Mars 2M No.522, also known as Mars M-69 No.522 and sometimes identified by NASA as Mars 1969B, was a Soviet spacecraft which was lost in a launch failure in 1969. It consisted of an orbiter. The spacecraft was intended to image the surface of Mars using three cameras, with images being encoded for transmission back to Earth as television signals. It also carried a radiometer, a series of spectrometers, and an instrument to detect water vapour in the atmosphere of Mars. It was one of two Mars 2M spacecraft, along with Mars 2M No.521, which was launched in 1969 as part of the Mars program. Neither launch was successful.
Mars 2M No.522 was launched at 10:33:00 UTC on 2 April 1969 atop a Proton-K 8K78K carrier rocket with a Blok D upper stage, flying from Baikonur Cosmodrome Site 81/24. One of the first stage engines caught fire almost immediately at liftoff. The remaining engines tried to compensate, but trying to lift a booster with a full load of propellant at low altitude was too much for them. The Proton climbed to about 100 meters (330 feet) before pitching over and nosediving into the ground just outside the launch complex. Subsequent examination found that a missing drain plug allowed nitrogen tetroxide to leak out and start a fire.
Following the crash of the Mars 2M No.522 launch vehicle, the wind spread toxic propellant back across the launch complex, which made the Baikonur Cosmodrome launch complex unusable until rain washed the toxic residuals away. By the time this had happened, the alignment of Earth and Mars necessary to launch spacecraft had ended, and the Soviet Union was unable to launch any further Mars probes until 1971. It also resulted in delays to a number of Luna spacecraft scheduled for launch in 1969.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.