A few months after the end of the Apollo-17 mission, NASA launched the last American lunar probe for 21 years: Explorer-49.
It wasn't in fact a real lunar probe, but a small lunar orbiting radio astronomy observatory. For this reason, the probe is also known as RAE-2 (Radio Astronomy Explorer). RAE-1 was launched as Explorer-38 on the 4th of July, 1968 from Vandenberg AFB, in California, on a Delta rocket and was put an a curious circular orbit 5,800 km high and with an inclination of 120 degrees with respect to the equator, i.e. a retrograde orbit.
The small satellite observed for months the "radio sky" in frequencies between 0.2 and 9.2 MHz, but it was subjected to the continuous radio interference coming from our planet, both natural (aurorae, thunderstorms) and artificial. The mission was a success, but it remained a strictly experimental one, and in fact no catalogue of observed radio sources was published.
RAE-1 pushed the technicians of NASA's Goddard Space Flight Center to built a second mission, designed to orbit the Moon to eliminate the Earth background noise.
RAE-2 was a small cylindrical satellite, having a diameter of 92 cm and 79 cm high. Electricity was generated by four solar panels and the radiotelescopes were two dipole antennae 18 meters long and four wire antennae each 229 meters long with a diameter of 0.005 cm that were unfurled once in orbit. The probe was also equipped with a 129 meters deployable boom for libration motion control. The on-board radiometers could observe the sky on nine frequencies between 25 kHz and 13.1 MHz, a part of the electromagnetic spectrum rarely observed from Earth, extending from VLF to LF, MF and HF. Three receivers with 32 channels each registered transitory events and a known impedance probe permitted calibration of the instruments. Two small vidicon camera observed the area of sky the radiotelecope was "listening" to.
The probe's weight at launch was 328 kg. RAE-2 was launched on a Delta 1913 rocket from Cape Canaveral on June 10, 1973 and entered Moon orbit five days later. The navigation to the Moon was very problematic because a new panoramic sensor that should have measured the attitude of the probe with respect to the Sun, the Moon and the Earth was subjected to stray light and the spacecraft controllers had to discriminate by hand between the real data and the spurious ones.
On June 15 the probe entered lunar orbit, with a periastron at 1,123 km, an apoastron at 1,334 km and 61.3 degrees of inclination. This orbit was then lowered to a 1,053 x 1,063 km, 38.7 degrees of inclination and 221 minutes of period one.
On arrival, RAE-2 ejected the solid rocket engine that braked it and deployed the dipole antenna for three weeks of observations. Up to that point the probe was spin stabilized at 4 rpm. The dipole was then retracted and the wire antennae were unfurled, stopping the rotation. The two antennae pointing at the Moon remained unfurled at 183 meters up to November 1974 when they finally completely deployed.
The probe could observe anyway many of its targets, starting from Earth, a powerful source of natural and artificial radio signals. Observing our planet was particularly easy at the time of full moon, when the Moon is inside the magnetosphere tail. The real advantage of being in lunar  orbit, however, was that the probe was shielded from our planet every time it flew over the farside and that the sharp limb of the Moon, that from RAE-2 vantage point had a 76 degrees angular diameter (150 times that from Earth) should have enabled the observation of the occultation of many radio sources. Between August 1973 and June 1974 RAE-2 observed occultations of the Sun and Jupiter, the most powerful radiosources of the Solar System. Other radiosources, such as Saturn, Fornax A, Pictor A, Taurus A, Hydra A, Virgo A, Hercules A and Cygnus A, were found to be too faint to be observed.
The mission was complete in June 1975 but contacts were sporadically maintained up to August 1977.

Bibliography
Alexander, J. K. et al: Scientific Instrumentation of the Radio Astronomy Explorer-2 Satellite; Astronomy & Astrophisics, Vol. 40, pp. 365-371 (This Article is available on NASA Astrophisic Data System)
Werz, J. R. (editor): Spacecraft Attitude Determination and Control; Dordrecht, Reidel
Wilson, A.: Solar System Log; London, Jane's
NSSDC Master Catalog

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