Radar Imaging and Characterization of the Binary Near-Earth Asteroid (185851) 2000 Dp107

The potentially hazardous asteroid (185851) 2000 DP107 was the first binary near-Earth asteroid to be imaged. Radar observations in 2000 provided images at 75 m resolution that revealed the shape, orbit, and spin-up formation mechanism of the binary. The asteroid made a more favorable flyby of the Earth in 2008, yielding images at 30 m resolution. We used these data to obtain shape models for the two components and to improve the estimates of the mutual orbit, component masses, and spin periods. The primary has a sidereal spin period of 2.7745 ± 0.0007 hr and is roughly spheroidal with an equivalent diameter of 863 m . It has a mass of kg and a density of 1381 ± 244 kg m −3 . It exhibits an equatorial ridge similar to the (66391) 1999 KW4 primary; however, the equatorial ridge in this case is not as regular and has a ∼300 m diameter concavity on one side. The secondary has a sidereal spin period of 1.77 ± 0.02 days commensurate with the orbital period. The secondary is slightly elongated and has overall dimensions of m (6% uncertainties). Its mass is kg and its density is 1047 ± 230 kg m −3 . The mutual orbit has a semimajor axis of 2.659 ± 0.08 km, an eccentricity of 0.019 ± 0.01, and a period of 1.7556 ± 0.0015 days. The normalized total angular momentum of this system exceeds the amount required for the expected spin-up formation mechanism. An increase of angular momentum from non-gravitational forces after binary formation is a possible explanation. The two components have similar radar reflectivity, suggesting a similar composition consistent with formation by spin-up. The secondary appears to exhibit a larger circular polarization ratio than the primary, suggesting a rougher surface or subsurface at radar wavelength scales.