List of manned Mars mission plans in the 20th century

This list of manned Mars mission plans in the 20th century is a listing of formal proposals, studies, and plans for a human manned mission to Mars during the 20th century. It is limited to serious studies done with engineering and scientific knowledge about the capabilities of then current technology, typically for high-budget space agencies like NASA. Mission profiles included manned flybys, manned landers, or other types of Mars system encounter strategies. For later plans, see Manned mission to Mars.

Many mission concepts for expeditions to Mars were proposed in the late 20th century. David Portree's history volume Humans to Mars: Fifty Years of Mission Planning, 1950—2000 discusses many of these. Portree notes that every 26 Earth months a lower energy Earth to Mars transfer opportunity opens, so missions typically coincide with one of these windows. In addition, the lowest available transfer energy varies on a roughly 16-year cycle, with a minimum in the 1969 and 1971 launch windows, rising to a peak in the late 70s, and hitting another low in 1986 and 1988. Also of note, the Mariner 4 Mars flyby in 1965 provided radically more accurate data about the planet; a surface atmospheric pressure of about 1% of Earth's and daytime temperatures of -100 degrees Celsius (-148 degrees Fahrenheit) were estimated. No magnetic field or Martian radiation belts were detected. The new data meant redesigns for planned Martian landers, and showed life would have a more difficult time surviving there than previously anticipated. Later NASA probes in the 1970s, 80s, and 90s confirmed the findings about Mars environmental conditions.

The first "engineering analysis" of a manned mission to Mars was made by Wernher von Braun in 1948. It was originally published as Das Marsprojekt in West Germany in 1952, and as The Mars Project in English in the United States in 1953. Von Braun's Mars "flotilla" included ten 4,000-ton ships with 70 crew members. The expected launch year was 1965.

The list is in semi-chronological order, with some groupings, as variation can exist in the dating of a given plan. Various references were consulted. LEO mass refers to how much hardware must be put in low Earth orbit for the mission. For comparison, the low Earth orbit payload capacity per launch of the U.S. Space Shuttle is about 25 metric tons, and that of the Saturn V, 120 metric tons.

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