Abstract

Arecibo S-band (2380 MHz, 13 cm) and Goldstone X-band (8560 MHz, 3.5 cm) radar observations from June 2-6, 2006 showed that Apollo asteroid 2004 DC is a binary system. Initial shape modeling efforts estimate the diameters of the components as 340 m for the primary and 60 m for the secondary. The mutual orbit has a semimajor axis of 4.4 primary radii and has an eccentricity of 0.24, which is atypical of near-Earth binary asteroids. The rotation period of the secondary also appears to be unsynchronized to its orbital period, 7 h versus 23 h, respectively. Although the precise rotation pole for the primary is undetermined, the best pole solutions from the the observed radar bandwidths of the primary are in good agreement with the best fit orbit normals for the secondary, suggesting an equatorial orbit. The 2.6-h lightcurve period of the primary [R. Behrend, pers. comm.] and the total angular momentum of the system suggest possible formation of the binary system through a spin-up mechanism such as the YORP effect or close planetary tidal encounter. Preliminary shape modeling suggests a shape for the primary reminiscent of 1998 KW4 including a pronounced circular equatorial belt and polar flattening.