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Can Small, Fast Spinning Asteroids be Rubble Piles?

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.

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Can Small, Fast Spinning Asteroids be Rubble Piles?

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  1. 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

  2. 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/d2.8from ~ millimeters to decameters • ~1/d2.8from 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

  3. 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 Cohesive regolith D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

  4. How Fast can a Cohesive Rubble Pile Spin? Monoliths? 2008 TC3 = Almahata Sitta Meteorites: Could have been a tumbling rubble pile! Rubble Piles? Strong Lunar Regolith Cohesion: 3 kPa Weak Lunar Regolith Cohesion: 100 Pa “Very Weak” Cohesion: 25 Pa D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

  5. How Fast Must a Boulder Spin to Clear Grains? Strength based on lunar regolith cohesion 10 Micron-sized grains 100 Micron-sized grains Millimeter-sized grains Centimeter-sized grains Decimeter-sized grains Even Fast-spinning “monoliths” can be covered with finer-grained regolith. D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

  6. 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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