The Solar Mesosphere Explorer (also known as Explorer 64) was a 1980s NASA spacecraft to investigate the processes that create and destroy ozone in Earth's upper atmosphere. 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.

Explorer 64 studied the processes that create and destroy ozone in the Earth's mesosphere. Over its 7.5 years mission, SME measured ultraviolet solar flux, ozone density, and the density of other molecules important to the understanding of ozone chemistry. During the mission over one hundred undergraduate and graduate students were involved in nearly every aspect of SME operations, including planning and scheduling spacecraft and science activities, controlling the spacecraft and its ground support system, and analyzing spacecraft subsystem performance.

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.

Characteristics:

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.

The spacecraft reentered Earth's atmosphere on 5 March 1991.

Explorer program