Mars 2M No.521, also known as Mars M-69 No.521 and sometimes identified by NASA as Mars 1969A, 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.522, which was launched in 1969 as part of the Mars programme. Neither launch was successful.
Mars 2M No.521 was launched at 10:40:45 UTC on 27 March 1969 atop a Proton-K 8K78K carrier rocket with a Blok D upper stage, flying from Baikonur Cosmodrome Site 81/23. The launch proceeded normally through the first and second stage burns, but during third stage flight, a rotor bearing failed and the turbopump caught fire. This resulted in an engine failure which occurred 438.66 seconds into the flight, followed by the third stage exploding. Debris fell over the Altai Mountains.
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.