MISSION CONCEPTS FOR THE SUPERCONDUCTING GRAVITY GRADIOMETER

The Superconducting Gravity Gradiometer (SGG), which can measure gravi tational field fluctuations to unprecedented precisions, is currently being evaluated for use in an Earth-orbiting platform. Two missions ar e planned: a Shuttle-based demonstration flight and a dedicated gravit y surveying mission. The Shuttle mission will feature a fully function al, self-contained instrument with an intrinsic noise level of 10(-3) E Hz(-1/2) where E is defined at 10(-9) m s(-2) m(-1). Calibration har dware, residual error correction techniques, and surface tension devic es which control liquid helium motion in the dewar will be tested. For a dedicated free-flier gravity mission, two concepts have been studie d. Both have been formulated to achieve low cost while maintaining an instrument sensitivity of 10(-3) E Hz(-1/2). One concept uses the spac ecraft developed for the Sub-mi Iii meter Wave Astronomical Satellite (SWAS), and would be in a polar orbit, decaying from an initial altitu de of 380 km. A mission lifetime of 6-9 months is planned. The other c oncept is based on a re-usable SPARTAN 400 carrier which would be depl oyed from, and retrieved by, the Shuttle. The carrier can be periodica lly reboosted and would maintain an altitude between 275 and 350 km. T he mission lifetime is restricted to about 3 months, and the orbit inc lination is limited to 57 degrees, the maximum the Shuttle can reach. Since the carrier is retrieved, the instrument could be periodially re flown. The free-flier missions, including experiment and spacecraft de sign, are described.