94 6.73 7.41 8.01 8.5 8.96 9.35 9.71

13.1 4.07 5.68 6.86 7.84 8.63 9.32 9.94 10.47 10.96 11.42

19.7 4.27 5.94 7.19 8.17 9.02 9.74 10.37 10.96 11.45 11.91

26.2 4.3 6 7.25 8.23 9.09 9.84 10.47 11.06 11.55 12.04

32.8 4.1 5.74 6.96 7.94 8.76 9.48 10.14 10.73 11.22 11.71

39.4 3.77 5.31 6.43 7.38 8.17 8.89 9.51 10.1 10.63 11.12

45.9 3.22 4.53 5.51 6.33 7.02 7.68 8.23 8.76 9.25 9.68

52.5 2.72 3.84 4.69 5.38 6 6.53 7.02 7.48 7.91 8.3

59.1 2.03 2.89 3.51 4.04 4.49 4.92 5.31 5.64 5.97 6.27

65.6 1.57 2.23 2.76 3.15 3.54 3.87 4.17 4.43 4.69 4.95

82 1.61 2.3 2.79 3.22 3.6 3.94 4.23 4.53 4.79 5.05

98.4 1.44 2.03 2.49 2.89 3.22 3.51 3.81 4.07 4.3 4.53

114.8 1.61 2.26 2.76 3.18 3.58 3.9 4.2 4.49 4.76 5.02

131.2 1.67 2.36 2.89 3.35 3.7 4.07 4.4 4.69 4.99 5.25

147.6 1.8 2.56 3.15 3.64 4.07 4.46 4.79 5.12 5.45 5.74

164 1.97 2.79 3.41 3.94 4.43 4.82 5.22 5.58 5.91 6.23

180.4 2.23 3.15 3.84 4.43 4.95 5.41 5.84 6.27 6.63 6.99

196.9 2.43 3.41 4.2 4.86 5.41 5.94 6.4 6.86 7.25 7.64

213.3 2.56 3.64 4.43 5.12 5.74 6.27 6.79 7.25 7.68 8.1

229.7 3.41 4.86 5.94 6.86 7.64 8.4 9.06 9.68 10.27 10.83

246.1 3.94 5.58 6.82 7.87 8.79 9.65 10.4 11.12 11.78 12.43

262.5 4.43 6.27 7.68 8.89 9.91 10.86 11.75 12.53 13.32 14.01

278.9 5.15 7.25 8.89 10.27 11.48 12.57 13.58 14.5 15.39 16.24

295.3 5.97 8.46 10.37 11.94 13.35 14.63 15.81 16.9 17.91 18.9

3.2.2 Discrete Gust Summary

NASA has developed a rationale for derivation of a discrete gust magnitude that is a function of altitude and gust half-width. This rationale is based on established methods that are 

included in the military specification (MIL-SPEC) of requirements for the flying qualities of 

piloted aircraft. This prior specification is significant because it is based on the same aircraft turbulence data used in studies that established the classical NASA 9 m/s (29.5 ft/s) discrete gust model. 

The program application of the MIL-SPEC model is for protection of the vehicle from elastic 

body loads response to gust during severe turbulence. The MIL-SPEC model produces larger gust 

magnitudes for severe turbulence than the NASA classical model from roughly 6 to 10 km (19.7 

to 32.8×103 ft) for gust half-widths between approximately 150 and 210 m (492.1 to 689 ft), and 

produces larger gust magnitudes than the NASA classical model from nearly 4 to 12 km (13.1 to 
111

39.4×103 ft) for gust half-widths greater than approximately 210 m (689 ft). Conversely, the NASA 

classical model produces larger gust magnitudes than the MIL-SPEC model at all other altitudes 

and gust half-widths. Using the MIL-SPEC model for moderate turbulence further decreases the 

gust magnitudes by roughly a factor of 3 to 3.5 compared to using severe turbulence. The conservatism of the classical model is attributed to the lack of a relationship between gust magnitude 

and gust half-width, which is illustrated by the ‘flat-top’ quasi-square waveform and the larger gust 

gradients that result from constraining the classical gust model build-up and back-off to a 30 m 

(98.4 ft) interval (fig. 46). The MIL-SPEC model eliminates this conservatism by building up the 

gust magnitude from the start of the gust to the gust half-width, and by backing off the gust magnitude from the gust half-width to the end of the gust. In addition, the MIL-SPEC model is not 

constrained to peaking at 9 m/s (29.5 ft/s) at certain altitudes and half-widths, as shown in tables 51

and 52. An illustration of applying the two models for two gust half-widths at 8 km (26.2×103 ft) 

is provided in figure 46. The NASA classical model exceeds the MIL-SPEC model for severe turbulence at all relevant gust distances for dm equals 60 m (196.9 ft), but the MIL-SPEC model for 

severe turbulence exceeds 9 m/s (29.5 ft/s) from roughly 120 to 180 m (393.7 to 590.6 ft) when dm

equals 150 m (492.1 ft) at the same altitude.

0

1

2

3

4

5

6

7

8

9

10

newF46

0 50 100 150 200 250 300

Distance (m)

Gust Magnitude (m/s)

Classical

MIL-SPEC

Figure 46. MIL-SPEC discrete gust at 8 km (26.2×103 ft) for severe turbulence 

(solid) and the NASA classical (dashed) discrete gust for half-widths 

of 60 and 150 m (196.9 and 492.1 ft).
112

3.3 Solar Radiation

Solar radiation intensity on a surface will increase with altitude above the Earth’s surface, 

with clear skies, according to the following equation:

IDN (Z) = IDN (0) + S ! IDN ( ) (0) 1! !z

!s

"

#

$

%

&

' , (45)

where

IDN(Z)= intensity of solar radiation normal to surface at required height Z

IDN(0) = intensity of solar radiation normal to surface at the Earth’s surface assuming 

clear skies (IDN(0)=ITN–IDH(Z))

ρz= atmospheric density at required height (from 1976 U.S. Standard Atmosphere 

or 1966 U.S. Standard Atmosphere Supplements) (kg/m3)

ρs= atmospheric density at sea level (from U.S. Standard Atmospheres or U.S. 

Standard Atmosphere Supplements) (kg/m3)

S= solar constant (e.g., 1,367 W/m2).

The diffuse solar radiation IdH decreases with altitude above the Earth’s surface. For clear skies, 

a good estimate of the value can be obtained from the following equation:

IdH (Z) = 523! 0.4076I

DN (Z) , (46)

where

IdH(Z) = intensity of diffuse radiation (W/m2) at height Z

IDN(Z) = intensity of solar radiation (W/m2) normal to surface at height Z.

Equation (46) is valid for values of IDN(Z) from equation (45) up to 1,283 W/m2. For values 

of IDN(Z) greater than 1,283 W/m2, IdH =0.

3.4 Temperature

The atmosphere from zero to 90 km (zero to 295.3×103 ft) has four distinct layers known 

as the troposphere, stratosphere, mesosphere, and thermosphere. The temperature profile contributes significantly to the characteristics of each layer. At the top of each layer, a transition zone 

exists where the temperature is quasi-isothermal with height. These transition zones are known 

as the tropopause, stratopause, mesopause, and thermopause.
113

The figures and tables in this section were derived from the 1983 Range Reference Atmosphere (RRA) for the particular site. Only the annual means are shown here. However, the designers are not constrained to using the annual means shown in this TM. Additional (monthly) data 

are available from the MSFC Natural Environments Branch and from the RRA documentation 

for each site.

3.4.1 Eastern Range

Seasonal variations in the temperature profile above the ER exist. For example, temperatures are colder in the layer from 70 to 90 km (229.7×103 to 295.3×103 ft) in the summer months 

than in the winter months above the ER. Figure 47 and table 55 show profiles of the annual 

mean and ±2σ values of temperature for the ER, as obtained from the Cape Canaveral RRA.79

Spatial and temporal atmospheric temperature profiles for specific purposes, such as temperature 

along a time-varying flight trajectory, can be derived using Earth-GRAM,71 which is discussed 

in section 3.9.

70

60

50

40

30

20

10

0

newF47

Altitude (km)

–100 –80 –60 –40 –20 0 20 40

Temperature (°C)

–2σ Mean 2σ

Figure 47. Annual mean and ±2σ of atmospheric temperature as a function 

of altitude at the ER.
114

Table 55. Annual mean and ±2s range of atmospheric temperature as a function

of altitude at the ER.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (°C) (°F) (°C) (°F) (°C) (°F)

– – 20.81 69.46 8.45 47.21 33.17 91.71

0.003 9.8 20.45 68.81 7.91 46.24 32.99 91.38

1 3.3 × 103 15.88 60.58 6.48 43.66 25.28 77.5

2 6.6 × 103 11.17 52.11 3.71 38.68 18.63 65.53

3 9.8 × 103 6.41 43.54 –0.11 31.8 12.93 55.27

4 13.1 × 103 0.87 33.57 –5.51 22.08 7.25 45.05

5 16.4 × 103 –5.06 22.89 –11.72 10.9 1.6 34.88

6 19.7 × 103 –11.35 11.57 –18.41 –1.14 –4.29 24.28

7 23 × 103 –17.9 –0.22 –25.28 –13.5 –10.52 13.06

8 26.2 × 103 –24.79 –12.62 –32.71 –26.88 –16.87 1.63

9 29.5 × 103 –32.14 –25.85 –40.3 –40.54 –23.98 –11.16

10 32.8 × 103 –39.69 –39.44 –47.53 –53.55 –31.85 –25.33

12 39.4 × 103 –53.84 –64.91 –60 –76 –47.68 –53.82

14 45.9 × 103 –63.82 –82.88 –69.7 –93.46 –57.94 –72.29

16 52.5 × 103 –69.28 –92.7 –74.84 –102.71 –63.72 –82.7

18 59.1 × 103 –68.66 –91.59 –75.64 –104.15 –61.68 –79.02

20 65.6 × 103 –62.61 –80.7 –69.09 –92.36 –56.13 –69.03

22 72.2 × 103 –57.23 –71.01 –62.91 –81.24 –51.55 –60.79

24 78.7 × 103 –53.18 –63.72 –59.04 –74.27 –47.32 –53.18

26 85.3 × 103 –49.57 –57.23 –55.81 –68.46 –43.33 –45.99

28 91.9 × 103 –46.08 –50.94 –52.62 –62.72 –39.54 –39.17

30 98.4 × 103 –42.67 –44.81 –49.25 –56.65 –36.09 –32.96

34 111.5 × 103 –38.01 –36.42 –48.09 –54.56 –27.93 –18.27

38 124.7 × 103 –33.80 –28.84 –44.56 –48.21 –23.04 –9.47

42 137.8 × 103 –28.94 –20.09 –40.48 –40.86 –17.4 0.68

46 150.9 × 103 –23.71 –10.68 –35.49 –31.88 –11.93 10.53

50 164 × 103 –18.07 –0.53 –29.93 –21.87 –6.21 20.82

54 177.2 × 103 –12.68 9.18 –24.84 –12.71 –0.52 31.06

58 190.3 × 103 –7.99 17.62 –19.61 –3.3 3.63 38.53

62 203.4 × 103 –5.62 21.88 –16.96 1.47 5.72 42.3

66 216.5 × 103 –4.81 23.34 –16.39 2.5 6.77 44.19

70 229.7 × 103 –5.71 21.72 –17.41 0.66 5.99 42.78
115

3.4.2 Edwards Air Force Base

Figure 48 and table 56 show profiles of the annual mean and ±2σ values of temperature 

for EAFB. Spatial and temporal atmospheric temperature profiles for specific purposes, such as 

temperature along a time-varying flight trajectory, can be derived using the Earth-GRAM.71 

Temperature data used in figure 48 and table 56 were obtained from the EAFB RRA.81

70

60

50

40

30

20

10

0

newF48

Altitude (km)

–100 –80 –60 –40 –20 0 20 40

Temperature (°C)

–2σ Mean 2σ

Figure 48. Annual mean and ±2σ of atmospheric temperature as a function 

of altitude at EAFB.
116

Table 56. Annual mean and ±2s range of atmospheric temperature as a function 

of altitude at EAFB.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (°C) (°F) (°C) (°F) (°C) (°F)

– – 13.27 55.89 –7.55 18.41 34.09 93.36

0.705 2.3 × 103 12.34 54.21 –5.12 22.78 29.8 85.64

1 3.3 × 103 14.62 58.32 –1.42 29.44 30.66 87.19

2 6.6 × 103 10.3 50.54 –5.58 21.96 26.18 79.12

3 9.8 × 103 4.41 39.94 –9.95 14.09 18.77 65.79

4 13.1 × 103 –1.94 28.51 –15.22 4.6 11.34 52.41

5 16.4 × 103 –8.68 16.38 –21.5 –6.7 4.14 39.45

6 19.7 × 103 –15.63 3.87 –28.25 –18.85 –3.01 26.58

7 23 × 103 –22.84 –9.11 –35.6 –32.08 –10.08 13.86

8 26.2 × 103 –30.29 –22.52 –43.15 –45.67 –17.43 0.63

9 29.5 × 103 –37.8 –36.04 –50.32 –58.58 –25.28 –13.5

10 32.8 × 103 –44.84 –48.71 –56.68 –70.02 –33 –27.4

12 39.4 × 103 –55.48 –67.86 –65.88 –86.58 –45.08 –49.14

14 45.9 × 103 –60.59 –77.06 –68.41 –91.14 –52.77 –62.99

16 52.5 × 103 –64.5 –84.1 –73.06 –99.51 –55.94 –68.69

18 59.1 × 103 –63.99 –83.18 –71.09 –95.96 –56.89 –70.4

20 65.6 × 103 –60.68 –77.22 –66.68 –88.02 –54.68 –66.42

22 72.2 × 103 –57.11 –70.8 –63.37 –82.07 –50.85 –59.53

24 78.7 × 103 –54.4 –65.92 –61.5 –78.7 –47.3 –53.14

26 85.3 × 103 –51.41 –60.54 –59.21 –74.58 –43.61 –46.5

28 91.9 × 103 –47.93 –54.27 –56.29 –69.32 –39.57 –39.23

30 98.4 × 103 –44.78 –48.6 –53.62 –64.52 –35.94 –32.69

34 111.5 × 103 –34.97 –30.95 –47.01 –52.62 –22.93 –9.27

38 124.7 × 103 –25.27 –13.49 –39.73 –39.51 –10.81 12.54

42 137.8 × 103 –14.24 6.37 –29.26 –20.67 0.78 33.4

46 150.9 × 103 –6.88 19.62 –20.66 –5.19 6.9 44.42

50 164 × 103 –6.29 20.68 –19.55 –3.19 6.97 44.55

54 177.2 × 103 –10.09 13.84 –22.87 –9.17 2.69 36.84

58 190.3 × 103 –15.31 4.44 –29.67 –21.41 –0.95 30.29

62 203.4 × 103 –24.68 –12.42 –44.56 –48.21 –4.8 23.36

66 216.5 × 103 –38.74 –37.73 –61.28 –78.3 –16.2 2.84

70 229.7 × 103 –54.7 –66.46 –80.8 –113.44 –28.6 –19.48
117

3.5 Pressure

Atmospheric pressure decreases approximately exponentially with height from zero to 

70 km (zero to 229.7×103 ft).

The figures and tables in this section were derived from the 1983 RRA for the particular site. 

Only the annual means are shown here. However, the designers are not to be constrained to use the 

annual means shown in this TM. Additional (monthly) data are available from the MSFC Natural 

Environments Branch and from the RRA documentation for each site.

3.5.1 Eastern Range

Pressure data used in figure 49 and table 57 were obtained from Cape Canaveral RRA.79

70

60

50

40

30

20

10

0

newF49

Altitude (km)

0 100 200 300 400 500 600 700 800 900 1,000 1,100

Pressure (hPa)

Figure 49. Annual mean atmospheric pressure as a function of altitude at the ER.
118

Table 57. Annual mean and ±2s range of atmospheric pressure as a function 

of altitude at the ER.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (hPa) (lbf/in2) (hPa) (lbf/in2) (hPa) (lbf/in2)

– – 1,017.4 14.7564 1,009.046 14.6352 1,025.754 14.8775

0.003 9.8 1,017.2 14.7535 1,008.736 14.6307 1,025.664 14.8762

1 3.3 × 103 906.15 13.1428 898.74 13.0353 913.56 13.2503

2 6.6 × 103 804.97 11.6753 797.296 11.564 812.644 11.7866

3 9.8 × 103 713.59 10.3499 705.426 10.2315 721.754 10.4683

4 13.1 × 103 631.03 9.1525 622.38 9.027 639.68 9.2779

5 16.4 × 103 556.63 8.0734 547.715 7.9441 565.545 8.2027

6 19.7 × 103 489.6 7.1012 480.303 6.9663 498.897 7.236

7 23 × 103 429.18 6.2248 419.66 6.0867 438.7 6.3629

8 26.2 × 103 374.92 5.4378 365.306 5.2984 384.534 5.5773

9 29.5 × 103 326.21 4.7313 316.537 4.5911 335.883 4.8716

10 32.8 × 103 282.63 4.0993 273.03 3.96 292.229 4.2385

12 39.4 × 103 209.22 3.0345 200.781 2.9121 217.659 3.1569

14 45.9 × 103 152.37 2.21 146.013 2.1178 158.727 2.3022

16 52.5 × 103 109.58 1.5893 105.383 1.5285 113.777 1.6502

18 59.1 × 103 78.532 1.139 75.494 1.095 81.57 1.1831

20 65.6 × 103 56.636 0.8214 54.194 0.786 59.078 0.8569

22 72.2 × 103 41.227 0.598 39.23 0.569 43.224 0.6269

24 78.7 × 103 30.211 0.4382 28.601 0.4148 31.821 0.4615

26 85.3 × 103 22.266 0.3229 20.972 0.3042 23.56 0.3417

28 91.9 × 103 16.495 0.2392 15.45 0.2241 17.54 0.2544

30 98.4 × 103 12.274 0.178 11.443 0.166 13.105 0.1901

34 111.5 × 103 6.871 0.0997 6.335 0.0919 7.407 0.1074

38 124.7 × 103 3.953 0.0573 3.618 0.0525 4.288 0.0622

42 137.8 × 103 2.328 0.0338 2.12 0.0308 2.536 0.0368

46 150.9 × 103 1.4 0.0203 1.272 0.0184 1.528 0.0222

50 164 × 103 0.848 0.0123 0.765 0.0111 0.931 0.0135

54 177.2 × 103 0.512 0.0074 0.457 0.0066 0.567 0.0082

58 190.3 × 103 0.306 0.0044 0.27 0.0039 0.342 0.005

62 203.4 × 103 0.179 0.0026 0.155 0.0022 0.203 0.003

66 216.5 × 103 0.1 0.0015 0.085 0.0012 0.115 0.0017

70 229.7 × 103 0.055 0.0008 0.043 0.0006 0.067 0.001
119

3.5.2 Edwards Air Force Base

Pressure data used in figure 50 and table 58 were obtained from EAFB RRA.81

70

60

50

40

30

20

10

0

0 100 200 300 400 500 600 700 800 900 1,000 1,100

Pressure (hPa)

Altitude (km)

F 51

Figure 50. Annual mean atmospheric pressure as a function of altitude at EAFB.
120

Table 58. Annual mean and ±2s range of atmospheric pressure as a function 

of altitude at EAFB.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (hPa) (lbf/in2) (hPa) (lbf/in2) (hPa) (lbf/in2)

– – 1,014.3 14.7114 1,000.979 14.5182 1,027.621 14.9046

0.705 2.3 × 103 932.26 13.5215 923.339 13.3921 941.181 13.6509

1 3.3 × 103 901.86 13.0806 893.422 12.9582 910.298 13.203

2 6.6 × 103 800.72 11.6136 791.585 11.4811 809.855 11.7461

3 9.8 × 103 709.17 10.2858 697.642 10.1186 720.698 10.453

4 13.1 × 103 626.21 9.0825 612.408 8.8824 640.012 9.2827

5 16.4 × 103 551.42 7.9978 536.094 7.7755 566.746 8.2201

6 19.7 × 103 484.23 7.0233 467.87 6.786 500.59 7.2606

7 23 × 103 423.33 6.14 406.476 5.8955 440.184 6.3844

8 26.2 × 103 368.76 5.3485 351.596 5.0995 385.924 5.5974

9 29.5 × 103 319.78 4.6381 302.747 4.391 336.813 4.8851

10 32.8 × 103 276.18 4.0057 259.558 3.7646 292.802 4.2468

12 39.4 × 103 203.5 2.9516 189.401 2.7471 217.599 3.1561

14 45.9 × 103 148.36 2.1518 138.014 2.0018 158.706 2.3019

16 52.5 × 103 107.36 1.5571 100.631 1.4595 114.089 1.6547

18 59.1 × 103 77.545 1.1247 73.042 1.0594 82.048 1.19

20 65.6 × 103 56.243 0.8157 52.887 0.7671 59.599 0.8644

22 72.2 × 103 41.074 0.5957 38.416 0.5572 43.732 0.6343

24 78.7 × 103 29.994 0.435 27.821 0.4035 32.167 0.4666

26 85.3 × 103 22.101 0.3206 20.345 0.2951 23.857 0.346

28 91.9 × 103 16.4 0.2379 14.986 0.2174 17.813 0.2584

30 98.4 × 103 12.196 0.1769 11.035 0.16 13.357 0.1937

34 111.5 × 103 6.84 0.0992 6.136 0.089 7.544 0.1094

38 124.7 × 103 3.925 0.0569 3.472 0.0504 4.378 0.0635

42 137.8 × 103 2.305 0.0334 2.007 0.0291 2.603 0.0378

46 150.9 × 103 1.381 0.02 1.187 0.0172 1.575 0.0228

50 164.0 × 103 0.835 0.0121 0.708 0.0103 0.962 0.0139

54 177.2 × 103 0.503 0.0073 0.42 0.0061 0.586 0.0085

58 190.3 × 103 0.3 0.0044 0.248 0.0036 0.352 0.0051

62 203.4 × 103 0.174 0.0025 0.143 0.0021 0.205 0.003

66 216.5 × 103 0.098 0.0014 0.081 0.0012 0.115 0.0017

70 229.7 × 103 0.054 0.0008 0.043 0.0006 0.065 0.0009
121

3.6 Density

Atmospheric mean density decreases exponentially with height, decreasing to half the MSL 

value at approximately 7 km (23×103 ft) above MSL. Density dispersions are of particular importance in the design of a launch vehicle. Dispersions at fixed altitudes have an influence on aerodynamic heating and dynamic pressure, and hence on trajectory designs and aerodynamic loads 

placed on a vehicle.

The figures and tables in this section were derived from the RRA for the particular site. 

Only the annual means are shown here. However, the designers are constrained to using the annual 

means shown in this TM. Additional (monthly) data are available from the MSFC Natural Environments Branch and from the RRA documentation for each site.

3.6.1 Eastern Range

Density data used in figure 51 and table 59 were obtained from the Cape Canaveral RRA.79

70

60

50

40

30

20

10

0

0 200 400 600 800 1,000 1,200 1,400

Density (g/m3

)

Altitude (km)

newF 51 Figure 51. Annual mean atmospheric density as a function of altitude at the ER.
122

Table 59. Annual mean and ±2s range of atmospheric density as a function 

of altitude at the ER.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (g/m3) (lb/ft3) (g/m3) (lb/ft3) (g/m3) (lb/ft3)

– – 1,197 7.473 × 10–2 1,136.88 7.098 × 10–2 1,257.12 7.848 × 10–2

0.003 9.8 1,198 7.479 × 10–2 1,136.94 7.098 × 10–2 1,259.06 7.86 × 10–2

1 3.3 × 103 1,086 6.78 × 10–2 1,047.04 6.537 × 10–2 1,124.96 7.023 × 10–2

2 6.6 × 103 982.7 6.135 × 10–2 957.82 5.98 × 10–2 1,007.58 6.29 × 10–2

3 9.8 × 103 886.8 5.536 × 10–2 869.85 5.43 × 10–2 903.75 5.642 × 10–2

4 13.1 × 103 800.6 4.998 × 10–2 787.178 4.914 × 10–2 814.022 5.082 × 10–2

5 16.4 × 103 722.1 4.508 × 10–2 710.38 4.435 × 10–2 733.82 4.581 × 10–2

6 19.7 × 103 650.6 4.062 × 10–2 640.242 3.997 × 10–2 660.958 4.126 × 10–2

7 23 × 103 585.1 3.653 × 10–2 575.902 3.595 × 10–2 594.298 3.71 × 10–2

8 26.2 × 103 525.4 3.28 × 10–2 516.65 3.225 × 10–2 534.15 3.335 × 10–2

9 29.5 × 103 471.1 2.941 × 10–2 462.876 2.89 × 10–2 479.324 2.992 × 10–2

10 32.8 × 103 421.5 2.631 × 10–2 413.424 2.581 × 10–2 429.576 2.682 × 10–2

12 39.4 × 103 332.4 2.075 × 10–2 321.168 2.005 × 10–2 343.632 2.145 × 10–2

14 45.9 × 103 253.6 1.583 × 10–2 239.13 1.493 × 10–2 268.07 1.674 × 10–2

16 52.5 × 103 187.3 1.169 × 10–2 177.526 1.108 × 10–2 197.074 1.23 × 10–2

18 59.1 × 103 133.8 8.353 × 10–3 127.928 7.987 × 10–3 139.672 8.72 × 10–3

20 65.6 × 103 93.72 5.851 × 10–3 90.212 5.632 × 10–3 97.228 6.07 × 10–3

22 72.2 × 103 66.52 4.153 × 10–3 63.916 3.99 × 10–3 69.124 4.315 × 10–3

24 78.7 × 103 47.84 2.987 × 10–3 45.802 2.859 × 10–3 49.878 3.114 × 10–3

26 85.3 × 103 34.69 2.166 × 10–3 33.084 2.065 × 10–3 36.296 2.266 × 10–3

28 91.9 × 103 25.31 1.58 × 10–3 23.996 1.498 × 10–3 26.624 1.662 × 10–3

30 98.4 × 103 18.55 1.158 × 10–3 17.485 1.092 × 10–3 19.615 1.225 × 10–3

34 111.5 × 103 10.03 6.262 × 10–4 9.268 5.786 × 10–4 10.792 6.738 × 10–4

38 124.7 × 103 5.534 3.455 × 10–4 5.058 3.158 × 10–4 6.01 3.752 × 10–4

42 137.8 × 103 3.118 1.947 × 10–4 2.82 1.761 × 10–4 3.416 2.133 × 10–4

46 150.9 × 103 1.823 1.138 × 10–4 1.652 1.032 × 10–4 1.994 1.245 × 10–4

50 164 × 103 1.106 6.905 × 10–5 1.005 6.275 × 10–5 1.207 7.534 × 10–5

54 177.2 × 103 0.678 4.233 × 10–5 0.612 3.821 × 10–5 0.744 4.645 × 10–5

58 190.3 × 103 0.414 2.585 × 10–5 0.369 2.301 × 10–5 0.459 2.868 × 10–5

62 203.4 × 103 0.25 1.561 × 10–5 0.22 1.376 × 10–5 0.28 1.746 × 10–5

66 216.5 × 103 0.148 9.24 × 10–6 0.127 7.941 × 10–6 0.169 1.054 × 10–5

70 229.7 × 103 0.087 5.431 × 10–6 0.07 4.358 × 10–6 0.104 6.505 × 10–6
123

3.6.2 Edwards Air Force Base

Density data used in figure 52 and table 60 were obtained from EAFB RRA.81

70

60

50

40

30

20

10

0

0 200 400 600 800 1,000 1,200 1,400

Density (g/m3)

Altitude (km)

newF52

Figure 52. Annual mean atmospheric density as a function of altitude at EAFB.
124

Table 60. Annual mean and ±2s range of atmospheric density as a function 

of altitude at EAFB.

Geometric Altitude Mean –2σ 2σ

(km) (ft) (g/m3) (lb/ft3) (g/m3) (lb/ft3) (g/m3) (lb/ft3)

– – 1,232 7.691 × 10–2 1,126.46 7.032 × 10–2 1,337.54 8.35 × 10–2

0.705 2.3 × 103 1,136 7.092 × 10–2 1,059.46 6.614 × 10–2 1,212.54 7.57 × 10–2

1 3.3 × 103 1,090 6.805 × 10–2 1,026.44 6.408 × 10–2 1,153.56 7.202 × 10–2

2 6.6 × 103 983 6.137 × 10–2 932.66 5.823 × 10–2 1,033.34 6.451 × 10–2

3 9.8 × 103 889.3 5.552 × 10–2 853.4 5.328 × 10–2 925.2 5.776 × 10–2

4 13.1 × 103 803.8 5.018 × 10–2 778.36 4.859 × 10–2 829.24 5.177 × 10–2

5 16.4 × 103 726 4.532 × 10–2 707.002 4.414 × 10–2 744.998 4.651 × 10–2

6 19.7 × 103 654.8 4.088 × 10–2 640.412 3.998 × 10–2 669.188 4.178 × 10–2

7 23 × 103 589 3.677 × 10–2 577.306 3.604 × 10–2 600.694 3.75 × 10–2

8 26.2 × 103 528.8 3.301 × 10–2 518.206 3.235 × 10–2 539.394 3.367 × 10–2

9 29.5 × 103 473.2 2.954 × 10–2 461.79 2.883 × 10–2 484.61 3.025 × 10–2

10 32.8 × 103 421.3 2.63 × 10–2 406.822 2.54 × 10–2 435.778 2.721 × 10–2

12 39.4 × 103 325.8 2.034 × 10–2 303.88 1.897 × 10–2 347.72 2.171 × 10–2

14 45.9 × 103 243.3 1.519 × 10–2 221 1.38 × 10–2 265.6 1.658 × 10–2

16 52.5 × 103 179.4 1.12 × 10–2 162.918 1.017 × 10–2 195.882 1.223 × 10–2

18 59.1 × 103 129.2 8.066 × 10–3 120.014 7.492 × 10–3 138.386 8.639 × 10–3

20 65.6 × 103 92.21 5.757 × 10–3 87.35 5.453 × 10–3 97.07 6.06 × 10–3

22 72.2 × 103 66.23 4.135 × 10–3 63.01 3.934 × 10–3 69.45 4.336 × 10–3

24 78.7 × 103 47.76 2.982 × 10–3 45.324 2.83 × 10–3 50.196 3.134 × 10–3

26 85.3 × 103 34.71 2.167 × 10–3 32.788 2.047 × 10–3 36.632 2.287 × 10–3

28 91.9 × 103 25.36 1.583 × 10–3 23.786 1.485 × 10–3 26.934 1.682 × 10–3

30 98.4 × 103 18.6 1.161 × 10–3 17.283 1.079 × 10–3 19.917 1.243 × 10–3

34 111.5 × 103 10.01 6.249 × 10–4 9.109 5.686 × 10–4 10.911 6.812 × 10–4

38 124.7 × 103 5.519 3.446 × 10–4 4.935 3.081 × 10–4 6.103 3.81 × 10–4

42 137.8 × 103 3.101 1.936 × 10–4 2.75 1.717 × 10–4 3.452 2.155 × 10–4

46 150.9 × 103 1.806 1.127 × 10–4 1.576 9.84 × 10–4 2.036 1.271 × 10–4

50 164 × 103 1.089 6.799 × 10–5 0.944 5.895 × 10–5 1.234 7.703 × 10–5

54 177.2 × 103 0.665 4.152 × 10–5 0.567 3.542 × 10–5 0.763 4.761 × 10–5

58 190.3 × 103 0.405 2.528 × 10–5 0.339 2.119 × 10–5 0.471 2.938 × 10–5

62 203.4 × 103 0.244 1.523 × 10–5 0.203 1.269 × 10–5 0.285 1.778 × 10–5

66 216.5 × 103 0.146 9.115 × 10–6 0.12 7.504 × 10–6 0.172 1.073 × 10–5

70 229.7 × 103 0.086 5.369 × 10–6 0.069 4.295 × 10–6 0.103 6.443 × 10–6
125

3.7 Humidity

In general, the vapor concentration decreases with altitude in the troposphere because 

of the decrease of temperature with altitude. Small amounts of moisture exist in the stratosphere 

and mesosphere. Figure 53 presents a reference model for the mean and variability of middle 

atmospheric water vapor.1 It represents mean, Northern Hemisphere, mid-latitude, springtime, 

and number concentration (parts per million by volume (ppmv)) along with its variability (bars) 

and accuracies (brackets).

The figures and tables that follow in this section were derived from the 1983 RRA for the 

particular site. Only the annual means are shown here. However, the designers are not constrained 

to using the annual means shown in this TM. Additional (monthly) data are available from the 

MSFC Natural Environments Branch and from the RRA documentation for each site.

0 2 4 6 8 10

80

65.5

48.1

31.2

16.2

0.1

0.01

0.1

1

10

100

1,000

(17%)

(17%)

(17%)

(20%)

(25%)

(25%)

Parts per Million by Volume

Altitude (km)

Pressure (hPa)

newF 53

Figure 53. Reference profile of middle atmosphere number concentration mean, 

variability, and accuracy; representative of Northern Hemisphere, 

mid-latitude, springtime conditions.
126

3.7.1 Eastern Range

Annual mean mixing ratio data for the ER used in figure 54 and table 61 were obtained 

from the Cape Canaveral RRA.79 The annual mean and ±2σ mixing ratio (w) data were derived 

using equation (47):

w =

Tv

T !1

0.61 , (47)

where the virtual temperature (Tv) and temperature (T) are the mean, ±2σ values for each 

calculation.

12

10

8

6

4

2

0

Altitude (km)

Mixing Ratio (g/kg)

0 2 4 6 8 10 12 14 16 18 20

Mean

2σ

–2σ

newF54

Figure 54. Annual mean and ±2σ range of mixing ratio versus altitude about the annual 

mean for the ER.
127

Table 61. Annual mean and ±2s range of atmospheric mixing ratio versus altitude

at the ER.

Geometric Altitude Mean Mixing Ratio

(g/kg)

–2σ Mixing Ratio

(g/kg)

2σ Mixing Ratio 

(g/kg) (km) (ft)

– – 12.94 5.36 19.91

3 × 10–3 9.8 12.67 4.96 19.76

1 3.3 × 103 9.3 3.75 14.5

2 6.6 × 102 6.34 2.84 9.66

3 9.8 × 103 4.57 2.4 6.65

4 13.1 × 103 3.53 1.78 5.2

5 16.4 × 103 2.87 1.32 4.36

6 19.7 × 103 2.38 1.16 3.54

7 23 × 103 1.99 0.86 3.06

8 26.2 × 103 1.72 0.68 2.69

9 29.5 × 103 1.43 0.35 2.43

10 32.8 × 102 0.84 < 0.001 2.45

11 36.1 × 102 0.07 < 0.001 0.21

12 39.4 × 103 <0.001 < 0.001 < 0.001
128

3.7.2 Edwards Air Force Base

Annual mean mixing ratio data for EAFB used in figure 55 and table 62 were obtained from 

EAFB RRA.81 The annual mean and ±2σ mixing ratio (w) data were derived using equation (47).

Mean

2σ

–2σ

12

10

8

6

4

2

0

0 1 2 3 4 5 6 7 8 9

Altitude (km)

Mixing Ratio (g/kg)

newF 55 Figure 55. Annual mean and ±2σ range of atmospheric mixing ratio versus altitude 

for EAFB.

Table 62. Annual mean and ±2s range of atmospheric mixing ratio versus altitude

at EAFB.

Geometric Altitude Mean Mixing Ratio

(g/kg)

–2σ Mixing Ratio 

(g/kg)

2σ Mixing Ratio

(km) (ft) (g/kg)

– – 4.64 0.8 7.95

0.71 2.3 × 103 4.48 1.59 7.03

1 3.3 × 103 4.16 2.11 5.99

2 6.6 × 102 3.07 1.53 4.44

3 9.8 × 103 2.36 0.87 3.71

4 13.1 × 103 1.81 0.64 2.88

5 16.4 × 103 1.36 0.26 2.36

6 19.7 × 103 1.08 0.2 1.88

7 23.0 × 103 0.85 0.07 1.56

8 26.2 × 103 0.74 0.07 1.35

9 29.5 × 103 0.56 <0.001 1.46

10 32.8 × 102 0.36 <0.001 1.16

11 36.1 × 102 <0.001 <0.001 <0.001

12 39.4 × 103 <0.001 <0.001 <0.001
129

3.8 Aloft Aerosols

Gases and particles in the atmosphere must be considered during vehicle development in 

order to avoid detrimental effects to the vehicle on the ground and in flight. Some of these effects 

include corrosion, abrasion, and optical hindrances. The intensity of damage depends on the 

source, location, and concentration of the particles. The particles, together with the air that 

suspends them, are termed aerosols.

3.8.1 Gaseous Constituents

Gaseous, as well as particulate matter, can cause detrimental effects on aerospace vehicles 

and ground equipment due to various chemical reactions/processes. Nitrogen (N2) and molecular 

oxygen (O2) make up approximately 99% by volume (98.6% by weight) of the lower atmosphere. 

These two atmospheric constituents, along with carbon dioxide (CO2), water vapor, and ozone (O3) 

are the gases of primary concern.

The variability of many atmospheric trace gases is quite large. However, table 63 provides 

the typical gas concentration values expected from the surface to 90 km (295.3×103 ft) altitude. 

Seasonal, diurnal, and location variations plus other changes can all add to the variability of the 

various atmospheric constituents. The mean values presented in table 63 are based on model information taken from references 82 and 83. The Earth-GRAM 2010 model, discussed in section 3.9, 

can be used to estimate atmospheric species concentrations.

3.8.2 Upper Atmospheric Aerosols

Atmospheric aerosols can exist at stratospheric levels (15 to 30 km, 49.2 to 98.4×103 ft) 

as well as in the troposphere.1 The stratospheric aerosols, consisting mainly of liquid sulfuric acid 

droplets, are divided into three categories: (1) Background aerosols, (2) volcanic aerosols, and 

(3) polar stratospheric cloud (PSC) particles.84 Table 64 presents the basic characteristics of 

stratospheric aerosols.83
130

Table 63. Average concentrations (standard atmosphere values) of various gaseous 

constituents from the Earth’s surface up to 90 km (295.3×103 ft) altitude.*

Constituent

Typical Concentration

Altitude (km) Altitude (ft) ppbv

N2 Zero and above Zero and above 7.81 × 108

O2 Zero and above Zero and above 2.09 × 108

Rare gases – – very small

O3 – – 27

7 23.0 × 103 50

22 72.2 × 103 3,650

40 131.2 × 103 7,300

75 246.1 × 103 250

90 295.3 × 103 700

N2O – – 320

9 29.5 × 103 320

32 104.9 × 103 117

49 160.8 × 103 5

90 295.3 × 103 0.5

NO** – – 0.3

12 39.3 × 103 0.3

18 59.1 × 103 0.2

40 131.2 × 103 11

70 229.6 × 103 11

90 295.3 × 103 213

NO2 – – 0.02

10 32.8 × 103 0.02

18 59.1 × 103 0.8

35 114.8 × 103 7.3

50 164 × 103 0.4

90 295.3 × 103 0.2

H2S** – – 0.1

2 6.6 0.03

10 32.8 × 103 0.01

26 and above 85.3 × 103 and above 10–15

CS2** – – 0.07

14 45.9 × 103 0.03

32 and above 104.9 × 103 and above 10–15

 * This table gives average values such that a constituent value at altitude can be obtained 

by linear interpolation between the listed altitude/concentration values. See references 80 

and 81 for more exact curves.

** These gases have a very large latitudinal and longitudinal gradient, due to short lifetimes, 

causing a large range of local concentrations with altitude to exist.
131

Table 63. Average concentrations (standard atmosphere values) of various gaseous 

constituents from the Earth’s surface up to 90 km (295.3×103 ft) altitude

(Continued).*

Constituent

Typical Concentration

Altitude (km) Altitude (ft) ppbv

COS – – <0.05

NH3** – – 0.5

12 and above 39.4 × 103 and above <0.01

H2 0 to 90 0 to 295.3 × 103 560

CH4 – – 1,700

10 32.8 × 103 1,700

40 131.2 × 103 564

50 164 × 103 210

90 295.3 × 103 140

SO2 ** – – 0.3

30 98.4 × 103 0.01

70 229.7 × 103 0.04

90 295.3 × 103 0.002

CO – – 150

10 32.8 × 103 100

21 68.9 × 103 12

50 164 × 103 46

90 295.3 × 103 5,840

CO2 – – 3.3 × 105

75 246.1 × 103 3.3 × 105

90 295.3 × 103 3.1 × 105

HNO3 (vapor)** – – 0.05

15 49.2 × 103 0.45

22 72.1 × 103 5.5

50 164.0 × 103 0.06

90 295.3 × 103 0.03

H2SO4 (vapor) 0 to 90 0 to 295.3 × 103 Small except in localized 

areas

* This table gives average values such that a constituent value at altitude can be obtained by linear

interpolation between the listed altitude/concentration values. See references 80 and 81 for more

exact curves.

** These gases have a very large latitudinal and longitudinal gradient, due to short lifetimes,

causing a large range of local concentrations with altitude to exist.
132

Table 64. Characteristics of stratospheric aerosols.

Particle Type Sulfate Aerosol Type I PSC Type II PSC Meteoric Dust Rocket Exhaust

Physical state Liquid or slurry with 

crystals

Solid nitric acid 

trihydrate, solid 

solutions

Solid crystal, 

hexagonal, or cubic 

basis

Solid granular 

irregular or spherical

Solid spheres or 

irregular surface 

ablated debris

Particle radius 

(mm, 10–6 m)

0.01–0.5, ambient 

0.01–10, volcanic

0.3–3 1–100 1–100, micrometeorites 

0.01–0.1, smoke

0.1–10

Number

(No. cm–3)

~ 1–10 ~ 0.1–10 <<1 10–6, 100 mm 10–3, 

1 mm

10–4, 10 mm 

10–2, 1 mm

Principal composition H2SO4/H2O

~ 70%/30%

HNO3/H2O

~ 50%/50%

H2O SiO2, Fe, Ni, Mg; C Al2O3

Trace composition NH4

+, NO3 HCl, SO42– HNO3, HCl SO42– (surface) Cl–, SO42– (surface)

Physical 

characteristics

Dust inclusions, 

in solution

Equidimensional 

crystalline or droplets

Elongated crystals 

with polycrystalline 

structure

Irregular mineral 

grains, grain defects

Homogeneous 

composition; smooth 

spheres

Distribution Global, ambient 

Regional, volcanic 

12–35 km alt.

Polar winter

14–24 km altitude 

Polar winter 

14–24 km altitude

~ Global

>12 km altitude

Global >12 km 

altitude

Residence time ~ 1–2 yr ambient 

~ 1–3 yr volcanic

~ 1 day to weeks ~ Hours <1 mo (micrometeorites) 

1–10 yr (meteoritic 

smoke)

<1 yr

3.9 Earth-Global Reference Atmospheric Model 2010

In the development of aerospace vehicles, standard or reference atmospheric models have 

been used to study the vehicle response to atmospheric variations in space and time. MSFC developed a GRAM, Version 2010 (Earth-GRAM 2010), that incorporates empirically-based models to 

provide complete global geographic variability, altitude coverage from the surface to orbital altitudes, and monthly and seasonal variability of wind components and thermodynamic variables. In 

addition to providing output of the monthly and seasonal mean atmospheric state, Earth-GRAM 

2010 can also simulate spatial and temporal perturbations of these atmospheric parameters. The 

models that constitute Earth-GRAM 2010 are the National Centers for Environmental Prediction 

(NCEP) Database, the Middle Atmosphere Program (MAP), and a choice between three thermosphere models:

(1) Marshall Engineering Thermosphere (MET).

(2) Naval Research Labs Mass Spectrometer, Incoherent Scatter Radar Extended Model 

thermosphere (NRL MSIS-E-00) with the associated Harmonic Wind Model (HWM-93). 

(3) Jacchia-Bowman 2008 thermosphere model (JB2008).

These are combined together for the purposes of providing a complete atmospheric profile. Figure 56 provides a graphical summary of the data sources and height regions.71 Additional information about each component and other user options are discussed in the following section.
133

MET2007, MSIS, or JB2008 Thermosphere Model Satellite Data

Fairing Between MAP and Thermosphere Model

Fairing Between MAP and NCEP Data

NCEP

Troposphere

Stratosphere

Mesosphere

Thermosphere

MAP Databases

Rocket

and Remote

Sensing Data

Balloon, Aircraft,

and Satellite Remote

Sensing Data

120

110

100

90

80

70

60

50

40

30

20

10

0

Altitude (km)

Figure 56. Graphical summary of the atmospheric regions in Earth-GRAM 2010 newF 56

and sources for the model and the data from which the mean monthly 

values are based.

3.9.1 National Centers for Environmental Prediction Database (Surface to 10 mb (0.15 lb/in2))

For the lower atmosphere, Earth-GRAM 2010 uses climatological data derived from 

an NCEP global reanalysis database. The NCEP data consist of means and standard deviations 

(at a global latitude-longitude resolution of 2.5°×2.5°) at four specific times of day (00, 06, 12, 

and 18 UTC) and for all four times of day combined at the surface and at each of 17 pressure 

levels. Averages and standard deviations are by month over a POR from 1990 through 2008. 

As part of validation studies conducted for Earth-GRAM 2010, NCEP hourly and daily averages 

of surface winds and temperatures were compared with statistics from more directly observed surface winds and temperatures. During these studies, it was found that NCEP average surface winds 

and temperatures did not have nearly as much variation with hour of day as did observed surface 

winds and temperatures. Consequently, a more detailed study of surface and near-surface NCEP 

data and observed winds and temperatures were undertaken. For more information on this study 

and the NCEP database, see Leslie and Justus.71

3.9.2 Middle Atmosphere Program (20 to 120 km (65.6×103 to 393.7×103 ft))

The MAP component of Earth-GRAM 2010 characterizes the monthly mean middle 

atmosphere from two data sets, the zonal mean and mean stationary wave patterns.71 The zonal 

monthly mean data set is merged from six separate data sets over an altitude range of 20 to 120 km 

(65.6×103 to 393.7×103 ft). This data set contains pressure, density, temperature, and mean easterly wind component data. The data are gridded in 10° latitude (0° to 80°) and 5 km (16.4×103 ft) 

height increments. For zonal mean values at ±90° latitude, data set values are computed by an 

across-the-pole interpolation scheme.71 The stationary perturbation data set contains pressure, density, temperature, and eastward and northward wind components gridded in the same latitude and 
134

height increments as the zonal mean data and 20° longitude increments. This data set was merged 

from three sources of data on planetary-scale standing wave patterns. At the poles, the stationary 

wave perturbations are identically zero. At altitudes where irregularly spaced data overlap between 

the lowest altitude MAP value at 20 km (65.6×103 ft) and the highest altitude NCEP data at 10 mb 

(0.15 lb/in2), fairing techniques are used to smooth the transition.

3.9.3 Marshall Engineering Thermosphere (Above 90 km (295.3×103 ft))

The MET (Jacchia) model calculates density, temperature, and pressure at altitudes above 

90 km (295.3×103 ft). The MET model accounts for temperature and density variations due to 

solar and geomagnetic activity, diurnal, seasonal, and latitude-longitude variations at altitudes 

above 90 km (295.3×103 ft).71 Geostrophic wind components are also evaluated by estimating 

horizontal pressure gradients. Fairing techniques are employed to ensure a smooth transition in the 

region of overlap, 90 to 120 km (295.3×103 to 393.7×103 ft), between the MAP and MET models.

3.9.4 Range Reference Atmosphere (0 to 70 km (0 to 229.7×103 ft))

Earth-GRAM 2010 can also incorporate RRA data as an alternative to Earth-GRAM 

climatology near RRA sites. Earth-GRAM 2010 uses a fraction of data contained within the 

RRA data set. The RRA data utilized in Earth-GRAM consists of an annual average data set and 

12 monthly data sets for the following: height; mean and standard deviation E-W wind; mean and 

standard deviation N-S wind; mean and standard deviation pressure; mean and standard deviation 

temperature; mean and standard deviation density; and mean and standard deviation dewpoint 

temperature. 

A practical application of using RRA data in Earth-GRAM 2010 is in vehicle trajectory 

simulations. For example, the vehicle can be launched from one RRA site (i.e., CCAFS), smoothly 

transition to an atmosphere characterized by the Earth-GRAM climatology, then smoothly transition into an atmosphere characterized by a different RRA site for landing (i.e., EAFB). Smooth 

transitions of an atmosphere characterized by an RRA site to Earth-GRAM climatology is based 

on user-selected inputs defining the size of the latitude-longitude region near any selected RRA 

site. Any latitude-longitude within a radius of 0.5° of any RRA site will use the RRA site data. If 

the location lies beyond a radius of 2.5°, the Earth-GRAM climatology is used. Between a radius 

of 0.5° and 2.5°, a weighted average of RRA data and Earth-GRAM data are used with the RRA 

data weighting factor smoothly changing from 1 at a radius of 0.5° to zero at a radius of 2.5°. The 

user can modify those values. If more than one RRA is located within a selected radius of influence, data from the nearest (highest weighted) site is used. Fairing techniques are used to transition 

from the RRA data to Earth-GRAM 2010 data near the top of the RRA site. For more information on how Earth-GRAM uses RRA data, refer to Leslie and Justus.71

3.9.5 Atmospheric Species Concentration Data

Earth-GRAM 2010 provides estimates of atmospheric species concentrations for water 

vapor (H2O), O3, nitrous oxide (N2O), carbon monoxide (CO), methane (CH4), CO2, N2, O2,
135

atomic oxygen (AO), argon (Ar), helium (He), and hydrogen (H). Air Force Geophysics Laboratory83 atmospheric constituent profiles are also used extensively for the constituents up to 120 km 

(393.7×103 ft) altitude. The MET (Jacchia) model provides the species concentrations for N2, O2, 

AO, Ar, He, and H above 90 km (295.3×103 ft). For more information on the atmospheric species 

concentration data in Earth-GRAM, see Leslie and Justus.71

3.9.6 Available Special Output

Earth-GRAM 2010 has the ability to calculate derived quantities at each grid point in addition to the standard output. Some examples of derived quantities are the speed of sound, pressure 

scale height, and mean free path. Refer to Leslie and Justus71 for more information on derived 

quantities.

3.10 Stratospheric and Mesospheric Clouds

Due to a presence of high-altitude cloud systems/particles above the troposphere, designers 

and planners need to account for these factors when vehicle reentry, launch, or horizontal flight 

above 12 km (39.4×103 ft) altitude is desired. Stratospheric clouds typically develop in the lower 

altitudes of the stratosphere, 15 to 30 km (49.2×103 to 98.4×103 ft), while mesospheric clouds 

develop in the upper altitudes of the mesosphere, 80 to 85 km (262.5×103 to 278.9×103 ft). General characteristics will be presented in the following section.

PSCs are frozen aerosol particles observed in local winter over both polar regions whenever 

the ambient temperature falls below –78 °C (–108.4 °F). The clouds are layered with the maximum 

amount near 20 km (65.6×103 ft), close to the region of minimum stratospheric temperature. The 

layers are thin; <1 to 2 km (<3.3×103 to 6.6×103 ft) thick (thicker in the Antarctic). The PSC base 

descends over the local winter polar region, falling to an altitude of approximately 15 km 

(49.2×103 ft) at the end of winter. Antarctic PSCs generally occur at lower altitudes, <17 km 

(< 55.8×103 ft) than Arctic PSCs (17 to 25 km (55.8×103 to 82 ×103 ft)), due to the strong gradient across the polar vortex that develops over the nighttime Antarctic, which lasts until the springtime breakup. The clouds are much more prevalent in the Antarctic with their colder, stratospheric 

temperatures, by 3.5 °C (6.3 °F), than they are in the Arctic. Although different kinds of PSCs 

exist, which may have different compositions, they exist as highly supercooled/supersaturated liquid 

drops.1

Nacreous clouds (NACs), thin stratospheric clouds appearing brilliantly colored and stationary (lenticular), occur in wintertime over high latitudes in both hemispheres, i.e., Scandinavia, 

Alaska, and Antarctica, when the Sun is below the horizon. There are over 155 dates in which 

northern hemispheric sightings (for undisturbed stratosphere only, no aircraft contrails included) 

of NACs have been observed over 100 years during winter (December to February). Somewhat 

more frequent are NACs over the Antarctic winter (June to September) where over 140 sightings 

in 100 years have occurred in these sparse reporting areas.1

NACs have been sighted between 17 to 31 km (55.8×103 to 101.7×103 ft) altitude (average 

23 km (75.5×103 ft)), and set up preferentially downwind of mountain ranges. This indicates 
136

orographic origin with lee waves producing up to 40 km (131.2×103 ft) wavelengths present in 

the NAC bands. NACs are a special subset of PSCs, but it is not yet clear that the two-cloud phenomena are the same. NACs are composed of micrometer-sized water ice particles (crystals) with 

sizes of the order of 1 to 2 μm in radius, and lifetimes are >10 min at 20 km (65.6×103 ft) (1 ppm 

of water is equivalent to 5 particles/cm3 (82 particles/in3) of size 1.5 μm). An approximate maximum radius of approximately 4 μm at 20 km (65.6×103 ft) altitude may be determined, assuming 

3 ppm of water condensing to form 1 particle/cm3 (16 particles/in3). It is generally believed that 

NACs form by deposition of H2O on preexisting stratospheric aerosol particles (sulfate), when 

stratospheric temperatures are typically at or below –85 °C (–121 °F). Therefore, the number concentration of NAC particles should be equal to that of the stratospheric aerosols, approximately 5 

to 20 cm3 at 20 km (82 to 328.0/in3 at 65.6×103 ft).1

Noctilucent clouds (NLCs) (also referred to as polar mesospheric clouds) were once thought 

to be very rare, especially in the Southern Hemisphere; however, observations from space have 

shown that they occur almost continuously during some periods of time.1 In both hemispheres, 

their coverage can be quite extensive. NLCs are composed of submicron-sized water ice particles 

growing in supersaturated air and occurring in a few kilometers thick layer, only during summer, 

over higher latitudes, (poleward of 45 °N. and 50 °S.) at cold (< –133 °C (< –207.4 °F)) mesopause 

altitudes (85 km (278.9×103 ft)). These clouds have been observed only from the ground over the 

past 100 years, at twilight (morning or evening), when the Sun is between 6° and 16° below the 

horizon, so that the 80 to 85 km (262.5×103 to 278.9×103 ft) level is still in sunlight. Whether NLC 

and polar mesospheric clouds both represent the same phenomenon currently remains an open 

question.

3.11 Aloft (150 m to 90 km) Ionizing Radiation Environments and Effects

Once the space vehicle is aloft, it will continue to be exposed to the terrestrial ionizing 

radiation environments that were presented in section 2.12.

3.11.1 Aloft Ionizing Radiation Environments

Section 2.12.1 described the processes involved in understanding the ionizing radiation 

environments that will be encountered in the terrestrial segments. Variation from the ground 

environments to the aloft environments will be discussed next.

3.11.2 Galactic Cosmic Ray Flux in the Atmosphere

Unlike the discussion in section 2.12 where the ionizing radiation environments from GCR 

and SPE were treated simply as a source for the terrestrial neutron production, in the aloft segment, these environments will directly impact the electronic systems. So an understanding of the 

nature of these environments is needed.

Both GCR and SPE constituents are similar conceptually in that they both contain protons 

and heavier ions produced outside the Earth’s geomagnetic field and must traverse the magnetic 

field and the upper atmosphere to impact the aloft segment. The other similarity is that both consist of a distribution of particle species, each with their own energy spectra. However, that is where
137

the similarity ends. With the source of the GCR postulated to be external to the solar system, the 

quantity of the particles is much lower but they have had sufficient acceleration encounters so they 

are at much higher energies than their Sun-produced equivalents. An SPE can constitute a significant event on the Sun and thereby throw a substantial amount of particle in Earth’s direction. This 

quantity can be orders of magnitude larger than during the nominal times of no SPE.

For atmospheric neutron environments, it is a much simpler process of presenting the environment. Once a maximum latitude is chosen and the minimum neutron energy required to produce the effect is determined (generally >10 MeV), then a simple plot of neutron flux as a function 

of altitude can be given (see fig. 57). For GCR and SPE environments, it is not this simple. 

12

10

8

6

4

2

0

0 5 10 15 20

Altitude (km)

Neutron Flux (No. / cm2–s)

newF 57

Figure 57. Plot of the atmospheric neutron flux as a function of altitude 

for >10 MeV neutrons.

These environments consist of protons that can provide a similar plot (see fig. 58) since 

they produce effects through spallation events as do the neutrons. The only difference for protons 

is that an altitude is selected and the proton energy spectrum is presented. However, these environments also contain heavier ions that produce effects through direct ionization that is a function of 

the ion’s atomic number, its energy, and the material it is interacting with. As a result, a different 

parameter is needed to deal with these issues. To deal with the ion species, energy, and target material, a parameter called the linear energy transfer (LET) is used. The LET is how much energy an 

ion deposits as a function of depth in the given target material. Since most electronics used currently are made from silicon, LET is typically presented for silicon. The calculation of this parameter is based on how ion stop in material; details of this can be found in reference 85. A typical 

example of this environment is shown in figure 59.
138

101

10

10–1

10–2

10–3

10–4

10–5

10–6

103 104

Energy (MeV)

Integral Proton Flux (No. / cm2–s)

105

SPE Protons

GCR Protons

newF58

Figure 58. Plot of the proton flux as a function of energy for an altitude >200 km.

SPE Protons

GCR Protons

10

Linear Energy Transfer (MeV–cm2/mg)

10–2

10–3

10–4

10–5

10–6

10–7

10–8

10–9

10–10

10–11

10–12

10–13

10–14

Integral Proton Flux (No. / cm2–s)

1 100

newF59 Figure 59. Plot of the heavy ion flux as a function of LET for an altitude >200 km.
139

3.11.3 Aloft Ionizing Radiation Effects

The effects of ionizing radiation on electronic systems can be divided into two categories: 

cumulative effects and transient effects. Cumulative effects are those that require exposure to the 

environments over time to produce an effect. Transient effects are those that can be induced at 

any time during a mission and can require nothing more than a single particle passing through an 

electronic device to produce the effect (that can either be temporary or permanent). As with the 

ground-based segment, the exposure time in the aloft segment is short and the ionizing radiation 

levels are small as compared to those in space, the cumulative effects for the terrestrial segments 

of a mission are negligible and will not be discussed here.

The only transient effect that is applicable to space systems is a SEE. SEEs are effects in 

microelectronics that are induced by the passage of a single particle through the part. Discussion 

of the basic concepts of SEE is presented in section 2.12.3. 

3.11.4 Aloft-Based Effects

In addition to the atmospheric neutrons producing SEEs, the aloft segment will also have 

to deal with high-energy protons and heavy ions (from GCR and SPE, as discussed above). Similar 

to the atmospheric neutrons, the protons must rely on spallation events to produce SEE. However, 

the direct interaction of the heavy ions depositing energy as they traverse sensitive regions of the 

electronic device will give every ion the possibility of producing an SEE. Therefore, it should be 

expected that the aloft segment will be dominated by the heavy ion-induced SEE. This will indeed 

be true for any electronics that will be expected to operate in the upper regions of the aloft segment. If, on the other hand, required operation is complete by 20 km in altitude, then, as with the 

ground effects, only the atmospheric neutrons will play a role (albeit at a much higher rate than on 

the ground).

It should be understood that the rate of SEEs will be an additive function of all the environments that a system will be exposed to. As discussed above, there will be regions of the aloft 

environment that will be dominated by one environment over the other so that its event rate only 

need be considered. The caveat here is that the required region of operation may encompass many 

or all of these regions, implying that all SEE-producing environments must be part of the analysis. 

So in addition to the neutron-based analysis reference given in section 2.12.4, references 86 and 87 

should be consulted for an understanding of proton-induced SEE and references 88–90 for heavyion-induced SEE. 

3.12 Lightning Aloft

Lightning, either triggered or natural, may occur in the vertical domain from ground level 

up to a typical maximum altitude of around 15 km (49.2×103 ft). The highest probability events 

occur between ground level and approximately 7 km (23×103 ft). Refer to section 2.8 for detailed 

information on lightning flash densities and considerations for external and internal lightning 

environment.
140

4. NEAR EARTH SPACE ENVIRONMENTS

The natural space environment refers to the environment as it occurs independent of the 

presence of a spacecraft; thus, it includes both naturally occurring phenomena (such as AO and 

atmospheric density, ionizing radiation, plasma, etc.), and a few man-made factors (such as orbital 

debris). The space environment covers all the environments ranging from approximately 90 km 

up to 10 Re.
141

5. LOW EARTH ORBIT SPACE ENVIRONMENTS 

(90 to 2,000 km)

5.1 Thermosphere

The region of the Earth’s atmosphere lying between approximately 90 and 500 km is known 

as the thermosphere. The neutral gases in the thermospheric region are in a diffusive equilibrium 

and are typically referred to as the D (50 to 90 km), E (90 to 160 km), and F (160 to 500+km) 

layers. At an altitude over 100 km, plasmas (charged particles) are generated due to photoionization of the ambient neutral atmosphere of the solar photons and by magnetospheric particles 

interacting with the thermosphere. Details of the plasma environment and its characteristics are 

described in section 5.4.

The neutral atmosphere in this region is most conveniently described in terms of a mean, 

with spatial and temporal variations about that mean. For space vehicle operations, the neutral 

atmosphere is significant because (1) even at its low density, it produces torques and drags on the 

vehicle, (2) the density height profile of the atmosphere above 100 km altitude modulates the flux 

of trapped radiation encountered, as explained in the ionizing radiation section, and the orbital 

debris, as explained in their respective sections, and (3) the AO may erode and chemically change 

those surfaces which are exposed to it.

5.1.1 Thermospheric Process

The temperature in the lower thermosphere increases rapidly with increasing altitude from 

a minimum at 90 km. Eventually it becomes altitude independent at upper thermospheric altitudes. 

This asymptotic temperature, known as the exospheric temperature, is constant with altitude due 

to the extremely short thermal conduction time. The thermospheric gases are heated by the absorption of the solar extreme ultraviolet (EUV) radiation. At the lowest thermospheric altitudes, the 

absorption of ultraviolet (UV) radiation is also important. The EUV and UV radiation initially 

heats only the dayside thermosphere, and although conductive and convective processes act to 

redistribute some of this energy, a large temperature gradient always exists between the daytime 

and the nighttime thermosphere. An average daytime exospheric temperature is 1,060 K, and an 

average nighttime exospheric temperature is 840 K. The longitudinal temperature gradient causes 

a wind to flow from the dayside to the nightside thermosphere, with speeds typically reaching 

100 m/s.

An additional heat source for the thermosphere is the interaction of the Earth’s magnetic 

field at distances of several Earth radii, in the region known as the magnetopause, with the solar 

wind. The solar wind is a stream of high-speed plasma emanating from the Sun. This interaction 

causes energetic particles to penetrate down into the lower thermosphere at high geographic latitudes and directly heat the thermospheric gas. These energetic particles are also responsible for 
142

the aurora seen at these high latitudes. In addition, electric fields mapped down from the magnetosphere onto the high latitude ionosphere cause electric currents to flow. The term ‘ionosphere’ 

refers to the small fraction of the thermosphere that remains ionized due to solar radiation. It 

never totally disappears at night, and during daylight hours the ionization density never exceeds 

more than 1% of the neutral density. These electrical currents lose energy through ohmic or joule 

dissipation and heat the neutral thermospheric gas. The ions also collide directly with the neutral 

gas, setting the whole gas into motion. At these high latitudes, the wind speeds generated by this 

process can be very large, at times as large as 1.5 km/s. Eventually viscous effects dissipate these 

winds, and their lost kinetic energy provides an additional heat source for the neutral thermospheric gas.

The high-latitude heat sources are effective during both the day and night. Although an 

intermittent source of energy for the thermosphere, they can at times exceed the global EUV 

energy absorbed by the thermosphere. In addition, although the energy is deposited at high latitudes (>60° or so), the disturbance effects are transmitted to lower latitudes through the actions 

of winds and waves. However, the disturbance effects at low latitudes are significantly smaller than 

they are at higher latitudes. The high latitude ionospheric currents that flow perturb the geomagnetic field, so that such disturbances, which can be detected by ground-based magnetometers, are 

referred to as geomagnetic storms. 

Whenever the neutral thermospheric gas is heated, it expands radially outward. Because the 

undisturbed thermospheric density decreases with increasing altitude, an outward expansion of the 

gas results in an increase of density at high altitudes. Thus, the daytime thermospheric density is 

greater than the nighttime density, while during times of geomagnetic storms, the high latitude density is greater than it is during undisturbed periods. This anisotropic heating leads to the so-called 

diurnal and polar bulges, which were first inferred from the increased drag experienced by orbiting 

satellites.

Below the turbopause (located at approximately 105 km altitude), the atmosphere is well 

mixed by turbulence, so that the composition of the atmosphere does not vary with altitude. Above 

the turbopause, however, diffusion becomes so rapid that the altitude variation of the various species becomes dependent on molecular mass, with the result that composition varies with altitude. 

Thus, the number densities of the heavier thermospheric species (N2 and O2) decrease with increasing altitude much faster than those of the lighter species (H and He). This means that the heavier 

molecular species dominate in the lower thermosphere, while the lighter atomic species dominate 

in the upper thermosphere. A typical altitude profile for the individual thermospheric constituents 

is shown in figure 60. Lifting of the thermosphere will cause the mean molecular weight at a given 

altitude to increase, while a sinking motion will cause it to decrease.
143

1,000

900

800

700

600

500

400

300

200

100

0

1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014

Number Density (cm–3)

OMin

OMax

HMax

HMin

HeMin

ArMax

ArMin

Altitude (km)

newF60

O2Max

N2Min

N2Max

HeMax

O2Min

Figure 60. Number density of atmospheric constituents versus altitude.

5.1.2 Variations

In addition to the diurnal variation in the neutral mass density at orbital altitudes, there is 

a semi-annual variation, a seasonal latitudinal variation in the lower thermosphere, and a seasonal 

latitudinal variation in the He number density, as well as nonperiodic perturbations associated with 

variations in solar activity, atmospheric waves, and thermospheric winds. All of these variations are 

discussed in the following sections.

5.1.2.1 Variations in Solar Activity. While the total amount of solar electromagnetic radiation is nearly constant, the short wavelength portion of the spectrum (EUV and UV) changes 

substantially with the overall level of solar activity, with the result that the thermospheric density, especially at orbital altitudes, is strongly dependent on the level of solar activity. Thus, there 

is a (roughly 11 year) variation in the thermospheric mass density, corresponding to the 11-year 

(more or less) variation in solar activity. Similarly, there is also, on the average, a 27-day variation 

in density that is related to the mean 27-day solar rotation period although the variation tends 

to be slightly longer than 27 days early in the cycle when active regions occur more frequently at 

higher latitude and slightly shorter than 27 days later in the cycle when active regions occur more 

frequently closer to the Sun’s equator. The appearance of coronal holes and active longitudes also 

affects this average 27-day variation. Changes in the thermospheric density related to changes 

in the solar output during active periods associated with flares, eruptions, coronal mass ejections 

(CMEs), and coronal holes can begin almost instantaneously (minutes to hours), although more 

often a day or more lag is seen. Figure 61 shows typical neutral densities for periods of high and 

low solar activity.
144

1,000

900

800

700

600

500

400

300

200

100

10–10 10–12 10–14 10–16 10–18

Atmospheric Density (g/cm3)

Altitude (km)

Low Solar

Activity

High Solar

Activity

newF61

Daytime Maximum

Nighttime Minimum

Figure 61. Typical neutral densities for low and high solar activities (the atmospheric 

density increases toward the left on the abscissa).

5.1.2.2 Variations During Periods of Increased Solar Activity (Flares, Coronal Mass 

Ejections, and Coronal Holes). As previously described, during periods of increased solar activity 

the enhanced interaction of the solar wind with the Earth’s magnetosphere leads to a high latitude 

heat and momentum source for the thermospheric gases. Some of this heat and momentum is convected to low latitudes. This episodic type of increased solar activity that usually causes variations 

in the Earth’s magnetic field varies over the solar cycle and usually has two or more major peaks; 

one during the rise of the cycle and the other with larger peaks during the decline of the cycle. 

Also, more intense solar cycles seem to have more intense episodic type activity. Finally, there is 

a seasonal variation associated with this episodic type activity with the density usually being greatest in March (± month) and September (± month) of each year. This variation is possibly related 

to the path of the Earth in its rotation around the Sun.

5.1.2.3 The Diurnal Variation. The rotation of the Earth with respect to the solar EUV heat 

source induces a diurnal (24-hr period) variation (or diurnal tide) in the thermospheric temperature 

and density. Due to a lag in the response of the thermosphere to the EUV heat source, the density 

at orbital altitudes maximizes around 14:00 LST at a latitude approximately equal to that of the
145

subsolar point. The lag, which is a function of altitude, decreases with decreasing altitude. Similarly, the density minimum occurs between 03:00 and 04:00 LST at approximately the same latitude 

in the opposite hemisphere. The diurnal variation is not a predominant effect in the lowest regions 

of the thermosphere (120 km and below) where the characteristic thermal conduction time is on 

the order of a day or more.

The various constituents of the thermosphere do not all respond to the diurnal variation 

of the solar EUV heat source with the same amplitude and phase. The time lag is longer, by as 

much as 2 hr at orbital altitudes, for the heavier constituents (N2, O2, and Ar) than for AO. By contrast, the lighter species’ number densities maximize in the early morning hours (03:00 and 

07:00 LST, for H and He, respectively). This is due to dynamical (buoyancy) effects.

Harmonics of the diurnal tide are also induced in the Earth’s atmosphere. In particular, 

a semidiurnal tide (period of 12 hr) and a terdiurnal tide (period of 8 hr) are important in the lower 

thermosphere (below 160 km for the semidiurnal tide and much lower for the terdiurnal tide). 

These tides are not important at orbital altitudes.

5.1.2.4 Semiannual Variation. This variation is believed to be a conduction mode of oscillation driven by a semiannual variation in joule heating in the high-latitude thermosphere (because 

of a semiannual variation in the effects on the Earth-atmosphere system of episodic-type variations 

in solar activity). The variation is latitude independent and is modified by composition effects. The 

amplitude of the variation is height dependent (30% to 20% and asymmetric about the mean density at 350 km) and variable from year to year, with a primary minimum in July, primary maximum 

in October, and a secondary minimum in January followed by a secondary maximum in April. It 

has been found that the magnitude and altitude dependence of the semi-annual oscillation vary 

considerably from one solar cycle to the next. This variation is important at orbital altitudes.

5.1.2.5 Seasonal-Latitudinal Variations of the Lower Thermosphere Density. This variation 

is driven in the thermosphere by the dynamics of the lower atmosphere (mesosphere and below). 

The amplitude of the variation maximizes in the lower thermosphere somewhere between about 

105 and 120 km, diminishing to zero at altitudes around 170 km. Although the temperature oscillation amplitude is quite large, the corresponding density oscillation amplitude is small. This variation is not important at orbital altitudes.

5.1.2.6 Seasonal-Latitudinal Variations of Helium. Satellite mass spectrometers have measured a strong increase of He above the winter pole. Over a year, the He number density varies by 

a factor of 42 at 275 km, 12 at 400 km, and 3 or 4 above 500 km. The formation of this winter He 

bulge is primarily due to the effects of global scale winds that blow from the summer to the winter 

hemisphere. The amplitude of the bulge decreases with increasing levels of solar activity, due to the 

increased effectiveness of exospheric transport above 500 km that carries He back to the summer 

hemisphere. There is also a very weak dependence of the He bulge amplitude on the magnitude of 

the lower thermospheric eddy diffusivity.
146

5.1.2.7 Thermospheric Waves. Fluctuations have been detected in temperature and density 

measurements throughout the atmosphere from the ground up to at least 510 km. Some of these 

fluctuations are caused by gravity waves, so named because they are primarily oscillations of the 

neutral gas for which the restoring force is gravity. A thermospheric gravity wave produces a corresponding wave in the ionosphere known as a traveling ionospheric disturbance.

Thermospheric gravity waves oscillate with periods typically in the range of 30 min to several hours, and have horizontal wavelengths in the range of hundreds of kilometers up to approximately 4,000 km. The density amplitudes of the larger scale waves are larger at higher latitudes and 

diminish toward the equator. At approximately 200 km altitude, typical values of these amplitudes 

are 15% of the mean at auroral latitudes and 5% of the mean at equatorial latitudes. The smallscale waves have amplitudes that are essentially independent of latitude. Gravity wave amplitudes 

generally decrease at greater altitudes in the thermosphere due to dissipation by molecular processes. The larger scale waves survive to greater altitudes than do the smaller scale waves.

5.1.3 Thermospheric Winds

Figures 62 and 63 show the general flow patterns of thermospheric winds between 100 and 

700 km as they are currently known. The wind speeds range from 100 to 200 m/s at low latitudes 

while at high latitudes (greater than approximately 65°) they can be as large as 1,500 m/s or more. 

Rapid (minutes) change in wind direction (of up to 180°), probably driven by gravity waves, has 

also been observed.

5.1.3.1 Austral (South Polar Region) Thermospheric Winds. Ground-based observations 

with Fabry-Perot interferometers have shown that there is a direct relationship between austral 

thermospheric winds at F-layer altitudes and the interplanetary magnetic field (IMF), particularly 

in the midnight magnetic local time (MLT) sector. The strength of the zonal component of the 

austral neutral wind in the MLT midnight sector is directly correlated with the strength and direction of the By component of the IMF. The zonal wind in the MLT midnight sector changes direction when the sign of the By component of the IMF changes and the speed of the zonal wind in the 

MLT midnight sector is directly proportional to the strength of the By component. Conversely, the 

meridional wind in the MLT midnight sector is correlated with the Bz component in the IMF. The 

meridional wind decreases as the Bz component turns northward. When Bz is positive, sunward 

winds develop in the MLT midnight sector and exceed the strength of the zonal winds.

5.1.3.2 Thermospheric Tide. Models of the tides in the lower thermosphere (90 to 140 km) 

show the midlatitude semidiurnal tide in the wind has an amplitude of tens of meters per second, 

maximizing at 50 to 70 m/s at approximately 110 km. Temperature tide amplitudes typically reach a 

maximum of 10 to 20 K; however, the models do not always agree with observations since a semidiurnal oscillation of approximately 70 K has been measured at low latitudes at 120 km altitude. The 

best agreement occurs at midlatitude; but in general, the models underestimate both the wind and 

temperature tidal amplitudes. Phase differences are relatively small for the temperature tide, but are 

large for the tidal winds, as much as 3 hr. Radio detection and ranging (RADAR) measurements of 

midlatitude semidiurnal wind tides show maximum amplitudes of 60 to 70 m/s in the 110 to

120 km altitude range.
147

600

500

400

300

200

100

0

(a)

600

500

400

300

200

100

0

(b)

600

500

400

300

200

100

0

(c) S. 80 60 40 20 0 20 40 60 80 N.

Latitude (deg)

Latitude (deg)

Latitude (deg)

newF62

Altitude (km) Altitude (km) Altitude (km)

S. 80 60 40 20 0 20 40 60 80 N.

S. 80 60 40 20 0 20 40 60 80 N.

Figure 62. Schematic of the zonal mean meridional circulation in the thermosphere 

at equinox for various levels of magnetic (auroral) activity (maximum 

velocities are typically hundreds of meters per second): (a) Quiet, (b) average, 

and (c) storm.
148

600

500

400

300

200

100

0

Latitude (deg)

(a)

Summer Winter

600

500

400

300

200

100

0

(c)

Summer Winter

600

500

400

300

200

100

0

Latitude (deg)

(b)

Summer Winter

S. 80 60 40 20 0 20 40 60 80 N.

Latitude (deg)

newF 63

Altitude (km) Altitude (km) Altitude (km)

S. 80 60 40 20 0 20 40 60 80 N.

S. 80 60 40 20 0 20 40 60 80 N.

Figure 63. Schematic of the zonal mean meridional circulation in the thermosphere 

at solstice for various levels of magnetic (auroral) activity (maximum 

velocities are typically hundreds of meters per second): (a) Quiet,

(b) average, and (c) storm.

Values of the maximum midlatitude, annual mean semidiurnal amplitudes, and the altitudes 

at which they occur are (1) temperature, 25 K at 115 km, (2) northward wind, 50 m/s at 125 km 

altitude, and (3) eastward wind, 50 m/s at 125 km altitude. The local times of these maxima generally decrease with increasing altitude; temperature leads the meridional wind by approximately 6 hr, 

while the zonal wind leads the temperature by a few hours.
149

At middle thermospheric altitudes (300 km), the mean winds are poleward in winter and 

equatorward in summer. The amplitude of the diurnal component of the mean wind is much 

greater at solar minimum than at solar maximum, showing that the influence of ion drag more 

than compensates for the greater EUV input at solar maximum. Typical seasonally averaged 

wind amplitude for the diurnal tide is approximately 50 m/s. The mean semidiurnal wind tide 

amplitude averaged over all seasons and solar cycle conditions is approximately 41 m/s.

5.1.3.3 Solar Storm-Induced Effects in the Thermosphere. During magnetic storms and 

substorms, particle precipitation and the joule dissipation of the high-latitude ionospheric currents 

heat the neutral gas at altitudes above 110 km. The thermosphere then expands neutral gas densities above approximately 120 km. The energy deposited in the high-latitude thermosphere at such 

times is significant and can exceed the global EUV absorbed by the thermosphere. It remains an 

important heat source locally at night in the auroral zones.

The energy generated during a substorm is transferred from high to low latitudes principally 

by the meridional winds assisted by gravity waves. This high-latitude heating sets up a circulation 

cell in which gas is lifted in the auroral zone, flows equatorward, sinks at middle to low latitudes, 

and then returns to the heating region at low altitudes. The sinking causes compressional heating 

(figs. 62 and 63).

Since the mean molecular weight decreases with altitude, lifting of the gas at auroral latitudes leads to increases in the mean molecular weight at a fixed altitude. Compositional changes 

also accompany geomagnetic storms and these changes are even observed at middle and low latitudes. In general, during a geomagnetic storm there is a large depletion in AO and an enhancement 

in O2 within the auroral ovals. The magnitude of both the AO depletion and the O2 enhancement 

is underestimated in current models, with the misrepresentation increasing with the use of daily values of Ap instead of the three hourly values. Analyses, which require instantaneous values of either 

AO or O2 within the auroral ovals, should take special precautions in using the available models 

of the thermosphere.

In one numerical simulation of a geomagnetic substorm it was found that at both 200 and 

450 km the latitudinal variations in the mass density during the substorm were relatively insignificant and did not at all reflect the total heat input into the auroral zone. In particular, it was apparent that the global density response was relatively significant (60%) when compared with the 40% 

increase between the equator and the pole at 450 km. This predominance in the global component 

was even more pronounced at 200 km and appeared to be in substantial agreement with the satellite drag data. The maximum in the density occurred at 02:00 UTC at high latitudes and shifted 

toward 08:00 UTC at the equator, with the average time lag of 6 hr deduced from satellite drag 

data.

In other simulations of geomagnetic substorms, one finds that the heat significantly perturbs 

the thermosphere at high latitudes and momentum deposited there, with wind speeds approaching 

1 km/s in the region of the polar cap and temperatures elevated approximately 500 K in the auroral 

zones. At lower latitudes the effects are far less dramatic but are still significant. The response is 

asymmetric with respect to longitude, reflecting the asymmetry of the high-latitude heat sources. 
150

Large-scale disturbances propagate from each auroral region toward equatorial latitudes with a 

time delay of ≈2 hr at 330 km altitude and ≈5 hr at 125 km altitude. The equatorial thermosphere 

at 330 km altitude is influenced mainly by the interactions of two converging gravity waves, while 

at 125 km the thermosphere is influenced by the mean motion of the waves. Equatorial temperature oscillations of magnitude 100 to 200 K occurred at 330 km altitude driven by oscillations in the 

mean meridional wind of approximately 200 m/s. Temperature enhancements at 125 km were much smaller 

(60 K) as were the meridional wind speed enhancements (8 m/s). The zonally averaged zonal wind 

enhancements were found to be confined primarily to the high latitudes.

5.1.3.4 Ionospheric Storm Effects. During and following a geomagnetic substorm, the 

ionosphere is altered as a consequence of several competing effects. Some of these effects will lead 

to increases in the ionization density of the F2-region (positive storm effects), and others will lead 

to decreases in the F2-peak (negative storm effects). These are briefly summarized for the subauroral ionosphere.

5.1.3.4.1 Positive Storm Effects. Traveling atmospheric disturbances, such as gravity waves, 

will cause a sudden uplifting of the F-layer. Through collisions the neutral disturbances impart to 

the ions a motion parallel to the geomagnetic field lines. These effects propagate equatorward at 

speeds of many hundreds of meters per second. The duration of the upward drift is ≈1 hr and is 

particularly pronounced in the noon/afternoon local time sector.

The large-scale wind circulation will also impart a motion to the ions that is parallel to the 

geomagnetic field. This effect may last a whole day and is more likely to occur in the daytime.

The expansion of the polar ionization enhancement toward lower latitudes, due for example 

to particle precipitation, will directly increase ionization densities. It is typically a nighttime 

phenomenon.

5.1.3.4.2 Negative Storm Effects. Changes in the neutral gas composition, and particularly 

an increase in the N2:O ratio, will lead to a depletion of ionization. This is most clearly observed 

in the morning sector, with an anomalously low rate of ionization after sunrise. The effect may last 

from many hours to days.

The displacement and expansion of the ionospheric trough region (plasma depletion region) 

toward lower latitudes creates a steep ionization density drop and is typically observed in the 

afternoon/evening sector.

Another ionospheric effect of geomagnetic disturbances is the propagation of disturbance 

effects known as traveling ionospheric disturbances (TIDs). They are characterized by oscillations 

in the ionization density at the height of the F-layer, driven by gravity wave-driven oscillations 

in the neutral density and neutral winds.
151

5.1.4 Solar and Geomagnetic Indices

Various surrogate indices are used to quantitatively assess the levels of solar activity. One 

of these is the 10.7 cm (2,800 MHz) solar radio noise flux, designated F10.7. It is the EUV radiation that heats the thermosphere, but the EUV cannot be measured at the ground. The F10.7 can be 

measured from the ground, and it also correlates quite well with the EUV radiation.

The planetary geomagnetic activity index ap (or kp, which is essentially the logarithm of ap) 

is an index that is used as a measure of episodic type solar activity. It is based on magnetic field 

fluctuation data reported every 3 hr at 12 stations between geomagnetic latitudes 48° and 63° and 

selected for good longitudinal coverage. Although it is the high latitude ionospheric current fluctuations that drive the magnetic field fluctuations as observed at these stations, it is not the magnetic 

field fluctuations which are driving the thermosphere. Therefore, the correlations between observed 

density changes and the ap index are not always good. The daily planetary geomagnetic index (Ap) 

is the average of the eight 3-hourly ap values for that particular day.

Table 65 lists 95th percentile, 50th percentile, and 5th percentile of 13-month smoothed 

values of solar radio noise flux (F10.7) and geomagnetic activity index (Ap) throughout a mean 

11 years and solar cycle. Figures 64 and 65 show data from this table. The F10.7 data are derived 

from sunspot records for the period 1749 to 1947 with direct F10.7 measurements thereafter. The 

standard deviation about the mean length is 1.23 years in the historical record.

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028.

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2001.92 194.8914 191.8801 188.925 12.506 11.7432 10.7939

2002 196.7669 189.1827 182.9999 13.1766 11.7679 10.5699

2002.08 196.9785 185.4429 176.684 14.4097 11.9812 10.1947

2002.17 195.6518 181.9426 171.0847 15.5131 12.1699 9.6501

2002.25 194.4169 178.7193 166.3541 16.1222 12.3507 9.6077

2002.33 194.6547 175.4454 161.7553 16.9907 12.5072 9.4901

2002.42 193.8348 171.6151 156.0731 17.2921 12.6248 9.1161

2002.5 190.7193 167.8858 150.27 16.9808 12.8019 8.9714

2002.58 186.5948 164.8723 146.5414 17.4043 12.9949 8.859

2002.67 182.6598 162.174 144.1609 17.1877 13.174 9.054

2002.75 179.5597 159.7874 141.324 16.8001 13.3728 9.2129

2002.83 175.7908 157.2723 140.5153 16.8457 13.578 9.6037

2002.92 169.702 153.3492 137.4805 17.0922 13.712 10.0902

2003 164.4646 148.436 132.9786 17.2449 13.7723 10.4484

2003.08 161.1737 144.2353 128.7686 17.3968 13.9019 10.9643

2003.17 159.4748 140.9991 125.2031 18.2537 13.9794 11.25

2003.25 157.5326 138.1079 121.2903 19.0982 13.9511 11.3494
152

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2003.33 154.2465 135.5067 118.2396 19.8833 14.0764 11.4433

2003.42 150.7187 133.3078 116.7739 20.1888 14.2407 10.7039

2003.5 147.9575 130.5241 114.2062 20.3186 14.2686 10.6321

2003.58 146.9935 127.5355 110.8836 20.8846 14.4016 10.9015

2003.67 146.2966 124.94 107.2988 21.5425 14.546 10.7647

2003.75 145.4693 122.1461 102.7479 21.614 14.664 10.6424

2003.83 143.2352 118.6863 98.7188 21.6039 14.8578 11.0628

2003.92 139.9465 115.5923 95.8111 21.9553 15.0998 11.4447

2004 138.4801 113.6259 94.0401 22.4415 15.5881 11.9079

2004.08 137.4264 112.6295 93.3168 22.6438 16.0604 11.9301

2004.17 135.031 111.6439 92.9643 22.065 16.3577 12.2559

2004.25 132.6903 110.6436 92.6502 21.2547 16.5438 12.4453

2004.33 131.172 109.4269 91.8763 21.3053 16.662 12.3896

2004.42 130.113 107.5554 90.2545 21.8257 16.6744 12.4603

2004.5 128.2233 105.2967 88.2664 21.7924 16.8382 13.1595

2004.58 125.0575 103.0319 86.3616 22.8151 17.2108 13.7104

2004.67 121.3224 100.8916 85.1261 23.4261 17.4597 13.7026

2004.75 117.5213 98.7849 84.1752 23.152 17.3417 13.7748

2004.83 114.0496 97.1131 83.5203 23.0656 17.1237 13.6257

2004.92 112.8977 95.9605 82.9042 23.1411 16.7074 13.0105

2005 111.2536 94.6416 82.1568 23.1416 16.1644 12.2215

2005.08 109.2016 92.8183 81.3941 23.7055 15.9137 11.3547

2005.17 106.1988 90.6926 80.2538 24.2181 15.5721 10.3515

2005.25 102.9697 88.6178 78.442 24.6073 15.2582 10.0012

2005.33 100.8023 86.8644 77.5514 24.9149 15.1366 9.749

2005.42 98.4352 85.5441 76.8469 25.1095 15.113 9.602

2005.5 96.0907 84.5975 76.342 25.0809 15.0273 9.8393

2005.58 95.5272 83.9043 75.8852 24.7025 15.0317 10.3193

2005.67 93.9364 82.897 75.251 24.0399 15.0532 10.9187

2005.75 92.3529 81.8942 74.6197 23.4342 15.1428 11.6073

2005.83 90.7838 80.9006 73.9942 23.4186 15.5166 11.5544

2005.92 89.2364 79.9207 73.3773 23.432 15.917 11.2871

2006 87.7179 78.9591 72.7719 23.5028 16.3904 11.4397

2006.08 86.2354 78.0203 72.1809 23.2244 16.6267 11.7461

2006.17 84.7963 77.109 71.6071 22.6938 16.6502 12.1742

2006.25 83.4077 76.2297 71.0536 22.003 16.5419 12.187

2006.33 82.0769 75.3869 70.523 21.045 16.4793 12.4141

2006.42 80.8111 74.5853 70.0183 19.9723 16.356 12.6623

2006.5 79.6174 73.8294 69.5425 19.1145 16.1904 13.226

2006.58 78.5032 73.1238 69.0983 18.788 16.0808 13.8318

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
153

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2006.67 77.4755 72.4731 68.6886 18.9404 16.0814 13.8694

2006.75 76.5418 71.8818 68.3163 19.1498 16.0465 13.8747

2006.83 75.7091 71.3545 67.9844 19.0994 15.9164 12.8981

2006.92 74.9848 70.8958 67.6956 18.6146 15.5736 12.155

2007 74.3759 70.5102 67.4529 18.2498 15.2775 11.6549

2007.08 73.8898 70.2024 67.259 18.0756 15.2119 11.6735

2007.17 73.5336 69.9768 67.1171 18.3813 15.2207 11.9373

2007.25 73.3146 69.8382 67.0297 18.7086 15.1398 11.8384

2007.33 73.24 69.7909 67 19.0527 15.0197 11.7872

2007.42 73.355 69.9227 67.13 18.985 14.8198 11.7797

2007.5 74 70.2773 67.26 18.6472 14.5115 11.7295

2007.58 74.84 70.6636 67.375 17.777 13.8757 11.1745

2007.67 75.665 71.0318 67.505 16.917 13.2474 10.6261

2007.75 76.735 71.4591 67.635 16.0777 12.6342 10.0908

2007.83 78.255 72.0227 67.85 15.2694 12.0436 9.5753

2007.92 79.6 72.6545 68.05 14.5023 11.4831 9.0861

2008 81.055 73.35 68.18 13.7868 10.9604 8.6299

2008.08 83.39 74.1636 68.21 13.1332 10.4828 8.213

2008.17 86.45 75.0864 68.255 12.5519 10.0581 7.8423

2008.25 91.325 76.2727 68.33 12.0531 9.6937 7.5242

2008.33 95.85 77.6682 68.285 11.6472 9.3971 7.2654

2008.42 99.315 79.1364 68.64 11.3445 9.176 7.0723

2008.5 104.58 80.8 68.64 11.1554 9.0378 6.9517

2008.58 110.68 82.7864 68.68 11.09 8.99 6.91

2008.67 116.65 84.9273 68.875 11.3 9.1 6.9

2008.75 124.025 87.1636 68.875 11.605 9.3 7.095

2008.83 132.525 89.5773 68.965 11.695 9.42 7.405

2008.92 139.015 92.1864 69.355 11.9 9.51 7.5

2009 143.575 94.9909 69.8 12.005 9.56 7.495

2009.08 147.915 97.8091 70.13 12.12 9.6 7.28

2009.17 152.24 100.5909 70.815 12.23 9.66 7.17

2009.25 156.96 103.3364 71.18 12.55 9.81 7.05

2009.33 162.115 106.3091 71.35 12.87 10.07 6.93

2009.42 167.195 109.5318 72.115 13.28 10.46 7.12

2009.5 171.485 112.7636 72.73 13.785 10.79 7.515

2009.58 177.305 115.9273 73.29 14.61 11.08 7.79

2009.67 185.155 119.0273 74.03 15.77 11.37 7.63

2009.75 190.17 122.1773 74.26 16.19 11.58 7.61

2009.83 192.75 125.0909 74.635 16.405 11.72 7.495

2009.92 195.38 127.4727 74.955 16.93 11.87 7.47

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
154

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2010 198.32 129.7273 75.275 18.08 12.2 7.52

2010.08 202.925 131.8955 75.455 19.02 12.56 7.58

2010.17 209.115 134.3455 75.45 19.23 12.77 7.57

2010.25 213.23 136.7818 75.13 19.33 13.03 7.67

2010.33 215.93 139.0636 74.895 19.125 13.23 7.575

2010.42 220.385 141.1545 75.19 18.705 13.29 7.595

2010.5 225.015 143.15 75.505 18.595 13.33 7.705

2010.58 227.27 144.85 76.26 19.005 13.5 7.895

2010.67 228.41 146.4909 77.48 18.89 13.57 8.11

2010.75 230.31 147.8773 78.215 17.93 13.63 8.47

2010.83 232.445 148.8136 79.135 18.03 13.84 8.57

2010.92 235.43 149.6545 80.705 18.55 14.05 8.65

2011 238.7 150.0773 82.63 19.165 14.34 8.935

2011.08 240.265 150.4545 84.675 19.67 14.8 9.33

2011.17 238.73 150.8364 85.92 20.395 15.11 9.505

2011.25 236.895 150.9864 86.86 20.395 15.16 9.505

2011.33 238.065 151.2227 88.81 20.385 15.16 9.715

2011.42 238.665 150.5682 90.195 20.585 15.22 9.915

2011.50 236.345 149.1591 92.16 20.875 15.39 10.425

2011.58 232.945 148.0591 93.54 21.265 15.44 11.035

2011.67 229.205 147.4864 94.03 21.345 15.28 11.555

2011.75 227.425 147.4409 95.33 21.465 15.09 11.235

2011.83 227.795 147.4 95.625 21.68 15.08 11.12

2011.92 227.93 147.1455 95.165 22.125 15.09 10.575

2012 226.525 146.7182 95.265 22.88 14.97 10.12

2012.08 224.845 146.2682 96.78 22.58 14.68 9.82

2012.17 224.275 145.6954 97.5 21.42 14.4 9.98

2012.25 224.155 145.1909 97.065 20.685 14.28 10.015

2012.33 223.625 144.7636 96.34 20.15 14.21 10.25

2012.42 222.645 144.0318 96.315 19.72 14.14 10.48

2012.5 220.255 142.9591 96.78 19.615 14.11 10.485

2012.58 216.76 141.3636 95.855 19.41 14.14 10.39

2012.67 213.915 139.7955 95.38 19.2 14.38 10.4

2012.75 211.07 138.6227 96.225 18.99 14.74 10.41

2012.83 207.55 137.6273 97.165 19 14.86 10.2

2012.92 206.55 136.3046 96.225 19.545 14.75 9.755

2013 204.505 134.6091 94.64 20.47 14.87 10.13

2013.08 201.09 132.7545 93.86 22.125 15.28 10.575

2013.17 197.535 131.3545 93.61 23.485 15.59 10.615

2013.25 195.195 130.2955 91.935 24.105 15.78 10.795

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
155

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2013.33 193.375 128.9273 88.495 25.25 16.06 10.95

2013.42 189.625 127.1455 87.745 25.765 16.28 11.135

2013.5 185.14 125.4955 88.815 25.02 16.28 11.38

2013.58 180.715 124.2182 88.35 24.905 16.25 11.595

2013.67 176.815 123.1136 86.96 24.06 16.16 11.74

2013.75 174.555 122.0182 86.045 22.175 16.12 11.725

2013.83 172.735 120.8273 85.715 22.48 16.2 11.92

2013.92 168.58 119.1636 84.79 23.225 16.28 11.675

2014 162.05 117.2045 83.51 23.26 16.27 10.94

2014.08 156.08 115.4 82.49 22.75 16.14 10.65

2014.17 152.515 113.4727 81.905 22.12 15.95 10.68

2014.25 150.32 111.4773 81.91 22.015 15.9 10.685

2014.33 147.975 109.7636 81.56 22.855 16.12 10.645

2014.42 145.065 108.3364 80.395 23.795 16.37 10.705

2014.5 141.97 106.9273 80.525 24.005 16.56 10.695

2014.58 138.355 105.4091 80.205 24.01 16.79 10.59

2014.67 134.54 103.9227 79.515 24.015 16.89 10.485

2014.75 130.145 102.4182 78.975 23.61 16.88 10.19

2014.83 124.09 100.8409 78.15 22.66 16.85 10.34

2014.92 119.11 99.3273 77.105 22.86 16.78 10.54

2015 118.185 98.0273 75.405 23.14 16.89 11.26

2015.08 118.725 96.9227 74.335 23.22 17.17 11.78

2015.17 119.28 95.8682 73.645 22.585 17.34 11.915

2015.25 119.65 95.0182 72.775 21.635 17.25 12.065

2015.33 118.84 94.0227 71.825 21.315 17.06 12.185

2015.42 117.435 92.8727 71.27 21.83 16.85 12.37

2015.5 115.965 91.75 70.815 21.79 16.74 13.21

2015.58 113.915 90.6727 70.375 22.22 16.82 12.98

2015.67 110.18 89.5182 70.275 22.65 16.95 12.75

2015.75 105.11 88.25 70.36 22.43 16.87 12.97

2015.83 102.72 87.2682 70.22 22.445 16.71 12.655

2015.92 101.68 86.5636 70.12 22.77 16.46 12.43

2016 100.195 85.7591 70.175 23 16.07 12

2016.08 98.11 84.7455 70.085 23.565 15.82 11.135

2016.17 96.3 83.5909 70.315 24.23 15.58 10.37

2016.25 94.48 82.5455 69.78 24.955 15.49 10.545

2016.33 93.435 81.7636 69.55 25.475 15.51 10.625

2016.42 92.125 81.0091 69.39 25.69 15.5 10.51

2016.50 91.67 80.2682 69.265 25.58 15.36 10.62

2016.58 91.055 79.5 68.995 25.045 15.26 10.855

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
156

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2016.67 90.46 78.7955 68.68 24.095 15.09 11.005

2016.75 89.77 78.1318 68.405 23.145 14.95 11.155

2016.83 88.885 77.5091 68.305 22.625 15.01 11.075

2016.92 88 76.9636 68.29 22.21 15.15 10.99

2017 87.1 76.4409 68.215 21.89 15.38 11.11

2017.08 85.775 75.8955 67.96 21.46 15.52 11.34

2017.17 84.355 75.4045 67.705 20.83 15.47 11.37

2017.25 82.27 74.8455 67.535 19.99 15.27 11.41

2017.33 79.87 74.1591 67.55 20.08 15.08 11.72

2017.42 78.37 73.4864 67.395 20.175 14.9 11.925

2017.5 77.47 72.8864 67.31 19.875 14.73 11.625

2017.58 77 72.4091 67.21 19.46 14.61 11.54

2017.67 76.87 72.0318 67.18 18.955 14.58 11.145

2017.75 76.655 71.6273 67.165 18.25 14.61 10.55

2017.83 76.485 71.2773 67.105 17.345 14.54 9.755

2017.92 76.165 70.9773 67.075 17.36 14.39 9.44

2018 75.18 70.6818 67.045 17.47 14.27 9.33

2018.08 74.2 70.3682 67.015 17.575 14.2 9.325

2018.17 73.985 70.1364 67.015 17.575 14.09 9.325

2018.25 73.5 69.9 67 17.79 14 9.21

2018.33 73.24 69.7909 67 19.0527 15.0197 11.7872

2018.42 73.355 69.9227 67.13 18.985 14.8198 11.7797

2018.50 74 70.2773 67.26 18.6472 14.5115 11.7295

2018.58 74.84 70.6636 67.375 17.777 13.8757 11.1745

2018.67 75.665 71.0318 67.505 16.917 13.2474 10.6261

2018.75 76.735 71.4591 67.635 16.0777 12.6342 10.0908

2018.83 78.255 72.0227 67.85 15.2694 12.0436 9.5753

2018.92 79.6 72.6545 68.05 14.5023 11.4831 9.0861

2019 81.055 73.35 68.18 13.7868 10.9604 8.6299

2019.08 83.39 74.1636 68.21 13.1332 10.4828 8.213

2019.17 86.45 75.0864 68.255 12.5519 10.0581 7.8423

2019.25 91.325 76.2727 68.33 12.0531 9.6937 7.5242

2019.33 95.85 77.6682 68.285 11.6472 9.3971 7.2654

2019.42 99.315 79.1364 68.64 11.3445 9.176 7.0723

2019.5 104.58 80.8 68.64 11.1554 9.0378 6.9517

2019.58 110.68 82.7864 68.68 11.09 8.99 6.91

2019.67 116.65 84.9273 68.875 11.3 9.1 6.9

2019.75 124.025 87.1636 68.875 11.605 9.3 7.095

2019.83 132.525 89.5773 68.965 11.695 9.42 7.405

2019.92 139.015 92.1864 69.355 11.9 9.51 7.5

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
157

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2020 143.575 94.9909 69.8 12.005 9.56 7.495

2020.08 147.915 97.8091 70.13 12.12 9.6 7.28

2020.17 152.24 100.5909 70.815 12.23 9.66 7.17

2020.25 156.96 103.3364 71.18 12.55 9.81 7.05

2020.33 162.115 106.3091 71.35 12.87 10.07 6.93

2020.42 167.195 109.5318 72.115 13.28 10.46 7.12

2020.5 171.485 112.7636 72.73 13.785 10.79 7.515

2020.58 177.305 115.9273 73.29 14.61 11.08 7.79

2020.67 185.155 119.0273 74.03 15.77 11.37 7.63

2020.75 190.17 122.1773 74.26 16.19 11.58 7.61

2020.83 192.75 125.0909 74.635 16.405 11.72 7.495

2020.92 195.38 127.4727 74.955 16.93 11.87 7.47

2021 198.32 129.7273 75.275 18.08 12.2 7.52

2021.08 202.925 131.8955 75.455 19.02 12.56 7.58

2021.17 209.115 134.3455 75.45 19.23 12.77 7.57

2021.25 213.23 136.7818 75.13 19.33 13.03 7.67

2021.33 215.93 139.0636 74.895 19.125 13.23 7.575

2021.42 220.385 141.1545 75.19 18.705 13.29 7.595

2021.5 225.015 143.15 75.505 18.595 13.33 7.705

2021.58 227.27 144.85 76.26 19.005 13.5 7.895

2021.67 228.41 146.4909 77.48 18.89 13.57 8.11

2021.75 230.31 147.8773 78.215 17.93 13.63 8.47

2021.83 232.445 148.8136 79.135 18.03 13.84 8.57

2021.92 235.43 149.6545 80.705 18.55 14.05 8.65

2022 238.7 150.0773 82.63 19.165 14.34 8.935

2022.08 240.265 150.4545 84.675 19.67 14.8 9.33

2022.17 238.73 150.8364 85.92 20.395 15.11 9.505

2022.25 236.895 150.9864 86.86 20.395 15.16 9.505

2022.33 238.065 151.2227 88.81 20.385 15.16 9.715

2022.42 238.665 150.5682 90.195 20.585 15.22 9.915

2022.5 236.345 149.1591 92.16 20.875 15.39 10.425

2022.58 232.945 148.0591 93.54 21.265 15.44 11.035

2022.67 229.205 147.4864 94.03 21.345 15.28 11.555

2022.75 227.425 147.4409 95.33 21.465 15.09 11.235

2022.83 227.795 147.4 95.625 21.68 15.08 11.12

2022.92 227.93 147.1455 95.165 22.125 15.09 10.575

2023 226.525 146.7182 95.265 22.88 14.97 10.12

2023.08 224.845 146.2682 96.78 22.58 14.68 9.82

2023.17 224.275 145.6954 97.5 21.42 14.4 9.98

2023.25 224.155 145.1909 97.065 20.685 14.28 10.015

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
158

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2023.33 223.625 144.7636 96.34 20.15 14.21 10.25

2023.42 222.645 144.0318 96.315 19.72 14.14 10.48

2023.5 220.255 142.9591 96.78 19.615 14.11 10.485

2023.58 216.76 141.3636 95.855 19.41 14.14 10.39

2023.67 213.915 139.7955 95.38 19.2 14.38 10.4

2023.75 211.07 138.6227 96.225 18.99 14.74 10.41

2023.83 207.55 137.6273 97.165 19 14.86 10.2

2023.92 206.55 136.3046 96.225 19.545 14.75 9.755

2024 204.505 134.6091 94.64 20.47 14.87 10.13

2024.08 201.09 132.7545 93.86 22.125 15.28 10.575

2024.17 197.535 131.3545 93.61 23.485 15.59 10.615

2024.25 195.195 130.2955 91.935 24.105 15.78 10.795

2024.33 193.375 128.9273 88.495 25.25 16.06 10.95

2024.42 189.625 127.1455 87.745 25.765 16.28 11.135

2024.5 185.14 125.4955 88.815 25.02 16.28 11.38

2024.58 180.715 124.2182 88.35 24.905 16.25 11.595

2024.67 176.815 123.1136 86.96 24.06 16.16 11.74

2024.75 174.555 122.0182 86.045 22.175 16.12 11.725

2024.83 172.735 120.8273 85.715 22.48 16.2 11.92

2024.92 168.58 119.1636 84.79 23.225 16.28 11.675

2025 162.05 117.2045 83.51 23.26 16.27 10.94

2025.08 156.08 115.4 82.49 22.75 16.14 10.65

2025.17 152.515 113.4727 81.905 22.12 15.95 10.68

2025.25 150.32 111.4773 81.91 22.015 15.9 10.685

2025.33 147.975 109.7636 81.56 22.855 16.12 10.645

2025.42 145.065 108.3364 80.395 23.795 16.37 10.705

2025.5 141.97 106.9273 80.525 24.005 16.56 10.695

2025.58 138.355 105.4091 80.205 24.01 16.79 10.59

2025.67 134.54 103.9227 79.515 24.015 16.89 10.485

2025.75 130.145 102.4182 78.975 23.61 16.88 10.19

2025.83 124.09 100.8409 78.15 22.66 16.85 10.34

2025.92 119.11 99.3273 77.105 22.86 16.78 10.54

2026 118.185 98.0273 75.405 23.14 16.89 11.26

2026.08 118.725 96.9227 74.335 23.22 17.17 11.78

2026.17 119.28 95.8682 73.645 22.585 17.34 11.915

2026.25 119.65 95.0182 72.775 21.635 17.25 12.065

2026.33 118.84 94.0227 71.825 21.315 17.06 12.185

2026.42 117.435 92.8727 71.27 21.83 16.85 12.37

2026.5 115.965 91.75 70.815 21.79 16.74 13.21

2026.58 113.915 90.6727 70.375 22.22 16.82 12.98

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
159

Year

F10.7 AP

95% Flux 50% Flux 5% Flux 95% AP 50% AP 5% AP

2026.67 110.18 89.5182 70.275 22.65 16.95 12.75

2026.75 105.11 88.25 70.36 22.43 16.87 12.97

2026.83 102.72 87.2682 70.22 22.445 16.71 12.655

2026.92 101.68 86.5636 70.12 22.77 16.46 12.43

2027 100.195 85.7591 70.175 23 16.07 12

2027.08 98.11 84.7455 70.085 23.565 15.82 11.135

2027.17 96.3 83.5909 70.315 24.23 15.58 10.37

2027.25 94.48 82.5455 69.78 24.955 15.49 10.545

2027.33 93.435 81.7636 69.55 25.475 15.51 10.625

2027.42 92.125 81.0091 69.39 25.69 15.5 10.51

2027.5 91.67 80.2682 69.265 25.58 15.36 10.62

2027.58 91.055 79.5 68.995 25.045 15.26 10.855

2027.67 90.46 78.7955 68.68 24.095 15.09 11.005

2027.75 89.77 78.1318 68.405 23.145 14.95 11.155

2027.83 88.885 77.5091 68.305 22.625 15.01 11.075

2027.92 88 76.9636 68.29 22.21 15.15 10.99

2028 87.1 76.4409 68.215 21.89 15.38 11.11

2028.08 85.775 75.8955 67.96 21.46 15.52 11.34

2028.17 84.355 75.4045 67.705 20.83 15.47 11.37

2028.25 82.27 74.8455 67.535 19.99 15.27 11.41

2028.33 79.87 74.1591 67.55 20.08 15.08 11.72

2028.42 78.37 73.4864 67.395 20.175 14.9 11.925

2028.5 77.47 72.8864 67.31 19.875 14.73 11.625

2028.58 77 72.4091 67.21 19.46 14.61 11.54

2028.67 76.87 72.0318 67.18 18.955 14.58 11.145

2028.75 76.655 71.6273 67.165 18.25 14.61 10.55

2028.83 76.485 71.2773 67.105 17.345 14.54 9.755

2028.92 76.165 70.9773 67.075 17.36 14.39 9.44

2029 75.18 70.6818 67.045 17.47 14.27 9.33

2029.08 74.2 70.3682 67.015 17.575 14.2 9.325

2029.17 73.985 70.1364 67.015 17.575 14.09 9.325

2029.25 73.5 69.9 67 17.79 14 9.21

Table 65. 13-month smoothed 10.7 cm solar radio noise flux and geomagnetic 

activity index over the mean solar cycle, 2002–2028 (Continued).
160

95 Percentile Values 50 Percentile Values 5 Percentile Values

50

75

100

125

150

175

200

225

250

newF64

13-Mo Smooth Solar Flux

2000 2005 2010 2015 2020 2025 2030

Year

Figure 64. 13-month smoothed values of solar flux (F10.7) over the mean solar cycle 

from 2002–2028.

95 Percentile Values 50 Percentile Values 5 Percentile Values

0

5

10

15

20

25

30

newF65

13-Mo Smooth 

Ap

2000 2005 2010 2015 2020 2025 2030

Year

Figure 65. 13-month smoothed values of geomagnetic activity index (Ap) over the mean 

solar cycle from 2002–2028.

5.1.5 Orbital and Suborbital Neutral Atmosphere Model

5.1.5.1 Marshall Engineering Thermosphere Model. The MET model has been developed 

to represent, insofar as is practical for engineering applications, the variability of the ambient mass 

density. It is the standard neutral atmospheric density model used for control and lifetime studies 

for most NASA spacecraft projects. A description of the model can be found in references 91–96. 

The MET model is an empirical model with coefficients that were obtained from satellite drag 

analyses. It is a static diffusion model and is essentially the Smithsonian’s Jacchia 1970 model with 

two additions from the Jacchia 1971 model.97,98 Inputs to the model are time (year, month, day, 

hour, and minute), position (altitude and geographic latitude and longitude), the previous day’s 

solar radio flux (F10.7), the centered solar radio flux averaged over six solar rotations (F10.7B), and 

ap index 6 to 7 hours before the time in question. (For some studies, the daily planetary geomagnetic index (Ap) may be used instead of the three hourly ap values.)
161

With the exospheric temperature specified, the temperature can be calculated for any altitude between the lower boundary (90 km) and the upper level (2,500 km) of the model from an 

empirically determined temperature profile. The density for all points on the globe at 90 km altitude is assumed constant, and mixing prevails to 105 km. Between these two altitudes, the mean 

molecular mass varies as a result of the dissociation of O2 to AO. At 120 km altitude, the ratio of 

AO to O2 is assumed to be 1:5. Density between 90 and 150 km is calculated by integration of the 

barometric equation. For altitudes above 105 km, the diffusion equation for each of the individual 

species (O2, AO, N2, He, and Ar) is integrated upward from the 105 km level. For H, the integration of the diffusion equation proceeds upward from 500 km altitude. The total mass density is 

calculated by summing the individual specie mass densities.

The total density is then further modified by the effects of the seasonal-latitudinal density 

variation of the lower thermosphere below 170 km altitude and seasonal-latitudinal variations 

of He above 500 km. These two effects have been incorporated in the MET model using equations developed by Jacchia for his 1971 thermospheric model. The final output of the MET model 

is total mass density, temperature, pressure, individual specie number densities, mean molecular 

weight, scale height, specific heats, and the local gravitational acceleration. The total mass density, 

the temperature, and the individual species all have the same phase variation in the MET model 

(i.e., they all maximize at the same local time). For some studies involving the effects of various 

species on an orbiting spacecraft where accurate phases of the various species within an orbit 

are required, it may be necessary to use the Mass Spectrometer Incoherent Scatter (MSIS) model 

developed by Goddard Space Flight Center. MSIS is a neutral atmosphere empirical model from 

the surface to lower exosphere.99 The values given in table 66 define the Earth pressure parameters 

for space vehicle design performance analyses.

Table 66. Pressure parameters in Earth orbit (ambient pressure in pascal; torr 

in parentheses).*

Minimum Nominal** Maximum

Altitude

(km)

4 × 10–5

(3 x 10–7)

8.5 × 10–5

(6.4 × 10–7)

2.3 × 10–4

(1.7 × 10–7)

200

1.8 × 10–7

(1.4 × 10–9)

1.5 × 10–6

(1.1 × 10–8)

1.7 × 10–5

(1.3 × 10–7)

400

3 × 10–8 

(2.3 × 10–10)

3.1 × 10–7

(2.3 × 10–9)

6.5 × 10–6

(4.9 × 10–8)

500

1.7 × 10–8

(1.3 × 10–10)

8.3 × 10–8

(6.2 × 10–10)

2.7 × 10–6

(2 × 10–8)

600

4.3 × 10–9

(3.2 × 10–11)

7.5 × 10–9

(5.6 × 10–11)

1.5 × 10–7

(1.1 × 10–9)

1,000

1 × 10–11

(7.5 × 10–14)

– – Geosynchronous

* Low- and high-pressure values were computed from the MET model using an orbit 

average value and assuming a 28.5° orbit inclination. The following inputs were 

used: 14:00 UTC, F10.7 = 230, Ap = 400.

** Nominal pressure values were taken from U.S. Standard Atmosphere 1976100

(cf., ref. 101). Geosynchronous pressure values were taken from reference 102.

To convert from pascal to torr, multiplyby 7.5×10 –3.
162

5.1.5.2 Statistical Analysis Mode. The statistical analysis mode (SAM) of the MET model 

was developed for use during those periods when actual values of the daily and 162-day mean values of the 10.7 cm solar radio flux and the global geomagnetic index (ap) are unknown or unavailable, e.g., when the time of application to a specific problem is sometime in the future. This is 

particularly true during the design, development, and testing phases of a space vehicle when there 

are only rough estimates of the 13-month smoothed mean values of these three parameters and no 

guesses at all about how they should be combined.

One possible procedure that has been used in spacecraft development work is to assume 

that the daily and the 162-day mean values of the 10.7 cm solar radio flux are both equal to the 

13-month smoothed value predicted using a technique developed through a statistical analysis of 

sunspot data from 1855 to the present.101 The 13-month smoothed values of ap are predicted in the 

same manner. This procedure provides useful estimates of the long-term mean density, subject 

primarily to the accuracy of the predicted 13-month smoothed input data. For periods shorter than 

90 days, the thermospheric properties defined by the use of the 13-month smoothed inputs are no 

longer representative of the original model outputs using the proper input parameters. For systems 

that are sensitive to thermospheric effects and variations over a period of a few days or less (e.g., 

control and pointing systems), representative samples100 of the proper model inputs taken from the 

historical data set should be used. Alternatively, for some applications the variations may be treated 

statistically.

The MET-SAM is the initial step in developing the statistics of these shorter period perturbations in orbital altitude density in addition to providing limits on the magnitudes of the variations that are averaged out through the use of 13-month smoothed solar activity input parameters 

to the original MET model.

The MET-SAM also provides answers to another frequently asked question: What is the 

percent of time that the recommended density value will be exceeded during the operational phase 

of the vehicle? Or, how confident are you that the recommended density value will not be exceeded 

more than 5% of the time? The answers are crucial in the design of the guidance and control capability, the selection of the altitude at which the vehicle will orbit, and the reboost strategy for payloads put into low Earth orbit (LEO) by the Shuttle. The MET-SAM is also based on the premise 

that most applications during the development phase primarily require detailed knowledge about 

the maximum and minimum densities that will be encountered with limits on the magnitude of the 

variations that occur during monthly periods.

All available daily values of the 10.7 cm solar flux and the three hourly values of ap were 

used to calculate values of the global minimum, mean, and maximum exospheric temperatures. The 

statistics of this new dataset were calculated and the results were indexed to five different levels of 

solar activity as defined by the values of the 13-month smoothed 10.7 cm solar flux. This makes it 

possible to statistically include the 3-hr to 90-day variations in these three temperatures using only 

known or estimated values of the 13-month smoothed 10.7 cm solar flux. MET-SAM is not available for release, so an output summary table of expected density variations is included in table 67. 

The data in this table present the statistics of the global (spatial) maximum densities for averaging 

periods as short as 3 hr, the temporal resolution of the ap dataset. Data in table 67 is reformatted 

from table C-1 in reference 102.
163

Table 67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range.

Height

(km)

Frequency

(percentile)

F10.7 Range

Bin 1

66–102

Bin 2

102–138

Bin 3

138–174

Bin 4

174–210

Bin 5

210–246

All 6

66–246

250 1 4.39 × 10–11 6.22 × 10–11 7.67 × 10–11 8.89 × 10–11 1.11 × 10–10 4.56 × 10–11

250 5 4.73 × 10–11 6.7 × 10–11 8.44 × 10–11 9.91 × 10–11 1.16 × 10–10 5.07 × 10–11

250 33 5.57 × 10–11 7.91 × 10–11 9.94 × 10–11 1.18 × 10–10 1.31 × 10–10 6.91 × 10–11

250 50 5.94 × 10–11 8.41 × 10–11 1.05 × 10–10 1.24 × 10–10 1.36 × 10–10 8.63 × 10–11

250 67 6.39 × 10–11 8.99 × 10–11 1.11 × 10–10 1.3 × 10–10 1.42 × 10–10 1.06 × 10–10

250 95 7.76 × 10–11 1.08 × 10–10 1.26 × 10–10 1.45 × 10–10 1.58 × 10–10 1.39 × 10–10

250 99 8.67 × 10–11 1.19 × 10–10 1.36 × 10–10 1.53 × 10–10 1.63 × 10–10 1.53 × 10–10

250 100 1.2 × 10–11 1.53 × 10–10 1.52 × 10–10 1.7 × 10–10 1.74 × 10–10 1.74 × 10–10

275 1 2.19 × 10–11 3.30 × 10–11 4.23 × 10–11 5.05 × 10–11 6.6 × 10–11 2.29 × 10–11

275 5 2.39 × 10–11 3.6 × 10–11 4.74 × 10–11 5.76 × 10–11 7.02 × 10–11 2.59 × 10–11

275 33 2.89 × 10–11 4.39 × 10–11 5.78 × 10–11 7.11 × 10–11 8.17 × 10–11 3.73 × 10–11

275 50 3.12 × 10–11 4.72 × 10–11 6.18 × 10–11 7.60 × 10–11 8.57 × 10–11 4.87 × 10–11

275 67 3.4 × 10–11 5.11 × 10–11 6.62 × 10–11 8.07 × 10–11 9.08 × 10–11 6.22 × 10–11

275 95 4.28 × 10–11 6.38 × 10–11 7.79 × 10–11 9.28 × 10–11 1.04 × 10–10 8.82 × 10–11

275 99 4.89 × 10–11 7.26 × 10–11 8.52 × 10–11 1 × 10–10 1.09 × 10–10 9.93 × 10–11

275 100 7.33 × 10–11 9.94 × 10–11 9.86 × 10–11 1.15 × 10–10 1.18 × 10–10 1.18 × 10–10

300 1 1.15 × 10–11 1.83 × 10–11 2.44 × 10–11 2.99 × 10–11 4.1 × 10–11 1.21 × 10–11

300 5 1.27 × 10–11 2.03 × 10–11 2.78 × 10–11 3.49 × 10–11 4.41 × 10–11 1.39 × 10–11

300 33 1.58 × 10–11 2.54 × 10–11 3.51 × 10–11 4.47 × 10–11 5.29 × 10–11 2.11 × 10–11

300 50 1.72 × 10–11 2.77 × 10–11 3.79 × 10–11 4.84 × 10–11 5.61 × 10–11 2.87 × 10–11

300 67 1.9 × 10–11 3.04 × 10–11 4.11 × 10–11 5.21 × 10–11 6.02 × 10–11 3.82 × 10–11

300 95 2.47 × 10–11 3.94 × 10–11 4.99 × 10–11 6.18 × 10–11 7.1 × 10–11 5.8 × 10–11

300 99 2.89 × 10–11 4.59 × 10–11 5.56 × 10–11 6.78 × 10–11 7.52 × 10–11 6.72 × 10–11

300 100 4.64 × 10–11 6.74 × 10–11 6.67 × 10–11 8.08 × 10–11 8.41 × 10–11 8.41 × 10–11

325 1 6.26 × 10–12 1.05 × 10–11 1.45 × 10–11 1.83 × 10–11 2.62 × 10–11 6.63 × 10–12

325 5 7 × 10–12 1.18 × 10–11 1.69 × 10–11 2.18 × 10–11 2.85 × 10–11 7.76 × 10–12

325 33 8.92 × 10–12 1.53 × 10–11 2.2 × 10–11 2.9 × 10–11 3.53 × 10–11 1.24 × 10–11

325 50 9.84 × 10–12 1.68 × 10–11 2.4 × 10–11 3.18 × 10–11 3.78 × 10–11 1.75 × 10–11

325 67 1.1 × 10–11 1.86 × 10–11 2.64 × 10–11 3.47 × 10–11 4.1 × 10–11 2.42 × 10–11

325 95 1.48 × 10–11 2.51 × 10–11 3.3 × 10–11 4.23 × 10–11 5 × 10–11 3.93 × 10–11

325 99 1.76 × 10–11 2.99 × 10–11 3.74 × 10–11 4.73 × 10–11 5.36 × 10–11 4.68 × 10–11

325 100 3.03 × 10–11 4.69 × 10–11 4.63 × 10–11 5.85 × 10–11 6.15 × 10–11 6.15 × 10–11

350 1 3.5 × 10–12 6.25 × 10–12 8.92 × 10–12 1.15 × 10–11 1.72 × 10–11 3.74 × 10–12

350 5 3.97 × 10–12 7.09 × 10–12 1.05 × 10–11 1.40 × 10–11 1.89 × 10–11 4.45 × 10–12

350 33 5.19 × 10–12 9.41 × 10–12 1.41 × 10–11 1.93 × 10–11 2.41 × 10–11 7.46 × 10–12

350 50 5.79 × 10–12 1.05 × 10–11 1.56 × 10–11 2.14 × 10–11 2.6 × 10–11 1.1 × 10–11

350 67 6.53 × 10–12 1.18 × 10–11 1.73 × 10–11 2.36 × 10–11 2.86 × 10–11 1.57 × 10–11

350 95 9.09 × 10–12 1.64 × 10–11 2.23 × 10–11 2.97 × 10–11 3.59 × 10–11 2.72 × 10–11
164

Height

(km)

Frequency

(percentile)

F10.7 Range

Bin 1

66–102

Bin 2

102–138

Bin 3

138–174

Bin 4

174–210

Bin 5

210–246

All 6

66–246

350 99 1.1 × 10–11 1.99 × 10–11 2.57 × 10–11 3.37 × 10–11 3.9 × 10–11 3.33 × 10–11

350 100 2.03 × 10–11 3.34 × 10–11 3.29 × 10–11 4.32 × 10–11 4.58 × 10–11 4.58 × 10–11

375 1 2.01 × 10–12 3.78 × 10–12 5.6 × 10–12 7.43 × 10–12 1.16 × 10–11 2.15 × 10–12

375 5 2.3 × 10–12 4.35 × 10–12 6.71 × 10–12 9.2 × 10–12 1.28 × 10–11 2.6 × 10–12

375 33 3.09 × 10–12 5.94 × 10–12 9.27- × 10–12 1.31 × 10–11 1.67 × 10–11 4.6 × 10–12

375 50 3.48 × 10–12 6.67 × 10–12 1.04 × 10–11 1.47 × 10–11 1.83 × 10–11 7.01 × 10–12

375 67 3.97 × 10–12 7.58 × 10–12 1.16 × 10–11 1.64 × 10–11 2.03 × 10–11 1.05 × 10–11

375 95 5.72 × 10–12 1.09 × 10–11 1.54 × 10–11 2.12 × 10–11 2.63 × 10–11 1.92 × 10–11

375 99 7.07 × 10–12 1.36 × 10–11 1.8 × 10–11 2.44 × 10–11 2.89 × 10–11 2.41 × 10–11

375 100 1.38 × 10–11 2.42 × 10–11 2.38 × 10–11 3.24 × 10–11 3.47 × 10–11 3.47 × 10–11

400 1 1.17 × 10–12 2.33 × 10–12 3.58 × 10–12 4.87 × 10–12 7.88 × 10–12 1.26 × 10–12

400 5 1.36 × 10–12 2.71 × 10–12 4.36 × 10–12 6.15 × 10–12 8.83 × 10–12 1.55 × 10–12

400 33 1.87 × 10–12 3.81 × 10–12 6.19 × 10–12 9.04 × 10–12 1.18 × 10–11 2.89 × 10–12

400 50 2.13 × 10–12 4.33 × 10–12 6.99 × 10–12 1.03 × 10–11 1.3 × 10–11 4.57 × 10–12

400 67 2.46 × 10–12 4.98 × 10–12 7.93 × 10–12 1.16 × 10–11 1.46 × 10–11 7.06 × 10–12

400 95 3.66 × 10–12 7.4 × 10–12 1.08 × 10–11 1.53 × 10–11 1.95 × 10–11 1.38 × 10–11

400 99 4.61 × 10–12 9.4 × 10–12 1.28 × 10–11 1.8 × 10–11 2.16 × 10–11 1.77 × 10–11

400 100 9.59 × 10–12 1.78 × 10–11 1.74 × 10–11 2.46 × 10–11 2.66 × 10–11 2.66 × 10–11

425 1 6.89 × 10–13 1.46 × 10–12 2.32 × 10–12 3.24 × 10–12 5.45 × 10–12 7.49 × 10–13

425 5 8.1 × 10–13 1.72 × 10–12 2.87 × 10–12 4.17 × 10–12 6.17 × 10–12 9.38 × 10–13

425 33 1.15 × 10–12 2.48 × 10–12 4.2 × 10–12 6.33 × 10–12 8.47 × 10–12 1.84 × 10–12

425 50 1.32 × 10–12 2.85 × 10–12 4.79 × 10–12 7.26 × 10–12 9.41 × 10–12 3.02 × 10–12

425 67 1.55 × 10–12 3.31 × 10–12 5.49 × 10–12 8.26 × 10–12 1.07 × 10–11 4.84 × 10–12

425 95 2.37 × 10–12 5.1 × 10–12 7.66 × 10–12 1.12 × 10–11 1.46 × 10–11 1 × 10–11

425 99 3.05 × 10–12 6.6 × 10–12 9.27 × 10–12 1.34 × 10–11 1.64 × 10–11 1.32 × 10–11

425 100 6.75 × 10–12 1.32 × 10–11 1.29 × 10–11 1.89 × 10–11 2.06 × 10–11 2.06 × 10–11

450 1 4.13 × 10–13 9.24 × 10–13 1.52 × 10–12 2.18 × 10–12 3.82 × 10–12 4.51 × 10–13

450 5 4.91 × 10–13 1.1 × 10–12 1.91 × 10–12 2.86 × 10–12 4.36 × 10–12 5.75 × 10–13

450 33 7.13 × 10–13 1.64 × 10–12 2.88 × 10–12 4.48 × 10–12 6.14 × 10–12 1.18 × 10–12

450 50 8.3 × 10–13 1.9 × 10–12 3.32 × 10–12 5.2 × 10–12 6.87 × 10–12 2.02 × 10–12

450 67 9.83 × 10–13 2.23 × 10–12 3.85 × 10–12 5.97 × 10–12 7.88 × 10–12 3.36 × 10–12

450 95 1.56 × 10–12 3.55 × 10–12 5.5 × 10–12 8.32 × 10–12 1.11 × 10–11 7.34 × 10–12

450 99 2.04 × 10–12 4.69 × 10–12 6.77 × 10–12 1.01 × 10–11 1.26 × 10–11 9.88 × 10–12

450 100 4.8 × 10–12 9.91 × 10–12 9.71 × 10–12 1.47 × 10–11 1.61 × 10–11 1.61 × 10–11

475 1 2.5 × 10–13 5.91 × 10–13 1.01 × 10–12 1.48 × 10–12 2.7 × 10–12 2.75 × 10–13

475 5 3.01 × 10–13 7.14 × 10–13 1.29 × 10–12 1.98 × 10–12 3.12 × 10–12 3.56 × 10–13

475 33 4.48 × 10–13 1.09 × 10–12 2 × 10–12 3.21 × 10–12 4.49 × 10–12 7.71 × 10–13

475 50 5.27 × 10–13 1.28 × 10–12 2.33 × 10–12 3.76 × 10–12 5.07 × 10–12 1.37 × 10–12

Table 67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range (Continued).
165

Height

(km)

Frequency

(percentile)

F10.7 Range

Bin 1

66–102

Bin 2

102–138

Bin 3

138–174

Bin 4

174–210

Bin 5

210–246

All 6

66–246

475 67 6.32 × 10–13 1.52 × 10–12 2.72 × 10–12 4.36 × 10–12 5.88 × 10–12 2.36 × 10–12

475 95 1.04 × 10–12 2.5 × 10–12 4 × 10–12 6.23 × 10–12 8.47 × 10–12 5.45 × 10–12

475 99 1.38 × 10–12 3.37 × 10–12 4.99 × 10–12 7.64 × 10–12 9.7 × 10–12 7.49 × 10–12

475 100 3.45 × 10–12 7.52 × 10–12 7.35 × 10–12 1.15 × 10–11 1.27 × 10–11 1.27 × 10–11

500 1 1.54 × 10–13 3.82 × 10–13 6.73 × 10–13 1.02 × 10–12 1.93 × 10–12 1.71 × 10–13

500 5 1.87 × 10–13 4.67 × 10–13 8.75 × 10–13 1.39 × 10–12 2.25 × 10–12 2.24 × 10–13

500 33 2.85 × 10–13 7.33 × 10–13 1.4 × 10–12 2.32 × 10–12 3.32 × 10–12 5.07 × 10–13

500 50 3.38 × 10–13 8.68 × 10–13 1.64 × 10–12 2.74 × 10–12 3.78 × 10–12 9.32 × 10–13

500 67 4.1 × 10–13 1.05 × 10–12 1.94 × 10–12 3.22 × 10–12 4.42 × 10–12 1.67 × 10–12

500 95 6.94 × 10–13 1.77 × 10–12 2.93 × 10–12 4.7 × 10–12 6.53 × 10–12 4.07 × 10–12

500 99 9.43 × 10–13 2.44 × 10–12 3.71 × 10–12 5.85 × 10–12 7.55 × 10–12 5.73 × 10–12

500 100 2.51 × 10–12 5.75 × 10–12 5.61 × 10–12 9.06 × 10–12 1.01 × 10–11 1.01 × 10–11

550 1 6.24 × 10–14 1.65 × 10–13 3.08 × 10–13 4.88 × 10–13 1 × 10–12 6.91 × 10–14

550 5 7.61 × 10–14 2.05 × 10–13 4.13 × 10–13 6.92 × 10–13 1.19 × 10–12 9.21 × 10–14

550 33 1.19 × 10–13 3.39 × 10–13 7 × 10–13 1.24 × 10–12 1.85 × 10–12 2.25 × 10–13

550 50 1.45 × 10–13 4.09 × 10–13 8.39 × 10–13 1.49 × 10–12 2.13 × 10–12 4.44 × 10–13

550 67 3.22 × 10–13 9.13 × 10–13 1.85 × 10–12 3.28 × 10–12 4.69 × 10–12 1.3 × 10–12

550 95 4.63 × 10–13 1.32 × 10–12 2.45 × 10–12 4.23 × 10–12 6.1 × 10–12 1.48 × 10–12

550 99 5.95 × 10–13 1.72 × 10–12 2.93 × 10–12 4.99 × 10–12 6.8 × 10–12 3.86 × 10–12

550 100 1.49 × 10–12 3.84 × 10–12 4.18 × 10–12 7.24 × 10–12 7.55 × 10–12 5.86 × 10–12

600 1 2.78 × 10–14 7.48 × 10–14 1.47 × 10–13 2.42 × 10–13 5.36 × 10–13 3.07 × 10–14

600 5 3.38 × 10–14 9.47 × 10–13 2.02 × 10–13 3.55 × 10–13 6.49 × 10–13 4.1 × 10–14

600 33 5.35 × 10–14 1.63 × 10–13 3.6 × 10–13 6.75 × 10–13 1.06 × 10–12 1.04 × 10–13

600 50 6.53 × 10–14 2 × 10–13 4.4 × 10–13 8.33 × 10–13 1.24 × 10–12 2.18 × 10–13

600 67 1.47 × 10–13 4.5 × 10–13 9.82 × 10–13 1.85 × 10–12 2.76 × 10–12 6.66 × 10–13

600 95 2.17 × 10–13 6.84 × 10–13 1.34 × 10–12 2.47 × 10–12 3.71 × 10–12 7.76 × 10–13

600 99 2.87 × 10–13 9.19 × 10–13 1.65 × 10–12 2.98 × 10–12 4.2 × 10–12 2.31 × 10–12

600 100 8.09 × 10–13 2.3 × 10–12 2.48 × 10–12 4.55 × 10–12 5.51 × 10–12 4.49 × 10–12

650 1 1.42 × 10–14 3.64 × 10–14 7.28 × 10–14 1.24 × 10–13 2.93 × 10–13 1.55 × 10–14

650 5 1.7 × 10–14 4.63 × 10–14 1.02 × 10–13 1.88 × 10–13 3.62 × 10–13 2.02 × 10–14

650 33 2.62 × 10–14 8.12 × 10–14 1.9 × 10–13 3.77 × 10–13 6.17 × 10–13 5.11 × 10–14

650 50 3.18 × 10–14 1.01 × 10–13 2.37 × 10–13 4.76 × 10–13 7.37 × 10–13 1.11 × 10–13

650 67 7.14 × 10–14 2.3 × 10–13 5.34 × 10–13 1.07 × 10–12 1.65 × 10–12 3.52 × 10–13

650 95 1.07 × 10–13 3.64 × 10–13 7.57 × 10–13 1.47 × 10–12 2.65 × 10–12 4.19 × 10–13

650 99 1.45 × 10–13 5.06 × 10–13 9.55 × 10–13 1.82 × 10–12 2.78 × 10–12 1.42 × 10–12

650 100 4.52 × 10–13 1.41 × 10–12 1.51 × 10–12 2.91 × 10–12 3.56 × 10–12 2.93 × 10–12

700 1 8.27 × 10–15 1.93 × 10–14 3.81 × 10–14 6.59 × 10–14 1.64 × 10–13 8.96 × 10–15

700 5 9.67 × 10–15 2.43 × 10–14 5.38 × 10–14 1.02 × 10–13 2.05 × 10–13 1.13× 10–14

Table 67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range (Continued).
166

Height

(km)

Frequency

(percentile)

F10.7 Range

Bin 1

66–102

Bin 2

102–138

Bin 3

138–174

Bin 4

174–210

Bin 5

210–246

All 6

66–246

700 33 1.42 × 10–14 4.26 × 10–14 1.03 × 10–13 2.15 × 10–13 3.66 × 10–13 2.67 × 10–14

700 50 1.7 × 10–14 5.32 × 10–14 1.3 × 10–13 2.76 × 10–13 4.45 × 10–13 5.87 × 10–14

700 67 3.78 × 10–14 1.22 × 10–13 2.97 × 10–13 6.26 × 10–13 1.01 × 10–12 1.92 × 10–13

700 95 5.66 × 10–14 1.99 × 10–13 4.34 × 10–13 8.92 × 10–13 1.45 × 10–12 2.34 × 10–13

700 99 7.66 × 10–14 2.85 × 10–13 5.63 × 10–13 1.13 × 10–12 1.7 × 10–12 8.85 × 10–13

700 100 2.58 × 10–13 8.86 × 10–13 9.33 × 10–13 1.92 × 10–12 2.39 × 10–12 2 × 10–12

750 1 5.48 × 10–15 1.14 × 10–14 2.14 × 10–14 3.67 × 10–14 9.43 × 10–14 5.86 × 10–15

750 5 6.24 × 10–15 1.4 × 10–14 3 × 10–14 5.74 × 10–14 1.2 × 10–13 7.08 × 10–15

750 33 8.65 × 10–15 2.38 × 10–14 5.82 × 10–14 1.26 × 10–13 2.22 × 10–13 1.53 × 10–14

750 50 1.01 × 10–14 2.97 × 10–14 7.41 × 10–14 1.64 × 10–13 2.72 × 10–13 3.27 × 10–14

750 67 2.23 × 10–14 6.79 × 10–14 1.7 × 10–13 3.75 × 10–13 6.23 × 10–13 1.08 × 10–13

750 95 3.23 × 10–14 1.13 × 10–13 2.56 × 10–13 5.52 × 10–13 9.32 × 10–13 1.36 × 10–13

750 99 4.33 × 10–14 1.66 × 10–13 3.39 × 10–13 7.15 × 10–13 1.11 × 10–12 5.66 × 10–13

750 100 1.52 × 10–13 5.66 × 10–13 5.9 × 10–13 1.27 × 10–12 1.6 × 10–12 1.36 × 10–12

800 1 3.94 × 10–15 7.33 × 10–15 1.29 × 10–14 2.16 × 10–14 5.58 × 10–14 4.17 × 10–15

800 5 4.4 × 10–15 8.79 × 10–15 1.78 × 10–14 3.36 × 10–14 7.14 × 10–14 4.9 × 10–15

800 33 5.8 × 10–15 1.43 × 10–14 3.41 × 10–14 7.51 × 10–14 1.36 × 10–13 9.54 × 10–15

800 50 6.63 × 10–15 1.76 × 10–14 4.35 × 10–14 9.91 × 10–14 1.7 × 10–13 1.93 × 10–14

800 67 1.44 × 10–14 4.01 × 10–14 1 × 10–13 2.28 × 10–13 3.92 × 10–13 6.38 × 10–14

800 95 2 × 10–14 6.66 × 10–14 1.54 × 10–13 3.47 × 10–13 6.06 × 10–13 8.2 × 10–14

800 99 2.62 × 10–14 9.93 × 10–14 2.08 × 10–13 4.6 × 10–13 7.31 × 10–13 3.67 × 10–13

800 100 9.2 × 10–14 3.68 × 10–13 3.8 × 10–13 8.7 × 10–13 1.1 × 10–12 9.52 × 10–13

850 1 2.99 × 10–15 5.17 × 10–15 8.49 × 10–15 1.36 × 10–14 3.42 × 10–14 3.14 × 10–15

850 5 3.3 × 10–15 6.05 × 10–15 1.13 × 10–14 2.07 × 10–14 4.39 × 10–14 3.64 × 10–15

850 33 4.22 × 10–15 9.28 × 10–15 2.1 × 10–14 4.62 × 10–14 8.56 × 10–14 6.49 × 10–15

850 50 4.74 × 10–15 1.12 × 10–14 2.67 × 10–14 6.15 × 10–14 1.08 × 10–13 1.22 × 10–14

850 67 1.02 × 10–14 2.53 × 10–14 6.14 × 10–14 1.43 × 10–13 2.51 × 10–13 3.95 × 10–14

850 95 1.35 × 10–14 4.13 × 10–14 9.56 × 10–14 2.22 × 10–13 4.01 × 10–13 5.21 × 10–14

850 99 1.71 × 10–14 6.16 × 10–14 1.31 × 10–13 3 × 10–13 4.9 × 10–13 2.42 × 10–13

850 100 5.75 × 10–14 2.43 × 10–13 2.48 × 10–13 5.94 × 10–13 7.59 × 10–13 6.63 × 10–13

900 1 2.33 × 10–15 3.86 × 10–15 5.98 × 10–15 9.11 × 10–15 2.18 × 10–14 2.45 × 10–15

900 5 2.56 × 10–15 4.44 × 10–15 7.74 × 10–15 1.35 × 10–14 2.79 × 10–14 2.81 × 10–15

900 33 3.22 × 10–15 6.48 × 10–15 1.36 × 10–14 2.94 × 10–14 5.5 × 10–14 4.72 × 10–15

900 50 3.58 × 10–15 7.67 × 10–15 1.71 × 10–14 3.92 × 10–14 6.97 × 10–14 8.28 × 10–15

900 67 7.62 × 10–15 1.71 × 10–14 3.92 × 10–14 9.11 × 10–14 1.63 × 10–13 2.58 × 10–14

900 95 9.73 × 10–15 2.69 × 10–14 6.11 × 10–14 1.44 × 10–13 2.68 × 10–13 3.48 × 10–14

900 99 1.2 × 10–14 3.97 × 10–14 8.45 × 10–14 1.99 × 10–13 3.33 × 10–13 1.62 × 10–13

900 100 3.71 × 10–14 1.63 × 10–13 1.65 × 10–13 4.17 × 10–13 5.38 × 10–13 4.76 × 10–13

Table 67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range (Continued).
167

Height

(km)

Frequency

(percentile)

F10.7 Range

Bin 1

66–102

Bin 2

102–138

Bin 3

138–174

Bin 4

174–210

Bin 5

210–246

All 6

66–246

950 1 1.86 × 10–15 3.01 × 10–15 4.48 × 10–15 6.51 × 10–15 1.46 × 10–14 1.95 × 10–15

950 5 2.04 × 10–15 3.43 × 10–15 5.63 × 10–15 9.26 × 10–15 1.85 × 10–14 2.23 × 10–15

950 33 2.54 × 10–15 4.8 × 10–15 9.37 × 10–15 1.94 × 10–14 3.62 × 10–14 3.63 × 10–15

950 50 2.8 × 10–15 5.58 × 10–15 1.16 × 10–14 2.58 × 10–14 4.61 × 10–14 5.98 × 10–15

950 67 5.95 × 10–15 1.23 × 10–14 2.64 × 10–14 6 × 10–14 1.08 × 10–13 1.78 × 10–14

950 95 7.39 × 10–15 1.85 × 10–14 4.05 × 10–14 9.62 × 10–14 1.82 × 10–13 2.43 × 10–14

950 99 8.85 × 10–15 2.67 × 10–14 5.61 × 10–14 1.35 × 10–13 2.29 × 10–13 1.1 × 10–13

950 100 2.49 × 10–14 1.11 × 10–13 1.12 × 10–13 2.91 × 10–13 3.79 × 10–13 3.39 × 10–13

1,000 1 1.5 × 10–15 2.41 × 10–15 3.50 × 10–15 4.89 × 10–15 1.02 × 10–14 1.57 × 10–15

1,000 5 1.64 × 10–15 2.73 × 10–15 4.3 × 10–15 6.73 × 10–15 1.27 × 10–14 1.8 × 10–15

1,000 33 2.04 × 10–15 3.73 × 10–15 6.81 × 10–15 1.34 × 10–14 2.45 × 10–14 2.87 × 10–15

1,000 50 2.25 × 10–15 4.27 × 10–15 8.26 × 10–15 1.76 × 10–14 3.12 × 10–14 4.54 × 10–15

1,000 67 4.77 × 10–15 9.3 × 10–15 1.86 × 10–14 4.07 × 10–14 7.36 × 10–14 1.29 × 10–14

1,000 95 5.83 × 10–15 1.34 × 10–14 2.79 × 10–14 6.55 × 10–14 1.26 × 10–13 1.77 × 10–14

1,000 99 6.83 × 10–15 1.88 × 10–14 3.84 × 10–14 9.26 × 10–14 1.6 × 10–13 7.63 × 10–14

1,000 100 1.74 × 10–14 7.67 × 10–14 7.71 × 10–14 2.09 × 10–13 2.75 × 10–13 2.48 × 10–13

Use of the data in this table also makes it possible for the design engineers to easily accomplish any required trade studies while managers will be able to determine just what risks they will 

be taking. For applications that require density values other than the global maximum or for 

questions concerning the application of the data in engineering analyses, contact personnel of 

MSFC Natural Environments Branch. Table 67 also shows the median value of the global maximum density for the altitude and the 10.7 cm solar flux bin indicated, computed over the POR. It 

also gives the median (50th percentile) global maximum density and several other percentile levels 

of the global maximum density. Percentile refers to the fraction of time the density was equal to 

or less than the indicated value. These data may be used directly to indicate the magnitude of the 

variations in the global maximum density that occurs over a period of a few days. If appropriate 

for the application, an additional increment may be added to account for model inaccuracies and 

variations related to other sources. To obtain a conservative (upper limit) estimate, recommendation is made to assume that these additional variations are Gaussian with a standard deviation 

equal to 0.2 times the total density. To understand the frequency of occurrence of these variations, 

table 68 provides the probabilities of encountering intervals of 10 and 30 days without exceeding 

the indicated percentile level. (As with table 67, only solar and geomagnetic variations are considered.)

Table 67. Global maximum density (kg/m3) for various frequencies of occurrence 

(percentile) versus altitude F10.7 range (Continued).
168

Table 68. Probabilities of achieving a time interval without encountering thermospheric 

density level above a given percentile value.

Probability of Going at Least 10 Days Without Exceeding the Indicated 

Percentile Density Level (Random Start)

13-mo Percentile Level

Smoothed F10.7 Range 66% 90% 95% 99%

66 to 102 0.13 0.46 0.63 0.86

102 to 138 0.21 0.55 0.71 0.9

138 to 174 0.28 0.55 0.68 0.89

174 to 210 0.35 0.6 0.75 0.93

210 to 246 0.38 0.65 0.76 0.87

Probability of Going at Least 30 Days Without Exceeding the Indicated

Percentile Density Level (Random Start)

13-mo Percentile Level

Smoothed F10.7 Range 66% 90% 95% 99%

66 to 102 0.03 0.23 0.4 0.72

102 to 138 0.05 0.28 0.48 0.76

138 to 174 0.08 0.33 0.44 0.77

174 to 210 0.12 0.37 0.52 0.82

210 to 246 0.12 0.38 0.57 0.75

Probabilities for other time intervals can be estimated from figure 66. If one of these higher percentile levels is exceeded, it typically drops back down within a short time (hours, a few days at most). 

The distributions are highly skewed; once a level has been exceeded, it is usually exceeded again 

within a few days. On the other hand, if the Sun is quiet, it may remain so for several months.

5.1.5.3 Earth Global Reference Atmospheric Model. The Earth-GRAM that was developed 

by MSFC’s Natural Environments Branch provides a single continuous representation of the neutral atmosphere from the Earth’s surface to orbital altitudes. Earth-GRAM is described more fully 

in section 3.9 and in reference 103.

5.1.6 Spacecraft Effects

Density of the neutral gas is the primary atmospheric property that affects a spacecraft’s 

orbital altitude, lifetime, and motion. Even though space is thought of as a vacuum, there is 

enough matter to impart a substantial drag force on orbiting spacecraft. Unless the vehicle’s propulsion system compensates for this drag force, the altitude will decay until reentry occurs. Density 

effects also directly contribute to the torques experienced by the spacecraft due to the aerodynamic 

interaction between the spacecraft and the atmosphere, and thus must be considered in the design 

of the spacecraft attitude control system.
169

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

newF66

Probability

0 5 10 15 20 25 30 35 40 45 50

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

Probability

0 5 10 15 20 25 30 35 40 45 50

Minimum Interval to Exceedance (days)

Bin 1

Bin 2

Bin 3

Bin 4

Bin 5

Bin 1

Bin 2

Bin 3

Bin 4

Bin 5

Minimum Interval to Exceedance (days)

(a)

(b)

Figure 66. Probability of meeting or exceeding a given time interval without exceeding 

the (a) 95th percentile density or (b) 99th percentile density (bins represent 

a range of F10.7: (1) 66–102, (2) 102–138, (3) 138–174, (4) 174–210, 

and (5) 210–246).

Many materials used on spacecraft surfaces are susceptible to attack by AO, a major constituent of the LEO thermosphere region. Due to photodissociation, oxygen exists predominantly 

in the atomic form and varies with altitude, solar activity, and other effects discussed in section 5.1.2. Simultaneous exposure to the solar UV radiation, micrometeoroid impact damage, sputtering, or contamination effects can aggravate the AO effects, leading to serious deterioration of 

mechanical, optical, and thermal properties of some material surfaces. Spacecraft glow is a related 

phenomenon, which may be of concern for optically sensitive experiments. Optical emissions are 

generated from metastable molecules that have been excited by impact on the surface of the spacecraft. Investigations show that the surface acts as a catalyst, thus the intensity is dependent on the 

type of surface material.
170

5.1.7 Atomic Oxygen

In the thermosphere, AO is formed by solar UV radiation dissociating oxygen molecules 

into free oxygen atoms and is the dominant neutral species from approximately 100–500 km 

altitude dependent on solar activity (fig. 60). At high solar activity AO can be ≈90% of the atmosphere at 500 km. Oxygen atoms alone are highly corrosive, combining with most materials they 

encounter. At LEO velocities, the AO flux experienced by a spacecraft is essentially a stream of AO 

with energies of ≈5 eV. Since the degree of spacecraft surface degradation is directly proportional to 

the total integrated flux or fluence, a spacecraft should be provided with AO protection optimized 

for the AO environment in which they will operate for the planned mission life, plus an operating 

margin to allow for variations in orbital conditions or for reasonable mission extensions. Thus, 

spacecraft operating for short periods in LEO and spacecraft bound for high orbits or interplanetary trajectories may require far less AO protection than spacecraft designed to operate for long 

durations in LEO.

The AO fluence for long-duration missions depends on several mission parameters including spacecraft altitude, attitude, orbital inclination, and mission duration. The spacecraft AO flux 

present during a mission can vary significantly for mission durations of a few months to years 

and thus affect AO fluence calculations. Thermosphere density variations such as the semiannual 

variation and those associated with solar activity (sec. 5.1.2) can be a major contributor to AO 

flux variations in addition to spacecraft altitude changes. To capture these variations, the AO flux 

can be determined from the MET or MSIS thermosphere models at the location of the spacecraft 

for appropriate time intervals and integrated over the mission timeline to obtain the AO fluence. 

This technique was used to determine the AO requirement for the International Space Station 

(ISS) by using the baseline 90 days to 277.8 km altitude strategy.104 An AO density time profile was 

determined for the lifetime of the ISS using the MSIS thermosphere model and then integrated to 

yield an annual average AO ram fluence value of 5×1021 particles/cm2-yr. For periods on the order 

of 30 days or less, short-term (daily) density variations become important in AO flux calculations. 

For the ISS, a short-term daily ram fluence value of 4.4×1019 particles/cm2-day was 

determined.

5.2 Thermal Environment

A summary of key design factors for the thermal environment is displayed in table 69.

The thermal environment definition provided here was extracted from NASA Technical 

Memorandum NASA/TM—2001–211221,105 which was based on analysis of the extensive database available from the Earth Radiation Budget Experiment (ERBE).106–108 Twenty-eight monthly 

datasets of 16-s resolution wide field data were obtained and analyzed to produce a new set of environmental parameters. One parameter set was derived specifically for 51.6° inclination orbits and 

incorporated into the environment definition and requirements for the ISS program.109,110 More 

general results keyed to three orbital inclination ranges, 0° to 30°, 30° to 60°, and >60°, were documented in NASA/TM—2001–211221,105 and these are repeated here. Because they were derived 

from the same datasets with essentially identical methodology, the definition presented here should 

lead to system designs that are fully compatible with ISS even though there are slight differences 

due to the different inclination ranges.
171

Table 69. Summary of key design factors.

Parameter Source Select Based On

For estimates of long-term average and hot/cold conditions:

Solar constant Section 5.2.1.1 Season of year or desired extreme

Albedo / long-wave radiance 

pair(s) 

Tables 72–74 Orbit inclination, α /ε ratio, desired 

extreme and averaging time 

Albedo correction Table 71 Orbital beta angle

For evaluation of thermal time constant dependent conditions (see secs. 5.2.2.4 to 5.2.2.6)

Solar constant Section 5.2.1.1 Season of year or desired extreme

Albedo / long-wave radiance 

pair(s)

Tables 72–74; or for noncritical 

applications, table 75

Orbit inclination, α/ε ratio, system 

criticality, desired extreme and 

averaging time. STEM analysis 

may be helpful

Albedo correction Table 76 and section 5.2.2.2.1 Orbital beta angle or SZA

Technically, the work with the ERBE data was undertaken to provide a more detailed 

statistical description of the variations of the thermal environment than previously was available. 

Greater use of lightweight structures sensitive to rapid thermal fluctuations greatly increased the 

importance of this information. Also, since application of these results depends upon the thermal 

time constants of the system being designed, a companion document, Simple Thermal Environment Model (STEM),111 was developed to aid in selecting environment parameters that most 

appropriately apply to a specific application.

5.2.1 Fundamental Thermal Environment Parameters

Space vehicles in Earth orbit receive radiant thermal energy from three sources and reflect 

or radiate it to the cold sink of space. The three primary sources are the incoming solar radiation, 

Earth-reflected solar energy (albedo radiation), and outgoing long-wave radiation (OLR) emitted by the Earth and atmosphere. If one considers the Earth and its atmosphere as a whole and 

averages over long periods, the incoming solar energy and outgoing long-wave radiant energy are 

essentially in balance; the Earth/atmosphere is very nearly in radiative equilibrium with the Sun. 

However, it is not in balance everywhere on the globe and important variations exist with respect 

to local time, geography, and atmospheric conditions. A space vehicle’s motion with respect to the 

Earth results in its viewing only a ‘swath’ across the full global thermal profile. Hence, the vehicle 

sees these variations as functions of time and responds in accordance with the thermal time constants of the hardware systems.

5.2.1.1 Solar Constant. The direct solar flux is the greatest source of heating for most 

spacecraft. The mean value of this solar flux at mean Earth-Sun distance is termed the ‘solar 

constant.’ Specifically, the solar constant is defined as the radiation that falls on a unit area of 

surface normal to the line from the Sun, per unit time and outside of the atmosphere, at 1 astronomical unit (AU) (mean Earth-Sun distance). However, as seen by an Earth-orbiting spacecraft, 

the incoming solar flux is not quite constant; two factors influence its variability. First, the amount 

of radiant energy emitted by the Sun is known to vary slightly throughout the 11-year solar cycle. 
172

The exact amount differs from cycle to cycle but is estimated to be only a fraction of a percent. 

Second, the slightly elliptical orbit of the Earth about the Sun results in a variation in the solar 

flux incident on the Earth or upon an Earth orbiting spacecraft. This ±1.7% departure from the 

mean distance leads to an approximate ±3.4% difference in radiation. That is, a few days following 

winter solstice, 3.4% more solar energy falls on a unit area normal to the line from the Sun at the 

outside of the atmosphere; just after summer solstice the amount is 3.4% less. The solar constant 

recommended in this TM corresponds to the value recommended by the World Radiation Center 

(WRC) in Davos, Switzerland, and is based on a summary of eight measurements made from 1969 

to 1980:112,113

Hot case: Shot = 1,414 W/m2

Median case: So = 1,367 W/m2 = solar constant

Cold case: Scold = 1,322 W/m2.

This variation from median to hot or cold case covers the Earth-Sun distance variation. An additional ±5 W/m2 could be added (subtracted) to account for measurement uncertainties and solar 

cycle variations, but is not included in the above values.

5.2.1.2 Solar Irradiance. Table 70 presents the solar spectral irradiance in tabular form for 

the range from 0.14 to 50 mm. This table was published in American Society for Testing and Materials Designation: E490-00a,114 entitled Standard Solar Constant and Zero Air Mass Solar Spectral 

Irradiance Tables. These tables define the zero air mass solar spectral irradiance for use in thermal 

analysis, thermal balance testing, and other tests of spacecraft and spacecraft components and 

materials. Typical applications include the calculation of solar absorptance from spectral reflectance data and the specification of solar UV exposure of materials during simulated space radiation testing. This standard does not purport to address all of the safety concerns, if any, associated 

with its use. It is the responsibility of the user of this standard to establish appropriate safety and 

health practices and determine the applicability of regulatory limitations prior to use.

5.2.1.3 Albedo. The fraction of incident solar energy reflected (or scattered) by a planet 

back into space is termed the albedo. Values typically are expressed as a fraction or percent. For 

spacecraft in LEO and in this TM, the term is more precisely defined as the ‘local bolometric’ 

albedo. Bolometric implies wavelength independence, i.e., the albedo representing the integrated 

short wavelength band. Local is used because it is a characteristic of only a small portion of the 

planetary surface; the portion viewed by a spacecraft close to Earth. Values presented in this TM 

are derived from wide field measurements made by the Earth Radiation Budget Satellite (ERBS) 

at 610 km altitude and the National Oceanic and Atmospheric Administration (NOAA) 9 and 10 

satellites at, respectively, 849 and 815 km. Altitude dependence has been removed by transforming 

the data to a standard surface, where the ‘top of atmosphere’ is considered to be approximately 

30 km above the Earth’s surface.100 ‘Top of atmosphere’ represents the virtual source of the albedo 

radiation and the outgoing long-wave radiation. 
173

Table 70. Solar spectral irradiance-standard curve, abridged version.

λ E λ D 0–λ λ E λ D 0–λ

0.14 9.833 × 10–2 – 0.57 1,797 31.39

0.16 0.3195 3.1 × 10–4 0.58 1,801 32.71

0.18 2.042 2 × 10–3 0.59 1,758 34.01

0.2 10.83 1.1 × 10–2 0.6 1,745 35.29

0.22 44.93 5.2 × 10–2 0.62 1,663 37.78

0.23 49.64 8.7 × 10–2 0.64 1,610 40.18

0.24 51.83 0.12 0.66 1,527 42.48

0.25 59.81 0.16 0.68 1,485 44.68

0.26 129.1 0.23 0.7 1,438 46.82

0.27 222.1 0.36 0.72 1,360 48.87

0.28 212.9 0.52 0.75 1,272 51.76

0.29 441 0.76 0.8 1,132 56.16

0.3 526 1.12 0.9 882.6 63.53

0.31 634.5 1.54 1 719.7 69.4

0.32 746.5 2.05 1.2 487.1 78.23

0.33 948.7 2.67 1.4 342.5 84.3

0.34 947.3 3.36 1.6 243.5 88.59

0.35 969.5 4.06 1.8 167.1 91.6

0.36 985.2 4.78 2 115 93.66

0.37 1,129 5.55 2.2 81.73 95.1

0.38 1,091 6.36 2.4 58.78 96.13

0.39 1,093 7.16 2.6 43.86 96.88

0.4 1,518 8.12 2.8 33.43 97.45

0.41 1,712 9.3 3 25.93 97.88

0.42 1,740 10.56 3.2 20.45 98.22

0.43 1,625 11.79 3.4 16.36 98.49

0.44 1,826 13.06 3.6 13.26 98.71

0.45 2,030 14.47 3.8 10.87 98.89

0.46 2,077 15.97 4 8.977 99.03

0.47 2,049 17.48 4.5 5.674 99.3

0.48 2,057 18.98 5 3.691 99.47

0.49 1,955 20.45 6 1.879 99.68

0.5 1,948 21.88 7 1.022 99.78

0.51 1,911 23.29 8 0.6041 99.84

0.52 1,806 24.65 10 0.2663 99.9

0.53 1,861 26 15 6.106 × 10–2 99.96

0.54 1,861 27.36 20 1.755 × 10–2 99.98

0.55 1,867 28.72 50 1.769 × 10–3 100

0.56 1,808 30.07

λ = wavelength (μm).

E λ = solar spectral irradiance averaged over small bandwidth centered at λ, (W • m–2 m–1).

D 0–λ = percentage of the solar constant (1,366.1 W • m–2) associated with wavelengths shorter than λ.
174

To evaluate the albedo radiation for any satellite at a known altitude the thermal analyst 

simply assumes the source is at this level, i.e., Earth radius +30 km, not Earth surface. Obviously, 

a spacecraft only receives reflected (albedo) radiation when a portion of the Earth or atmosphere 

seen by the spacecraft is sunlit. Albedo radiation has approximately the same spectral shape as the 

Sun’s spectrum, which approximates a blackbody with a characteristic temperature of 5,777 K.

Albedo is highly variable across the globe and is dependent on the distribution of reflective properties of the surface and the amount and type of cloud cover. Reflectivity increases with 

increased cloud cover. Continental areas generally have higher albedo values than ocean areas. 

Because of snow and ice cover, decreasing solar elevation angle, and increasing cloud cover, albedo 

tends to increase with latitude if viewed on a large scale. From the spacecraft design perspective, 

the most important systematic albedo variation is with SZA that, when averages are used, depends 

on the spacecraft’s beta angle. Care must be taken to correctly account for this effect, especially 

near the terminator (reference sec. 5.2.1.5 on geometric factors).

5.2.1.4 Outgoing Long-Wave Radiation. In addition to direct solar and reflected (albedo) 

solar radiation, the third primary component of a spacecraft’s thermal environment is the OLR 

emitted by the Earth itself. This Earth-emitted thermal radiation is a combination of radiation 

emitted in infrared wavelength bands by atmospheric gases and radiation emitted by the Earth’s 

surface and cloud tops but is partially absorbed in the atmosphere. Thus, the spectral distribution 

is somewhat complex. For the purpose of spacecraft thermal analysis, however, it is generally sufficient to assume a graybody spectrum corresponding to a temperature in the 250 K to 300 K range.

OLR is not constant over the globe, but the localized variations are much less severe than 

for albedo. Outgoing long-wave radiation is principally influenced by the temperature of the 

Earth’s surface and the amount of cloud cover. A warmer region of the Earth’s surface emits more 

radiation than a colder area. On a large scale, highest values of OLR occur in tropical and desert 

regions (regions of the globe receiving the maximum solar heating) and decrease with latitude. 

Increasing cloud cover tends to lower OLR because cloud tops are cold and clouds effectively block 

upwelling radiation from the Earth’s warmer surface below.

The diurnal effects on OLR as experienced by a satellite were studied in this analysis. 

For low inclination orbits (<60°), inclusion or exclusion of the nighttime data and changes in SZA 

cutoff made no significant difference in the net OLR distribution functions. Thus, for these orbits 

no special accounting for diurnal effects is needed. This is not true, however, for high-inclination 

orbits. Passage over the nighttime and lighted but high SZA polar regions contributes significantly 

to the low (cold case) OLR populations. This effect was overlooked in earlier studies, which only 

checked diurnal variations at low inclination.111,112

5.2.1.5 Geometric Factors

5.2.1.5.1 Orbital Altitude and ‘Top of Atmosphere.’ The OLR and albedo radiation 

received on a satellite surface diminishes as its altitude increases, i.e., as the satellite moves away 

from the source. This effect is accounted for as part of the ‘view factor’ in thermal calculations. 
175

Derived OLR and albedo data measurements from satellites at several altitudes (610, 815, and 

849 km) were corrected to the apparent source surface (30 km above Earth surface) or ‘top of 

atmosphere.’ Thus, in applying these data, the analyst should assume a source at Re+30 km, where 

Re is the Earth’s radius, 6,378.14 km equatorial. Failure to do so leads to a slight underestimate of 

the OLR and albedo radiation by a factor of:

Fa = Re ( ) + A 2

Re ( + 30 km + A)

2 , (48)

where A is the orbital altitude. The error is quite small (Fa=0.9911 at A=300 km) and decreases 

(Fa approaches 1) with increased altitude.

5.2.1.5.2 Orbital Inclination, Beta Angle, and Solar Zenith Angle. Orbital inclination refers 

to the angle between the Earth’s polar vector and the vector normal to the satellite’s orbit plane. 

Thus, an equatorial orbit has an inclination of zero; a perfect polar orbit has an inclination of 90°. 

The orbital beta angle is the minimum angle between the satellite’s orbit plane (the closest to a Sunpointing vector possible in the plane) and the Sun-Earth vector. The beta angle can be thought of 

as the solar elevation angle with respect to the orbit plane. The angle between the Sun-Earth vector 

and the Earth-satellite vector is termed the SZA. The SZA is zero when the Sun is directly above 

the satellite (Earth-satellite-Sun in a straight line) and 90° when a satellite is directly over the terminator. Except for special Sun synchronous cases, the SZA varies rapidly over an orbit; the minimum SZA is equal to the absolute value of the beta angle.

5.2.2 Technical Background

This TM provides recommendations on the selection of natural thermal environment 

parameters for use in design of spaceborne systems. The recommendations are based on analysis 

of data from the ERBE. The results provide a comprehensive and accurate statistical picture 

of the thermal environment encountered in LEO. In addition to the material summarized below, 

additional details are available in the STEM User’s Guide.111

5.2.2.1 Earth Radiation Budget Experiment. The ERBE is a multisatellite experiment with 

the primary objective of global data collecting such Earth radiation budget parameters as incident 

sunlight, reflected sunlight (albedo), and OLR. This experiment was selected because of its thorough coverage and high-quality data. The experiment consists of three satellites: the lowinclination ERBS and two NOAA Sun-synchronous satellites. The data used here are from the 

active cavity, flat plate radiometers in the fixed, nonscanning, and wide field-of-view mode. This 

type of instrument was chosen because it directly measures the albedo and OLR as they would be 

received by a spacecraft surface. The available, separated data sets are daily averaged values (S-4), 

hourly averaged values (S-10), and raw 16-s instrument measurements along the ERBS or NOAA 

satellite trajectory (S7). The S-4 and S-10 data products were inappropriate for this application 

because the averaging times are too long compared to the thermal time constant of typical space 

systems. Therefore, the design criteria presented below are based on 28 files, representing 1 month
176

of 16-s data each of S-7 data obtained directly from the ERBE Program Office. The measurements, 

made from November 1984 through July 1987, represented all seasons well.

5.2.2.2 Details of Application

5.2.2.2.1 Albedo Correction for Solar Zenith Angle. Often in scientific studies of the 

Earth and Earth radiation balance results are based on albedo data associated with a restricted 

range of SZA centered about zero for improved accuracy. In these cases and to a first approximation, albedo may be assumed to be independent of SZA, i.e., the scattering is Lambertian or equal 

in all directions. This approximation was also assumed in the past for most spacecraft engineering applications even though it is less appropriate for this application. For large SZA, the Earth 

reflects radiation more strongly in the forward direction than to the sides or backward. The effect 

on albedo can be significant; thus, ERBE data quality and the capability of current engineering 

analysis methods warrant an improved approach. For low and medium inclination orbits, which 

see a wide range and rapidly changing SZA, the correction is most significant for subsystems with 

short thermal time constants (<10 min). For certain Sun-synchronous orbits (those flying near the 

terminator), the SZA is always relatively large and the correction is important to even orbit-average 

albedos.

Treatment of this topic in the scientific literature is generally ‘scene specific,’ e.g., it depends 

on geographic features in the field of view, data generally not available to the design engineer. Also, 

the algorithms tested did not fully remove the zenith angle dependence from this data set. Therefore, a zenith angle correction was derived specifically for this data set in a manner specifically 

tuned to the analysis tools most commonly used for engineering analysis: Thermal Radiation Analyzer System, Thermal Desktop, and Thermal Synthesizer System. Used with this correction and 

the proper parameters for the ERBS and NOAA satellites, these tools should accurately reproduce 

the measurements.

The correction term, derived from 4 months of data restricted to the –30 to 30 latitude 

band, was verified by testing another 4 months of data to wider latitude bands. This removes 

the SZA dependence to within +0.04. The correction is:

Albedo(SZA)=albedo(SZA=0)+correction (49)

 Correction = C4(SZA)4 +C3(SZA)3 +C2(SZA)2 +C1(SZA) !

" #$ , (50)

where SZA is the solar zenith angle in degrees and the albedo is expressed as a fraction:

 C4=4.9115×10–9 (51)

 C3=6.0372×10–8 (52)

 C2=–2.1793×10–5 (53)

and

 C1=1.3798×10–3. (54)
177

Figure 67 illustrates the albedo correction term as a function of SZA. To evaluate the 

albedo at a specific SZA, this term must be added to the SZA=0 albedo values presented in the 

tables. Correction terms to obtain orbital average albedo values are provided in table 71.

0.05

0

0.1

0.15

0.2

0.25

0.3

0.35

newF67

Albedo Correction Term

0 10 20 30 40 50 60 70 80 90

Solar Zenith Angle (deg)

Figure 67. Albedo correction term, C(SZA), as a function of SZA.

Table 71. Values of orbital average albedo correction term, <c>113 (add this correction

to the SZA=0 albedo value).

Orbital Beta Angle

(°)

Orbital Average Albedo

Correction <c>

0 0.04

10 0.04

20 0.05

30 0.06

40 0.07

50 0.09

60 0.12

70 0.16

80 0.22

90 0.31
178

5.2.2.2.2 Temporal and Orbital Variations. For any satellite system, different portions of 

the hardware have different thermal response times, ranging from a few minutes to hours. Conceptually, the thermal performance of these systems could be analyzed by inputting either a real or 

simulated time series of albedo and OLR values, solar exposure, and internal heat sources, all having a time resolution finer than the shortest system time constant. In practice, however, the usual 

design objective is only to ensure that the system remains within a selected operational temperature 

range, not to model its detailed temporal variability. Thus, high resolution time series analysis is 

not warranted. The problem can be solved with adequate accuracy by modeling only the primary 

orbital variations (light to dark cycles) and assuming extreme albedo and OLR values appropriate to the thermal time constant(s) of the system. The systematic dependence of albedo on SZA, 

discussed above, must also be considered, especially for short time-constant systems.

To provide appropriate values of extreme albedo and OLR for this analysis approach, running means of the albedo and OLR variations, transformed to top of the atmosphere (30 km) to 

remove the altitude dependence, were derived from the ERBE data sets. The results are latitude 

dependent, so the choice of appropriate values for a particular mission depends on the orbital inclination of the satellite in question. Since the satellite inclination seldom equals the inclination of 

the ERBE satellites, approximation is required to deal with this mismatch. Fortunately, the latitude 

dependence is sufficiently weak that adequate resolution is obtained by dividing the possible range 

of inclinations into three regions and deriving a single set of albedo and OLR extremes for each 

region. The inclination ranges selected are 0° to 30°, >30° to 60°, and >60°. 

Running averages are presented for times ranging from a few seconds to 24 hr. Averaging periods 

to 10 days failed to show substantial variation from the 24-hr results.

There is no albedo radiation, and thus no albedo data, when the satellite is over the dark 

side of the Earth. Also, at high SZA values the albedo showed a great deal of scatter that would 

have distorted the distributions, thus these data were excluded from consideration. Thus, there 

were two data sets: a paired set containing both OLR and concurrent albedo values and a larger 

unpaired set of OLR data that included the dark side and high SZA data. Comparison of the total 

(unpaired) OLR data set with the paired OLR subset shows a significant difference in the OLR 

distributions for high inclinations. As illustrated by figure 68, the difference between the paired 

and unpaired data sets is negligible for the low inclination orbits. For the middle (30° to 60°) inclinations, the difference is becoming obvious but hardly significant for engineering applications. 

The mean differs by only 8 W/m2 between the unpaired and paired 60° data and, more importantly, 

the upper and lower limits are very similar and are also very similar to the 30° data limits. The 

separation is more significant at 90° inclinations. In this case the unpaired data set is approximately 

three times the size of the paired set. A large fraction of the data is excluded because much of the 

time the SZA is greater than 65° over polar regions. More problematic is the substantial difference 

in the extreme cold-side values, which clearly must be accounted for in the thermal design process.
179

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

newF68

Normalized Fraction per 5 W/m2 Internal

100 150 200 250 300 350

Outgoing Longwave Radiation (W/m2)

Low Inclination

Medium Inclination

Paired

Unpaired

Figure 68. OLR distributions for low inclination (≤30°) and medium inclination 

(30° to ≤60°) orbits, 128-s averaged paired and unpaired data.

5.2.2.3 Engineering Values for Critical Applications. For critical applications, the designer 

generally likes to select environment design points associated with some defined risk of occurrence 

during the mission lifetime. For example, he might select a design point associated with a 5% 

(or 1% or 0.01%...any reasonable value for the particular application) probability of being 

encountered during the design life of the system. Unfortunately, this type of information cannot be derived from the limited data set available for this study. The required data set would cover 

a very long time compared to the mission design life—so long that multiple independent samples, 

each the duration of the design life, could be drawn from the data set. A distribution function can 

then be formed of the extreme values encountered in each draw. In this work, 28 monthly data 

sets drawn from a 33-month period were studied; 10 months were sampled twice by two separate 

satellites. Typical mission design lifetimes are usually 1 to 10 years. This data set is large enough to 

provide only a distribution of extremes for mission lifetimes of approximately 1 week or less. Thus, 

this data resource is limited to a selection of design point recommendations of expected extremes 

of the distributions (one for each system time constant—averaging time) of the total data. These 

worst case values are appropriate for critical applications, i.e., situations where the temperature 

limit is not to be exceeded. Use should be coupled with a design margin selected considering the 

application, level of confidence in the design analysis, and comparison of mission life to duration 

of database.

For noncritical applications, i.e., applications where the design limit may be exceeded, 

a certain fraction of the time, less extreme environmental parameters based on the information 

provided in the next section may be selected.
180

In all cases, the percentiles indicated represent the distributions from this single data set.

Thus, they are to be associated with the fraction of time the values are expected to be exceeded; 

not the probability that they will be exceeded over a mission lifetime.

Selection of the extremes of these distributions to define critical engineering minima and 

maxima involves an arbitrary process of deciding where to cut the tails of the distributions. As 

with any data set of this type, the larger the data set the more extreme the values that appear in the 

distribution tails. Generally, the 0.04 and 99.96 percentile values (equivalent to ~ ±3.3 σ if a Gaussian distribution) were selected as engineering worst case points for practical applications. One point 

in 2,500 is above this upper limit and one point is below this lower limit. OLR values in the tails 

of the distributions reached 20 to 25 W/m2 beyond these percentile values for the 16- and 128-s 

running mean data, and ~5 W/m2 for the 30-min running means. At longer averaging times the 

number of independent data points diminishes to <2,500 and collapses the tails. The interpolated 

0.04 or 99.96th value may actually lie slightly outside the minimum (or maximum) value observed. 

In these cases the extreme observed value was selected. For the long averaging times there is usually an approximate 5 W/m2 difference between the first (or 99th) percentile values, extremes of the 

data set, and the interpolated 0.04 (or 99.96th). Likewise, the albedo data tends to show the same 

characteristics on the hot side of the distributions. The albedo data reaches 0.04 to 0.08 above the 

engineering values for the shortest averaging times and less for the longer average data. The cutoff 

is naturally sharp on the cold side of the albedo distribution.

Once one engineering extreme value has been identified, either OLR or albedo, the next step 

is to determine the proper value of the other parameter used to form a pair. As illustrated by the 

contour plots of 128-s running mean paired-data distributions (figs. 69–71), the albedo and OLR 

data are partially correlated. Low OLR values tend to be paired with high albedos; high OLR 

values tend to be paired with low to moderate albedos. To select an appropriate albedo to pair with 

the extreme hot OLR, for example, paired data sorted into bins ranked by both OLR and albedo 

value were done. The highest OLR value bins were selected until at least 0.04% of the data set was 

accumulated; the associated albedo values were then averaged to find the match for the OLR engineering maximum pair (tables 72–74). The same process is used to find the engineering minimum 

OLR, high and low albedo cases.
181

2,000

1,000

1,500

100

500

10

0.05

0

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

newF71

Albedo

100 150 200 250 300 350

Outgoing Longwave Radiation (W/m2)

Figure 69. Albedo-OLR correlation for low inclination orbits, 128-s averaged data 

(contour intervals indicate relative frequency of occurrence).
182

100

10

2,000

1,000

1,500

500

0.05

0

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

Albedo

100 150 200 250 300 350

Outgoing Longwave Radiation (W/m2)

Figure 70. Albedo-OLR correlation for medium inclination orbits, 128-s averaged data

(contour intervals indicate relative frequency of occurrence).

0.05

0

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

newF71

Albedo

100 150 200

100

500

10

250 300 350

Outgoing Longwave Radiation (W/m2)

2,000

1,000

1,500

Figure 71. Albedo-OLR correlations for high inclination orbits, 128-s averaged data 

(contour intervals indicate relative frequency of occurrence).
183

Table 72. Albedo, OLR pairs for critical systems in low inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re + 30 km).

Averaging

Time

Cold Cases

Minimum Albedo

Alb ⇔ OLR (W/m2)

Combined Minimum 

Alb ⇔ OLR (W/m2)

Minimum OLR

Alb ⇔ OLR (W/m2)

16 s 0.06 ⇔ 273 0.13 ⇔ 225 0.4 ⇔ 150

128 s 0.06 ⇔ 273 0.13 ⇔ 226 0.38 ⇔ 154

896 s 0.07 ⇔ 265 0.14 ⇔ 227 0.33 ⇔ 173

30 min 0.08 ⇔ 261 0.14 ⇔ 228 0.3 ⇔ 188

90 min 0.11 ⇔ 258 0.14 ⇔ 228 0.25 ⇔ 206

6 hr 0.14 ⇔ 245 0.16 ⇔ 232 0.19 ⇔ 224

24 hr 0.16 ⇔ 240 0.16 ⇔ 235 0.18 ⇔ 230

Averaging

Time

Hot Cases

Maximum Albedo

Alb ⇔ OLR (W/m2)

Combined Maximum 

Alb ⇔ OLR (W/m2)

Maximum OLR

Alb ⇔ OLR (W/m2)

16 s 0.43 ⇔ 182 0.3 ⇔ 298 0.22 ⇔ 331

128 s 0.42 ⇔ 181 0.29 ⇔ 295 0.22 ⇔ 326

896 s 0.37 ⇔ 219 0.28 ⇔ 291 0.22 ⇔ 318

30 min 0.33 ⇔ 219 0.26 ⇔ 284 0.17 ⇔ 297

90 min 0.28 ⇔ 237 0.24 ⇔ 275 0.20 ⇔ 285

6 hr 0.23 ⇔ 248 0.21 ⇔ 264 0.19 ⇔ 269

24 hr 0.22 ⇔ 251 0.2 ⇔ 260 0.19 ⇔ 262

Mean Albedo: 0.18 Mean OLR: 246
184

Table 73. Albedo, OLR pairs for critical systems in medium inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re+30 km).

Averaging

Time

Cold Cases

Minimum Albedo

Alb ⇔ OLR (W/m2)

Combined Minimum 

Alb ⇔ OLR (W/m2)

Minimum OLR

Alb ⇔ OLR (W/m2)

16 s 0.06 ⇔ 273 0.15 ⇔ 213 0.4 ⇔ 151

128 s 0.06 ⇔ 273 0.15 ⇔ 213 0.38 ⇔ 155

896 s 0.08 ⇔ 262 0.17 ⇔ 217 0.34 ⇔ 163

30 min 0.12 ⇔ 246 0.18 ⇔ 217 0.27 ⇔ 176

90 min 0.16 ⇔ 239 0.19 ⇔ 218 0.3 ⇔ 200

6 hr 0.18 ⇔ 238 0.19 ⇔ 221 0.31 ⇔ 207

24 hr 0.19 ⇔ 233 0.2 ⇔ 223 0.25 ⇔ 210

Averaging

Time

Hot Cases

Maximum Albedo

Alb ⇔ OLR (W/m2)

Combined Maximum 

Alb ⇔ OLR (W/m2)

Maximum OLR

Alb ⇔ OLR (W/m2)

16 s 0.48 ⇔ 180 0.31 ⇔ 267 0.21 ⇔ 332

128 s 0.47 ⇔ 180 0.3 ⇔ 265 0.22 ⇔ 331

896 s 0.36 ⇔ 192 0.28 ⇔ 258 0.22 ⇔ 297

30 min 0.34 ⇔ 205 0.28 ⇔ 261 0.21 ⇔ 282

90 min 0.31 ⇔ 204 0.26 ⇔ 257 0.22 ⇔ 274

6 hr 0.31 ⇔ 212 0.24 ⇔ 248 0.21 ⇔ 249

24 hr 0.28 ⇔ 224 0.24 ⇔ 247 0.21 ⇔ 245

Mean Albedo: 0.22 Mean OLR: 234
185

Table 74. Albedo, OLR pairs for critical systems in high inclination orbits (albedo 

and OLR values are referenced to the ‘top of the atmosphere,’ Re+30 km).

Averaging

Time

Cold Cases

Minimum Albedo

Alb ⇔ OLR (W/m2)

Combined Minimum 

Alb ⇔ OLR (W/m2)

Minimum OLR

Alb ⇔ OLR (W/m2)

16 s 0.06 ⇔ 273 0.16 ⇔ 212 0.4 ⇔ 108

128 s 0.06 ⇔ 273 0.16 ⇔ 212 0.38 ⇔ 111

896 s 0.09 ⇔ 264 0.17 ⇔ 218 0.33 ⇔ 148

30 min 0.13 ⇔ 246 0.18 ⇔ 218 0.31 ⇔ 175

90 min 0.16 ⇔ 231 0.19 ⇔ 218 0.26 ⇔ 193

6 hr 0.18 ⇔ 231 0.2 ⇔ 224 0.27 ⇔ 202

24 hr 0.18 ⇔ 231 0.2 ⇔ 224 0.24 ⇔ 205

Averaging

Time

Hot Cases

Maximum Albedo

Alb ⇔ OLR (W/m2)

Combined Maximum 

Alb ⇔ OLR (W/m2)

Maximum OLR Alb

⇔ OLR (W/m2)

16 s 0.5 ⇔ 180 0.32 ⇔ 263 0.22 ⇔ 332

128 s 0.49 ⇔ 184 0.31 ⇔ 262 0.22 ⇔ 331

896 s 0.35 ⇔ 202 0.28 ⇔ 259 0.2 ⇔ 294

30 min 0.33 ⇔ 204 0.27 ⇔ 260 0.2 ⇔ 284

90 min 0.28 ⇔ 214 0.26 ⇔ 244 0.22 ⇔ 250

6 hr 0.27 ⇔ 218 0.24 ⇔ 233 0.22 ⇔ 221*

24 hr 0.24 ⇔ 224 0.23 ⇔ 232 0.2 ⇔ 217*

Mean Albedo: 0.23 Mean OLR: 211

* Dark side OLR data were included to reach these figures; thus, they may underestimate the maximum 

 to always-daylight Sun synchronous satellites by perhaps 15 W/m2.

From the spacecraft perspective, however, what proves to be the extreme hot or cold case 

for a particular system depends on the emissivity of the spacecraft surfaces and its absorptivity for 

solar radiation. Depending on the ratio between these parameters, the extreme spacecraft temperatures may be associated with extreme OLR cases, extreme albedo cases, or some intermediate 

‘combined’ case where both OLR and albedo run high (or low) together, but neither is near its 

individual extreme. To provide hot and cold combined extremes, points where normalized variates 

for albedo and OLR are equal were determined and the 0.04 and 99.96 percentile points from 

this subset were identified to provide the cold and hot combined extremes. A normalized variate 

is the deviation from the mean value divided by the standard deviation of the distribution, i.e., 

xN = (x–xm)/σx, where xN is the normalized variate, xm is the mean of x in the distribution, and σx

is the standard deviation. Basically, take the distributions of two variables, OLR and albedo (illustrated in figs. 71–73), select the subset defined by OLRN =ALBN, and find the 0.04 and 99.96 percentile points in the tails. The resulting values for engineering extreme cases of albedo and OLR 

are given in tables 72–74 for low, medium, and high inclination orbits. These tables provide engineering values for critical applications of albedo and OLR for various averaging times (time constants) and  extreme types (extreme albedo case, extreme OLR case, and extreme ‘combined’ case).
186

5.2.2.4 Engineering Margin for Critical Systems. Examination of the OLR distribution tails 

for the 16- and 128-s averaging times reveals a similarity. The most extreme observations average 

17 W/m2 outside the 0.04 or 99.96 percentile values, while the 1 and 99 percentile values average 

26 W/m2 to the inside. Reference figure 72 for an illustration. Considering that LEO satellites traverse about 120 km per 16-s interval, this is to be expected. The wide field viewing satellite sees the 

same scene for several sequential 16-s intervals. For longer averaging times, the numerical averaging process causes the tails to shrink. In an 896-s interval the satellite traverses over 6,600 km, thus, 

many scene types are included in the average. The extreme data point is about 8 W/m2 outside the 

0.04 or 99.96 percentile values while the 1 and 99 percentile values are displaced about 18 W/m2

toward the inside. The distributions for longer intervals cut off even more sharply.

0.025

0.02

0.015

0.01

0.005

0

newF72

Normalized Fraction per 5 W/m2 Interval

270 280 290

95%

99%

Max

300 310 320 330 340 350

Outgoing Longwave Radiation (W/m2)

99.96%

Figure 72. Hot side tail of the 128-s average OLR distribution for low inclination orbits.

Note: The basic confidence limit in the OLR data is about 5 W/m2; widths of the distribution tails are at most a few multiples of the measurement uncertainty.

Examination of the albedo distribution tails reveals the same characteristics on the hot 

(high albedo value) side, except the tails are narrower in comparison to the basic measurement 

uncertainty of approximately 0.02. For the 16- and 128-s intervals, the separations between the 

99th and 99.96th percentiles average 0.07, between the 99.96th and the extreme observations, 0.06. 

Because the cold side cuts off very sharply, there is no tail; separations between the minimum 

observation, the 0.04, and first percentiles are 0.02 or less.
187

One key factor in selecting the critical system design point was the shape of the tails and 

the relationship to the measurement uncertainty. Another key factor was the probable encounter frequency for exceeding the worst case. This is also termed the ‘return time’ or the mean time 

between exceedances. Selecting the 0.04 percentile implies that one in 2,500 data points lays outside 

the design point. If these points were uncorrelated and random, the expectation is to encounter 

one every 2,500 by 16 s or 11 hr (for the 16-s averaging time data). In fact, the points are not usually isolated, so the return time will be longer. However, comparing the 16- and 128-s distributions 

reveals the 16-s points exceeding the worst case are unlikely to be in groups larger than eight, thus, 

the average return time must be <2,500 by 128 s (89 hr). Eighty-nine hours is short compared to the 

duration of most space experiments and missions. Selecting the 0.04 and 99.96 percentile values as 

design points is not being overly conservative. A reasonable expectation exists that these values will 

actually be encountered. In fact, if a system is critically sensitive to such short-duration fluctuations in the environment, i.e., a tether might become brittle and break, it is appropriate to design 

not only for the worst case limits but also with some appropriate margin. On the other hand, if the 

system is sensitive in only a noncritical sense, i.e., a temperature exceedance results in a momentary 

loss of science data, it may suffice to have little or no design margin at the worst case environment 

and simply tolerate the occasional loss of data. Similar arguments can be made for longer averaging times. Coupled with the collapse of the tail width to within a factor of 2 or less of the measurement uncertainty, the same conclusion is apparent.

5.2.2.5 Design Conditions for Noncritical Systems. For applications where thermal excursions beyond the design point can be easily tolerated, design values associated with the 5th and 

95th percentiles are provided in table 75 and as an optional output from the STEM tool. Table 75 

gives albedo (Alb)-OLR pairs for thermal analysis sensitivity studies. The albedo extreme type 

cases are 5th percentile (cold) and 95th percentile (hot) albedos and an associated OLR (obtained 

by averaging the OLRs paired to the albedos outside the indicated percentile). Combined (Comb) 

types are pairs with equal normalized variates identified with the 5th or 95th percentile (reference 

sec. 5.2.3). OLR extreme types were obtained in the same manner as the albedo extreme types but 

with the parameters switched. One may expect system temperatures to lie outside values derived 

with parameters from this table for a small fraction of the total mission duration. This fraction 

cannot be specified but should be on the order of a few percent. Table 76 provides pulse-averaged 

SZA correction terms for albedo for pulses beginning at or symmetrical about solar noon. That is, 

the average was taken from solar noon to the time indicated, assuming a 90-min circular orbit and 

using the method in STEM. As the numbers indicate, the correction term is not a strong function 

of orbit position because the average has been weighted with cosine (SZA) in accordance with 

variation of albedo energy. Example albedo-OLR profile for low inclination orbit, beta of 40o. 

(This is an ‘albedo extreme’ hot case with pulse averaged SZA corrections included. The initialization and base albedo is the 5,400 s value, 0.28, plus the 0.07 orbit average SZA correction. Data are 

from tables 75 and 76.)
188

Table 75. Albedo-OLR pairs for noncritical applications.

Extreme 

Type

Avg. Time 

(s)

Cold Case Data Hot Case Data

30° 60° 90° 30° 60° 90°

Alb OLR Alb OLR Alb OLR Alb OLR Alb OLR Alb OLR

Alb 16 0.09 270 0.1 267 0.1 267 0.29 205 0.36 201 0.38 197

Alb 128 0.09 267 0.1 265 0.1 265 0.29 211 0.35 202 0.37 199

Alb 896 0.1 261 0.13 252 0.14 252 0.26 225 0.29 213 0.28 213

Alb 1,800 0.12 257 0.16 242 0.17 244 0.24 234 0.27 223 0.26 223

Alb 5,400 0.13 249 0.18 238 0.18 230 0.22 246 0.26 229 0.24 219

Alb 21,600 0.15 241 0.19 233 0.19 230 0.2 252 0.25 231 0.23 224

Alb 86,400 0.16 240 0.19 235 0.19 230 0.2 252 0.25 232 0.23 224

Comb 16 0.15 236 0.19 227 0.2 225 0.21 260 0.23 240 0.24 237

Comb 128 0.16 237 0.19 227 0.2 225 0.21 260 0.23 240 0.24 238

Comb 896 0.16 237 0.2 226 0.2 227 0.21 261 0.23 241 0.23 240

Comb 1,800 0.16 237 0.2 225 0.2 226 0.21 258 0.23 240 0.23 242

Comb 5,400 0.16 237 0.2 225 0.21 224 0.2 258 0.23 241 0.23 232

Comb 21,600 0.17 237 0.2 226 0.21 226 0.19 255 0.23 242 0.22 230

Comb 86,400 0.17 236 0.2 226 0.2 225 0.19 257 0.23 241 0.23 230

OLR 16 0.3 195 0.33 183 0.35 164 0.17 285 0.17 280 0.17 280

OLR 128 0.29 198 0.33 184 0.34 164 0.17 284 0.17 279 0.17 279

OLR 896 0.26 209 0.28 189 0.27 172 0.18 279 0.18 264 0.18 263

OLR 1,800 0.23 216 0.25 200 0.25 190 0.18 274 0.2 258 0.2 258

OLR 5,400 0.2 225 0.23 209 0.24 202 0.19 268 0.21 254 0.21 242

OLR 21,600 0.18 231 0.23 212 0.23 205 0.19 261 0.21 242 0.21 216

OLR 86,400 0.17 233 0.23 212 0.23 207 0.18 258 0.21 241 0.21 215

Table 76. Pulse-averaged SZA correction terms for albedo assuming a 5,400-s orbit.*

Beta Angle

MaximumTime From Solar Noon

128 s 448 s 896 s

1,350 s or More

(Orbit Average)

0 0.01 0.02 0.03 0.04

10 0.01 0.02 0.03 0.04

20 0.02 0.02 0.04 0.05

30 0.03 0.03 0.04 0.06

40 0.04 0.04 0.05 0.07

50 0.05 0.06 0.07 0.09

60 0.08 0.09 0.1 0.12

70 0.13 0.13 0.15 0.16

80 0.2 0.21 0.22 0.22

90 0.31 0.31 0.31 0.31

* Add the indicated correction to the value for SZA = 0.
189

Fifth and 95th percentile reference points are associated with the percentile from the distribution of a single parameter, albedo or OLR. System temperature is determined by several parameters including albedo and OLR, plus the absorptance/emittance ratio. Thus, it cannot be said that 

system temperatures derived using 5th/95th percentile reference points will be exceeded 5% of the 

time, for the correlation is only approximate. Also, the reference here is to the fraction of time the 

conditions will be exceeded, not to risk of exceedance during mission lifetime.

5.2.2.6 Applications and the Simple Thermal Environment Model. A simple method for 

incorporating this information into design specifications and requirements documents was devised 

by Dr. Eugene Ungar and his colleagues at Johnson Space Center on behalf of the ISS Program. 

For ISS, they were dealing with a complex vehicle with multiple components requiring thermal 

analysis and control, each with different thermal time constants. The approach was to specify two 

sets of albedo and OLR value profiles that would drive the thermal analyses. Each profile covered 

an initialization time plus one orbit. One set was a functional set: conditions during which the 

components must function within specification. The other was an extreme set: components were 

required to return to a proper functioning condition after the conditions were applied and then 

removed. Each set consisted of four albedo-OLR profiles, two hot (max albedo and max OLR 

cases), and two cold (low albedo and low OLR cases). Each profile consisted of albedo-OLR pairs 

from tables 72–74. An example, not from the ISS, is provided in figure 73. The figure illustrates an 

albedo extreme hot case for low inclination orbit with pulse-averaged SZA correction terms added 

to the albedo. For specification writing, typically, profiles without SZA corrections are provided 

because the system will usually fly at multiple beta angles.

Several points are important about this approach. First, the profiles are tied to specific 

orbits with the hot or cold short duration pulses modeled as step functions. The hot case albedo 

and OLR step functions are specified to encompass orbital noon so the maximum short-term 

values are encountered at the time of greatest heating. Likewise, the cold case step functions are 

applied at the opposite side of the orbit, orbital noon plus 180°, when the vehicle is in a shadow or 

at least the albedo radiation is at minimum. By locking the pulse locations with respect to the orbit, 

the SZA correction is known and can be applied at the time the specification is derived rather than 

requiring the analyst to determine the correction. In this illustration, and in the ISS cases, each 

pulse is modeled as a double square step function, one on top of the other to represent two time 

constants. Of course, actual variations are not simple step functions. The TESTSTEM routine in 

STEM gives a good indication of hardware response when using step functions compared to measured profiles. Finally, the orbit is preceded by an initialization period with conditions set at the 

90-min average conditions (hot or cold) of interest. Thus, the model temperatures are stabilized 

before the short period pulses are applied.
190

180

200

220

240

newF73

OLR (W/m2)

–2,000 –1,000 0 1,000 2,000 3,000 4,000 5,000 6,000

Initialization 5,400-s ‘Alb Extreme’ Hot OLR

(b) Time (s)

0.3

0.35

0.4

0.45

0.5

Albedo

–2,000 –1,000 0 1,000 2,000 3,000 4,000 5,000 6,000

Initialization

896-s Pulse and 128-s Pulse

Beta = 40 deg

Solar Noon

(a) Time (s)

Figure 73. Example low inclination orbit, beta of 40o: (a) Albedo profile and (b) OLR

profile.

The ISS approach is especially useful for a situation like ISS where a single set of albedo/

OLR conditions need to be specified for a variety of hardware elements. However, in simpler cases, 

i.e., a single hardware element and surface treatment, this approach can lead to unneeded analysis when the extreme hot and cold cases can be selected directly.To aid this process and to identify 

cases when a combined albedo/OLR case leads to the temperature extremes, a STEM was developed. STEM is fully documented111 and provides a shortcut by:

(1) Estimating the thermal time constant for the hardware.

(2) Selecting, based on the absorptance/emittance ratio for the surface, whether extreme 

albedo, extreme OLR, or combined extreme conditions, yields extreme system temperatures.

(3) Providing the complete set of thermal parameters to model the thermal environment 

with correct time constant, SZA correction, etc.

STEM accomplishes this shortcut by analyzing the energy balance for a simple spherical 

satellite. Provision is made for inclusion of internal energy sources. Data in tables 72–74 are used 

and return values suitable for returning the extreme temperatures, hot and cold, when used in 

actual system analysis. As an option, STEM also outputs reference points associated with the 5th 

and 95th percentile extremes (reference sec. 5.2.2.5). This option is included primarily to aid the 
191

analyst in determining the sensitivity of system temperature to environmental inputs near the limits 

of the distribution. Use as a design specification is only recommended for rare, noncritical circumstances where periodic excursions beyond the design limit can be easily tolerated. In STEM output, 

the SZA corrections are applied to the albedo values and associated locations are identified in the 

orbit.

Note that since STEM models the system as a sphere, the environmental parameters selected 

might not yield the extreme temperatures in a thermal analysis of a system with complex geometry. 

For example, reflected radiation from adjacent spacecraft surfaces could alter the balance. Thus, 

when the system involves complex geometry and the extreme temperatures derived from STEM for 

the different types of environmental conditions are close to one another, the analyst should check 

system performance using alternate types of environmental extremes, not just conditions identified 

by STEM. STEM will identify the alternate extremes if the user resets the cold case and hot case 

parameter switches.

5.3 Geomagnetic Field

The Earth’s natural magnetic field (or geomagnetic field) comes from two sources: (1) Currents inside the Earth, which produce 99% of the field at the surface, and (2) currents in the magnetosphere. The Earth’s magnetic field can shield the Earth from certain radiations and it can trap 

charged particles. The magnetosphere has a lower boundary of the ionosphere and an upper one 

defined by the magnetopause, which is the interface between the Earth’s magnetic field and the 

solar wind. Most (99.9%) of the solar wind particles do not penetrate the magnetopause but are 

deflected and flow around the magnetosphere.

A dipole magnetic field configuration dominates the magnetic field topology within a few Re

of the planet. The dipole center is approximately 400 km from the center of the planet. The geomagnetic axis is inclined at approximately 11.7° to the Earth’s rotational axis. The Earth’s magnetic 

field greatly affects thermal, plasma, radiation, and other environments in space, particularly in 

the LEO region. The specific environment is affected by the charged particles trapped through the 

magnetic field lines and the deflections of the low energy cosmic rays. These effects will be specifically described in each respective environment section. In this section, a few simple approximations 

to the Earth’s main geomagnetic field used in the past to curve fit simple radiation models will be 

discussed. Recent spherical harmonic models used to curve fit the Earth’s main geomagnetic undisturbed field from zero to 2,000 km in altitude will be presented.

The strength of Earth’s dipole magnetic field varies. Surface magnetic fields are approximately 0.3 G (3×10–5 T) at the equator and 0.6 G (6×10–5 T) at the poles. Due to the difference 

between the Earth’s rotational axis and the magnetic axis, and the offset of the dipole described 

above, the field intensity is low in the region near 20 °S. latitude and 50 °W. longitude. This is 

referred to as the South Atlantic Anomaly (SAA). The magnetic field direction at the equator 

is nearly horizontal pointing north. At the magnetic North Pole, it points down into the Earth. 

Beyond an altitude of approximately 2,000 km, strong currents in the magnetosphere cause deviations from the near-Earth dipolar field. In addition to the general magnetic field variation, the 

geomagnetic storms caused by solar activity can change the magnetic field strength.
192

The interaction of the Earth’s magnetic field at very great distances (several Re) with the 

solar wind is one of the heat sources for the neutral thermospheric region. The solar wind is 

a stream of high-speed plasma emanating from the Sun. This interaction causes energetic particles 

to penetrate down into the lower thermosphere at high geographic latitudes and directly heat the 

thermospheric gas.

Based on magnetic field fluctuation data reported every 3 hr at 12 stations between geomagnetic latitudes 48° and 63° and selected for good longitudinal coverage, the planetary geomagnetic 

activity index ap (or kp, which is essentially the logarithm of ap) as a measure of episodic type solar 

activity is used. Although it is the high latitude ionospheric current fluctuations that drive the magnetic field fluctuations as observed at these stations, it is not the magnetic field fluctuations that are 

driving the thermosphere. Therefore, the correlations between observed density changes and the ap

index are not always good. The daily planetary geomagnetic index, Ap, is the average of the eight 

3-hourly ap values for that particular day.

The aurora is primarily produced by high energy charged particles precipitating into the 

atmosphere along magnetic field lines. At high latitudes, the ionosphere is strongly coupled to the 

magnetosphere and to the solar wind. The transition from closed to open field lines and the influx 

of energetic particles profoundly affects the ionospheric plasmas. The boundary region, the auroral oval, is marked by the beautiful display of auroras. Surrounding the magnetic poles, the oval 

extends to near 75° geomagnetic latitude at noon and 65° at local midnight. On the nightside, the 

oval is well marked by a depletion of electron density, the so-called trough. On the dayside, one 

finds a region of enhanced densities just inside the oval, the so-called magnetospheric cleft. The 

electron density at the tip of the cleft is almost an order of magnitude greater than it is at the bottom of the trough. During magnetic storms, the trough moves equatorward approximately 2° per 

unit increase in Kp. The region inside the oval is the polar cap.

5.3.1 Models

The magnetic field of the Earth has been investigated by satellite for several decades. Data 

obtained were sufficient to apply for model development with its average strength and shape during different levels of overall magnetic activity. A summary of key design factors is displayed in 

table 77.

Table 77. Summary of key design factors (geometric field).

Parameter Source Select Based On

For prediction of geomagnetic field:

Geomagnetic field IGRF (sec. 5.3.1.1) Plasma, charged particles

Geomagnetic field effects and guidelines:

Plasma effects Section 5.3.2 Section 5.4

EMF potential Section 5.3.2 Section 5.4

Trapped high energy particle Section 5.3.2 Section 5.5
193

5.3.1.1 International Geomagnetic Reference Field. A current version of the International 

Geomagnetic Reference Field (IGRF) is recommended to predict the magnetic field strength for 

regions where the field retains the dipolar configuration (typically for altitudes less than approximately geostationary orbit). The IGRF model is the empirical representation of the Earth’s magnetic field for scientific use recommended by the International Association of Geomagnetism and 

Aeronomy. The IGRF model represents the main (core) field without external sources. The model 

employs the usual spherical harmonics expansion of the scalar potential in geocentric coordinates. 

The IGRF model coefficients are based on all available data sources, including geomagnetic measurements from observatories, ships, aircraft, and satellites. In combination with the IGRF coefficient sets, different subroutines have been used to determine the components of the magnetic field 

vector and the L-value at a given location. The National Space Science Data Center (NSSDC) 

version uses a FORTRAN field calculation subroutine to calculate magnetic field components 

and SHELLG to calculate McIIwain’s L parameter. Both subroutines were developed by G. Kluge 

at the European Space Operations Center. The program BILCAL produces tables of the geomagnetic field strength, vector components (B-abs., B-north, B-east, B-down, declination, inclination), 

equatorial/ minimum field strength (B0), dipole moment, and L-value in latitude, longitude (geodetic), altitude, or year (decimal). The code can calculate the magnetic field at a given position and 

time or it can calculate the maximum and average magnetic fields during an orbit. Most Earth/Sun 

parameters remain constant for hundreds of years; however, the magnetic field strength changes 

significantly every 5 to 10 years. Thus, it is important to use a recent version of the model to correctly determine the magnetic field. The IGRF prediction of the Earth’s magnetic field is shown 

in section 5.3.2. 

5.3.2 Geomagnetic Field Effects

The Earth’s magnetic field exerts a strong influence on space environmental phenomena, 

such as plasma motions, electric currents, and trapped high energy charged particles. This influence 

has important consequences to spacecraft design and performance. The IGRF prediction of the 

Earth’s magnetic field is shown in figures 74 and 75.

The geomagnetic field influences the motions of particles within the Earth’s orbital environment and also deflects incoming high energy particles, such as those associated with cosmic 

rays. These high energy particles may charge spacecraft surfaces causing failure of or interference 

with spacecraft subsystems. Due to the dipole field geometry, the magnetic field strength is lowest 

over the southern Atlantic Ocean, which leads to a higher concentration of trapped radiation in 

this region. It is in the vicinity of the SAA that a spacecraft may encounter electronic ‘upsets’ and 

instrument interference. An accurate depiction of the geomagnetic field is also needed to properly size magnetic torquers, which are used in guidance, navigation, and control (GN&C) systems. 

Potentials can be induced across spacecraft through the V×B•L electromagnetic force (EMF) or 

magnetic induction effect. The magnitude of this potential depends on the magnetic field strength 

(B) direction relative to the spacecraft velocity vector (V) and the orientation and size of the spacecraft (L).This effect has been used to generate power and propulsion with long conductive tethers.
194

F76

0.5

0.5

0.3

0.35

0.4

0.45

0.5 0.55

0.55

0.55

0.6

0.6

0.4

0.6

0.65

0.45

0.45

0.35

0.4

0.35

0.3

0.25

90° N.

60° N.

30° N.

0

30° S.

60° S.

90° S.

180° W. 150° W. 120° W. 90° W. 60° W. 30° W. 0 30° E. 60° E. 90° E. 120° E. 150° E. 180° E.

Longitude (deg)

Latitude (deg)

Figure 74. IGRF prediction of the Earth’s magnetic field at sea level.

F77

0.386

0.36

0.335

0.309

0.283

0.283 0.257 0.257 0.231

0.205

0.309

0.335 0.36

0.386

0.412

0.412

0.438 0.438

0.438

0.464

0.464

90° N.

60° N.

30° N.

0

30° S.

60° S.

90° S.

180° W. 150° W. 120° W. 90° W. 60° W. 30° W. 0 30° E. 60° E. 90° E. 120° E. 150° E. 180° E.

Longitude (deg)

Latitude (deg)

Figure 75. IGRF prediction of the Earth’s magnetic field at 650 km 

altitude.

Geomagnetic storms may also affect orbiting spacecraft. Disturbances in the geomagnetic field which last one or more days are called geomagnetic storms. When a geomagnetic storm 

occurs, large numbers of charged particles are dumped from the magnetosphere into the atmosphere. These particles ionize and heat the atmosphere through collisions. The heating is first 

observed minutes to hours after the magnetic disturbance begins. The effects of geomagnetic heating extend from at least 300 km to well over 1,000 km and may persist for 8 to 12 hours after the 

magnetic disturbance ends.

The response of the Earth’s field during a full geomagnetic storm cycle is a combination 

of effects that includes both an initial distortion and field compression followed by an overall 

decrease in the field intensity. When the CME (or enhanced solar wind) first arrives, the initial 

effect is to compress the Earth’s magnetic field which increases the field intensity. This is variously 
195

called a ‘storm sudden commencement’ or ‘geomagnetic sudden impulse’ but the event is transient 

and only lasts for a few minutes to tens of minutes but rarely longer than an hour. Geosynchronous orbit (GEO) spacecraft may end up outside of the magnetosphere during this phase if they 

are located on the dayside of the Earth and the magnetopause (boundary of the magnetosphere) 

is compressed inside of GEO. However, this is not the main phase of the storm that comes next.

Large storms are typically driven by CMEs during the phase when the IMF inside the CME 

has a strong southward-directed field component (the IMF Bz component). Since a Bz<0 field orientation is opposite that of the Earth’s field, magnetic field annihilation occurs at the dayside of the 

Earth’s magnetosphere and magnetic flux is transferred to the tail, increasing the size of the polar 

cap and moving the GCR and SPE cutoff latitude towards the equator. In addition, the excess flux 

in the tail is unstable and the excess energy is released in the form of a geomagnetic storm that 

drives plasma into the Earth’s inner magnetosphere, increasing the intensity of the Earth’s ring 

currents. The ring current generates a perturbation magnetic field in the opposite direction of the 

Earth’s ambient field so the total field intensity is reduced. This phase can last for many hours to 

days.

5.4 Plasma Environment

Plasmas are quasi-neutral gases in space with an approximately equal number density of 

ions and electrons. The charged species interact through the electromagnetic field and exhibit a collective behavior. Movements of individual particles are controlled to a great extent by the Earth’s 

magnetic field and electric field. Their collective behavior and movement, however, generate electric and magnetic fields that, in turn, affect the particle’s motion and the motion of other charged 

particles. Only a few percent of neutral gases are ionized in the lower ionosphere. Essentially all are 

ionized in the outer regions of the Earth’s magnetosphere. In some areas, the plasma can be relatively cold (~1,000 K), and dense, such as in the ionosphere region, while the hot (~108 K) tenuous 

plasma exist at very high orbits.

Slightly below the region discussed in the thermospheric environment section that marks the 

lower boundary of the thermosphere, and associated changes in composition of the neutral species, 

there is an important transition related to the electromagnetic properties of the gas. At roughly 

90 km altitude, there is a division between the lower turbulent neutral gas mixture region where 

all the meteorological processes occur and the upper region where solar irradiation produces partially ionized plasma composed of AO, N2, O2, He, H, atomic oxygen ion (O+), hydrogen ion (H+), 

helium ion (He+), nitrous oxide ion (NO+), molecular oxygen ion (O2

+), nitrogen ion (N2+), and 

electrons. This upper region is electrically neutral, with the most abundant neutral being AO and 

the most abundant ion being O+ up to approximately 1,000 km altitude where H+ and He+ become 

dominant elements. The properties of the nonionized fraction are discussed in section 5.1.

Section 5.4.1 defines the plasma regimes in the near Earth space. Derivations of plasma 

characteristics from basic distribution functions are introduced in section 5.4.2. Sections 5.4.3 

to 5.4.5 describe the plasma properties in the ionospheric LEO region, the auroral region, and 

the polar region.
196

Plasma interactions can be quite complicated, and there are significant differences between 

a space vehicle’s interactions with the relatively low energy plasma in the ionosphere and at very 

high orbits, and in the auroral regions where the higher energy plasma characteristic of higher 

altitudes penetrates to LEO. Examples of plasma interaction effects with space vehicles are solar 

array/power system degradation, contamination, ionospheric scintillation, and spacecraft charging. 

Details of the plasma effects are shown in sections 5.4.6 and 5.4.7. Most commonly used analysis 

tools and computer models to obtain the environmental properties and to assess the engineering 

performance effects are described in section 5.4.8. A summary of key design factors for plasma 

environments is displayed in table 78.

Table 78. Summary of key design factors (ambient/energetic plasma condition).

Parameter Source Select Based On

For prediction of ambient/energetic 

plasma conditions:

 Plasma density IRI, GeoSpace models (sec. 5.4.8) Spacecraft charging

 Plasma temperature IRI/GeoSpace models (sec. 5.4.8) Spacecraft charging

Plasma measurements and spacecraft

charging guidelines evaluation

for spacecraft charging:

 Plasma density Tables 79–81 Spacecraft charging

 Plasma temperature Tables 79–81 Spacecraft charging

 Charging predictions NASCAP/NASCAP-2K (sec. 5.4.8) Spacecraft charging

Other plasma effects and guidelines:

 Power system / solar array Section 5.4.6.1 Electrical power

 Contamination Section 5.4.6.2 Spacecraft

 Plasma emissions Section 5.4.6.3 Communication

 Ionospheric scintillation Section 5.4.6.4 Communication/GPS

 Secondary plasma effects EMF Section 5.4.6.5 Charging/communication 

Section 5.4.6.6 Charging

5.4.1 Plasma Ionosphere

The ionosphere is characterized by its low temperature and high density relative to the 

plasma populations within the magnetosphere and its predominantly O+ composition due to the 

terrestrial origin of the ions. The equatorial extension of the ionosphere is predominantly composed of lighter ions, H+ and He+, and is usually referred to as the plasmasphere (sec. 5.4.1.1).

5.4.2 Plasma Characteristics

Plasmas are usually described by their density (expressed as electron number density, ne 

/

m3), the chemical composition of the ions (often expressed as percentage of total ions), and the 

electron and ion temperatures (expressed in Kelvin, or as energy, in electron volts). The high energy 

particles (megaelectron volt range) that also may be present cannot be so described, and they interact differently with a vehicle than the plasma. They are discussed in section 5.5, ionizing radiation 

environment.
197

Maxwellian velocity distribution functions have been shown to provide a useful mathematical approximation of the plasma characteristics. Collisions between species in LEO are sufficiently 

frequent for the gas to assume a thermodynamic equilibrium and the distribution of velocities for 

individual species may be described by a function of the form:115

f

i

(v) = ni

mi

2!kTi

!

"

#

$

%

&

3/2

exp '

mi v ' u ( )i

2

2kTi

(

)

*

*

+

,

-

- , (55)

where v is particle velocity and the ith species (i=electron and ion) is characterized by the number 

density ni , mass mi

, temperature Ti, and bulk flow velocity ui.

The advantage of using equation (55) as a description of the plasma environment rather 

than specifying a complete velocity or energy dependent flux spectra is that the conditions in the 

plasma are characterized for each species by only a few parameters: density, mass, temperature, 

and bulk flow velocity.

Plasma characteristics are obtained from equation (55) by computing the velocity ‘moments’ 

of the velocity distribution. Of greatest interest are the first four moments:

(1) Number density: M1= ni = v

0 f

i

(v)d3v , ! (56)

(2) Number flux: M2 = EFi = v

1 f

i(v)d3

v , ! (57)

(3) Energy density: M3 = EDi = v

2 f

i(v)d3

v , ! (58)

and

(4) Energy flux: M4 = EFi = v

3 f

i(v)d3

v . ! (59)

Multiple Maxwellian distributions may be used in the case where plasma may not be 

adequately described by equation (55) and its moments in equations (56)–(59). For example, the 

low-density plasma of GEO collisions are very rare and the gas will never be truly Maxwellian. 

Even in this environment it has been shown that a double Maxwellian provides a useful description 

of the plasma:115,116

f

i(v) = ni,1

mi,1

2!kTi,1

!

"

#

$

%

&

3/2

exp '

mi,1 v ' u ( ) i,1

2

2kTi,1

(

)

*

*

*

+

,

-

-

-

+ ni,2

mi,2

2!kTi,2

!

"

#

$

%

&

3/2

exp '

mi,2 v ' u ( ) i,2

2

2kTi,2

(

)

*

*

*

+

,

-

-

-

. (60)

Typically, equations (55) or (60) are implemented in standard charging analysis codes and 

the engineer simply needs to input the appropriate set of plasma density and temperatures along 

with an estimate of the relative velocity between the plasma and the spacecraft. Plasma characteristics will therefore be described in terms of these parameters throughout the rest of this discussion.
198

5.4.3 Low Inclination Ionosphere

The ionospheric plasma is generated principally by photoionization of the ambient neutral 

atmosphere by solar photons on the sunlit side of the Earth and by magnetospheric particles interacting with the thermosphere at high latitudes over an altitude range of 100 to 200 km. Short-term 

temporal variations in the solar photon input at a fixed location on the Earth may vary from hours 

in the case of solar flares to the diurnal variation in solar illumination due to the rotation of the 

Earth. Long-term modulation of the UV-EUV photon flux over the approximately 11-year solar 

cycle leads to substantial variations in the composition and density of the ionospheric plasma. 

Transport of the plasma is strongly controlled by the geomagnetic field and magnetospheric electric fields. Hence, plasma characteristics vary with geomagnetic latitude, altitude, local time, season, and level of solar and geomagnetic activity, but only weakly with geomagnetic longitude. The 

plasma density decreases at night when the loss of solar UV photons fails to maintain the production of free ions and electrons. The plasma density tends to be greater in the equatorial regions than 

in the polar regions, with a local minimum near the geomagnetic equator at the center of the equatorial anomaly. Plasma temperatures (thermal energy) are approximately 1,000 K during the day, 

but two to three times that in the morning and evening.

Composition of the ionospheric plasma mirrors that of the neutral atmosphere since it is 

photoionization of the neutrals that produces the most common ionic species. The most common 

neutral species found in the upper atmosphere include AO, N2, O2, He, and H so it is not surprising to find that the most common ionospheric constituents are O+, H+, He+, NO+, O2

+, N2+, and 

electrons. The dominant constituent of the neutral atmosphere is AO while ion populations are 

dominated by O+ up to altitudes of 500 to 1,000 km, where H, He, H+, and He+ finally occur in 

greater numbers.

Ionospheric structure is generally discussed in terms of three fundamental altitude regimes, 

which differ, in addition to differences in neutral gas properties and in the depth of penetration of 

solar UV radiation. The regions are referred to as the D (50 to 90 km), E (90 to 160 km), and F 

(160 to 500+ km) regions.

5.4.3.1 Ionospheric Density. Above the D layer, the neutral gases are in diffusive equilibrium; however, layering of the electron density profile is exhibited within these regions due to competing particle production, loss, and transport processes. The highest electron densities are at the 

F2 layer peak, 250 to 350 km at mid-latitudes and 350 to 500 km at equatorial latitudes. The density in the E layer, typically located at 100 to 160 km altitude, is an order of magnitude less than 

the F2 layer peak. Under certain conditions, a valley or an F1 ledge can be observed between these 

two layers. The density in the D layer, below the E layer peak, decreases rapidly with decreasing 

altitude. Figure 76 demonstrates the global plasma density (m–3) at 400 km of altitude using the 

International Reference Ionosphere (IRI90) model for plasma prediction. Values are for June 21 

and solar minimum conditions (F10.7=70) at 00:00:00 UTC. Figure 77 describes the noon-midnight 

cross section of plasma density (m–3) as a function of altitude from 150 to 700 km for June 21 and 

solar minimum conditions (F10.7=70) at 00:00:00 UTC from IRI90 model prediction.
199

7.5 × 1010 7.5 × 1010

1.1 × 1011

6.1 × 10 Subsolar 11

Point

2.6 × 1011

4 × 1011

1.7 × 1011

1.1 × 1011

7.5 × 1010

4.9 × 1010

3.2 × 1010

3.2 × 1010

1.1 × 1011

1.1 × 1011

1.7 × 1011

2.6 × 1011

4 × 1011

6.1 × 1011

newF76

Day Night Day

90

60

30

0

–30

–60

–90

–180 –150 –120 –90 –60 –30 0 30 60 90 120 150 180

Longitude (deg)

Latitude (deg)

Figure 76. Plasma density (m–3) at 400 km.
200

3.3×1010

3.3×1010

3.3×1010

3.3×1010

3 ×109

1.4×109

3.3×1010

7.2×1010

7.2×1010

1.5×1010

7.2×1010 1.6×1011

1.6×1011

1.6×1011 1.6×1011

3.5×1011

1.5×1010

1.5×1010 6.7×109

6.7×109

1.5×1010

F79 

150 km

700 km

Sun

Figure 77. Noon-midnight cross section of plasma density (m–3) as a function

of altitude from 150 to 700 km for June 21 and solar minimum 

conditions (demonstration only).

In the D layer, ionization is primarily caused by solar x-rays and depends strongly upon the 

SZA. The highest values, 108 to 109 m–3, occur around summer noon. Below approximately 70 km 

altitude, ionization by cosmic rays is the major electron source; therefore, the electron density is 

negatively correlated with the solar cycle below 70 km and positively correlated above.

The E layer is composed primarily of AO ions and electrons, the result of ionization of the 

predominantly AO neutral atmosphere by solar EUV radiation. The daily maximum is near noon, 

the seasonal maximum is in the summer, and density varies directly with solar activity. The nighttime density is more than an order of magnitude less than the daytime density due to loss of the 

ionized species through recombination. Although thin and patchy, a sporadic E layer, Es, with 

a density that can exceed the E and F2 peaks, occurs irregularly.

The F region consists of the overlapping F1 and F2 layers. The F1 layer, at approximately 

150 to 200 km altitude, is of lesser importance. It is under strong solar control. The density distribution in the F layer is generally determined by transport processes, ambipolar diffusion, electrodynamic drift, and neutral wind drag because neutral densities decrease rapidly with increasing 

altitude. Therefore, the F2 layer peak and the topside ionosphere are highly variable, with 10% 

to 30% day-to-day variations in density. The F2 layer peak density maximizes in the afternoon in 

winter. There are two crests at ±15° magnetic latitude with a minimum at the magnetic equator in 

a latitudinal profile of the F layer. At night and higher altitudes, the two crests merge into a single 
201

crest at the magnetic equator. The so-called ‘fountain effect’ causes this ‘equatorial anomaly.’ The 

charged particles are pushed upward by the equatorial electric field where they then drift downward 

along magnetic field lines.

A wide variety of ionospheric irregularities have been observed, predominantly at high 

latitudes and during the equatorial nighttime. The plasma fluctuations range in scale from hundreds of kilometers down to centimeters. Plasma instabilities play an important role in the generation of medium scale (kilometers) and small scale (meters) irregularities. Examples of irregularities 

are patches of enhanced ionization in the E region (sporadic E) and of depleted ionization in the 

F region (spread F). Spread F is most frequently observed in the equatorial nighttime ionosphere. 

The irregularities cause signal fluctuations in traversing radio waves, known as scintillations.

5.4.3.2 Ionospheric Temperature. The main source of energy for the terrestrial ionosphere 

is EUV radiation from the Sun. Ionospheric electrons are heated most efficiently, and their temperature exceeds the temperature of the ions and neutrals. Electron temperatures increase from 

approximately 300 K at 100 km altitude to approximately 3,500 K at 800 km altitude. Ion temperatures are close to the neutral temperature below approximately 400 km altitude and increase toward 

the electron temperature above that altitude. Below 150 km altitude, the high neutral densities and 

the high collision frequencies result in the same temperature for electrons, ions, and neutrals. During nighttime, the temperatures of all species are similar in magnitude. Figure 78 demonstrates the 

electron temperature at 400 km of altitude. Values are for June 21 and solar minimum conditions 

(F10.7=70) at 00:00:00 UTC. Results are from IRI90 with default options. Figure 79 describes 

the noon-midnight cross section of electron temperature (electronvolt) as a function of altitude 

from 150 to 700 km for June 21 and solar minimum conditions and solar minimum conditions 

(F10.7=70) at 00:00:00 UTC from IRI90 model prediction.

In general, plasma temperatures are lowest at the geomagnetic equator and increase toward 

higher latitudes. The greater temperatures at high latitudes are due to the additional energy input 

to the atmosphere from precipitating particles and Joule heating (due to currents flowing in the 

ionosphere) in the auroral zones. At low altitudes, however, the electron temperature peaks at the 

magnetic equator, reaches minimum values at approximately ±20° latitude, and then increases 

toward higher latitudes. This behavior is the mirror image of the equatorial anomaly of the electron density and illustrates the strong anti-correlation between electron density and temperature.

On the average, ionospheric temperatures increase from an almost constant nighttime value 

to an almost constant daytime value. The most significant departure from this behavior is the early 

morning peak in electron temperature. It is most pronounced at the magnetic equator at approximately 300 km altitude (the peak temperature exceeds the daytime value by a factor of 2 or 3); its 

magnitude decreases rapidly toward higher and lower altitudes and toward higher latitudes. The 

temperature peak is a result of the sharp increase in solar heating coupled with the still low electron 

densities from the preceding night.
202

0.22

0.25

0.22

0.25

0.19

0.19

0.19

0.19

0.16

0.16

0.16

0.16

0.12

0.12

0.12

0.31

0.12

0.22

0.22

0.22

0.22 0.25

0.28

0.09

0.09

0.09

Subsolar

Point

newF78

Day Night Day

90

60

30

0

–30

–60

–90

–180 –150 –120 –90 –60 –30 0 30 60 90 120 150 180

Longitude (deg)

Latitude (deg)

Figure 78. Electron temperatures (eV) at 400 km.
203

newF79

150 km

700 km

Sun

0.22

0.22

0.22

0.22

0.19

0.19

0.19

0.19

0.1

0.1

0.1

0.14

0.14

0.14

0.17

0.17

0.17

0.17

0.12

0.12

0.12

0.12

0.24

0.24

0.24

0.07

0.07

Figure 79. Noon-midnight cross section of plasma temperature (eV) as a function 

of altitude from 150 to 700 km.

The electron temperature is almost unaffected by the solar cycle, in contrast to the increase 

of almost all other neutral and ionized parameters. This again is a result of the close coupling with 

the electron density, which determines both energy gain and loss of the electron gas. The simultaneous increase of both terms leaves the electron temperature nearly unchanged.

5.4.3.3 Spacecraft Charging in Ionosphere. The plasma electrons have much less mass than 

the positive ions. Since, at a given energy, the thermal velocity is inversely related to the square root 

of the particle mass, the thermal electron flux to passive spacecraft surfaces and structure is greater 

than the corresponding thermal ion flux. As a result, surfaces tend to accumulate negative charge. 

At low latitudes in LEO, where the plasma is relatively dense and of low energy, equilibrium is 

established within a few volts negative to the reference plasma potential. Thus, charging of passive surfaces is usually not a problem in this regime. However, for active surfaces, e.g., solar arrays 

and structure tied electrically to them, arcing and related significant effects can occur, depending 

upon the grounding scheme and the magnitude of the spacecraft-imposed voltages. More details 

of specific plasma interactions are discussed in section 5.4.6, plasma effects, and in section 5.4.7, 

spacecraft charging.

5.4.4 Auroral Region

The auroral region is defined as the area between approximately 60° to 75° magnetic 

latitude. Along auroral field lines a variety of phenomena is observed, including high energetic 

ionospheric upward ions, high energetic downward acceleration of magnetospheric electrons, perpendicular electrostatic shocks, and intense wave emissions. Sharply defined regions of low electron
204

densities are a common feature of auroral zone crossings from the predusk hours until the early 

morning hours. The energy of these precipitated electrons can be on the order of several tens of 

kiloelectron volts. Electron densities in this region are strongly depleted in relation to the adjacent 

polar cap and plasmaspheric densities and form a low-density cavity. Minimum densities in this 

auroral cavity frequently fall to values below 3×105 m–3 and rarely exceed 3×106 m–3.

The plasmas in the polar region (above 60° geomagnetic latitude) have a significant energy 

contribution from both magnetospheric particle precipitation and current-driven heating. Plasma 

in the high latitude regions is often more similar to the magnetospheric plasma than to the ionospheric plasma at low latitudes. The ionosphere is strongly coupled to the magnetosphere and to 

the solar wind. The transition from closed to open field lines and the influx of energetic particles 

profoundly affects the ionospheric plasma. The boundary region, the auroral oval, is marked by the 

beautiful display of auroras. Surrounding the magnetic poles, the oval extends to near 75° geomagnetic latitude at noon and approximately 60° at local midnight. On the nightside, the oval is well 

marked by a depletion of electron density, the so-called trough. On the dayside, one finds a region 

of enhanced densities just inside the oval, the so-called magnetospheric cleft. The electron density 

at the tip of the cleft is almost an order of magnitude greater than it is at the bottom of the trough. 

During magnetic storms, the trough moves equatorward 2° per unit increase in Kp. The region 

inside the oval is called the polar cap.

Spacecraft in high inclination orbits will encounter the precipitating auroral particle fluxes 

as they pass through the auroral oval. One result of these fluxes is the increase of local plasma 

density by a factor of up to 100 over regions of tens of kilometers in latitudinal dimension and 

hundreds or thousands of kilometers in longitudinal dimension in the auroral regions (60° to 

75° magnetic latitude). These enhancements occur between approximately 100 and 250 km altitude 

(reference fig. 80). Above 250 km, the thermal plasma may be depleted above intense auroral in the 

midnight sector, falling far below 1×1010/m3. In this region, there is no clear distinction between 

magnetospheric and ionospheric phenomena. There is a very wide range of scales, both spatially 

and temporally, present in the dynamics. Small scale (meters to decameters) irregularities exist 

and move with the prevailing ambient plasma drift. Ionion and ion-neutral collisions in the lower 

thermosphere tend to make the temperature distribution isotropic while anisotropies still remain 

at higher altitudes (near 600 km). Plasma property anisotropies are introduced by the geomagnetic 

field.

5.4.4.1 Auroral Morphology. Aurora occurs in all three altitude regimes, D, E, and F, with 

the E layer dominant in terms of total auroral precipitation energy deposition. Electron density 

profiles in the auroral region are dependent upon the energy distribution of the incident precipitating particles, while the auroral emissions are dependent upon the photochemistry of the auroral 

ionosphere, as well as the energy distribution of the precipitating particles. The spectrum of the 

precipitating particles determines the altitude at which the particle energy is deposited and, therefore, the applicable photochemistry since neutral composition and density depend strongly on 

altitude. Enhanced electron densities are produced by impact ionization due to precipitating electron fluxes. The auroral E region extends approximately 2° farther equatorward than the F region, 

and this extension is formed by proton precipitation, in contrast to the mainly electron precipitation 

that forms the F region. There is a strong seasonal and local time control of the auroral ionosphere.
205

320

280

240

200

160

120

80

60

newF80

Altitude (km)

0

10

10 6

6

20

40 40

0820

0815

Rocket

Echo

0818

60

10

10

6

Density 104 cm–3

Scan Limit

3202802402001601208040

Distance North of Chatanika (km)

March 9. 1978

0815:00–0820:09 UT

Figure 80. Ionospheric electron density in an aurora (×104 ne/cm3).117

The aurora occurs at all local times in the auroral ovals that are rings at approximately 65° 

magnetic latitude around each magnetic pole. The most intense and energetic fluxes occur near 

local midnight. The location of the aurora and its intensity vary with solar (and resulting magnetospheric) activity. Plots of the average integral energy flux and the average energy of precipitating 

electrons are presented in polar spectrogram format in an MLT-corrected geomagnetic latitude 

coordinate system for each of four levels of Kp in figures 81–84. Plots apply to both poles. A statistical analysis of the probability of encountering various levels of aurora particle flux may be found 

in reference 118, and a detailed discussion of auroral occurrence of morphology may be found in 

reference 119.
206

newF81

10+6

10+6

10+6

10+6

10+6 90

70

60

50

80 10+6

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Hours in MLT

Magnetic

Latitude

18 06

12

24

(a)

(b)

10+6

10+6

10+6 90

70

60

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80

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32+6

32+6

32+4

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32+6 32+7

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32+5

32+5

Magnetic

Latitude

18 06

12

24

Figure 81. Average integral precipitating electron energy flux: (a) Kp=0 and (b) Kp=2 

(polar plots are in corrected geomagnetic coordinates, electron flux is in units 

of KeV/cm2-s-sr—demonstration only).
207

newF84

10+6

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90

70

60

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Magnetic

Latitude

18 06

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24

(a)

(b)

10+6

90

70

60

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80

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32+7

32+7

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10+7

10+7

32+6

32+7 32+7

32+7

Magnetic

Latitude

18 06

12

24

Figure 82. Average integral precipitating electron energy flux: (a) Kp=4 and (b) Kp=6 

(the polar plots are in corrected geomagnetic coordinates, electron flux 

is in units of KeV/cm2-s-sr—demonstration only).
208

newF83

90

70

60

50

80

Hours in MLT

Magnetic

Latitude

18 06

12

24

(a)

(b)

90

70

60

50

80

Magnetic

Latitude

18 06

12

24

0.6

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4

1

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0.6

Figure 83. Average integral energy (KeV) of precipitating electrons: (a) Kp=0 

and (b) Kp=2 (polar plots are in corrected geomagnetic coordinates

—demonstration only).
209

newF84

90

70

60

50

80

Hours in MLT

Magnetic

Latitude

18 06

12

24

(a)

(b)

90

70

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Latitude

18 06

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Figure 84. Average integral energy (KeV) of precipitating electrons: (a) Kp=4 and (b) Kp=6

(polar plots are in corrected geomagnetic coordinates—demonstration only).
210

In the dayside auroral zone, magnetic field lines extend to the dayside magnetopause and 

provide direct access for the shocked solar wind plasma in the magnetosheath to the topside ionosphere. Strong coupling between the neutral and ionized components of the thermosphere drives 

winds in the dayside auroral ionosphere anti-sunward across the polar cap toward the nightside 

ionosphere. The strong ionospheric heating associated with this process generates the largest outflows of ionospheric plasma anywhere at the Earth and forms a global scale ‘ion fountain’ across 

the polar cap. Global electric current systems flow along magnetic field lines into and out of the 

auroral ionosphere. Where these are most intense and in the upward direction, they exceed the 

capacity of the plasma to carry them with thermal motions, and discharges occur in which electrons are accelerated downward and ions are accelerated upward. These discharges greatly enhance 

the brightness of the aurora where the electrons are incident and produce outward-flowing ion 

beams that are far out of local thermodynamic equilibrium and unstable, generating natural 

plasma wave noise at a variety of frequencies.

There are three distinct magnetospheric states that govern the auroral environment 

activities: (1) Quiet magnetosphere state, (2) active polar cap state, and (3) active auroral oval state, 

described in sections 5.4.4.1.1 through 5.4.4.1.3.

5.4.4.1.1 Quiet Magnetosphere State. In this state, the energy input from the solar wind 

is minimized by a configuration that minimizes magnetic recombination on the dayside. A weak 

but northward IMF Bz is typical. The auroral oval has the following characteristics:

• Visually small in diameter and circular.

• Somewhat expanded near the dawn and dusk regions. 

• Relatively thin near midnight.

5.4.4.1.2 Active Polar Cap State. At this time, characteristics of this state are still controversial, and more detailed observational data are required before a consensus can be reached. 

However, it is believed that it occurs during strongly northward IMF Bz conditions and is characterized by the presence of:

• Sun-aligned arcs.

• Additional magnetic field-aligned current systems.

• Distortions or disruptions in the traditional two-cell polar convection patterns.

5.4.4.1.3 Active Auroral Oval State. The southward IMF Bz plays a prominent role in this 

state. Auroral boundaries are active, with rapid poleward movement of the poleward boundary 

and less rapid equatorward movement of the equatorward boundary during the expansion phase 

of magnetic substorms.

Although there is some controversy, it appears as if the optical signature of this poleward 

movement ends before the ground signature of the electrojet activity ceases. The equatorward 

particle precipitation boundaries move equatorward with increasing geomagnetic activity. The ion 

boundaries are equatorward of the electron boundaries in the disk/evening sector, while the electron boundaries are ahead in the morning sector. The electron boundaries are equatorward of the 
211

ion boundaries at all local times except the disk when the geomagnetic activity is very low, very 

quiet times.

Auroral features are also very active in this state. During the expansion phase of an auroral 

substorm, the poleward and equatorward motions of the nighttime aurora can begin at localized 

sites in local time, primarily between 22:00 and 24:00 hr LST. Sometimes an ‘eye-shaped’ structure 

with substantial north-south structures embedded can form as the expansion proceeds in the eastwest as well as north-south directions. The configuration of the east-west expansion is a function 

of the condition of the IMF.

At the most extreme portion of the poleward expansion, the arcs can intensify rapidly and 

form vortices along the arc’s length. Westward traveling surges (WTSs) have been observed to 

propagate nearly 7,000 km. Sometimes WTS activity is the predominant auroral response to substorm activity.

5.4.4.2 Electrodynamics and Convection. There are many statistical or empirical, as well as 

several theoretical, models of the convection and/or electric fields in the auroral zones. Specific use 

will dictate which model should be used in engineering analyses. Most of these models stress the 

importance of the IMF By, the dawn-dark component of the IMF, in controlling the dayside convective patterns. Results of analyses have shown that substantial structure can be present within the 

distribution of field-aligned currents. Results of studies have also shown that:

(1) In the morning sector, the westward electrojet is dominated on the poleward side by 

strong electric fields (with low auroral luminosity and conductivity) and on the equatorward side 

by large conductivity values. The equatorward side is also the region of precipitating electrons 

(upward electrical currents).

(2) On the evening side of the eastward electrojet, the reverse trend is observed.

(3) The westward electrojet is also centered in the region of highest auroral luminosity, 

while the eastward electrojet is in the region of the lowest luminosity.

(4) The brightest auroral regions are associated with upward electrical currents, both 

in the evening and morning sectors.

(5) The polar cap potential is modified rapidly by changing IMF conditions; however, 

the nightside auroral zone response is delayed substantially.

(6) Conductivity gradients play crucial roles in the formation of auroral surges.

(7) Once again, understanding the characteristics of the global aurora is progressing 

rapidly. However, there are many differences between models and observational data, and the 

use of any empirical/statistical/theoretical model will be highly dependent upon the application. 

Care should be utilized in the selection of the ‘proper’ model and experts in the field should be 

consulted.
212

5.4.4.3 Transient Fluxes in Polar Low Earth Orbit. In polar LEO, the important transient 

and energetic fluxes occur in the auroral zone. Figure 85 shows typical aurora fluxes from rocket 

measurements at approximately 320 km.

104

101 102 103 104 105

105

106

107

108

109

1010

newF87

log10 (Intensity)

(Electrons/cm2-s-sr-keV)

0818:36.6–0818:40.7 UT

log10 (Electron Energy) (eV)

Input

Calculated

0°–5°

60°–65°

Figure 85. Auroral electron intensity (center of arc).

Auroral fluxes have been shown to be responsible for spacecraft charging at low altitudes 

in the polar regions.120,121

Examples of the plasma environments during intervals when the Defense Meteorological 

Satellite Program (DMSP) weather satellites have experienced charging to negative potentials less 

than –100 V are given in table 79. A common feature in all these examples is:

• Thermal plasma density <104 cm–3. • Large integral number flux of energetic electrons.

• Spacecraft in eclipse.
213

Table 79. Charging environments in low Earth polar orbits.

Ambient Plasma

N1,e (cm–3) kT1,e (keV) N1,i

 (cm–3) kT1,i (keV)

125 ~ 0.15 2.35 29

190 ~ 0.15 2.35 29

180 ~ 0.15 2.35 29

Energetic Plasma

N2,e (cm–3) kT2,e (keV) N2,i (cm–3) kT2,i (keV)

3.9 10.1 0.16 0.327

3.2 14.4 0.16 0.327

4.9 4.2 0.16 0.327

Gussenhoven et al.121 defined the ‘high energy’ electrons as E ≥ 14 keV and found that 

charging typically occurs when the energetic electron flux exceeds 108 electrons/cm2-s-sr. Anderson 

and Koons120 similarly examined a strong charging event observed on a DMSP spacecraft that 

was associated with an instrumentanomaly. The ambient plasma density was <100 cm–3 and fluxes 

of energetic electrons peaked in the 30 keV channel on the DMSP particle detectors, the greatest 

energy sampled by the instruments.

5.4.4.4 Spacecraft Charging in Auroral Region. In the near auroral ionospheric region, 

the important transient and energetic fluxes occur in the auroral zone. Spacecraft passing through 

the auroral zone can be charged to large negative potentials by energetic electrons precipitating from the magnetosphere. This occurs because large surface potentials are required to retard 

this flux and allow equilibrium (defined in this context as no net current to the spacecraft) to be 

achieved. Also in this region, conditions occur in the wake of large structures, or they may occur 

naturally so that the entire vehicle is involved, where the low energy plasma density is depleted. 

This makes it ineffective in balancing the current from the high energy electron flux, and the charging process is enhanced. This is similar to the situation in GEO where the plasma is very energetic 

but tenuous. Sun/shade effects become important to the point that potentials as large as several 

kilovolts can develop between sunlit and shaded surfaces (depending on geometry and material 

properties).

Table 80 lists examples of ambient plasma at 350 to 600 km altitude in three latitude zones 

(0° to 30°, 30° to 50°, and 50° to 70°). Density (Ne, #/m3), and temperature (Te, eV) values are 

placed in latitude bins based on the absolute value of the spacecraft latitude (i.e., Southern and 

Northern Hemisphere data from equivalent latitudes are grouped into a single latitude range). 

Within each latitude range the percentile Ne, Te values are computed by sorting the data into 

monotonically increasing order and identifying the data value that corresponds to 1%, 10%, 

...90%, and 99% of the Ne, Te values.
214

Table 80. Ambient plasma at 350–600 km altitude in three latitude 

zones (0°–30°, 30°–50°, and 50°–70° inclination).

Plasma Environment—Day

Latitude Range 1% 5% 10% 50% 90% 95% 99%

10° to 30° Ne (m–3) 1.62 × 1010 3.27 × 1010 4.26 × 1010 2.3 × 1011 1.09 × 1012 1.61 × 1012 2.57 × 1012

Te (eV) 0.082 0.097 0.106 0.195 0.307 0.386 0.537

30° to 50° Ne 2.1 × 1010 2.62 × 1010 4.36 × 1010 3.16 × 1011 1.3 × 1012 1.83 × 1012 2.71 × 1012

Te 0.086 0.092 0.107 0.211 0.351 0.41 0.574

50° to 70° Ne 9.2 × 109 3.19 × 1010 5.92 × 1010 1.61 × 1011 8.57 × 1011 1.48 × 1012 2.65 × 1012

Te 0.074 0.097 0.116 0.174 0.267 0.374 0.558

 Plasma Environment—Night

Latitude Range 1% 5% 10% 50% 90% 95% 99%

10° to 30° Ne 3.4 × 109 1.47 × 1010 1.67 × 1010 1.18 × 1011 5.32 × 1011 8.97 × 1011 1.7 × 1012

Te 0.057 0.068 0.069 0.104 0.214 0.334 0.416

30° to 50° Ne 9.69 × 109 1.76 × 1010 2.91 × 1010 2.68 × 1011 8.06 × 1011 1.37 × 1012 3.05 × 1012

Te 0.065 0.07 0.076 0.135 0.317 0.388 0.533

50° to 70° Ne 5.18 × 109 7.75 × 109 2.56 × 1010 8.72 × 1010 8.11 × 1011 1.61 × 1012 2.77 × 1012

Te 0.06 0.063 0.074 0.097 0.318 0.409 0.516

Table 81 demonstrates high energy plasma environment extremes for the spacecraft charging design consideration at auroral region (50° to 70° inclination) and at 350 to 600 km of altitude. 

More details of specific plasma interactions are discussed in section 5.4.6, plasma effects, and in 

section 5.4.7, spacecraft charging.

Table 81. Plasma environment at 50°–70° inclination and 350–600 km altitude.

Energetic Plasma

Nnominal

(cm–3)

Tnominal

(eV)

Charging Time 

(min)

Nextreme

(cm–3)

Textreme

(eV)

Charging Time 

(min)

1.05 10,000 1 1.05 30,000 5

5.4.5 Plasmas in Polar Cap

The polar cap is the region of the Earth’s ionosphere and magnetosphere poleward of the 

auroral oval where magnetic field lines do not connect to the conjugate hemisphere as they do at 

lower latitudes. Instead, magnetic field lines connect to the solar magnetic field in interplanetary 

space or extend deep into the magnetotail. Particle fluxes on polar cap field lines tend to be much 

smaller than at lower latitudes because the open field lines do not support the trapped particle populations that exist on closed field lines. Electric fields, currents, and particle precipitation within the 

polar cap respond quickly to changes in solar wind conditions due to the relatively direct connection 

between the terrestrial field within the polar cap region to the solar field in interplanetary space.
215

Spacecraft within the polar cap encounter plasma environments originating from a number 

of sources. Magnetosheath electrons with energies of approximately 100 eV penetrate to ionospheric altitudes where they are referred to as the ‘polar rain.’ Polar rain electrons typically exhibit 

energy fluxes on the order of approximately 10–2 erg/cm2s and densities of only approximately 

1/m3. These values are much less than the energy flux and density within the magnetosheath source 

region because only a fraction of the population can reach the ionospheric altitudes. Polar rain 

precipitation is most intense near the cusp and weakens toward the central polar cap. An upwardmoving polar wind electron and ion flux is also observed on polar cap field lines. The source of 

these particles is ionospheric plasma escaping from the polar ionosphere, which is accelerated 

upward along open magnetic field lines. The low energy ion component of the polar wind exhibits both field-aligned and conical distributions. Finally, sporadic events associated with polar cap 

auroral arcs are encountered within the polar cap, characterized by an intense localized precipitating electron flux and upward-flowing ions consistent with the acceleration of electrons and positive 

ions by parallel electric fields.

5.4.5.1 Ionosphere and Polar Wind. Density structures in both E and F regions showed 

that, in darkness, the ratio of enhanced density to the background density remained constant for 

many tens of hours and that the vortices only disappeared when they were convected into regions 

of sunlight or auroral precipitation.

There are systematic differences between the winter electron density signatures in the topside ionosphere in the southern and northern polar regions with the Southern Hemisphere having 

the lower densities. The region above the polar ionosphere contains mainly ionospheric plasma 

flowing generally away from the Sun across the polar cap and upward into the magnetosphere. 

Only weak fluxes of energetic components of the solar wind plasma enter the polar cap region, 

sometimes referred to as ‘polar rain.’

The light ion component of the ionospheric plasma, which flows upward into the magnetosphere even without solar wind energy inputs, is referred to as the polar wind. The heavy ion 

component responds to the following:

• Energy inputs from the solar wind.

• Heat-driven thermospheric dynamics.

• Solar cycle in UV inputs to the thermosphere.

• Magnetic activity.

This combined light and heavy ion source of plasma to the magnetosphere provides much 

of the material from which the energetic particle populations are generated. Theoretical research 

on this area has shown that results are highly model dependent and that 2D models produce different results than 3D models. Caution in use of models is strongly advised.

The polar cap region becomes active in Sun-aligned auroral arcs when the IMF is northward. Field-aligned electrodynamic systems, which close in the ionosphere, are associated with 

these arcs. Plasma drift inside the arcs is primarily anti-sunward, while outside the arcs both 

sunward and anti-sunward drifts occur.
216

A wide variety of ionospheric irregularities have been observed at high latitudes at nighttime. The plasma fluctuations range in scale from hundreds of kilometers down to centimeters. 

Plasma instabilities play an important role in the generation of medium scale (kilometers) and 

small scale (meters) irregularities. Examples of irregularities are patches of enhanced ionization in 

the E region (sporadic E) and of depleted ionization in the F region (spread F). Spread F is most 

frequently observed in the equatorial nighttime ionosphere. The irregularities cause signal fluctuations in traversing radio waves, known as scintillations.

Influx of solar plasma into the tail of the magnetosphere, sometimes preceded by solar 

flares, can cause complex ionospheric disturbances (storms). The most consistent pattern is an 

enhancement in D region ionization. These effects are most dramatic at auroral latitudes, but 

significant modifications in the ionosphere occur at all latitudes. A particularly severe event, called 

a polar cap absorption event, is due to energetic protons from the Sun producing ionization at altitudes as low as 40 to 60 km over the polar cap and causing a radio communications blackout over

a considerable period.

5.4.6 Plasma Effects for Design Consideration

This section describes the plasma effects that might degrade the spacecraft performance 

except for spacecraft charging. The major plasma effect, spacecraft charging, will be separately 

discussed in the next section.

5.4.6.1 Power System/Solar Array. Negatively grounded solar arrays operating in the 

relatively dense plasmas in LEO will experience a current drain on the power system as a result 

of losses through coupling to the plasma. This will become more severe as systems are enlarged and 

operating voltages are raised. Arcing of solar arrays in plasmas could lead to damage and electrical 

noise. Arcing becomes likely when the array is in a contaminated environment. Attraction of the 

ambient ions by negatively charged areas of the array could lead to sputtering and mass loss rates 

that may limit the lifetime of components, especially those designed for long duration operation.

5.4.6.2 Contamination. Contamination will be a critical issue in future missions and can 

be divided into two areas of concern. The first is that of spacecraft contamination, where material 

properties may be changed, thermal control systems affected, delicate sensing equipment damaged, 

etc. The relatively dense atmospheric pressure in LEO is important here. The second aspect is that 

of modification of the ambient atmosphere by outgassing from the spacecraft structure, thruster 

firings, water dumps, etc. Plasma effects and spacecraft charging can enhance the collection of 

contaminants especially at higher latitudes where the Debye length is larger.

5.4.6.3 Plasma Emissions. There will be a large scope for the generation and emission of 

plasma waves, not just by active beam emissions experiments but also by other spacecraft interactions. Plasma wake, spacecraft power leakage, and contaminant ions all provide a source of radio 

frequency emissions. As spacecrafts become larger and produce more contamination, the possibilities of disturbance, and of a wider range of generation mechanisms, frequencies, and power levels, 

will also increase.
217

5.4.6.4 Ionospheric Scintillation. Rapid electron density variations over a period of minutes 

affect radio waves propagating through the Earth’s ionosphere. Changes in the electron density 

along electromagnetic waves propagating through the ionosphere changes their velocity and transit 

time. This ionospheric scintillation effect degrades the performance of spacecraft air-to-ground 

communications systems and adversely affects the accuracy of the Global Positioning System 

(GPS). Large numbers of solar energetic particles, such as solar protons, can cause rapid changes 

in the ionospheric density that produces an ionospheric scintillation effect at high latitudes. Particle 

precipitation in the auroral zone results in increased scintillation in the ionosphere. The auroral 

oval is usually near the regions of increased ionospheric scintillation.

5.4.6.5 Secondary Plasma Effects. Many plasma interactions not only produce primary 

effects but also are responsible for secondary effects, acting in synergism with other interactions. 

As an example, a large structure with a large ion-free wake zone in a flux of energetic electrons 

could experience high negative charging levels. These in turn could attract contaminant ions to sensitive surfaces and modify the local plasma environment, causing plasma wave emissions that could 

disrupt sensors and monitors, or even in extreme cases, onboard computers and communications.

5.4.6.6 Electromagnetic Force. While not a plasma effect, discussion of potential differences produced by the v × B force is given here since the result is an additional potential on spacecraft that must be added to the electron-ion current balance charging to determine the absolute 

magnitude of the spacecraft potential.

Charge carriers in conductors moving with a velocity (v) through a magnetic field (B) 

are subject to the Lorenz force,

F=q (E+v×B) , (61)

that will separate charge. Ionospheric electric fields are typically sufficiently small that the magnetic 

term dominates and the electric field effects can be neglected. The result is an induction voltage 

given by:

V=(v×B·L) (62)

where L is a vector along the length of the conductor.

At completion of ISS construction, the dimensions of the photovoltaic array wings and 

truss structures are sufficient to result in induction voltages of ±30 V at high latitudes where the 

spacecraft velocity is nearly perpendicular to the vertical geomagnetic field. On the Tethered Satellite System-1R shuttle mission, a 20-km-long conductive tether generated voltages of ≈4,000 V, 

sufficient to arc and destroy the tether.

5.4.7 Spacecraft Charging

Spacecraft charging results from the differential collection of electron and ion currents on 

or in spacecraft materials when exposed to space plasma and radiation environments. The electric 
218

potential of a spacecraft varies (either positive or negative) as charge accumulates until an equilibrium is established where the net currents to and from the plasma environment are equal. Accumulation of a net negative charge density on spacecraft surfaces (surface charging) and buried inside 

insulators (bulk charging) generate electric potentials and fields. Electrostatic discharge (ESD) arcs 

occur when the electric fields associated with the potential gradients exceed the breakdown strength 

of the insulating materials. ESD is a well known source of spacecraft anomalies and failures. Both 

the plasma properties and the spacecraft design and operating characteristics influence the process, 

so knowledge of material electrical properties, spacecraft configuration, and grounding and bonding schemes are all required to determine if there is a spacecraft charging threat to a vehicle design.

Surface charging currents include the collection of ions and electrons from the plasma 

environment, conduction currents through the surface materials, active current sources such as 

electron guns or plasma contactors, and a loss of charge from the spacecraft surface due to the 

photoelectric effect and secondary emissions generated when energetic particles collide with the 

spacecraft surface. In addition, electrons with energies ranging from approximately 50 keV through 

a few MeV are capable of penetrating the outer surface of the spacecraft producing internal charging of spacecraft systems and materials. All of these processes are governed by the properties of the 

spacecraft materials including conductivity, thickness, density, composition, and work function as 

well as the plasma density, temperature, bulk velocity, presence of energetic particles, and intensity 

of solar flux within the space environment.

Generally, the space plasma environment can affect spacecraft charging behavior. An accurate definition of plasma properties is essential in determining the severity of these effects because 

the charging level of a spacecraft is directly related to plasma properties encountered on orbit. The 

primary charging environment in LEO is believed to be the low energy (<50 keV) particle deposition causing the surface charging which was found in the Maritime European Communications 

Satellite (MARECS) upsets. The second mechanism, although rarely happening in LEO, is due to 

the more energetic particle flux (>50 keV). The charged particles can penetrate into dielectric materials or surfaces and discharge at random times when external conditions change or with a dielectric breakdown.

Exposure to high energy (tens of kiloelectronvolts) auroral electron fluxes in the polar 

regions can lead to high levels of charging on spacecraft, particularly if the current collection 

occurs in the ion-depleted wake zones. The orientation of the magnetic field will complicate the 

interaction mechanisms, with fields parallel to the collection surface inhibiting the escape of secondary emitted electrons and enhancing charging.

5.4.7.1 Types of Spacecraft Charging. For the spacecraft designer, all types of charging 

(i.e., absolute, differential, and internal charging) are of concern:

• Absolute charging is the development of a potential on the spacecraft frame relative 

to the surrounding space plasma.

• Differential charging is the change in the potential on one part of the spacecraft with respect 

to another.

• Internal charging (or buried charging) is the potential buildup in the dielectric.
219

All charging, particularly buried and differential, may produce strong local electrical fields that 

can give rise to discharges. Arc discharging can cause problems for operating spacecraft. A primary problem is the occurrence of electronic switching anomalies that can be triggered by differential charging related discharges. The discharge-induced transients can cause system failures 

and, potentially, material damage. A more common anomaly is a phantom command that requires 

intervention from the ground, possibly results in loss of data and/or expendables, and shortens the 

operational lifetime of the spacecraft. Discharging can cause increased levels of contamination that 

result in changes in surface characteristics. The contaminants are attracted to oppositely charged 

surfaces and the material expelled during a discharge can be deposited on other surfaces. The surface charging on spacecraft can also bias plasma measurements of the space environment.

Recently, it has been found that the transient discharges due to spacecraft charging can lead 

to continuous arcing, referred to as a sustained arc. This occurs when the initial discharge transitions into a discharge between two closely spaced conductors at different potentials. The sustained 

arcs, powered by one or more solar array strings, can completely and permanently short out whole 

array strings. This phenomenon was first seen on the PAS-6 and Tempo-II satellites in GEO, but 

has also been seen in ground tests under LEO conditions.122

A few severe charging events have been observed in the auroral region. During 1983, instruments onboard the DMSP 7 observed in the polar region an absolute potential of –800 V.123 Since 

then a few events with higher potentials up to –1.2 kV were observed. The Advanced Earth Observing Satellite-II polar orbiting satellite lost over 80% of its power due to a discharge in an auroral 

event that led to arc tracking in a cable bundle.124 Furthermore, theory predicts that the larger 

spacecraft of the future will develop even higher potentials. Auroral charging differs from geosynchronous charging in that charging currents tend to be much higher; the vehicle is in a charging 

environment for only seconds, and the charging rate and the potential reached depend on the vehicle size. Auroral charging is more severe the lower the ambient thermal plasma density is because it 

tends to slowly discharge the charge caused by the fluxes of high energy auroral electrons. In addition, two-body and wake effects can become important. Also, differential charging between vehicles 

or a shuttle orbiter and an astronaut during extravehicular activity (EVA) is of concern.

Spacecraft charging in LEO equatorial orbits is produced by currents collected by exposed 

high voltage conductors, and is thus a function of the spacecraft power system voltages. While 

spacecraft charging is usually less severe in LEO than the GEO charging produced by solar substorms, it may be just as damaging as studies have shown that arcing voltage thresholds are lower 

in the LEO plasma than in GEO.125–127

Combining extreme plasma conditions (e.g., maximum/minimum temperature and density) 

for each region does not constitute the worst case situation because the extreme plasma density 

and the extreme temperature may not occur coincidentally. This worst case environment depends 

on variations of solar activities and the geomagnetic storm levels. The plasma environment in 

high altitude orbit is a dynamic medium that exhibits large, daily property variations. Essentially 

all major parameters can vary over at least 2 orders of magnitude daily. Dynamic changes in the 

plasma environment can produce large potential differences on spacecraft that might not otherwise 
220

develop during time-stationary plasma conditions. Figure 86 presents the NASA worst case 

environmentto be used in analysis of internal charging issues. Analysis of discharge events on 

spacecraft in geostationary or geostationary transfer orbits have shown that electron fluxes at internal dielectric breakdown must typically be sufficient to provide a fluence of 2×1010 electrons/cm2

in 10 hr.128 However, lower charging rates can lead to breakdown under extremely cold 

temperatures.

1×103

1×104

1×105

Suggested Worst Case

Environment

NASA AE8min 

(see text)

1×106

1×107

0.1 1 10

Flux (

Ne /cm2-s-sr)

Electron Energy (MeV)

Figure 86. NASA worst case spacecraft charging environment 

for internal charging issues.

5.4.7.2 Spacecraft Charging Mitigation. Mitigation of charging effects is possible through 

sound design of the spacecraft systems and structure.129A fundamental rule is to ground all conducting surfaces to a common point to reduce the possibility of differential charging. Ideally, 

all surface materials should be at least partially conductive and the use of dielectric materials 

on spacecraft surfaces should be avoided as much as possible. If the use of dielectric materials is 

required (e.g., for a thermal characteristic), the materials can be treated with conductive coatings 

or impregnated with conductive materials. Electrical filters should be used to protect sensitive 

circuits from stray currents produced by electrical discharges.

Processes for consideration of the plasma environment by the satellite designer and its 

possible impact on a spacecraft system follow:

• Determine the plasma environment to which the spacecraft will be exposed during its mission 

lifetime.

• Determine the magnitude of the satellite floating potential, differential charging, and internal 

charging in the average and extreme environments. A spacecraft charging analysis utilizing 

a charging computer code is generally required to obtain useful results.
221

• Determine if the magnitude of the charging is sufficient to potentially lead to detrimental effects 

to the spacecraft materials and systems.

• Determine what design steps are necessary to mitigate the effects of charging.

• Determine if charging is likely to be a problem to a spacecraft is a complex problem. Estimates 

of the average and extreme environments the spacecraft will encounter throughout its intended 

mission are required to determine the range of floating and differential potentials that are to be 

expected on the spacecraft structure as well as levels of internal charging.

Knowledge of the materials to be used in the construction of the spacecraft is required and 

insight into the design of individual systems is required to determine if the charging and associated 

effects can be tolerated by the spacecraft systems. Designers considering spacecraft charging for the 

first time may consult a number of excellent references on the subject including Kasha,123 DeForest,130 Garrett,131 Purvis et al.,129 and Garrett and Spitale.132 The collection of papers edited by 

Garrett and Pike,133 Kasha,123 and Tribble134 provide outstanding introductions to the subject of 

the plasma environment and spacecraft charging.

5.4.8 Plasma Environments and Spacecraft Charging Models

Most commonly used models both for specifying the conditions in the plasma environment and for analyzing spacecraft interactions with the environment are described in the following 

subsections. Users may also select from a variety of models that have been accepted by the plasma 

community for spacecraft design purposes.

5.4.8.1 Plasma Environment Models. A wide variety of ionospheric models are currently 

available.135 Selection of individual models and their use will require consultation with an ionospheric specialist since a variety of inputs and assumptions are required to obtain meaningful 

and valid results.

5.4.8.1.1 International Reference Ionosphere. One of the most commonly used computer 

codes for design work is the IRI model.136 The IRI model describes the ionosphere plasma properties in the 85 to 1,000 km altitude range for geomagnetic latitudes up to approximately 60°. Both 

the electron and ions (O+, NO+, O2

+, O2– ) densities and temperatures can be calculated as a function of latitude, longitude (geomagnetic or geocentric), time of day, day of year, altitude, and solar 

F10.7 radio flux. The error bars on the model during quiet times are a factor of 2 to 4 of the indicated values at altitudes below the F2 peak, and these are primarily the result of small-scale variations in the ionosphere. Above the F2 peak, problems with the scale height could lead to error bars 

of up to a factor of 10.

The IRI90 was used to generate figures 76–79. The IRI2000 is the most current version and 

is frequently updated by the plasma community. Different options are suitable to different applications; care should be exercised in this selection. Models of the ionosphere have not progressed to 

the point where they can be used for reliable forecasts of future conditions, especially those related 

to magnetic storm and substorm dynamics. Major questions of convection patterns are still 
222

unanswered. Even the static conditions predicted by the IRI90 model frequently do not match 

observational data perfectly. IRI90 is a climatic model giving average conditions. Great care must 

be exercised in the use of all ionospheric models.

5.4.8.1.2 GeoSpace. AF-GeoSpace, v.2.0 is a computer model developed by the U.S. Air 

Force Research Laboratory (AFRL) to facilitate the assessment of space environmental hazards 

by bringing often-used environmental models together under the umbrella of a user-friendly and 

graphics-intensive master program. One of the modules recommended here for the auroral environment is the Auroral Precipitation Model in GeoSpace. The AURORA science module is a compilation of statistical models of auroral ion and electron precipitation derived from measurements 

made by USAF particle detectors flown on the DMSP and P78-1 satellites. PL-GEOSpace uses 

functional representations for the following: 

• Integral electron number and energy flux.

• Electron-produced, height-integrated Hall and Pederson conductivities.

• Integral ion number and energy flux, which were derived using the statistical models presented.

The PL-GEOSpace code is distributed on CD–ROM and is available free of charge upon request 

through the AFRL.

5.4.8.2 Space Charging Analysis Models. Charging calculations are typically complex 

and require the use of time-dependent computer models. The most common analysis program in 

use today is NASA/Air Force Spacecraft Charging Analyzer Program-2000 (NASCAP-2k). Other 

codes include the Sensit, Environments Work Bench, Spacecraft Charging Handbook, and the 

Space Environment Information System codes (Equilibrium Potential code (EQUIPOT), and 

others).

5.4.8.2.1 NASCAP-2k. NASCAP-2k is the next generation spacecraft charging analysis 

code. It is a comprehensive update to the original NASCAP spacecraft charging codes written 

20 years ago. NASCAP-2k is a collaborative project between NASA and the AFRL that builds 

upon the Air Force’s DynaPAC charging algorithms. The model replaces 3D spacecraft charging 

codes for all environments in GEO, LEO, Auroral Charging, and interplanetary environments.

Satellites orbiting in LEO (excluding polar orbiting satellites) should use the spacecraft 

charging code NASCAP-2k LEO option for any charging analysis.117 LEO is typically defined as 

a satellite orbiting within an altitude of 100 to 1,000 km. NASCAP-2k LEO option is the appropriate code for these satellites, less than ±51° latitude, and where the Debye length (typically in the 

order of 1 cm for this region) is small compared to the size of the spacecraft. This 3D code utilizes 

space charge limited particle collection. This type of current collection is appropriate in regions 

where the plasma is denser and means the space charge actually limits the range of potentials on 

the spacecraft. NASCAP-2k LEO option is appropriate to use for satellites with solar arrays configuration. It contains a solar array power system model that can determine the solar array currents. It is strongly encouraged for any satellite with solar arrays biased greater than approximately 

–100 V orbiting in a LEO to perform a spacecraft charging analysis.
223

Subsets of a low Earth-orbiting spacecraft are those in polar orbits (greater than ±51° ±5° 

latitude) where spacecraft are exposed to fluxes of auroral particles. The auroral part of NASCAP2k code also utilizes space charge limited particle collection. It is a 3D code specifically designed 

to incorporate the ambient LEO orbit plasma density, as well as the hotter environment from 

the plasma brought to low altitudes by the open magnetic field lines. The auroral component of 

NASCAP-2k also incorporates the satellite wakes into charging calculations. This charging code 

is appropriate for satellites orbiting in short Debye-length plasmas.

5.5 Low Earth Orbit (90 to 2,000 km) Ionizing Radiation Environments and Effects

Once the space vehicle is above 200 km it is considered to be in space. Under 2,000 km, it 

would be considered to be in the low Earth region and if in orbit, it would be in LEO. This section 

represents the transition from the upper reaches of the terrestrial environment to the LEO region 

of space.

5.5.1 LEO Ionizing Radiation Environments

Sections 2.12.1 and 3.11.2 described the processes involved in understanding the ionizing radiation environments that will be encountered in the terrestrial segments. The discussion in 

section 3.11.2 is still valid for the region from 90 to 200 km of this section. Additionally, the discussion in that section on the GCR and SPE environment is applicable to this segment with the 

exception that those environments will now be seen without atmospheric shielding effects. Variation 

from the terrestrial environments to the space environments will be discussed next.

5.5.2 LEO Environments

The near-Earth radiation environment can be divided into a trapped radiation environment 

and transient radiation environments. A depiction of these environments is shown in figure 87. 

The trapped environment is due to the Earth’s magnetic field confining charged particles to certain 

regions of space. These regions are termed the Van Allen Belts. Nominally there will be one proton 

and two electron belts (inner and outer) though this can temporarily change with large solar events. 

The transient environments are due to the effects of the Sun (solar wind and solar particle events) 

and GCRs. A 2D artist’s depiction is shown in figure 88 and a 3D model-based view of these 

trapped belts is shown in figure 89.

Sections 2.12.1 and 3.11.2 have dealt with the transient ionizing radiation environment 

in sufficient detail. Therefore, this section will concentrate on the trapped particle environments.

Figure 90 shows the approximate regions of space the various radiation environments 

occupy. Since boundaries of the bands are not sharp transitions, the indicated numbers should 

be considered approximations. The trapped radiation belts extend from approximately 500 km to 

about 12 Re (roughly 76,000 km). Normally, over this range there are two electron bands (with different population and energy spectra) and one proton band. As shown in figure 89, however, 

sometimes a departure from normalcy occurs. Figure 89 is based on data from a severe solar event 

in 1989 that formed a third electron belt (between the inner and outer zones) and a second proton 

belt (higher in altitude) lasting for many weeks.
224

newF87

Cosmic Rays Spacecraft

Trajectory

SPE

Earth’s

Radiation

Belts

Figure 87. Depiction of all the radiation types that a spacecraft can experience, 

including the inner and outer trapped radiation belts, solar particle event, 

and galactic cosmic radiation.

newF88

Figure 88. Two-dimensional artist’s depiction of the trapped radiation environment.
225

newF89

(a)

(b)

Figure 89. Three-dimensional views of the trapped radiation environment: 

(a) The true toroidal shape of the radiation belts, with transparent 

green representing the outer electron belt and solid green the proton 

belts (data from a particularly strong storm event where a second proton 

belt was formed), and (b) edge-on view shows cross-sectional data for 

the belt regions with low to high density represented by blue to red.
226

Inner Zone Electrons

Outer Zone Electrons

Trapped Protons

SPE/GCR

newF90

Region

1 2 3 4 5 6

~2:8

~3:8

~5

Geosynchronous

~12

7 8 9 10 11 12 13 14 15

Equatorial Radius (Earth Radii)

~2.8

~3.8

~5 ~12 Geosynchronous

Figure 90. Pictogram showing regions of space where radiation types are significant. 

Regions are plotted as a function of the equatorial radius, in Earth radii 

(~6,370 km), because the belts vary with the magnetic field that changes 

with increasing inclination off the equator.

A specific region of space that warrants special coverage is the SAA. The Earth’s magnetic 

field axis does not point to geographic north (and does not pass directly through the center of the 

Earth. The combination causes a deformation of the magnetic field over the South Atlantic (magnetic field lines dip lower in altitude) and over Southeast Asia (field lines are higher in altitude). 

The net effect, the harshness of the radiation belts, is seen at lower altitudes in the SAA region 

(see fig. 91). Thus, for spacecraft in LEO, this region tends to dominate the observed radiation 

environment. For systems at higher altitudes, however, this effect is less significant.

Another important point is that the orbit of a spacecraft can take it into and out of the 

radiation belts. Figure 92 shows the electron environment as a function of time for a satellite in low 

Earth polar orbit.It is evident there are times of radiation exposure followed by times of no 

exposure.

Therefore, depending on the orbit or trajectory of the space vehicle, and mission time, the 

vehicle will be exposed to trapped electrons and/or trapped protons environments. The exposure 

levels and the respective energy spectra for these particles will be mission specific but will be presented as a differential and integral energy spectra that can be used to analyze for effects that will 

be discussed next.

5.5.3 LEO Ionizing Radiation Effects

5.5.3.1 Total Ionizing Dose. The term total ionizing dose (TID) implies the dose is deposited to the electronics through ionization effects only. The energy deposited by ionizing radiation 

moves the electrons of the material to a higher energy state, thus making them available for conduction and mobile inside a nonconductive material. These electrons, or more correctly the positive 

charge created by the ionization, are the prime cause of the total ionizing dose effects.
227

new F91

Figure 91. Contour plot of the trapped proton environment overlaying a map

of the Earth. Proton density increases from blue to green to yellow 

to red. Contours are highlighting the SAA where the radiation particle 

population is high due to the offset nature of the Earth’s magnetic field.

101

102

103

104

105

106

107

F94

Positional Electron Flux (No./cm2-s)

0 20

North

Polar

Region

Equatorial

Region

Northern

Horn Northern

Horn Southern

Horn

Southern

Horn

North

Polar

Region

South

Polar

Region

Electron Flux

Flux Free

40 60 80 100 120

Relative Time (min)

Figure 92. Plot of trapped electron flux as a function of relative orbital time 

for a 900 km polar orbit. The graph shows the regions of the 

radiation belts and the noncontinuous exposure (horn identifies 

regions of space (at high latitudes) where the outer electron belt 

is seen at lower altitudes).
228

In general, all types of electronics are susceptible to this ionization but the charge generated 

inside the semiconductor material can quickly be collected and removed without ill effect (assuming the radiation interaction rate is at the low level of the space environment). If a semiconductor 

device contains, for example, a silicon dioxide/silicon interface (as in all modern integrated circuits 

based on complementary metal oxide semiconductor (CMOS) technology), charge generated inside 

the oxide can become trapped at the interface. This trapped charge, by changing the potential of 

the interface structure, can lead to increased ‘leakage’ current or changed operational characteristics of any device using this structure (if this interface exists at a biasing point). Details of this type 

of cumulative effect can be found in references 137 and 138.

A special consideration for TID comes for a specific technology called linear bipolar devices. 

Unlike the CMOS technology that uses field effect transistors, linear bipolar devices use bipolar 

transistors as the switching and amplification elements. For many years it was felt that the sensitivity of this technology to ionizing radiation was good. However, it was discovered that the technology can be susceptible to a cumulative effect of ionizing radiation that is truly dependent on the 

rate at which the dose is deposited. It was found that the same device would degrade faster with 

a dose under low dose rate conditions than at higher dose rates. This effect is termed the enhanced 

low dose rate sensitivity (ELDRS). ELDRS is still a topic of research and the most current information can be found in the December issues of the IEEE Transactions on Nuclear Science, containing the papers presented at the Nuclear and Space Radiation Effects Conference held that year. 

The most current review paper on the ELDRS topic can be found in reference 139.

From an engineering perspective, these cumulative macroscopic effects are important in 

determining the reliability of electronics to the ionizing dose environment. To determine a part’s 

reliability in this radiation environment, a detailed set of test data is required. Simply testing a part 

for functionality after exposure to a given radiation level may not be sufficient. Parametric data 

must be taken with functionality testing over smaller dose steps than one-shot, go/no-go testing. 

Test data for all parts in a given design must be reviewed for meeting all specifications (not just 

functionality). This radiation performance change information must be combined with aging and 

temperature effects. Finally, overall degradation must be compared with actual circuit specification 

to ensure proper operation throughout mission lifetime.

Test data must also show information useful for mission procured electronics. A high degree 

of variability in test results is possible for the same parts from the same lot from the same manufacturer. Actual testing should include a number of parts from the same lot and the data should be 

treated statistically based on observed variability and sample size. Variability results from the attention paid by the manufacturer to designing and building parts while being mindful of the radiation 

environment. A manufacturer producing a ‘rad-hard’ part will have a small amount of variability. 

A mass-produced part for the commercial market, however, can have extreme variations. Therefore, 

testing should be tailored for the radiation quality of the commercial part (i.e., from accepting 

previous test data for rad-hard parts to performing piece-part testing). As more and more manufacturers leave the radiation-tolerant market, testing of parts becomes more critical to system design. 

This process of understanding the test data and the impact on system design is called radiation 

hardness assurance. A detailed review of this process can be found in reference 140.
229

In general, TID effects are mitigated through proper use of shielding materials. These effects 

depend on transfer of energy from the radiation environment. If the particle environment population or its energy is decreased, the effect is lessened, i.e., exactly what shielding does. By forcing the 

particle to transport through an intervening material, the environment interacts and loses energy 

(sufficient energy loss can mean the particle never enters the electronics). By appropriately selecting the shielding material and its thickness, mitigation is optimized. Since nothing is free, the cost 

associated with shielding is weight. Putting weight into orbit costs money. Therefore, the tradeoff 

is survivability or reliability of electronic systems versus added cost to get systems into space. Also, 

the effect of shielding is not linear (as shown in fig. 93). At some point, depending on the environment and relative ‘hardness’ of the electronics, addition of reasonable shielding is no longer effective. It is easily seen from figure 93 that the first few millimeters of aluminum reduces the daily dose 

by over 2 orders of magnitude. However, to reduce the dose from this point by another factor of 2 

requires an additional 50 mm of aluminum, a significant weight addition.Therefore, the only reliable mitigation is replacement of the part with a more tolerant version. A discussion on radiation 

shield design for space systems can be found in reference 141.

newF93

Dose (cGy(Si)/day)

0 10 20 30 40 50 60

Depth (mm)

10–1

100

101

102

103

104

Figure 93. Plot of the total ionizing dose as a function of effective 

aluminum shield thickness.

5.5.3.1.1 Displacement Effects. The second area of the cumulative effects of radiation is 

displacement damage. When radiation interacts with the material, either electronically or via direct 

nuclear interaction, energy is imparted to the atom as a whole. This energy is typically seen as heat 

by the increasing vibrational motion of the atoms. Or if the energy transfer is sufficiently high, the 

atom can overcome the binding energy of the crystalline lattice of the material. If this occurs, the 
230

atom is ‘displaced’ from its normal position to various end locations. Unless the end location is 

an exact duplicate of the former position, the regular order of the crystalline lattice is disturbed.

Since this regular order gives semiconductor materials their unique properties, the disturbance causes changes in the operation of any device exposed to this environment (level of dose 

a device is susceptible to varies, but all electronics are affected). This change may add a current 

path that previously did not exist (allowing increased leakage current) or make conduction more 

difficult in regions designed for flow. For example, diodes become less effective (two-way current 

flow becomes easier) and the inherent amplification capabilities of transistors diminish. Similar 

to the total ionizing dose, effects of displacement damage are cumulative. With a small exposure 

to a radiation environment, observable effects are small but effects build with exposure time.

The solar cell is an example for this category since it displays these described effects and 

is a common element to most spacecraft. The solar cell is basically a diode with one side exposed 

to the illumination of the Sun; photons hit the device, creating electrons. The electric field of the 

diode (cell) allows the electrons to be collected, thus, generating power. With exposure to radiation, 

diodes become less effective. Diode leakage current increases, generated electrons ‘live’ for much 

shorter time periods, and the internal electric field decreases. All these effects combine to make the 

solar cell less efficient producing power. Power reductions of 50% are possible depending on solar 

cell construction and the environment to which the cell is exposed. In general, shielding is effective for mitigating displacement effects, but adding shielding to resolve this solar array problem 

is difficult. The shielding (called cover glass in this application) must be transparent to the optical 

photons and make a good optical interface at the solar cell (minimize refractive effects). Even with 

these conditions met, transmission losses increase with increasing cover glass thickness. Therefore, 

a balance must be reached between allowable radiation degradation and cell power production 

efficiency.

A thorough treatment of displacement damage theory and the hardness assurance processes 

needed for space systems is described in reference 142.

5.5.4 Low Earth Orbit-Based Effects

Both cumulative and transient radiation effects are important for this segment of a space 

mission. The SEE impacts and analyses are the same as those discussed in section 3.11, so no further discussion is needed here. Section 5.5.3 discussed the cumulative effects of ionizing radiation. 

Since particles trapped in the Van Allen Belts can produce both TID and displacement damage, 

this mission segment requires an analysis for both of these types of effects.
231

5.6 Meteoroid and Orbital Debris Environments

5.6.1 Meteoroid Environment

Meteoroids are natural particles that continually enter space from a variety of sources 

within the solar system: comets, asteroids, and planets. The cometary particles form the bulk of 

the particles in the size regime of interest to engineers (>100 mm), of the ‘sporadic’ meteoroid 

background, with asteroids contributing the remainder. The cometary/asteroidal ratio is still being 

investigated; future revisions of this TM will discuss any modifications to this ratio. The planetary 

component is insignificant by comparison, though it certainly does exist (as evidenced by the Allan 

Hills 84001 Mars meteorite). For an object in orbit around the Sun in the inner solar system (such 

as the Earth), the sporadic meteor sky looks similar to figure 94. Note that there are six sources 

or radiants that can be divided into three groups according to origin. The helion sources, which 

account for the bulk of the flux, are comprised of particles from a subset of the short period 

comets called Jupiter family comets (e.g., comets Halley and Encke), whereas the toroidal sources 

probably derive their material from another subset of the short period comets called Halley family 

comets. The apex sources derive their material from long period comets. As might be expected, the 

average speed of meteoroids from the apex sources (~46 km/s) is higher than that of particles coming from the helion radiant (~24 km/s) since the Earth is hitting the apex meteoroids nearly head-on.

newF94

90

180

–90

–60 –60

–30 –30

0 0

30 30

60 60

8

8

8

8

8

8

8

–90 90

18

North

Torodial

North

Apex

South

Apex

South

Torodial

Helion

Source

Anti

Helion

Figure 94. Radiant distribution of meteor sky according to radar (the coordinate 

frame places the Earth’s apex of motion at the center, the solar (helion)

direction at a longtitude of –90°, and the north ecliptic pole 

at latitude 90°).
232

Figure 94 displays the radiant distribution of the sporadic meteor sky as seen by radar for 

meteoroids with mass >10–4 g encountering the Earth.143,144 The radiant distribution is based 

on the Harvard Radio Meteor Project (HRMP) 1969 meteor orbit survey, which has been corrected for various selection effects. Other radar surveys such as the Adelaide radio surveys from 

1960–1969, the Kharkov radio survey of 1975, and the recent Canadian Meteor Orbit Radar145

survey also confirm the locations of these radiants. Particles of sizes >100 μ can do considerable 

damage to thermal protection systems (TPSs), radiators, windows, and pressurized containers. 

Some secondary effects might include partial penetration or pitting, local deformation, and surface 

degradation that cause a failure upon reentry. The speed, mass, density, and directionality of these 

small particles are important factors for design considerations and mitigation during operations. 

The shape of the meteoroid and its orientation relative to its trajectory can also affect the degree 

of the damage, but present knowledge of the meteoroid environment is inadequate to specify these 

parameters.146 For the purposes of this environment definition, the assumption is made that meteoroids are spherical in shape.

5.6.1.1 Meteor Showers. In addition to the normal influx of background meteoroids, the 

Earth (and spacecraft near it) also encounters quasi-periodic meteoritic enhancements caused 

by streams of material ejected from short period comets that pass near the Earth’s orbit. These 

streams of debris produce the meteor showers observed here on Earth, and normally represent 

only a modest enhancement over the background (a few percent). However, the density of material in a stream can be increased by a recent entry of the parent comet into the inner solar system, 

thereby resulting in an enhanced shower (meteor rates of several hundred per hour) or a meteor 

storm (rates in excess of 1,000 meteors per hour). As might be expected, an enhanced shower or 

storm presents a time of increased risk for spacecraft. Streams that are notorious for producing 

enhanced showers or storms at Earth are the Perseids, the Draconids (also called the Giacobinids), 

and the Leonids. The Leonids are especially famous for producing spectacular meteor storms at 

33-year intervals.

Table 82 presents information on those meteor streams that are known to have the potential for causing outbursts at Earth and the Earth-Sun L1 and L2 points. It would be appropriate 

to devote some thought on how to deal with stream activity, whether through design or mitigation 

procedures, since at least one satellite has been damaged and one effectively killed by encounters 

with meteor streams.

Table 82. Meteor streams known to produce enhanced or storm level activity.

Stream

Radiant

Time of 

Maximum Activity

RA

(deg)

Declination

(deg)

Speed

(km/s)

Quadrantids 230 49 41 January 3

Cygnids 286 59 25 August 18

Lyrids 271 34 49 April 22

Draconids 262 54 20 October 9

Perseids 46 58 59 August 13

Leonids 152 22 71 November 17–18
233

In mid-September 1967, the Mariner IV spacecraft was located midway between the orbits 

of Earth and Mars when it encountered an unknown meteor stream. The onboard meteor counter 

registered a thousandfold increase in flux for ≈45 min, during which time the spacecraft was slightly 

torqued about its roll axis and had some of its thermal insulation ripped away. Mariner IV was 

lucky in that it suffered no major damage; such was not the case with the European Olympus communications satellite in August 1993. During the enhanced shower experienced that year, the Olympus satellite was struck by a Perseid somewhere near its electronics bay. The resulting plasma 

discharge generated enough current to disable a gyro, causing the satellite to tumble. By the time 

control was restored some days later, the spacecraft had used practically all of its fuel and was 

effectively dead as far as its mission was concerned.

It should be noted that even though there are a small number of reported failures, NASA 

and other scientific spacecraft represent a small fraction of the total satellite surface area; commercial and military satellites account for the majority of exposed surfaces. It is likely that there have 

been other spacecraft failures resulting from meteoroid impacts; however, insurance or national 

security concerns often prevent the public release of such information. Still there exist numerous 

anecdotal reports about attitude anomalies etc.; are these issues the result of a meteoroid strike or 

faulty thruster? More often the lack of engineering data prevents absolute confirmation of a meteoroid strike. Fortunately, current design practices for Earth-orbiting spacecraft tend to mitigate the 

meteoroid threat since the primary threat from solid particles is from orbital debris.

5.6.2 Orbital Debris Environment

Orbital debris is man-made material induced by spacecraft that can be as large as spent 

rocket motors and as small as the dust particles ejected from the nozzles of maneuvering thrusters. Spent satellites, accidental explosions, and collisions between orbiting bodies are contributing 

towards a rapid increase in this hazard. A serious problem with orbital debris is that it can occur 

in a size range that is large enough to destroy a spacecraft but is too small to be tracked from the 

ground. The average impact speed of debris on a spacecraft is 10 km/s, only half that of meteoroids, but the population of debris in typical Earth orbit is much higher than that of meteoroids, 

making debris a greater hazard for most spacecraft.

NASA’s orbital debris program at JSC routinely gathers information about the debris 

population from the Haystack, Haystack Auxiliary, and Goldstone radars. The debris population 

is described using statistical sampling from these ground-based radars, which can detect particles 

as small as a few millimeters in size. Tracking particles smaller than 10 cm is difficult, so statistically sampling the environment provides the most efficient means to develop an engineering debris 

model. Much of the debris environments are in families of near circular orbits but some are in 

more eccentric orbits. Using statistical methods, population ‘fits’ are used to construct debris 

populations in the Orbital Debris Engineering Model 2000 (ORDEM2000). The populations 

are functions of altitude, eccentricity, inclination, and size. The biggest unknown with the debris 

environment along with meteoroids is the shape. Shape and material properties for the debris are 

important for calculating collision penetrability.147
234

5.6.3 Models/Analysis

5.6.3.1 Meteoroid Environment Models and Analysis. Any accurate model of the meteoroid 

environment in near-Earth space must properly describe these three aspects of the environment: 

flux, directionality, and velocity distribution. These three parameters are inseparably intertwined 

in both the development of a meteoroid model (where crater counts from surfaces exposed in space 

are converted to the flux versus mass, directionality, and velocity distributions of the model), and 

in the reverse process where the model is applied to estimate the crater counts to be expected on 

a future spacecraft.

For example, velocity enters the relationship between crater dimensions and impactor size, 

as well as the estimation of gravitational focusing. Directionality affects the exposure and crater 

size evaluations. Unfortunately, it is now known that there are deficiencies in the velocity distributions/assumptions used in older NASA meteoroid models. Improvements in computational 

techniques, along with new research efforts, have allowed us to incorporate the directionality of 

the sporadic environment. The previous NASA meteoroid model documented in Space Station 

Program (SSP) 30425, which assumed an isotropic directionality and three-step speed distribution, 

should be replaced.

Several researchers in this field have published directional models such as Taylor and 

McBride,143 Brown and Jones,148 and the historical work presented by Elford et al.149 Also, new 

models developed by NASA’s Meteoroid Environment Office (MEO) and European Space Agency 

have been designed to remedy many of the earlier mentioned deficiencies. The MEO model is 

known as the Meteoroid Engineering Model (MEM)150 and is applicable to spacecraft in Earth 

orbit and trans-lunar space, spacecraft in lunar orbit, and spacecraft in interplanetary space 

between 0.2 and 2 AU.

MEM reports the overall meteoroid flux relative to a spacecraft’s trjectory; it also reports 

the flux per second interval, per angular interval, and per input state vector. The output files are 

designed to be compatible with the BUMPER151 risk analysis tool. MEM was developed to replace 

the older Grün model that did not address the directionality of the environment and assumed one 

average speed at Earth. The directionality and velocity distributions are an important part of accurately modeling the risk to spacecraft and should not be ignored.

5.6.3.1.1 Flux. The interplanetary submodel of MEM (which has no gravitational focusing 

and planetary shielding effects) yields a flux of meteoroids greater than 10–6 g of 5.7 particles/m2/

yr or 6.5×10–4 particles/m2/hr at 1 AU. This is a cross-sectional area flux for a randomly tumbling 

satellite, where the relevant area is the time-averaged, cross-sectional area. Figure 95 shows the sporadic flux as a function of mass from MEM at 1 AU.
235

1×10–13

1×10–12

1×10–11

1×10–10

1×10–9

1×10–8

1×10–7

1×10–6

1×10–5

1×10–4

1×10–3

newF95

Flux (No. m–2 hr –1)

1×10–6 1×10–5 1×10–4 1×10–3 0.01 0.1 1 10

Mass (g)

Figure 95. Sporadic meteoroid flux as a function of mass.

This is the flux in terms of mass; meteoroid densities are needed in order to convert to the 

particle sizes required by penetration equations. Discussion later in this section will cover current 

efforts to characterize the meteoroid density distribution.

5.6.3.1.2 Directionality. It is now widely documented that sporadic meteors radiate from 

six distinct sources rather than approaching uniformly from all directions. Imagine a coordinate 

system in the plane of the Earth’s orbit (the ecliptic), with 0° longitude being located at the position of the Sun and 270° being the approximate direction of the Earth’s motion. In this system, 

and without the disturbing effects of Earth’s gravity and shielding, background meteors would 

then be seen radiating from the following six sources:

• One source near the Sun (the helion source).

• One source nearly opposite the Sun (the anti-helion source).

• Two sources near the direction of Earth’s velocity (the apex sources).

• Two sources located toward the apex, but 60° above and below the plane of the ecliptic 

 (the toroidal sources).

As it is currently modeled, the flux of meteors from the two helion sources account for 

approximately 46% of the total flux. The apex sources provide only 3%, and the toroidal sources 

contribute 45%. These source strengths are currently being updated and at time of publication, 

this is the best estimate available. The asteroidal component is severely underrepresented and 

is currently modeled as contributing approximately 6% of the total flux. Table 83 summarizes this 

directionality information.
236

Table 83. The six background meteor radiants.

Source

Longitude

(deg)

Latitude

(deg) Fraction of Flux

Helion 342 0 0.23

Anti-helion 98 0 0.23

North apex 270 15 0.015

South apex 270 –15 0.015

North toroidal 270 60 0.225

South toroidal 270 –60 0.225

MEM can also calculate the average flux on the surfaces of an oriented spacecraft modeled 

initially as a basic cube structure. This orientation displays the directionality of the sporadic meteoroids encountered in a 1 AU orbit. Figure 96 shows the importance of surface orientation and 

directionality effects.

0

0.5

1

1.5

2

2.5

newF96

Flux (No./m2/yr)

Ram

Wake

North

South

Starboard

Port

Sun

Anti-Sun

Earth

Spacecraft Surface

Figure 96. Flux on spacecraft surfaces for limiting mass, 10–6 g 

at a 1 AU orbit (no gravitational focusing or planetary 

shielding effects are considered).

The basic cube modeled in MEM is oriented with the Ram surface along the velocity vector 

and the North surface oriented along the angular momentum vector. One thing to notice is that, 

for a nearly circular orbit, the position vector and velocity vectors will be nearly perpendicular, 

and therefore the port and Sun surfaces will see the same flux. This will change if the orbit is more 

eccentric, as will the effects of directionality become more enhanced.
237

Directionality is a very important issue for spacecraft, as less shielding can be used if the 

meteoroid threat is shown to be significant on specific surfaces and not over the whole spacecraft. 

Less shielding saves valuable resources (weight and money). Spacecraft orientation is also another 

important risk mitigation strategy, and using a directional model to assess orientation will improve 

any designer’s assessment.

5.6.3.1.3 Velocity. Previous velocity distributions and average speeds associated with older 

meteoroid models suffered from various biases and misapplications between models. Based on best 

estimates as of this publication, the velocity distribution at Earth as modeled by MEM is given in 

figure 97.

newF97

Frequency

0 10 20 30 40 50 60 70 80

Speed (km/s)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Figure 97. Average meteoroid speed distribution at 1 AU (no gravitational focusing 

or planetary shielding effects are considered).

This distribution represents the composite of all of the meteoroid sources, weighted for 

source strength in a circular 1 AU orbit, basically representing what is seen by the Earth but ignoring gravitational focusing and Earth shielding effects.

The composite distribution was derived from individual distributions associated with the 

sporadic meteoroid sources. If symmetry is assumed among sources of the same type (helion/

anti-helion, north apex/south apex, etc.), the velocity distributions can be reduced to four, one 

for each source. The speed distributions behind MEM have been compared to individual source 

speed distributions from the HRMP149 and are in good agreement with the only known published 

meteor survey, provided the biases in the old HRMP data is accounted for as described in Taylor 

and McBride.143 Table 84 shows the average speeds coming from each source, excluding any Earth 

disturbing forces.
238

Table 84. Meteoroid average speeds at 1 AU, without Earth 

focusing or shielding included.

Source

Vavg 

(km/s)

Helion 24

Anti-helion 24

North apex 46

South apex 46

North toroidal 20

South toroidal 20

Asteroidal 5

Notice that while the helion and toroidal sources have nearly the same average velocity, the 

apex meteoroids have an average speed of approximately 46 km/s, which is twice that of the other 

sources. Some consideration should be given to surfaces that might face in the apex direction for 

an extended period of time. Masses being equal, the average apex meteoroid will have 4 times the 

striking power (relates to penetration) and 16 times the plasma production potential (plasma produced by a meteoroid impact is proportional to v4) than a meteoroid from the helion sources.

Also, in table 84 there is an asteroidal source average speed. This value came from analysis 

of orbital distributions of asteroids taken from infrared measurements and was provided by J.C. 

Liou of JSC.152 Although the strength of the asteroidal component is underrepresented in current 

meteoroid models, the average speed is presented for completeness.

The strengths of the individual sources are still a highly debated topic and most important 

in creating a complete picture of the meteoroid threat. Again, future releases of this definition 

document will contain revisions as necessary to update the environment based on new information.

5.6.3.1.4 Gravitational Focusing and Planetary Shielding. Meteoroid trajectories can be 

dramatically altered when approaching a planet. The size of the gravitating body and the flyby 

geometry will cause the meteoroid trajectories to converge or diverge leaving regions of space 

behind the planet relatively free of meteoroids or intensely populated.153 The probability of 

encounters with meteoroids increases for spacecraft whose orbits cross into these intensely populated regions. The effects of focusing and shielding on sporadic meteoroids (sporadics) is complicated by the fact that sporadics approach from all directions with varying speeds. Therefore, the 

gravitational field does not focus the sporadic meteoroids to a point. Instead, sporadics are focused 

to varying distances on the opposite sides of the Earth. Slower moving meteoroids such as those 

from an asteroidal source are focused more and their trajectories are altered more significantly than 

fast meteoroids from the apex source.

There have been several methods published and presented that describe the equations and 

geometries involved in planetary focusing and shielding (M. Matney, Private Communication, 

2006).154,155 While almost all methods accurately describe how the meteoroid trajectories are 

deflected, the focusing factors do not agree. 
239

The Earth submodel of MEM can then be used to model the gravitationally focused and 

shielded meteoroids in Earth space out to the lunar sphere of influence. At that point, the Moon 

becomes the central influencing body on meteoroids, and, for first order calculations, the Earth’s 

focusing effects at that distance are second order at best.

The following figures are examples of how sporadic meteoroids are influenced by the gravity field of the Earth. Plot (a) in figure 98 shows what the sporadic meteoroid environment looks 

like at 1 AU in a circular orbit around the Sun without the effects of focusing or shielding. Plot (b) 

shows the sporadic meteoroid environment in LEO at an altitude of 400 km. Both plots are relative 

to the local spacecraft frame and motion, and show flux intensity as a function of local azimuth 

and elevation. The darker red colors indicate a higher concentration of meteoroid flux. Also notice 

in plot (b) that the Earth is obviously shielding a large portion of the meteoroids, and that it is not 

a straight line cutoff but has a spherical shape due to the aberration effects of spacecraft motion

newF98

Degrees Elevation

40°

–40°

20°

0°

–20°

180° 210° 240° 270° 300° 330° 0° 30° 60° 90° 120° 150° 180°

60°

–60°

80°

–80°

Degrees Azimuth From Ram (V)

Flux by Direction (in Local Spacecraft Coordinates)

–2

–3

–4

–5

–6

–7

–8

–9

Degrees Elevation

40°

–40°

20°

0°

–20°

180° 210° 240° 270° 300° 330° 0° 30° 60° 90° 120° 150° 180°

60°

–60°

80°

–80°

Degrees Azimuth From Ram (V)

(a)

(b)

Flux by Direction (in Local Spacecraft Coordinates) –2

–3

–4

–5

–6

–7

–8

–9

Figure 98. Flux intensity plots of the sporadic meteoroid directionality relative to local 

spacecraft coordinates moving in circular orbits at (a) 1 AU from the Sun 

and (b) 400 km above the Earth, respectively.
240

Table 85 shows the effects of focusing and shielding on a spacecraft in LEO compared to 

a spacecraft in a circular orbit around the Sun. The specifics of the orbits were described in the 

preceding paragraph.

Table 85. Flux and average speed values for ram/wake surfaces of spacecraft 

in circular orbits at 1 AU from the Sun and 400 km above the Earth,

respectively.

Circular Orbit

Total Flux 

From All 

Directions

(No./m2/yr)

Ram Flux

(No./m2/yr)

Average Ram 

Speed

(km/s)

Wake Flux

(No./m2/yr)

Average 

Wake Speed 

(km/s)

1 AU orbit (no focusing 

or shielding)

5.75 1.52 23.8 0.29 20.9

400 km Earth orbit (with 

focusing and shielding)

7.59 4.21 24.6 0.09 23.9

5.6.3.1.5 Meteoroid Densities. Meteoroid models of the past have assumed various densities for penetration calculations. It is currently accepted that most meteoroids are cometary in 

origin; based on the available evidence, such bodies are not homogenous spheres. They are porous 

conglomerates of ice and dust with a mean bulk density close to water. The hypervelocity effects 

of these cometary particles probably cannot be adequately represented by the aluminum projectiles used in orbital debris studies; experiments are encouraged to find substitutes that approximate the bulk density. The remainder of the meteoroid population originates from asteroids or, 

much less frequently, planets. These meteoroids will have a bulk density of approximately 2 g/cm3

(4.8×10–4 lb/in3); most experimenters use boro-silicate glass or olivine projectiles in hypervelocity 

tests, as these are thought to be adequate representations of this meteoroid type.

A recent meteoroid radar survey has compiled a database of over 1,000 meteoroids and 

presented a useful meteoroid density distribution.156,157 Figure 99 shows these densities.

The following equation and table of values (table 86) are used to fit the density distributions 

presented in figure 101:

y = a exp !0.5

ln x / x ( ) o

b

"

#

$

%

&

'

2 (

)

*

*

*

+

,

-

-

-

. (63)

The mean bulk densities determined by the 3D spherical and overdense theories were 

0.6±0.5 and 0.7±0.6 g/cm3, respectively. These data are valid over the 10–6 to 10–4 g mass range. 

These densities are consistent with those determined by other researchers, a summary of which 

can be seen in SEE/TP-2004-400, section 2.6.157
241

0

0.2

0.4

0.8

0.6

3D Data, 636 Meteors

Lognormal Fit, 3 Param

1

newF99

Number of Meteors (Normalized)

0 0.5 1 1.5 2 2.5 3

Bulk Density (g/cm3 (a) )

0

0.2

0.4

0.8

0.6

Ovr Data, 649 Meteors

Lognormal Fit, 3 Param

1

Number of Meteors (Normalized)

0 0.5 1 1.5 2 2.5 3

Bulk Density (g/cm3 (b) )

Figure 99. Bulk density histogram of very high frequency head echoes 

observed by Advanced Research Project Agency long-range 

tracking and identification radar calculated by (a) the third 

spherical model and (b) the overdense model.
242

Table 86. Parameters of the lognormal distribution.

Parameter

3D Spherical Model, R2=0.9561 Overdense Model, R2=0.96

Value Standard Error Value Standard Error

a 0.9786 0.03362 0.9598 0.02903

b 0.799 0.03185 0.7698 0.02762

xo 0.4086 0.01612 0.4927 0.01633

To convert from meteoroid mass to size, a spherical shape of particle can be assumed; 

see the following equation:

D = 6!Mp

! ! "

"

#

$

%

&

'

1/3

, (64)

where D is the size of the particle in centimeters, Mp is the mass of the particle in grams, and r is 

the density of the meteoroid found by applying the distributions above.

5.6.3.2 Orbital Debris Environment Model and Analysis. The baseline model for defining 

the orbital debris environment will be ORDEM2000. ORDEM2000 is an empirical model based 

on data from a variety of sources. The model describes the orbital debris environment in the LEO 

region between 200 and 2,000 km altitude. The model is appropriate for those engineering solutions requiring knowledge and estimates of the orbital debris environment (debris spatial density, 

flux, etc.). ORDEM2000 can also be used as a benchmark for ground-based debris measurements 

and observations. The executable code for this model is available at the following Web site: <http://

orbitaldebris.jsc.nasa.gov/model/engrmodel.html>.158

For all documentation, refer to the ORDEM2000 documentation. Particularly important 

is NASA/TP––2002–210780, the New NASA Orbital Debris Engineering Model ORDEM2000.152

This Technical Publication documents the technical basis, verification, and validation of the model, 

and provides the best available information for estimating confidence limits on the environment 

definition. A detailed study of confidence limits has not been undertaken in the past because of the 

complexity of the problem and the dependencies on traffic model, satellite breakup rate, breakup 

locations, and debris population shape and material, all factors where the state of knowledge is 

very poor. These issues are currently being reviewed and new information will be presented in 

a forthcoming orbital debris handbook.

A large set of observational data (both in situ and ground based), covering the object size 

range from 10 mm to 10 m, was incorporated into the ORDEM2000 debris database. A new analytical technique, utilizing a Maximum Likelihood Estimator, was employed to convert observations 

into debris population probability distribution functions. The small particle portions of the model 

were based on the orbit distributions derived from the analysis of impact craters on the Long 

Duration Exposure Facility (LDEF) and on flux measured from the returned surfaces of the space 

shuttles, as well as other materials returned from space. To relate impact crater dimensions
243

to the properties of the debris particles, an assumption was made that the particles had the density 

of aluminum, 2.8 g/cm3. Therefore, to be consistent with the model, a particle density of 2.8 g/cm3

should be assumed for orbital debris for particles smaller than 1 cm diameter.

Figure 100 illustrates the orbital debris total flux prediction using ORDEM2000 for an ISS 

orbit: 51.6° inclination and 407 km circular orbit altitude for the year 2015 for various particle 

sizes. Figure 101 shows the orbital debris flux in a 28.5° inclination circular orbit at 277.8 km for 

the year 2015.

1×10–7

1×10–6

1×10–5

1×10–4

1×10–3

1×10–2

1×10–1

0

1×10

1×102

1×103

1×104

newF100

Total Flux (m2/yr)

1×10–6 1×10–5 1×10–4 1×10–3 1×10–2 1×10–1 1×10 1×101

Debris Size (m)

Figure 100. Orbital debris flux prediction using ORDEM2000 for an ISS orbit 

for the year 2015.

1×10–8

1×10–7

1×10–6

1×10–5

1×10–4

1×10–3

1×10–2

1×10–1

0

1×101

1×102

1×103

1×104

newF101

Total Flux (m2/yr)

1×10–6 1×10–5 1×10–4 1×10–3 1×10–2 1×10–1 1×10 1×101

Debris Size (m)

Figure 101. Orbital debris flux prediction using ORDEM2000 for the 28.5° inclination 

and 277.8 km circular orbit for the year 2015.
244

The previous figures are only ‘snapshots’ of the orbital debris environment at specific inclinations and years. ORDEM2000 also produces a directionality plot called the ‘butterfly’ plot, 

which is very useful in understanding the directionality of debris in LEO. Changing these inputs 

to the model will produce different results. Spacecraft designers and users of the environment models are encouraged to understand all the assumptions, limitations, and features of the environment 

models so that they will be an aid and not a hindrance in performing risk assessments and design 

options.

5.6.4 Meteoroid and Orbital Debris Protection

The design considerations for meteoroids and orbital debris are much the same, except that 

orbital debris provides the greatest penetration threat because impact from large (5 to 15 mm) particles are much more likely to be debris, while smaller but very high-speed particles are more likely 

to be meteoroids. These have the potential to cause electrical damage from impact-induced plasma. 

In either case, hypervelocity impacts release large amounts of energy with associated flash, plasma, 

shock waves, secondary debris, etc. capable of causing extensive damage. Even impacts from small 

particles ‘erode’ surfaces over time, resulting in degraded performance of solar arrays, optical surfaces, and other exposed components.

The reader should be aware of the following terms frequently used within the meteoroid 

and orbital debris (M/OD) technical community:

• Critical item: An item whose loss of function or destruction could cause loss of crew or loss of 

vehicle. Enumeration of these items should be consistent with the Failure Modes and Effects 

Analysis/Critical Items List usage within the program.

•Penetration: ‘Through hole’ damage or detached spall damage to an item of interest. The focus 

on complete through hole damage is a result of historical usage, and confusion can result concerning partial penetration (cratering) that can still result in critical failures of some systems. 

Perforation is another frequently used word to describe complete penetration creating a through 

hole. It is best to add clarification if there is any chance of misinterpretation.

•Probability of no penetration: Probability that no M/OD will penetrate an item 

of interest. To be properly quantified the associated exposure period must also be specified.

•Probability of no critical failure: Probability that M/OD impact will not cause loss of any critical 

function, either immediately or by secondary or delayed effects. Such loss may be by any mechanism, complete or partial penetration, spall, plasma discharge, shock damage, etc. Application 

is to the total of all critical functions and failure modes, not individual function or mechanism. 

Again, the exposure period must also be clearly specified.

Following is a list of options for crew and space vehicle protection that may be useful. 

Arriving at a safe and cost-effective design for any future space vehicles will doubtless require 

systems level analysis and planning to arrive at a successful integrated solution.
245

5.6.4.1 Collision Avoidance. The easiest way to avoid critical damage from a large orbital 

debris impact is, in many cases, to not get hit. Currently, the Air Force Space Command tracks 

orbital debris larger than approximately 10 cm in diameter and works with NASA to provide collision avoidance warnings to high value assets, i.e., the Shuttle and the ISS. More importantly, the 

Air Force has undertaken programs to greatly improve the tracking and warning capabilities of 

the space surveillance network. The goal is to concurrently increase the precision of the process so 

that the number of maneuvers needed to maintain the current level of safety does not dramatically 

increase.

5.6.4.2 Robust Design. Selection of impact-resistant materials and design concepts, especially for the Thermal Protection System (TPS) and other subsystems located on or near the vehicle 

exterior, goes a long way toward adequate impact M/OD protection. Internal thermal barriers 

that increase allowable TPS damage before failure; redundant, self-healing, or self-sealing coolant 

loops; self-shielding structures, etc., may all prove effective. Use of redundant systems is one of the 

most effective means of increasing overall system reliability from all types of failure modes, and 

M/OD impact is certainly no exception. Redundant and primary hardware components should be 

physically separated so that they are unlikely to be harmed by the same impact.

5.6.4.3 Shielding. Shielding is the most common protection system implemented with 

vehicle design and may be added internally, externally, or it may be deployable. While shielding provides an easy and cost-effective means of protection from the extremely numerous small particles, 

it becomes significantly more difficult and costly for particles 5 mm in diameter and larger. For 

each system, the level of shielding to be added must be weighed against the added weight, complexity, and cost. For simple metallic structures (single surfaces and bumper configurations), extensive 

test data and empirical equations allow the shielding effectiveness to be easily evaluated. However, 

for complex structures (honeycomb, meshes, and foams) and nonmetallic structures, the body of 

existing test data is very limited and equations are typically not available. Thus, for these cases, 

shielding effectiveness must be evaluated by test.

5.6.4.4 Orientation. Both the meteoroid and orbital debris environments are directional, 

but meteoroids approach from several directions fixed in Earth-Sun coordinates, so a vehicle’s 

orbital motion smoothes out much of the directionality (viewed in vehicle coordinates) over long 

periods of time; for short duration missions, this may not be the case. On the other hand, the 

orbital debris flux is highly directional. The flux is nearly all in the local horizontal plane with the 

specific distribution a function of orbit inclination and altitude. In the ISS orbit at a 51.6° inclination and a 450 km altitude, the flux peaks from 30° to 60° either side of the spacecraft velocity 

direction. The selection of a vehicle aspect ratio and adoption of an operating orientation that 

minimizes the area presented to the flux will help minimize the threat. Shield location, thermal 

radiator orientation, and the location of critical components with respect to other structures 

should all be determined with the directionality of the debris threat in mind. Clearly, the orientation during the greatest duration of on-orbit segments of the mission, e.g., while docked to the ISS, 

is the most important.

5.6.4.5 Crew Escape. A hypervelocity impact from a large object on the core of a vehicle 

would most certainly be so energetic that the spacecraft would be completely and instantaneously 
246

destroyed. However, such impacts are very rare compared to impacts from smaller objects, or 

impacts to exterior structure, which cause extensive damage to critical systems but leave much of 

the vehicle intact. Therefore, it is important to consider M/OD impact when developing the overall 

configuration of the vehicle, especially the crew compartment and life support systems. If the crew 

is protected from the initial impact and life support can be maintained, various options may be 

available to save the crew, including:

• Safe haven with rescue by another vehicle.

• Safe abort, if there is still reentry capability.

• Abort to ISS if the vehicle and station are docked, or if rendezvous is possible.

Two important aspects are:

• Configuration development and contingency planning that allow for rescue.

• Damage isolation and protection/inerting of onboard stored energy in order to prevent the 

amplification and propagation of damage following the initial impact.

5.6.4.6 Operational Protection––Meteoroids. In effect, one designs a vehicle to withstand 

only the sporadic meteor background and the average meteor shower environment. However, 

models such as MEM incorporate a yearly average shower flux for design purposes. The occasional 

meteor outburst or storm is mitigated operationally. For the most important showers, modern computer models enable meteor shower activity to be forecast with adequate accuracy well in advance. 

These forecasts project both the magnitude and duration of the enhanced shower or storm period, 

enabling the vehicle managers to decide upon the appropriate course of action. Actions that have 

been adopted by current spacecraft include:

• No launches during enhanced meteor activity (Space Shuttle during 1993 Perseids and 1998 

to 2001 Leonid showers).

• No EVAs during enhanced meteor activity (ISS, 2001 Leonids).

• Solar arrays turned edge-on to direction of meteor stream (numerous unmanned vehicles).

• Sensitive surfaces and optics pointed away from direction of meteor stream (Hubble Space 

Telescope, Chandra X-ray Observatory, other vehicles).

5.6.4.7 Pressurized Tanks and Volumes. Hypervelocity impacts release very large amounts 

of energy in a very small space and time. One of the key considerations in M/OD protection is 

ensuring that the hot plasma flash and pressure wave created by this energy do not trigger additional damage by interaction with other onboard stored energy sources. Examples include burning 

of flammable materials; explosion of stored propellants, high-pressure bottles, high-pressure lines, 

or batteries; and release of pressurized gasses leading to unwanted propulsion and ‘unzipping’ of 
247

the walls of pressurized volumes. Design measures for mitigating these risks include, in addition 

to adding shielding, dumping unneeded propellants, inerting stored energy devices, using selfsealing tanks and lines, employing automatic leak detection, and implementing isolation systems.

5.6.4.8 Damage Detection and Fault Isolation. Automatic damage detection and fault isolation capability are standard methods of improving system reliability that are also important for the 

M/OD problem. Quick detection and isolation of a breached system may prevent a serious loss of 

critical fluids. On-orbit damage detection and perhaps repair could be very important for protecting systems such as large propellant tanks where the failure might not be otherwise evident until 

a later phase of the flight, reentry for instance.

5.6.4.9 Wire Routing and Shielding. One easily overlooked problem area is the vulnerability 

of unprotected wires to meteoroid or debris impact. Very small (and thus numerous) particles are 

often capable of penetrating wire insulation and coaxial shielding, as well as cutting the wire itself. 

On low-voltage wires, breaching the insulation may not be problematic, but for voltages above 

≈100 V, the plasma created by the impact can initiate a sustained short that would burn out the 

wire or power source. Structure penetrations by larger particles create a cone of secondary debris 

that can also damage wires and cable bundles. For these reasons, redundant wires/cables in critical 

systems should be separated by sufficient distance to ensure that they are not taken out by a single 

impact, more than 30 cm, if possible. In addition, cable shielding should be reviewed and augmented if necessary to prevent damage to the insulation from small particle and secondary 

particle impact.

5.6.4.10 Radiator Design. Design measures have been taken to shield STS thermal radiators 

from M/OD penetrations, as shown in figure 102. Similar considerations may be useful for future 

space programs.

Additional radiator system upgrades include:

• Double-layer beta cloth sleeve added to panel interconnect flex lines and hard external lines.

• Automatic isolation valves added to each loop (port and starboard).

5.6.4.11 Window and Surface Degradation. Impact craters from the multitude of small 

particles that continually bombard external vehicle surfaces are also small, typically 3 to 5 times 

the diameter of the generating particle. Thus, damage to surfaces accumulates relatively slowly 

and degradation is often not a problem; however, there are some exceptions. One exception is the 

windows, where optical properties are important and crack propagation is an issue. On the average, three windows were replaced on the Shuttle for every two flights. Thought should be given to 

window protection and easy replacement. Another exception is certain types of laminated materials. Experience on LDEF showed that impacts caused delamination in some materials to diameters 50 to 100 times the diameter of the impact crater. A third potential issue is synergistic effects. 

Atomic oxygen and plasma (arcing) effects may be triggered or enhanced by M/OD impacts in 

some cases.
248

newF102

0.011-in Face Sheet

3/16-in Cell 3.1 Pcf Al Core

0.005-in Silver-Teflon

Type IV Tape

1.9 in

0.9 in

0.5 in

0.005-in Silver-Teflon

Type IV Tape

A1 Doubler

(0.02 in H×0.4 in W)

0.011-in Face Sheet

0.02-in Wall

F21 Tube

F21 Tube

FWD Radiator (Typ.)

34 0.187-in OD Tubes/Panel/Side

15.1 ft×10.5-in ft/Panel

Four Panels/Vehicle

AFT Radiator (Typ.)

26 0.236-in OD Tubes/Panel

15.1 ft×10.5-in ft/Panel

Four Panels/Vehicle

(a) (b)

Figure 102. Design measures to shield STS thermal radiators from M/OD penetrations:

(a) Before M/OD and (b) after M/OD.

5.6.4.12 Plasma Effects. The apparent fate of the Olympus spacecraft in 1993 (discussed 

in sec. 5.6.1.2) illustrates a less obvious, yet potentially dangerous effect of meteoroid impact upon 

a vehicle surface: plasma discharge. Plasma is released in all hypervelocity impacts, but the plasma 

produced goes as v4, whereas the total energy goes as v2. High-speed meteoroids from streams 

such as the Leonids and Perseids (especially if they strike a spacecraft with a substantial surface 

potential), can produce intense conducting plasma that can result in a large electrical discharge 

in and around the spacecraft surface. Should this discharge be near electronic devices, disruption 

or destruction of the devices may result. This threat is mitigated by both the operational methods 

noted above, and, more importantly, by good electrical design from a spacecraft charging, electromagnetic interference, and compatibility point of view.

5.7 Gravitational Field

Since the advent of Earth satellites, there has been a considerable advancement in the accurate determination of the Earth’s gravitational field. The current knowledge regarding the Earth’s 

gravitational field has advanced far beyond the normal operational requirements of most space 
249

missions. Adequate accuracy for determining most spacecraft design values of gravitational interactions is obtained with the central inverse square field:

 

!

F = μEm

r

2 rˆ ,

 

 (65)

where

!

F = vector force acting on a particle (N)

μE = gravitational constant for the Earth

m = mass of particle (kg)

rˆ = unit vector pointing from the center of the Earth to the particle

r = distance between particle and center of Earth (m).

The above central force model accurately represents the gravitational field to approximately 

0.1%. If this accuracy is insufficient for particular program needs, a more detailed model of the 

gravitational field can be used that accounts for the nonuniform mass distribution within the Earth, 

the primary effect of which is the precession of the plane of the orbit. This model gives the gravitational potential, V, to an accuracy of approximately a few parts in a million. The gravitational 

acceleration is expressed in terms of the negative gradient of the potential. The potential is then 

expressed using harmonic expansion:

!

F = m!g = m !

!

( ) "V , (66)

where

!

F = vector force acting on a particle (N)

m = mass of particle (kg)

!

g = accelerator vector !

! = vector differential operator.

The formula for V is shown below:

V (r,!,") = μe / r !!Re ( ) / r

n

Pnm(sin!) Cnm cos(m") +Snm ! sin(m!) " # { $} , (67)

where

φ = geocentric declination

λ = east longitude.
250

For the Marsh et al.159 model discussed below,

 mE = 3.9860064×1014 Nm2/kg

Re = 6,378.137 km

Z = the Z component of the spacecraft geocentric position vector.

The coefficients Cnm and Snm are the harmonic coefficients of the potential function, and 

Pnm represents the associated Legendre functions of the first kind of degree n and order m. Because

sinφ=Z/r=v, it is simply a function cosine and the associated Legendre function may be expressed 

as:

Pnm = 1!!2 ( )m / 2

2n n!

d n+m !2 ( ) !1

d!n+m . (68)

5.7.1 Spacecraft Effects

Accurate predictions of the Earth’s gravitational field are a critical part of mission planning 

and design for any spacecraft. The Earth’s gravitational field will affect spacecraft orbits and trajectories. Gravity models are used to estimate the gravitational field strength for use in designing the 

GN&C pointing subsystem, and for designing the telemetry, tracking, and communications. The 

available gravitational models have sufficient accuracy for estimating the gravitational field strength 

for spacecraft planning and design including rendezvous and docking missions.

5.7.2 Gravity Recovery and Climate Experiment Earth Gravity Model GGM02C

Gravitational models of the Earth are continually being updated and upgraded. Currently, 

gravitational models are available for varying degrees of accuracy. For operational applications and 

other situations where computer resources are a premium, ‘best fit’ approximations to the highest 

accuracy model may be utilized, which minimize the computational requirements yet retain the 

required accuracy for the specific application.

The Earth gravity field recommended for use is the Gravity Recovery and Climate Experiment (GRACE) Earth gravity model (GGM02C). This is a 200th spherical harmonic degree and 

order model that combines approximately a year of GRACE K-band range rate, attitude, and 

accelerometer data with surface gravity and mean sea surface information. The model was produced by Byron Tapley et al. at the University of Texas160 and released on October 29, 2004.
251

6. MEDIUM AND HIGH EARTH ORBITS (2,000 km TO 10 Re)

6.1 Thermosphere

Reference section 5.1 for more information about the thermosphere. Gases are negligible 

in the region 2,000 km to 10 Re. 

6.2 Thermal Environment

The description of the thermal environment in section 5.2 still applies above 2,000 km. 

However, the dominant cause of the very short duration variations in long wave and albedo are 

caused by the motion of the satellite over the changing terrain below. The data in section 5.2 were 

derived from measurements made by satellites in the 610 to 850 km range. When orbiting above 

2,000 km, the greater distance to Earth and increased area viewed will tend to smooth out the short 

period variations of albedo and long-wave radiation.

6.3 Geomagnetic Field

A dipole magnetic field configuration dominates the magnetic field topology within a few 

Re of the planet. The dipole field is generated in the Earth’s liquid outer core and controls plasma 

and energetic particle populations throughout the inner magnetosphere. Significant distortions 

appear in the dipole field configuration at distances beyond 4 to 6 Re from the Earth due to currents produced by the interaction of the solar wind with the outer regions of the magnetosphere. 

The boundary between the magnetosphere and the solar wind, the magnetopause, is characterized 

by a current sheet (the ‘Chapman-Ferraro current’) that provides the force balance between the 

inward-directed dynamic pressure of the external solar wind and the outward-directed stress of the 

Earth’s magnetic field. The dipole center is approximately 400 km distance from the center of the 

planet. The geomagnetic axis is inclined at approximately 11.7° to the Earth’s rotational axis.

The interaction of the geomagnetic field at very great distances (several Re) with the solar 

wind is one of the heat sources for the neutral thermospheric region. The solar wind is a stream of 

high-speed plasma emanating from the Sun. This interaction causes energetic particles to penetrate 

down into the lower thermosphere at high geographic latitudes and directly heat the thermospheric 

gas.

A general description of the geomagnetic field is provided in section 5.3. While the description provided there is generally applicable above 2,000 km, different models may be used to provide 

quantitative information about the field in the 2,000 km to 10 Re region.
252

6.3.1 Geomagnetic Field Model

Satellites (IMP, Highly Eccentric Orbiting Satellite, International Sun-Earth Explorer, 

Potentials of Large Spacecraft in Auroral Regions (POLAR), GEOTAIL, etc.) investigated the 

magnetic field of the Earth for several decades. Data obtained were sufficient to apply to model 

development with its average strength and shape during different levels of overall magnetic activity.

6.3.1.1 Tsyganenko Model. The Tsyganenko model is the most used empirical global magnetic field model for applications above 2,000 km. The Tsyganenko model is a semi-empirical, 

best-fit representation for the magnetic field based on a large number of satellite observations. The 

model includes the contributions from external magnetospheric sources: ring current, magnetotail 

current system, magnetopause currents, and large-scale system of field-aligned currents. The latest 

models include T95 and T96_01.161,162 All of these realistic models start from the official IGRF 

model, which is improved by adding magnetospheric current systems that modify it toward the 

true, observed configuration. In the series of figure 103, the magnetospheric topology is seen to be 

dependent on the season. The Kp index in this study is taken to be 2.

6.3.2 Geomagnetic Field Effects

The Earth’s magnetic field exerts a strong influence on space environmental phenomena 

including ionizing radiation and plasma activities, electric currents, and trapped high-energy 

charged particles. This influence has important consequences to spacecraft design and performance. Details of the geomagnetic field effects are discussed in section 5.3.2.
253

–20

–15

–10

–5

0

5

10

15

20

GSM z

10 0 –10 –20

GSM x

–20

–15

–10

–5

0

5

10

15

20

GSM z

10 0 –10 –20

GSM x

–20

–15

–10

–5

0

5

10

15

20

GSM z

10 0 –10 –20

GSM x

–20

–15

–10

–5

0

5

10

15

20

GSM z

10 0 –10 –20

GSM x

newF103

(a) (b)

(c) (d)

Figure 103. Earth’s magnetic field from Tsyganenko model during Kp=2 

for different seasons: (a) Winter, (b) spring, (c) summer, and (d) fall.
254

6.4 Plasma Environment

A summary of key design factors for the plasma environment is displayed in table 87.

Table 87. Summary of key design factors.

Parameter Source

Spacecraft Design 

Consideration Factors

For prediction of ambient/

energetic plasma conditions:

 Solar winds Section 6.4.3/table 88 Spacecraft charging

 Plasma density Figure 108 Spacecraft charging

 Plasma temperature Figure 108 Spacecraft charging

Plasma measurements 

and spacecraft charging

guidelines evaluation

for spacecraft charging:

 Plasma density Figure 109 Spacecraft charging

 Plasma temperature Figure 109 Spacecraft charging

 Charging predictions NASCAP/GEO/section 6.4.7 Spacecraft charging

Note: For other plasma effects and guidelines see section 5.4.6.

6.4.1 Plasma Regimes in Near Earth Space

Figure 104 illustrates the main features of the magnetosphere, the region of space dominated by the terrestrial magnetic field. The dipole field, dominant near the Earth, is compressed 

by the solar wind flow on the dayside and stretched into the extended magnetotail on the nightside 

of the Earth. The dipole field is generated in the Earth’s liquid outer core and controls plasma and 

energetic particle populations throughout the inner magnetosphere. This schematic illustrates the 

main features of the near-Earth (<20 Re) magnetosphere in the noon-midnight plane. The field is 

offset from the Earth’s center of mass by approximately 400 km and inclined approximately 11°

with respect to the rotation axis of the solid Earth.134 Significant distortions appear in the dipole 

field configuration at distances beyond 4 to 6 Re from the Earth due to currents produced by the 

interaction of the solar wind with the outer regions of the magnetosphere. The magnetopause, 

a boundary between the magnetosphere and the solar wind, has a current sheet (the ChapmanFerraro current). The current sheet provides the force balance between the inward-directed dynamic 

pressure of the external solar wind and the outward-directed stress of the Earth’s magnetic field.
255

newF104

Bow Shock

Magnetopause

Radiation Belt 

and Ring Current

Polar Wind

Plasma Mantle

Magnetosheath

Plasmasphere

Solar Wind

Polar

Cusp

Plasma Sheet

Figure 104. Near-Earth magnetosphere.

Above the ionopause/plasmapause boundary (defined by the sharp density gradients at the 

outer reaches of the plasmasphere), but within the region of closed magnetic field lines, the plasma 

is still dominated by photoionized terrestrial particles, but energy input from the solar windmagnetosphere interaction may accelerate particle-producing regions of energetic ions and electrons within the magnetosphere.

The solar wind significantly compresses the geomagnetic field on the dayside of the Earth, 

and the dayside magnetopause is typically 10 to 14 Re from the Earth in the direction of the 

Sun. Approximately 2 to 3 Re upstream of the dayside magnetosphere is a shock wave (the ‘bow 

shock’) formed where the supersonic solar wind flow is abruptly decelerated upon encountering 

the magnetosphere. The temperature of the solar wind plasma increases upon traversing the bow 

shock, as its forward motion is converted to random thermal energy, and its density increases as 

the plasma stagnates and builds up in front of the magnetosphere in the subsolar region. Plasma 

in the magnetosheath (the region between the bow shock and the magnetopause) flows around the 

magnetopause in the same sense as the free solar wind flow. Anti-sunward of the Earth, the solar 

wind interaction stretches the geomagnetic field for at least several hundred Earth radii, forming 

the extended magnetotail. The magnetotail has been detected by satellites as far as 500 to 1,000 Re

from the Earth.

The GEOTAIL satellite was launched on July 24, 1992, to study the structure and dynamics 

of the tail region of the magnetosphere with a comprehensive set of scientific instruments. For this 

purpose, the orbit has been designed to cover the magnetotail over a wide range of distances, 

8 to 210 Re from the Earth. This orbit also allows us to study the boundary region of the magnetosphere as it skims the magnetopause at perigees. Another spacecraft mission, the Chandra spacecraft, flies with a perigee of 10,000 km, an apogee of 140,000 km with the magnetic latitude of the 
256

orbit inclination at the beginning and the end of the mission between 28.5° and 52°. The spacecraft 

may encounter the solar wind, plasma sheet, polar cap, auroral, and radiation belt regions within 

a single orbit. This indicates that the spacecraft will experience several different regimes in orbit.

Some regions in the magnetosphere are well defined and have distinguishable boundaries. 

Others have particle populations that overlap, making identification difficult. Further complicating the picture are the dynamic properties of this area of space. For example, during a geomagnetic storm, plasma sheet particles can rapidly move in the midnight sector, which causes many 

of the regions to change size, shape, and plasma composition. Plasma regions that may encounter 

the charging phenomenon include polar cusp, auroral region, radiation belt, magnetotail, plasma 

sheet, and plasmasphere. The characteristics of plasmas in each region vary depending upon time, 

location, geomagnetic storm, and solar activity. The magnetospheric region and characteristics 

have been described in several publications.115,163,164 A brief description of the plasma environment characteristics in each region of the magnetosphere is provided in the following sections.

6.4.1.1 Plasmasphere. The plasmasphere extends out to as little as 2 to 3 Re and, under 

quiet conditions on the evening side, to more than 6 Re depending on space conditions and varying 

region boundaries. The actual extent of the plasmasphere is variable and depends on space weather 

activity. High levels of activity erode the plasmasphere and the plasmapause (the outer edge of 

the plasmasphere) moves inwards towards Earth; long periods of quiet allow the plasmasphere to 

expand outward towards geostationary orbit and beyond. The plasma within the plasmasphere 

contains cold (1 to 30 eV) and dense (>10 cm–3) plasma surrounding the Earth. During periods of 

high magnetic activity, the plasmasphere recedes inward, well within the geosynchronous altitude. 

The plasmapause is characterized by a steep gradient in low energy plasma density. The plasma 

density can drop by over 3 orders of magnitude within approximately a 1,000 km band in the equatorial plane. Data obtained from Orbiting Geophysical Observatories’ five measurements indicated 

a composite of several typical plasmapause crossings representing different levels of magnetic 

activity.165 The plot of H+ concentration versus L value represents all of the outbound passes in 

the local time region from midnight to 04:00 hours during the Dynamics Explorer-1 mission 

(reference fig. 105).

6.4.1.2 Plasma Sheet. The plasma sheet lies beyond the plasmapause and is clearly identifiable on the nightside, where it extends down tail to several tens of Re. The temperature of charged 

particles in this region can go to several tens of kiloelectronvolts. The plasma density is normally 

<1 cm–3. The plasma sheet expands and contracts during solar and magnetic activity. During high 

activity periods, the earthward edge of the plasma sheet in the equatorial plane can extend inward 

to <5.5 Re; during prolonged magnetically quiet periods, the inward edge can lie beyond 10 Re.

6.4.1.3 Ring Current. The ring current, defined as a western electric current that flows 

above the geomagnetic equator, is located in the outer Van Allen radiation belt. The ring current, 

composed primarily of hot trapped ions with energies approximately 10 to 100 keV, lies mostly 

earthward of the inner edge of the plasma sheet. The outer boundary of the ring current is dependent on particle energy, equatorial pitch angle, and local time. A partial ring current can also exist 

in the midnight region near geosynchronous altitude.
257

10–2

10–1

0

101

Vehicle Turn

102

103

104

105

newF105

H+ions/cm3

1 2 3 4 5 6 7

L

Kp=4–5 Kp=3 Kp=2 Kp=1+

4/12/68: Kp<1+

4/4/68: Kp=2

4/22/68: Kp=3

3/25/68: Kp=4–5

Figure 105. A composite of several typical plasmapause crossings representing different 

levels of magnetic activity (plots of H+ versus L represent all of the outbound 

passes in the local time region from midnight to 04:00.74).

6.4.1.4 Magnetotail. The magnetotail is an extended magnetospheric region on the antisunward side of the Earth to at least 100 Re. The magnetotail is essentially parallel to the solar 

wind flow in the magnetosheath region. In order to sustain this structure, the tail itself has a twolobed structure where magnetic field lines from the Earth are divided by the plasma sheet that 

carries a larger current separating the two regions of oppositely directed magnetic fields.

6.4.1.5 Radiation Belt. The radiation belt of the magnetosphere roughly lies between 

1.2 and 6 Re above the equator in which charged particles (electrons with energies approximately 

0.1 to 500 MeV and trace quantities of heavier ions) are trapped by the geomagnetic field. The 

outer boundary is near the magnetopause on the sunward side (~10 Re under quiet conditions, 

~6 Re on the nightside); however, due to the distortion of the magnetic field, the boundary may lie 

well beyond that altitude.

6.4.2 Solar Wind

Solar wind is the plasma source in this region. The solar wind is an extension of the Sun’s 

corona into interplanetary space. As the plasma moves away from the Sun, it expands and cools,
258

 with the electron temperature decreasing from 1 million K (~ 100 eV) in the corona to ~ 100,000 K 

(~10 eV) near the Earth. The solar wind is supersonic (with speed ranges from 200 to 900 km/s) 

and consists of electrons, protons, and alphas. These particles coming from the Sun cannot pass 

through the Earth’s magnetosphere and are forced around it. Thus, the solar wind piles up and 

creates a bow shock in front of the Earth. These parameters vary considerably with time, but are 

consistent with gross charge quasi-neutrality. The ionic constituents are chiefly protons with a small 

percentage (~4) of alpha particles. The solar wind can vary markedly on an hourly basis and is 

highly structured throughout the solar system because of time variations, shocks, CMEs, and flares. 

Despite this marked variation, it is useful to provide average values for the parameters describing 

the interplanetary medium near the Earth. Table 88 compares the plasma characteristics near the 

Earth for low speed, high speed, and average solar wind conditions.

Table 88. Solar wind parameters near the Earth.

Plasma Parameter Average Low High

n (cm–3) 8.7 11.9 3.9

u (km/s) 468 327 702

Tp (K) 1.2 × 105 0.34 × 105 2.3 × 105

Te (K) 1.4 × 105 1.3 × 105 1 × 105

6.4.3 Plasma Characteristics

The plasma characteristics, generally described in terms of plasma density and temperature, can be used to define the appropriate particle distribution function shown in section 5.4. The 

plasma environment in high altitude orbit is a dynamic medium that exhibits large, daily property 

variations. Essentially, all major parameters can vary over at least 2 orders of magnitude daily. 

Dynamic changes in the plasma environment can produce large potential differences on spacecraft 

that might not otherwise develop during time-stationary plasma conditions. The extent of the 

variations and the expected resulting effects on levels of expected spacecraft charging have been 

well documented for GEO. In GEO, plasma thermal current densities can be 3 orders of magnitude 

less than in LEO, so that photoelectron emission from surfaces can play a significant role in balancing currents to a spacecraft. Sun/shade effects become important to the point that potentials as 

large as several kilovolts can develop between sunlit and shaded surfaces (depending on geometry 

and material properties).

Currently, the GEOTAIL satellite is orbiting in 9×30 Re with an inclination of –7° to the 

ecliptic plane. Figure 106 describes plasma characteristics and its variation during the GEOTAIL 

mission from February 25, 2005, through March 1, 2005 (plot courtesy of University of Iowa 

Comprehensive Plasma Instrument team). Each survey plot is a 5-day record of the electron 

and ion intensities and several parameters derived from those intensities. The measurements are 

given in a sequence of five panels. From top to bottom, the panels display the following:

• N (cm–3), the ion number density.

• Vx (km/s), the component of the ion bulk-flow velocity along the Earth-Sun line. Positive flow 

is towards the Sun.
259

10–2

102

103

104

newF106

E/Q (V)

N (cm–3) T (K)

Vx (km/s)

2/25 2/26 2/27 2/28 3/1

L

102

103

104

106

107

108

–800

0

800

0.1

1

10

2 Counts

Electrons

10 102 103

2

2/25

10 102 103

Ions

Ions

Electrons

Figure 106. Demonstration of plasma density and temperature variation during 

the GEOTAIL mission from February 25, 2005, through March 1, 2005 

(updates to the plasma environments are planned when new data are 

available).

• Ti and Te (K), the respective temperatures of ions and electrons.

• The intensities of positive ions as a function of particle energy.

• The intensities of electrons as a function of particle energy.

6.4.3.1 Geosynchronous Altitude Plasma. The geosynchronous altitude plasma environment 

is very complex and dynamic. Charged particle flux in geostationary orbit can be quite energetic 

and is highly variable with magnetic activity, reaching the highest density and temperature during geomagnetic substorms. Plasma parameters listed in table 89 (adapted from ref. 129) represent 

the average, 90th percentile, and worst case geostationary orbit plasma environments derived from 

the spacecraft charging at high altitudes (SCATHA) spacecraft mission, assuming bi-Maxwellian 

representations for the environment. The strongest charging conditions will be encountered in the 

extreme environments where the high electron flux in the hot plasma environment drive spacecraft 

potentials to large negative values before equilibrium potentials are established on the spacecraft 

structure.
260

Table 89. Geoplasma parameters (adapted from ref. 29).

Parameter

Average Environment* 90% Environment**

SCATHA Worst Case

Environment***

Electrons Ions Electrons Ions Electrons Ions

Number density (No./cm3) 1.09±0.89 0.58±0.35 2.55 1.16 3 3

Current density (nA/cm2) 0.115±0.1 3.3±2.1 0.28 6.75 0.501 0.016

Number density, 

population 1 (No./cm3)

0.78±0.7 0.19±0.16 1.93 0.453

 Parallel 1 1.1

 Perpendicular 0.8 0.9

Temperature, 

population 1 (eV)

550±320 800±100 1,076 965

 Parallel 600 400

 Perpendicular 600 300

Number density, 

population 2 (No./cm3)

0.310±0.37 0.39±0.26 0.919 0.818

 Parallel 1.4 1.7

 Perpendicular 1.9 1.6

Temperature, 

population 2 (eV)

8,680±4,000 15,800±5,000 15,260 24,025

 Parallel 25,100 24,700

 Perpendicular 26,100 25,600

* Error is 1σ.

** The 90% case is the average + 1.645σ.

*** SCATHA worst case from reference 129.

Spacecraft often encounter the greatest potentials in, and immediately after, eclipse passage.166 Photoelectron currents are typically significant (if not dominant) contributors to the charging process in the low-density geostationary orbit environments, resulting in positive spacecraft 

potentials (or at least reducing the magnitude of negative potentials in hot electron environments). 

The absence of the outgoing photoelectron current in eclipse conditions removes one of the primary currents that balance the incoming electron current from the plasma environment, and the 

spacecraft must charge to large negative potentials before current balance is established on a spacecraft surface. Differential charging is often a significant issue during eclipse conditions. The electrical resistivity of insulating materials is inversely proportional to temperature, so localized charge 

densities can accumulate in cold materials where conduction processes are ineffectual in removing 

the excess charge as the spacecraft surfaces cool during eclipse passage. Charging analyses using 

the environments provided in table 89 should therefore be examined both for sunlight and eclipse 

conditions to determine the full range of spacecraft potentials on a spacecraft surface.

6.4.4 Spacecraft Charging Environment

Generally, the space plasma environment can affect spacecraft charging behavior. Combining extreme plasma conditions (e.g., maximum/minimum temperature and density) for each region 

does not constitute the worst case situation because the extreme plasma density and the extreme 
261

temperature may not occur coincidentally. This worst case environment depends on variations 

of solar activities and the geomagnetic storm levels. An accurate definition of plasma properties 

is essential to determine the severity of these effects because the charging level of a spacecraft is 

directly related to plasma properties encountered on orbit. Properties in GEO orbits have been 

quantified from various satellite and ground measurement programs. However, for spacecraft with 

orbits outside these regions, plasma characterization is far more difficult. At geosynchronous altitudes with spacecraft enveloped in a plasma cloud, charge is injected during a magnetic substorm. 

These plasma clouds have particle densities on the order of 106 to 107 m–3 and energies of tens of 

kiloelectronvolts. For calculation purposes, measured fluxes can usually be fitted by a Maxwellian 

or biMaxwellian distribution function shown in section 5.4.

Two regions, geosynchronous altitudes (~6.6 Re) and low altitude auroral regions (~60° to 

75° geomagnetic latitude, reference sec. 5.4), have been identified with plasma energy sufficiently 

high to charge spacecraft to kilovolts or higher. Probably two different charging mechanisms are 

involved in GEO charging. Lower energy (<50 keV) particle deposition is believed to cause the 

surface charging typically found in the MARECS upsets. The second mechanism, due to the more 

energetic particle flux (>50 keV), can be buried in the dielectric and discharge at random times 

when external conditions change or with dielectric breakdown. Once charged in this manner, 

the dielectric can hold its charge for long periods of time.

6.4.4.1 Geostationary Transit and Geostationary Orbits and Internal Charging. Spacecraft 

in geostationary transfer orbits and geostationary orbits will encounter the strong fluxes of the 

electron radiation belts. Energetic electrons in the belts with energies of a few megaelectronvolts or 

greater can penetrate spacecraft shielding and deposit energy in internal systems, creating a susceptibility to anomalies and failures. Electric fields produced by the charge buildup create a hazard to 

electronic systems from electrostatic discharges and surface materials from arcing. Since electrons 

of a few hundred kiloelectronvolts deposit their energies near surfaces, and electrons with energies of a few megaelectronvolts can penetrate into materials and deposit energy internally, internal 

charging can impact internal systems or exposed dielectrics. Many charge-related anomalies in 

geostationary orbit have been identified with internal charging.

Figure 107 presents the NASA worst case environment to be used in analysis of internal 

charging issues. Analysis of discharge events on spacecraft in geostationary or geostationary transfer orbits have shown that electron fluxes at breakdown must typically be sufficient to provide 

a fluence of 2×1010 electrons/cm2 in 10 hr.

6.4.5 Charging Consideration for Spacecraft Design

GEO are used frequently for scientific missions that require long viewing periods without 

the interference of the Earth’s atmospheric effects and magnetic field effects. Typically, satellites flying these orbits are concerned with spacecraft charging and the potentially adverse effects of charging on particular systems. Defining the plasma environment that spacecraft will encounter is the 

first step in analyzing spacecraft charging in space. Spacecraft charging models require specific 

characteristics of the plasma environment for analyzing worst case charging levels. These plasma 

characteristics are generally described in terms of plasma density and temperature and can 
262

1×103

1×105

1×106

1×107

0.1 1 10

newF107

Flux (

Ne/cm2-s-sr)

Electron Energy (MeV)

1×104

Suggested Worst Case

Environment

NASA AE 8 min

(see text)

Figure 107. NASA worst case environment for internal charging.

be used to define an appropriate particle distribution for the spacecraft charging analysis. However, 

defining the worst case plasma/charging environment for the various regions becomes difficult to 

quantify because of different altitude, latitude, solar activity, geomagnetic storms, sunlight/eclipse 

effects, and ram/wake effects. Also, several different plasma regions may be encountered during 

every orbit, each having specific plasma environment characteristics and each bringing about its 

own detrimental effects. Transient events occur as a spacecraft passes through the boundaries of 

different regions because of tremendous changes in space plasma characteristics. Therefore, the 

time spent in crossing different regions needs to be precisely quantified to accurately define overall 

levels of spacecraft charging. Also, large orbital changes due to gravitational perturbations of the 

Earth and Moon significantly alter the orbit of high altitude spacecraft from beginning through 

end of life. All these variations should be considered when specifying worst case plasma environment characteristics for spacecraft charging analyses.

6.4.6 Other Plasma Effects for Design Consideration

Other than the spacecraft charging, plasma effects that might degrade the spacecraft performance include effects to the power system/solar array, thermal control/contamination, plasma 

emissions, ionospheric scintillation, and secondary plasma effects. Details of these plasma effects 

are described in section 5.4.6.

6.4.7 Spacecraft Charging Models

NASA/Air Force Spacecraft Charging Analyzer Program-2k (NASCAP-2k) for GEO is one 

popular spacecraft charging model that is used in evaluating charging analysis in geosynchronous 

orbits.117 This 3D code uses an orbit-limited particle collection algorithm appropriate for the low 
263

density and high temperature geosynchronous orbit charging environments. The orbit-limited algorithm is used in low density conditions where the dimensions of the plasma sheath adjacent to the 

spacecraft structure are sufficiently large and the collection of particles from the plasma depends 

on the orbital parameters of the individual particles. These conditions are satisfied when the Debye 

length in the plasma is greater than the radius of curvature of the spacecraft surface, a condition 

typically met for spacecraft in geostationary orbit.167

6.5 Ionizing Radiation Environment

In the region of space from 2,000 km to 10 Re, the ionizing radiation environment will be 

composed of the trapped radiation belts, the GCR and SPE. The details of these environments and 

the effects on electronics are presented in sections 2.12, 3.11, and 5.5.

The middle Earth orbits (MEOs), which range to about 4 Re, are in the heart of the 

trapped radiation belts with very high radiation levels. From a TID and displacement damage 

(DD) perspective, the trapped radiation environments will dominate the GCRs and SPEs (except 

for an extremely large solar event). In the high Earth orbits (HEOs), which range from 4 to 10 Re, 

the trapped electron belt still provides a significant source of the ionizing radiation but the orbits 

are now beyond the trapped proton belt. For TID, the trapped electron belt will still be the primary 

source of dose for the lightly shielded electronics. Once shielding thicknesses reach about 5 mm 

of equivalent aluminum thickness, the electrons are effectively screened out and the dose driver for 

both TID and DD will be solar particle events. It should also be pointed out that, at these altitudes, 

the geomagnetic screening is weakening significantly and the SPE particles are almost entirely 

unscreened.

From a SEE point of view, the rate drivers for the MEO will be the SPEs, GCRs (under 

non-SPE conditions), and the trapped protons, for those electronics that are sensitive to proton 

SEE. Upon reaching the HEO altitudes, the trapped protons are no longer a SEE threat and therefore the only sources for SEE are GCRs (under nominal conditions) and SPEs. It should be noted, 

as stated above, that the HEO have very little protection from the Earth’s geomagnetic field. Therefore, the GCR rates will be higher than those at MEO levels and the rates due to SPE can increase 

substantially (orders of magnitude). For these mission segments, it is often best to be conservative 

in design by having systems that can deal with event rates from a worst case unscreened SPE (at 

least the high criticality systems). If the system is capable of surviving and/or operating under these 

conditions, then performing under the lower rates of GCR and trapped protons will already be in 

the design.

6.6 Meteoroid and Orbital Debris Environments

6.6.1 Meteoroid Environment

Refer to section 5.6.1 for general discussion of the meteoroid environment. This description 

is still valid for this region of space.
264

6.6.2 Orbital Debris Environment

Refer to section 5.6.2 for general discussion of the orbital debris environment. This 

description is still valid for this region of space with the exception of the modeling capabilities. 

ORDEM2000 does not model the debris region above 2,000 km, although debris is still present out 

to GEO. The orbital debris model is being expanded to cover as much of this unmodeled region as 

possible.

In a 1999 United Nations (UN) report on space debris, it is stated that the population of 

large space debris in and near the GEO region is well known for spacecraft and upper stages. The 

probability for collision between these objects is much lower than in LEO because of their fewer 

numbers, leading to a wider spatial distribution with lower average relative speeds (500 m/s). Since 

spacecraft and upper stages continue to be left in orbits above and below GEO, the number of 

uncontrolled intact objects intersecting this GEO region is increasing at a very slow rate. Collisions 

between objects in GEO are still possible because of the close distances between active spacecraft 

at specific longitudes.

Smaller debris less than 1 m in diameter near the GEO region is poorly defined. At the time 

of the 1999 UN report,168 two breakups of a spacecraft and an upper stage were recorded. It was 

anticipated that the resulting smaller debris pieces would be perturbed into new orbits (possibly 

reducing the time in GEO, but increasing the relative collision velocity), making the flux contribution near constant with inclination change. It is believed that in many cases the debris fragments 

would be widely dispersed in both altitude and inclination.

Since there is no natural removal mechanism for spacecraft in GEO, there is a risk that 

active spacecraft could be damaged by these dead satellites or their fragments from breakup events. 

This annual collision probability for an average operational satellite with other catalogued objects 

is estimated at 10–5.

168

6.7 Gravitational Field

Reference section 5.7 for the gravitational field in LEO and section 8.5 for lunar gravity.
265

7. CISLUNAR SPACE ENVIRONMENTS

The cislunar section covers those environments in lunar space (10 to 60 Re).

7.1 Lunar Exosphere (Lunar Atmosphere)

The Moon has an atmosphere attributed to interactions of the solar wind and solar photons 

with the lunar surface, coupled with contributions from volatile materials introduced by meteor 

impact.169 The total number density of neutral species that have been detected on the lunar surface 

is ≤105/cm, although undetected species may contribute additional density.170 The lunar atmosphere is more properly termed an ‘exosphere’ due to the low density of gas atoms. Interactions 

of individual atoms are dominated by the lunar surface and altitude distributions are governed 

by ballistic motions rather than atom-atom interactions. The dominant species is Ar at densities 

of approximately 4×105/cm3 with He being the dominant minor species contributing 2×103/cm3

to 4×105/cm3 to the exospheric density. Other minor species include <5×102/cm3 oxygen atoms, 

approximately 70/cm3 sodium (Na) atoms, approximately 16/cm potassium atoms, and <17/cm3

hydrogen atoms.170 The density of Na and potassium (K) are time dependent with variations 

attributed to solar wind modulated photo-desorption171 and enhancements in metallic species 

during meteor storms.172–174 Searches for additional exospheric species have been conducted with 

negative results including magnesium (Mg),175 silicon (Si), aluminum, calcium (Ca), iron (Fe), and 

titanium (Ti).176

7.2 Solar Wind (Interplanetary Fields)

Solar wind is the primary source of lower energy charged particles in the interplanetary 

region of space. Its outflow starts in the lower corona and the velocity steadily increases as the 

plasma moves radially outward away from the Sun. The formation of shocks in the planetary 

medium can have important consequences for planetary magnetospheres and ionospheres because 

the strong impulsive force associated with shock impact on planetary magnetic fields induce electric 

fields within the magnetosphere and magnetic fields associated with the shock structures control 

the rate at which energy flows from the solar wind into the Earth’s magnetosphere, ionosphere, and 

thermosphere. Shocks can form where CME interact with the ambient solar wind and when a fast 

solar wind stream overtakes a slower moving solar wind. The solar wind density and velocity can 

vary markedly on time scales from minutes to hours and even days, giving rise to variations in solar 

wind structure throughout the solar system. Solar wind plasma generally takes 2 to 3 days to reach 

the Earth from the Sun at average speeds, although high-speed CME flows may arrive in times of 

less than a day. Near the Earth, the speed ranges from 200 to 900 km/s and the density varies from 

1 to 80 cm–3 with extreme low densities <0.1 cm–3 and high densities over 100 cm–3. The highest 

solar wind flow speeds on the order of a few 1,000 km/s are found in CMEs. As the plasma moves 

away from the Sun, it expands and cools, with the electron temperature decreasing from 1 million K
266

in the corona to approximately 100,000 K near the Earth. The interplanetary magnetic field (IMF) 

also decreases with distance from the Sun, from approximately 1 G at the Sun’s surface to approximately 3×10–5 G near the Earth. Table 90 compares the plasma characteristics near the Earth for 

low speed, high speed, and average solar wind conditions.

Table 90. Solar wind parameters near the Earth.

Parameter Average Low Speed High Speed

n (cm–3) 8.7 11.9 3.9

u (km/s) 468 327 702

Tp (K) 1.2 × 105 0.34 × 105 2.3 × 105

Te (K) 1.4 × 105 1.3 × 105 1 × 105

7.3 Plasma Environments

Earth’s magnetotail extends to distances well beyond lunar orbit at 60 Re from the Earth 

(magnetotail environments are well ordered at L2 distances of 236 Re and have been detected at 

distances of 500 to 1,000 Re). The Moon is immersed in the solar wind flow approximately 73.5% 

of the time,177 with the remaining time approximately equally split between the Earth’s magnetosheath and magnetotail.178 This section describes the characteristics of the solar wind, magnetosheath, and magnetotail plasma environments that will impact vehicles in transit to the Moon 

from the Earth. Plasma characteristics will be perturbed in the vicinity of the Moon due to charging of the lunar surface and formation of the lunar wake (sec. 8.2.1), and the properties provided 

in this section should be applied in those conditions.

7.3.1 Solar Wind

A summary of the solar wind plasma environments is given in table 91 (adapted from 

ref. 179).

Table 91. Average solar wind properties.

Parameter Mean σ Most Probable Median 5%–95% Limit

N (# /cm3) 8.7 6.6 5 6.9 3 to 20

V (km/s) 468 116 375 442 320 to 710

Tp × 104 (K) 12 9.1 5 9.5 0.98 to 30

Te × 104 (K) 14 3.9 12 13 8.9 to 20

Tα × 104 (K) 58 50 12 45 6 to 154

Nα /Np 0.047 0.019 0.048 0.047 0.017 to 0.078
267

Statistical differential flux spectra for solar wind plasma at 1 AU are given in figure 108 for 

(a) electrons and (b) ions. These environments are computed from data obtained by the Ulysses 

spacecraft solar wind plasma experiment plasma and heliosphere instrument for spectra, composition, and anisotropy at low energy energetic particle instruments. Ulysses data were obtained over 

a range of distance from 1 to 5.4 AU, but all data in this figure have been scaled to 1 AU for computation of the statistical flux. Differential flux values from over 12 years of the Ulysses mission 

are sorted within individual energy bins into increasing flux order, and values are extracted for the 

50% (black), 90% (blue), 95% (green), and 99% (yellow) flux environments. In addition, the red 

line in both plots indicates the maximum (100%) value in each energy bin. The solid (dashed) lines 

indicate flux impacting the Sun side (dark side) of the Moon. The electron flux in this model environment is isotropic at the highest energies and only exhibits flux directionality at energies below 

0.01 keV. In contrast, the solar proton flux is anisotropic with the largest flux, ions of a few kiloelectronvolts, impacting the illuminated side of the Moon. Plasma flux approaching the backside 

of the Moon is approximately 5 orders of magnitude less than the flux on the Sun side (see sec. 8).

Charging of the lunar surface due to differential collection of ion and electron currents 

from the plasma environment and emission of photoelectrons from the illuminated surface 

will modify the spectrum of particles that reach the surface. Halekas et al.180 report nightside

100

105

1010

1015

newF108

No./cm2-s-sr-keV

10–3 10–2 10–1 0 101

Ulysses

1 AU

SSRE_RevA

102 103 104

(a) Energy (keV)

100

105

1010

1015

No./cm2-s-sr-keV

10–2 10–3 10–1 0 101

Ulysses

1 AU

SSRE_RevA

102 103 104

(b) Energy (keV)

Figure 108. Statistical solar wind flux: (a) Electrons and (b) ions are represented 

Maxwellian environments at low energies with nonthermal tails 

extending megaelectronvolt energies.
268

potential differences of approximately –35 V between the Lunar Prospector (LP) satellite and the 

lunar surface and note that the satellite may in fact be charged relative to the ambient plasma, suggesting even greater lunar surface potentials. These results are consistent with ion measurements 

from the Apollo 14 Superthermal Ion Detector Experiment (SIDE) that suggested potentials of 

approximately 100 V negative exist near the terminator.181 Recently, Halekas et al.182 report statistical analyses of the LP electron reflectometer (ER) data that demonstrate the existence of >500 V 

negative lunar surface potentials. Most of the charging events occur in shadow and the large charging events are rare, although they do occur in both sunlight and shadow. The extreme charging 

events appear to be correlated with exposure of the lunar surface to plasma sheet environments 

within the magnetotail. They are strongly correlated with characteristics of the incident electrons 

instead of localized variations in properties of the lunar surface, suggesting the charging flux is the 

primary control of the surface charging phenomenon.

7.3.2 Magnetosheath and Magnetotail

Figure 109 shows plasma environments for a 6-month period in 1993, obtained from the 

Comprehensive Plasma Instrument Hot Plasma analyzer within 25 Re of the Earth-Sun line, over 

distances from –50 Re to beyond –200 Re. Data are from the University of Iowa/Comprehensive 

Plasma Instrument onboard the GEOTAIL satellite for the first 180 days in 1993. Values for all 

GSE Y and Z distances are plotted on the same axis. Data gaps are due to the sparce sampling of 

deep tail plasma regimes during the four deep tail passes represented in the data set.183 Environments represented in the figure are primarily magnetosheath and magnetotail plasma environments 

due to the large dimensions of the bow shock at distances beyond –50 Re. Radial trends are largely 

lacking in the plasma moments, and parameters appropriate for lunar distances of 60±10 Re are 

essentially the same throughout the distant magnetotail to nearly L2 at –236 Re. Statistics from the 

entire data set are included in figure 109 and in table 92.

7.3.2.1 Extreme Electron Flux Environment for Internal (Bulk) Charging Evaluations. The 

electron flux environment at the lunar surface is estimated by analyzing solar wind measurements 

taken by the Charged Particle Measurement Experiment (CPME) on the Interplanetary Monitoring Platform 8 (IMP-8) satellite. The satellite was placed in a near-circular orbit around the Earth 

in October 1973 with a radius of approximately 35 Re. The period of the satellite is approximately 

12 days, of which 60% of its period is in the solar wind and the remaining time is in the magnetosheath and magnetosphere. IMP-8 provides a reasonable estimate of extreme internal charging 

environments for lunar operations even though the spacecraft orbits the Earth at approximately 

half the distance to the Moon. This happens because the origins of the high flux environments are 

typically solar energetic particle events (flares and CMEs) that exhibit minimal spatial variations 

over the scale of Earth-to-Moon distances. It should be noted that charging behavior of the lunar 

surface (positive in the sunlit hemisphere and negative in the dark hemisphere) will perturb plasma 

populations and may alter the free-field environments reported in this section. More detailed analyses of the surface electron populations are required to obtain bulk charging environments for the 

lunar surface.
269

–800

–400

0

0

400

–800

–400

400

–800

–400

400

0

0.01

0.10

1

10

105

106

107

108 105

106

107

108

newF109

Vz

(km/s) Vy (km/s) Vx (km/s) Ni (No./cm3) Ti (K) Te (K)

–200 –150 –100 –50

XGSE (Re)

Figure 109. Survey of magnetotail and magnetosheath plasma characteristics (data 

are from the University of Iowa/comprehensive plasma instrument (CPI) 

onboard the GEOTAIL satellite for the first 180 days in 1993. Values for all 

GSE y and z distances are plotted on the same axis.
270

Table 92. Magnetosheath and magnetotail plasma parameters.

Cumulative 

Probability

Ni

(#/cc)

Ti × 10

 (K)

Ti × 104

(K)

Vx

(km/s)

Vy

(km/s)

Vz

(km/s)

Magnetosheath Parameters*

5 0.283 18.6 3.12 –519 –93.6 –52.3

33 0.687 75.5 20.6 –383 –50 –15.8

50 1.006 92.7 31.2 –311 -32.9 –7.21

67 1.645 110 41.8 –272 –12.1 –0.88

99 8.684 461 111 –112 75.1 35.3

Boundary Layer Parameters*

5 0.108 30.9 6.03 –297 –61.2 –68.8

33 0.235 90.3 39.8 –199 –18.8 –23.6

50 0.316 114 60.7 –167 –5.67 –12

67 0.44 165 84.9 –139 6.87 –1.9

99 2.286 1,220 272 2.49 74.4 58.7

Lobe Parameters*

5 0.01 93.7 75.2 –194 –119 –202

33 0.069 405 151 –50.5 –23 –79.3

50 0.107 627 205 –24 –5.62 –55.4

67 0.166 1,180 303 11.5 –33.6 –28.5

99 0.62 5,880 1,650 330 208 124

Plasma Sheet Parameters*

5 0.015 185 60 –378 –104 –148

33 0.096 472 130 –134 –25.4 –58

50 0.146 708 169 –62 –4.87 –36

67 0.196 1,100 228 –10.1 14.7 –17.4

99 1.361 4,740 753 308 154 81

* Based on analysis of GEOTAIL CPI records.183

Data collection ended in October 2001, providing approximately 28 years of data covering two complete solar cycles. The CPME instrument has three electron detection channels that 

measure electron fluxes in three energy ranges (0.22–2.5, 0.5–2.5, and 0.8–2.5 MeV). From these 

measurements, maximum electron flux environments are determined for various durations of interest. Daily average fluxes are used in the analysis. Higher temporal resolution data were analyzed 

for variations in flux due to measurements in the magnetosphere. No significant variations were 

observed due to magnetosphere boundary crossings.

Figure 110 shows the daily average electron flux bounds and the maximum flux observed 

for various durations as a function of electron energy. The CPME measurements are fitted with 

the function of form: F=AEγ. F is the flux (#/cm2-s-ster-MeV), E is the electron energy (MeV), and 

A and γ are fitting parameters. Fluxes of higher electron energies are obtained by extrapolation of 

the CPME data. Table 93 provides values of A and γ for the curves in figure 110.
271

newF110

1 Day

2 Days

7 Days

14 Days

1-Day Lower Bound

105

104

103

102

101

0

10–1

10–2

10–3

10–4

0 0.5 1 1.5 2 2.5

Energy (MeV)

Flux (No./(cm2–s–ster–MeV))

Figure 110. Maximum measured electron flux by the CPME instrument from 

1973 to 2001 corresponding to durations equal to the number 

of days indicated. CPME instrument measurements are plotted 

along with curve fit (see table 93).

Table 93. Equation and coefficients for the maximum electron flux events 

(refer to fig. 110).*

Event

Duration

(days)

Flux (#/(cm2-s-ster-MeV))=A Eg

1 2 7 14

A 3,700 2,000 660 16

g –1 –1.1 –1.2 –1.7

E = Electron Energy (MeV)

* Refers to maximum measured flux by the CPME instrument for events with durations equal

to the number of days indicated.
272

Figures 111–114 show the flux associated with each of the durations in figure 112 for a given 

number of observed events in the data set. This provides an indication of the frequency of events 

for lower fluxes less than the maximum values provided in figure 110. Tables 94–97 provide the 

function parameters associated with the curves in the associated figures.

105

104

103

102

101

0

10–1

10–2

10–3

10–4

Flux (No./(cm2–s–ster–MeV))

0 0.5 1 1.5 2 2.5

Energy (MeV)

newF111

1 Event

20 Events

50 Events

100 Events

500 Events

Lower Bound

Figure 111. Flux values obtained from the CPME instrument data associated with 

the number of 1-day events the given flux was greater than or equal to that 

indicated by the number of events curve. CPME instrument measurements 

are plotted along with curve fit (see table 94).

Table 94. Equation and coefficients for average daily electron flux bounds 

(refer to fig. 111).*

Number

of Events

Flux (#/(cm2-s-ster-MeV))=A Eg

1 20 50 100 500 All

A 3,700 610 170 65 2.5 0.0055

g –1 –1.2 –1.2 –1.2 –1.5 –1.5

E = Electron Energy (MeV)

* Flux values obtained from the CPME instrument data associated with the number of days the given flux was greater than or equal 

 to that indicated by the number of events curve.
273

105

104

103

102

101

0

10–1

10–2

10–3

10–4

Flux (No./(cm2–s–ster–MeV))

0 0.5 1 1.5 2 2.5

Energy (MeV)

1-Day Upper Bound

1 Event

10 Events

20 Events

50 Events

100 Events

200 Events

1-Day Lower Bound

Figure 112. Flux values obtained from the CPME instrument data associated with newF112

the number of 2-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 95).

Table 95. Equation and coefficients for 2-day duration electron flux events 

(refer to fig. 112).*

Number

of Events

Flux (#/(cm2-s-ster-MeV))=A Eg

1 10 20 50 100 200

A 2,000 610 170 65 2.5 0.4

g –1.1 –1.2 –1.3 –1.4 –1.4 –1.4

E = Electron Energy (MeV)

* Flux values obtained from the CPME instrument data associated with the number of 2-day duration events that the given flux

was greater than or equal to that indicated by the number of events curve.
274

105

104

103

102

101

0

10–1

10–2

10–3

10–4

Flux (No./(cm2–s–ster–MeV))

0 0.5 1 1.5 2 2.5

Energy (MeV)

newF113

1-Day Upper Bound

1 Event

10 Events

20 Events

50 Events

100 Events

200 Events

1-Day Lower Bound

Figure 113. Flux values obtained from the CPME instrument data associated with 

the number of 7-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 96).

Table 96. Equation and coefficients for 7-day duration electron flux events 

(refer to fig. 113).*

Number

of Events

Flux (#/(cm2-s-ster-MeV))=A Eg

1 2 10 20 50

A 660 40 5.5 1.9 0.32

g –1.2 –1.4 –1.8 –1.7 –1.7

E = Electron Energy (MeV)

* Flux values obtained from the CPME instrument data associated with the number of 7-day duration events

 that the given flux was greater than or equal to that indicated by the number of events curve.
275

newF114

1-Day Upper Bound

1 Event

10 Events

20 Events

50 Events

100 Events

200 Events

1-Day Lower Bound

105

104

103

102

101

0

10–1

10–2

10–3

10–4

Flux (No./(cm2–s–ster–MeV))

0 0.5 1 1.5 2 2.5

Energy (MeV)

Figure 114. Flux values obtained from the CPME instrument data associated with 

the number of 14-day duration events that the given flux was greater than 

or equal to that indicated by the number of events curve. CPME instrument 

measurements are plotted along with curve fit (see table 97).

Table 97. Equation and coefficients for 14-day duration electron flux events 

(refer to fig. 114).*

Number of 

Events

Flux (#/(cm2-s-ster-MeV))=A Eg

1 2 4 12 20

A 16 11 2.3 0.9 0.32

g –1.7 –1.7 –1.7 –1.7 –1.7

E = Electron Energy (MeV)

* Flux values obtained from the CPME instrument data associated with the number of 14-day duration 

events that the given flux was greater than or equal to that indicated by the number of events curve.
276

7.4 Ionizing Radiation Environment

In the region of space from 10 to 60 Re, the ionizing radiation environment will be composed of GCRs and SPEs. For the small section between 10 to 12 Re, there is some exposure to the 

outer electron belt, but it is weakening rapidly with altitude. The details of these environments and 

the effects on electronics are presented in sections 2.12, 3.11, and 5.5.

The main impact at these altitudes is that the Earth’s geomagnetic screening is no longer 

there. All the external environments (GCRs and SPEs) will be at full strength when interacting with 

any electronic systems. For total ionizing dose (TID), the trapped electron belt will only be a source 

of dose for the lightly shielded electronics and for altitudes <12 Re. Beyond this altitude, the dose 

driver for both TID and DD will be unscreened SPEs.

From a SEE point of view, the only sources for SEE are unscreened GCRs (under nominal 

conditions) and unscreened SPEs. For these mission segments it is often best to be conservative 

in design by having systems that can deal with events rates from a worst-case unscreened SPEs (at 

least the high criticality systems). If capable of surviving and/or operating under these conditions, 

then performing under the lower rates of GCRs will already be in the design.

7.5 Meteoroid Environment

Refer to section 5.6.1 for general discussion of the meteoroid environment. This description is still valid for this region of space. For mission trajectories from the Earth to the Moon, the 

meteoroid environment can be described by the Earth submodel of MEM. Once the mission trajectory reaches the lunar sphere of influence, roughly 60,000 km from the Moon’s center, meteoroids 

are primarily gravitationally focused and shielded by the Moon, so the lunar submodel of MEM 

would be applicable. Within the lunar sphere of influence, the Earth’s gravity field has a small effect 

on meteoroids and is not modeled in the lunar submodel of MEM. For discussion of the meteoroid 

environment in lunar orbit, see section 9.8.
277

8. LUNAR SPACE ENVIRONMENTS

This section covers those environments in lunar space, lunar orbit, and on the lunar surface 

(10 to 60 Re).

8.1 Thermal Environments

The lack of atmosphere, water, and slow rotation rate of the Moon leads to a substantially 

more severe thermal environment on the lunar surface than on the Earth. The same conditions 

result in an equally severe thermal environment—both hot and cold—in lunar orbit. In eclipse 

above the dark side, a satellite lacks direct solar radiation and receives only a few watts per square 

meter from the cold lunar surface. On the sunlit side near the subsolar point, the satellite receives 

both direct solar radiation and a nearly equal amount of energy in the infrared spectrum from the 

lunar surface.

8.1.1 Solar Constant

The solar constant, as defined in section 5.2.1.1, applies to lunar orbit. However, there is 

an enhanced range of variation due to changes in Moon-Sun distance caused by the Moon orbiting the Earth. This motion adds or subtracts up to 405,504 km to the variations in the Earth-Sun 

distance considered in section 5.2.1.1, so the effective solar constant range is as follows (does not 

include a +5 W/m2 measurement uncertainty):

• Maximum solar flux: 1,421 W/m2.

• Mean solar flux: 1,367 W/m2 (solar constant).

• Minimum solar flux: 1,315 W/m2.

For the near side of the Moon, average values of the normal albedo are provided by Dollfus 

and Bowell184 for wavelengths from 0.327 to 1.050 μ.To convert these data to an average bolometric value, the data were averaged weighted by the solar spectrum as approximated by a blackbody 

curve for 5,780 K. The resulting value is:

• Average bolometric normal albedo = 0.12 (lunar near side).

Local surface variations will cause an orbiting spacecraft to experience short-term variations about this value. These could range from a low of 0.07 to a high of 0.2.The average reflectivity is 164 W/m2 with a maximum of 285 W/m2 and minimum of 91 W/m2 for the near side.

The surface of the far side of the Moon is dominated by ‘lunar highlands’ terrain that tends 

to be somewhat brighter than the near side, which has numerous darker maria. For the far side, 

local normal albedo data are available for the waveband at 750 nmi from the UV/visible camera 
278

(UV/Vis) instrument from the Clementine mission. Noting that the curve fit of albedo against 

wavelength for the near side184 data crossed 750 nmi essentially right at the average value, 0.12, 

it can be assumed that the 750 nmi normal albedo will be a close approximation to the normal 

bolometric albedo. Thus, the resulting value is:185

• Average bolometric normal albedo = 0.15 (lunar far side).

Temporal variation about this value will be ±0.05. The average reflectivity is 154 W/m2 with 

a maximum of 285 W/m2 and minimum of 131 W/m2 for the far side.

The ranges of variation quoted here should bound the values seen by low-orbiting spacecraft. In higher orbits, some spatial averaging will occur and the range of variation will diminish 

somewhat.

8.1.2 Lunar Long-Wave Radiance

Analysis of the long-wave infrared camera data from the Clementine mission185 indicates 

that the radiance from the Moon, as viewed by a nadir-looking spacecraft, can be well represented 

by a simple Lambertian thermal balance model. The term Lambertian implies equal scattering in 

all directions. Given the slow rotation rate of the Moon, effects of thermal inertia on the surface 

can be neglected for this application. On this basis, the thermal balance at the lunar subsolar point 

can be expressed by the Stefan-Boltzmann law:

!"TS

4 ! (1" a )So / RL2 , (69)

where

ε = long-wave emissivity of the surface, assumed to equal 1

σ = Stefan-Boltzmann constant (5.67×10–8 W/m2K4)

Ts = lunar surface temperature at the subsolar point

a = average lunar normal bolometric albedo = 0.15

So = average solar constant at 1 AU from section 5.2.1.1.

On the sunlit side, the solar irradiance varies as cosine (cos)(i), where i is the solar incidence 

angle (the angle between the Moon-Sun vector and the Moon-satellite vector). A good approximation for the gross lunar surface temperature variation is therefore:

T !Ts cos1/4(i) . (70)

As a result the lunar long-wave radiance on the sunlit side is:

ILW ! !"T4 ! (1" a )So cos(i) . (71)

The uncertainty and range of variation due to local albedo effects is approximately 270 W/m2 

at the subsolar point. This corresponds to a ±20 K surface temperature variation. As the solar 
279

incidence angle increases to ~80°, the range of surface temperature variation increases to ~60 K but 

the irradiance variation is much less than at the subsolar point because of the power law.

On the dark side, the first part of this expression still applies:

ILW ! !"T4 , (72)

with the dark side temperature equal to 100 K, ±20 K. The analyst may wish to add an interpolation over the small discontinuity at i=90°.

8.2 Plasma Environments

8.2.1 Lunar Plasma (Solar Wind, Magnetosheath, and Magnetotail) Environments

Solar wind plasma energies are too low to produce bulk damage in thick materials but may 

degrade surface properties of thermal control coatings and multilayer insulation (including optical 

properties required for heat rejection). They may also degrade material integrity of thin materials 

used for thermal control systems, regolith ‘sandbags,’ or other lunar system design features. Should 

surface optical, thermal, or electrical properties be identified as critical to crew exploration vehicle 

(CEV) function, then evaluation of solar wind and magnetospheric plasma radiation damage 

should be conducted to determine the magnitude of the effect.

There are three types of lunar space plasma environments that the designer must consider:

solar wind, magnetotail, and magnetosheath.

The Moon’s orbit is inclined approximately 5.1° from the ecliptic plane, yielding a maximum 

distance of ±5.3 Re that the Moon can be located above or below the ecliptic plane. The magnetotail is approximately circular in cross section at lunar distances and the radius of the magnetopause 

(the boundary between the magnetotail and the magnetosheath) ranges from 20 to 30 Re.

186

The magnetotail is aligned with the solar wind, approximately 4° from the Sun-Earth line 

in the plane of the ecliptic, so the Moon must past through the magnetotail once each month. 

Bow shock dimensions are even larger, with the mean radius varying from 40 to 70 Re (or more) 

at lunar orbit.187 Using values of Re for the magnetopause (RMP)=25 Re and Rozetka Bortovoi 

Seti (bow shock radii) =50 Re for the magnetopause and bow shock radii, respectively, the fraction 

of the lunar orbit inside the bow shock is 26.5%, the fraction inside the magnetotail is 13.2%, and 

the fraction between the bow shock and magnetopause is 13.3%. The balance of the Moon’s orbit, 

73.5%, is spent in the solar wind. For many reasons, one will see a more severe charging environment inside the bow shock than in the solar wind.

Plasma environments are based on mean solar wind values from Feldman et al.188 and magnetosheath and plasma sheet plasma moment statistics reported by Minow et al.189 Plasma electron 

and ion spectra are reconstructed from the moments given in table 98 using Kappa distribution 

functions with a value of κ=3.5 for all species to provide a nonthermal tail to the distribution 

functions and corresponding differential flux spectra. Magnetotail encounters are all conservatively
280

Table 98. Moments for lunar plasma environment.

Environment Np (# /cm3) Nα /Np Ne (#/cm3) Tp (×104 K) Tα (×104 K) Te (×104 K) Vc

Solar wind 8.7 0.047 Np+2Nα 12 58 14 468

Magnetosheath 1 0.047 Np+2Nα 92.7 370.8 31.2 311

Plasma sheet 0.15 0.047 Np+2Nα 708 2,832 169 62

assumed to be plasma sheet environments, which add a conservative high-energy environment 

(although many magnetotail encounters are likely to be the low flux, lower energy lobes above or 

below the plasma sheet).

The electron environment shown in figure 115(a) is a power law dominated by high flux at 

low energies. Ion fluences given in figure 115(b) exhibit two peaks: a lower energy peak near 1 keV 

due to protons and a higher energy peak due to He ions. Assumptions are that the Moon spends 

73.5% of the time in the solar wind, 13.3% of the time in the magnetosheath, and 13.2% of the 

time in the magnetotail—taken to be the plasma sheet. The lunar 10-year plasma fluence environments are given in tabular form in table 98 for use in developing TID tests of material surface properties or bulk properties of thin materials. 

1010

1015

1020

newF115

Differential Fluence

(No./cm2-sr-keV)

10-yr Fluence SW: 73.5%

MS: 13.3%

PS: 13.2%

(a)

010–1 101 103 102

Energy (keV)

1010

1015

1020

Differential Fluence

(No./cm2-sr-keV)

10-yr Fluence SW: 73.5%

MS: 13.3%

PS: 13.2%

(b)

010–1 101 103 102

Energy (keV)

Figure 115. Lunar 10-year, free field plasma fluence: (a) Electron and (b) ion fluence 

environment for sunward-facing surfaces of the lunar surface.

These free-field environments provide the plasma flux at 60 Re from the Earth in the absence 

of the perturbations caused by the Moon. Spacecraft in lunar orbit or systems used in lunar surface operations will see significant perturbations to the free-field environments due to shielding 

by the Moon, as well as photoelectron production on illuminated surfaces.
281

8.2.2 Lunar Wake Plasma

The lunar wake is a plasma cavity in the solar wind on the anti-solar side of the Moon. 

With no intrinsic magnetic field and only a tenuous lunar atmosphere, the solar wind plasma is 

absorbed, for the most part, on the dayside when it comes into contact with the lunar surface, 

leaving a plasma void on the nightside.190 When the solar wind plasma directly collides with the 

Moon, a very low-density region called a lunar wake is formed behind the body of the Moon. The 

solar wind IMF passes through the Moon with only a slight inductive interaction with the more 

electrically conducting interior and reemerges into the plasma void of the solar wind wake. The 

IMF variation is expected to cause significant differences in the structure of the lunar wake. The 

extent of the wake and its topology is dependent upon the IMF direction. The system of currents 

that result from formation of the plasma void slightly amplifies the magnetic field within the wake 

and depresses the field at the wake boundary. At the boundary of the wake, electrons with greater 

thermal speed rush into the wake region faster than ions, creating an ambipolar electric field and 

a potential drop along the flanks of the wake that slows the energetic electrons and accelerates ions 

into the central wake region.

Al’pert,191 using a kinetic approach, started the theoretical study of the lunar wake. 

Explorer 35 and the Apollo subsatellites made the first measurements of the lunar wake, albeit 

with very low resolution. They were able, however, to detect a significant depletion in density 

behind the Moon. Recently, the WIND and LP satellites observed the plasma density structure 

of the lunar wake when the satellites passed behind the Moon at a distance of 6 to 8 lunar radii 

(RL). The WIND spacecraft made transits across the wake at distances beyond approximately 

6 RL. The result showed that a depletion of plasma density occurred widely behind the Moon.192

The observation of the lunar wake by the 3D Plasma and Energetic Particle investigation 

(WIND/3DP) experiment showed:

• Significant ion and electron depletions.

• Rarefaction waves traveling away from the wake. 

• Increased electron temperatures and constant ion temperatures. 

• Counterstreaming ion beams.

• An increase in magnetic field strength.

• A number of electromagnetic and electrostatic waves.193–196

The various regions encountered through the wake at approximately 6 RL are shown schematically in figure 116. A trailing compression wave is introduced in the plasma penumbra region 

to account for the changes in flow direction. The results can be summarized as follows:

• A plasma cavity (or plasma umbra), not entirely empty of plasma, is located within the Moon’s 

optical shadow and particle densities are low, <0.5 cm–3.

• The plasma penumbral region, externally bounded by a rarefaction wave (Mach cone inclined 

of 11°), is populated by plasma with increasing temperatures, decreasing densities, and variable 

flow velocities.

• The isotropic background of energetic solar particles follows IMF lines and is partially occulted 

by the Moon.
282

4

2

0

–2

–4

–6

–8

140,000

Trailing Compression Wave

Magnetic

Shadow

Interplanetary

Magnetic Field

143,000

153,000

160,000

Mach Cone

Mach Cone

163,000

170,000

Umbra

Penumbra

Penumbra

Optical

Shadow

173,000

180,000

150,000

newF116

(signY).rad (RM)

46 2 0 –2 –4 –6 –8 –10 –12 –14

X336 (RM)

Vsw

Figure 116. Schematic view of the solar wind-Moon interaction as deduced from 

the WIND/3DP results on December 27, 1994.

As the plasma is convected downtail, the wakeward beams fill the lunar tail with cold 

plasma. The rate of the wake refilling depends on the orientation of the IMF. The evolution 

of the lunar wake in simplified geometry can be simulated via an electromagnetic particle-in-cell 

code.197 This simulation begins immediately downstream of the Moon, before the solar wind has 

refilled the wake region, then evolves in the solar wind rest frame. The electrons immediately begin 

to move into the void but are trapped by two potential wells, thus generating vortices in phase 

space on both sides of the wake, between which counterstreaming electron beams interact. Ion 

beams are generated after the lighter electrons have moved into the void, creating a two-stream 

distribution that mixes in phase space due to the potentials created by the electron two-stream 

instability. The simulations are consistent with both Wind observations and the results of earlier 

electrostatic simulations that focus only on the ion dynamics.

8.2.3 WIND Observations of the Distant Wake 

Launched on November 1, 1994, the WIND spacecraft passed behind the Moon on 

December 27, 1994. At the time, the Moon was immersed in the planetary medium and a large 

amount of new data were collected by the WIND/3DP experiment. Wind in situ observations192,196

show a plasma density dropout region extending to at least 6 RL down tail. The solar wind properties observed before and after the WIND pass in the lunar environment are Np ~ 5 cm–3, Vp ~ 

500 km/s, and Tp ~ 8×104 K. Near-Earth IMF variations show an almost constant IMF magnitude, slightly increasing from 7 to 8 nT during the Wind pass near the Moon.196 This demonstrated 

the plasma characteristics (density and velocity) during the lunar wake crossing, as shown in 

figure 117.
283

6

(a)

(b)

(c)

(d)

4

2

0

0

–200

–400

–600

–800

6

4

2

0

400

200

0

–200

–400

–600

–800

–1,000

UT

Xsse

Ysse

Zsse

RAD

14:00

–6.40

2.70

0.82

2.82

Yx,Vy,Vz Vx,Vy,Vz Np Ne

14:30

–6.63

1.37

0.61

1.49

15:00

–6.81

0.02

0.39

0.39

15:30

–6.92

–1.32

0.18

1.33

16:00

–6.22

–2.65

0.04

2.65

F119

PESA-L Optical Shadow

PESA-H

PESA-H

EESA-L

Mach Cone

EESA-L

Vy

Vx

Vx

Vy

Yz

Vz

Figure 117. Plots of WIND/3DP plasma data during the lunar wake crossing 

in December 27, 1994:196 (a) Np, (b) Vx, Vy, Vz, (c) Ne, and (d) Vx, 

Vy, Vz.

As shown in panel (a) of figure 117, the ion density Np is approximately 4 to 5 cm–3 in the 

interplanetary medium, before 14:15 UTC. Beginning at 14:15 UTC, the ion density increases 

slightly up to 5 cm–3. At 14:32 UTC, Np begins to decrease toward a marked minimum of 0.5 cm–3, 

reached at about 14:52 UTC, i.e., inside the optical umbra region. Between 14:55 and 15:15 UTC, 

the ion densities reach minimum and then gradually increase, finally reaching its interplanetary 

value of 2.5 cm–3 at approximately 15:36 UTC. Panel (b) shows the directions of the ion flow. Only 

small modifications of external flow are noted in the wake; deflection of the flow direction as measured by the Vy component is outward directed during the inbound penumbra crossing. This result 

indicates that no significant flow component is needed to partially fill the plasma cavity. Indeed, it 

may be due to the expansion of the solar wind plasma owing to its thermal motion. The ion plasma 

temperature is increased at 14:40 UTC, followed by a sharp cooling between 15:00 and 15:12 UTC 

(not shown in fig. 117).
284

In panel (c), the crossing of the wake is evidenced by a sharp decrease in the electron density 

Ne down to <0.5 cm–3, between 14:50 and 15:17 UTC, roughly simultaneous with the crossing of 

the optical shadow. In panel (d), a significant Vy component away from the wake is again detected 

within the plasma void, well correlated with the associated ion flow. It is worth mentioning that 

during the entire wake crossing, the magnetic field direction was not strongly influenced by the 

downstream flow.

Similar results reported by Ogilvie et al.,192 shown in figure 118, indicate the observations 

of the lunar plasma wake from the WIND spacecraft on December 27, 1996, at a downstream distance of 6.5 RL. The plasma number densities show reductions clearly associated with the Moon. 

The minimum densities measured are approximately 20 times lower than the solar wind densities 

measured just before entry into the wake. While the electron temperature increases in the wake by 

a factor of approximately 4, the ion temperature remains fairly constant. As the system attempts 

to restore pressure balance,198 the magnetic field exhibits a corresponding gradual increase. The 

magnetic field rises slightly as reported earlier. Within the region where the plasma is significantly 

depleted, two distinct proton distributions are simultaneously detected. The ions are accelerated 

along the direction of the IMF by an electric field of the order 2×104 V/m. Each appears as a cold 

beam, narrow in velocity space, one convecting with a speed slightly faster than the ambient solar 

wind and the other convecting slightly slower.

1.0

0.1

1,000

100

10

600

550

500

450

400

350

30

–30

20

–20

10

–10

0

80

60

40

20

0

10

9

8

7

6

5

13.5 14 14.5 15

Hour (UT) December 27, 1994

IBI (nT) B (°) E/W (°)

V1 (km/s) T (103 K)

N1 (cm–3)

15.5 16 16.5

newF118

Shadow

Plasma Density

Reduced

Longitude (°)

160

180

200

φ θ

Figure 118. Plasma and magnetic field parameters for lunar wake crossing.192
285

Figure 119199 shows the in situ observations in the lunar wake by the WIND spacecraft at 

~25 RL on November 13, 1996. Figure 119(a) shows a gray-scale plot of the magnetic field fluctuation spectrum as a function of time and frequency. The overlaying dark curve denotes the local 

proton gyrofrequency. Panel (b) shows the gray-scale plot of the ion spectrum as a function of time 

and energy. Panels (c) and (d) show the three components and magnitude of the magnetic field.

Panel (e) shows the density profile. Panel (b) clearly shows counterstreaming ion beams192 as an 

important feature of the wake tail region.

1,000

100

10,000

0.01

0.10

(a)

(b)

(c)

(d)

(e)

0

6

8

10

12

4

2

0

6

8

6

Bz

By

Bx

–8

–6

–4

–2

4

4

2

2

0

0000 0100

n (cm–3) |B| (nT) Ion Energy (eV) B (nT) f (Hz)

0200

newF119 Figure 119. WIND spacecraft data from the moon wake tail encounter on 

November 13, 1996 (spacecraft distance from the Moon center 

was approximately 25 RL).199
286

Results of the simulation study by Trávníček et al.,199 with the focus on the deep lunar 

wake (up to 40 RL), show that two beams of counterstreaming plasma fill the lunar wake with the 

relatively cool, inhomogeneous and highly anisotropic plasma. As the plasma moves into the cavity, two rarefaction waves propagate away from the wake’s center, forming edges of the lunar tail 

structure. The streaming plasma is subject to velocity filtering as hotter plasma moves faster into 

the cavity than colder plasma. The current associated with this plasma expansion is responsible 

for the enhancement of the magnetic field in the cavity of rarefied plasma. The enhanced magnetic field cools down the streaming plasma. The plasma convects further downtail and a regionenhanced density is formed in the wake’s center where the magnitude of the magnetic 

field decreases.

8.2.4 Plasma Interactions and Effects

Counterstreaming electrons were reported during NOZOMI’s second lunar swing-by.200

These events can be categorized into two types from the characteristics of the electron distribution 

function:

(1) Counterstreaming electrons with a velocity distribution similar to that of the solar 

wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function 

of velocity. 

(2) Counterstreaming electrons that are thermalized and have a flux comparable 

to or dominating that of the solar wind electrons.

The origin of counterstreaming electrons observed in the first event is the lunar wake region, 

where the electrostatic potential drop associated with the ambipolar plasma expansion reflects the 

solar wind electrons. The energy of the reflected electrons reaches 480 eV, so that the magnitude of 

ambipolar electrostatic potential is estimated to be at least 480 V. The value of 480 V is consistent 

with the value of 400 V concluded by Ogilvie et al.192 based on the accelerated solar wind proton 

data from the WIND spacecraft. As for the second event, the electron energy spectrum implies that 

the solar wind electrons have thermalized at the downstream region. There are two candidates for 

the thermalization mechanisms: the terrestrial bow shock and the interaction region of the solar 

wind ions in the downtail of the Moon. When solar wind protons passing both sides of the terminator of the Moon interact with each other behind the void region, the two-stream instability can 

easily develop. After the thermalization of the solar wind electrons, the considerable amount of the 

solar wind electrons can be reflected back. Both cases can account for the observed counterstreaming electrons of the second event.

Ogilvie et al.192 detected the counterstreaming beam distribution, creating a two-stream 

instability. Analysis of the two-stream instability generated by the counterstreaming particle beams 

from either side of the wake197,201 revealed that the instability is largely due to the presence of the 

counterstreaming electrons. Previously, it was believed the instability was generated by the presence 

of counterstreaming ions.
287

Various types of electrostatic and electromagnetic plasma waves, in both upwake region 

and in the waketail itself, are reported.193–195 In general, these observations show that kinetic processes are important in the lunar plasma environment. A characteristic feature of the lunar wake 

is the presence of two counterstreaming ion beams drawn in from either flank.

Clack et al.202 observed both ion components exhibit an extreme temperature anisotropy, 

often with T(⊥) ~ 10T( !). The anisotropy is greatest in the central wake region. It appears that the 

anisotropy arises through the conservation of adiabatic invariants as solar wind plasma expands 

to fill in the cavity behind the Moon. Despite their large anisotropy, the proton distributions appear 

stable to the cyclotron instability. Correlated field and flow directional changes show that the wake 

geometry is dependent upon the prevailing magnetic field orientation.

8.2.4.1 Lunar Prospector Observations of the Inner Wake. In contrast to WIND that passes 

through the outer wake region, the LP spacecraft made a large number (>6,000) of crossings 

through the wake close to the Moon (between 20 to 115 km). The LP had a magnetometer and 

electron spectrometer onboard, and thus could detect the electron depletion and magnetic perturbations from the wake. Further, based on the systematic shift in the electron energy distribution 

function, LP could infer the potential change in crossing from solar wind into the wake plasma 

void. These near-surface transits through the wake flanks are valuable, providing unique insight 

into the plasma expansion process in a location close to where the wake is formed. The primary 

results of LP include the following:180

• The electron density was found to exponentially decrease by a factor of 1001,000 from 

a subsolar angle of 90° (terminator) to 180° (antisolar point).

• The electron temperature was found to progressively increase by a factor of 6–8 from a subsolar 

angle of 90° (terminator) to 180° (antisolar point).

• The inferred electrostatic potential was found to be slightly positive on the dayside (tens of volts), 

pass through zero volts at a subsolar angle of 75°, and become progressively more negative with 

increasing subsolar angle. At 90° (terminator), the potential was typically approximately –20 V 

but dropped as low as –300 V over the lunar nightside (subsolar angles >150°).

Figures 120–122 summarize the results stated above. In essence, with >6,000 passages, 

LP effectively mapped out the wake region close to the Moon. The views are centered as if looking 

down the tail of the wake back towards the Moon, with 0° latitude and 0° longitude being the  

antisolar point.
288

10

n/ne

1

0.1

0.01

–90 0.001

0 30 60 90 120 150 180 210 240 270 300 330 360

–60

–30

0

30

60

90

Wake Latitude (deg)

Wake Longitude (deg)

newF120

Figure 120. Normalized LP electron density measurements from altitudes 

of 20 to 115 km, binned by wake longitude and latitude 

(white circle shows the solar wind flow terminator).203

0

2

4

6

8

10

–90

0 30 60 90 120 150 180 210 240 270 300 330 360

–60

–30

0

30

60

90

Wake Latitude (deg)

T/Te

Wake Longitude (deg)

newF121

Figure 121. Normalized LP electron temperature measurements from altitudes 

of 20 to 115 km, binned by wake longitude and latitude (white 

circle shows the solar wind flow terminator).203
289

200

100

–100

0

–200

–300

–400

–90 –500

0 30 60 90 120 150 180 210 240 270 300 330 360

–60

–30

0

30

60

90

Wake Latitude (deg)

Wake Longitude (deg)

∆U (V)

F124

Figure 122. Electrostatic potential measurements from altitudes of 20 to 115 km,

binned by wake longitude and latitude (white circle shows the solar 

wind flow terminator). 203

Halekas et al.180 demonstrated that the modified self-similar plasma expansion process 

described by Samir et al.204 best fits the observations. The modification involved the use of solar 

wind kappa distributions rather than Maxwellians, which then gave a realistic fit to the observed 

solar wind temperature increase. Given a plasma void immediately following plasma absorption 

by the Moon, electrons will thermally diffuse into the wake ahead of the more massive ions. As 

a consequence, an ambipolar electric field develops that attempts to slow the electron inward flow 

and accelerate ions into beams that then catch up to the electrons. The self-similar model predicts 

the plasma density given this progressive charge neutrality loss, E-field development, and ion 

beam formation. Halekas et al.180 found that observations and the theoretical model matched very 

closely.

Unfortunately, LP did not include a direct current (DC) E-field measurement system or an 

ion spectrometer. Theories have also suggested that the ions accelerated via the plasma expansion 

process into the wake never fully catch up to the electrons, creating an electron cloud and a region 

of enhanced E-fields.These added instruments would be able to discern any subtle loss of plasma 

neutrality.

8.2.4.2 Simulations. Based on the WIND transits of the wake,194,197 a series of simulations 

of the plasma expansion process were run and compared to actual measurements. The codes were 

fully kinetic allowing the development of electrostatic fields that operate in the plasma expansion 

region. Figure 123 shows the results of a particle-in-cell simulation. Farrell et al.205 assumed that 

the solar wind magnetic field lay across the wake. The following simulated features were noted:

• A deep plasma void that extended back to beyond 32 RL.

• The development of an intense ambipolar electric field along the wake flank, in the region 

<4 RL of the Moon.

• Wake replenishment by an ion-ion beam instability that acts to slow the ion beams emitted into 

the central region by the wake flank. The structure of the trailing streamers is associated with 

wake replenishment that starts in the central region and progressively moves toward the flanks.
290

–4

0

4

–0 –8 –16 –24 –32

Low

Density

Streamers

Wake

Fill-in

Via

ES Instability

(a)

(b)

Y (Obstacle Radii)

Ambipolar E Field

ES Tail Via Instability

X (Obstacle Radii) = V-convect* t

newF123

–4

0

4

–0 –10 –20 –30

Y (Obstacle Radii)

X (Obstacle Radii) = V-convect* t

Figure 123. Kinetic simulation of the lunar wake showing the formation of the void 

region and associated electrostatic fields (development of strong ambipolar 

E-fields appear along the wake flank near the Moon): (a) Ion wake density 

and (b) wake electric field.205

Both the density and temperatures simulated in the model compared almost identically 

to those actually measured by the WIND spacecraft at the 6.5 RL crossing location. Figure 124 

is a comparison of WIND measured and simulated densities and ion beam velocities across 

the feature. Note that the density dropout is nearly one-to-one and the ion flow speeds (two beams 

coming from opposite flanks) vary nearly identically between measurement and simulation.
291

0.1

1

Density

Lunar Shadow

13:00

8 6 4 2 0

0

–2

–2

–4

–6

–8

–10

–4 –6 –8

Y (Lunar Radii)

0.2-hr Increments

newF124

300

–300

+ = 1st H* Stream

 = 2nd H* Stream

0

VY-GSE (km/s) Simulated Ion Velocity

0.01

0.1

1

Simulated

Ion Density

Figure 124. Comparison of wind density and simulation density along with wind 

ion beams and simulation ion distributions for the December 24, 1994, 

wake encounter near 7 RL (development of two separate beams appears 

in both measured and modeled sets).205
292

8.2.5 Plasma Wake Interactions and Spacecraft Effects

Traditional spacecraft surface charging, which can occur for the CEV, lunar landers, lunar 

EVAs, and lunar habitats occurs from high-energy electrons impinging on spacecraft surfaces. The 

following factors influence the amount of charging:

• Potential of the surface. 

• Secondary and photoemission characteristics of the surface materials.

• Bulk and surface resistivities of surface materials.

• Flux and spectrum of the impinging electrons.

• Geometry of the spacecraft (i.e., where the spacecraft dielectrics and conductors are located, 

which are in sunlight and which are in shade).

• Grounding schemes and floating conductors. 

• Spacecraft wakes. 

• Other factors.

For a description and mitigation techniques for spacecraft charging, see the references listed 

in section 5.4.7.2.

Passages of spacecraft into the lunar wake region are made both in darkness (no photoemission) and into a very tenuous plasma environment. As a consequence, the most serious concern is 

spacecraft charging effects. Halekas et al.203 demonstrated that the LP spacecraft charges to a few 

hundred volts negative during wake passages. During a solar storm when a flux of energetic particles is expected, the spacecraft could charge even more negative. As such, there should be a special emphasis on spacecraft charging practices, including tying all surfaces to a chassis ground and 

verifying that any carbon composite systems are coated with conductive paints (that tie to ground). 

The objective is to keep any part of the spacecraft from charging to a differential voltage relative 

to other well-grounded systems.

Recent analyses by Halekas et al.180 of the lunar surface charging measured from counterstreaming electrons on LP in 1998 have shown that in solar energetic particle events, when the 

Moon is in the magnetotail, shadowed areas can charge up to potentials of ~ –4.5 kV. It is expected 

that spacecraft or landers deeply immersed in the lunar wake may charge to similar levels. Such 

large potentials may have important consequences for surface expeditions near the lunar poles.

8.2.6 Magnetosphere Plasma Interactions and Spacecraft Effects

As shown in table 98, the plasma sheet consists of a low density but very warm plasma component. A concern is spacecraft charging in the warm plasma, which will tend to make unlit surface 

charge to large negative values (relative to sunlit surface that have photoemission processes). As 

such, there should be a special emphasis on spacecraft charging practices, including tying all surfaces to a chassis ground and verifying that any carbon composite systems are coated with conductive paints (that tie to ground). The objective is to keep any part of the spacecraft from charging 

to a differential voltage relative to other well-grounded systems.
293

8.3 Ionizing Radiation Environment

The lunar space ionizing radiation environments are the same as those discussed in section 6.5 for 60 Re. Since the Moon does not have any magnetic field that could provide any geomagnetic screening, the ionizing radiation environment will be composed of the unscreened 

GCRs and SPEs. The details of these environments and the effects on electronics are presented 

in sections 2.12, 3.11, and 5.5.

8.4 Lunar Gravity

Detailed gravity models of the type needed for precise orbit determination are derived from 

tracking data from orbiting spacecraft. For Earth there are, of course, many satellites and extensive 

data sets. However, for the Moon this is not the case. Suitable data are available from the Lunar 

Orbiters (I–V), Apollo 15 and 16 and their subsatellites, Clementine, and LP. These data are limited in two important aspects: (1) Orbit inclinations are limited to low inclination (<30°) and polar 

inclinations and (2) no direct tracking data are available from the backside passages. As a result, 

expected errors in the geoid reach 30 mm on the far side at mid-high latitudes, and orbit propagation errors will be greatest for the 40° to 70° and 130° to 160° inclination ranges. The situation 

cannot be improved until additional data from new missions become available.

Details and accuracy of the lunar gravity field require more careful consideration for lunar 

spacecraft design than is typically needed for Earth-orbiting spacecraft. Because there is no lunar 

atmosphere, lunar missions may use lower orbits (≤100 km) where the gravity variations have the 

greatest effect. Also, the variations represent a greater fraction of the total field. Without station 

keeping, a satellite placed in circular polar lunar orbit at 100 km will impact the surface in approximately 160 days. If the initial polar orbit is at 50 km, the lifetime is only approximately 50±5 days, 

depending upon initial longitude. A more extreme example, the propulsionless Apollo 16 subsatellite, which was released into a near circular 100 km orbit at a low (10°) inclination, impacted the 

surface after only 36 days. For higher altitude orbits (≥200 km), the details of the field become 

much less critical for design applications and it becomes appropriate to use truncated or simplified 

models for many applications.

8.4.1 Lunar Gravity Design Model

The Gravity Recovery and Interior Laboratory lunar gravity model is currently under 

evaluation for use by the Exploration program.

The gravitational potential of the Moon is expressed in terms of a spherical harmonic 

expansion with normalized coefficients Cnm,S ( ) nm in the form:

U = GM

r

+

GM

r

ac

r

!

"

# $

%

&

n

P

m=0 nm

n

n=2' ( ' (sin!) Cnm cos(m") +Snm ! sin(m!) " #

$ , (73)
294

where GM is the lunar gravitation constant (universal gravitation constant times the lunar mass), 

Pnm represents the fully normalized associated Legendre polynomials, ac is the reference radius 

of the Moon (1,738 km) for the model specified, φ is the latitude, and l is the longitude.

The model selected for design applications is the LP150Q model. The LP150Q lunar gravity field is a 150th spherical harmonic degree and order model that was developed using all available data from past U.S. missions to the Moon, including the Lunar Orbiter missions (I–V), the 

Apollo 15 and 16 subsatellites, Clementine, and all LP Doppler and range data. The LP data 

include all 100 km altitude data from the primary mission (which lasted for approximately 1 year) 

and all extended mission data consisting of 1 month at an average altitude of 40 km and 6 months 

at an altitude of 30 km. The model was produced by Alex Konopliv at the Jet Propulsion 

Laboratory.

Trajectory and navigation related model information must be used in the design of reference trajectories and in the development of baseline operational navigation strategies. Adequate 

sensitivity analysis is needed to ensure that the design will cover the range of DRMs. The gravity 

field is available on-line at <https://pds-geosciences.wustl.edu/geodata/lp-l-rss-5-gravity-v1/lp_1001/

sha/>.206

The LP150Q model should not be simply truncated to fewer terms for applications requiring lower fidelity. A lower order model must be generated by fitting a complete data set with fewer 

terms.

It is important that the proper astrodynamic constants and coordinates be used in conjunction with the model when optimum accuracy is needed. For the DE403 JPL ephemeris, the Earth 

gravitation constant is:

GMEarth = 398,600.435608 km3 /s2 = 0.888769245056500600 !10"9 AU3 /day2 . (74)

The ratio of the mass of the Earth to the mass of the Moon is 81.300585 and the lunar 

radius is 1,738 km. Carranza et al.207 discuss conversion between the DE403 coordinates and the 

International Astronomical Union 1994 coordinates. At the lunar surface, the shift between these 

two systems is approximately 700 m.

Errors intrinsic in the model are predominately due to data limitations. Thus, resulting 

errors in orbit propagation are very dependent on the orbit being considered. The errors will be 

small when the orbit matches closely to one of the satellite orbits used in developing the model, but 

they are much larger for inclinations far from those covered by the data set or when the orbit periapse is on the lunar far side. Near-polar and near-equatorial inclinations are represented in the data 

set, so errors for similar orbits can be anticipated to be only approximately 20 m radially and on 

the order of 1 km along track for a 2-day simulation at 100 km.However, for inclinations between 

30° and 80°, the errors increase such that the radial error can reach 500 m for a similar 2-day simulation. Orbit determination errors are discussed in detail in Carranza et al.207
295

Note that this specification is not intended for operations, and hopefully new models incorporating direct measurements from the lunar backside will be available prior to crewed missions.

As discussed above, for high orbit applications, this 100×100 coefficient model is more complex than will be needed for many hardware design applications and it may be appropriate to use 

a truncated set of coefficients, especially in preliminary and noncritical applications. The average 

truncation error in orbit eccentricity for the last 7 days of a 30-day orbit propagation from initially 

circular polar orbits is illustrated in figure 125. Three curves are presented for orbits beginning 

at 50, 100, and 400 km altitude and 0° longitude. For the 50 km case, the effect of various initial 

longitudes (initial right ascension) was also investigated. The error variation due to initial longitude 

was ±2% at 40°×40° resolution, and ±10% at 20°×20° resolution, about the values indicated on the 

graph. Thus, the variations, with respect to initial right ascension, are roughly equivalent in magnitude to going from 50 to 100 km initial altitude. Truncation errors for nonpolar orbits may be 

different.

30

25

20

15

10

5

0

0 100×100 80×80 60×60 40×40 20×20

Model Resolution

Mean % Error

newF125

50 km

100 km

400 km

Figure 125. Indication in the growth in truncation error relative to orbit propagation 

with the 100×100 model (30-day propagation of circular polar orbits 

beginning at 0° longitude and 50, 100, and 400 km altitude. The graph 

shows average error in eccentricity over the last 7 days of the simulation).

8.5 Lunar Eclipse

A lunar eclipse occurs when the Earth is between the Sun and the Moon, and the Earth’s 

shadow falls on the Moon. The Moon’s orbit is 5° inclined with respect to the Earth’s orbit around 

the Sun. This results in a lunar eclipse occurring only when the ascending or descending nodes fall
296

in the ecliptic during a full moon. Lunar eclipse tables can be found at <http://eclipse.gsfc.nasa.gov/

lunar.html> for the years 1901 through 2100.208 There may be up to five lunar or solar eclipses in a 

year but the combined total of both will never exceed seven.

There are three types of lunar eclipses: penumbral, partial, or total. During a penumbral 

eclipse, the Moon only encounters the penumbral shadow. A partial eclipse will have only part of 

the Moon going through the penumbral and umbral shadows. However, during a total eclipse, the 

Moon experiences all three types of events during the eclipse: penumbral, partial, and total. Light 

is refracted in the Earth’s atmosphere and will give the Moon an orange, red, blue-green, or purple 

hue while totally immersed in the umbra. The color is dependent on atmospheric conditions on 

Earth while the Moon is experiencing totality.

The stellar apparent magnitude is a numerical value of the observed brightness of a celestial 

object as seen from Earth. A full moon has an apparent magnitude of –12.6 (the brighter an object 

the lower the numerical value), while the Moon during totality ranges from –3.5 to 1.5.209 These 

numbers are derived using an eclipse model209 during mid eclipse. Using these computed values, 

the solar flux can be calculated during an eclipse from the following equations:

!m = meclipse " mfull moon = m1 " m0 (75)

calculates the decrease in apparent magnitude during an eclipse and

I0 = 10

!m

2.5 (76)

calculates the ratio of brightness of a full moon to the eclipsed Moon I1.

By setting the eclipseflux equal to 1,367 W/m2, the solar flux during a lunar eclipse can then 

be calculated using the average I0 /I1 solar flux at the Moon of 1,367 W/m2. This corresponds 

to 0.03% to 0% of solar flux reaching this region. This is of concern to a spacecraft orbiting the 

Moon if the spacecraft is dependent on solar energy.

During the years 2018–2035, the worst case lunar eclipse occurs on June 26, 2029. Using 

the data from this eclipse as an example, a spacecraft in a circular orbit would experience totality 

for the following altitudes versus times around the Moon and would experience worst case minimum solar flux for times indicated in table 99.

Table 99. Projected worst case minimum solar flux during lunar eclipse, 

June 26, 2029.

Orbit Altitude

(km)

Totality

(min)

No Solar Flux 

(Worst Case)

(min)

100 188 233

500 197 242

1,000 177 234
297

9. LUNAR SURFACE ENVIRONMENTS

This lunar environment section covers those environments in lunar space 10 to 60 Re.

The lunar surface is covered with a ‘soil’ called the regolith. It is formed from continuous 

bombardment of the surface materials by micrometeoroid impacts, and is modified by the solar 

UV flux, the solar wind, radiation, and other weathering factors.

The Moon can be divided into two major terrain classes on the basis of composition and 

age: (1) The Highlands and (2) the Mare (plural Maria). The Highlands and Mare terrains have 

very different morphologies due to the length of time each type of terrain has been exposed to 

meteorite bombardment (the ‘exposure age’). See figure 126.

newF126

Figure 126. Near side of the Moon—the bright rayed crater in the southern highlands 

is Tycho (fig. 127) (courtesy of NASA).
298

9.1 Principal Surface Features—Lunar Highlands

Lunar highlands were originally formed as the Moon accreted and consist of plutonic rocks 

that have been modified by impact. The shallow crust consists of a megaregolith (kilometer thickness) formed during the early heavy bombardment composed of thick deposits of basin ejecta, and 

a meter scale regolith. Compositionally, the highlands are ‘anorthositic.’ See figure 127.

newF127

Figure 127. Central southern highlands—the fresh crater at center left with the central 

peak and terraced rim is Tycho (courtesy of NASA).

The maria have relatively smooth lava surfaces that formed after the end of the heavy bombardment, hence they are relatively less cratered than the older highland terrain. Compositionally, 

the maria can be considered ‘basaltic.’ Because they postdate the heavy bombardment, they have 

a relatively thin regolith compared to the highland terrain. See figure 128.
299

F130

Figure 128. A portion of Oceanus Procellarum, including the craters Schiaparellic 

and Schiaparellie—a typical mare surface with relatively few impact craters

(courtesy of NASA).

9.2 Surface Feature Properties

9.2.1 Crater Size Distributions

Crater size frequency distributions have been compiled for the highlands and mares areas 

on the lunar surface from both orbital and surface imaging data. These data were compiled primarily for scientific purposes (e.g., to estimate the relative and absolute age of an area). As a result, 

such compilations typically focus on craters having diameters >1 km. A few of those compilations 

have been made which include craters down to the submeter size range. These analyses were done 

with Surveyor surface images, a few very high resolution lunar orbiter images, and now using the 

Lunar Reconnaissance Orbiter (LRO) Lunar Reconnaissance Orbiter Camera (LROC) images. It 

is the craters whose diameters are in the tens of centimeters to hundreds of meters scale that are 

relevant to landing and surface operations.

On a level surface, at diameters less than about 300–500 m, the craters are in steady state, 

in equilibrium, or saturated in terms of the frequency. This means that, as time passes, each new 

impact destroys an existing impact and the frequency remains more or less constant. Figure 129 

illustrates the size frequency distribution for a number of sites. The red line denotes the steady state 

level and it can be seen that the observed frequency of craters does not extend above this value. For 

a typical intercrater area, the frequency of 1-m-diameter craters is ~10/100 m2; for 10-m craters, it is 

~0.1/100 m2.
300

newF131

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Lunar Orbiter 3

(M-155)

Lunar Orbiter 4

(H-125)

Surveyor 3

Site Map. 1:2.000

NASA

Photograph

AS12-48-7075

Surveyor I

Surveyor III

Apollo 11

Apollo 12

Production Curve

Steady State Curve

Apollo 11

Apollo 11

Apollo 12

Apollo 12

Fall

Log (Cumulative Number of Craters/106km2)

–2 –1 0 1 2 3 4 5

Log Crater Diameter (m)

Figure 129. Crater size frequency distributions for several areas. The red line denotes 

the steady state or equilibrium condition. At diameters <200–300 m, 

the surface is in approximate equilibrium.210
301

On a surface having slopes, the frequency of craters at the relevant diameters is significantly 

less than the value that would be expected based on the absolute formation age of that surface. Figure 130 illustrates a mare surface under low Sun. The roughness is exaggerated by the low illumination angle; the Sun is 2.8° above the horizon. The area labeled A is a very old 1,500-m-diameter 

impact crater which is almost completely infilled. The gentle slopes on the interior wall have very 

few impact craters. This lack of craters and the texture on the surface (‘elephant skin’) is the result 

of micrometeoroid-induced downslope creep of the regolith. This creep is sufficiently active to 

remove most craters. It is also observed on steeper slopes covering larger area.

newF130

Figure 130. Mare surface in northwest mare imbrium under very low illumination; Sun 

is illuminating from the right. The cratered nature of mare surface is evident.

Due to the low Sun, even the older shallow craters are apparent. ‘A’ is an old 

1,500 m crater. Creep of the regolith down the interior slopes has removed 

small diameter craters and produced the observed texture (LROC M116465524RE)

(courtesy of NASA/GSFC/Arizona State University).
302

The topography of an impact crater is a function of its age; as the crater ages, the rim height 

decreases and the floor becomes shallower due to erosion by micrometeoroid impact.

For craters ~<700–800 m diameter, the initial depth/diameter relationship is:211

Ra = 0.15Da

0.88 , (77)

where Ra=depth (km), relative to external surface, not from the rim, and Da=diameter (km).

9.2.2 Rock Size Distributions

The distribution of rocks on the lunar surface is not random but rather is tied directly 

to the geology and geologic history of the region. Rocks are typically found associated with the 

ejecta of fresh craters, within the interiors of craters, and along scarps of exposed bedrock (e.g., 

rille walls). There are a few random rocks scattered across the surface that represent distal ejectaclasts launched from impact events. Compilations of the size-frequency distribution of rocks for 

different areas using Lunar Orbiter images have been compiled by Cintala and McBride,212 Bart 

and Melosh,213,214 and Cameron and Coyle,215 (G. Bart, “Lunar surface geology from analysis of 

impact crater and their ejecta,” unpublished dissertation, University of Arizona, 2007).

On the lunar mare, the surface is composed of a fragmental, impact-derived regolith that 

is of the order of a few meters thick. Impact craters that formed entirely within the regolith (i.e., 

those with depths less than a few meters) excavate only regolith material and typically do not have 

rocks scattered around the rim. As the diameter of the crater increases and its depth increases, the 

crater excavates through the regolith into the underlying basalt and ejects rocks around the crater 

rim (fig. 131).

newF131

Figure 131. West Crater—fresh impact crater east of the Apollo landing site with rocks 

scattered around the rim. A11: Apollo 11 LM descent stage, LWC: Little West 

Crater (visited by Armstrong), and West Crater: WC (M117338434R)

(courtesy of NASA/GSFC/Arizona State University).
303

Figure 132 illustrates rock maps for a number of craters showing the distribution of rocks 

around the craters. These figures from Bart and Melosh214 were compiled from Lunar Orbiter 

images and illustrate the variation in the total number of rocks as well as the spatial distribution 

of rocks around the craters.

newF132

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2

X Location (km)

–6

–4

–2

0

2

4

6

Y Location (km)

–6 –4 –2 0 2 4 6

X Location (km)

–30

–20

–10

0

10

20

30

Y Location (km)

–30 –20 –10 0 10 20 30

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1 –1.5 –0.5 0 0.5 1 1.5 2 –1

X Location (km)

2

1.5

1

0.5

0

–0.5

–1

–1.5

2

Y Location (km)

–2 –1.5 –0.5 0 0.5 1 1.5 2

X Location (km)

Figure 132. Map of rock locations around lunar craters showing the range of spatial 

distributions. Red circle denotes the crater rim (three from Bart and

Melosh).214
304

Over time, the flux of micrometeorites slowly breaks down the rocks into smaller and 

smaller particles. A number of craters are observed on the mare in which rocks are not present 

(or rarely present) around the rim but whose interiors have large number of rocks (fig. 133). There 

is no reason to suspect that the degradation process of rocks within the crater and around the rim 

should be any different, thus in these cases, the rocks presumably are being derived from bedrock 

exposed in the crater walls.

newF133

Figure 133. Impact crater with rocky inner wall; note the presence of rocks 

along the inner wall compared with the exterior where rocks are 

rare to absent (LROC image M119449091l) (courtesy of NASA/

GSFC/Arizona State University).

Within the highlands, the same impact-generated regolith is formed as on the mare. However, in this case, the original target material is an impact-derived debris formed by the ejecta of 

large basins (the so-called megaregolith). In this case, there is not an intact bedrock surface that 

can be excavated and thus rocks are not as frequent.
305

Surface rock counts have been compiled using orbital and landed images. These analyses 

have suffered from two different types of problems. The orbital data have typically been of a resolution too low to resolve the rocks that are the most common hazards to landing and operations 

(i.e., the large rocks tens of meters are easy to recognize and avoid). Images from landed robotic 

spacecraft (Surveyor, Luna) have a limited range over which data can be acquired and thus may 

not be representative of a larger area (i.e., Surveyor) or of poor quality (i.e., Luna). Surface images 

from the Apollo program allowed for detailed high-resolution data to be collected. Figure 134 illustrates the average cumulative size-frequency distributions for rocks near the Surveyor spacecraft.

newF134

10

102

103

104

105

106

107

Surveyor I

Surveyor III

Surveyor V

Surveyor VI

Surveyor VII

1

Cumulative Number of Particles Per 100 m2

1 2 4 8 1.6 3.2 6.2

12.8

25.6

51.2

1 2

mm cm m

4

Particle Size

Figure 134. Cumulative size-frequency distributions 

for rocks near the Surveyor spacecraft 

derived from surface images.

During the Apollo traverses, panoramic image sequences were acquired at each of the sampling stations. From these image sequences the number and distribution of rocks within 10 m 

(32.8 ft) were compiled to produce maps of the rock distribution. These maps were presented in 

the Preliminary Science Reports of each mission. Unfortunately, the original data have been lost 

and the figures show only two size classes (10–20 cm and 20–50 cm) as well as the actual size of the 

largest rocks. Figure 135 illustrates the maps for three of the stations (4–6).
306

newF135

AS16-108-17606

17612

17611

17610

17609

17608

17607

17626

17625

0.5 m

66055

66040

x 66080 and

66085 

66030

17624

17623

(c)

17614

17615

17616

17617

17618

17619

LRV Shadow

22.7 m2

17620

17621

Near Limit

Field of View

15.6 m2

0 1 2 3 4 5

m

N

18018 AS16-110-17992

18016

18014

18013

18011

18009

18008

LRV

30.8 m2

(b)

17994 0.7 m

Near Limit

Field of View

36.5 m2

17996

17998

18000

18003

18005 18006

0 1 2 3 4 5

m

N

AS16-110-17952

Lunar Roving 

Vehicle Shadow

8.7 m2

0.8 m2

Penetrometer 2

17957

17956

17955

17954

17974

17972

17971

17970

(a)

17968

17966

17965

17964

17963

17962

64475

64420 (Trench)

17961

17960

0.5 m

Near Limit

Field of View

22 m2

17959

0 1 2 3 4 5

m

Station 4 Station 5

Station 6

N

Round 

Subangular

Angular

0.7 m

0.7 m

0.7 m

Fragment Symbols

10 to 20 cm 20 to 50 cm Measured Size

Figure 135. Maps of the rock distributions at the Apollo 16 landing site. Figures are from 

the Apollo 16 Preliminary Science Report: (a) Station 4, (b) station 5, 

and (c) station 6.216
307

The data that are illustrated in a few of the rock maps were also published as cumulative 

size-frequency curves. But again, the original data have been lost and only a few points are plotted 

on each curve. Figure 136 illustrates the cumulative size-frequency distributions for Apollo 16 

stations 4–6.

newF136

10

100

Station 5

Station 4

Station 6

1

Cumulative Number of Fragments/100 m2

6.4 12.8 25.6 51.2 102.4

Size (cm)

Figure 136. Cumulative size-frequency distributions for rocks at stations 4 –6 

at the Apollo 16 landing site. 216
308

Cintala and McBride212 compiled data for rocks at the Surveyor sites. They grouped the 

rocks as those within craters (such as those shown in fig. 135) and those between craters. Figure 137 

illustrates their data for the Surveyor 7 site. A large counting was surveyed, such that the largest 

block counted was of the order 70 m.

newF137

1 × 10–6

1 × 10–5

1 × 10–4

1 × 10–3

1 × 10–2

Cumulative Number/100 m2

0.1 1 10 100 1,000

Diameter (m)

Series 1

Figure 137. Cumulative size-frequency distribution for rocks around the Surveyor 7 

landing site based on lunar orbiter data (data from Cintala and McBride212).

The LROC provides much higher resolution data than were available for most of the lunar 

orbiter images;217 these data are also better controlled in terms of position and image calibration. 

In addition, multiple lighting angles are available for many sites to allow for an assessment of the 

influence on lighting angle on rock recognition. The image scale for an LROC narrow angle camera image is 0.5 m/pixel from the nominal mapping orbit (50 km). Thus, rocks of the order of 2 m 

can be resolved. Rock frequency data collected for a number of sites illustrate the variation in size 

range of the rocks, the spatial distribution, and the statistics of the population.

Figures 138–150 present a number of different areas, followed by table 100 summarizing the 

data. These data represent a range of areas with variable geology and rock distribution. In some 

cases the data are for rocks around craters; in other cases, the data indicate the distribution for 

intercrater areas (plains units between craters excluding rocks directly associated with ejecta).
309

newF138

Figure 138. Western rim of North Ray Crater at the Apollo 16 landing site: A, lobes 

of blocky ejecta extending from the rim; B, areas between rocky lobes with 

fewer blocks; and C, partially buried blocks (LROC image: M117392541l) 

(courtesy of NASA/GSFC/Arizona State University).

newF139

Figure 139. NRC at the Apollo 16 landing site. Counting area is the area 

to the west of the crater rim. (LROC image M117392541l)

(courtesy of NASA/GSFC/Arizona State University).
310

newF140

0.0001

0.001

0.01

0.1

1

Cumulative Number/100 m2

0.01 0.1

y = 6.4187×–3.073

1 10 100

Diameter (m)

NRC/West

Power (NRC/West)

Figure 140. Cumulative size frequency distribution for rocks on the western rim 

of NRC (area shown in fig. 138).

newF141

Figure 141. Concentric crater with diameter of 130 m. Area outlined 

in red indicates the total area counted; area outlined in blue is 

subarea with higher rock frequency (LROC image M119449091l), 

crater at –11.8721 °S./.339.432 °E. (courtesy of NASA/GSFC/ 

Arizona State University).
311

newF142

0.001

0.01

0.1

1

10

Cumulative Number/100 m2

0.1

y = 118.45×–5.519

y = 122.88×–4.231

1 10 100

Diameter (m)

Subarea

Total Area

Power (Subarea)

Power (Total Area)

Figure 142. Size frequency distribution of ejected rocks for the crater illustrated

in figure 143. Subarea is outlined in blue; total area in red in figure 141.

newF143

Figure 143. Counting area is the northwest quadrant of West Crater

(LROC image M117338434R) (courtesy of NASA/GSFC/

Arizona State University).
312

newF144

0.001

0.01

0.1

1

Cumulative Number/100 m2

0.1

y = 4.7648×–4.302

1 10 100

Diameter (m)

West Crater NW – M117338434

Power (West Crater NW – 

M117338434

Figure 144. Cumulative size-frequency distribution for the northwest quadrant 

of the ejecta of West Crater.

newF145 Figure 145. Area surrounding the Surveyor VII landing site (LROC image M157668488RE)

(courtesy of NASA/GSFC/Arizona State University).
313

newF146

0.001

0.01

0.1

1

Cumulative Number/100 m2

0.1

y = 3.4339×–4.107

1 10 100

Diameter (m)

M157668488R S7

Power (M157668488R S7)

Figure 146. Cumulative size-frequency for rocks in the area outlined in figure 143.

Counts exclude rocks within craters.

newF147

Figure 147. Area surrounding the Apollo 16 landing site; LM descent stage

(LROC image M117392541L) (courtesy of NASA/GSFC/

Arizona State University).
314

newF148

0.001

0.01

0.1

1

Cumulative Number/100 m2

0.1

y = 0.0306×–1.578

1 10 100

Diameter (m)

A16 LM Area

Power (A16 LM Area)

Figure 148. Cumulative size-frequency distribution of rocks observed around 

the Apollo 16 landing site.

newF149

(a) (b)

Figure 149. Apollo 16 landing sites: (a) Station 5 and (b) station 6 (LROC image: 

M122108795L) (courtesy of NASA/GSPC/University of Arizona).
315

newF150

0.001

0.01

0.1

1

Cumulative Number/100 m2

0.1 1 10 100

Diameter (m)

Station 5

Station 6

Figure 150. Cumulative size-frequency distributions of rocks at stations 5 and 6 

of the Apollo 16 landing site.

Table 100. Rock frequency data.

Region

Frequency of Rocks Distribution

10 cm 1 m Slope

North Ray West Rim 7,558 6 –3.1

West Crater NW quadrant 95,509 5 –4.3

Concentric Crater total 39,132,272 118 –5.5

Concentric Crater subarea 40,595,809 122 -4.3

Surveyor VII 43,932 3 –4.1

Apollo 16 landing area 1.2 <0.1 –1.6

Apollo 16 station 5 13 <0.1 –2.3

Apollo 16 station 6 183,986 0.4 –5.6

9.2.3 Slope Distributions

The distribution of slopes on the lunar surface is primarily a function of the baseline over 

which the slope is measured. For long baselines, the cratered highlands are rough with a large distribution of slopes whereas the maria are smooth. However, at short baselines, both the mare and 

the highlands are rough with similar slope distributions. It is the presence of small diameter craters 

(in the meter to hundreds of meters size range) which controls the slope distribution, and both the 

maria and the highlands are saturated with craters in this diameter range.
316

9.3 Regolith Composition and Characteristics

9.3.1 Lunar Regolith Properties

The lunar regolith is a fragmentary material produced on the lunar surface over billions of 

years by the impact of meteoroids. Each impact event shatters the surface material creating a finegrained layer whose thickness and properties are a function of age. There are, in fact, two competing processes, the impact events break up the surface into the smaller pieces while at the same time 

melting small particles and fusing them into composite particles (agglutinates).

Regolith is taken to mean the entire fragmental layer on the lunar surface. A somewhat 

inconsistent nomenclature has developed that refers to specific size fractions. The definitions 

adopted here are listed in table 101.

Table 101. Particle size nomenclature.

Particle Name

Size Range

(mm) (φ)

Rock >10 < –3.32

Coarse fine 1–10 0 to –3.32

Fine fines/soil <1 >0

Dust <0.05 >4.32

φ = –log2(d), where d is the particle diameter

When considering analyses of the regolith (e.g., chemical, mineralogic, and physical properties), it is critical to recognize that the data were compiled from very small samples of specific size 

fractions. Most regolith samples were sieved at JSC after return from the Moon into the following 

size fractions: >10 mm, 10–4 mm, 4–2 mm, and <1 mm. Most of the laboratory data have been 

collected using particles <1 mm. Study of the >1 mm size fraction is so limited that it is difficult to 

compare with analyses of the <1 mm size fraction. For Luna and Apollo core material, data are for 

the <0.25 mm fraction. No data on size fractions >1 mm have been included in attempts to infer 

bulk properties of the regolith. Except for a few rock fragments, the average size of a rock clast in 

the regolith is <250 m. It is difficult to determine the petrology of such fragments and hence the 

chemistry has been used to infer the petrology of the rock fragment (i.e., individual minerals have 

not actually been determined).203,205,218–272
317

9.3.1.1 Size-Frequency Distributions. Particle size-frequency distributions were compiled 

on a large number of lunar regolith samples (e.g., fig. 151). These data were collected for small 

aliquots of size subsampled material. They do not represent the size-frequency distribution for 

the bulk regolith.

newF151

0

10

20

30

40

50

60

70

80

90

100

Percent Finer by Weight (%)

10 1

Apollo 12

Coarse Layer

Double Core

Apollo 14

Trench Bottom

Apollo 16

Station 11 Apollo 11, 12,

14, 15 Range

Apollo 16 (Bags)

Luna 16

0.1 0.01

Particle Size (mm)

Figure 151. Cumulative size-frequency distribution of regolith grain size from a variety 

of samples. Note the very coarse material from the Apollo 12 double core 

and the Apollo 14 trench.216

Figure 151 shows the size-frequency distribution for several lunar samples. Bands illustrate 

the variability in samples from a given site. Also illustrated are two very coarse samples (lower layer 

Apollo 14 trench; coarse layer Apollo 12 double core tube). Table 102 lists the dust component 

particle size frequency as illustrated in figure 151.
318

Table 102. Particle size frequency distribution for the dust component as illustrated 

in figure 151.

Sample

Fraction of Total Mass

Total Sample 

Mass (g)

Mean Grain Size

>10 mm 4 –10 mm 2–4 mm 1–2 mm <1 mm

<1 cm

Fraction

(μ)

<1 mm 

Fraction

(μ)

A10002 0.039 0.016 0.023 0.031 0.891 476.3 – 52

A14003 0.021 0.031 0.03 0.039 0.879 1,077.9 129 99

A14141 – 0.154 0.14 0.113 0.594 48 616 123

A14163 – 0.038 0.038 0.056 0.867 5,126.3 76 56

A15220 – 0.023 0.019 0.008 0.95 305.2 – 43

A15270 – 0.005 0.016 0.025 0.954 837.1 – 94

A15400 0.83 0.013 0.01 0.008 0.14 618.3 330 61

A61180 – 0.034 0.035 0.053 0.878 177.9 94 64

A61220 0.055 0.114 0.104 0.069 0.658 92.7 216 68

A62280 0.043 0.051 0.047 0.078 0.781 279.6 134 70

A64500 0.052 0.04 0.04 0.047 0.821 603.6 104 65

A68500 0.002 0.029 0.042 0.063 0.865 602.6 106 68

A70180 0.737 0.003 0.005 0.007 0.248 633.1 67 58

A71500 0.074 0.019 0.025 0.032 0.85 706.6 83 65

A72140 0.005 0.011 0.008 0.022 0.953 237.1 57 50

A72500 0.004 0.011 0.018 0.033 0.935 735.3 67 57

A73240 0.007 0.091 0.059 0.061 0.784 245.9 127 51

A74220 – 0.102 0.018 0.071 0.809 9.6 – 41

A78220 – 0.006 0.011 0.022 0.96 236.5 50 45

A78500 0.124 0.022 0.018 0.024 0.812 884.7 46 41
319

9.3.1.2 Particle Shape. Regolith particles exhibit a range of sizes and shape because of the 

manner in which they form. Meteoroids of various sizes have bombarded the surface for billions 

of years, shattering the rocks and creating finer and finer particles over time. Micrometeoroids, 

which also break down particles, can also create larger particles (agglutinates) due to the welding 

of particles by the impact-induced melt. Table 103 lists grain-specific properties.

Table 103. Summary of grain-specific properties (<1 mm size fraction).

Property Value Units Notes Sources

Sorting 1.99 to 3.73 range φ Very poorly sorted Reference 255

Skewness 0 to 0.3 range Nearly symmetric 

to slightly coarsely 

skewed

Elongation 01.32 to 1.3835 range

1.35 average

Somewhat 

elongated

Flatness 01.17 to 1.37 range

Aspect ratio 0.3 to 0.9 range

0.55 average

Slightly to medium 

elongation

Roundness 0.19 to 0.29 range

0.21 average

Subangular to 

angular

Volume coefficient 0.32 to 0.35 range

0.3 average

Specific surface area 0.4 to 0.78 range

0.5 average

m2 g–1

Depending upon the size fraction considered <1 cm versus <1 mm, the mean value can vary 

by a few to a hundred microns. There can also be variations between splits of individual samples. 

Pyroclastic material is typically much finer with more spherical particles and better sorting:

• Elongation: Elongation is defined as the ratio of the major to intermediate axes of the particle, 

or length to width. Particles with values of the ratio <1.3 are considered equant, and particles 

whose ratio is >1.3 are elongate. Measurements of elongation on individual particles (sized 

44–2,300 μm) ranged from 1.31 to 1.39.

• Aspect ratio: In geotechnical studies, aspect ratio is inversely related to elongation. It is defined 

as the ratio of the minor axis to the major axis of an ellipse fitted to the particle by a leastsquares approximation. Measurements of the aspect ratio of individual particles (sized 

1.25–30 μm) range from 1 (equant) to <0.1 (very elongate), with most values falling in the range 

0.4 to 0.7 (slightly to moderately elongated).

• Roundness: Roundness is defined as the ratio of the average of the radii of the corners of the 

particle image to the radius of the maximum inscribed circle. Measurements of roundness on 

individual particles (sized 44–2,300 μm) had average roundness values from 0.19 to 0.26.
320

• Volume coefficient: Volume coefficient is defined as the volume of a particle divided by the cube 

of the diameter of the circle that encloses the same area as the particle profile. Measurements of 

volume coefficient of individual particles (sized 60–733 μm) range from 0.24 to 0.37, with an average value of approximately 0.3; this value corresponds approximately to a prolate spheroid with 

a major-to-minor axis ratio of 3:1. The volume coefficient for a sphere is >0.52, well above the 

measured values for lunar particles.

• Specific surface area: Specific surface area (SSA) is defined as the surface area of a particle 

divided by its mass. It is a measure of both the size and the shape of the particle. A ‘soil’ consisting of spheres with the same submillimeter particle size distribution as lunar soil would have 

an SSA of approximately 0.065 m2/g. Lunar soil particles have measured SSA values of 0.02 to 

0.78 m2/g, with a typical value of 0.5 m2/g. The relatively large SSA of lunar soils is indicative 

of the extremely irregular, reentrant particle shapes.

9.3.2 Physical Properties

The mechanical properties of the lunar regoliths have been established by a combination of 

in situ measurements on the lunar surface, laboratory measurements of the properties of returned 

lunar samples, and theoretical analysis. Table 104 lists a summary of bulk geotechnical properties 

with notes indicating the source of the information.

• Bulk density: The bulk density (ρ) of the regolith is defined as the mass of material in a given 

volume usually expressed as g/m3 or kg/m3. The density of the regolith has been determined 

by analysis of the returned core and drill tubes, by analysis of boot, boulder and rover tracks, 

penetration resistance, and in situ measurements by Surveyor, Luna, and Lunokhod spacecraft. 

Figure 152 illustrates the density as a function of depth for several Apollo drill cores. There is 

a general increase in the bulk density with depth. These values represent minimum values. If 

larger volumes were considered, intact rocks would be incorporated into the volume and the net 

bulk density would be greater.

• Specific gravity: The specific gravity (G) of a particle is a ratio of its mass to the mass of an equal 

volume of water at 4 oC. To determine the specific gravity of the regolith, a volume of material is 

weighed and then immersed in a fluid to determine its volume. Since it is a ratio, it is unitless.
321

 Table 104. Summary of bulk regolith properties.

Property Value Units Notes Sources

Bulk density (ρ) 1.58 ± 0.05: 0–30 cm 

1.74 ± 0.05: 30–60 cm

g/cm3 Intercrater areas Reference 273 

(table 9.4)

Relative density (DR) 74 ± 3: 0–30 cm 

92 ± 3: 30–60 cm

% Intercrater areas Reference 273 

(table 9.6)

Specific gravity (G) 3.1 – Based on limited number 

of bulk samples

Reference 273 

(table 9.3)

Porosity (n) 49 ± 2: 0–30 cm 

44 ± 2: 30–60 cm

% Calculated Reference 273 

(table 9.5)

Void ratio (e) 0.96 ± 0.07: 0–30 cm 

0.78 ± 0.07: 30–60 cm

– Reference 273 

(table 9.5)

Permeability (Q) 1–7 × 1012 m2 Firing of Surveyor vernier 

engines on surface

Reference 274

Diffusivity 7.7 He, 2.3 Ar, 1.8 Kr cm2/s Measured on simulant 

function of gas species

Reference 275

Friction angle (φ) 30–50 o Reference 273 

(table 9.11, 9.12)

Cohesion (c) 0.1–1 kPa Reference 273 

(table 9.11, 9.12)

Thermal conductivity (k) 1.5 × 102 A15 

1.72–2.95 × 10–2 A17 

1.5 × 10–3 for 0–2 cm

W/mK Langseth et al.276, 277

Thermal diffusivity (κ) 0.002–0.008 @ 300 K cm2/s Temperature and density 

dependent

Horai and Winkler278

Heat capacity (specific heat) TBD J/gK Temperature dependent TBD

DC electrical conductivity <10–10 @ 400 K 

<10–25 @ 100 K 

<10–9 upper 5 km

Mho/m Rock values, lower for soil. 

Temperature dependent lab 

analysis and in situ

Reference 273 

(table 9.15), 

Reference 279

Relative dielectric permittivity (k) 2.8 @ 1.58 g cm–3

3.11 @ 1.74 g cm–3

– Function of density, 

temperature, and frequency

Reference 273, p. 536, 

Reference 280

Loss tangent 0.0057 @ 1.58 g cm–3

0.0066 @ 1.74 g cm–3

– Function of density, 

temperature, frequency, 

and chemistry

Reference 273,

p. 545.

Compression index (Cc) 0.3: loose 

0.05: dense 

0.01 to 0.11: range

– Lab measurement on 

1.2 to 200 g samples

Reference 277 

(tables 9.9 and 9.10)

Recompression index (Cr

) 0.003: average 

0 to 0.013: range

– Lab measurement on 

1.2 to 200 g samples

Reference 273 

(table 9.9)

Coefficient of lateral stress (Ko) 0.45: normally consolidated 

3–5: below a few meters 

0.7: recompacted

– Lab measurement on 

1.2 to 200 g samples

Reference 273 

(table 9.9)

Modulus of subgrade reactions (k) 8: average 

1 to 100: range

kPa/cm Based on in situ 

observations of boot prints

Reference 273

(fig 9.37)
322

newF152

300

200

Depth Below Lunnar Surface (cm)

Bulk Density of Lunar Soil, (g/cm3)

Apollo Drill Core Samples

100

0

1 1.5 2 2.5

Apollo 17

Apollo 15

Apollo 16

ρ

Figure 152. In situ bulk density of the regolith based on drill core samples from 

the Apollo 15, 16, and 17 missions. The abrupt increases and decreases 

result from different layers at different depths273 (Copyright ©1991, 

Cambridge University Press, reprinted with permission of Cambridge 

University Press).
323

• Relative density: The relative density (DR) is a function of the degree of packing of the individual particles in a granular medium. For mass of spherical particles, the relative density would be 

a minimum if the particles were arranged in face-centered packing and a maximum in a hexagonal close packing. Figure 153 illustrates the average relative density as a function of depth. The 

parameter is expressed as a percentage,

DR = emax ! e

emax ! e ( ) min

!

"

#

#

$

%

&

&

'100% , (78)

where

e = void ratio of regolith its present configuration

emax = maximum void ratio at which the regolith can be placed

emin = minimum void ratio at which regolith can be placed.

newF153

80

Very

Loose Loose Medium

0 20 40 60 80 100

Average Relative Density (%)

Dense Very

Dense

60

Depth Below Lunar Surface (cm) 40

20

0

Figure 153. Relative density as a function of depth (modified from fig. 9.20 in ref. 273)

 (Copyright ©1991, Cambridge University Press, reprinted with 

permission of Cambridge University Press).
324

• Porosity: The porosity (n) is defined as the volume of void space divided by the total volume, and 

related to the bulk density and the specific gravity. The parameter is expressed as percent:

n = 1! !

G! ( ) w

!

"

#

#

$

%

&

&

, (79)

where

ρ = bulk density

G = specific gravity

ρw = density of water.

• Void ratio: The void ratio (e) is the volume of the void space between particles to the volume 

of the solid particles (including subgranular porosity). The parameter is unitless:

e = n

(1! n)

"

#

$ %

&

' , (80)

where

n = porosity.

The relations between void ratio (e), relative density (DR), and porosity is illustrated in 

figure 154 for a number of lunar samples, a lunar simulant, and for uniform spheres with different 

packing.

newF157

0

0.4

0.8

1.2

1.6

2

2.4

70

60

50

40

30

20

10

0 

Void Ratio

Porosity (%)

0 20

Apollo 14

Apollo 15

Apollo 11

Basaltic Lunar

Soil Simulant

Uniform Spheres

Maximum

Void

Ratio

Minimum

Void

Ratio

40 60 80 100

Relative Density (%)

Very

Loose Loose Medium Dense VeryDense

Figure 154. Void ratio and porosity as a function of relative density for lunar samples. 

Also shown is a function for ideal, variably packed uniform spheres. 

There is considerable scatter among the lunar samples (modified from fig. 9.18 

in ref. 273) (Copyright ©1991, Cambridge University Press, reprinted with 

permission of Cambridge University Press).
325

• Permeability and diffusivity: These parameters relate to the movement of a fluid (either liquid 

or gas) through the regolith. These values have not been directly measured on lunar regolith but 

rather are inferred from other observations.

• Permeability: The coefficient of permeability defines the flow quantity through the regolith

as a function of the fluid pressure gradient. The value cited in table 104 was derived from the 

test firing of the Surveyor 5 vernier engine on the lunar surface and the permeability to depth of 

25 cm was deduced.281 This value is considered to be consistent with the fine-grained nature of 

the regolith.

The simplest case is an incompressible, nonreactive fluid and is given by Darcy’s law:

Q = K !g ( ) m

μ

!

"

#

#

$

%

&

&

iA , (81)

where

Q = flow rate (m3/s)

K = absolute permeability (m2)

ρ = density of the fluid (kg/m3)

gm = gravity (m/s2)

μ = fluid viscosity (Ns/m2)

i = hydraulic gradient

A = cross-sectional area normal to fluid flow direction (m2).

• Diffusivity: Gas diffusivity defines molecular flow through the regolith in response to a gradient 

in the concentration of a species. Diffusivity depends on the species composition, pressure, and 

temperature and the particle size and shape distribution. Data in table 104 were derived using He, 

Ar, and Kr in with a lunar simulant under vacuum conditions.275 These data indicate that the 

diffusivity for these gases is proportional to the absolute temperature and molecular weight of the 

gas.

•Friction angle: The angle of internal friction relates the shear stress and normal stress at which 

failure occurs. It is a measure of the regolith’s ability to withstand shear stress, and determined 

experimentally. The manner in which the angle is calculated is illustrated in figure 155.
326

newF155

0

1

2

3

4

5

6

7

Shear Strength (kPa)

0

Basaltic Simulant DR = 30%

Basaltic Simulant DR =90%

Surveyor

Model

Apollo and

Lunokhod

1 2 3 4 5 6 7 8 9 10

0 50 100 150 200 250 300

Normal Stress (kPa) 

Corresponding Depth Below Lunar Surface (cm)

r

c

r=c+ +

Figure 155. Normal shear stress versus shear strength. The slope of the relation defines 

the friction angle (inset) (modified from fig. 9.26 in ref. 273) (Copyright ©1991, 

Cambridge University Press, reprinted with permission of Cambridge 

University Press).

• Cohesion: Cohesion is the component of shear strength that is independent of interparticle 

friction; it is the strength when the confining pressure is zero. Variations in the cohesion 

of the regolith can be accounted for by differences in the relative density of the regolith.

•Thermal conductivity: Thermal conductivity (k) is the ability of the material to conduct heat. 

Thermal conductivity data were collected in situ on the lunar surface as part of the heat flow 

experiment and for returned samples in the laboratory. 

The upper 1–2 cm of the regolith has extremely low conductivity, about a factor of 10 lower than 

the bulk regolith. Overall, the bulk lunar regolith has very low conductivity; it is similar to aerogel 

and about a factor of 10 less than dry silica sand.

• Heat capacity: Heat capacity (C) is a measure of the amount of heat that is required to change 

the temperature of a body by a given amount. Specific heat capacity refers to the amount of heat 

required to change the temperature a given amount of mass.
327

Heat capacity is defined as 

C = !Q

!T , (82)

where

DQ = amount of heat

DT = temperature change.

For comparison, water has a specific heat capacity of 4.1855 J/gK.

• Thermal diffusivity: The thermal diffusivity (κ) is a measure of the rate at which material adjusts 

to changes in the temperature of the surroundings:

! = k

( ) "Cp

, (83)

where

k = thermal conductivity (W/mK)

ρ = density (g/m3)

Cp = heat capacity (J/kgK).

Materials with high thermal diffusivity adjust to the changes in the surrounding thermal 

environment rapidly.

Thermal diffusivity has been measured on a number of lunar samples. The values measured 

at atmospheric pressure are greater than those measured in a vacuum; values measured in vacuum 

are relevant to the Moon. Data indicate that basalts, which are denser and less fractured, have 

higher values than breccias, which are less dense and fractured.

Each investigation compiles the data for an individual rock sample and then a curve is fit 

to the data. Typically the expression has the form of:

! = A+ BT +

C

T2 + DT2 , (84)

where

T= temperature (K)

A, B, and C = constants.

Lunar samples have diffusivities at 300 K of 0.001 to 0.008/cm2s. For comparison, rocks 

have a thermal diffusivity of ~10–2/cm2s.
328

9.3.2.1 Derived Physical Properties. Following are some physical property descriptions:

• Compressibility: The compressibility of the material is an indication of the volume change that 

occurs when a confining stress is applied. At low stresses, the volume change is accommodated by 

particle movement and reorientation. At higher stresses, the volume change results from particle 

deformation or breakage. 

• Compression index: The compression index (Cc) is defined as the decrease in void ratio when 

the confining stress is increased by an order of magnitude (fig. 156):

Cc = ! "e

"log! ( ) v

#

$

%

%

&

'

(

(

, (85)

where

∆e = change in void ratio

 ∆ logsv= log change of applied vertical stress.

newF156

0.6

0.8

1

1.2

One-Dimensional

Oedometer Test

Apollo 12: 12001, 119

1.4

Void Ratio

0.01 0.1

Cc: Compression

Index = –

1 10 100

Applied Vertical Stress (kPa)

∆e

∆log σ v

σv

Figure 156. Compressibility measurements on Apollo 12 samples. Inset shows a diagram 

of a 1D oedometer for measuring compressibility (modified from fig. 9.21 in 

ref. 273) (Copyright ©1991, Cambridge University Press, reprinted with 

permission of Cambridge University Press).

• Recompression index: The recompression index (Cr) indicates the slope of the rebound-reload 

curve. It results when the sample expands after unloading and then is compressed again 

in the next compression cycle, as illustrated in figure 157.
329

newF157

0.7

0.68

0.72

0.74

0.76

0.78

0.8

0.82

Apollo 12: Sample 12001, 119

Initial Void Ratio = 0.854

Void Ratio

0.01 0.1 1 10 100

Applied Vertical Stress (kPa)

Cr: Recompression Index

= ∆e / ∆logσv

Figure 157. Change in void ratio as a function of applied vertical stress. The arrows 

indicate the changes in void ratio as the sample is loaded and then unloaded.273

(Copyright ©1991, Cambridge University Press, reprinted with permission 

of Cambridge University Press).

• Coefficient of lateral stress: The coefficient of lateral stress (K0) defines the ratio of horizontal stress to vertical stress. This value has not been measured on the Moon, but the assumption 

is made that the lunar regolith is normally consolidated. As the parameter is a ratio, it is unitless:

K0 = ! h

! v

, (86)

where

 sh = horizontal stress

 sv = vertical stress.

• Shear strength: The shear strength (τ) of the regolith is measure of the stability of the regolith 

against failure. It is typically defined in terms of the Mohr-Coulomb equation:

t  = c+s tanf , (87)

where

c = cohesion (kPa)

σ = normal stress (kPa)

f = friction angle (°).
330

• Bearing capacity: Bearing capacity is the ability of the regolith to support a load. Bearing capacity is discussed in two categories: ultimate bearing capacity and allowable bearing capacity. Ultimate bearing capacity is the maximum possible load that can be applied without causing gross 

failure. For a 1-m footing, the ultimate bearing capacity is approximately 6,000 kPa. Allowable 

bearing capacity defines a smaller load that can be applied without exceeding a given amount of 

settlement. Both ultimate and allowable bearing capacity can be further defined into static and 

dynamic regimes.

A parameter that is used to calculate the allowable static bearing capacity is the modulus 

of subgrade reaction (k). This parameter has been calculated from astronaut boot prints and has 

an average value of 8 kPa/cm.

• Slope stability: Slope stability is a factor of consideration in construction, either in the excavation 

of the regolith or the construction of an embankment or pile. For stability analyses, a factor of 

safety (FS) is calculated:

FS = N !gm ( ) h

c

!

"

#

#

$

%

&

&

, (88)

where

N = stability number (function of friction angle and the slope angle)

ρ = regolith density

gm = lunar gravity

h = slope height.

9.3.3 Regolith Composition

The composition of the regolith can be defined in terms of chemistry and mineralogy/ 

lithology. In terms of both chemistry and mineralogy/lithology, the composition depends upon 

both basement rock (basaltic mare, anorthositic highlands, or pyroclastics) and the grain size. 

With a given site (e.g., the Apollo 11 mare site), the chemistry and mineralogy/lithology 

of the material in the regolith is a direction function of the grain size range under consideration.

9.3.3.1 Chemistry. An analysis of the chemistry does not necessary imply a specific mineralogy, although a specific mineralogy does imply an approximate chemistry. In a simple example, 

both graphite and diamond are composed of carbon. Knowing the sample was composed of 

carbon does not allow a determination of the mineralogy and hence the history of the sample. On 

the other hand, if one knows the mineralogy, both the bulk chemical composition and the geologic 

history of the material can be determined.

Chemical compositions are typically expressed as oxides, although the elements do not necessarily occur in such forms. Trace elements are usually reported as ppm (parts per million) or ppb 

(parts per billion), depending upon the species. Again, this does not necessarily indicate that the 

elements occur as native species. Tables 105 and 106 list the bulk and trace element compositions 

for a number of Apollo samples, respectively.
331

Table 105. Bulk element compositions of Apollo samples.

Major Oxides

(wt %)

Element Apollo 11 Apollo 12

Apollo 15 

KREEP-rich

Apollo 17 

Orange Glass

Apollo 17 

Light Mantle

Apollo 14 

Fra Mauro

Apollo 16 

Cayley Fm

Luna 20 

Highlands

SiO2 42 46.1 46.7 38.7 45.1 47.7 44.9 45.2

TiO2 7.5 2.7 1.5 8.8 1.3 1.7 0.59 0.49

Al2O3 13.5 12.6 16.4 6.5 21.3 17.4 26.7 22.8

FeO 15.8 16.5 12.2 22.3 8.3 10.5 5.44 7.3

MnO 0.21 0.21 0.16 0.29 0.11 0.14 0.07 0.11

MgO 7.9 10.2 10.4 14.5 9.8 9.4 6 9.5

CaO 12 10.3 11.2 7.5 12.9 10.9 15.3 14.4

Na2O 0.44 0.46 0.46 0.37 0.43 0.7 0.46 0.35

K2O 0.14 0.24 0.21 0.077 0.144 0.52 0.121 0.07

Cr2O3 0.3 0.38 0.34 0.7 0.22 0.2 0.111 0.19

P2O5 0.1 0.3 0.22 0.06 0.13 0.49 0.12 0.12

S 0.11 0.08 0.08 0.05 0.06 0.1 0.07 –

∑ 100.1 100 99.8 99.8 99.9 99.8 99.9 100.5

Is/FeO* 78 55 65 1 65 65 82 –

 * Is/FeO is a measure of regolith maturity; increasing maturity with increasing number.282
332

Table 106. Trace element compositions of Apollo samples.

Trace Element Composition

(ppm)

Element Apollo 11 Apollo 12

Apollo 15 

KREEP-rich

Apollo 17 

Orange Glass

Apollo 17 

Light Mantle

Apollo 14 

Fra Mauro

Apollo 16 

Cayley Fm

Luna 20 

Highlands

Li 12 17 9 11 10 27 8 6

Na 3,200 3,400 3,400 2,800 3,200 5,200 3,400 2600

P 400 1,300 1,000 300 600 2,100 540 500

K 1,100 2,000 1,700 640 1,200 4,300 1,010 600

Sc 63 39 24 49 17 22 9.5 16

V 65 110 85 130 40 55 25 45

Cr 2,100 2,600 2,300 4,800 1,500 1,400 760 1,300

Mn 1,600 1,700 1,300 2,200 900 1,100 540 900

Co 29 43 41 62 90,032 35 31 30

Ni 200 200 260 90 270 370 440 230

Zn 24 7 25 250 15 25 26 50

Ga 4.5 4.2 4.2 – 2.8 6 3.6 –

Rb 2.8 6.2 5.8 1.1 3.4 15 2.9 1.6

Sr 163 137 140 209 152 179 176 139

Y 115 131 90 49 58 242 46 26

Zr 300 480 380 190 220 880 180 110

Nb 19 29 23 15 13 55 13 –

Cs 0.11 0.3 0.26 0.12 0.18 0.65 0.14 0.09

Ba 170 390 280 80 170 800 140 90

La 16 34 25 6.4 15 67 13 6.9

Ce 47 89 68 19 39 176 34 17

Pr 7 12 9 3 5 24 4.5 2.4

Nd 38 55 39 18 26 105 21 11

Sm 13 16 12 6.6 7.3 30 6 3.1

Eu 1.77 1.75 1.45 1.84 1.24 2.5 1.2 0.91

Gd 17 20 13 9 9 35 8 4

Tb 2.9 3.5 2.3 1.5 1.5 6.2 1.2 0.64

Dy 19 22 15 9 10 39 8 4.2

Ho 4.6 4.8 3.4 1.8 2.1 8.7 1.7 0.9

Er 11 13 9 5 6 23 5 2.7

Tm 1.7 1.9 1.3 0.7 0.9 3.5 0.7 0.4

Yb 10.6 12.3 8.5 4.2 5.5 22 4.3 2.5

Lu 1.54 1.74 1.24 0.61 0.78 3.1 0.61 0.38

Hf 10 13 10 5 6 23 4.5 2.5

Ta 1.3 1.5 1.1 1 0.7 3 0.55 0.3

W 0.2 0.7 1.2 – – 1.7 <0.5 64

Ir 7 7 7 0.8 12 13 15 10

Au 2.9 3.5 4 0.3 5 7 10 3.4

Th 2 5.8 4.2 0.5 2.6 13 2.2 1.3

U 0.51 1.5 1.2 0.16 0.73 3.5 0.62 0.33
333

• Apollo 11: Data are based on sample 10084 and some data for 10085 and 10010, both of which 

are nearly identical to 10084.283

• Apollo 12: Data from typical samples 12001, 12023, 12029, 12041, 12042, 12044, 12057, 

and 12070 which are very similar in composition (e.g., FeO range: 15.9%–17.1%; Th range: 

5.3–6.3 ppm).

• Apollo 15 KREEP-rich: Data are from six samples from station 6. Station 6 regolith contains 

the greatest proportions of KREEP-rich materials and thus has the largest concentration 

of incompatible elements.

• Apollo 17 Orange Glass: Data from 74220 taken near Shorty Crater at station 4. This soil is the 

most anomalous in the collection as it consists almost entirely of pyroclastic glass beads with 

some ‘normal’ soil (drive tube 74001/74002).

• Apollo 14 Fra Mauro: Based on data for all samples except 14141. Apollo 14 are very similar 

in composition (e.g., Fe range: 10.1%–10.9%; Th range: 12.3–13.6 ppm).

• Apollo 16 Cayley Fm: Mean of data from all 24 samples of mature soil. This composition probably best represents surface of Cayley Fm (Korotev284). Compositional range of mature soils 

is moderate (e.g., FeO range: 4.4%–6%, Th range: 1.6–2.5 ppm) and mainly represents variations 

in the relative abundance of anorthosite.

• Luna 20 Highlands: Based on all available data.

9.3.3.2 Mineralogy/Lithology. Since the regolith is simply a layer of fragmental material 

derived largely from the underlying bedrock, it is a mixture of fragments of the bedrock, individual 

mineral grains derived from rocks, and agglutinates (small fragments formed by welding together 

of other grains) (table 107). Figure 158 illustrates these data in a graphical form.
334

Table 107. Modal volume abundance of particles in the 1,000–90 μ size range 

for different sites.

Modal Abundance (Vol. %) for 1,000–90 μ Size Fraction

10,084 12,001 12,033 15,221 14,163 64,501 67,641 76,501 22,001

Mineral Fragments

Pyroxene + 

olivine

4.2 18.3 26.3 16.1 2.6 1 0.5 17.3 8.9

Plagioclase 1.9 3.9 9.9 13.1 5.1 32.1 12.2 15.2 14.7

Opaque 1.1 0.2 1.3 0.1 – – 1.1 2.8 0.1

Lithic Fragments

Mare basalt 24 12.9 7.5 3.1 2.2 0.3 0.5 9.2 6.9

ANT 0.4 1 1.3 2.6 2.9 5 21.7 0.5 3.5

LMB 0.8 0.1 0.3 0.6 0.3 2.1 30.7 6.3 0.5

Feldspathic 

basalt 

(KREEPY)

1.1 0.5 – 0.4 0.6 1.6 1.6 0.2 1.4

RNB/POIK – 2.3 3.7 2.7 10.9 8.3 7.9 8.1 2.2

Fused Soil

DMB 7.5 9.5 11.9 13.3 19.3 13.9 11.1 4.2 10.6

Agglutinate 52 40.1 17 36.9 45.7 29.1 8.5 29.2 16.6

Glass Fragments

Orange/ 

black

2.7 0.5 1.5 0.4 – 0.7 0.5 1.6 –

Yellow/ 

green

0.8 2.8 0.2 4.5 2.9 1.2 – 1.3 0.9

Brown – 1.5 7.8 0.3 – – – – 0.2

Clear 1.3 1 – 1.5 1.3 1.4 – 0.8 0.6

Devitrified 

glass

1.8 5 10.8 4.1 6.1 3.4 3.2 2.2 5.4

Others 0.3 0.5 0.5 0.3 – – 0.5 1.1 0.3

Total 99.9 100.1 100 100 99.9 100.1 100 100 100.1

N 625 823 666 1,000 311 942 189 820 634

ANT= anorthosite, norite, troctolite

LMB = light matrix breccia

RNB/POIK = recrystallized norite breccia / poikilitic breccia
335

newF161

40

30

20

10

0

Pyroxene + Olivine

Plagioclase

Opaque

Mare Basalt

ANT

LMB

Feldspathic Basalt (KREEPY)

RNB/POIK

DMB

Agglutinate

Orange/Black

Yellow/Green

Brown

Clear

Devitrified Glass

Others

 

 14163

 15221

 12033

 12001

 10084

 

 67641

 64501

 22001

 76501

Clast Composition

Figure 158. Grain composition in regolith samples. x axis lists the component 

composition, y axis lists the different samples (data from table 107).

9.3.4 Lunar Regolith Electrical Properties

9.3.4.1 Introduction. Theoretical considerations and observational evidence acquired from 

Apollo, as well as subsequent lunar missions, indicates that the lunar surface and dust grains are 

electrostatically charged by the incident solar UV radiation and the solar wind plasma.285,286 On 

the lunar dayside of the Moon, the dust is believed to be charged positively by photoelectric emissions with the incident solar UV radiation, and predominantly negatively by the incident solar 

wind electrons on the nightside. There is considerable evidence to indicate that the charged fine 

lunar dust grains, smaller than a few microns in size, are levitated and transported to high altitudes 

and transported over long distances across the lunar terminator.287–290 The lunar dust, with its 

toxic nature and high adhesive characteristics, constitutes a major source of hazard for humans 

and the mechanical systems in human and robotic exploration of the Moon.

Although the basic principles and the underlying sources of the observed lunar dust phenomena are recognized, the extent and the details of the lunar dust charging, levitation, and transportation process remain poorly understood. The current theoretical models do not satisfactorily 

explain the observed lunar dust phenomena. A more definitive knowledge of the lunar dust phenomena with acquisition of the basic data is needed for engineering solutions and development 

of mitigating strategies.
336

9.3.4.2 Lunar Dust Charging Processes. The charging of the lunar regolith is a complex 

process and can be accomplished by many different means. The lunar regolith is typically a nonconductive material suggesting that it can be charged readily by many different means. Electrostatic 

charging of the lunar regolith and dust can be done by photoelectric emissions produced by UV 

radiation at wavelengths near 200 nm on the dayside, leading to positively charged grains, with 

substantial electrostatic charging taking place when the dust is bombarded by soft x-rays with 

a wavelength <100 nm. Electron or ion collisions on the nightside of the lunar surface produce negatively charged dust grains due to low-energy electrons (<100 eV) impact, and positively charged 

dust grains due to high-energy electron impact. These different charge states are typically driven by 

variations in the secondary electron yield of the dust grains.

Triboelectric charging is the other charging process that must be considered. Triboelectric 

charging of dust grains by contact charging is a process in which electrons are transferred from 

a solid material with high work function to one with a lower work function, and occurs during 

landing of a lunar vehicle or movement of an astronaut over the regolith. Triboelectric charging 

can be exacerbated by trying to remove the dust through brushing, dusting, or blowing. Triboelectric charges can build up rapidly because there is no atmosphere to discharge through, and 

the regolith is electrically insulating (i.e., there is no common ‘ground’ for electrical equipment). 

The dust forms unique morphologies, is loosely packed, and is electrically and thermally insulating. The experience of the Apollo astronauts was that the dust is very adherent and abrasive, and 

hindered the effectiveness of even those very short missions. Mitigation of the effects of charge 

and dust must be a priority for any mission planned for a long stay on the lunar surface. The source 

of the problem is twofold: induced charging through triboelectric effects and interactions with 

naturally occurring background charge.

9.3.4.3 Electrical Properties for Charging Analysis. The typical electrical properties 

required to conduct a surface charging analysis of the lunar regolith and lunar dust using the 

NASCAP and NASCAP-2K are bulk resistivity of the lunar regolith, surface resistivity of the 

lunar regolith, secondary electron data, photoemission data, and physical properties. This section 

is designed to address only the bulk resistivity, surface resistivity, secondary electron data, and 

photoemission data of the bulk lunar regolith and dust grains. The physical properties of the lunar 

regolith such as atomic number, atomic mass, density, and size distribution are addressed in sections 9.3.1 and 9.3.2.

The lunar surface temperature typically varies from approximately –150 to 150 °C, depending upon shadowing of the Sun. Because electrical properties of insulators are highly dependent 

upon temperature, the electrical properties needed for charging analysis should include the temperature dependence. Where possible, this dependence has been incorporated.

9.3.4.3.1 Measured Bulk (Dust) Electrical Properties. Most of the known bulk electrical 

properties of the lunar surface fines have been made on samples taken in the Apollo era, either on 

the lunar surface or on samples returned to the Earth and held in the Lunar and Planetary Institute, a division of the Universities Space Research Association. These measurements are few and 

were made on samples from the locations of the sites of the Apollo landings. Thus, the coverage 

over the lunar surface is limited. The values given below are generally based on direct measurements. Where this is not so, it is noted.
337

9.3.4.3.2 Bulk Conductivity of Lunar Regolith. All of the electrical conductivity data that 

are known were measured from samples retrieved during Apollo missions 14–16.291 The samples 

were brought back to the Earth where they were tested in the laboratory. One sample retrieved 

during the Apollo 15 missions was tested over a wide temperature range and showed temperature 

dependence. For the most part, the data show the lunar regolith to be a good insulator. Table 108 

shows data measured on various lunar samples.291 The crust of the Moon has an average conductivity of about 10–7 to 10–8 mho/m.

Table 108. Direct current electrical conductivity of lunar rocks.

Lunar Sample

σ0*

(mho/m)

E0**

(eV)

σ1

(mho/m)

E1

(eV)

10048 3.5 0.896 2.66 × 10–3 0.559

12002,85 13 1.09 1.8 × 10–4 0.48

15058 134 1.374 2.78 × 10–3 0.593

15418 137 1.509 9.84 × 10–2 0.971

15555 36.8 1.04 1.27 × 10–2 0.604

68415 1.27 × 108 2.64 – –

68815 14.2 1.366 – –

 * Conductivity.

** Activation energy.

Data for sample 12002 is from Olhoeft et al.;247 the rest of the data are from Schwerer et al.243

Figure 159 shows a temperature dependence of electrical conductivity measured on a soil 

from the Apollo 15 site (sample 15301,38).251,291 The temperature dependence shown is characteristic of amorphous type samples typical of the lunar regolith. Also shown in figure 159 is the 

temperature dependence of an Apollo 16 rock (sample 65015,6),247 which was similar in form to 

that of soil. However, the temperature dependence of the electrical conductivity measured on the 

remaining lunar rocks was found to be given by table 108.

9.3.4.3.3 Bulk Conductivity (Cryogenic Temperature). The conductivity is temperature 

dependent. Using data from a variety of samples, Olhoeft et al.250 and Schwerer et al.245 derived 

the following relation:

DC conductivity = ! 0e

!E0

kT

"

#

$ %

&

'

+! 1e

!E1

kT

"

#

$ %

&

'

mho/m , (89)

where

k = Boltzmann’s constant = 8.6176×10–5 eV/K

E0, E1 = activation energies

σ0, σ1 = conductivities.
338

newF159

–14

–12

–10

–8

–6

–4

–2

65015,6

12002,85

15301,38

Log DC Conductivity (mho/m)

0.5

10 8 6 4 3 2 1

1 1.5 2 2.5 3 3.5

1,000/T

Temperature (×100 °C)

Figure 159. Temperature dependence of lunar soil samples (Apollo 15)

and lunar rock samples (Apollo 16) (Copyright ©1991, Cambridge 

University Press, reprinted with permission of Cambridge 

University Press).

9.3.4.3.4 Dielectric Constant. 

• Dielectric permittivity: Dielectric permittivity (k) is a measure of the ability of the regolith 

to maintain electrical charges physically separated by a distance. The relative permittivity is the 

ratio of permittivity of the material to the permittivity of a vacuum. The value is related to the 

density of the material. Unlike the loss tangent, the permittivity does not show a relation with 

the amount of titanium dioxide (TiO2) and ferrous oxide (FeO).

Laboratory measurements on a variety of returned lunar materials indicate that for all 

of the data, the following relation is observed:

k=1.919r , (90)
339

where ρ = material density (g/cm3).

For soils alone, they define a somewhat different relation:

k=1.871r , (91)

where ρ = material density (g/cm3).

• Loss tangent: The loss tangent of a material quantifies its dissipation of electromagnetic energy. 

The parameter refers to the angle in a complex plane between the lossy component of an EM field 

and its lossless component. For the laboratory analysis of lunar regolith samples, the loss tangent 

is a function of both the density of the material and the amount of FeO + TiO2.

Expressed as a function of only density for all data:

Loss tangent = 10[0.44*(r –2.943)] , (92)

where ρ = density (g/cm3).

Expressed as a function of only density for lunar soils:

Loss tangent = 10[0.42*(r –2.903)] , (93)

where ρ = density (g/cm3).

When the FeO + TiO2 composition is considered, as well as the density, the following two 

expressions are determined:

• For all data:

Loss tangent = 10[(0.038×M)+(0.312*(r –3.26))] , (94)

where

ρ = density (g/cm3) 

M= % FeO + TiO2.

• For only soil data:

Loss tangent = 10[(0.027×M)+(0.273*(r –3.058))] , (95)

where

ρ = density in g/cm3

M = % FeO+TiO2.
340

9.3.4.4 Lunar Surface Potential. Solar radiation and plasma are directly incident on the 

lunar surface, affecting the near-surface electrical environment. The situation is especially complex 

at the polar regions where surface photoelectric currents are reduced (due to large solar radiation/ 

surface incidence angles), and solar wind plasma currents are reduced in the trailing lunar plasma 

void or ‘wake’ region (sec. 8.2.2). As recent LP measurements indicate, the unlit lunar surface 

potential is also affected strongly by solar storms. The impact on any landed system will be 

described below.

9.3.4.4.1 Surface and Near-Surface Electrical Environments. Figure 160 shows the nearsurface electrical environment. It is characterized by three major regions: dayside, nightside, and 

terminator/polar regions.

newF160

Lunar Surface Charging

and Electric Fields

Sheath

Solar Wind Ions

Solar Wind Ions

Electron-Driven

Negative Charging

Photon-Driven

Positive Charging

Terminator

Region

Photoelectrons

Plasma Electrons

Horizontal and/or Vertical

Dust Transport

Wake Boundary

Secondary

Electrons

Figure 160. Lunar near-surface electrical environment.292
341

9.3.4.4.1.1 Dayside region. Solar photon radiation is directly incident on the lunar surface, releasing a population of low-energy electrons from the surface called photoelectrons. 263 The 

nominal emitted electron current, Jph, for direct solar incidence is approximately 4 μA/m2, assuming a photo-conversion of the lunar regolith of approximately 0.1. These photoelectric currents 

dominate over incoming solar wind electron and ion currents. Since the surface boundary requires 

current balances (i.e., the net flux of all currents equals zero), the surface will charge a few volts 

positive to draw the photoemitted electrons back toward the surface. Typically, the surface potential is shielded by the electrons and ions in the plasma on the spatial scale of a Debye sheath length, 

which for the lunar dayside is approximately 0.5 m.263

9.3.4.4.1.2 Nightside region. Since there are no photoelectrons, the nightside surface potential is defined by the current balance between inflowing plasma electrons and ions. For comparable 

temperatures (i.e., solar wind temperature of kT/e ~ 10 eV), the electrons will have a higher thermal 

velocity than the more massive ions, making the nightside electron flux greater than the ion flux. 

As a consequence, the nightside surface potential will be negative to slow the electrons and  

attractions.

However, the situation is compounded by the nightside surface’s exposure to the low-density 

lunar wake trailing the Moon. As a consequence, the surface can charge strongly negative to values 

< –100 V. Figure 161 shows a model of the surface potential and electric field at the terminator and 

in the plasma void/wake region.259 In this region, the ambient plasma density is getting progressively lower (to <1% of that in the solar wind) and the electron temperature is progressively increasing, reaching many factors times their solar wind ambient temperature. The nightside surface 

potential varies approximately as:

! ! ! kTe

e

ln(A,B,C) , (96)

where A is the ratio of electron density to ion density (ne 

:ni ), B is the square root of the ratio 

of electron to ion temperatures ((Te :Ti)1/2 and progressively increases from 1 to 4 as a function 

of subsolar angle), and C is the square root of the ratio of ion-to-electron mass ((mi :me)1/2 ~ 43 for 

protons). For a quasi-neutral plasma (ne ~ ni), the value of ln (A.B.C) is on the order of ~5. Thus, 

the surface potential is φ ~–5 kTe/e, and can become strongly negative deep in the nightside as 

illustrated in figure 161. Clearly, the increase in potential is a strong function of electron temperature, which is found to steadily increase with subsolar angle (or distance from the wake flank) due 

to velocity filtration of non-Maxwellian solar wind electrons. This Te increase is confirmed with 

LP observations.180 This progressive temperature increase is illustrated in figure 161(d). The Debye 

sheath in the nightside region varies from ~10 m just nightside of the terminator 

to >500 m at the antisolar point.180
342

newF161

–4

–3

–2

–1

0

1

Ez, Ex (V/m)

(e) 60

(d)

(c)

(b)

(a)

70 80 90 100

Ex

Wake

Rarefaction

Ez

110 120

Angle From Subsolar Point

Vertical

Distance

(m)

0

10

20

30

40

Electron Temperature (eV)

0.01

0.1

1

10

Electron Density (No./cc)

–250

–200

–150

–100

–50

0

0

100

50

Surface Potential (V)

Figure 161. A model of the near-surface E-field, associated surface potential, ambient 

electron plasma density and temperature above the sheath, and surface 

electric field in the terminator. 260
343

The model calculations shown in figure 161 assumes a quasi-neutral plasma with ne ~ ni. 

However, in the plasma expansion process into the void (described in sec. 8.2.3), the low mass 

electrons diffuse into the void region ahead of the ions and establish an ambipolar E-field to force 

the ions to ‘keep up.’264 However, simulation of the process197,258,260 suggests that there are regions 

near the leading edge of the expansion where the ions never fully catch up, creating an electron 

cloud along this leading edge where ne ! ni

.

205,266 This electron cloud region will also be incident 

with the lunar surface, creating a local region of very strong negative charging where the ion-devoid 

electron cloud intercepts the surface. For ne ~ 10 ni, the surface potential decreased by another 

factor of 2.3, and φ ~ –11 kTe /e, extending to values in the many hundreds of volts negative for 

a surface immersed in the electron cloud region.

9.3.4.4.1.3 Terminator/polar region. Due to the transition from dayside/positive to nightside/negative potentials, terminator regions (including the poles) possess very complicated electrical 

geometries. As indicated in figure 161, the surface potential makes a steep transition and the local 

E-field in the near-surface region can be 1–5 V/m. Local topography plays a large role in the formation of the surface potential, since both photoemission and solar wind (which is flowing quasiperpendicular to the surface normal) will be influenced by terrain. Mountains and crater walls will 

both obscure sunlight and block solar wind flow (a solar wind orographic effect) leading to large 

negative potentials on leeward-facing unlit surfaces.259

9.3.4.4.2 Validation of Model Results—Past Data Sets. Current balance models259,265 like 

that shown in figure 161 indicate evidence for strong nightside surface charging. These models have 

been validated, in part, by previous missions to the Moon. The SIDE flown as an Apollo lunar 

surface experiments package (ALSEP) on Apollo 12, 14, and 15 had the ability to measure surface 

potentials via the acceleration of local ions by surface fields. Benson et al.256 mapped the surface 

potentials across the terminator and found variations very similar to those modeled with potentials 

dropping below –100 V at terminator crossings. The orbiting LP ER can remotely sense nightside 

negative surface potential via surface-ejected electron beams and surface-reflected electrons. The 

electron beams are created by secondary processes at the surface, but then are repulsed from the 

negatively charged surface and move along connecting magnetic field lines out to the spacecraft. 

Large negative potentials are consistently detected in nightside regions (many hundreds of volts 

negative)203 and terminator regions had potentials that were moderately negative (~ –35 V).262

9.3.4.4.3 Temporal Effects: Solar Storms and Passages Into the Magnetosphere. The LP 

ER recently detected surface potentials on the lunar nightside that were below –4 kV in association with a passing solar energetic particle event, typical of that which occurs with the most intense 

solar storms.203 The detection is from electron beams that are created by secondary processes at 

the surface and ejected via the surface potential. When LP is magnetically connected to the Moon, 

these electron beams stream directly to the spacecraft along the low conductivity field line. The 

nightside surface potential just prior to the storm was ~ 700 V and the change in potential to 

>–4 kV storm values occurs on time scales of hours. The Moon was located in the solar wind 

during these times. It is believed that the large influx of energetic electrons charge the surface to 

anomalous large negative values. Hence, the same energetic relativistic particles that pose a radiation hazard to astronauts are also capable of altering the surface potential, posing a system 

charging risk.
344

The Moon spends approximately 73.5% of its orbit in the solar wind but passes through 

the magnetotail for approximately 13%–25% of its orbit. In this plasma, there is not a substantial 

flow, but the plasma is both of very low density and very warm (see table 98). This combination 

makes the anti-sunward surface charge to large negative values approaching –1,000.0 V.263 The 

LP confirms the presence of these large negative potentials with geomagnetic tail crossing, with 

the number of surface-ejected electron beams with energy >500 eV increasing during the crossings 

(indicating an electron ejecting surface potential of <–500 V). See figure 162.

May Jul Sep Nov

newF165

1998

0

20

40

60

80

100

120

(No. >500 eV Events)/Day

Figure 162. Number of surface-ejected electron beams with energies >500 eV 

as a function of time (white bars at the bottom indicate times 

of solar storm events (solar energetic particle events with solar storms); 

gray bars indicate passages through the geomagnetic tail).263

9.3.4.4.4 Effects on Landed Operations: Roving, Power Systems, and Dust. The surface 

potential and its fluctuations in solar storms can affect landed systems in three ways: roving and 

charge dissipation, lunar surface power system, and lunar dust.

9.3.4.4.4.1 Roving and charge dissipation. As astronauts rove, they will accumulate triboelectric charge (frictional or contact electricity) with the regolith. In sunlit regions, the photoelectric 

and ambient plasma currents can dissipate astronaut charge on time scales of <0.01 s. However, 

in unlit regions where solar wind flow is obstructed (by a large mountain or inside a crater such as 

Shackleton or Shoemaker), there are little natural environmental currents to remediate any 

tribocharge buildup. Within the cold craters, the conductivity of the regolith can become as low as 

10–14 S/m, rendering them insulators (unable to deliver the needed currents). Recent studies indicate that the dissipation time in unlit craters could be as large as 10,100 s. Hence, a rover (continuous charging) or astronaut (charging with each step with a cadence of a second) will charge faster 

than can be dissipated. As a consequence, roving systems in unlit regions can become ESD hazards.

9.3.4.4.4.2 Lunar surface power system. Any polar base will spend some fraction of time in 

darkness and away from the photoelectric sheath. Consequently, the region is susceptible to solar
345

storm-induced variations in surface potential. Given the poor conductivity of the lunar regolith 

(making it a poor electrical ‘ground’), a potential difference can develop between the surface and 

objects located on the unlit surface during storms. To date, there are no direct measurements of this 

effect and modeling is just beginning to address this issue. In essence, an object in the unlit region 

is sitting on an insulator. Consequently, it is recommended that there are clear ground paths for 

all landed system components to reduce the effect of differential charging, especially during solar 

storms when the ground and landed components will develop potential differences relative to each 

other.

Any system venturing into a polar crater should also be aware that the surface in the unlit 

region is strongly negative relative to any voltage referenced at the crater rim (in daylight). Thus, 

the use of a tether for power will have a ground reference to the sunlit region where the power 

source is located, but the surface surrounding the exploration system could be many hundreds of 

volts negative relative to the system ground (referenced to a topside location). Some consideration 

should be given to mitigating this ESD risk.

9.3.4.4.4.3 Lunar dust. At terminator/polar regions, where electric fields are expected to be 

large (see fig. 161), the dust environment becomes active. The Lunar Ejecta and Meteorite (LEAM) 

experiment, an Apollo 17 lunar surface experiments package, detected incidence of highly energetic 

lifted dust grains with speeds >100 m/s at both terminator crossings Berg et al.267 The dust was 

detected in all directions but was primarily moving in nightside directions. The activity peaked at 

the terminators but extended well into the nightside. Naturally lifted dust grains were detected by 

LEAM at a rate of one every couple of minutes. Since LEAM was relativity insensitive to lowenergy dust, it is anticipated that there is a progressively larger flux of natural dust at progressively 

lower speeds (a distribution of dust that increases with density at decreasing velocities) and that 

LEAM is detecting only the most energetic lofted grains at the ‘tail’ of the distribution. However, 

landed instrumentation is required to confirm this possibility. The naturally lifted lunar dust is an 

indicator of locations where near-surface electric fields may become large.

Figure 161 indicates that driving E-fields may indeed peak where LEAM dust is most active 

at the terminator. Such an electrical environment would also have an impact on any anthropogenically lofted dust since there is an induced potential on both the roving astronaut and the dust that 

may further increase their electrostatic attraction.

9.3.4.5 Photoelectric Emission of Lunar Fines. Photoelectron emission is affected by surface 

properties. Attempts were made to mitigate possible contamination of lunar samples from Apollo 

14 and 15 before measurements of photoemission measurements from the lunar fines were made. 

References 268 and 269 give information on laboratory results for the photoelectric yield versus 

wavelength for samples of lunar surface fines from Apollo 14 and 15. In situ behavior is likely to 

be different because of potentials on the particles in the fines not present in the laboratory environment and possibly a different packing on the lunar surface. There are no measurements of this 

property from the lunar surface. This yield function has to be folded with the Sun’s spectra in the 

500 to 2,500 Å range in order to obtain the total photoemission from the lunar surface.
346

9.3.4.6 Secondary Electron Emission—Bulk Material. Secondary electron emission from 

a semi-infinite slab of material as a result of electron impact can be represented approximately by 

the Sternglass formula:

! ! C! max

E

Emax

!

"

#

$

%

& exp '2 E

Emax

!

"

#

$

%

& , (97)

where 

δ = number of secondary electrons emitted per incident electron

C = constant (7.4)

δmax = maximum number of secondary electrons emitted per incident electron

E = energy of incident electron

Emax = energy at which the maximum yield occurs.

Preliminary secondary electron emissions from Apollo 14 and 15 can be found in references 293 

and 269.

9.3.4.7 Triboelectric Charging. As a result of motion both through and over lunar regolith, a moving body, such as a rover or an astronaut, will experience triboelectric charging. The 

basic mechanism involved in triboelectric charging is well understood. Triboelectric charging, 

also known as tribocharging or the triboelectric effect, is an example of contact charging, wherein 

surfaces become electrically charged after making contact with each other. Often associated with 

frictional contact, such as rubbing, the effect actually requires only that the materials come into 

contact and then separate. The strength of the resulting charge as well as the polarity acquired 

by each of the two materials depends on material properties. While the most important of these 

is known to be electron work function, such properties as surface roughness and temperature can 

compound the problem. For this reason, attempts to predict the actual response of engineering 

systems are often only partly successful and generally require testing under realistic conditions.

At its source, then, the magnitude of the effect depends on the difference in work function 

between the materials involved. While the effective work function of lunar dust is not known, and 

probably differs from that of available simulant, it is believed to be approximately equivalent to 

that of platinum. What is not known is how sensitive vehicle charging is to this parameter. It is 

conceivable that careful attention to work function in the selection of materials and coatings could 

considerably reduce the charging problem at its source. While most of the emphasis has been paid 

to moving rovers, and it is this case that the best information is currently available, it is increasingly 

recognized that contact and movement of crewmembers’ boots with regolith results in charge transfer that is implicated in significant dust adhesion to boots and the lower parts of space suits.

For the case of triboelectric charging of rovers, experiments at Glenn Research Center 

(GRC) in support of Mars Pathfinder showed that a rover wheel turning in simulant in a Martian atmospheric environment would lead to overall charging in excess of 100 V with transients 

observed that were 2 to 3 times that much.239,240 Mars has a tenuous atmosphere, therefore it was 
347

possible to partially mitigate the charging through the addition of a microdischarge point. GRCprovided discharge points were installed on the Pathfinder rover and have been installed on both 

Mars Exploration Rovers now operating on Mars. A more efficacious solution was developed for 

the Mars 2001 rover, Marie Curie. That package, the Mars Experiment on Electrostatic Charging 

(MEEC), used a small dot of radioactive Americium 241 to partially ionize the atmosphere in front 

of the rover. MEEC was built and delivered to JPL, but the mission was cancelled.241

Triboelectric charging is expected to be more severe on the lunar surface. On Mars, it is the 

presence of roughly 7 torr of CO2 that allows charge to be bled off. On the Moon, the presence of 

hard vacuum does not allow mitigation that might be achieved through discharge points, whether 

they are added or consist of naturally occurring sharp points on the vehicle body. The radioactive 

dot solution also works by partially ionizing atmosphere and will be ineffective on the Moon.

In addition to triboelectric charging caused by motion, operations on the lunar surface must 

deal with a naturally occurring background charge. The distribution of charge in the lunar environment is not well understood. Tribocharging has been studied to date assuming that the natural 

environment was initially neutral and that charge separation and transfer resulted in charging. This 

is the case for Mars, but on the Moon the environment may already be charged.

9.3.5 Contamination

Contaminants on the lunar surface are described in table 109.

Table 109. Contamination of the lunar surface.

Contaminant 

Type Description Prevalence Systems at Risk

Particulate Adhered regolith/

dust

Ubiquitous Mechanical, EVA, life support, power, 

thermal control surfaces, optics

Particulate Dust fallout Sporadic Power, thermal control surfaces, optics

Particulate Micrometeoroid Sparse/continuous Power, thermal control surfaces, optics

Molecular Impact melt vapor Sparse/continuous Power, thermal control surfaces, optics

Particulate Micrometeoroidinduced spallationVery sparse Power, thermal control surfaces, optics

Molecular Condensed gas Shadowed regions Optics

Molecular Solar nuclei Dayside/sporadic Optics

Molecular GCR nuclei Sparse/continuous Optics

9.3.5.1 Lunar Regolith as a Contaminant. By far, the greatest natural contamination source 

on the lunar surface is the regolith. From a contamination perspective, the whole Moon is a sea of 

potential foreign object debris, which is of concern to any mechanical systems. Additionally, even 

moderate size fines (>5 μ) can tend to adhere tenaciously to surfaces due to charging. Bulk charging is predicted to be positive on the dayside and negative on the nightside, so adhesion properties 

will change from day to night. Similarly, any triboelectric charging of the regolith may differ from 

day to night.
348

At the leading and trailing terminators, rapid changes in charging conditions may allow 

dust dynamics to occur, lofting submicron particulate kilometers above the surface. Such lofted 

particulate would then settle in a fairly broad fallout zone. The LEAM experiment of Apollo 17 

accidentally detected particle events at lunar dawn and dusk which were determined to be charged 

dust. Measurements of actual diurnal fallout rates would be necessary to predict the level and size 

distribution of this natural particulate contamination. Preliminary estimates of the column density 

of dust to produce Apollo-observed horizon glow would allow for as high as 1% obscuration of 

surfaces from fallout of 0.1–0.5 μ particulate. Fallout onto some surfaces could allow accumulation 

of particulate on every cycle (where the particulate tends to stick, or is shaded, etc.), such that in 

1 year there could be over 10% total obscuration. But even after 10 years a surface completely covered in submicron dust particles might appear clean to the unaided eye, though it would be absorbing over wavelengths determined by the dust mineral species.

9.3.5.2 Micrometeoroids. There is a somewhat higher flux of micrometeoroids on the lunar 

surface than the interplanetary flux of surrounding space. Over time, this may allow a significant 

buildup of micrometeoroid contamination of surfaces exposed on the Moon. The constituent 

elements of this particulate (as described in the micrometeoroid section) will become embedded 

in surface layers but will cover the surface at a cumulative level of <1% over 10 years of exposure 

(estimated from LunaRef meteoroid fluence of 5,000/yr m2 for masses in the 10–12 g range). However, localized contamination can be quite high near a rare, larger impact.

The high velocity of micrometeoroids imparts energy upon impact that produces, at minimum, a localized melt vaporization zone. Vaporized ejecta may then condense on cold surfaces 

in the immediate vicinity. For micrometeoroids of greater mass, affected area of impact increases 

accordingly, eventually reaching the point where the energy of impact can cause spallation of 

debris from the surface. In cases where the surface is loose regolith with fines, the ejecta will include 

such dust particles, which will create a fallout field in the vicinity (with submicron diameter dust 

having the widest fallout zone).

9.3.5.3 Condensed Gas. In the cold, shadowed regions and on the nightside, some gas 

species will tend to condense on surfaces. The densities of these gases on the lunar surface are 

extremely low, so it would take years to accumulate an appreciable molecular layer, and then only 

if permanently shadowed. However, the lunar poles are likely repositories of these condensed and 

frozen gases.

9.3.5.4 Implanted Nuclei. The solar wind, made up mostly of protons (and electrons) will, 

over time, implant a subsurface layer of hydrogen atoms. The flux of protons has a fairly continuous lower energy component but can increase dramatically with SPEs. There is a similar but more 

tenuous flux from GCR that would penetrate deeper and be more dispersed than the higher density, lower energy solar protons. Such a layer of implanted hydrogen could eventually affect optical 

systems, though it would more likely be a detriment to surface finishes, perhaps causing flaking of 

metals through hydrogen embrittlement. Though hydrogen would be the predominant implanted 

species, iron nuclei could actually cause more of a problem as an eventual contaminant.
349

9.3.6 Transient Lunar Phenomena

Transient activity on the Moon has been reported by visual observers for many years 

although documented events are very rare and are often disputed. Hazes and red glows are the 

most frequent. Flashes from meteoroid impacts on the Moon have been recorded,243 but the other 

transient lunar phenomena (TLPs) are so poorly documented and uncertain that no design or 

operational planning regarding these questionable phenomena is appropriate.

9.3.7 Lunar Interior and Moonquakes

9.3.7.1 Introduction. The seismic activity of the Moon was determined using the Passive 

Seismic Experiment (PSE) that was part of the ALSEP package on missions 11, 12, 14, 15, and 16. 

The seismometer at the Apollo 11 site failed a few weeks after deployment. The remaining stations 

continued in operation until October 1977. The stations form an approximate equilateral triangle 

approximately 1,100 km on a side; the Apollo 12 and 14 sites forming the southwest corner, 

Apollo 15 the northern corner, and Apollo 16 the southeast corner (fig. 163). In addition to the 

passive experiments, active seismic experiments were conducted on the Apollo 17 Lunar Seismic 

Profiling Experiment and the impact of the S-IVB stages and upper stages of the lunar module 

(LM) were also used as active sources.

Figure 163. Location of ALSEP seismometers. Lunar image with landers courtesy of NASA

GSFC Visualization Studio.
350

9.3.7.2 Seismometer. Each station consisted of four seismometers: three low-frequency 

components that formed a triaxial set (one vertical, two horizontal) with a peak sensitivity of 

0.45 Hz and a fourth seismometer sensitive to vertical motion with a peak sensitivity of 8 Hz (highfrequency component). The instruments are capable of detecting ground motion of 0.05 nm. The 

deployed instrument is illustrated in figure 164. 

newF164

Figure 164. Passive seismic experiment at the Apollo 16 site (courtesy of NASA).

9.3.7.3 Seismic Events. During the nearly 8 years of operation, the PSE recorded in excess 

of 12,500 events (table 110). These events can be categorized as deep moonquakes, shallow moonquakes, and impact events. Table 110 lists the events. The seismic energy released in the Moon is 

approximately 2×1010 J/yr compared with 1017–1018 J/yr on the Earth. The short interval of time 

during which data were collected suggests that the lunar energy release might be as high as 1014 J/yr 

but still many orders of magnitude lower than the Earth.
351

Table 110. Seismic events on the Moon detected by the Apollo 16 PSE.

Number of Seismic

Events Detected Total* Major Events**

Artificial impacts 9 5 

Meteoroid impacts 1,743*** 95 

Shallow moonquakes 28*** 7 

Deep moonquakes 3,145*** 9 

Unclassified events 7,300*** – 

 * These data do not include signals detected only by short-period

 seismometer or large number of events recorded only at the 

 Apollo 16 site.

 ** Signal amplitude >10 nm at two or more stations.

*** Numbers as of 1982. 294 Subsequent analysis has indicated 

 that additional events may be retrievable from the data.

9.3.7.3.1 Deep Moonquakes. These are the most abundant type of events. They have small 

magnitude (typically ~1) and occur approximately half-way between the surface and the center 

of the Moon at depths of 800 to 1,000 km (range 700 to 1,200 km). Their occurrence is strongly 

linked to tides raised by the Earth and the Sun and are suggested to be caused by dissipation of 

tidal energy. Almost all events have been located on the front side; the highlands terrain on the far 

side either lacks such events or the signals are attenuated and not detectable. These quakes occur in 

localized groups (‘nests’) within a volume of approximately 1×1×1 km. There are approximately 2 

to 300 quakes in each nest.

9.3.7.3.2 Shallow Moonquakes. These events have the highest energies but are rare 

(approximately four events per year). A total of 28 events were recorded in 8 years, 2 of which had 

magnitudes close to 5. Focal depths were not determined but are estimated to be 50 to 200 km. The 

occurrence of these events is unrelated to tides, hence is considered tectonic in origin and shows 

similar characteristics to terrestrial intraplate earthquakes. The events produce P and S arrivals 

suggesting that the events occur at some depth below the surface. The temporal frequency of shallow quakes can be found in reference 294; The location of the shallow moonquakes can be found in 

reference 295.

9.3.7.3.3 Thermal Moonquakes. Thousands of very small seismic events that are detectable 

only to distances of a few kilometers are caused by temperature variations near or at the surface. 

They may be spatially associated with young craters and large rocks.

9.3.7.3.4 Natural Impact Events. Meteoroids in the size range of 0.1 to 1,000 kg have been 

detected in the long-period data; more numerous events have been detected by the short-period 

seismometers. These are not all random events but tend to be clustered in time (20% of the events 

occurred within 3% of the time). Large events were detected between April and July, while smaller 

events were associated with meteor showers. The largest events indicate meteoroids of ~5 tons; 

seven events were recorded that were formed by bodies >1 ton, impacting the Moon.

9.3.7.3.5 Artificial Impact Events. For several Apollo missions, the S-IVB booster and the 

upper stage of the LM were targeted at the Moon to produce artificial impact events with sufficient 

energy to probe the lunar interior.
352

9.3.7.3.6 Unclassified Events. Unclassified events are those whose origin could not be determined. Table 111 lists the frequency and energy of moonquakes and earthquakes.

Table 111. Frequency and energy of moonquakes and earthquakes.

Parameter Moon Earth 

Event frequency 5 per year (shallow, m > 2.2) 

500 per year (deep, m> 1.6)

104 

(m> 4) 

Energy released by largest event 2 × 1012 J (shallow) 

1 × 1018 J (deep)

1021 J 

Magnitude of largest event 4.8 (shallow) 

3 (deep)

9.5 

Seismic energy release 2 × 1012 J/yr (shallow) 

8 × 108 J/yr (deep) 

1025 J/yr 

9.3.7.4 Seismic Risk. The seismic events that present risk to a lunar base are those associated with relatively strong shallow moonquakes and impacts. The primary issue with impacts is the 

impact event itself rather than the resulting seismic energy.

Shallow quakes occur at depths of less than 100 km and average four per year. The likelihood that a randomly chosen site would experience a quake with magnitude Mb>4.5 within 100 km

of that point is estimated at 1 in 400 year.

9.3.8 Lunar Volcanism

Lunar volcanism is the result of partial melting of the lunar interior (mantle) and eruption of these melts onto the lunar surface. Volcanic products include lava flows (called ‘maria’ 

when they fill large impact basins) and pyroclastic deposits that created volcanic glass beads such 

as the Apollo 17 orange glass. Lava flows cover about 17% of the lunar surface, primarily filling 

topographic lows on the nearside.296 Lunar basalt classification is based on three distinguishing 

geochemical indices (Ti, Al, and K);297 however, remote sensing data sets have shown that the full 

range of mare basalt composition and ages has not yet been sampled.298 Knowledge of the duration of lunar volcanism comes from (1) radiometric dating of meteorites and samples from the 

Apollo and Luna missions, and (2) crater counting statistics of mare surfaces from remote sensing 

data. Lunar volcanism reached its maximum volumetric output between 3.8 and 3.2 giga years ago 

(Ga),299 but began as early as 4.3 Ga300–303 and may have persisted until as recently as 1.2 Ga.304

There is currently no evidence of volcanic activity on the Moon.

9.4 Lunar Poles

The lunar poles have a unique environment because of the small value of the lunar obliquity 

(1.5°). Thus, the Sun never rises much above (or sets much below) the horizon. The relatively constant illumination geometry creates locations of near-permanent (possibly permanent) sunlight and 

areas of permanent shadow (crater interiors). Areas of permanent shadow have very low temperatures (perhaps approximately 50 K).
353

The heavily cratered morphology of both poles suggests they are ‘anorthositic’ highlands 

terrain. The South Pole lies just beyond the rim of the South Pole Aitken Basin. Figures 165 

and 166 show the north and south polar regions in visible and in radar illumination.

9.4.1 Solar Illumination Conditions 

Topographically high areas in the polar region have the potential to be permanently or 

nearly permanently illuminated if they are high enough to remain illuminated by the Sun when it 

is 1.5 below the nominal horizon. Similarly, depressions such as craters can remain in permanent 

shadow if they are deep enough.

Analysis of images acquired over the North and South Poles (primarily Clementine, with 

additional data from Lunar Orbiter, Galileo, and Small Missions for Advanced Research in Technology-1, have allowed an assessment of the lighting conditions.305

Figure 165. North Pole mosaic from Lunar Reconnaissance Orbiter 

(courtesy of NASA/GSFC/Arizona State University).
354

Figure 166. South Pole mosaic from Clementine (courtesy of NASA).

9.4.2 Volatile Traps 

Areas within permanent shadow are thought to have such low temperatures that they 

become cold traps for normally volatile elements and compounds. Any atomic or molecular species 

that enters the cold trap will be stored there permanently unless energized with sufficient kinetic 

energy to escape ballistically (e.g., meteor impact).

Because of the presence of water at the poles, there may be some significant water or hydration in the minerals there. Hydrated minerals are very rare or nonexistent at other lunar sites.

Because of the differences in the temperature, chemistry, lighting, electrical, and other 

conditions at the poles, results of robotic precursors to manned missions will help preclude unfortunate surprises when manned missions land. A robotic lander would help immensely in finding 

out how much the polar surface differs from that in the equatorial regions and would greatly help 

reduce the risks involved with landing in unknown conditions.

The Lunar Crater Observation and Sensing Satellite impacted a Centaur upper stage followed by a sensing satellite into the permanently shadowed crater Cabeus in October 2009. There 

were significant water and other volatile signatures detected by the sensing satellite.249 In addition, 

a significant amount of mercury (Hg) was also detected in the plume by the Lyman-alpha mapping 

project instrument on the LRO.306,307

9.5 Lunar Dust

What is popularly called the lunar dust is actually the very fine material in the lunar 

regolith. For its properties, see sections 9.3.1 through 9.3.4.
355

9.6 Thermal Environment

Since there is no moderating atmosphere, surface temperatures are extreme. At the 

Apollo 17 site the temperature ranged from 384 K during the day to 102 K just before sunrise. 

A detailed study of infrared radiation at 11 μ from the eclipsed lunar surface revealed over 

a thousand small anomalously warm regions.

There is an approximate anticorrelation between infrared eclipse temperature and albedo. 

Local high temperature anomalies have been interpreted as due to the presence of an excess of bare 

rocks on the surface.

The spectral distribution of sunlight is approximately blackbody at 5,760 K. Energy flux 

at the Earth-Moon system is 1.97 cal/(cm2min) or 1,371±5 W/m2. Other than direct sunlight, the 

most important sources of radiant flux at the Moon are earthshine (sunlight reflected from the 

Earth) and thermal radiation from the Earth’s surface and atmosphere.

9.6.1 Surface Temperature

The surface temperature on the lunar surface varies as a function of latitude due to the fact 

that toward the poles, the Sun illuminates the surface at a lower angle. Figure 167 illustrates generalized temperature as a function of time and latitude.

newF177

120

140

160

180

200

220

240

260

280

300

320

340

360

380

Temperature (K)

0 4 8 12

88°

75°

45°

Equator

16 20 28

Time (Earth days)

Figure 167. Generalized temperature as a function of time and latitude.
356

The radiative heat input into the surface Qin (W/m2):

Qin=(1 –A(m))Scosq , (98)

where 

A(μ) = directional hemispherical albedo (reflectivity)

S = solar constant

q = angle relative to surface normal. 

The solar constant (S) varies from 1,315 to 1,412 W/m2 (minimum in July; maximum 

in January). This is orders of magnitude greater than the heat flow from the interior of 2 to 

4×10–6 W/cm2.

The surface temperature is a function of the thermal loading and the thermal properties 

of the regolith. The thermal loading varies over a period of 709 hr (a lunar day). For a given set 

of thermal properties, the surface temperature curve depends only on the thermal inertia (I) in units 

of J/(m2 s1/2 K):

I=√(KrCp) , (99)

where

K = thermal conductivity (J/(smK))

ρ = density (kg/m3)

Cp = specific heat (J/(kg K)).

Thermal conductivity data for the lunar regolith were determined in several ways: modeling 

the decay of the thermal disturbance caused by the insertion of the heat flow probes, heat dissipation from active heating of regolith, and analysis of returned samples. Data from the heat flow 

probes may not accurately reflect values for the bulk regolith due to disturbance of the physical 

properties during insertion of the heat flow probes. Values determined in the laboratory may also 

not reflect the bulk regolith because of the limited sample size and lower densities. Conductivity 

also has a temperature dependence. Thermal diffusivity is estimated from models of the attenuation 

of annual temperature amplitude as a function of depth and transient thermal events associated 

with probe insertion.

Thermal conductivity and diffusivity both vary as a function of temperature. The relation 

among these different parameters for Apollo 12, sample 12001 and two different densities can be 

found in reference 308. Table 112 shows the thermal properties of the lunar regolith.

The resulting surface temperatures vary very little within the range of these parameters 

except for small differences at night. Figure 168 shows the calculated lunar surface temperature 

for rock and regolith over a lunar day. Table 113 lists the temperature variations across the lunar 

surface. The lunar thermal inertia is so low that the surface daytime temperature is in thermal equilibrium with absorbed solar radiation flux. At the equator, the subsolar point temperature would 

be approximately 387 K, but a radiometric measurement would indicate a temperature of 409 K 

because the surface does not radiate energy uniformly.
357

Table 112. Thermal properties of the lunar regolith.

Parameter Units Value Notes 

Conductivity <6 cm W/cmK 0.6 × 10–4 Very low at the surface 

Conductivity >50 cm W/cmK 0.9–1.3 × 10–4 Increases with depth 

Diffusivity Cm/s 0.74–1 × 10–4 Attenuation of annual temperature amplitude 

Thermal gradient K/m 0.79–2.52 Significant variation between Apollo 15 

and 17, and between probes at each site 

Specific heat W s/gK 0.67 

newF168

50

100

150

200

250

300

350

400

Surface Temperature (K)

0 60 120 180 240 300 360

Rock Surface

Regolith

Zenith Angle of the Sun (deg)

Figure 168. Surface temperature as a function of time; when the thermal properties 

of the regolith are considered the lowest, nighttime temperature can vary.

(Circles represent lunar sample values; the lower solid line represents values 

for a mature regolith; and the upper solid line represents values for 

a theoretical rock surface. The blue area illustrates the difference 

in temperature between immature and mature regolith; the yellow area 

illustrates the difference between a regolith-covered and rock-covered surface.)

Table 113. Lunar surface temperatures.

Temperature

Shadowed

Polar Crater 

Other

Polar Areas 

Nearside

Equatorial 

Farside

Equatorial Mid-Latitude 

Average

temperature 

40 K 220 K 254 K 256 K 220 < T < 255 K 

Monthly range None ±10 K ±140 K ±140 K ±110 K 

Clementine long-wavelength infrared data (8–9.5 μ) shows that a Lambertian temperature 

model provides a good approximation of the temperature observed from orbit below a spacecraft. 

The relation is a function of cos1/4(i) where i is the solar incidence angle.
358

9.6.2 Subsurface Temperature

The Apollo 15 and 17 heat flow experiments provide information on the temperature and 

thermal conductivity of the regolith with depth over different time scales. Long-term monitoring 

of the subsurface temperature shows that it consists of three components; diurnal, annual, 

and aperiodic.

The diurnal variation (period 29.53 days) is observed close to the surface and affects the 

upper 30 cm. The annual variation extends to a depth of 200 cm at the Apollo 17 site. An additional aperiodic effect caused by the disturbance of the surface by the crew resulted in a slow, 

long-term increase in the subsurface temperature. Figure 169 illustrates temperature variation as a 

function of depth at the Apollo 15 and 17 landing sites.

newF169

200

100

150

250

50

0

Depth (cm)

248 249 250 251 253 256 257

Apollo 15

Probe 2 Apollo 15

Probe 1

Apollo 17

Probe 2

Gradients in K/m

3.0

2.0

0.4

0.7

1.0

1.3

1.7

Apollo 17

Probe 1

252 254 255

Temperature (K)

Figure 169. Temperature variation as a function of depth at the Apollo 15 and 17 landing 

sites (hatched areas show day-to-night temperature variations). Below 

approximately 50 cm, there is essentially no temperature variation 

due to the day-night cycle (adapted from fig. 3.9)291 (Copyright © 1991,

Cambridge University Press, reprinted with permission of Cambridge 

University Press).

9.7 Ionizing Radiation Environment

With one exception to be discussed next, the lunar surface ionizing radiation environments 

are the same as those discussed in section 9.3 for lunar space. Since the Moon does not have any 

magnetic field that could provide any magnetic screening, the ionizing radiation environment will 

be composed of the unscreened GCRs and SPEs. The details of these environments and the effects 

on electronics are presented in sections 2.12, 3.11, and 5.5.
359

9.7.1 Lunar Neutron Environment

GCRs generate neutrons through inelastic collisions with atoms in the lunar soil. These 

neutrons lose energy through collision with other atoms, a process that is called thermalization. 

The lunar neutron environment has a continuous energy spectrum from the thermal range, approximately 2.5×10–8 MeV through the high-energy range at approximately 1,000 MeV. The peak flux 

occurs at the most probable spallation energy of approximately 2 MeV.

Albedo neutrons produced by GCR interactions with the lunar surface were measured by 

the neutron spectrometer (NS) and gamma ray spectrometer (GRS) instruments on the LP mission while the spacecraft was in orbit around the Moon in 1998 and 1999. The spacecraft was 

inserted into a 100 km altitude circular, polar lunar orbit on January 11, 1998, and the first set of 

neutron observations was obtained at an altitude of 100 km from January 16, 1998 to October 5, 

1998. A second set of lower altitude measurements between 10 km and 50 km from the lunar surface 

was obtained between December 20, 1998 and July 28, 1999.308 The mission was terminated on July 

31, 1999, with disposal of the spacecraft through a commanded crash of the vehicle into the lunar 

surface in the Moon’s southern polar region.

During the mission, thermal neutrons (>0.4 eV) and epithermal neutrons (0.4 to 100 eV) 

were measured by the NS. Two other channels, the wide band (0 to 500 eV) and fast neutrons (0.6 

to 8 MeV) were measured by the GRS. Maurice et al.308 and Feldman et al.309 describe the instrument and data sets.

The lunar surface neutron spectrum was not measured directly, but those indirect measurements of the escaping flux at 30 km, along with knowledge of the GCR environment, were used 

to model the lunar surface neutron environment. The Monte Carlo N-Particle eXtended code was 

used to predict the spallation response of lunar soil to the GCR environment and the results were 

matched to the measured neutron spectra at 30 km.310 Figure 170 presents the flux as a function of 

landing site soil composition.

106

104

102

10

10–2

10–4

10–10 10–8 10–6 10–4 10–2 10 102 104

Energy (MeV)

Flux (n cm–2 s–1 MeV–1)

A11

A12

A14

A15

A16

A17

FAN

L16

L20

L24

newF170 Figure 170. Lunar neutron spectrum in terms of neutron differential flux (n/cm2/s/MeV)

as a function of energy) MeV and lunar soil composition.310
360

9.8 Meteoroid Environment

For a general description of the meteoroid environment including showers, flux, directionality, speed, and densities, see section 4.6. The general description of the environment does not 

change near the Moon, just the effects of shielding and focusing due to the Moon’s mass.

9.8.1 Flux

The Moon’s relatively weak gravitational pull does not significantly focus or enhance the 

meteoroid flux in its vicinity; indeed, simple calculations assuming an isotropic environment show 

that the flux enhancement is of the order of 2% or less, much smaller than other uncertainties in 

the environment. Shielding is still significant, though. Still, for accuracy in modeling the shielding 

and focusing effects, a lunar submodel of MEM is available. The lunar submodel of MEM models 

the primary meteoroid flux, speed, and directionality for spacecraft in orbit around the Moon out 

to the Moon’s sphere of influence (which extends out to roughly 60,000 km from the lunar center). 

These output files can then be incorporated into BUMPER (meteorid and orbit debris risk assessment tool) for risk analysis. Figure 171 shows the average flux on the surfaces of an oriented cubelike spacecraft. Figure 171 was generated using 1,500 randomly drawn state vectors over a 2-week 

period corresponding to a lunar equatorial 100 km altitude orbit.

newF171

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Surface Flux (No./m2/yr)

Surface of Spacecraft

Ram

Wake

Port

Starboard

Zenith

Nadir

Moon

Sun

Anti-Sun

Figure 171. Effects of directionality for an oriented cube-like spacecraft in low lunar orbit, 

with focusing and shielding.
361

Figure 172 represents the flux of meteoroids as a function of diameter. To convert diameter 

to mass in grams, use the following relationship (also found in sec. 5.6):

D = 6!Mp

! ! !

!

"

#

$

%

&

1/3

, (100)

where ρ is the density of the meteoroid and for most purposes can be assumed to be 1 g/cm3.

1× 10–11

1× 10–10

1× 10–9

1× 10–8

1× 10–7

1× 10–6

1× 10–5

1× 10–4

1× 10–3

0.1

0.01

1

10

Flux (No./m2/yr)

0.01 0.1 1 10

Diameter (cm)

newF172

Figure 172. Meteoroid surface area flux near the Moon as a function of particle size 

(no correction has been made for lunar shielding, and the particle sizes 

were computed assuming a density of 1 g/cm3).

Table 114 shows the interplanetary meteoroid flux at Earth’s distance (1 AU) as a function 

of particle size.
362

Table 114. Interplanetary meteoroid flux at Earth’s distance (1 AU) as a function 

of particle size.

Diameter

(cm)

Mass

(g)

Flux

(No. m–2 yr–1)

0.01 5.24 × 10–7 2.639

0.03 1.41 × 10–5 0.1

0.05 6.55 × 10–5 1.7 × 10–2

0.07 1.8 × 10–4 5.09 × 10–3

0.1 5.24 × 10–4 1.3 × 10–3

0.3 0.014 1.96 × 10–5

0.5 0.065 2.61 × 10–6

0.7 0.18 6.87 × 10–7

1 0.524 1.66 × 10–7

3 14.137 2.05 × 10–9

5 65.45 2.64 × 10–10

7 179.594 6.84 × 10–11

Note: Diameters are computed assuming a meteroid 

 density of 1 g/cm3.

Figure 172 ignores the directional nature of the meteoroid environment, which could have 

significant effects. This is currently being investigated. See figure 173.

Figure 173 shows the directional nature of the meteoroid environment relative to a stationary observer at the lunar pole and lunar equator. The sporadic meteoroid environment is tied 

to the solar direction, therefore the intensity of incoming meteoroid fluxes varies by latitude, 

longitude, and lunar phase.

F184

Degrees Elevation

40°

60°

80°

20°

–20°

y 0°

–40°

–60°

–80°

–9

–8

–7

–6

–5

–4

–3

–2

Degrees Azimuth From Ram (V)

Flux by Direction (in Local Spacecraft Coordinates)

Degrees Elevation

40°

60°

80°

20°

–20°

y

–40°

–60°

–80°

–9

–8

–7

–6

–5

–4

–3

–2

Degrees Azimuth From Ram (V)

(a) (b)

Flux by Direction (in Local Spacecraft Coordinates)

Figure 173. Example of the directionality of the meteoroid environment for stationary

observer at (a) lunar Equator and (b) lunar North Pole.
363

9.8.2 Lunar Shielding

For a discussion involving shielding and focusing from a primary body, see section 5.6. An 

example of lunar shielding and focusing at the surface is shown in figure 173, specifically the zero 

flux from negative elevations.

9.8.3 Density

For a discussion on meteoroid densities, see section 5.6.

9.8.4 Speed

The velocity of meteoroids in near-Earth space is still the subject of considerable debate; 

the Grün model used to derive the fluxes given previously assumes a single speed, 20 km/s, for all 

meteoroids. An actual speed distribution, derived from photographic observations of large meteors, is given in NASA TM-4527,311 Natural Orbital Environment Guidelines for Use in Aerospace 

Vehicle Development, and NASA Special Publication (SP) 8013, Meteoroid Environment Model1969312 (Near-Earth to Lunar Surface), but many researchers now believe that it underestimates 

the number of fast meteoroids. The speed distribution given by the new LunarMEM is used, which 

seems to best match the available observational data. This distribution for particles greater than 

and including 10–6 g is shown in figure 174.

newF174

Frequency

0 10 20 30 40 50 60 70 80

Speed (km/s)

0

0.05

0.1

0.15

0.2

0.25

Figure 174. Average impacting meteoroid speed distribution in lunar space, 

no gravitational focusing no lunar shielding.

The velocity distribution of meteoroids impacting the Moon is roughly the same as that of 

meteoroids just outside the Earth’s gravitational influence. Figure 175 compares the normalized 

velocity distribution for the lunar surface to that of meteoroids in lunar space or roughly interplanetary space at Earth’s distance from the Sun. The only noticeable difference is at the slower speeds, 

where the Moon’s weak gravity can accelerate the meteoroids by an additional kilometers per second. The minimum meteoroid impact speed with the lunar surface is set by conservation of energy 

to be the same as the lunar escape speed –2.38 km/s.
364

newF175

Frequency

0 20 40 60 80

Speed (km/s)

0

0.05

0.1

0.15

0.2

0.25

Lunar Surface

Lunar Space, 1 AU

Figure 175. Comparison of meteoroid speed distributions at the lunar surface and in lunar 

space at 1 AU.

9.8.5 Meteor Showers

The meteor showers experienced by lunar orbiting spacecraft or surface sites are essentially 

the same as for Earth-orbiting spacecraft; however, the times of maxima will be offset by as much 

as several hours and the meteor activity could be orders of magnitude different, depending on the 

Moon’s proximity to the meteor stream. The Moon’s average distance is over 3 times greater than 

the width of a typical meteor stream, so it is quite possible for lunar space to experience much 

more activity than recorded by Earth-bound observers.

9.8.6 Lunar Secondaries

Meteoroids striking the surface produce craters, thereby generating ejecta, which comprise 

an additional component of the lunar meteoroid environment. The following is taken from section 3.2 of NASA SP-8013,312 the model used in the Apollo program, which may overestimate the 

flux of secondaries. Work is currently proposed to refine this environment definition.

Ejecta bulk density is assumed to be 2.5 g/cm3 and the average velocity is set to 0.1 km/s 

(100 m/s). With these assumptions, the average annual cumulative flux of ejecta or secondaries 

having masses greater than m is given by:
365

Fejecta = 10!10.75!1.2 logm , (101)

where Fejecta is No./m2s and m is in g.

A more detailed analysis of the risk posed by the ejecta requires consideration of the cumulative flux as a function of speed. NASA SP–8013313 gives a flux expression for each of three velocity ranges as shown in table 115.

Table 115. Cumulative ejecta flux versus velocity.

Velocity Range

(km/s) Cumulative Ejecta Flux

Adopted Ejecta Speed 

(km/s)

0.1 ≤ Vejecta <0.1 Fejecta=10–10.75–1.2 log m 0.05

0.1 ≤ Vejecta <0.25 Fejecta=10–11.88–1.2 log m 0.175

0.25 ≤ Vejecta<1 Fejecta=10–13.41–1.2 log m 0.625

Measured: Fejecta (No./m2s) and m (g).
366

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