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Spacecraft at Small NEO. D.J. Scheeres Department of Aerospace Engineering The University of Michigan. The Asteroid Dynamical Environment is …. one of the most perturbed environments found in the solar system

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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

Contributors to the Dynamical Environment

Solar Radiation Pressure

Asteroid Gravity

Solar Tide

Solar Tide

Asteroid Rotation

Solar Radiation Pressure


SRP can strip a spacecraft out of orbit

View in the terminator plane

View from the Sun

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 from the sun

View in asteroid orbit plane

Terminator Orbit in above propagated over 100 days

Looking down on asteroid orbit plane

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

Inertial Hovering







Control Volume





∆V = constant

Practical Inertial Hovering

Control Strategy





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

Muses Sea

TD1 Site

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