Size matters: The rotation rates of small near-Earth asteroids

Abstract We present results from a program of optical light curve observations of near-Earth asteroids (NEAs) with diameters under 1 km, designed to detect, and determine the distribution of, rotation periods shorter than a few hours. We obtain measurements or estimates of rotation period P for approximately one third of the 83 NEAs observed. Most of the measured periods are in the fast-rotating asteroid (FRA) regime ( P H . We find that the FRA fraction F rises sharply from zero to a value statistically consistent with unity from H  = 21.4 to H  = 23.6, a span corresponding to a factor of only 2.8 in nominal diameter. Almost nothing larger than 170 m, and almost everything smaller than 60 m, is a fast rotator, assuming a mean S class albedo of 0.17. The formal 95% confidence limits are F H F  > 56% for 23.6  H H . Determining this distribution to the accuracy needed to constrain the physical properties of NEAs and their dynamical evolution will require larger samples, and homogeneous, unbiased reporting of the data, including accurate errors, for all objects observed, not just those with measured periods.

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