|Mission type||Earth science|
|Launch mass||98.9 kg (218 lb)|
|Start of mission|
|Launch date||29 November 1965, 04:48UTC|
|Rocket||Thor SLV-2 Agena B|
|Launch site||Vandenberg SLC-2E|
|End of mission|
|Last contact||15 January 1971|
|Perigee altitude||505 kilometers (314 mi)|
|Apogee altitude||2,978 kilometers (1,850 mi)|
|Epoch||29 November 1965|
Explorer 31 (also called DME-A) was an American satellite launched as part of the Explorers program of NASA. Explorer 29 was launched on 29 November 1965 from Vandenberg Air Force Base, California, United States, with Thor Agena rocket. Explorer 31 was released along with the Canadian satellite Alouette 2.
Explorer 31 was a small ionospheric observatory instrumented to make direct measurements of selected ionospheric parameters at the spacecraft. Since the spacecraft had no tape recorder, data could be observed at the spacecraft only when the spacecraft was in sight of the telemetry station and when commanded on. Experiments were operated either simultaneously or sequentially, as desired. The satellite was spin-stabilized with the spin axis perpendicular to the orbit plane. The spin rate and spin axis were controlled by an onboard magnetic torquing system. The attitude and spin rate information were observed by a sun sensor and a three-axis magnetometer.
Satellite performance was satisfactory except for a partial power failure in May 1966, which reduced data acquisition time to about half the nominal amount. Some difficulties were encountered in obtaining attitude information that was necessary for the reduction of the experiment observations. On July 1, 1969, the satellite data observations were terminated with five of the seven experiments operating. Responsibility for standby monitoring of the satellite was given to the ESSA telemetry station at Boulder, Colorado, on July 8, 1969. During this standby operation, experiment data were collected only once on October 1, 1969, for 9 min from the electrostatic probe for use in studying a red arc event. On January 15, 1971, no response was received from a variety of satellite commands, and the satellite was abandoned.