Spacecraft above Moon.

Artist's concept of Lunar Orbiter at the Moon.

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Lunar Orbiter 5 was the last in a series of highly successful missions to map the Moon for potential landing sites and to conduct general observational surveys. This last mission’s objectives were both photographic (the primary mission) and non-photographic (the secondary).

The former involved taking additional pictures of Apollo sites, broad surveys of unphotographed areas of the far side, imaging the Surveyor landing sites, and photographing areas of scientific value.

The secondary goals included acquisition of precise trajectory information for improving the definition of the lunar gravitational field, measurement of the micrometeoroid flux and radiation dose in the lunar environment, and helping to prepare the Manned Space Flight Network for the Apollo missions.

After a single mid-course correction on Aug. 3, 1967, Lunar Orbiter 5 entered lunar polar orbit two days later after an engine firing at 16:48 UT that lasted 8 minutes, 28 seconds. Initial orbital parameters were about 121 x 3,743 miles (194.5 × 6,023 kilometers) at 85.01 degrees inclination. The orbital period was 8.5 hours.

Lunar Orbiter 5 began its photography mission at 01:22 UT on Aug. 7, 1967 before executing a maneuver to bring it to its operational orbit at about 61 x 3,743 miles (100 × 6,023 kilometers).

The spacecraft photographed 36 different areas on the near side and mapped most of the far side via a set of 212 frames until the photography mission ended on Aug. 18, 1967. These included five potential Apollo landing sites, 36 science sites, and 23 previously unphotographed areas of the far side, as well possible targets for future Surveyor missions.

Controllers also extensively used the spacecraft to map the Moon’s gravitational field in order to predict orbital perturbations on future lunar orbital missions. The probe also obtained a spectacular high-quality photo of Earth showing Africa and the Middle East at 09:05 UT on Aug. 8, 1967.

A further orbital change on Aug. 9, 1967 brought the orbit down to about 932 x 61.5 miles (1,499.37 × 98.93 kilometers) at 84.76 degree inclination.

Lunar Orbiter 5 was commanded to impact on the lunar surface on Jan. 31, 1968, which it did at 0 degrees north latitude and 70 degrees west longitude.

In total, the five Lunar Orbiters photographed 99 percent of the lunar surface. Perhaps the most important finding credited to data from the Lunar Orbiters (and Lunar Orbiter 5 in particular) is the discovery of “mascons” or lunar mass concentrations under the lunar ringed maria, first published by JPL researchers P. M. Muller and W. L. Sjogren in mid-1968.

Source

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

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