| Mission type | Technology |
|---|---|
| Operator | US Air Force/ARPA |
| Harvard designation | 1961 Zeta 1 |
| COSPAR ID | 1961-006A |
| SATCAT no. | 00084 |
| Spacecraft properties | |
| Bus | Agena-B |
| Manufacturer | Lockheed |
| Launch mass | 1,110 kilograms (2,450 lb) |
| Start of mission | |
| Launch date | 18 February 1961, 22:58 UTC |
| Rocket | Thor DM-21 Agena-B 261 |
| Launch site | Vandenberg LC-1 launch pad 75-3-5 |
| End of mission | |
| Decay date | 20 April 1962 |
| Orbital parameters | |
| Reference system | Geocentric |
| Regime | Low Earth |
| Perigee altitude | 243 kilometers (151 mi) |
| Apogee altitude | 1,026 kilometers (638 mi) |
| Inclination | 80.7 degrees |
| Period | 97.4 minutes |
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Discoverer 21, also known as RM-2, was an American satellite which was launched in 1961. It was a technology demonstration spacecraft, based on an Agena-B.[1]
The launch of Discoverer 21 occurred at 22:58 UTC on 18 February 1961. A Thor DM-21 Agena-B rocket was used, flying from launch pad 75-3-5 at the Vandenberg Air Force Base.[2] Upon successfully reaching orbit, it was assigned the Harvard designation 1961 Zeta 1.
Discoverer 21 was operated in a low Earth orbit, with a perigee of 243 kilometres (151 mi), an apogee of 1,026 kilometres (638 mi), 80.7 degrees of inclination, and a period of 97.4 minutes.[3] The satellite had a mass of 1,110 kilograms (2,450 lb),[4] and was used to demonstrate an engine restart,[5] and to test infrared sensors for the Midas programme.[4] It remained in orbit until 20 April 1962,[3] when it decayed and reentered the atmosphere.
