In January 1999—the next minimum-energy Mars transfer opportunity—two more Ares rockets would lift off. One would carry a cargo lander identical to the one already on Mars. The other would carry a "manned spacecraft looking somewhat like a giant hockey puck 27.5 feet in diameter and 16 feet tall" based on Martin Marietta designs developed for the NASA Office of Exploration. 8 The top floor would comprise living quarters for the four-person crew, while the bottom floor would be stuffed with cargo and equipment, including a pressurized rover. Zubrin and Baker estimated the piloted spacecraft's weight at 38 tons.
The upper stage would launch the "hockey puck" spacecraft on course for Mars and separate, but the two would remain attached by a 1,500-meter tether. This assemblage would rotate once per minute to produce acceleration equal to Martian surface gravity in the piloted spacecraft. A similar lightweight artificial gravity concept was proposed by Robert Sohn in 1964. Near Mars the upper stage and tether would be discarded.
The piloted spacecraft would aerobrake into Mars orbit, then land near the 1996 cargo lander. No part of the ship would remain in orbit. Landing the entire crew on the surface would help minimize risk. Once on Mars, the Martian atmosphere would provide some radiation protection, and the crew could use Martian dirt as additional shielding. They would also experience Martian gravity. Though only a third as strong as Earth's gravity, it seemed likely that even that small amount would be preferable to a long weightless stay in Mars orbit.
As in the SAIC split-sprint plan, the crew would have to rendezvous at Mars with propellants for their trip home. This was seen by some as increasing risk. Unlike the SAIC crew, however, the Mars Direct astronauts would have options if they could not reach their Earth-return propellants.
Baker and Zubrin pointed out that the crew had their rover to drive to the 1996 cargo lander, though ideally they would land within walking distance. If some gross error meant they landed more than 600 miles from the 1996 cargo lander—beyond the range of their rover— they could command the cargo lander launched with them in 1999 to land nearby. It would then manufacture propellant for their return to Earth. If the 1999 cargo lander failed, the Mars Direct astronauts would have sufficient supplies to hold out until a relief expedition arrived in two years. Assuming that the crew landed near the 1996 cargo lander as planned, the 1999 cargo lander would set down 500 miles from the first Mars landing site and begin to make propellants for the second Mars expedition, which would leave Earth in 2001.
Eleven of the 107 tons of propellants manufactured by the 1996 cargo lander would be set aside to power the pressurized rover. During their 500-day stay on Mars, the explorers would conduct long traverses—up to 600 miles round-trip—thoroughly characterizing the region around their landing site. This impressive capability would maximize science return by allowing the crew to survey large areas, though with some increased risk. If the rover broke down, the crew could become stranded beyond hope of rescue, hundreds of kilometers from base.
At the end of the 500-day Mars stay, the ERV engine would ignite, burning methane and oxygen propellants manufactured using the Martian atmosphere. The small ERV spacecraft would use the cargo lander as a launch pad to perform ascent and direct insertion onto a trajectory to Earth. After six weightless months in the cramped ERV, the crew would reenter Earth's atmosphere and perform a parachute landing. The small ERV was considered by many to be a weak link in the Mars Direct plan.
The 2001 expedition crew would land near the 1999 cargo lander. If all went as planned, the 2001 cargo lander would land 500 miles away. The 2003 crew would land next to the 2001 cargo lander, while the 2003 cargo lander would touch down 500 miles away for the 2005 expedition, and so on. After several expeditions, a network of bases would be established. "Just as towns in the western U.S. grew up around forts and outposts," wrote Baker and Zubrin, "future Martian towns would spread out from some of these bases. As information returns about each site, future missions might return to the more hospitable ones and larger bases would begin to form." 9
SEI’s Last Gasp
In SEI's last days, the Stafford Synthesis Group report formed the basis of NASA's Mars planning. From 1991 to 1993, the Agency performed the First Lunar Outpost (FLO) study, which took as a point of departure the lunar elements of the Synthesis Group's four architectures. In the summer of 1992, the NASA Headquarters Exploration Office under Michael Griffin, the successor to the Office of Exploration first headed by Sally Ride, launched a NASA-wide study to determine how FLO might find hardware commonality with a follow-on Mars expedition, thereby reducing the costs of both programs. 10
The Mars Exploration Study Team workshop held in August 1992 produced a plan containing elements of both Mars Direct and the Synthesis Group Mars plan. It was briefed to Griffin in September. 11 The May 1993 Mars Exploration Study Team workshop produced a Mars expedition Design Reference Mission (DRM) with little overt FLO commonality beyond a common heavy-lift rocket and outwardly similar vehicles for lunar and Mars ascent. In fact, the DRM was modeled on Mars Direct. Robert Zubrin was an advisor to the Mars Exploration Study Team in late 1992 and 1993. He briefed Griffin on Mars Direct in June 1992, then briefed the JSC Exploration Program Office in October 1992. 12
The Mars Exploration Study Team DRM was reported in a workshop summary and in technical papers in September and November 1993. 13, 14 It included the following:
- no low-Earth orbit operations or assembly— that is, no reliance on a space station as a Mars transportation element,
- no reliance on a lunar outpost or other lunar operations,
- heavy-lift rocket capable of launching 240 tons to low-Earth orbit, 100 tons to Mars orbit, and 60 tons to the Martian surface (more than twice the capability of the Saturn V),
- short transit times to and from Mars and long Mars surface stay times beginning with the first expedition (conjunction-class missions),
- six crewmembers to ensure adequate manpower and skills mix,
- early reliance on Mars ISRU to minimize weight launched to Mars, and
- common design for surface and transit habitats to reduce development cost.
The most significant difference between Mars Direct and the Mars Exploration Study Team's DRM was the division of the Mars Direct ERV functions between two vehicles. In the Mars Direct plan, the ERV lifted off from Mars at the end of the surface mission and flew directly to Earth. In the judgment of many, however, the Mars Direct ERV was too small to house four astronauts during a six-month return from Mars, let alone the DRM's six astronauts. 15 In the DRM, therefore, only a small Mars Ascent Vehicle (MAV) would rely on ISRU. The crew would use it to reach Mars orbit at the end of their surface stay and dock with the orbiting ERV. The addition of a rendezvous and docking in Mars orbit was seen by some as increasing risk to crew, but there seemed to be little alternative if a realistically large ERV was to be provided.
Figure 23—NASA's 1993 Mars mission plan: after landing on Mars, the automated propellant factory manufactures liquid methane and liquid oxygen propellants for the conical Mars Ascent Vehicle it carries on top. (NASA Photo S93-50643)
<!-- image -->The September 2007 Mars transfer opportunity was used for the study because it would be challenging in terms of time and energy required for Mars transfer, not necessarily because an expedition was planned for that time. The first expedition would begin with launch of three heavy-lift rockets, each bearing one unmanned spacecraft and one nuclear propulsion upper stage. The three spacecraft were the cargo lander, the ERV orbiter, and an unmanned Habitat lander. They would weigh between 60 and 75 tons each, a weight estimate considered more realistic than the 30 to 40 tons quoted in Mars Direct.
The ERV and Habitat designs were based on a common crew module design resembling the Mars Direct "hockey puck." The cargo lander would carry the MAV, ISRU propellant factory, and hydrogen feedstock, along with 40 tons of cargo, including the pressurized rover. All would reach Mars during August and September 2008. The ERV would aerobrake into Mars orbit, while the cargo lander and Habitat would land on Mars. The cargo lander would then set about manufacturing 5.7 tons of methane and 20.8 tons of oxygen for the MAV and a 600-day cache of life-support consumables.
Figure 24—The crew Habitat lands near the propellant factory with empty propellant tanks. Note wheels for moving the Habitat on the martian surface. (NASA Photo S93-050645)
<!-- image -->As in Mars Direct, the crew would follow during the next Mars launch opportunity 26 months later (October-November 2009), accompanied by unmanned vehicles supporting the next expedition or providing backup for those already on Mars. The explorers would land near the 2007 cargo lander and Habitat. The Habitats would include wheels to allow the explorers to move them together so they could be linked using a pressurized tunnel. The 2007 Habitat would also provide a backup pressurized volume if the 2009 Habitat was damaged during landing and rendered uninhabitable.
Figure 25—Mars Base 1: the crew docks its Habitat on the surface with a second Habitat and begins a 600-day stay. They use a pressurized rover (left) to explore up to 500 kilometers from base. (NASA Photo S93-45582)
<!-- image -->The first Mars outpost thus established, the crew would unpack the pressurized rover from the 2007 cargo lander. During their 600-day stay on Mars, the crew would carry out several 10-day rover traverses ranging up to 500 kilometers from the outpost.
In October 2011, the 2009 crew would lift off from Mars in the 2007 MAV. They would dock in Mars orbit with the 2007 ERV and fire its twin liquid methane/liquid oxygen rocket engines to leave Mars orbit for Earth, retaining the MAV capsule. Near Earth the explorers would enter the MAV capsule and detach from the ERV, which would sail past Earth into solar orbit. They would then reenter Earth's atmosphere and perform a parachute landing.
Figure 26—Using the propellant factory as a launch pad, the Mars Ascent Vehicle blasts off burning propellants made from terrestrial hydrogen and Martian atmospheric carbon dioxide. (NASA Photo S93-050644)
<!-- image -->The Mars Exploration Study Team effort was SEI's last gasp. Before it was completed, NASA had begun to dismantle its formal Mars exploration planning organization. The Headquarters Exploration Office was abolished in late 1992. The JSC Exploration Directorate, created soon after The 90-Day Study's release, was trimmed back and re-created as the JSC Planetary Projects Office. 16
Figure 27—Mars Orbit Rendezvous: The Mars Ascent Vehicle docks with the Earth Return Vehicle in Mars orbit. The Earth Return Vehicle's rocket engines would place the crew on a six-month low-energy trajectory homeward. (NASA Photo S93-27626)
<!-- image -->As the apparatus for piloted Mars planning within NASA shrank, automated Mars exploration also suffered a cruel blow. Mars Observer, the first U.S. automated Mars mission since the Vikings, had left Earth on 25 September 1992. On 21 August 1993, three days before planned Mars orbit arrival, the spacecraft's transmitter was switched off as planned to protect it from shocks during propellant system pressurization. Contact was never restored. An independent investigation report released in January 1994 pointed to a propulsion system rupture as the most probable cause of Mars Observer's loss, the first post-launch failure of a U.S. planetary exploration mission since Surveyor 4 in 1967. 17