Mars 5 was the sister Mars orbiter to Mars 4. After two midcourse corrections on Aug. 3, 1973, and on Feb. 2, 1974, Mars 5 successfully fired its main engine at 15:44:25 UT to enter orbit around the planet. Initial orbital parameters were about 1,094 x 20,248 miles (1,760 × 32,586 kilometers) at 35 degrees, 19 feet and 17 inches inclination.
Soon after orbital insertion, ground controllers detected the slow depressurization of the main instrument compartment on the orbiter, probably as a result of an impact with a particle during or after orbital insertion. Calculations showed that at the current rate of air loss, the spacecraft would be operational for approximately three more weeks.
Scientists drew up a special accelerated science program that included imaging of the surface at 328-foot (100-meter) resolution. Five imaging sessions on Feb. 17, 21, 23, 25 and 26, 1974, produced 108 frames comprising only 43 usable photographs. Both the high-resolution Vega-3MSA and the survey Zufar-2SA TV cameras were used. Additionally, Mars 5 used the OMS scanner to take five panoramas of the surface.
The last communication with Mars 5, when the final panorama was transmitted back to Earth, took place Feb. 28, 1974, after which pressure in the spacecraft reduced below working levels.
Mars 5’s photos, some of which were of comparable quality to those of Mariner 9, clearly showed surface features which indicated erosion caused by free-flowing water. The first of these images taken by both the television cameras were published in the Academy of Sciences’ journal in the fall of 1974. Among significant achievements claimed for Mars 5 was “receipt of mean data on the chemical composition of rocks on Mars for the first time.”
The vehicle was supposed to act as a data relay for the Mars 6 and Mars 7 landers which arrived in March 1974 but was obviously unable to do so.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.