Explorer 21

Explorer 21
	Explorer 21 satellite
Names	IMP-B; IMP-2; Interplanetary Monitoring Platform-2
Mission type	Space physics
Operator	NASA
COSPAR ID	1964-060A
SATCAT no.	00889
Spacecraft properties
Bus	IMP
Manufacturer	Goddard Space Flight Center
Launch mass	138 kg (304 lb)
Power	4 deployable solar arrays and batteries
Start of mission
Launch date	4 October 1964, 03:45:00 GMT
Rocket	Thor-Delta C; (Thor 392 / Delta 026)
Launch site	Cape Canaveral, LC-17A
Contractor	Douglas Aircraft Company
Entered service	4 October 1964
End of mission
Last contact	13 October 1965
Decay date	1 January 1966
Orbital parameters
Reference system	Geocentric orbit
Regime	Highly elliptical orbit
Perigee altitude	917 km (570 mi)
Apogee altitude	94,288 km (58,588 mi)
Inclination	33.70°
Period	2080.00 minutes
Instruments
Instruments
	Cosmic-Ray Range versus Energy Loss
	Cosmic Rays
	Faraday Cup
	Fluxgate Magnetometers
	Ion Chamber and Geiger–Müller Counters
	Retarding Potential Analyzer
	Solar Wind Protons
	Explorers program ← Explorer 20 Explorer 22 → Interplanetary Monitoring Platform ← IMP-A IMP-C →

Explorer 21, also called IMP-B, IMP-2 and Interplanetary Monitoring Platform-2, was a NASA satellite launched as part of Explorer program. Explorer 21 was launched on 4 October 1964, at 03:45:00 GMT from Cape Canaveral (CCAFS),^[5] Florida, with a Thor-Delta C launch vehicle. Explorer 21 was the second satellite of the Interplanetary Monitoring Platform, and used the same general design as its predecessor, Explorer 18 (IMP-A), launched the previous year, in November 1963. The following Explorer 28 (IMP-C), launched in May 1965, also used a similar design.^[6]

^ "Jonathan's Space Report". 21 July 2021. Retrieved 7 November 2021.
^ "Letter dated 22 December 1964 from the Permanent Representative of the United States of America addressed to the Secretary-General" (PDF). COMMITTEE ON THE PEACEFUL USES OF OUTER SPACE (64-28156). Retrieved 5 December 2022.
^ "NASA'S YEAR". Flight International. Retrieved 9 June 2018.
^ ^a ^b Cite error: The named reference Trajectory was invoked but never defined (see the help page).
^ "Explorer-21 Satellite Falls Far Short Of Orbital Goal". The Palm Beach Post. Vol. 56, no. 202. UPI. 5 October 1964. p. 8. Archived from the original on 25 January 2024. Retrieved 9 June 2018 – via Newspapers.com.
^ "Explorer-series reference images". Retrieved 4 July 2021.

[jonathan-1] "Jonathan's Space Report". 21 July 2021. Retrieved 7 November 2021.

[2] "Letter dated 22 December 1964 from the Permanent Representative of the United States of America addressed to the Secretary-General" (PDF). COMMITTEE ON THE PEACEFUL USES OF OUTER SPACE (64-28156). Retrieved 5 December 2022.

[3] "NASA'S YEAR". Flight International. Retrieved 9 June 2018.

[Trajectory-4] Cite error: The named reference Trajectory was invoked but never defined (see the help page).

[5] "Explorer-21 Satellite Falls Far Short Of Orbital Goal". The Palm Beach Post. Vol. 56, no. 202. UPI. 5 October 1964. p. 8. Archived from the original on 25 January 2024. Retrieved 9 June 2018 – via Newspapers.com.

[explorer-series-images-6] "Explorer-series reference images". Retrieved 4 July 2021.

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