Speckle interferometry of asteroids IV. Reconstructed images of 4 Vesta

The first glimpses of an asteroid's surface have been obtained from images of 4 Vesta reconstructed from speckle interferometric observations made on November 16 and 17, 1983, using Steward Observatory's 2.3-m telescope coupled with Harvard's PAPA camera. From power spectrum analysis of the 10 images Vesta is found to have a “normal” triaxial ellipsoid shape of 584(±16) × 531(±11) × 467(±12) km. Its rotational pole lies within 4° of RA = 21 h 00 m , Dec = +41° (Ecliptic long = 336°, lat = +55°) Our observations definitely support a 5-hr 20.5-min rotational period and do not fit one twice as long. Reconstructed images reveal dark and bright patterns, reminiscent of the Moon, which can be followed across the disk as the asteroid rotates. By placing circular “spots” with diameters of 135 km (= 0.11 arcsec, the effective resolution) over three dark and three bright features, and assigning albedos (relative to the surrounding material) of 0 to the dark spots and 2 to the bright spots (except one with an albedo of 1.2), we are nearly able to match its visible lightcurve. It only requires an additional bright spot deep in Vesta's southern hemisphere, an area not visible during our observations, to provide a near perfect match to all low solar phase angle lightcurves ever obtained of this asteroid. At phase angles greater than about 10° the observed amplitude becomes greater by up to 0.02 mag. The dark areas so dominate one face of Vesta that a minimum in the lightcurve occurs when the maximum cross-sectional area is visible. Its lightcurve is determined primarily by albedo structure rather than shape, leading to one maximum and one minimum per rotation instead of the expected two of each associated with its triaxial ellipsoid shape.