On April 13, 1970, an oxygen tank exploded in the Apollo 13 Command and Service Module Odyssey, badly damaging the spacecraft 200,000 miles from Earth. NASA had no choice but to scrub the planned third Apollo moon landing and return the Apollo 13 crew to Earth as quickly as possible. Astronauts James Lovell, Fred Haise, and Jack Swigert used the Lunar Module Aquarius as a backup propulsion system and lifeboat, swung around the moon, and splashed down safely in Odyssey‘s Command Module on April 17, about three and a half days after the explosion.

Amidst the drama of Apollo 13′s mission abort, mathematician A. A. VanderVeen, with NASA planning contractor Bellcomm, drafted a memorandum. In it, he pointed out that the time needed to return to Earth following a malfunction during the outbound leg of a Mars or Venus mission would nearly always be measured in months ...

VanderVeen concluded that, all things considered, successfully aborting a Mars mission after departure from Earth orbit would be extremely challenging. His memorandum’s chief recommendation reflected the essential hopelessness of an abort in interplanetary space: he wrote that “all major [spacecraft] systems should be thoroughly checked out very early in the mission while a short return abort opportunity exists.”