* Astronomy

Title: Radar Imaging and Characterisation of Binary Near-Earth Asteroid (185851) 2000 DP107
Author: Shantanu P. Naidu, Jean-Luc Margot, Patrick A. Taylor, Michael C. Nolan, Michael W. Busch, Lance A. M. Benner, Marina Brozovic, Jon D. Giorgini, Joseph S. Jao, Chris Magri

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 favourable 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 h and is roughly spheroidal with an equivalent diameter of 863 m ± 5%. It has a mass of 4.656 ± 0.56 x 10^11 kg and a density of 1381 ± 268 kg/m³. 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 377 x 314 x 268 m (6% uncertainties). Its mass is 0.178 ± 0.021 x 10¹¹ kg and its density is 1047 ± 230 kg/m³. The mutual orbit has a semi-major 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.

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