A NASA spacecraft launched in December 1995 to explore the variability of X-ray sources. It carried the proportional counter array (PCA) to cover the lower part of the energy range, and the high-energy X-ray timing experiment (HEXTE) for the upper energy range. It also carried the all-sky monitor (ASM) that scans about 80% of the sky on every orbit. RXTE was expected to operate for up to five years, but it far exceeded that goal, being decommissioned 16 years later in January 2012. Its many discoveries include the closest black hole to the Earth, 1 600 light years away, detected in February 2000, and the first transient magnetar, a rare class of extremely magnetic neutron star, discovered in January 2004.

RXTE was launched from the Kennedy Space Center by a Delta II rocket that placed it into a low Earth orbit of 580 km from the Earth. The mission is managed by the Goddard Space Flight Center.

RXTE was named the X-Ray Timing Explorer when launched, and the Rossi name was added in 1996 to honour Professor Bruno Rossi of the Massachusetts Institute of Technology who, with his colleagues, discovered the first non-solar source of X-rays in 1963.