Forty-five years ago this week, the Explorer 35 satellite was launched on a mission to investigate the space environment in the vicinity of the Moon. Among other discoveries, the 230-lb satellite would find that the Moon does not have a magnetosphere like Earth.
Explorer is the longest running satellite program in the history of American spaceflight. Indeed, the first satellite orbited by the United States was Explorer I in January of 1958. Since that time, a total of ninety (90) Explorer satellites have been launched into space for the purpose of investigating the space environments of the Earth, Moon and Sun.
Explorer 35, also known as Anchored Interplanetary Monitoring Platform 6 (AIMP-6), was designed to expand our knowledge concerning a variety of interplanetary space phenomena. The characteristics of the solar wind, interplanetary magnetic field, lunar gravitational field and lunar radiation environment were of particular focus.
Explorer 35 carried an array of scientific instruments to probe the lunar space environment. Key spacecraft instrumentation included multiple magnetometers, geiger tubes, and ion chambers. The spacecraft instrument package also included thermal ion and micrometeoroid detectors as well as a Faraday cup.
On Wednesday, 19 July 1967, Explorer 35 was launched towards the Moon by a Thor-Delta launch vehicle; the 50th such vehicle flown. Lift-off from LC-17B at Cape Canaveral, Florida occurred at 14:19:02 UTC. Flying a direct ascent trajectory, the spacecraft arrived in lunar orbit on Friday, 21 July 1967.
Explorer 35′s orbit about the Moon was highly-elliptical; featuring an apolune of 4,152 nm and a perilune of 432 nm. During its operational lifetime, the satellite found that the Moon has no magnetosphere, that the solar wind impacts the lunar surface directly and that a void in the solar wind exists in the lee of the Moon.
Explorer 35 provided a wealth of scientific data that greatly expanded our knowledge of the Moon and its space environment. After continuously probing that environment for nearly six (6) years, Explorer 35 was permanently turned-off via command from Earth on Sunday, 24 June 1973. While the craft has long since fallen from lunar orbit, the exact time and location of impact are not known.