After 16 years in space, NASA's Rossi X-ray Timing Explorer made its last observation on January 4, 2012. Rossi opened a new window into the workings of neutron stars and black holes. Using its data, astronomers established the existence of highly magnetized neutron stars (known as magnetars) and discovered the first accreting millisecond pulsars. The observatory also provided the first observational evidence of "frame-dragging" in the vicinity of a black hole, an effect predicted by Einstein's general theory of relativity. The 7,000-pound satellite is expected to re-enter the atmosphere between 2014 and 2023.