Gravity and Extreme Magnetism SMEX
Artist's impression of GEMS.
|Major contractors||Orbital Sciences
|Launched from||Kennedy Space Center
|Launch vehicle||Pegasus Class TBD|
|Mission length||9 months|
|Mass||288.7 kg (636 lb)|
|Location||575 km from Earth, 28.5 degree inclination, circular
(Low Earth orbit)
|X-ray Polarimeter Instrument (XPI)||Two co-aligned telescopes|
The Gravity and Extreme Magnetism SMEX (GEMS) mission was a planned space observatory. The main scientific goal of GEMS is to be the first mission to systematically measure the polarization of cosmic X-Ray sources. GEMS would have provided data to help scientists study the shape of space that has been distorted by a spinning black hole's gravity and the structure and effects of the magnetic fields around Neutron stars. It was cancelled by NASA in June 2012 for cost overruns.
GEMS was managed by the National Aeronautics and Space Administration NASA, the United States space agency, at the Goddard Space Flight Center. The project was an astrophysics program reporting to NASA's Science Mission Directorate (SMD) in Washington, D.C..
The spacecraft would have been launched on in July 2014 on a nine-month mission with a possible 15 month extension for a guest observer phase; but the mission was terminated at the Confirmation Review stage on May 10, 2012 due to expected cost overruns.
Cancelled missions can be reinstated— for example NuSTAR was cancelled in 2006, but reinstated a year later and launched in June 2012. However, NuSTAR was not cancelled due to project overruns, but rather due to changes in the overall NASA budget, so the circumstances for cancellation were very different.
The GEMS X-ray telescope was designed to indirectly measure the regions of distorted space around spinning black holes through a measurement of the polarization of X-rays emitted. It will thereby probe the structure and effects of the magnetic field around magnetars and dead stars with magnetic fields trillions of times stronger than Earth's.
GEMS could reveal:
- How spinning black holes affect space-time and matter as they are drawn in and compressed by strong gravitational fields.
- What happens in the super strong magnetic fields near pulsars and magnetars.
- How cosmic rays are accelerated by shocks in supernova remnants.
Current missions cannot do this because the required Angular resolution is limited and magnetic fields are invisible.
The detector in GEMS would have been a small chamber filled with gas. When an X-ray is absorbed in the gas, an electron carries off most of the energy, and starts out in a direction related to the polarization direction of the X-ray. This electron loses energy by ionizing the gas; the instrument measures the direction of the ionization track, and thereby the polarization of the X-ray. The GEMS detector readout will employ a time projection chamber to image the track. The GEMS instrument is about 100 times more sensitive than previous X-ray polarization experiments.
Mission costs were be capped at US$105 million (in Fiscal Year 2008 dollars), excluding the launch vehicle, but an independent confirmation review board at NASA claimed it would grow to an estimated $150M, leading to cancellation of the mission.
The cancellation of GEMS marks the end of multi-year long binge of cancellations and attempted cancellations of current and future missions: it was the last funded future U.S. space telescope besides JWST. The cancellation of GEMS may also doom the successful Pegasus XL launcher.
Project Status 
GEMS was one of six Small Explorer missions selected in May 2008 for the NASA Small Explorer (SMEX) Program Phase A study. In June 2009, GEMS was chosen to be the second of these missions to go forward into Phase B, starting in October 2010 for a launch in April 2014.
GEMS did not pass a confirmation review conducted on 2012-05-10, which effectively cancelled the project. The project team intends to appeal the cancellation.
NASA on June 7, 2012 announced the cancellation of the GEMS project. The Gravity and Extreme Magnetism Small Explorer mission was supposed to blast off in 2014 to study black holes and neutron stars. But external reviews found the project would likely come in over budget. GEMS was supposed to hold at $119 million, not counting the rocket. NASA's astrophysics director, Paul Hertz, says the technology needed for the instrument took longer to develop than expected, and that drove up the price.
Project and Science Team 
Project Team 
- GSFC will be responsible for the GEMS instrument, system engineering, spacecraft contract, and the overall program management.
- Ames Research Center will provide Co-Investigators and perform the Education and Public Outreach(EPO)support.
- The satellite will be built by Orbital Sciences using its LEOStar-2 TM spacecraft bus design, and will also conduct mission operations, under a US$40 million contract.
- Alliant Techsystems (ATK) wwill build a deployable boom to place the X-ray mirrors at the proper distance from the detectors or polarimeters.
- The University of Iowa will provide instrument calibration assistance and will have students prepare a small instrument that could be part of the mission.
Science Team 
- NASA Goddard Space Flight Center
- NASA Ames Research Center
- University of Iowa
- Massachusetts Institute of Technology
Science Collaborators 
- Johns Hopkins University
- Cornell University
- Rice University
- University of Oulu (Finland)
- North Carolina State University
- Washington University
- "GEMS Project Website". NASA. Retrieved May 22, 2011.
- "NASA Science Mission Directorate GEMS Site". NASA. Retrieved May 22, 2011.
- GEMS, Smithsonian Astrophysical Observatory, accessed 2011-05-23
- W. Graham - Orbital’s Pegasus successfully launches NASA’s NuSTAR spacecraft (June 13th, 2012)
- "Proposed NASA Mission Could Explore Twisted Space Around Black Holes". NASA. 2008-06-30. Retrieved 2009-08-014.
- "NASA Awards Two Small Explorer Development Contracts". NASA. 2009-06-19.
- D. Leone - Demise of GEMS May Cost Orbital 150 Jobs, Doom Pegasus (2012) - Space News
- NASA Selects Small Explorer Investigations for Concept Studies, NASA media release C08-029, 2008-05-29, accessed 2009-08-14
- NASA SMD Astrophysics Program Letter, NASA, 2011-04-28, accessed 2010-05-23
- "NASA To Cancel GEMS X-Ray Telescope". Space News. 4 June 2012.
- NASA kills GEMS X-ray telescope, blames project's cost
- "Orbital To Build New Space Science Satellite To Study X-Ray Polarization". SpaceMart.com. 2009-07-09. Retrieved 2009-08-14.
- GEMS home page, accessed 2009-08-14
Science Instrument Co-Investigator Sites