Determining shape of a seasonally shadowed asteroid using stellar occultation imaging

Abstract

A key objective in exploration of small, asteroidal bodies is to determine global shape and volume. The accuracy to which volume can be determined limits determination of bulk density, an important measurement for understanding internal structure. A special case for a rendezvous mission that uses stereo imaging to determine shape is a body with high obliquity encountered near solstice: half of the body is in shadow, and imaging of illuminated terrain alone under-constrains global shape. In this paper we demonstrate the use of stellar occultation imaging to place an upper bound on volume of such a shadowed hemisphere. Thirty-three sets of images of the night side limb of Mercury, acquired by the Mercury Dual Imaging System (MDIS) wide-angle camera (WAC) on MESSENGER, were used to bound the radius of that planet's night side. The maximum radius determined from this limited image set agrees with the actual radius to within ~0.1%. We show, by simulation, expected performance of a campaign of such night side limb images to bound the shape of an irregular, high-obliquity asteroid encountered at solstice. We assumed a body the size and shape of Deimos imaged from a 40-km radius orbit by an imager having specifications of the MDIS/WAC but an updated detector sensitive to mv~10 stars, and a day-side stereo imaging campaign by a well-calibrated camera system. From an equatorial orbit, with one hemisphere in shadow, a campaign of ≥150 night side limb images determines volume of the shadowed hemisphere to ~4 to 6% accuracy. Increasing orbital inclination to improve sampling of high latitudes decreases residuals for the dark hemisphere by 2 to 3%, for the same number of images. A ~2 to 3% uncertainty in global volume – from stereo imaging of illuminated terrain and stellar occultation imaging of shadowed terrain – compares favorably to uncertainty of up to ±25% in the absence of direct measurements of the radius of the shadowed hemisphere.