Revival: A Piloted Mars Flyby in the 1990s (1985)

An Orbital Transfer Vehicle (OTV) carrying a drum-shaped Command Module aerobrakes in Earth's atmosphere in this NASA painting by Pat Rawlings. 
In the 1960s, NASA expended nearly as much study money and effort on piloted Mars and Venus flyby mission planning as it did on its more widely known plans for piloted Mars landings. Italian aviation and rocketry pioneer Gaetano Crocco had described a free-return piloted Mars/Venus flyby mission in 1956. Piloted flyby studies within NASA began with the EMPIRE study the Marshall Space Flight Center (MSFC) Future Projects Office initiated in 1962 and culminated in the NASA-wide Planetary Joint Action Group (JAG) piloted flyby study of 1966-1967.

The Planetary JAG, led by the NASA Headquarters Office of Manned Space Flight, brought together engineers from MSFC, Kennedy Space Center, the Manned Spacecraft Center (MSC), and Washington, DC-based planning contractor Bellcomm. It issued a Phase I report in October 1966 and continued Phase II study work in Fiscal Year (FY) 1967. The Phase I report emphasized a piloted Mars flyby mission in 1975, but included Mars and Venus flyby missions tailored to low-energy mission opportunities through 1981. All would be based on hardware developed for the Apollo Program and its planned successor, the Apollo Applications Program (AAP).

The piloted flyby spacecraft would carry automated probes, including one that would land on Mars, collect a sample of surface material and launch it back to the flyby spacecraft for immediate analysis. A leading point in favor of the piloted flyby mission was, in fact, the ability of the flyby crew to examine a Mars sample for signs of life less than an an hour after it left the martian surface.

Red planet off the port bow: a piloted flyby spacecraft based on Apollo spacecraft hardware releases probes as it passes Mars. Image credit: Douglas Aircraft Company.
According to Edward Clinton Ezell and Linda Neumann Ezell, writing in their 1984 NASA-published history On Mars: Exploration of the Red Planet, 1958-1978, NASA MSC was largely responsible for the demise of 1960s piloted flyby mission planning. On 3 August 1967, the Houston, Texas-based center, home of the astronaut corps and Mission Control, distributed to 28 aerospace companies a Request for Proposal (RFP) for a piloted Mars flyby spacecraft sample-returner design study. By doing this, MSC appeared to disregard warnings from Congress that no new NASA program starts would be tolerated.

In the summer of 1967, NASA was preoccupied with recovery from the 27 January 1967 Apollo 1 fire, which had killed astronauts Virgil Grissom, Roger Chaffee, and Ed White. Many in Congress felt that NASA had been lax in enforcing quality and safety standards at North American Aviation, the Apollo Command and Service Module spacecraft prime contractor, so deserved to be "punished" for the accident. Other members of Congress were angered by NASA's apparent failure to share its concerns regarding NAA's performance so they could exercise Congressional oversight. They did not, however, wish to cut Apollo funding and endanger accomplishment of Apollo's very public goal of a man on the Moon by 1970.

In addition, by August 1967, the Federal budget deficit for FY 1967 had reached $30 billion. Though negligible by modern standards, this was a shocking sum in 1967. The deficit was driven in large part by the cost of fighting in Indochina, which had reached more than $2 billion a month, or the entire Apollo Program budget of $25 billion every 10 months.

After learning of the MSC RFP, long-time House Space Committee Chair and NASA supporter Joseph Karth declared angrily that "a manned mission to Mars or Venus by 1975 or 1977 is now and always has been out of the question — and anyone who persists in this kind of misallocation of resources. . .is going to be stopped." On 16 August, the House cut all funding for advanced planning from NASA's FY 1968 budget bill and slashed the budget for AAP from $455 million to $122 million. Total cuts to President Lyndon Baines Johnson's January 1967 FY 1968 NASA budget request amounted to more than $500 million, or about 10% of NASA's FY 1967 budget total.

Though he opposed the cuts, President Johnson bowed to the inevitable and signed the budget into law. The Planetary JAG and Bellcomm tied up loose ends of the piloted flyby study during FY 1968, but most work on the concept ended within a few months of the Houston center's ill-timed RFP.

It is ironic, then, that NASA's next piloted Mars flyby study took place in Houston, at Johnson Space Center (JSC), as MSC had been re-christened following President Johnson's death in January 1973. Barney Roberts, an engineer in the JSC Engineering Directorate, reported on the study to the joint NASA-Los Alamos National Laboratory (LANL) Manned Mars Missions workshop in June 1985.

The workshop, held at NASA MSFC, was a significant step in the revival of piloted Mars exploration planning within NASA after the long drought of the 1970s. Unfortunately, in their plan for a piloted Mars flyby in the 1990s, NASA JSC engineers demonstrated little sign of awareness of the 1960s piloted flyby studies. As a result, their proposed mission was less credible than it might have been.

Roberts explained that the NASA JSC flyby plan aimed to counter a possible Soviet piloted Mars flyby. He cited a 1984 Central Intelligence Agency (CIA) memorandum that suggested (without citing much in the way of evidence) that the Soviet Union might attempt such a mission in the 1990s — possibly as early as the 75th anniversary of the Bolshevik Revolution in 1992 — in order to garner international prestige. The CIA study had been performed at the request of Apollo 17 moonwalker Harrison Schmitt, whose chief spaceflight interest in the early-to-mid 1980s was a piloted Mars mission.

NASA's piloted Mars flyby would be based on space hardware expected to be operational and readily available in the late 1990s. Space Shuttle Orbiters would deliver to NASA's Space Station an 18-ton Mission Module (MM) and a pair of expendable propellant tanks with an empty mass of 11.6 tons each. The MM, derived from a Space Station module, would carry a 3000-pound solar-flare radiation shelter, 7000 pounds of science equipment, and 2300 pounds of food and water.

Going for a ride: a piloted Mars flyby spacecraft prepares for launch from Earth orbit in the late 1990s. A = twin Orbital Transfer Vehicles (OTVs); B = twin strap-on propellant tanks; C = Command Module; D = Mission Module. Image credit: NASA/David S. F. Portree.
The MM would be docked to a six-ton Command Module (CM) and two 5.75-ton Orbital Transfer Vehicles (OTVs). The OTVs would each include an aerobrake heat shield and two rocket engines derived from the Space Shuttle Main Engine. The JSC engineers had assumed that the CM and OTVs would be in space already as part of a late 1990s NASA Lunar Base Program. The strap-on tanks would be joined to the MM/CM stack by trunnion pins similar to those used to anchor payloads in the Space Shuttle Orbiter payload bay, then Space Station astronauts would perform spacewalks to link propellant pipes and electrical and control cables.

Shuttle-derived heavy-lift rockets would then deliver a total of 221 tons of cryogenic liquid hydrogen and liquid oxygen propellants to the Space Station to fill the piloted flyby spacecraft's twin tanks. The propellants would be pumped aboard just prior to departure from Earth orbit to prevent liquid hydrogen loss through boil off. Mass of the piloted flyby spacecraft at the start of its Earth-departure maneuver would total 358 tons.

As the launch window for the Mars flyby opportunity opened, the piloted flyby spacecraft would move away from the Space Station using small thrusters on retractable arms, then the four OTV engines would ignite and burn for about one hour to put it on course for Mars. The only propulsive maneuver of the baseline mission, the burn would empty the OTV and strap-on propellant tanks. Roberts advised retaining the spent tanks to serve as shielding against meteoroids and radiation for the MM and CM during the year-long flight.

Roberts told the workshop that the flyby spacecraft would spend two-and-a-half hours within about 20,000 miles of Mars. Closest approach would bring it to within 160 miles of Mars. At closest approach, the spacecraft would be moving at about 5 miles per second.

The spacecraft would then begin its long return to Earth. Roberts provided few details of the interplanetary phases of his piloted Mars flyby mission.

As Earth grew from a bright star to a distant disk, the Mars flyby astronauts would discard the twin strap-on tanks. They would then undock one OTV by remote control and re-dock it to the front of the CM. After entering the CM and sealing the hatch leading to the MM, they would discard the MM and second OTV, then would then strap into their couches to prepare for aerobraking in Earth's upper atmosphere and capture into Earth orbit. After the OTV/CM combination completed the aerobraking maneuver, the crew would pilot it to a docking with the Space Station.

Almost home: the piloted Mars spacecraft prepares for the aerobraking maneuver in Earth's atmosphere at the end of its epic year-long interplanetary voyage. A = OTVs; C = Command Module bearing crew; D = discarded Mission Module (attached to discarded OTV). Image credit: NASA/David S. F. Portree.
Roberts told the NASA/LANL workshop that Earth return would be the most challenging phase of the piloted Mars flyby mission. The OTV/CM combination would encounter Earth's upper atmosphere at a speed of 55,000 feet (10.4 miles) per second, producing reentry heating well beyond the planned capacity of the OTV's heat shield. In addition, the crew would suffer "exorbitant" deceleration after living for a year in weightlessness.

Roberts proposed a "brute-force" solution to these problems: use the OTV's twin rocket motors to slow the OTV/CM to lunar-return speed of 35,000 feet (6.6 miles) per second. The braking burn would, however, increase the Mars flyby spacecraft's total required propellant load to nearly 500 tons. He calculated that, assuming that a Shuttle-derived heavy-lift rocket could be designed to deliver cargo to LEO at a cost of $500 per pound (an optimistic assumption, as it would turn out), then Earth-braking propellant would add $250 million to his mission's cost.

Roberts briefly considered partially compensating for the large mass of braking propellants by substituting an MM derived from a five-ton Space Station logistics module for the 18-ton MM. This would mean, however, that the crew would have to spend a year in cramped quarters with no exercise or science equipment.

Planners in the 1960s had wrestled with and prevailed over the same problems of propellant mass and Earth-return speed that NASA JSC engineers faced in their 1985 study. Bellcomm had, for example, proposed in June 1967 that the Planetary JAG's piloted Mars flyby mission conserve propellants through assembly of the flyby spacecraft in an elliptical orbit, not circular Space Station orbit. The elliptical assembly orbit would mean, in effect, that the flyby spacecraft would begin Earth-orbit departure even as it was being assembled.

In addition, returning the crew directly to Earth's surface in a small Apollo-type capsule with a beefed-up heat shield would greatly reduce the quantity of braking propellants required; it could eliminate the braking maneuver entirely. It would also enable an aerodynamic "skip" maneuver that would reduce deceleration stress on the astronauts.

TRW Space Technology Laboratory had proposed as early as 1964, during the EMPIRE follow-on studies, that NASA use a Venus flyby to slow spacecraft returning from Mars. Crocco had described the concept in 1956, in fact, though in a form that turned out to be unworkable because of errors he made when he calculated his flyby spacecraft's orbit about the Sun.

Exploiting a Venus flyby to reduce speed would, of course, limit Earth-Mars-Earth transfer opportunities to those that would intersect Venus on the return leg, but would also eliminate the costly end-of-mission braking burn and enable Venus exploration as a bonus. The Planetary JAG's October 1966 report described Mars-Venus and Venus-Mars-Venus flyby missions in the late 1970s. Bellcomm determined in late 1966 and 1967 that Mars/Venus flyby opportunities are not rare.


"Soviet Plans for a Manned Flight to Mars," C. Cravotta and M. DeForth, Office of Scientific and Weapons Research, Central Intelligence Agency, 2 April 1985.

"Concept for a Manned Mars Flyby," Barney B. Roberts, Manned Mars Missions: Working Group Papers, Volume 1, NASA M002, NASA/LANL, June 1986, pp. 203-218; proceedings of a workshop held at NASA Marshall Space Flight Center, Huntsville, Alabama, 10-14 June 1985.

On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, Edward Clinton Ezell & Linda Neuman Ezell, NASA History Office, 1984, pp. 117-118.

Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA SP-2001-4521, David S. F. Portree, NASA History Division, February 2001, pp. 11-12, 15, 60-62.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple Flyby: Venus-Mars-Venus Piloted Flyby Missions in the Late 1970s/Early 1980s (1967)


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