Mars Design Reference Mission
The NASA Mars Design Reference Mission ("DRM") refer to a series of NASA conceptual design studies of the missions to send humans to Mars. The related term, Design Reference Architecture (DRA), refers to the entire sequences of missions and supporting infrastructure. These are a reference baseline studies summarizing the current technology and possible approaches for a human mission to Mars, and are not actual mission program. According to NASA, the documents "represent a 'snapshot' of work in progress in support of planning for future human exploration of the Martian surface." The design reference missions are used  for technology trade studies, to analyze the effect of different approaches to the mission. Reference Design Mission (1993). The first Mars Design Reference Mission was a NASA study completed in May 1993, under the auspices of the Space Exploration Initiative (SEI). The objective was to develop a "Reference Mission" based on previous studies and data, where the Reference Mission serves as a basis for comparing different approaches and criteria from future studies. The study was based on Robert Zubrin's Mars Direct mission design. Thus dubbed Mars Semi-Direct by Zubrin, it also made several significant changes, for instance accounting  for a larger crew and a dedicated Mars Ascent Vehicle that was to do an Apollo-style Mars-orbit rendezvous with the Earth Return Vehicle, which was to remain in orbit. The Design Reference Mission replaced the preceding SEI as the standing mission plan. Approach and results. The conclusions of the study were that the total mission mass was approximately 900 metric tons for the first crew (3 cargo vehicles, 1 piloted vehicle). The study pointed out that development of the large launch vehicle is a long-lead and expensive system, and approaches using smaller launch vehicles should be investigated . Design Reference Mission 2.0. In 1997 a NASA Mars Exploration Study Team was put together and made a more detailed version of the original design reference mission. The plan describes the first human missions to Mars with concept of operations and technologies to be used as a first cut at an architecture. According to the report: Personnel representing several NASA field centers have formulated a "Reference Mission" addressing human exploration of Mars. This report summarizes their work and describes a plan for the first human missions to Mars, using approaches that are technically feasible, have reasonable risks, and have relatively  low costs. The architecture for the Mars Reference Mission builds on previous work of the Synthesis Group (1991) and Zubrin's (1991) concepts for the use of propellants derived from the Martian Atmosphere. In defining the Reference Mission, choices have been made. In this report, the rationale for each choice is documented; however, unanticipated technology advances or political decisions might change the choices in the future. According to Portree "Subsequent DRM evolution focused on minimizing spacecraft weight in an effort to reduce estimated mission cost." Although not officially given a designation "2.0", the 1997 "scrubbed" (that is, mass reduced) version  is referred to by the designation 2.0 in later documents. Design Reference Mission 3.0. Design Reference Mission 3.0 was a continuation of the 1997 study performed by the NASA Mars Exploration Team, with the report published in June 1998 as an addendum to the 1997 study. The stated purpose of the Reference Mission was to stimulate further thought and development of alternative approaches: "It is intended to identify and clarify system "drivers", or significant sources of cost, performance, risk, and schedule variation. It does not represent a final or recommended approach to human Mars missions. Several alternative scenarios, including human  exploration missions to the Moon, Asteroids, or other targets beyond Earth orbit as well as employing different technical approaches to solving mission and technology challenges, are currently under study by the Exploration Team. Comparing alternative approaches provides the basis for continual improvement to technology investment plans and general understanding of future human exploration missions. The Addendum represents a "snapshot" of work in progress in support of planning for future human exploration missions through May 1998. The report of the Reference Mission Version 3.0 states: From the work of the original Reference Mission (Version 1.0), the strategy for the human exploration  of Mars has evolved from its original form to one of reduced system mass, use of a smaller, more reasonable launch vehicle, and use of more current technology. The steps which have been taken by the Exploration Team are motivated by the need to reduce the mass of the payload delivery flights, as well as the overall mission cost, without introducing additional mission risk. By eliminating the need for a large heavy-lift launch vehicle and deleting the redundant habitat delivery flight in Version 3.0, two launches from the Earth were eliminated. The net result is a current Version 3.0  Reference Mission which requires an injected mass of approximately one-half that of the 1993/94 Reference Mission." Design Reference Mission 4.0. The objective of Design Reference Mission 4 in 1998 was to refine DRM 3.0 to improve identified weaknesses, provide further refinement of systems design and concepts, and improve risk abatement strategy. DRM 4.0 examined both nuclear thermal propulsion and solar electric propulsion variants of the Mars transportation system. Principal results. Principal results of the study were Design Reference Architecture 5.0. NASA Design Reference Mission 5.0 was done in 2009. with an addendum in July 2009, and a second addendum  in March 2014 There is also a version of DRA5 called Austere Human Missions to Mars, produced in 2009 that has a reduced amount of hardware and goals. As of 2017, 5.0 is the most recent version of the Design Reference mission.