The Space Exploration Initiative (SEI), announced by President George H. W. Bush amid much fanfare on the steps of the National Air and Space Museum on 20 July 1989, failed utterly in its stated purpose. As President Bush described it, SEI was to have led to a permanent moon base and humans on Mars. Instead, the initiative, which was at least in part an effort to give U.S. aerospace companies new work to do as the Cold War wound down, became a short-lived but powerful idea generator.
SEI promoted development of moon and Mars mission concepts in two ways. First, NASA answered Bush's call with a moon and Mars plan that pleased no one, driving many to propose alternatives. The product of the NASA Johnson Space Center (JSC)-led 90-Day Study, the plan seems to have had as its goal not to put American astronauts on the moon and Mars, but rather to provide SEI opponents with ammunition.
In a major miscalculation, NASA suppressed its own 90-Day Study plan cost estimates out of fear that they would prove politically unpalatable. The estimates leaked, however, and soon SEI opponents - who were not privy to the cost-estimation methodology because NASA had deleted that section of the 90-Day Study report - declared, wrongly, that Bush's initiative would cost at least a trillion dollars. In fact, the most ambitious variant of the 90-Day Study plan was estimated to cost $541 billion between 1991 and 2025 with a 55% "cushion" built in to compensate for anticipated cost overruns.
Second, in what was ostensibly an effort to get at faster, cheaper concepts for moon bases and Mars missions, Vice President Dan Quayle forced on NASA an "Outreach Program" that solicited ideas for carrying out SEI from anyone who cared to offer them. As chair of Bush's National Space Council, Quayle, whose experience included nothing connected with spaceflight, was in charge of SEI. Eventually the Outreach Program pulled together some 2000 inputs from industry and individuals. Many of the concepts were unworkable, while others boasted respectable pedigrees dating back to the 1950s. The Outreach Program seems also to have been a weapon in a personal feud between Quayle and NASA Administrator and former astronaut Richard Truly.
The SEI Synthesis Group was responsible for sifting through the mish-mash of concepts that the Outreach Program collected. Chaired by former astronaut Thomas Stafford, it became the equivalent of a sausage machine. With help from a staff of 40 and a team of 22 experts that included retired Apollo-era luminaries Maxime Faget, Robert Seamans, and Christopher Kraft, Stafford turned the crank, and out popped the report America at the Threshold in May 1991.
By then it was clear to many that SEI was beyond saving. Congressional opposition remained vehement and other space program issues - U.S.-Russian space cooperation and problems with the Space Shuttle and Space Station - had taken center stage. By early in the election year of 1992, even President Bush sought to distance himself from SEI. Bush failed to win a second term as President and SEI studies ground to a halt by early 1994.
The Synthesis Group report invoked heavy-lift rockets, nuclear-thermal propulsion stages, and a strict adherence to the letter of the SEI plan as laid out by Bush (that is, it contained little that could be construed as fat). From it emerged the First Lunar Outpost (FLO) plan and the Mars Exploration Study Team's 1993 Design Reference Mission (DRM). The former will be described in future Beyond Apollo posts. The latter, meant to grow from FLO, constituted a marriage of the Synthesis Group's proposed Mars architecture and the Mars Direct plan.
Of the concepts that emerged during SEI, Mars Direct, the brainchild of Denver-based Martin Marietta engineers Robert Zubrin and David Baker, has had the most staying power and exercised the most profound influence on subsequent NASA Mars planning. In part this was because it is a clever plan. Equally important, however, was that one of its authors - Zubrin - made himself its zealous champion.
At the time Bush made his SEI announcement Martin Marietta was no stranger to moon and Mars planning. After the Ride Report led NASA to establish the Office of Exploration in 1987, the company had become its de facto support contractor. The 90-Day Study ignored Office of Exploration plans, but some aspects of Mars Direct - for example, reliance on heavy-lift and launch of fully assembled systems - reflect this earlier work.
Zubrin and Baker unveiled their plan in a briefing to NASA engineers at Marshall Space Flight Center in Huntsville, Alabama, in April 1990, then went public with it in late May at a National Space Society conference in Anaheim, California. Mars Direct first received significant media attention in early June when Zubrin presented it to the Case for Mars IV conference in Boulder, Colorado.
In the summer of 1990, Zubrin made the rounds of aerospace meetings. Among his first stops was the NASA Office of Space Flight-sponsored Space Transportation Propulsion Technology Symposium held 25-29 June 1990, at Pennsylvania State University. His Penn State presentation - provocatively titled "Humans to Mars in 1999!" - saw print in a 1991 NASA Conference Publication. It was, as best this author can determine, the first time Mars Direct appeared in a public NASA document.
Zubrin outlined the Mars Direct mission architecture for his Penn State audience. In December 1996, a heavy-lift rocket based on Space Shuttle hardware would boost a 40-metric-ton propellant factory and unmanned Earth Return Vehicle (ERV) toward Mars. Zubrin and Baker dubbed their hypothetical launcher Ares. The 82.3-meter-tall rocket would comprise twin Advanced Solid Rocket Boosters (ASRBs), a heavily modified Space Shuttle External Tank with a side-mounted "pod" containing four Space Shuttle Main Engines, and a new-design 172-metric-ton chemical-propellant stage. The ASRBs counted as Stage 0, the modified ET as Stage 1, and the new-design stage as Stage 2. The Stage 1 and Stage 2 rocket motors would burn liquid hydrogen and liquid oxygen propellants.
The propellant factory made Mars Direct the latest in a long series of Mars mission plans that included on In-Situ Propellant Production (ISPP). ISPP mission concepts first appeared in the early 1960s, when very little was known about Mars's surface and atmospheric composition. While the Viking landers operated on Mars in the late 1970s, an ISPP Mars Sample Return plan was formulated based on the Mars surface and atmosphere data they returned. No piloted Mars plan had, however, relied quite so heavily on ISPP as did Mars Direct.
Eight months after Earth departure, the propellant factory/ERV would aerobrake into Mars orbit behind a 23-meter-diameter, 5.26-metric-ton umbrella-like "flex-fabric" heat shield. Soon after capture into Mars orbit, the landing propulsion module would ignite its rocket motors to decelerate the propellant factory/ERV for reentry into the martian atmosphere, which is about 1% as dense as Earth's. The heat shield would fall away after reentry, then a 5.85-metric-ton landing propulsion module would ignite its rockets and lower the first Mars Direct payload to a gentle touchdown on the rust-red martian surface. Zubrin told his audience that all Mars Direct payloads would use the same heat shield and landing propulsion module design.
Immediately after landing, the propellant factory/ERV would deploy a robot "utility truck" rover carrying a 4.5-metric-ton SP-100 nuclear reactor. The rover, which would burn methane fuel and oxygen oxidizer, would carry the reactor a few hundred meters away and place it into a natural crater or one blasted "with the aid of a few sticks of dynamite." The crater wall and rim would prevent the reactor from irradiating the landing area. Thermal radiators would deploy from the SP-100, then the rover would run a cable from the reactor back to the propellant factory/ERV.
The SP-100 would supply 100 kilowatts of electricity to compressors in the ERV. These would draw in martian air, which is mostly carbon dioxide. The carbon dioxide would be reacted in the presence of a catalyst with 5.8 metric tons of liquid hydrogen brought from Earth, yielding 37.7 metric tons of methane and water. Electricity from the reactor would split the water into hydrogen and oxygen. The oxygen would be chilled until it became liquid and stored, along with the methane, in the two-stage ERV's propellant tanks. The hydrogen would then be reacted with more martian carbon dioxide. Meanwhile, carbon dioxide would be split into oxygen, which would be chilled and stored, and carbon, which would be discarded. In a year, these chemical processes would produce 107 metric tons of methane and oxygen for the ERV's rocket motors.
In January 1999, NASA would launch two more Ares rockets. One payload would be identical to the 1996 propellant factory/ERV. The other would take the form of a drum-shaped, two-deck habitat lander measuring about 8.4 meters in diameter and 4.9 meters tall. The top deck of the 38-metric-ton habitat lander would provide its four-person crew with a total of about 55.2 square meters of floor space (about as much as a one-bedroom apartment). A closet-sized airlock at the center of the top deck would provide access to the bottom deck and double as a radiation shelter during solar flares. The bottom deck would contain supplies and equipment, including a 1.6-metric-ton methane/oxygen-driven pressurized rover and an inflatable Mars surface greenhouse. Oddly enough, the habitat lander appeared to lack any sort of flight controls.
Zubrin envisioned that the habitat lander would separate from the spent Ares rocket Stage 2, but would remain attached to it by a cable about 1400 meters long. The assemblage would revolve once per minute, creating acceleration in the lander that the astronauts would feel as gravity. He assumed that acceleration equal to the pull of martian gravity - that is, equal to about 38% of Earth gravity - would be sufficient to ensure astronaut health during the six-month journey to Mars. Stage 2 and the tether would be cast off as the habitat lander neared Mars.
The habitat lander would aerobrake into Mars orbit, then would descend to a landing near the 1996 propellant factory/ERV. The 1999 propellant factory/ERV would land a few days later about 800 kilometers away, deploy its reactor, and begin making propellants. If all went as planned, the second Mars crew, launched in 2001, would land near the 1999 propellant factory/ERV.
Zubrin and Baker opted for a conjunction-class Mars mission; that is, one in which the crew would travel to Mars on a six-month minimum-energy trajectory, remain at Mars for about 500 days, and return to Earth on a six-month minimum-energy trajectory. In practice, travel and stay times would vary from one conjunction-class mission opportunity to the next because Mars has a somewhat elliptical orbit about the Sun. Cautious NASA planners have generally opted for opposition-class Mars missions, which would see the crew stay at Mars for perhaps a month. Though a bold choice, the conjunction-class mission - the type, incidentally, that Wernher von Braun used in his 1950s Mars mission plans - would mean that the crew would spend more time on Mars than they would in traveling to and from the planet. It would also need less energy - hence propellants - than an opposition-class mission.
Eleven of the 107 metric tons of methane and oxygen in the 1996 ERV's tanks would be set aside to power the pressurized rover, which could rack up a mileage total of 16,000 kilometers. Two-person crews would take turns exploring up to about 500 kilometers from their base camp. They might even reach the 1999 ERV, Zubrin told his Penn State audience, and use it as a field camp for more distant forays. If a condenser were added to recover the water vapor in the rover's exhaust for recycling, then rover mileage might reach 160,000 kilometers.
One of the mission's objectives would be to seek water ice. This would, Zubrin explained, be split to form hydrogen and oxygen, eliminating the requirement that liquid hydrogen be brought from Earth for propellant maufacture. Mars water could also supplement and eventually replace life-support consumables brought from Earth, permitting the early development of Mars settlements.
As they neared the end of their pioneering 500-day stay on Mars, the 1999 mission crew would load 100 kilograms of Mars samples into the 1996 ERV's crew cabin. They would then ignite the ERV's first-stage rocket motors. After the ERV first stage exhausted its propellants, the second stage would take over, placing the 1999 crew on a direct path back to Earth. Six months after Mars departure, they would arrive at Earth, where they would aerobrake into Earth orbit and rendezvous with the Space Station or a waiting Space Shuttle. The ERV, which Zubrin explained would be reusable, would include as part of its structure a biconic ("bent cone") aerobrake.
The 2001 crew would land near the 1999 propellant factory/ERV shortly after the 1999 crew left Mars. The 2001 propellant factory/ERV would, meanwhile, touch down about 800 kilometers away and begin manufacturing propellants for the 2003 crew. The 2001 crew might use the 1999 crew's habitat lander and the 2003 crew's propellant factory/ERV as field camps.
Zubrin told the engineers gathered at Penn State that the Mars Direct program could continue indefinitely, with two Ares rockets launching during every minimum-energy Mars-Earth transfer opportunity (that is, roughly every two years). This would establish a "string" of bases that might become "the seeds for future Martian towns," much as forts had become the nuclei of towns in the Old West.
The Martin Marietta engineers scheduled establishment of the first permanent Mars settlement for soon after a Nuclear-Thermal Rocket (NTR) engine became a part of the Mars Direct plan. Replacement of the chemical Stage 2 with an NTR version expending only hydrogen propellant would enable a single Ares rocket to boost both a habitat lander and a propellant factory/ERV toward Mars. Zubrin told his audience that the NTR-Ares could boost "large habitations and massive amounts of equipment" to Mars. If NASA opted to continue to launch a pair of rockets in each minimum-energy Earth-Mars transfer opportunity, then as many as 12 astronauts might reach Mars about every two years. If NTR-Ares-launched habitat lander and propellant factory/ERV payloads landed at one site on Mars over several opportunities, then a 100-person Mars settlement could be founded in the 2010-2020 decade.
According to one NASA engineer, Mars Direct was more a "parable" (that is, a story that teaches a lesson) than a fully developed mission plan. To assume that it would be otherwise would be to expect too much from a pair of engineers, no matter how brilliant they might be. Even so august a figure as Wernher von Braun proposed Mars mission scenarios that overlooked or downplayed important challenges.
Several difficulties became obvious to NASA planners as they sought to integrate Mars Direct with their FLO-derived Mars plans. For example, the 7.1-metric-ton ERV cabin was too small to support all the needs of four people during a six-month weightless journey from Mars to Earth. Zubrin did not reveal to his Penn State audience the ERV's planned habitable volume, though he did compare it to the Space Shuttle's roughly 70-cubic-meter two-deck crew cabin. Controls, pilot and commander seats, equipment, and storage compartments occupied a large portion of the Shuttle crew cabin volume; Zubrin did not indicate whether the same would apply to the ERV cabin. In addition, the lone pressurized rover could not be used to its full potential; if it ventured beyond the range of an astronaut on foot and became stuck or broke down, then its occupants would become stranded without hope of rescue.
Despite such shortcomings, Mars Direct caused a paradigm shift in NASA Mars planning. In the two decades since the Mars Exploration Study Team completed its 1993 DRM, NASA Mars planning has evolved in response to internal and external pressures. Despite this, current NASA plans - the Design Reference Architecture 5.0 (2007) and its Human Exploration using Real-time Robotic Operations (HERRO) variant (2009) - contain echoes of Zubrin and Baker's 1990 Mars Direct plan.
References:
"Humans to Mars in 1999!" Robert Zubrin and David Baker, Martin Marietta Astronautics; paper presented at the NASA Office of Space Flight Space Transportation and Propulsion Technology Symposium at Pennsylvania State University, State College, Pennsylvania, 25-29 June 1990, published as Presentation 4.1.4 in NASA Conference Publication 3112, 1991, pp. 881-891.
Humans to Mars: Fifty Years of Mission Planning, David S. F. Portree, Monographs in Aerospace History #21, NASA SP-2001-4521, February 2001, pp. 77-99.
Mars Wars: The Rise and Fall of the Space Exploration Initiative, Thor Hogan, NASA SP-2007-4410, August 2007.
The Case for Mars: The Plan to Settle the Red Planet and Why We Must, Robert Zubrin with Richard Wagner, Free Press, 1996.
