Radar observations of asteroid 1986 JK

Abstract

Echoes from this near-Earth asteroid were obtained in May and June 1986, three weeks after its discovery, using the Goldstone 3.5-cm-wavelength radar. The asteroid's minimum distance during the observations was less than 0.029 AU, only 11 times further than the Moon and closer than for any other asteroid or comet radar onbervation to date. 1986 JK's circular polarization ratio μ c, of echo power received in the same sense of circular polarization as transmitted (the SC sense) to that in the opposite (OC) sense, averages 0.26 ± 0.02, indicating that single backscattering from smooth surface elements dominates the echoes, although there is a moderate degree of wavelength-scale, near-surface roughness. Variations in μ c and in the shapes of the OC and SC echo spectra suggest that the surface is at least moderately heterogeneous at structural scales no smaller than the wavelength and probably much larger. The asteroid's echo bandwidth provides the constraint D max ≥ P/5, where P is the apparent spin period, in hours, and D max, in kilometers, is the maximum width of the asteroid's polar silhouette. Our estimate of 1986 JK's average OC radar cross section is 0.022 ± 0.007 km 2. Combining that result with an indirect size constraint based on W.Z. Wisniewski's (1987, Icarus 70, 566–572) photometry yields an interval estimate for 1986 JK's radar albedo that overlaps values reported to date for comets and the radar-darkest asteroids. A “working model” of 1986 JK postulates a 1- to 2-km object whose shape is not extremely irregular, with little elongation but some polar flattening; the rotation period is not more than a few hours longer than 10 hr and the near-surface bulk density is within a factot of 2 of 0.9 g cm −3. The orbital and physical characteristics of 1986 JK are somewhat comet-like. However, the Earth passes within 0.005 AU of the asteroid's orbit, and evidence for recent meteor shower activity associated with this object is lacking. Estimates of the asteroid' echo Doppler frequencies (i.e., its radial velocities) were used in conjunction with the available optical astrometric data to provide refined orbital elements and ephemeris predictions. The radar astrometric data are extremely powerful for orbit improvement. At the next Earth close approach (0.12 AU in mid-2000), a search ephemeris based upon all optical and radar data will have a plane-of-sky, solid-angle uncertainty an order of magnitude smaller than that for an ephemeris based upon the optical data alone. A recovery attempt made on June 17, 2000, would have a plane-of-sky position uncertainty ≈20′, so prospects for recovering 1986 JK are good.