Discoverer 21

{{short description|Reconnaissance satellite}}

{{Infobox spaceflight

| name = Discoverer 21

| image =

| image_caption =

| mission_type = Technology

| operator = US Air Force/ARPA

| Harvard_designation = 1961 Zeta 1

| SATCAT =

| mission_duration =

| spacecraft =

| spacecraft_type =

| spacecraft_bus = Agena-B

| manufacturer = Lockheed

| launch_mass = {{convert|1110|kg}}

| launch_date = {{start-date|18 February 1961, 22:58|timezone=yes}} UTC

| launch_rocket = Thor DM-21 Agena-B 261

| launch_site = Vandenberg LC-1 launch pad 75-3-5

| decay_date = {{end-date|20 April 1962}}

| orbit_epoch =

| orbit_reference = Geocentric

| orbit_regime = Low Earth

| orbit_periapsis = {{convert|243|km|mi|sp=us}}

| orbit_apoapsis = {{convert|1026|km|mi|sp=us}}

| 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

}}

File:Thor Agena B with Discoverer 21 (Feb. 18, 1961).gif

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.{{cite web|url=http://space.skyrocket.de/doc_sdat/rm-1.htm|title=RM 1, 2 (Discoverer 19, 21)|first=Gunter|last=Krebs|publisher=Gunter's Space Page|accessdate=30 June 2010}}

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.{{cite web|url=http://planet4589.org/space/log/launchlog.txt|title=Launch Log|first=Jonathan|last=McDowell|author-link=Jonathan McDowell|publisher=Jonathan's Space Page|accessdate=30 June 2010}} 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 {{convert|243|km}}, an apogee of {{convert|1026|km}}, 80.7 degrees of inclination, and a period of 97.4 minutes.{{cite web|url=http://planet4589.org/space/log/satcat.txt|title=Satellite Catalog|first=Jonathan|last=McDowell|publisher=Jonathan's Space Page|accessdate=30 June 2010}} The satellite had a mass of {{convert|1110|kg}},{{cite web|url=http://www.astronautix.com/craft/midas.htm|title=Midas|first=Mark|last=Wade|publisher=Encyclopedia Astronautica|accessdate=30 June 2010|url-status=dead|archiveurl=https://web.archive.org/web/20101120040549/http://astronautix.com/craft/midas.htm|archivedate=20 November 2010}} and was used to demonstrate an engine restart,{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-006A|title=Discoverer 21|work=NSSDC Master Catalog|publisher=NASA|accessdate=30 June 2010}} and to test infrared sensors for the Midas programme. It remained in orbit until 20 April 1962, when it decayed and reentered the atmosphere.

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