Title:Arecibo and Goldstone Radar Imaging of Contact Binary Asteroids
Author:L. A. M. Benner, M. C. Nolan, J. L. Margot, S. J. Ostro, M. K. Shepard, C. Magri, J. D. Giorgini, and M. W. Busch

Arecibo (2380 MHz, 13 cm) and Goldstone (8560 MHz, 3.5 cm) delay-Doppler radar observations have yielded images of sixteen candidate contact binary near-Earth asteroids (NEAs). Here we define a contact binary as an an object consisting of two lobes that are in contact, have a bimodal mass distribution, and that may have once been separate. The objects in our sample range from ~0.25 to ~3 km in average diameter; have rotation periods as short as 2.7 hours to as long as weeks; and several are probably non-principal axis rotators. Some could be former binaries that collapsed together or objects that could spin up into binaries in the future. Since completion of the Arecibo upgrade in 1999, ~11% (14/132) of near-Earth asteroids larger than 200 m in diameter have had contact binary shapes. Given that ~17% of NEAs observed by radar are binaries, among NEAs larger than ~200 m in diameter, binary and contact binary objects constitute more than 25% of the population.