# Presentation: 559748

## LSSO: Precision Lunar Laser Ranging 

*LSSO: **Precision Lunar Laser Ranging *

- Stephen Merkowitz (663), Philip Dabney (694), Jeffrey Livas (663), Jan McGarry (694), Gregory Neumann (698), Thomas Zagwodzki (694)
- October 18, 2007

## Lunar Laser Ranging Background

*Lunar Laser Ranging Background*

- Apollo arrays used fused silica “circular opening” cubes, 3.8 cm diameter each
- Apollo 11 and 14 arrays used 100 cubes
- Apollo 15 used 300 cubes

- First suggested by R. H. Dicke in early 1950s.
- MIT and soviet Union bounced laser light off lunar surface in 1960s.
- Retroreflectors proposed for Surveyor missions but not flown.
- Retroreflectors flown on 3 Apollo missions.

## Slide 3

## Slide 4

## Slide 5

## Soviet Luna Missions

- Lunokhod

*Soviet Luna Missions*

- Lunokhod arrays consist of 14 triangular shaped cubes, each side 11cm
- Only Lunokhod 2 is still visible

## 38 Years of LLR and Still Going

- Lick Observatory in California got first light in 1969.
- McDonald Observatory in Texas 1969 to present!
- Other early ranges:
- Crimean astrophysical observatory in the Soviet Union,
- Orroral Observatory in Australia,
- Air Force Cambridge Research Laboratories Lunar Ranging Observatory in Arizona,
- The Pic du Midi Observatory in France (Calame et al., 1970),
- Tokyo Astronomical Observatory
- Orroral Observatory in Australia 1978 to 1980.
- Observatoire de la Côte d’Azur (OCA) in France 1984 to present.
- Haleakala Observatory on Maui in Hawaii 1984 to 1990.
- Apache Point Observatory in New Mexico is starting operation.

## History of LLR

- J. G. Williams* et al.*, gr-qc/0507083

## Satellite Laser Ranging (SLR)

*Satellite Laser Ranging (SLR)*

- http://ilrs.gsfc.nasa.gov/

- LAGEOS

- ~35 stations in operation tracking over 100 satellites

- Satellite Laser Ranging began in 1964 at
- NASA Goddard Space Flight Center.

## Ranging System

## Modeling

- Modeling orbit dynamics (r)
- Gravitational interaction between Sun, Moon, Earth, Planets.  Includes masses and relativity parameters.
- Asteroid attractions
- Newtonian attraction between bodies and gravitational harmonics
- Tidal effects
- Lunar rotation dynamics
- Torques from other bodies
- Dissipative torque from fluid core
- Effects at Earth station
- Plate motion
- Tidal effects
- Orientation of Earth’s rotation axis

- Effects at lunar reflector
- Tidal effects
- Relative lunar orientation
- Time delays
- Atmospheric
- Relativistic time delay
- Other effects
- Solar radiation pressure
- Thermal expansion of reflectors

- J. G. Williams* et al.*, gr-qc/0507083

## Science of LLR

- Lunar ephemerides are a product of the LLR analysis used by current and future spacecraft missions.
- Lunar ranging has greatly improved knowledge of the Moon's orbit, enough to permit accurate analyses of solar eclipses as far back as 1400 B.C.
- Gravitational physics:
- Tests of the Equivalence principle
- Accurate determination of the PPN parameter ,
- Limits on the time variation of the gravitational constant G,
- Relativistic precession of lunar orbit (geodetic precession).
- Lunar Science:
- Lunar tides
- Interior structure

## Testing the Equivalence Principle 

*Testing the Equivalence Principle *

- A violation of the Equivalence Principle would cause the Earth and Moon to fall at different rates toward the Sun resulting in a polarization of the lunar orbit.
- This polarization shows up in LLR as a displacement along the Earth-Sun line with a 29.53 day synodic period.
- Torsion pendulum measurements at UW on Earth & Moon like test bodies separates out composition dependence.
- The current limit on the Strong Equivalence Principle:
- (MG/MI)EP = (-1.0±1.4)x10-13= (4.4±4.5)x10-4

- J. G. Williams *et al.*, Phys. Rev. Lett. **93**, 261101 (2004).
- S. Baeßler* et al.*, Phys. Rev. Lett. **83**, 3585 (1999).

## Time Variation of G

- The strength of gravity is given by Newton’s gravitational constant G.
- Some scalar-tensor theories of gravity predict some level of time variation in G.  This will lead to an evolving scale of the solar system and a change in the mass of compact bodies due to a variable gravitational binding energy.
- The current limit on the time variation of G is given by LLR:

- J. G. Williams *et al.*, Phys. Rev. Lett. **93**, 261101 (2004).

## Geodetic (de Sitter) precession

*Geodetic (de Sitter) precession*

- A gyroscope moving through curved spacetime will precess with respect to the rest frame.
- Earth-Moon system behaves as a gyroscope with a predicted geodetic precession of 19.2 msec/year.
- Observed using LLR by measuring the lunar perigee precession.
- The current limit on the deviation of the geodetic procession is: Kgp=(-1.9±6.4)x10-3.

## Parameterized Post-Newtonian Limits 

- In weak field limit (post-Newtonian) the metric can be parameterized to describe most metric theories.
-  indicates how much spacetime curvature is produced per unit mass,
-  indicates how nonlinear gravity is (self-interaction).
-  =  =1 in General Relativity.
- Limits on  can be set from geodetic procession measurements, but the best limits come from measurements of the gravitational time delay of light (Shapiro effect).  Ranging measurement to the Cassini spacecraft set the current limit on : (–1) = (2.1±2.3)10-5
-  combined with LLR data provides the best limit on : (–1) = (1.2 ±1.1)10-4.

- B. Bertotti *et al.*, Nature **425**, 374 (2003).
- J. G. Williams *et al.*, Phys. Rev. Lett. **93**, 261101 (2004).

## Lunar Science

*Lunar Science*

- Lunar tides, characterized by Love numbers, are measured by LLR.
- Love numbers give information on the elastic properties of the lunar interior.
- k2 has an accuracy of 11%.
- The LLR solutions are also sensitive to the lunar tidal dissipation
- Strong tidal and lunar rotation dissipations suggest a fluid core of ~20% the Moon's radius.
- Evidence for the oblateness of the lunar fluid-core/solid-mantle boundary may be reflected in a century-scale polar wobble frequency.

## What’s Next?

- Better Ground Stations
- More & Better Retroreflectors
- Active Systems (i.e. Laser Transponders
- Moon, Mars, and Beyond

**Concept Studies for Lunar Sortie Science Opportunities (LSSO):**
- “The first science opportunities enabled by the human exploration program may be autonomous instrument packages that are deployed on the lunar surface by astronauts late in the next decade.  In order to begin planning for such an opportunity, and to identify any requirements that may impact planning for human exploration of the moon, NASA is soliciting concept studies for lunar surface experiments.”

## New Sites - New Retroreflectors

- Available retroreflectors all lie within 26 degrees latitude of the equator, and the most useful ones within 24 degrees longitude of the sub-earth meridian.
- Operating LLR stations are at similar northern latitudes.
- The addition of one or more reflectors would improve the geometrical precision of a normal point by a factor of 1.5 to nearly 4 at the same level of ranging precision.
- Better retroreflectors
- Hollow cubes which weigh much less than their solid counterparts so arrays could be made larger.
- Smaller thermal distortions, especially in hollow cubes made of beryllium, so the cubes can be made larger without sacrificing optical performance.

- S. M. Merkowitz* et al.*, in Proceedings of the International Workshop
- "From Quantum to Cosmos: Fundamental Physics Research in Space"  (2006).

## Transponders

- J. J. Degnan, J. Geodyn. **34**, 551 (2002).

*Asynchronous Transponder*

*Echo Transponder*

*Transponders*

- Transponders have only r2 signal loss compared to r4 for retroreflectors.
- Transponder would not have large orientation errors.
- Asynchronous Transponder could be used with existing SLR systems with little modification

## Two-Station Demonstration

- Goddard 1.2 m facility

- SLR 2000

- Satellites500 to 25,000 km altitude

- LAGEOS

- Demonstrate both laser communication and PN code ranging between the two SLR stations via passive relay satellite

## Interplanetary Range Demonstrations

*Interplanetary Range Demonstrations*

- Two-Way Transponder Experiment to the Messenger Spacecraft
- Ranging demonstrated over 24.3 Million Km (May/June 2005)
- Laser communications and ranging demo attempted at 100 Million Km (June 2007).
- Next attempt at 100 Million Km scheduled for March 2008.
- One-Way Earth-to-Mars Transponder Experiment to MOLA on Mars Global Surveyor (September 2005)
- 80 Million Km
- 100’s of pulses observed at Mars

- D. E. Smith* et al.*, Science **311**, 53 (2006).

## Combine with Optical Communications

- With an optical link it is natural to use it for communications in addition to ranging.
- Mission data requirements are increasing; free-space optical communications has potentially higher capacity over large distances than RF communications.
- Several methods currently under development at GSFC.

## Moon to Mars

*Moon to Mars*

- Current Mars ranging achieves only meter level accuracy.
- Sun-Earth-Mars-Jupiter system tests SEP qualitatively different from LLR.
- With 1 cm precision ranging, the PPN parameter   can be measured to about 10-6, ten times better than the Cassini result.
- SEP polarization effect is ~100 times larger for Earth-Mars orbits than for lunar orbit.
- With 1 cm precision ranging,  can be measured to between 610-6 and 210-6 for observations ranging between one and ten years.
- Combined with the time delay measurements this leads to a measurement of PPN parameter  to the 10-6 level.
- Mass of Jupiter can be determined more accurately than from Pioneer & Voyager data combined.
- Better measurements of Mars’ rotational dynamics could provide estimates of the size of the core.

## LSSO Summary

- Define gravitational and lunar science return as a function instrument performance and site location.
- Define instrument requirements to make maximal use of existing and next generation SLR stations.
- Analyze various retroreflector designs and configurations and identify associated resource, cost, operation, and technology requirements.
- Analyze asynchronous laser transponder options and identify associated resource, cost, operation, and technology requirements.
- Analyze combined optical communication and ranging system options and identify associated resource, cost, operation, and technology requirements.
- Identify the TRL of all major instrument subsystems and define any technology development that is necessary to mature the concepts and operate in the lunar environment.
- Compile options and derive science return as a function of cost, resource requirements, etc.