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 mu m at solar phase angles from 40 degrees to 136 degrees. These data, combined with telescopic observations at phase angles from 1 degrees to 16 degrees were used to derive a photometric model for Mathilde, The photometric prope rties 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 mu m, making it the darkest minor planet yet observed by spacecraft. (2) Ma thilde is remarkably homogeneous in reflectance across the surface. Normal reflectance ranges from 0.041 to 0.053, a distribution at the low end of th e 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 leve l 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 backsc attering than Phobos, and Mathilde may have a less porous regolith or a dif ferent distribution of regolith grain sizes than Phobos. (C) 1999 Academic Press.