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

• Published in: Icarus (New York, N.Y. 1962) Vol. 225; no. 1; pp. 141 - 155
• Main Authors: Statler, Thomas S., Cotto-Figueroa, Desireé, Riethmiller, David A., Sweeney, Kevin M.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.07.2013
• Subjects: Asteroids, Dynamics; Asteroids, Rotation; Asteroids, Surfaces; Near-Earth objects; Asteroids, Surfaces; Asteroids, Rotation; Asteroids, Dynamics; Near-Earth objects
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2013.03.010

•Light curve survey of fast rotating (period <2h) near-Earth asteroids.•New de-biasing algorithm to constrain intrinsic spin rate distribution.•Almost nothing bigger than 170m is a fast rotator.•Nearly everything smaller than 60m is a fast rotator. We present results from a program of optical light curve observations of near-Earth asteroids (NEAs) with diameters under 1km, 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<2h). We assess our detection sensitivity using simulated light curves and a new Monte Carlo algorithm (SALSA), which allows us to de-bias the counts of detected FRAs and determine the fraction of objects that are fast rotators as a function of 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 170m, and almost everything smaller than 60m, is a fast rotator, assuming a mean S class albedo of 0.17. The formal 95% confidence limits are F<13% for 18.5<H<21.4 and F>56% for 23.6<H<26.3. Relative to a distribution with the same FRA fraction that is uniform in frequency (as implied by models of evolution following the YORP cycle) up to a size-dependent upper cutoff, the actual spin distribution extends to shorter periods. Approximately two thirds of our sample shows ambiguous light curves from which no period estimate can be obtained. Finite photometric errors account for some of these, but do not explain the steep increase in the ambiguous fraction toward larger objects, which suggests an increase in the actual fraction of very slow rotators, very fast rotators, or nearly axisymmetric objects. With a significantly larger data set, our fully general SALSA procedure will be able to extract the NEA spin rate distribution as a function of absolute magnitude 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.