Explorer 49


Explorer 49 was to be placed into lunar orbit in order to be able to use the Moon as a shield against radio interference from Earth as it listened for radio emissions from the rest of our solar system, the galaxy and beyond.


The spacecraft achieved lunar orbit and was used successfully to study low-frequency radio emissions from sources including the Sun, Jupiter, the galaxy and extra-galactic sources.

10 Jun 1973: Launch

15 Jun 1973: Lunar Orbit Insertion

Mission Type: Orbiter

Launch Vehicle: Delta 1913 (no. 95 / Thor no. 581)

Launch Site: Eastern Test Range / launch complex 17B, Cape Canaveral, USA

NASA Center: Goddard Space Flight Center

Spacecraft Mass: about 330 kg at launch, 200 kg in lunar orbit (after solid braking motor was ejected)

Spacecraft Instruments:

  1. galactic studies experiment
  2. sporadic low-frequency solar radio bursts experiment
  3. sporadic Jovian bursts experiment
  4. radio emission from terrestrial magnetosphere experiment
  5. cosmic source observation experiment

Spacecraft Dimensions: Radio-antenna array: 183 meters from tip to tip. Main body: truncated cylinder 92 cm in diameter, about 79 cm high

Spacecraft Power: solar panels which charged six nickel-cadmium batteries

Maximum Power: 38.3 W

Antenna Diameter: 183 meters (would have been 457 m if fully extended)


Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi
National Space Science Data Center, http://nssdc.gsfc.nasa.gov/
Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

After launch on a direct-ascent trajectory to the Moon and one midcourse correction on 11 June, Explorer 49 fired its insertion motor at 07:21 UT on 15 June to enter orbit around the Moon. Initial orbital parameters were 1,334 x 1,123 kilometers at 61.3? inclination. On 18 June, the spacecraft jettisoned its main engine and, using its Velocity Control Propulsion System, circularized its orbit.

The spacecraft, with a partially deployed radio-antenna array measuring 183 meters from tip to tip, was the largest spacecraft in physical dimensions to enter lunar orbit. Although the antennas did not deploy to full length, the mission goals were not affected. During its mission, Explorer 49 studied low-frequency radio emissions from the solar system (including the Sun and Jupiter) and other Galactic and extra-galactic sources. It was placed in lunar orbit to avoid terrestrial radio interference. NASA announced completion of the mission in June 1975. Last contact was in August 1977.

This was the last spacecraft the U.S. sent to the Moon for 21 years (until Clementine in 1994).

Additional Resources

Goddard Space Flight Center's Explorers Program

NSSDC Master Catalog: Explorer 49

Basics of Radio Astronomy

Summary of Lunar Exploration Missions

Far Travelers: The Exploring Machines

Solar System News