Schedule constraints on manned Mars missions (1966)

In March 1966, at the height of 1960s optimism about America's future in space, Robert Riedesel and John Wall, respectively Project Manager and Chief Engineer of the Future Systems Department at Douglas Aircraft Company, presented a paper in which they discussed the constraints which they believed made a piloted Mars mission unlikely before 1981. Though the specifics have changed, in general the constraints they listed 43 years ago are the same ones faced by planners of costly space missions today.

Their first group of constraints, those related to natural phenomena in the Solar System, included the cycle of minimum-energy Mars launch opportunities and the 11-year solar activity cycle. Minimum-energy Mars launch opportunities occur every 26 months, the Douglas engineers noted. The less energy needed to reach Mars, the less propellant would be required. The less propellant required, the fewer costly heavy-lift rockets would be needed to launch Mars spacecraft hardware and propellant into Earth orbit for assembly.

Riedesel and Wall noted that not all minimum-energy opportunities are created equal; the amount of energy required to reach Mars follows a roughly 15-year cycle. Unfavorable opportunities would occur in 1977 and 1979, they wrote, then opportunities would improve in 1981 and 1984. The year 1986 would see the most favorable Mars launch opportunity since 1971.

The 11-year solar cycle would reach maximum intensity in 1981, then would decline to a minimum in 1986-1987. The potential for "giant" solar flares during solar maximum meant that astronauts traveling to Mars in 1981 would need a massive radiation shelter. Riedesel and Wall recommended that NASA launch its first Mars expedition in 1986, when the solar minimum and a highly favorable Earth-Mars transfer opportunity would coincide.

Some schedule constraints were based on needed experience. Before engineers could develop a piloted Mars spacecraft, they would need more data on the martian environment and the effects on humans of long exposure to weightlessness. Riedesel and Wall expected that the planned Voyager automated Mars probe and astronaut stays on board an Earth-orbiting space station would provide the necessary data as early as 1973. They cited Apollo development experience when they estimated that Mars spacecraft development would require at least seven years after the needed data became available. This would mean that the first Mars expedition could set out no earlier than the 1981 launch opportunity.

Economics would also constrain the piloted Mars mission schedule. Riedesel and Wall cited published Mars program cost estimates ranging from $40 billion to $100 billion. Their own cost estimate - $62 billion - included the cost of automated precursor probes, an Earth-orbiting space station, a heavy-lift rocket more powerful than the Saturn V, and eight piloted missions leading to one piloted Mars landing. They assumed that NASA's budget would remain at its Apollo peak of about $5 billion per year, with $3 billion going to piloted spaceflight. They concluded that, if astronauts were to reach Mars in the 1980s, then either a "major increase" in the space agency's budget or "a less expensive approach to interplanetary exploration" would be necessary.

Finally, piloted Mars exploration faced political constraints. Riedesel and Wall predicted that, if President Lyndon B. Johnson were re-elected in 1968, then he would have little incentive to commit funds and political capital to a piloted Mars program that would not succeed until long after he left office. The Mars program might start when Johnson's successor took office in January 1973. The new president might, however, find no personal benefit in championing a Mars expedition, for even if he initiated it immediately after he took office, it would leave Earth for Mars no earlier than the 1981 launch opportunity, after his second term had ended in January 1981.

If, on the other hand, President Johnson were not re-elected in 1968, then his successor could initiate the automated Mars probe and space station programs in 1969 with a good chance of seeing them succeed before his second term ended. Commitment to a piloted Mars expedition would probably have to wait until another president took office in early 1977, however. Given the time required for hardware development, this would postpone launch of the first U.S. piloted Mars expedition until at least 1984.

"Scheduling Constraints on Manned Exploration of Mars," Robert Riedesel and John Wall, A Volume of Technical Papers Presented at the AIAA/AAS Stepping Stones to Mars Meeting, pp. 99-106; paper presented in Baltimore, Maryland, March 28-30, 1966.