It was the first of NASA's three High Energy Astronomy Observatories, launched on August 12, 1977 aboard an Atlas rocket with a Centaur upper stage, operated until 9 January 1979.

[2] It was designed, operated, and managed at the Naval Research Laboratory (NRL) under the direction of Principal Investigator Dr. Herbert D. Friedman, and the prime contractor was TRW.

[3] The A2, or Cosmic X-ray Experiment (CXE), from the Goddard Space Flight Center, covered the 2–60 keV energy range with high spatial and spectral resolution.

[4] The A3, or Modulation Collimator (MC) instrument, provided high-precision positions of X-ray sources, accurate enough to permit follow-up observations to identify optical and radio counterparts.

[7] Each detector was actively shielded by surrounding CsI scintillators, in active-anti-coincidence, so that an extraneous particle or gamma-ray event from the side or rear would be vetoed electronically, and rejected.

The largest, or High Energy Detector (HED), occupied the central position and covered the upper range from ~120 keV to 10 MeV, with a field-of-view (FOV) collimated to 37° FWHM.

Results of the experiment included a catalog of the positions and intensities of hard X-ray (10–200 keV) sources,[8] a strong observational basis for extremely strong magnetic fields (of order 1013 G) on the rotating neutron stars associated with Her X-1[9][10] and 4U 0115+634, a definitive diffuse component spectrum between 13 and 200 keV, discovery of the power-law shape of the Cygnus X-1 power density spectrum, and discovery of slow intensity cycles in the X-Ray sources SMC X-1 and LMC X-4, resulting in approximately 15 Ph.D theses and ~100 scientific publications.