GEMS

Gravity and Extreme Magnetism Small Explorer

Mission Profile

GEMS (Gravity and Extreme Magnetism Small Explorer) was a proposed NASA SMEX-class X-ray polarimetry mission that competed in the Explorer program selection process, designed to measure the polarization of X-rays from black holes, neutron stars, and supernova remnants to probe the structure of magnetic fields and the geometry of emission regions in extreme astrophysical environments. The mission was not selected in its competed round, but the scientific objectives it embodied — X-ray polarimetry of compact objects — were ultimately realized by the IXPE mission launched in 2021, which has produced the groundbreaking results GEMS originally proposed to achieve.