Gravity and Extreme Magnetism Small Explorer (GEMS or SMEX-13) mission was a NASA space observatory mission. The main scientific goal of GEMS was to be the first mission to systematically measure the polarization of X-ray sources. GEMS would have provided data to help scientists study the shape of spacetime 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 potential cost overruns due to delays in developing the technology and never moved into the development phase.
GEMS was managed by the NASA Goddard Space Flight Center (GSFC). The project was an astrophysics program reporting to NASA's Science Mission Directorate (SMD) in Washington, D.C.
Cancelled missions can be reinstated - for example, NuSTAR was cancelled in 2006, but reinstated a year lat