|Cislunar abort: the crew of Apollo 13 on the deck of the aircraft carrier Iwo Jima after their safe return to Earth. Pictured are Lunar Module Pilot Fred Haise (left), Commander Jim Lovell, and Command Module Pilot Jack Swigert. Image credit: NASA.|
Amidst the drama of Apollo 13's mission abort, A. A. VanderVeen, a mathematician 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 analyzed aborts for a piloted Mars orbiter ("capture") mission launched in 1981. An abort would begin with a rocket burn to slow the Mars spacecraft in its orbit around the Sun and cause it to fall back toward Earth. Because of the great cost of launching propellants off the Earth, he assumed that the spacecraft would carry no propellants dedicated entirely to an abort; it would perform its abort burn using only the propellants which, in a normal mission, would slow the spacecraft by 12,000 feet per second (3658 meters per second) at the end of its Earth-Mars transfer so that the gravity of Mars could capture it into orbit (that is, its Mars orbit insertion propellant).
A 270-day flight to Mars could be divided into three abort phases, VanderVeen found. Phase 1 would span from late in mission day 1 through mission day 60. During this period, the spacecraft would move slowly out from the Sun but remain relatively close to the Earth. A 12,000 feet per second (3658 meters per second) abort burn on mission day 60 would return the crew to Earth in 80 to 110 days.
Phase 2 would span mission days 60 through 180. Earth would pull ahead of the spacecraft during this period. Following the abort burn, the spacecraft would have to dip inside the orbit of Venus to gain speed and complete nearly one full orbit of the Sun in order to catch up with Earth from behind. An abort burn on mission day 180 would return the crew to Earth in 290 days, VanderVeen calculated.
Phase 3 would span mission days 180 through 270. The spacecraft would be playing catch up with Mars during this period. Aborts in Phase 3 would result in Earth returns longer than those in Phase 1 but shorter than those in Phase 2.
Increasing the quantity of abort propellant available would not meaningfully decrease Earth-return time, VanderVeen found. An abort on mission day 100 using only the spacecraft's Mars orbit insertion propellant would yield a 260-day Earth return. Adding enough propellant to change the spacecraft's speed by an additional 1000 feet per second (305 meters per second) would shave only 10 days off that return time. Providing enough propellant to change its speed by 24,000 feet per second (7315 meters per second) — that is, twice the velocity change needed for Mars orbit insertion in a normal mission — would yield a disappointingly lengthy 140-day Earth return.
Abort timing would also affect the speed at which the returning spacecraft would reach Earth. VanderVeen assumed that the Mars crew would reenter Earth's atmosphere directly in an Apollo-derived Earth entry module. The Apollo reentry system was designed to withstand reentry at up to 40,000 feet per second (12,192 meters per second). This would suffice for aborts initiated during the first 130 days of flight.
For aborts initiated between 130 and 200 days after Earth departure, however, an improved Apollo heat shield or braking rockets designed to slow reentry would be required. Aborts during the remainder of the Earth-Mars transfer would demand "advanced [reentry] systems or a high retro-fire maneuver" requiring much additional propellant.
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."
"Abort from Mars and Venus Missions - Case 103-8," A. A. VanderVeen, Bellcomm, Inc., 15 April 1970.