Mars 7, also known as 3MP No.51P was a Soviet spacecraft launched in 1973 to explore Mars. A 3MP bus spacecraft which comprised the final mission of the Mars programme, it consisted of a lander and a coast stage with instruments to study Mars as it flew past. Due to a malfunction, the lander failed to perform a maneuver necessary to enter the Martian atmosphere, missing the planet and remaining in heliocentric orbit along with the coast stage. Mars 7 spacecraft carried an array of instruments to study Mars. The lander was equipped with a thermometer and barometer to determine the surface conditions, an accelerometer and radio altimeter for descent, and instruments to analyse the surface material including a mass spectrometer.

Wikipedia: https://en.wikipedia.org/wiki/Mars_7. Related NASA reference: https://nssdc.gsfc.nasa.gov/planetary/chronology.html#1973. Description adapted from Wikipedia contributors (CC BY-SA 4.0; https://creativecommons.org/licenses/by-sa/4.0/).