Search

We present both a review of earlier data and new results on non-principal axis rotators (tumblers) among asteroids. Among new tumblers found, the best data we have are for 2002 T[D.sub.60], 2000 W[L.sub.107], and (54789) 2001 M[Z.sub.7]--each of them shows a lightcurve with two frequencies (full terms with linear combinations of the two frequencies are present in the lightcurve). For 2002 TD60, we have constructed a physical model of the NPA rotation. Other recent objects which have been found to be likely tumblers based on their lightcurves that do not fit with a single periodicity are 2002 N[Y.sub.40], (16067) 1999 R[H.sub.27], and (5645) 1990 SP. We have done a statistical analysis of the present sample of the population of NPA rotators. It appears that most asteroids larger than ~ 0.4 km with estimated damping timescales (Harris, 1994, Icarus 107, 209) of 4.5 byr and longer are NPA rotators. The statistic of two short-period tumblers (D = 0.04 and 0.4 km) with non-zero tensile strength suggests that for them the quantity [micro] Q/T, where [micro] is the mechanical rigidity, Q is the elastic dissipation factor, and T is a spin excitation age (i.e., a time elapsed since the last significant spin excitation event), is greater by two to four orders of magnitude than the larger, likely rubble-pile tumblers. Among observational conditions and selection effects affecting detections of NPA rotations, there is a bias against detection of low-amplitude (small elongation) tumblers. Keywords: Asteroids; Excited rotation; Photometry

Abstract: The spin rate distribution of main belt/Mars crossing (MB/MC) asteroids with diameters 3–15 km is uniform in the range from to 9.5 d−1, and there is an excess of slow rotators with d−1. The observed distribution appears to be controlled by the Yarkovsky–O''Keefe–Radzievskii–Paddack (YORP) effect. The magnitude of the excess of slow rotators is related to the residence time of slowed down asteroids in the excess and the rate of spin rate change outside the excess. We estimated a median YORP spin rate change of for asteroids in our sample (i.e., a median time in which the spin rate changes by 1 d−1 is ), thus the residence time of slowed down asteroids in the excess is . The spin rate distribution of near-Earth asteroids (NEAs) with sizes in the range 0.2–3 km (∼5 times smaller in median diameter than the MB/MC asteroids sample) shows a similar excess of slow rotators, but there is also a concentration of NEAs at fast spin rates with . The concentration at fast spin rates is correlated with a narrower distribution of spin rates of primaries of binary systems among NEAs; the difference may be due to the apparently more evolved population of binaries among MB/MC asteroids. [Copyright &y& Elsevier]