NEAR Photometry of Asteroid 253 Mathilde

NEAR's Multispectral Imager (MSI) recorded over 300 images of asteroid 253 Mathilde during a flyby on June 27, 1997. Images were acquired at 0.70 μm at solar phase angles from 40° to 136°. These data, combined with telescopic observations at phase angles from 1° to 16° were used to derive a photometric model for Mathilde. The photometric properties of Mathilde were then compared with those of similar small bodies. We find that: (1) Mathilde has a geometric albedo of 0.047±0.005 at 0.55 μm, making it the darkest minor planet yet observed by spacecraft. (2) Mathilde is remarkably homogeneous in reflectance across the surface. Normal reflectance ranges from 0.041 to 0.053, a distribution at the low end of the range of reflectance measurements of carbonaceous chondrites (0.03–0.11) and in the middle of the range of albedo measurements of C-type asteroids (0.03–0.06). (3) Typical reflectance contrasts on Mathilde occur at the level of ±6% from the mean, as compared to Phobos which exhibits variation at the level of ±20% from the mean. (4) As a whole, Mathilde is more backscattering than Phobos, and Mathilde may have a less porous regolith or a different distribution of regolith grain sizes than Phobos.