Mars 6 was one of two combination flyby-landers launched by the Soviet Union during the 1973 launch period. The landers were very similar in design to the Mars 2 and Mars 3 landers dispatched by the Soviets in 1971, except the spacecraft was now comprised of a flyby vehicle (instead of an orbiter) and a 2,200-pound (1,000-kilogram) lander.
Mars 6 completed its first midcourse correction en route to Mars at 23:45 UT on Aug. 12, 1973, but immediately an onboard tape recorder failed, forcing controllers to use a backup recorder. Then on September 3, there was a major failure in the telemetry system that transmitted scientific and operations data from the spacecraft. Only two channels remained operational, neither of which provided the ground with any direct data on the status of the flyby vehicle’s systems. Controllers could only use a time-consuming “playback” mode for the reception of data.
Ultimately, the flyby spacecraft automatically performed all its functions and at 05:01:56 UT (signal reception time) March 12, 1974, and the lander successfully separated from its mother ship at a distance of about 28,600 miles (46,000 kilometers) from Mars.
About four hours later, at 09:05:53 UT, it entered the Martian atmosphere at a velocity of about 3.5 miles (5,600 meters) per second. The parachute system deployed correctly at an altitude of about 12 miles (20 kilometers) at 09:08:32 when speed had been reduced to about 2,000 feet per second (600 meters per second), and scientific instruments began to collect and transmit data (to the flyby vehicle) as the probe descended.
The only useful data was, however, directly from the lander to Earth, and its information was rather “weak” and difficult to decode. It appeared that the lander was rocking back and forth under its parachute far more vigorously than expected. Nevertheless, Mars 6 returned the first direct measurements of the temperature and the pressure of the Martian atmosphere as well as its chemical composition (using the radio-frequency mass spectrometer) to Earth. The data indicated that argon made up about one-third of the Martian atmosphere.
Moments before the expected landing, the ground lost contact with the probe. The last confirmed data was information on ignition of the soft-landing engines received about two seconds before impact. The probe landed at 09:11:05 UT at 23.9 degrees south latitude and 19.5 degrees west longitude.
Later investigation never conclusively identified a single cause of loss of contact. Probable reasons included failure of the radio system or landing in a geographically rough area. The Mars 6 flyby bus, meanwhile, collected some scientific information during its short flyby (at a minimum range of about 990 miles or 1,600 kilometers to the surface) before heading into heliocentric orbit.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.