smallNEO

Spacecraft at Small NEO: 

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

The Asteroid Dynamical Environment is …: 

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

Characterization Missions: 

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

Orbiting vs. Hovering: 

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

Slide5: 

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

Solar Radiation Pressure (SRP) Effects: 

Solar Radiation Pressure (SRP) Effects

Slide7: 

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

Slide8: 

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

Stable orbits do exist for SRP : 

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

Terminator vs. Non-Terminator Orbit: 

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

Terminator vs. Non-Terminator Orbit: 

Terminator vs. Non-Terminator Orbit

Gravity Effects: 

Gravity Effects

Mixed Perturbations: 

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

Very Small NEO: 

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

Slide19: 

Inertial Hovering Sun Thrust SRP Gravity

Slide20: 

Practical Inertial Hovering Control Strategy

Slide21: 

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

Instrument Placement: 

Instrument Placement Hovering Boresight Placement Terminator Orbit Boresight Placement

Challenges for Mitigation Missions: 

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

Surface operations at small bodies: 

Surface operations at small bodies All images courtesy JAXA/ISAS

Example: Stability of Close Motion: 

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

What is needed?: 

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