Goals: Mars 4 was to orbit Mars (along with Mars 5), taking pictures and conducting remote-sensing experiments on the atmosphere and surface, and to serve as a communications relay for the Mars 6 and 7 landers.

Accomplishments: Faulty transistors disabled the spacecraft's ability to slow down and enter an orbit around Mars, so Mars 4 became a flyby mission. It transmitted the results of one imaging session and some radio occultation data which constituted the first detection of the nightside ionosphere on Mars. Following the flyby, Mars 4 returned data about the interplanetary environment.

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