Radar Observations and Physical Model of Asteroid 6489 Golevka

Abstract

We report 8510-MHz (3.5-cm) radar observations of the Earth-crossing asteroid (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 estimate the object's convex hull, refine the ephemeris, and yield four possible pole directions. Three-dimensional modeling using two-dimensional delay-Doppler images and published lightcurves unambiguously defines the pole and reveals an extraordinarily angular shape with flat sides, sharp edges and corners, and peculiar concavities. The equivalent diameter of the object is 530±30 m, with moments of inertia about the (long, intermediate, short) axes proportional to (1.00, 1.38, 1.39) ±0.1. The asteroid's pole direction is λ=202±5°, β=−45±5°, and its sidereal period is P=6.0289±0.0001 h. The asteroid's circular polarization ratio, SC/OC=0.23±0.02, is lower than the average for radar-detected near-Earth asteroids and reveals only a modest degree of near-surface roughness at scales near the 3.5-cm wavelength. However, the approximately Lambertian radar scattering law implies considerable surface roughness at larger scales. The asteroid's radar scattering law is modeled as ρcos n θ, with ρ=0.25±0.12 and n=1.7±0.7 giving 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 imaged by radar. The Hapke parameters describing the object's optical scattering properties are w=0.173±0.006, h=0.024±0.012, B 0=1.03±0.45, g=−0.34±0.02, and θ =20±5°. Both the 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 provide astrometry with uncertainties less than 0.1 arcsec. Integration of an orbit based on all available radar and optical astrometry shows that Golevka has an insignificant probability of collision with any planet during at least the next nine centuries. We investigate Golevka's dynamical environment, assuming uniform density. Some areas of the surface are characterized by large enough slopes that we expect that they are exposed, solid, monolithic rock.