Explorer 49


Explorer 49 was the final U.S. lunar mission for 21 years until Clementine in 1994.

The spacecraft, part of a duo of Radio Astronomy Explorer (RAE) missions (the other being Explorer 48), was designed to conduct comprehensive studies of low frequency radio emissions from the Sun, the Moon, the planets, and other galactic and extragalactic sources, while in a circular orbit around the Moon. Its location was driven by the need to avoid terrestrial radio interference.

After launch on a direct ascent trajectory to the Moon and one midcourse correction June 11, 1973, Explorer 49 fired its insertion motor on 07:21 UT June 15 to enter orbit around the Moon. Initial orbital parameters were 829 x 698 miles (1,334 × 1,123 kilometers) at 61.3 degree inclination.

On June 18, 1973, the spacecraft jettisoned its main engine and, using its velocity control propulsion system, circularized its orbit to about 661 x 654 miles (1,063 × 1,052 kilometers) at a 38.7-degree inclination.

The spacecraft was the largest human-made object to orbit the Moon with its deployed antennas measuring about 1,500 feet (457.2 meters or nearly half a kilometer!) tip-to-tip. These antennas, as well as a 630-feet (192-meter) long damper boom and a 120-foot (36.6-meter) dipole antenna, were all stored away on motor-driven reels which allowed them to unfurl in lunar orbit.

Once in lunar orbit, the spacecraft deployed its various antennae in stages, assuming its full form by November 1974. During its mission, Explorer 49 studied low-frequency radio emissions from the solar system (including the Sun and Jupiter) and other galactic and extragalactic sources.

NASA announced the completion of the mission in June 1975 although contact was maintained until August 1977.


Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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