Photometric stability of the lunar surface

The rate at which cratering events currently occur on the Moon is considered inlight of their influence on the use of the Moon as a radiometric standard. Theradiometric effect of small impact events is determined empirically from the studyof Clementine images. Events that would change the integral brightness of the moonby 1% are expected once per 1.4 Gyr. Events that cause a 1% shift in one pixel forlow Earth-orbiting instruments with a 1-km nadir field of view are expectedapproximately once each 43 Myr. Events discernible at 1% radiometric resolutionwith a 5 arc-sec telescope resolution correspond to crater diameters ofapproximately 210 m and are expected once every 200 years. These rates areuncertain by a factor of two. For a fixed illumination and observation geometry,the Moon can be considered photometrically stable to 1 x [10.sup.-8] per annumfor irradiance, and 1 x [10.sup.-7] per annum for radiance at a resolution commonfor spacecraft imaging instruments, exceeding reasonable instrument goals by sixorders of magnitude.