Explorer 49 (also called Radio Astronomy Explorer-2, RAE-B) was a NASA 328 kg (723 lb) satellite launched on 10 June 1973, for long wave radio astronomy research. It had four 230 m (750 ft) X-shaped antenna elements, which made it one of the largest spacecraft ever built. The Radio Astronomy Explorer B (RAE-B) mission was the second of a pair of RAE satellites. It 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. The experiment complement consisted of two Ryle-Vonberg radiometers (nine channels each), three swept-frequency burst receivers (32 channels each), and an impedance probe for calibration.

Wikipedia: https://en.wikipedia.org/wiki/Explorer_49. Original timeline Wikipedia link: https://en.wikipedia.org/wiki/Radio_Astronomy_Explorer_2. Related NASA reference: https://science.nasa.gov/mission/explorer-49/. Description adapted from Wikipedia contributors (CC BY-SA 4.0; https://creativecommons.org/licenses/by-sa/4.0/).