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Let's go to Mars

The Soviet 1973 armada fails four different ways

Four spacecraft launched in 19 days. Mars 4 flew past at 1,844 km, Mars 5 leaked after nine days, Mars 6 died at touchdown, and Mars 7 missed the planet by 1,300 km.

An orbital view of rust-red Martian highlands, dominated by a large eroded crater whose floor is filled with pale broken blocks, with branching channels cutting the cratered plains around it.
Margaritifer Terra, in the southern hemisphere of Mars, is the region Mars 6 was descending into when it stopped transmitting, after returning 224 seconds of data. The mosaic combines Mars Express High Resolution Stereo Camera images from 2007 and 2017. · Image: ESA/DLR/FU Berlin

The Soviet Union launched four spacecraft at Mars between 21 July and 9 August 1973, the largest interplanetary launch campaign anyone has attempted. The 1973 window was energetically worse than 1971, and a Proton could not lift the orbiter-plus-lander combination flown as Mars 2 and Mars 3, so the mission split: Mars 4 and Mars 5 orbiters with no landers, Mars 6 and Mars 7 flyby buses carrying descent modules and no way of stopping themselves.

Mars 4 arrived first, on 10 February 1974. Its braking engine never fired; it swept past at 1,844 kilometres into solar orbit, returning one swath of imagery and a radio occultation that caught the nightside ionosphere for the first time. Mars 5 entered orbit on 12 February, an ellipse of 1,755 by 32,555 kilometres with a period just under 25 hours; over nine days it returned 43 usable images and found an ozone layer near 40 km. Its pressurised instrument compartment had been leaking since arrival, taking the transmitter housing with it; controllers blamed a micrometeoroid struck during insertion.

Mars 6's descent module separated 48,000 km out and entered the atmosphere at 09:05:53 UT on 12 March 1974, slowing from 5,600 to 600 metres per second before the parachute opened. For 224 seconds it measured the Martian atmosphere from inside: about 25 km altitude at 150 K down to a surface at 230 K, a pressure near 6 millibars, and more water vapour than anyone expected. At 09:11:05 UT, it stopped, either as the retrorockets fired or on impact near Margaritifer Sinus at an estimated 61 metres per second; much of what it had already sent proved unreadable.

Mars 7's descent module separated four hours before encounter on 9 March, but its retro-rocket never fired; bus and probe went by the planet together, 1,300 km wide.

The common cause was a transistor: the 2T-312s in the onboard computers used aluminium contacts instead of gold-plated ones as a cost-saving measure, and degraded during the seven-month cruise, taking out Mars 4's insertion burn, scrambling Mars 6's telemetry and very likely causing Mars 7's retro-rocket failure.

Having launched at nearly every window since 1960, the Soviet Union then sent nothing for fifteen years, until Phobos 1 and 2 in 1988. Mars 6's 224 seconds stayed the only direct measurement of the Martian atmosphere until Viking 1 landed in July 1976.