Explorer 49
Artist's impression of Explorer 49 in orbit around the Moon
Mission typeRadio astronomy
COSPAR ID1973-039A
SATCAT no.6686
Mission duration4 years and 2 months (launch date to last contact)
Spacecraft properties
Launch mass328 kilograms (723 lb)
Start of mission
Launch dateJune 10, 1973, 14:13:00 (1973-06-10UTC14:13Z) UTC
RocketDelta 1913[2]
Launch siteCape Canaveral LC-17B[2]
End of mission
Last contactAugust 1977 (1977-09)[3]
Orbital parameters
Reference systemSelenocentric
Periselene altitude1,123 kilometers (698 mi)
Aposelene altitude1,334 kilometers (829 mi)
Inclination61.3 degrees
EpochJune 15, 1973
Lunar orbiter
Orbital insertionJune 15, 1973, 07:21 UTC

Explorer 49 (also called Radio Astronomy Explorer-B (RAE-B)) was a 328-kilogram (723 lb) satellite launched on June 10, 1973, for long wave radio astronomy research. It had four 230-metre-long (750 ft) X-shaped antenna elements, which made it one of the largest spacecraft ever built.[2]


Explorer 49 was launched after the termination of the Apollo program, and although it did not examine the Moon directly, it became the last American lunar orbital mission until the launch of Clementine spacecraft in 1994. It was launched on June 10, 1973, 14:13:00 UTC in the Rocket Delta 1913 from the Launch site Cape Canaveral LC-17B.


This mission was the second of a pair of Radio Astronomy Explorer (RAE) satellites, Explorer 38 or RAE-A being the first. Explorer 49 was placed into lunar orbit to provide radio astronomical measurements of the planets, the Sun, and the galaxy over the frequency range of 25 kHz to 13.1 MHz. Since the spacecraft's design used gravity gradient booms, the lumpy lunar gravity field was a problem for the mission scientists.


Explorer 49 was placed in lunar orbit to record radio measurements from 25 kHz to 13.1 MHz of the Milky Way galaxy. Explorer 49 was placed in to lunar orbit so that radio waves from Earth would not be as big of an interference as Explorer 38 had discovered.[4]


The principal investigator for all the experiments was Dr. Robert G. Stone.

  1. Step Frequency Radiometers: Explorer 49 was equipped with two Ryle-Vonberg (RV) receivers one on the upper V-antenna and the other on the lower V-antenna. The radiometers were designed to measure at nine frequencies from .45 to 9.18 MHz.[5]
  2. Rapid-Burst Receivers: Explorer 49 had three rapid burst receivers, one on the upper V-antenna one the lower V-antenna and the third on the middle dipole antenna. The Burst Receivers were composed of a pair of IF Amplifiers and detectors one of the pair was used for back up. They detected on 32 channels between 25 kHz and 13.1 MHz.[6]
  3. Impedance Probe: This was to test the engineering to calibrate the upper V-antenna.[7]


Data were returned to Earth via either a low-power UHF/(400 MHz) transmitter, in real time, or stored in an onboard tape recorder and transmitted to Earth via a high-power UHF transmitter (400 MHz). Two tape recorders provided backup storage.[8]


The third burst receiver on the dipole antenna failed after the first week, and no data resulted from the receiver.[9]

A mechanical flaw in the lower V-antenna caused the leg to only deploy to a length of 183 metres (600 ft) instead of 229 metres (751 ft). This was corrected in November 1974, and the leg extended to the full intended length.[10]

See also


  1. ^ Nick Greene. "Explorer 49 Information". About.com. Retrieved December 25, 2010.
  2. ^ a b c Gunter Dirk Krebs. "Explorer: RAE B". Retrieved December 25, 2010.
  3. ^ Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958–2016. NASA History Program Office, 2018.
  4. ^ http://articles.adsabs.harvard.edu//full/1975A%26A....40..365A/0000368.000.html
  5. ^ https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1973-039A-01
  6. ^ https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1973-039A-02
  7. ^ https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1973-039A-03
  8. ^ https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-039A
  9. ^ http://articles.adsabs.harvard.edu//full/1975A%26A....40..365A/0000371.000.html
  10. ^ https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1973-039A-02