Spacecraft above Moon.

Artist's concept of Lunar Orbiter at the Moon.

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Lunar Orbiter 4 was the first in the series dedicated to scientific surveys of the Moon. Its goal was to acquire contiguous photographic coverage of the lunar surface of at least 80 percent of the near side at 164-to-328 feet (50-to-100 meter) resolution.

After a midcourse correction on May 5, 1967, Lunar Orbiter 4 fired its engine at 15:08 UT on May 8 to insert the spacecraft into an initial lunar polar orbit of 3,797 x 1,681 miles (6,111 × 2,706 kilometers) at 85.5 degree inclination, thus becoming the first spacecraft to go into polar orbit around the Moon. The orbital period was about 12 hours.

The spacecraft began its photographic mission at 15:46 UT on May 11, 1967. A potentially serious problem threatened the mission when on May 13, 1967 controllers found a problem with a camera thermal door that failed to close, leaking light onto exposed images. They were able to devise a fix that worked and the spacecraft continued its imaging mission.

During its two-month mission, Lunar Orbiter 4 took pictures of 99 percent of the near side and 75 percent of the far side of the Moon in a total of 163 frames. The imaging mission ended on the orbiter’s 34th orbit due to worsening readout difficulties. Fortunately, all but 30 of the 163 images collected, many with a resolution down to 197 feet (60 meters), were successfully transmitted to Earth by June 1, 1967.

In early June, controllers lowered the spacecraft’s orbit to match that of Lunar Orbiter 5 so that scientists could collect gravitational data in support of the latter mission.

Before losing contact on July 17, 1967, Lunar Orbiter 4 took the first photos of the lunar south pole and discovered a 149-mile (240-kilometer) long crustal fault on the far side. Since contact was lost before controlled impact, the spacecraft naturally crashed onto the Moon on October 6, 1967 due to gravitational anomalies.

Sources

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

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