Radar Imaging of binary near-Earth asteroid (66391) 1999 KW4

High-resolution radar images reveal near-Earth asteroid ( 66391) 1999 KW4 to be a binary system. The similar to 1.5-kilometer-diameter primary ( Alpha) is an unconsolidated gravitational aggregate with a spin period similar to 2.8 hours, bulk density similar to 2 grams per cubic centimeter, porosity similar to 50%, and an oblate shape dominated by an equatorial ridge at the object's potential-energy minimum. The similar to 0.5-kilometer secondary ( Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius similar to 2.5 kilometers and period similar to 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.