Lightcurves of 26 Near-Earth Asteroids☆

Abstract We present the results of our photometric observations of 26 near-Earth asteroids (NEAs) in the range of absolute magnitudesH= 13.6–20.0 (diameters approximately 0.4–8 km). The synodic periods in the range 2.3–230 h were detected for 25 of them; 21 periods are new and in 4 cases we confirmed earlier determinations. In 20 cases the synodic periods are interpreted as being the rotation periods. Among the 5 exceptions, in two cases there remains an uncertainty whether the detected period is not half or twice that of the rotation period, and in another two cases—(3691) 1982 FT and 1997 BR—there were found large deviations of the lightcurve points from the mean curves that can be due to possible complex rotations of the small, slowly rotating asteroids. Overall, the short period end (2.3–3.3 h) of the spin rate distribution shows characteristics that are consistent with the hypothesis of their “rubble pile” structure, as noted by Harris (LunarPlanet. Sci.XXVII, 493–494); specifically, there is a “barrier” against spins faster than 2.3 h and the amplitudes of the fast rotating NEAs are smaller in comparison with the other, longer period NEAs. In the group of slow rotators (P> 12 h), the suggested presence of objects in excited rotation states must be confirmed by further observations using also different techniques. This slow rotators group may be actually more abundant than our results suggest (6 of 25 objects, i.e., 20–30%), since there is a bias against low-amplitude slow rotators in the groundbased photometric program.

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