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.

GEMS could reveal: Current missions cannot do this because the required angular resolution is limited and magnetic fields are invisible.

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 mission was supposed to launch in July 2014 to study black holes and neutron stars, but external reviews found the project would likely exceed its budget.

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.

[3] The GEMS principal investigator was Dr Jean H. Swank, of NASA's Goddard Space Flight Center, Greenbelt, Maryland.