Asteroids with satellites are observed throughout the Solar System, from subkilometre near-Earth asteroid pairs to systems of large and distant bodies in the Kuiper belt. The smallest and closest systems are found among the near-Earth and small inner main-belt asteroids, which typically have rapidly rotating primaries and close secondaries on circular orbits. About 15 per cent of near-Earth and main-belt asteroids with diameters under 10 km have satellites1,2. The mechanism that forms such similar binaries in these two dynamically different populations was hitherto unclear. Here we show that these binaries are created by the slow spinup of a ‘rubble pile’ asteroid by means of the thermal YORP (Yarkovsky–O’Keefe–Radzievskii–Paddack) effect. We find that mass shed from the equator of a critically spinning body accretes into a satellite if the material is collisionally dissipative and the primary maintains a low equatorial elongation. The satellite forms mostly from material originating near the primary’s surface and enters into a close, low-eccentricity orbit. The properties of binaries produced by our model match those currently observed in the small near-Earth and main-belt asteroid populations, including 1999 KW4 (refs 3, 4).
We thank W. F. Bottke and A. Harris for their constructive reviews. K.J.W. and D.C.R. acknowledge support from the National Science Foundation under grants AST0307549 and AST0708110. K.J.W. and P.M. also had the support of the European Space Agency’s Advanced Concepts Team on the basis of the Ariadna study 07/4111 ‘Asteroid Centrifugal Fragmentation’, and of the French Programme National de Planétologie. K.W. is also supported by the Henri Poincaré fellowship at the Observatoire de la Côte d’Azur, Nice, France. We acknowledge the use of the Mésocentre de Calcul-SIGAMM hosted at the Observatoire de la Côte d’Azur, Nice, France. Some simulations were performed at the University of Maryland with the Department of Astronomy borg cluster and the Office of Information Technology High Performance Computing Cluster. Raytracing for Figs 1 and 3 was performed with the Persistence of Vision Raytracer.