Small Spacecraft Design for the GRAIL Mission

On September 10, 2011, two identical spacecraft were launched from the Kennedy Space Center Space Launch Complex-17B on their 4-month, low-energy trajectory to the moon. The primary objective of the Gravity Recovery and Interior Laboratory (GRAIL) mission was to collect a global gravity map of the moon with a resolution approximately 1000 times better than existing knowledge. Lockheed Martin had the responsibility of designing, developing, assembling, testing, launching, and operating the twin spacecraft. With a dry mass of 200.6 kg each, these GRAIL spacecraft were among the lightest ever to be selected for a NASA Discovery-class mission. This paper discusses some of key trade studies performed and the resulting design features of these two small spacecraft. Among the areas of discussion are the following: • Spacecraft architecture and its significant heritage from Experimental Small Satellite #11 (XSS-11) • Increasing the delta-v capability required for the lunar orbit insertion • Solar array sizing for the science collection phase • Mounting position for the Ka-band antenna, one of the key components of the science instrument • Launch configuration trade study: stacked design vs. side-by-side design • Spacecraft similarity trade study: mirror image buses vs. identical buses with a rotated science orientation • Limited redundancy approach and its associated fault protection This paper also discusses some of the on-orbit performance during GRAIL’s primary mission including: • Spacecraft performance and anomalies • Science results from the primary mission • Analysis performed to justify and gain approval for a 6-month extended mission Not only have the GRAIL spacecraft returned a wealth of scientific data, but they paved the way for future Lockheed Martin small satellite applications including an entry in NASA’s Rapid Spacecraft Development Office (RSDO) catalog