Mars Direct is a proposal for a human mission to Mars which purports to be both cost-effective and possible with current technology. It was originally detailed in a research paper by Martin Marietta engineers Robert Zubrin and David Baker in 1990, and later expanded upon in Zubrin's 1996 book The Case for Mars. It now serves as a staple of Zubrin's speaking engagements and general advocacy as head of the Mars Society, an organization devoted to the colonization of Mars.

On July 20, 1989, US President George H. W. Bush announced plans for what came to be known as the Space Exploration Initiative (SEI). In a speech on the steps of the National Air and Space Museum he described long-term plans which would culminate in a human mission to the surface of Mars.

By December 1990, a study to estimate the project's cost determined that long-term expenditure would total approximately 450 billion dollars spread over 20 to 30 years. The "90 Day Study" as it came to be known (also “90 Day Report” from people such as Zubrin), evoked a hostile Congressional reaction towards SEI given that it would have required the largest single government expenditure since World War II. Within a year, all funding requests for SEI had been denied.

Dan Goldin became NASA Administrator on April 1, 1992, officially abandoning plans for near-term human exploration beyond Earth orbit with the shift towards a "faster, better, cheaper" strategy for robotic exploration.

While working at Martin Marietta designing interplanetary mission architectures, Robert Zubrin perceived a fundamental flaw in the SEI program. Zubrin came to understand that if NASA's plan was to fully utilize as many technologies as possible in support of sending the mission to Mars, it would become politically untenable. In his own words:

The exact opposite of the correct way to do engineering.

Zubrin's alternative to this "Battlestar Galactica" mission strategy (dubbed so by its detractors for the large, nuclear powered spaceships that supposedly resembled the science-fiction spaceship of the same name) involved a longer surface stay, a faster flight-path in the form of a conjunction class mission, in situ resource utilization and craft launched directly from the surface of Earth to Mars as opposed to be being assembled in orbit or by a space-based drydock. After receiving approval from management at Marietta, a 12-man team within the company began to work out the details of the mission. While they focused primarily on more traditional mission architectures, Zubrin began to collaborate with colleague David Baker's extremely simple, stripped-down and robust strategy. Their goal to "use local resources, travel light, and live off the land" became the hallmark of Mars Direct.

The first flight of the Ares rocket (not to be confused with the similarly named rocket of the now defunct Constellation program) would take an uncrewed Earth Return Vehicle to Mars after a 6-month cruise phase, with a supply of hydrogen, a chemical plant and a small nuclear reactor. Once there, a series of chemical reactions (the Sabatier reaction coupled with electrolysis) would be used to combine a small amount of hydrogen (8 tons) carried by the Earth Return Vehicle with the carbon dioxide of the Martian atmosphere to create up to 112 tonnes of methane and oxygen. This relatively simple chemical-engineering procedure was used regularly in the 19th and 20th centuries, and would ensure that only 7% of the return propellant would need to be carried to the surface of Mars.