Radar observations and physical model of asteroid 6489 Golevka

We report 8510-MHz (3,5-cm) radar observations of the Earth-crossing astero id (ECA) 6489 Golevka (1991 JX) obtained between June 3 and June 15, 1995, at Goldstone, the Very Large Array and the Evpatoria (Ukraine) and Kashima (Japan) radio antennas. One-dimensional Doppler spectra are used to estimat e the object's convex hull, refine the ephemeris, and yield four possible p ole directions. Three-dimensional modeling using two-dimensional delay-Dopp ler images and published lightcurves unambiguously defines the pole and rev eals an extraordinarily angular shape with flat sides, sharp edges and corn ers, and peculiar concavities. The equivalent diameter of the object is 530 +/- 30 m, with moments of inertia about the (long, intermediate, short) ax es proportional to (1.00, 1,38, 1.39) +/- 0.1. The asteroid's pole direction is lambda = 202 +/- 5 degrees, beta = -45 +/- 5 degrees, and its sidereal period is P = 6.0289 +/- 0.0001 h, The asteroi d's circular polarization ratio, SC/OC = 0.23 +/- 0,02, is lower than the a verage for radar-detected near-Earth asteroids and reveals only a modest de gree of near-surface roughness at scales near the 3.5-cm wavelength. Howeve r, the approximately Lambertian radar scattering law implies considerable s urface roughness at larger scales. The asteroid's radar scattering law is m odeled as rho cos(n) theta, with rho = 0.25 +/- 0.12 and n = 1.7 +/- 0.7 gi ving an equivalent spherical albedo of 0.18 +/- 0.09. This value is in the middle of the distribution of albedos of S-class asteroid's previously imag ed by radar. The Hapke parameters describing the object's optical scatterin g properties are w = 0.173 +/- 0.006, h = 0.024 +/- 0.012, B-0 = 1.03 +/- 0 .45, g = -0.34 +/- 0.02, and <(<theta>)over bar> = 20 +/- 5 degrees. Both t he optical and the radar scattering properties are consistent with those of a typical S-class asteroid. Goldstone-VLA plane-of-sky images do not resolve the asteroid but do provid e astrometry with uncertainties less than 0.1 arcsec. Integration of an orb it based on all available radar and optical astrometry shows that Golevka h as an insignificant probability of collision with any planet during at leas t the next nine centuries. We investigate Golevka's dynamical environment, assuming uniform density. S ome areas of the surface are characterized by large enough slopes that we e xpect that they are exposed, solid, monolithic rock, (C) 2000 Academic Pres s.