APEX – Asteroid Probe Experiment

J.B. Plescia1, O. Barnouin1, M. Paul1, N. Schmerr2, D.C. Richardon2, H. Yu3,4, W. Schlei1, 

M. Ozmiek1, F. Siddique1, A. Mautino2, E. Bell2, and J.V. DeMartini2

1Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723

2University of Maryland, College Park, MD

3Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287

4The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 

March 18, 2018
Outline

• Target

• Science Objectives

• Spacecraft – Mission

• Issues

• Alternative Approaches

• Conclusions: 

• APEX does not fit within the PSD SmallSat Study Constraints – 180 kg / $100M. 

• APEX fits a ESPA grande mass limit and larger (much larger) fiscal cap. 

• But, such a mission achieves only minimal science.

• NASA better served by a Discovery class APEX mission – more science, less risk.
Apophis

Table 1. (99942) Apophis Basic Information

Parameter Value Units Source

0.43 ± 0.04

Dimensions 0.30 

± 0.03

0.26 ± 0.03

km Brozovic et al. (2015)

Volume

2.1 x10

7

2.0 x 10

7

m

3 Brozovic et al. (2015)

this analysis

Density

Mass

Surface gravity

Escape velocity

Rotation

2.4

4.9 x 1010

0.00027

0.14

27.38 ± 0.7 

retrograde

kg m-3

kg

m sec-2

m sec-1

hr.

assumed

calculated from above

calculated from above

Brozovic et al. (2015)

Pravec et al. (2014)

Aphelion

Perihelion

1.099

0.747

AU

AU

Orbital period

Inclination

Precession

323.6

3.331

6304.8

day

deg

hr. Brozovic et al. (2015)

Pravec et al. (2014)

Radar albedo 0.25 

± 0.11

Visible albedo

Absolute magnitude

0.33 + 0.05 / -0.06

19.09 ± 0.19

Müller et al. (2014)

Pravec et al. (2014)

Surface temperature

250

K Müller et al. (2014)

Thermal inertia 600 + 200 / 

-350

50-500 J m-2

s

-0.5

K-1 Müller et al. (2014)

Licandro et al. (2016)

Brozovic et al. (2015)
Apophis

Reedy et al., 2018 IRTF

Binzel et al., 2009 Müller et al., 2012; Brozovic et al., 2017 

12.3 cm Radar

Goldstone

Herschel

70, 100, 160 μm 
Science Objectives

• Level 1 objectives

• Determine the rotational state and 

bulk properties of Apophis

• Determine the interior structure

• Determine the geology and 

geologic history of Apophis

• Determine the tidal effects on 

surface morphology, interior 

structure, rotation.

• Level 2 objectives

• Rotation period and orientation

• Shape and volume

• Topography

• Mass and density

• Internal tectonic stress seismicity

• Surface thermal stress seismicity

• Impact induced seismicity

• Geologic history

• Surface morphology

• Physical properties

• Calibration of remote sensing data
Friday the 13th April 2029
Internal Structure

Britt and Consolmagno, 2001

Perera et al., 2016
Surface Character and Modification

Expose fresh (unweathered) material

Movement downslope

Seismic motion

Richardson et al., 2004

Walsh et al., 2008

Binzel et al., 2010

Gaffey, 2010

Nesvorný et al., 2010;

DeMeo et al., 2014

Garcia et al., 2015

Keane and Matsuyama, 2015

Polishook et al., 2014a, 2014b
Rotational Dynamics
Trade Studies

• Propulsion: Chemical vs. SEP

• Propellant: Iodine vs. Xenon

• Trajectory Origin: LEO/GEO vs. L1

• Communication: Dish vs. Radial Slot Array Antenna (RLSA)

• Seismometer Deployment: Propulsive Impact vs. Direct Emplacement

• Boom: Stick vs. Deployable

• Payload: Charlie Brown vs. Rockefeller Center

• Panchromatic imaging

• Seismometer

Scientist Perspective Engineer / Program Manager Perspective
Spacecraft

APEX Spacecraft Mass Summary

Component Unit Mass (kg)

Spacecraft Bus 159

Primary Structure 50

All other Structure9

Propulsion Hardware 26

Avionics6

Power Electronics2

Solar Cells 24

Battery2

Attitude Control (all sensors) 13

RF Communications 12

Harness 10

Thermal5

Payload7

Imaging System1

Seismometer6

Contingency (30%) 49

Dry Mass Total 166

Propellant 62

Total Mass 228

Total Mass vs. ESPA (180 kg)-48

Total Mass vs. EPSA -28

requalified (200 kg)

Total Mass vs. ESPA Grande 72

SEP BHT-600 Busek

Antenna
Trajectory

∆V Budget

Mission Phase m s-1

Early Ops through Earth Escape/Lunar Flyby 307

Heliocentric Cruise 3408

Station Keeping

Total

50

3765

Launch data is flexible. Loiter in the Earth-Moon environment.

Departure of Earth-Moon environment is the real constraint 6/20/2025

Arrival at Apophis October 2028, Flight time is 3+ years.

Assume co-manifest with IMAP – launch by December 2024
Spacecraft
Spacecraft

700 W power generation
Imaging

Imaging System Details

Parameter NAC Proxops

Focal Length 550 mm 7 mm

Pupil 25 mm 2.3

Diameter

Detector 3D Plus 2650 x 1944 

3DCM681 pixels

2048 x 2048 2.2 μm 

pixels pitch

5.5 μm pitch

F / number 22 3.5

FOV 1.17° 44° x 33°

IFOV 10 μrad 286 μrad

SNR 100 60-100

Spectral Bands 400-700 nm 400-700 nm

Mass (kg)

Optics 0.45 0.6

Electronics 0.15 1.5

Structure 0.11 0.2

Total 0.71 2.8

Power (W)

Average 2 17

Peak 19

Standby 8

NAC

Proxops ECAM-C50

Malin Space Science

Imaging Strategy

3D Plus detector – Bayer RGB
Seismometer

Fluid filled sensor. Motion detected by fluid flow / MEMs 

sensor. No moving part, orientation independent. High G 

tolerance.

Package needs to be self-contained: power, comm, data 

storage, long-lived.
Seismometer Deployment

Roccor extendable tubular boom
Seismometer Deployment

Descend to the surface, insert the seismometer, release, depart.

Allows for verification of deployment and failure recovery (back off and try again).

Similar to NEAR, Hayabusa, OSIRIS-REx trajectories.
Assumption: Apophis ≈ Itokawa

535 x 294 x 209 m

Thermal and radar properties consistent with collection of fine and coarse particles –

regolith. 

Thermal inertia of ~ 600 J m-2 s-0.5 K-1. 

Radar SC/OC for Apophis is 0.33 ± 0.11

0.27 ± 0.04 for Itokawa
Surface Package Anchoring

Apollo 15 Cone Penetrometer Data - mare

Penetration Resistance Data

Assume fine-grained regolith occurs.

Thermal inertia suggests fine-grained 

material present.

Thickness unknown

Itokawa
Deformation
Seismology

Peak ground motion (displacement) for a source situated at the surface of Apophis 

[0,150,150]. The source is located at the tip of the model in the lower right.

Seismic events:

Interior stress relief

Thermal cracking

Impact

Tidal deformation

Similar studies:

Blanchette-Guertin et al., 2015

Garcia et al., 2015

Kharvroshkin and Tsyplakov, 2010

Walker et al., 2009
Seismology

Our 3-D simulation of seismic

waves propagating within asteroid

Apophis for a 5 N source at the

surface. Each panela snapshot of

the evolution within the asteroid

for ground motion in m/s

2

. Note

the strong antipodal focusing at the

“peaks” in the asteroid cross-

section.
Seismology
Seismology

Peak amplitudes of ground motion that would

be recorded by a seismometer at Apophis for a

5 Newton, 1-second duration impact force at

the surface.

Ground motion near the source (peak at 2

m/s2) and antipode (peak at 3.5 m/s2) are not

shown.

Data points represent maximum amplitude of

acceleration observed in Apophis for this force.
Issues – Good and Bad

• Mass Constraints – 228 kg wet

• No obvious technology changes to reduce mass

• Close to the Earth – 1 AU

• Communications and power are not a problem

• Seismometer housing and deployment

• Lifetime of BHT-600

• Radial line slot array antenna
Summary - APEX

• Important science, unique opportunity – tidal interactions with a large planet

• Challenging mission within PSDS constraints 

• CONOPS at a small no-g body – Autonomous operations

• Rendezvous

• Touch the surface

• Emplacement of seismometer (tiny self-contained “spacecraft”) – ensure coupling

• Data downlink – high data volume from seismometer (40 Gb / Apophis day)

• Close to the Earth, close to the Sun 

• Acknowledgements: 

• NASA Planetary Science Deep Space SmallSat Studies Program

• NASA Innovative Advanced Concepts Program