Goddard's GEMS proposal is part of NASA's Explorer program. The proposal was submitted in response to NASA's Announcement of Opportunity for Small Explorers (SMEX) and Missions of Opportunity issued on September 28, 2007.
NASA Goddard will be responsible for the GEMS instrument and the overall program management. Orbital Sciences Corporation, Dulles, Va., will be responsible for building the spacecraft and mission operations. ATK Space, Goleta, Calif., will build a boom to place the X-ray telescopes the proper distance from the detectors. NASA's Ames Research Center, Moffett Field, Calif., will assist the project by providing science co-investigators and education and public outreach support
The University of Iowa will provide instrument calibration assistance, and will have students prepare an experiment that could be part of the mission. GEMS includes collaborators from universities including MIT, Johns Hopkins University, Cornell University, Rice University, the University of Oulu (Finland), North Carolina State University and Washington University.
GEMS mission operations uses simple and robust operational concept. It features once a day downlink and a once a week uplink, utilizes existing facilitiesprocesses, procedures and ground system located at Orbital Sciences Corporation in Dulles, VA for the Mission Operations Center (MOC), along with an existing Science Operations Space Flight Center. Spacecraft communication contacts are carried out through the Near Earth Network.
After the 30-day checkout is complete, the GEMS baseline operational phase continues for nine months. During this time, GEMS measures the polarization of x-ray emissions from cosmic x-ray sources spanning about three orders of magnitude in brightness. Observation durations are calculated to produce useful limits on the polarization of the x-ray flux. Observations last from hours to weeks depending on the brightness of the source. The observatory rotates at 0.1 rpm to measure and remove systematic errors in the polarization measurements. Targets are chosen within a 60 degree band normal to the sun to maintain a worst-case 30 degree sun incidence angle to the solar array. Measurements do not take place during parts of the orbit when the target is occulted behind the Earth or the observatory passes through the South Atlantic Anomaly (SAA), resulting in an estimated 50% observing efficiency.