| Discoverer 21 | |
| Mission Type: | Technology |
| Operator: | US Air Force/ARPA |
| Harvard Designation: | 1961 Zeta 1 |
| Spacecraft Bus: | Agena-B |
| Manufacturer: | Lockheed |
| Launch Date: | UTC |
| Launch Rocket: | Thor DM-21 Agena-B 261 |
| Launch Site: | Vandenberg LC-1 launch pad 75-3-5 |
| Orbit Reference: | Geocentric |
| Orbit Regime: | Low Earth |
| Orbit Periapsis: | 243km (151miles) |
| Orbit Apoapsis: | 1026km (638miles) |
| Orbit Inclination: | 80.7 degrees |
| Orbit Period: | 97.4 minutes |
| Apsis: | gee |
| Programme: | Discoverer |
| Previous Mission: | Discoverer 20 |
| Next Mission: | Discoverer 22 |
| Programme2: | Midas |
| Previous Mission2: | RM-1 |
| Next Mission2: | Midas 3 |
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, an apogee of, 80.7 degrees of inclination, and a period of 97.4 minutes.[3] The satellite had a mass of,[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.