Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

Ostro, Steven. J.
Margot, Jean-Luc
Benner, Lance A. M.
Giorgini, Jon D.
Scheeres, Daniel J.
Fahnestock, Eugene G.
Broschart, Stephen B.
Bellerose, Julie
Nolan, Michael C.
Magri, Christopher
Pravec, Petr
Scheirich, Petr
Rose, Randy
Jurgens, Raymond F.
De Jong, Eric M.
Suzuki, Shigeru

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA. ²Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853-6801, USA. ³Department of Aerospace Engineering, University of Michigan, 1320 Beal Avenue, Ann Arbor, MI 48109-2140, USA. ⁴Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA. ⁵University of Maine at Farmington, 173 High Street-Preble Hall, Farmington, ME 04938, USA. ⁶Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 1, CZ-25165 Ondřejov, Czech Republic.

*To whom correspondence should be addressed. E-mail: [email protected]

Supporting Online Material: www.sciencemag.org/cgi/content/full/1133622/DC1; Methods; Figs. S1 to S4; Table S1 to S4; References

8 August 2006; accepted 4 October 2006

Published online 12 October 2006; 10.1126/science.1133622

Include this information when citing this paper.

Science 314(5803):p 1276-1280, November 24, 2006.

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ∼1.5-kilometer-diameter primary (Alpha) is an unconsolidated gravitational aggregate with a spin period ∼2.8 hours, bulk density ∼2 grams per cubic centimeter, porosity ∼50%, and an oblate shape dominated by an equatorial ridge at the object's potential-energy minimum. The ∼0.5-kilometer secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius ∼2.5 kilometers and period ∼17.4 hours, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.