Explorer 49

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Explorer 49 (also called RAE-B) was a 328 kilogram weighing satellite launched on June 10th, 1973 for radio astronomy research in the long wave range. It had four 230-meter long X-shaped antenna elements, which made it to one of the largest spacecrafts ever built.

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 made for some interesting problems for the mission scientists.

Explorer 49 was launched after the termination of the Apollo Program, so it became the last USA lunar mission (although it did not examine the moon directly), until the launch of Clementine spacecraft in 1994.