Spacecraft at Small NEO

1. Spacecraft at Small NEO D.J. Scheeres Department of Aerospace Engineering The University of Michigan

2. The Asteroid Dynamical Environment is … • one of the most perturbed environments found in the solar system • Solar tide and radiation pressure perturbations can easily strip a spacecraft out of orbit about an asteroid. • Asteroid gravity and rotational effects can rapidly destabilize a spacecraft orbit, causing impact or escape on time scales of less than a day. • Gravity is so weak as to allow a spacecraft to “hover” above the surface for extended periods of time, yet strong enough to require frequent correction and reaction. • Our real experience for operating in this environment is limited: • The NEAR mission provided the first set of precision measurements of such an environment, and established the baseline for all such future missions. • The Hayabusa mission provided the first view of small asteroids and confirmed their rubble-pile structure. • Special challenges exist for characterization and mitigation missions to small NEO

3. Characterization Missions • Any serious attempt at mitigation must be preceded by a characterization mission • Enables the mitigation mission to be more efficiently designed • Needed for guaranteed results • Needed for precision verification • A characterization mission must establish: • A precise orbit for the asteroid • Measurements of the asteroid environment at a level of precision necessary to design a successful mitigation mission • Total mass • Mass distribution • Rotation state • Shape • Surface morphology • Interior morphology

4. Orbiting vs. Hovering • Currently there are two competing mission approaches: • Orbital missions (e.g. NEAR) • Hovering missions (e.g. Hayabusa) • How do these missions compare relative to characterization goals • NEAR provided: • high precision determination of mass, mass distribution, shape, rotation state, asteroid trajectory • Intimately tied to its being an orbital mission, allowing for long periods of no thrusting • Hayabusa provided: • high precision determination of shape and rotation state • low precision determination of mass and trajectory update • No determination of mass distribution • Intimately tied to its being a hovering mission, requiring frequent thruster firings and only brief periods close to the asteroid • In principle, an orbiting mission can provide a more precise characterization

5. Contributors to the Dynamical Environment Solar Radiation Pressure Asteroid Gravity Solar Tide Solar Tide Asteroid Rotation Solar Radiation Pressure

6. Solar Radiation Pressure (SRP) Effects

7. A 100 meter difference in initial conditions can change escape to impact

8. SRP can strip a spacecraft out of orbit View in the terminator plane View from the Sun A maximum orbit size for stability exists

9. Stable orbits do exist for SRP • Orbits lie in the sun-terminator plane • Orbit radius must be small enough to not be stripped away • SRP force makes them sun-synchronous • Very robust and stable

10. Terminator vs. Non-Terminator Orbit View from the sun View in asteroid orbit plane Terminator Orbit in above propagated over 100 days Looking down on asteroid orbit plane

11. Terminator vs. Non-Terminator Orbit

12. Gravity Effects

17. Mixed Perturbations • As smaller orbit sizes are considered, destabilizing interactions between SRP effects and gravity field effects occur • Becomes a challenge for orbital missions at small asteroids

18. Very Small NEO • For very small NEO, SRP and gravity are simultaneously effective • Creates difficulties for an orbital mission • Can be mitigated by decreasing spacecraft area/increasing mass to make SRP less important • May require a hovering approach for a characterization mission • Higher precision orbit determination and characterization may be possible by carrying out repeated slow hyperbolic flybys

19. Inertial Hovering Gravity Sun SRP Thrust

20. Sun Control Volume Vbefore Vafter ∆V Earth ∆V = constant Practical Inertial Hovering Control Strategy

21. Vbefore ∆V Vafter Higher Precision Hovering Control Strategy Sun Slow, close hyperbolic flybys at a range of sub-solar latitude Controlled maneuvers to repeat, a few days after every close approach Earth

22. Instrument Placement Hovering Boresight Placement Terminator Orbit Boresight Placement

23. Challenges for Mitigation Missions • By definition, a mitigation mission involves close proximity interactions between “something” and the asteroid • Close hovering of a large spacecraft (gravity tug) • Mechanical interaction with the surface (space tug) • Precise targeting of an impactor • Precise placement of an explosive device • Etc… • Design of the mitigation technology must account for the extreme dynamics that exist in the asteroid environment • Binary asteroids • Loose regolith that is easily mobilized into orbit • Influence of asteroid shape and interior morphology on impactor/explosive effect • Effect of SRP and gravity

24. Muses Sea TD1 Site Surface operations at small bodies All images courtesy JAXA/ISAS

25. Example: Stability of Close Motion • Gravity gradient S/C at Earth are stable • Large S/C close to small bodies are not • Major implications for the design and operation of such vehicles

26. What is needed? • We do not know what is really feasible for close proximity operations at NEO for mitigation • A direct way to address this is to fly a dedicated technology mission to an NEO that will: • Address spacecraft orbit and hover operations issues • Evaluate basic properties of an asteroid surface and interior • Test landed operations on an asteroid • Validate navigation and tracking technologies • Spur focused and adequately supported research • Produce scientific benefits • Enable realistic development of mitigation technologies