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Mars 7 was the last of the four Soviet spacecraft sent to Mars in the 1973 launch period (although it arrived at Mars prior to Mars 6).

On its way to Mars, the spacecraft performed a single midcourse correction at 20:00 UT on Aug. 16, 1973. En route to Mars, there were failures in the communications systems, and controllers were forced to maintain contact via the only remaining radio-communications complex.

On March 9, 1974, the flyby spacecraft ordered the lander capsule to separate for its entry into the Martian atmosphere. Although the lander initially refused to accept the command to separate, it eventually did. Ultimately, the lander’s main retro-rocket engine failed to fire to initiate entry into the Martian atmosphere. As a result, the lander flew by the planet at a range of about 800 miles (1,300 kilometers) and eventually entered heliocentric orbit.

The flyby probe did, however, manage to collect data during its encounter with the Red Planet and contact was maintained until March 25, 1974. Both the failures on Mars 4 (computer failure) and Mars 7 (retro-rocket ignition failure) were probably due to the faulty transistors installed in the circuits of the onboard computers which were detected prior to launch.

Data from Mars 7 was being analyzed as late as 2003 when researchers published results based on data collected by the KM-73 cosmic ray detector in September 1973 en route to Mars.

Source

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

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