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Identical to Mariner 6, Mariner 7 had a similar mission of flying by Mars. After Mariner 6 had returned intriguing photos of Mars’ south polar cap, controllers reprogrammed Mariner 7’s control system to increase the number of scans of the south pole for the second spacecraft from 25 to 33.

After a perfect midcourse correction on the way to Mars on April 8, 1969, on July 30, 1969, just 7 hours before its twin was scheduled to fly by Mars, the deep space tracking station at Johannesburg, South Africa, lost contact with the spacecraft’s high-gain antenna. One of two stations in Madrid, Spain, was diverted from their original missions of tracking Pioneer 8 and joined the search for Mariner 7. Fortunately, the Pioneer station at Goldstone picked up faint signals from the spacecraft. Controllers sent commands to Mariner 7 to switch to the low-gain antenna that worked well afterwards.

Mission controllers later speculated that the spacecraft had been thrown out of alignment when struck by a micrometeoroid (although later speculations centered on the silver-zinc battery on board which might have exploded, with venting electrolytes acting like a thruster). As a result, 15 telemetry channels were lost. Despite problems with positional calibration, Mariner 7 recorded 93 far-encounter and 33 near-encounter images of the planet, showing heavily cratered terrain very similar to Mariner 6.

Closest approach to Mars was at 05:00:49 UT on Aug. 5, 1969 at a distance of 2,131 miles (3,430 kilometers). Oddly, despite the high resolution of 984 feet (300 meters), Mariner 7 found the center of Hellas to be devoid of craters. The spacecraft found a pressure of 3.5 millibars and a temperature of minus 90 degrees Fahrenheit (minus 67.8 degrees Celsius) at 59 degrees south latitude and 28 degrees east longitude in the Hellespontus region, suggesting that this area was elevated about 4 miles (6 kilometers) above the average terrain.

Post-flight analysis showed that at least three photos from Mariner 7 included the moon Phobos. Although surface features were not visible, the pictures clearly showed the moon to be irregularly shaped.

Mariner 7 entered heliocentric orbit (1.11 × 1.70 AU) and NASA maintained continued to receive data from the vehicle until mid-1971.

Source

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

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