Martian settlers on a weekend excursion. Painting by Pat Rawlings, courtesy NASA.
At the National Space Society International Space Development Conference in Anaheim, California, on April 20, 1990, Robert Zubrin brought down the house with the first public presentation of the "Mars Direct" scenario developed by Zubrin and his colleague David Baker of the Martin Marietta company. The audience had not heard such an innovative proposal in many years. The Mars Direct scenario held new potential of lowering costs and of bringing the human exploration and settlement of the Red Planet into the realm of feasibility.
The Mars Direct scenario lowered cost in two key ways: (1) by extensively relying whenever possible on in situ (on site) resource utilization (ISRU), such as the automated manufacture of return fuel from the Martian atmosphere before the first humans even leave for Mars; and (2) by launching directly from Earth, making it unnecessary to first develop an infrastructure in Earth orbit or on the Moon (the hardware developed for Mars Direct could also be used on the Moon, however).
Zubrin describes his ideas quite effectively in the following three articles he wrote for the NSS magazine, Ad Astra:
1996: The Case for Mars: the Plan to Settle the Red Planet and Why We Must, by Robert Zubrin with Richard Wagner. The Free Press. Carl Sagan wrote: "Bob Zubrin really, nearly alone, changed our thinking on this issue." [Review] [Buy from Amazon]
2003: A Traveler's Guide to Mars: The Mysterious Landscapes of the Red Planet, by William K. Hartmann. Workman Publishing. Kim Stanley Robinson called this book "a masterpiece of scientific writing for the general reader." We agree. Profusely illustrated. [Review] [Buy from Amazon]
2011: Access to Mars, by John K. Strickland, Jr. Presentation at the International Space Development Conference, Huntsville, Alabama, May 18-22, 2011. 109 pages. [PDF 3.1 MB]
2003: Space Colonization Using Space-Elevators from Phobos, by Leonard M. Weinstein, NASA Langley Research Center. 9 pages. [PDF 180K]
2002: Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface. National Academy of Sciences. 64 pages. [HTML]
2001: The Mars Surface Reference Mission: A Description of Human and Robotic Surface Activities, by Stephen J. Hoffman. NASA/TP—2001–209371. 114 pages. [PDF 1.3 MB]
2001: Humans to Mars: Fifty Years of Mission Planning, 1950-2000, by David S. F. Portree. NASA SP-2001-4521. 151 pages. [PDF 2.4 MB]
1998: Resource Utilization and Site Selection for a Self-Sufficient Martian Outpost. NASA/TM-98-206538. 54 pages. [PDF 6.4 MB]
1997: Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Team. NASA Special Publication 6107. 237 pages. [PDF 2.3 MB] This Reference Mission was heavily influenced by the Mars Direct scenario and has been called "Mars Semi-Direct."
1993: Resources of Near-Earth Space, Part IV: Mars. University of Arizona Press.