Derek Richardson
"Rotational breakup as the origin of small binary asteroids"
About 15% of near-Earth and small inner Main Belt asteroids are binaries or multiples. Lightcurve measurements and radar observations indicate that the primaries of many of these systems are oblate and rapidly rotating, near the breakup limit for unconsolidated material. We show that gradual spin up of small asteroids with little to no cohesion can account for the physical characteristics of many of the observed systems. The so-called "YORP effect", which causes a small body's spin rate and axis orientation to change as a result of solar illumination, may provide the required spin-up mechanism. (Tidal disruption, though a viable means of binary formation, cannot explain the small binaries in the inner main belt nor the tendency toward rounded, fast-spinning primaries.) We present numerical simulations demonstrating the spin-up, break-up, and reshaping of a progenitor leading to binary formation. (Related article: Walsh et al. 2008, Nature 454, 188.)