The mesosphere is a layer of the atmosphere extending from the top of the stratosphere to an altitude of about 80 km (50 mi).

The spacecraft carried five instruments to measure ozone, water vapor, and incoming solar radiation.

[1][2] Explorer 64 studied the processes that create and destroy ozone in the Earth's mesosphere.

[3] Managed for NASA by the Jet Propulsion Laboratory, the Solar Mesosphere Explorer was built by Ball Space Systems and operated by the Laboratory for Atmospheric and Space Physics of the University of Colorado Boulder.

[3] Characteristics:[1] Launched on 6 October 1981, on a Thor-Delta 2310 from Vandenberg Air Force Base, in California, the satellite returned data until 4 April 1989.