Mars Direct
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Mars Direct is a proposal for a relatively low-cost manned mission to Mars with current rocket technology. The mission was developed by Robert Zubrin and David Baker and is detailed in Zubrin's book, The Case For Mars.
The plan involves launching an unmanned Earth Return Vehicle (ERV) directly from Earth's surface to Mars using a heavy-lift booster (no bigger than the Saturn V used for the Apollo missions), containing a supply of hydrogen, a chemical plant and a small nuclear reactor.
The ERV return vehicle would take some 8 months to reach Mars. Once there, a relatively simple set of chemical reactions (the Sabatier reaction coupled with electrolysis) would combine a small amount of hydrogen carried by the ERV with the carbon dioxide of the Martian atmosphere to create up to 112 tonnes of methane and oxygen propellants, 96 tonnes of which would be needed to return the ERV to Earth at the end of the mission. This process would take approximately 10 months to complete.
Some 26 months after the ERV was originally launched from Earth, a second vehicle, the Mars Habitat Unit would be launched on a high-energy transfer to Mars carrying a crew of 4. This vehicle would take some 6 months to reach Mars. During the trip, artificial gravity would be generated by tying the spent upper stage of the booster to the Habitat Unit, and setting them both rotating about a common axis.
On reaching Mars, the useless spent upper stage would be jettisoned, with the Habitat Unit aerobraking into Mars orbit before soft-landing in close proximity to the ERV.
Once on Mars, the crew would spend 18 months on the surface, carrying out a range of scientific research, aided by a small rover vehicle carried aboard their Habitat Unit, and powered by excess methane produced by the ERV.
To return, they use the ERV, leaving the habitat for the possible use of subsequent explorers. The propulsion stage of the ERV would be used as a counterbalance to generate artificial gravity for the trip back.
The initial cost estimate for Mars Direct was put at $20 billion, including development costs. In today's terms, this equates to some $30-35 billion. In 2004, NASA and ESA undertook cost modelling exercises to review the cost of human space missions. While the exercise was not an endorsement of Mars Direct, both cost models found Zubrin and Baker's cost estimate to be remarkably accurate.
Revisions
Since Mars Direct was initially conceived, it has undergone considerable review by The Mars Society, NASA and Stanford University.
The NASA model, referred to as the Design Reference Mission, currently on version 3, calls for a significant upgrade in hardware (up to 3 launches per mission, not two), and sends the ERV to Mars fully fuelled, parking it in orbit above the planet, where it is reached by a small ascent craft.
The Mars Society and Stanford studies retain the original 2-vehicle mission profile of Mars Direct, but increase the crew size to 6.
Zubrin has since written a fictional account of how his plan might unfold in the novel, First Landing.
The Mars Society has demonstrated the viability of the Mars Habitat Unit concept through their Mars Analogue Research Station programme.