Thermal Design and Test of the Gravity Recovery and Interior Laboratory (GRAIL) Ka-Band Ranging (KBR) Payload

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) mission is a part of the NASA Discovery Program and is managed by California Institute of Technology’s Jet Propulsion Laboratory. Scheduled to launch aboard a Delta-2 l aunch vehicle on September 8th, 2011, GRAIL will map the lunar gravitational field in unprecedented detail for 90 days. Gathered measurements will allow scientists to better unders tand the evolution of our Moon as well as the composition of the lunar core. GRAIL will accomplish these science objectives by accurately measuring the distance between two co-orbiting spacecraft spaced by approximately 100km. The GRAIL mission primary payload accomplishes this by means of a Ka-Band Ranging (KBR) RF Horn. Measurement time between the two orbiters is synchronized using an S-Band link. Doppler ranging of the two orbiters from Earth also places both orbiters within the same reference frame. Changes in the local gravitational field will yield a change in the distance between t he two orbiters. By accurately recording this change in distance, a detailed gravitational m ap can be resolved. Given the sensitivity of these measurements, GRAIL’s science quality depends on minimizing thermal, structural, and mass perturbations as well as being able to qua ntify other “small forces” such as the reradiation of thermal energy and the out-gassing of materials. Reducing these perturbations is particularly challenging for lunar spacecraft due to intense changes in IR-heating into and out of eclipse. This manuscript describes the luna r environmental assumption, KBR thermal design, analysis, and test program used to successfully meet these science requirements using only passive thermal control. A “small forces” analysis was also performed to better understand how the re-radiation of thermal energy from spacecraft external surfaces would affect the quality science measurement.