Discoverer 21

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]

Discoverer 21

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 (1961-02-18UTC22:58Z) 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 (1962-04-21)
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
Discoverer ← Discoverer 20 Discoverer 22 → Midas ← RM-1 Midas 3 →

The launch of Discoverer 21

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.