Forming Binary Near-Earth Asteroids From Tidal Disruptions

arrier around 2.2 h. The excess of both fast and slow rotators in the NEA populationhas been suggested as a possible manifestation of the YORP thermal effect (Pravec et al.2002; Bottke et al. 2006).Scheeres et al. (2004) analyzed the nature of rotation changes due to the close encounterswith planets that NEAs undergo. This study quantified the overall increase in spinrate that a population would gain through these encounters by modeling a steady-statesystem where Main Belt asteroids become NEAs, have encounters over their lifetime andare then replaced by new MBAs. This Monte Carlo simulation used the rotation ratesfrom Donnison & Wiper (1999) derived from collisional experiments as the initial MBArotation rate distribution. This distribution was then compared to the steady-state distributionof NEAs that evolved over time through planetary encounters. Overall a slightspin-up of rotation rates was noted, and the maximum spin achieved from an encounterwas near the classical critical breakup limit of 2 h. This study confirmed that the NEApopulation will have a spin-rate distribution different from its parent bodies in the MainBelt.1.3.3 Motivation for work on small MBAsDue to observational constraints it is significantly more difficult to obtain lightcurvesfor kilometer-sized MBAs than for the closer NEAs. Thus direct comparisons betweensimilarly sized NEAs and MBAs is nearly impossible, as the number of lightcurves forsmall MBAs is currently inadequate. Each population has differing dynamical environments,with short lifetimes and frequent planetary encounters for NEAs and much shortercollisional than dynamical lifetimes in the Main Belt. They also have differing thermalenvironments, with the more distant MBAs less affected by the Yarkovsky or YORP thermaleffects. Because of the sometimes powerful effects these phenomena may have onthe shape and spin of small asteroids, detailed studies of lightcurves are potentially diag-19