Mars Direct is a plan for a manned Mars exploration program, first outlined by Robert Zubrin and David Baker in 1990. Mars Direct focuses on getting to Mars quickly, using current space technologies, direct launch from Earth, and the Martian atmosphere in place of chemicals brought from Earth. Mars Direct was later reviewed by NASA and Stanford University, and although several NASA plans based on Mars Direct have been published, none have been given official status as NASA's plan for getting humans to Mars.
Under Mars Direct, the vehicle used to return to Earth is launched separately from the crew, two years beforehand. This vehicle, carrying a crew compartment, rocket engines and a nuclear reactor, lands on Mars well in advance of the crew's arrival and starts to produce rocket fuel for the journey home. The fuel used is methane, or natural gas, which can be produced by using hydrogen brought from Earth and carbon dioxide from the Martian atmosphere. The methane can also be used to fuel generators and engines on Mars, and the chemical reactions can also produce oxygen for use by the crew.
Two years later, the Mars Direct crew would launch, using a heavy-lift vehicle similar to Project Apollo's Saturn V. The crew's spaceship is launched in one piece from the Earth's surface, instead of being assembled in space, and lands in one piece on the surface of Mars. After the crew has finished their exploration of Mars, they drive to the return vehicle, which by now is fully fueled, and blast off for Earth.
Several alternate versions of Mars Direct have been proposed by NASA and others. The NASA version of Mars Direct uses three launches; the extra launch is used to place an Earth return vehicle in Mars orbit, which the crew would dock with as they were coming up from the Martian surface. Although Mars Direct deliberately avoids speculative or undeveloped technologies to make the mission simpler to carry out, several untested technologies such as nuclear thermal rockets and artificial gravity can be incorporated into the design if they are available.