The JSC flyby plan for countering this possible Soviet move was prepared by Barney Roberts, who performed lunar base studies in the JSC Engineering Directorate. 30 Roberts' year-long Mars flyby mission would begin with orbital assembly at the Space Station. Shuttles would deliver two expendable strap-on propellant tanks and an 18-ton Mission Module to the Station. The latter would dock with a 6-ton Command Module (not to be confused with the Apollo CM) and two 5.75-ton OTVs assumed to be in space already as part of the Space Station Program. Shuttle-derived heavy-lift rockets would then deliver 221 tons of liquid hydrogen/liquid oxygen propellants. The propellants would be loaded into the strap-on and OTV tanks just prior to departure for Mars. Spacecraft weight at Earth-orbit departure would come to 358 tons.
Chapter 7: The Case for Mars
At the proper time, the OTV engines would ignite and burn for about one hour to put the flyby craft on course for Mars. This would empty the strap-on tanks, but Roberts advised retaining them to provide additional meteoroid and radiation shielding for the crew modules. After a six-month Earth-Mars transfer, the flyby spacecraft would spend two and one-half hours within about 20,000 miles of Mars. Closest approach would occur 160 nautical miles above the Martian surface with the flyby craft moving at 5 miles per second.
As Earth grew from a bright star to a distant disk, the astronauts would discard the strap-on tanks, then undock one OTV and redock it to the Command Module. They would enter the Command Module and discard the Mission Module and the second OTV. The OTV/Command Module combination would slow to a manageable reentry speed using the OTV's engines, aerobrake to Earth-orbital speed, then dock at the Space Station.
Roberts found (as had planners in the 1960s) that Earth return was the most problematical phase of the flyby mission because the OTV would hit Earth's atmosphere at 55,000 feet per second, producing friction heating beyond the planned limits of the OTV heat shields. In addition, the crew would experience "exorbitant" deceleration levels after spending a year in weightlessness.
Figure 19—During return to Earth the flyby spacecraft discards empty propellant tanks, revealing cylindrical Command and Mission Modules between twin almond-shaped Orbital Transfer Vehicles. ("Concept for a Manned Mars Flyby," Barney Roberts, Manned Mars Missions: Working Group Papers, NASA M002, NASA/Los Alamos National Laboratories, Huntsville, Alabama/Los Alamos, New Mexico, June 1986, Vol. 1, p. 210.)
<!-- image -->In the 1960s, planners proposed a Venus flyby to reduce reentry speed without using propellant, but Roberts did not mention this possibility. He proposed instead to slow the OTV and Command Module to 35,000 feet per second using the former's engines. Adding this burn would nearly double spacecraft weight at Earth-orbit departure. Roberts calculated that, assuming the Space Shuttle-derived heavy-lift rocket could deliver cargo to Earth orbit at a cost of $500 per pound, Earth-braking propellant would add $250 million to mission costs. 31
Figure 20—The flyby crew prepares to aerobrake in Earth's atmosphere. As Earth grows from a bright star to a disk, they undock the Command Module and one Orbital Transfer Vehicle, abandoning the second Orbital Transfer Vehicle and their home for the previous year, the Mission Module. ("Concept for a Manned Mars Flyby," Barney Roberts, Manned Mars Missions: Working Group Papers, NASA M002, NASA/Los Alamos National Laboratories, Huntsville, Alabama/Los Alamos, New Mexico, June 1986, Vol. 1, p. 213.)
<!-- image -->Interplanetary Infrastructure
Some Mars planners envisioned the NASA Space Station in low-Earth orbit as merely the first in a series of stations in logical places serving as Mars transportation infrastructure, much like trails, canals, railways, and coaling stations formed transportation infrastructure in bygone days. They looked ahead to solar-orbiting space stations, known as cyclers, traveling a regular path between Earth and Mars, and to spaceports at the Lagrange gravitational equilibrium points. Apollo 11 Lunar Module Pilot Buzz Aldrin, the second man on the Moon, described cyclers in a popular-audience article in Air & Space Smithsonian magazine in 1989:
Like an oceanliner on a regular trade route, the Cycler would glide perpetually along its beautifully predictable orbit, arriving and departing with clock-like regularity. By plying the solar system's gravitational "trade winds" it will carry mankind on the next great age of exploration . . . . For roughly the same cost as getting humans safely to Mars via conventional expendable rocketry (because the problems to be solved would be largely the same), the Cycler system would provide a reusable infrastructure for travel between Earth and Mars far into the future. 32
In the 1960s, Massachusetts Institute of Technology professor Walter Hollister and others studied "periodic" orbits related to Crocco flyby orbits but indefinitely repeating. A space station in such an orbit would cycle "forever" between Earth and the target planet. In January 1971, Hollister and his student, Charles Rall, described an Earth-Mars transport system in which at least four cycling periodic-orbit stations would operate simultaneously, permitting opportunities every 26 months for 6-month transfers between Earth and Mars. 33
As the large periodic-orbit station flew past Earth or Mars, small "rendezvous shuttle vehicles" would race out to meet it and drop off crews and supplies for the interplanetary transfer. After several Mars voyages, the cycler approach would yield a dramatic reduction in spacecraft mass over the MOR mission mode because the cycler would only need to burn propellants to leave Earth orbit once; after that, only the small shuttles would need to burn propellants to speed up and slow down at Earth and Mars.
The Case for Mars II conference (10-14 July 1984) included a workshop that planned "a permanent Mars research base using year 2000 technology" as a "precursor to eventual colonization." The Case for Mars II workshop took advantage of the long-term weight-minimization inherent in cyclers and Mars ISRU. The Boulder Center for Science and Policy published a JPL-funded report on the workshop results in April 1986. 34
The Case for Mars had begun to gather steam. Participants in the second conference included Harrison Schmitt with a paper on his Mars 2000 project, Benton Clark, and Christopher McKay, who had earned his Ph.D. and gone to work at NASA Ames. Former NASA Administrator Tom Paine presented a timeline of Mars exploration spanning 1985 to 2085. It predicted, among other things, a lunar population in the thousands in the 2025-2035 decade and a Martian population of 50,000 in the 2055-2065 decade. 35 Barney Roberts, Michael Duke, and lunar scientist Wendell Mendell presented a paper called "Lunar Base: A Stepping Stone to Mars," 36 while NASA Space Station manager Humboldt Mandell presented a paper called "Space Station: The First Step." 37
In the Case for Mars II plan, the cycler's Earth-Mars leg lasted six months, followed by a Mars-Earth leg lasting 20 to 30 months. Each crew would spend two years on Mars, and new crews would leave Earth every two years. The first crew would leave Earth in 2007 and return in 2012; the second crew would depart in 2009 and return in 2014; and so on. This schedule would require at least two cyclers. As Hollister and Rall proposed, small Crew Shuttle vehicles would transfer crews to and from the passing cycler. The Crew Shuttles were envisioned as two-stage biconic vehicles designed for aerobraking at Earth and Mars. Their proposed shape was derived from ballistic missile warhead research.
A heavy-lift rocket capable of launching at least 75 metric tons, possibly based on Shuttle hardware, would place cycler components, some based on Shuttle and Space Station hardware, into Earth orbit for assembly. The 1984 Case for Mars plan called for cycler assembly at the low-Earth orbit Space Station; in a subsequent version, a dedicated assembly facility was proposed. The first Mars expedition would require 24 heavy-lift rocket launches and 20 Shuttle launches.
ISRU would supply the Case for Mars II base with many consumables, including propellant. "Mars is abundantly endowed with all the resources necessary to sustain life," the report stated, adding that "propellant production on the surface of Mars is critical to reducing the cost of the program" because it "reduces the Earth launch weight by almost an order of magnitude." 38 Each Crew Shuttle would require 150 tons of Mars ISRU-manufactured carbon monoxide/oxygen propellant to catch up with the passing Earth-bound cycler. The Case for Mars II workshop proposed that an automated probe should test ISRU propellant production on Mars before the Mars base program began.
Lagrangia
Like the cycler concepts, the notion of siting infrastructure at the Lagrange points dates to the 1960s. Its theoretical roots, however, date to 1772. In that year, French mathematician Joseph Lagrange noted that gravitational equilibrium points exist in isolated two-body systems.
Lagrange points exist in space—for example, in the two-body Earth-Moon system. In theory, an object placed at one of these points will remain as if nesting in a little cup of space-time. In practice, Lagrange points in space are unstable or quasi-stable because planets and moons do not exist as isolated two-body systems. In the case of the Earth-Moon system, the Sun's gravitational pull introduces instability. Objects placed at the Earth-Moon Lagrange points thus tend to move in "halo orbits" around the Lagrange point and require modest station keeping to avoid eventual ejection.
Robert Farquhar, an engineer at NASA's Goddard Space Flight Center in Greenbelt, Maryland, first wrote about using the Lagrange equilibrium points of the Earth-Moon system in the late 1960s. 39 For the NASA MMM workshop in June 1985, Farquhar teamed up with David Dunham of Computer Sciences Corporation to propose using Lagrange points as "stepping stones" for Mars exploration. 40
Farquhar and Dunham envisioned a large, reusable Interplanetary Shuttle Vehicle (ISV) in halo orbit about the quasi-stable Earth-Sun Lagrange 1 point, 1.5 million kilometers in toward the Sun. A Mars transport spacecraft parked there would be gravitationally bound to Earth much more weakly than if parked in low-Earth orbit. A mere propulsive burp would suffice to nudge the ISV out of halo orbit; then gravity-assist swingbys of the Moon and Earth would place it on course for Mars with little additional propellant expenditure. This meant, of course, that the amount of propellant that would need to be launched from Earth was minimized. To save even more propellant, the ISV might park at the Mars-Sun Lagrange 1 point, about 1 million kilometers Sunward from Mars, and send the crew to the Martian surface using small shuttle vehicles.
Farquhar and Dunham pointed out that an automated spacecraft had already left Earth-Sun Lagrange 1 on an interplanetary trajectory. The International Sun-Earth Explorer-3 spacecraft had entered Earth-Sun Lagrange 1 halo orbit on 20 November 1978. After completing its primary mission it was maneuvered during 1984 through a series of Earth and Moon swingbys to place it on course for Comet Giacobinni-Zinner. Farquhar supervised the effort. The maneuvers consumed less than 75 kilograms of propellant. Renamed the International Comet Explorer, the spacecraft successfully flew past Giacobinni-Zinner, 73 million kilometers from Earth, on 11 September 1985.
Paul Keaton elaborated on Farquhar and Dunham's MMM paper in a "tutorial" paper published in August 1985. He wrote that "[a]n evolutionary manned space program will put outposts along routes with economic, scientific, and political importance" to serve as "'filling stations' for [making and] storing rocket fuel" and "transportation depots for connecting with flights to other destinations." 41
The first outpost would, of course, be NASA's planned Space Station in low-Earth orbit, where Earth's magnetic field would help protect travelers from galactic cosmic rays and solar flare radiation, and medical researchers would learn about the effects of long-term weightlessness on the human body. Keaton then looked beyond Earth orbit for the next outpost site. He proposed placing it in halo orbit around the Earth-Moon Lagrange 2 point, 64,500 kilometers behind the Moon, or at Farquhar's Earth-Sun Lagrange 1 site. He wrote,