Mars 2M No.521

At first, a modified Luna E-8 bus was to be used for the spacecraft, however it had a number of limitations that made it unsuitable for the long journey to Mars.

However, the 2M probes ended significantly heavier than intended and engineers also ran out of time to conduct drop tests of the lander, so that part was abandoned which left only the orbiter.

Mars 2M No.521 was launched at 10:40:45 UTC on March 27, 1969, atop a Proton-K 8K78K carrier rocket with a Blok D upper stage, flying from Baikonur Cosmodrome Site 81/23.

However, third stage ignition did not occur on time and reports filtered back to the launch center that the booster had exploded.

Investigation found that an imbalanced rotor in the third stage oxidizer turbopump had started a fire, which led to loss of thrust and vehicle breakup.