NEAR Encounter with Asteroid 253 Mathilde: Overview

On June 27, 1997, the NEAR spacecraft carried out the first-ever encounter with a C-type asteroid, flying by 253 Mathilde at a distance of 1212 km. We summarize findings derived from 330 images obtained by NEAR's MSI camera which cover about 60% of the surface of the asteroid. The highest resolution achieved was about 160 m/pixel. Mathilde is a low-reflectance object (geometric albedo = 0.047) with principal diameters of 66 × 48 × 44 km. The mean radius of 26.4 ± 1.3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data. Mathilde's surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The two largest, Ishikari and Karoo, have diameters of 29.3 and 33.4 km, respectively. No evidence of layering is exposed in the crater walls, but suggestions of downslope movement are present. The surface density of craters in the diameter range from 0.5 to 5 km is close to equilibrium saturation, a situation in which as many craters are being destroyed as are being produced. Observed depth-to-diameter ratios for craters in this size range are close to those observed on the lunar surface. A disruption lifetime of about 4 billion years has been estimated for Mathilde. Based on the mass determination obtained from Doppler tracking (D. K. Yeomans et al., 1997, Science 278, 2106-2109) and the volume derived from MSI images, the average density of Mathilde is remarkably low: 1.3 ± 0.3 g/cm³, a value consistent with a rubble pile structure for the interior. Assuming that Mathilde's rock type is similar to that found in CM meteorites, the porosity of the interior must be some 50%. Shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by Mathilde's underdense interior, a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events. Except for the lower geometric albedo (0.047 ± 0.005), the photometric properties of Mathilde are closely similar to those of Phobos. The surface is extremely homogeneous in terms of both color or albedo: specifically, no color or albedo variations associated with craters have been identified.