36th DPS Meeting, 8-12 November 2004
Session 48 Asteroid Dynamics
Oral, Friday, November 12, 2004, 3:30-5:00pm, Clark

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[48.08] A Transition Size in Physical Properties?

A. F. Cheng (JHU APL)

In a new synthesis of asteroid collisional evolution [A F Cheng, Icarus 169: 357, 2004] the well-known Dohnanyi result, that collisional evolution produces a self-similar power law size distribution, is overturned because of scale-dependent collision physics. The new picture reveals the possibility of a transition size around 5 km, below which asteroids would be primarily collisional breakup fragments, whereas much larger asteroids are mostly eroded or shattered survivors of collisions. If so, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. This picture has critical consequences for regolith evolution on asteroids, because the collisional breakup event should remove pre-existing regolith and thereby ‘reset’ the surface. Such resetting would be typical of the small asteroids below the transition size. A transition in the spectral properties of asteroids in the 0.1 km to 5 km size range has recently been reported [Binzel et al., Icarus 170, 259, 2004]. A key question is, whether these small asteroids as observed today have regenerated significant regolith cover. A possible mechanism whereby this could have occurred is via giant impacts wherein gravity scaling may apply; this likely occurred on Eros. Observational tests will be discussed, including the first visit of a spacecraft to an asteroid in the small size range: that of spacecraft Hayabusa to asteroid Itokawa. What may Hayabusa find?

This work was funded by NASA.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.