Can Small, Fast Spinning Asteroids be
Rubble Piles?
D.J. Scheeres and P. Sánchez
Department of Aerospace Engineering Sciences
The University of Colorado Boulder
Research support from NASA’s NEOO and PG&G programs is acknowledged
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
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What is a Rubble Pile?
• A size distribution of boulders and grains.
– Extends from ~ microns to several decameters
– Measurements of Itokawa suggest cumulative size distributions of:
• ~1/d2.8 from ~ millimeters to decameters
• ~1/d2.8 from microns to 100 microns
• Small regolith “dominates” in surface area, but not in volume
– Implies that larger boulders and grains are coated in a matrix of finer grains
• What are the consequences of this?
– The fine grains can serve as a “cement” that holds larger blocks in place.
– Allows us to apply basic properties of cohesive grains measured on Earth and
the Moon to provide predictions for cohesive strength of a rubble pile.
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
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Example Computation
• Cohesive forces can strengthen rubble piles, allowing them to
spin faster
– 1-meter boulders with cohesive interstitial regolith van der Waals forces
– Equal pull forces applied to each... very different outcomes
Cohesionless regolith
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
Cohesive regolith
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How Fast can a Cohesive Rubble Pile Spin?
2008 TC3 = Almahata Sitta Meteorites:
Could have been a tumbling rubble pile!
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
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How Fast Must a Boulder Spin to Clear Grains?
Strength based on lunar regolith cohesion
Even Fast-spinning “monoliths” can
be covered with finer-grained regolith.
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
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Summary on Small Body Cohesion:
Strength and Surfaces
• Rubble pile asteroids can be
strengthened by cohesive forces
between their smallest grains
• Cohesive strength less than found in
the upper lunar regolith can allow
~10 meter rubble piles to spin with
periods less than 5 minutes!
• “Monolithic boulders” ~ 10 meters in
size and spinning with periods much
faster than ~ 1 minute can have
millimeter to micron grains on their
surfaces.
D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder
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