The ERV would take some eight 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 ten months to complete.
Some 26 months after the ERV is originally launched from Earth, a second vehicle, the Mars Habitat Unit (MHU), would be launched on a high-energy transfer to Mars carrying a crew of four. This vehicle would take some six 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 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 MHU, and powered by excess methane produced by the ERV.
To return, they would use the ERV, leaving the MHU for the possible use of subsequent explorers. The propulsion stage of the ERV would be used as a counterweight to generate artificial gravity for the trip back.
The initial cost estimate for Mars Direct was put at $55 billion, to be paid over ten years.
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 six.
The Mars Society has argued the viability of the Mars Habitat Unit concept through their Mars Analogue Research Station program.
Mars Direct was featured on a Discovery Channel programs Mars: The Next Frontier in which were discussed, in part, issues surrounding NASA funding of the project, and on Mars Underground, where the plan is discussed more in-depth.
Currently NASA is in the final stages of implementing a modified Mars Direct approach to both Lunar and Martian exploration. Zubrin's ultimate goal of a fully terraformed and colonized Mars is a long-term, multigenerational goal, but eventually, if NASA's 30 year architecture is carried to fruition, that end might be seen as plausible.
A modified proposal, "Mars for Less" , was developed by Grant Bonin and has been adopted as the design reference mission for a new umbrella group of advocates, the MarsDrive consortium The design retains most of the essential features of Mars Direct, but uses multiple medium-lift rocket launchers that are commercially available today (such as the Ariane V or the Delta rocket) to launch the crew vehicles, and their propulsion, separately, and mate them in orbit. By doing so, the multi-billion dollar development cost of a new launch vehicle is avoided.
A modified proposal has also been offered by Dean Unick, to not return the first immigrant/explorers immediately, or ever. The cost of sending a four or six person team is one fifth to one tenth the cost of returning that same four or six person team. A quite complete lab can be sent and landed for less than the cost of sending back even 50 kilos of rocks. Twenty or more persons can be sent for the cost of returning four. It is far less expensive, makes for better science, makes far more sense, to colonize. Returns would be far easier and less expensive, and far safer, once there is a developed and operating base. Send brick masons, not pilots. Send older explorers, not young.