Space Shuttle External Tank (ET) Applications: ET as Space Facility (1982)

Big tank: External Tank-1, with the Space Shuttle Orbiter Columbia and twin Solid Rocket Boosters attached, arrives at Launch Pad 39-A at NASA Kennedy Space Center, Florida, after its roll-out from the Vehicle Assembly Building on 29 December 1980. Note the fire truck for scale. Image credit: NASA.

NASA announced in August 1973 that it had awarded Martin Marietta Corporation a $107-million contract to develop the Space Shuttle External Tank (ET). The initial contract called for the manufacture of three ground test ETs and six flight test ETs. The first Shuttle flight test was expected as early as 1977.

Four years later (9 September 1977), the first ET rolled off the Martin Marietta assembly line at NASA Michoud Assembly Facility, near New Orleans, Louisiana. By the next day, the space agency had moved the tank the short distance to the National Space Technology Laboratories (NSTL — now called NASA Stennis Space Center) in southern Mississippi. 

The 153.8-foot-long (46.9-meter-long), 27.5-foot-diameter (8.4-meter-diameter) ET included three major parts, all made mostly of aluminum alloy. Its forward third, shaped like a fat teardrop for streamlining, was the 19,500-cubic-foot (552-cubic-meter), 55-foot-long (16.8-meter-long) liquid oxygen (LOX) tank. Its aft two-thirds was the 53,500-cubic-foot (1515-cubic-meter), 97-foot-long (29.6-meter-long) liquid hydrogen (LH2) tank, a cylinder with dome-shaped ends. The two pressure vessels partially nested in the drum-shaped intertank, which measured 22 feet (6.7 meters) in length. The nine ETs delivered under the initial Martin Marietta contract each weighed about 38.6 U.S. tons (35 metric tons) empty. 

First tank: the Main Propulsion Test Article (MPTA) External Tank (ET) rolls off the Martin Marietta assembly line at Michoud Assembly Facility, Louisiana, on 9 September 1977. The three major ET components are discernible; the ribbed intertank separates the cylindrical liquid hydrogen (LH2) tank, the largest component, from the streamlined liquid oxygen tank at left. Please note the LH2 tank aft dome just clearing the door at right. Image credit: NASA.

Though unveiled amid much ceremony, the first ET was not intended for flight. Instead, it became the largest component of the Main Propulsion Test Article (MPTA). Other MPTA parts included a sturdy truss that stood in for the Shuttle Orbiter and a cluster of three Space Shuttle Main Engines (SSMEs) attached to the truss. The MPTA was hoisted vertical, mounted on an NSTL test stand, and put to work in SSME tests. 

On 29 June 1979, Martin Marietta rolled out the first flight ET. NASA loaded ET-1 onto a barge and shipped it across the Gulf of Mexico, around the southern tip of Florida, and up the Atlantic coast to NASA Kennedy Space Center (KSC). There the tank was moved to the Vehicle Assembly Building (VAB) and mated to a pair of Solid Rocket Boosters (SRBs) and the Orbiter Columbia in preparation for the first mission of the Space Transportation System (STS), which was aptly designated STS-1.

NASA rolled the STS-1 stack out of the VAB on 29 December 1980. Four months later (12 April 1981), it lifted off from Launch Complex 39-A. On board Columbia for her maiden flight were astronauts John Young and Robert Crippen. Shortly after the first Orbiter's triumphant return to Earth, NASA reduced the number of flight tests to four, freeing two of the flight test ETs for operational flights. 

The ET performed two critical functions during every Shuttle flight. It carried about 800 U.S. tons (725 metric tonnes) of LH2 fuel and LOX oxidizer for the three SSMEs in the Orbiter's tail; in addition, it bound together and provided thrust load paths for the 120-U.S.-ton (109-metric-tonne) Orbiter and twin 650-U.S.-ton (590-metric-tonne) SRBs. Together the three SSMEs on the Orbiter and the SRBs generated about seven million pounds (31,100,000 newtons) of thrust at liftoff.

The SRBs expended their propellants and separated from attachment fixtures on either side of the ET about two minutes after liftoff. They fell into the ocean and were recovered for reuse. The ET supplied propellants to the SSMEs for a further six and a half minutes; then, shortly after SSME shutdown, it was cast off and made to tumble to hasten its fall into Earth's atmosphere. When the ET separated from the Orbiter, it typically contained about 15 tons of leftover propellants (weight is approximate, so U.S. and metric units both apply). Reentry destroyed the ET; surviving pieces fell in remote ocean areas.

Orbiter and ET attained about 98% of orbital velocity before the latter was discarded. Two small Orbital Maneuvering System (OMS) engines in the Orbiter's tail then supplied the remaining 2% of the velocity needed to boost it, its crew, and its payload into a stable circular orbit about the Earth.

The process by which NASA arrived at the Shuttle design was complex. Until mid-1971, most designs paired a reusable, winged, piloted Orbiter with a reusable, winged, piloted Booster. The latter would have released the former just short of orbit. In most designs, the Booster would then have performed a wide 180° turn, deployed jet engines, and flown to a runway landing near its launch site. The semi-reusable Orbiter/ET/SRB stack, forced on NASA by funding limits imposed by President Richard Nixon, was, by comparison, a kludge — but in the minds of some spaceflight planners, it created an opportunity.

Beginning about the time the MPTA ET rolled out at Michoud, planners proposed that NASA boost ETs into orbit and put them to use. Some assumed that the ET would supply the SSMEs with LOX and LH2 until orbit was attained. Others assumed that the SSMEs would shut down just short of orbital velocity as during a normal flight, but that the Orbiter would retain the ET; then, when the twin OMS engines ignited to complete injection into orbit, it would bring the ET along for the ride.

When one reads of plans to exploit the ET in space, it is important to recall the giddy optimism many felt during Shuttle development in the 1970s. It started early — for example, the aerospace industry publication Aviation Week & Space Technology reported at the time Martin Marietta won its initial ET contract that NASA anticipated that 439 flight ETs would be manufactured through 1984. Assuming a first launch at the start of 1977, this implied a Shuttle launch every six days. 

The Shuttle, it was expected, would fly so cheaply that NASA would be able to spend the lion's share of its human spaceflight budget on payloads the Orbiter could carry to orbit in its 15-by-60-foot (4.6-by-18.3-meter) payload bay, not on transportation costs. At a bare minimum, such payloads would include government and commercial satellites and components and supplies for an expansive Space Station that Orbiter crews would assemble in orbit.

Proposed ET uses fell into three categories: propellant scavenging, exploitation of ET aluminum, and conversion of ET structures. LOX and LH2 scavenged from the ET could, some estimated, economically supply Space Tugs based at the Space Station; they would transport astronauts and cargo throughout cislunar space. Ground up or melted down, ETs could become propellant for aluminum-burning rocket engines, aluminum girders and trusses for large space structures, and reaction mass for electromagnetic mass drivers. Partially disassembled or clustered, ETs might be converted into space habitats, telescopes, propellant depots, warehouses, greenhouses, space warfare decoys, and platforms for instruments and weapons.

Brown tank: liftoff of Columbia at the start of STS-4, the final Orbital Flight Test mission (27 June-4 July 1982). Only STS-1 and STS-2 flew with white tanks; starting with STS-3, NASA opted not to paint the ETs. Image credit: NASA.

In July 1982, shortly after STS-4, the last Shuttle flight test, Martin Marietta completed a study for NASA Marshall Space Flight Center of the Aft Cargo Carrier (ACC) (see "More Information" below). Structurally similar to the ET — the company envisioned that it would be manufactured at Michoud using ET tooling and jigs — the ACC would ride to orbit attached to the dome-shaped aft end of the ET LH2 tank. As might be expected given Martin Marietta's ET expertise, the ACC proposal was among the most technically credible of the many ET exploitation schemes put forward in the late 1970s and 1980s.

As its name implies, the ACC, which would include two sections, was intended chiefly to augment Shuttle payload capacity. Use of the 27.5-foot-diameter (8.4-meter-diameter), 31.9-foot-long (9.7-meter-long) ACC with the Shuttle Orbiter payload bay would nearly double maximum Shuttle payload diameter and volume. Other ACC applications were possible, however; its lower section might, for example, serve as a protective shroud covering a "Space Facility Module" bolted to the ET LH2 tank aft dome. The ACC shroud would shield the drum-shaped pressurized module from the harsh thermal and acoustic environment the SRBs would create at the aft end of the ET during Shuttle ascent.

This image of the two-part Martin Marietta Aft Cargo Carrier (ACC) shows its proximity in flight to the three Space Shuttle Main Engines mounted to the Orbiter's tail. The Solid Rocket Boosters can be assumed to have detached; typically they would obstruct the view of the ACC from this angle. The ACC is mounted to and covers the aft dome of the ET liquid hydrogen tank. Image credit: Martin Marietta.

Space Facility Modules would have different functions, but all would include a vertical cylindrical airlock that would enable astronauts to take advantage of a circular 36-inch (91.4-centimeter) "manhole" in the LH2 aft dome. A feature of all ETs, the manhole was designed to permit technicians on the ground to access the LH2 tank interior during ET checkout and launch preparation. In space, it would enable astronauts to enter and convert the LH2 tank for a range of purposes.

Space Facility Modules would thus resemble the Spent Stage Experiment Support Module (SSESM) proposed in the early 1960s for use with Apollo Saturn S-IVB rocket stages. The S-IVB, the second stage of the two-stage Saturn IB rocket and the third stage of the three-stage Saturn V, included in its upper two-thirds an LH2 tank. The drum-shaped SSESM, launched attached to the top of a Saturn IB S-IVB, would have enabled astronauts to enter the empty LH2 tank to outfit it in orbit as an Earth-orbiting space station. A 1966 plan proposed landing a Saturn V-launched SSESM/S-IVB combination on the Moon (see "More Information" below). 

Space Facility Module: the Service Module. Please note the off-center, slanted port at top, just left of center; conforming to the shape of the aft dome of the ET liquid hydrogen tank, it would enable access to the manhole located there. The Service Module has five additional ports; two radial ports with petal-type docking units and the tunnel leading to the aft port are visible. Image credit: Martin Marietta.

The company described the rapid growth of an Earth-orbiting Space Facility space station. The first Space Facility launch would see an Orbiter boost an ET with attached Space Facility Module — configured as a "Service Module" — into a 215-nautical-mile-high (398.2-kilometer-high) orbit. During ascent, 15 seconds after the SRBs separated from the Shuttle stack, the lower section of the ACC shroud would separate and fall away, exposing the Service Module. The Orbiter would retain the ET, firing its SSMEs until the desired orbit was achieved.

The Orbiter crew would vent residual ET propellants through the SSMEs and would hand off ET stabilization to an attitude control/orbit-maintenance propulsion system in the Service Module, then would separate their spacecraft from the ET/Service Module combination and perform station-keeping with it. The Service Module would deploy a pair of electricity-producing solar arrays and orient them toward the Sun. 

The Space Facility would include three Docking/Service Tunnels. Image credit: Martin Marietta.

The astronauts would next open the Orbiter payload bay doors and use the Remote Manipulator System (RMS) robot arm to hoist a "Docking/Service Tunnel" out of the payload bay. After linking the tunnel to an aft-facing port on the Service Module, they would dock the Orbiter with the tunnel. They would then enter the newly established Space Facility.

In addition to its propulsion system, power system, and airlock linking it to the ET LH2 tank, the Service Module would contain life support systems and living and working space for several astronauts. Its single pressurized volume would, however, only be occupied if an Orbiter were docked to it; this was a safety measure meant to ensure that the crew could reach a safe haven in the event of Space Facility depressurization, fire, or atmospheric contamination.

Space Facility Module: the Habitat Module. Image credit: Martin Marietta.

Addition of a second ET with Space Facility Module — this time configured as a "Habitat Module" — would remove that restriction. The Orbiter and ET/Habitat Module would rendezvous with the Space Facility; then, after separation, the crew would hoist a second Docking/Service Tunnel out of the payload bay and link it to one of four radial (side-mounted) ports on the Service Module. The ET/Habitat Module would then move or be moved (by a means not described) so that it could link one of its radial ports with the second tunnel, binding the two Space Facility Module/ET combinations together.

The astronauts would next use the RMS to hoist a Logistics Module out of the payload bay. They would attach the small module, which would contain supplies and small experiment apparatus, to one of the four Habitat Module radial ports. With that task completed, they would dock with and enter the Space Facility. With the addition of the Habitat Module, astronauts could remain on board after the Orbiter departed.

The third Space Facility assembly flight would see a Shuttle Orbiter arrive with a full payload bay and no ET or Space Facility Module. A third Docking/Service Tunnel would be hoisted from the payload bay and linked to a Service Module radial port, then a small piloted space tug designed for satellite deployment, retrieval, and repair would be docked to the new tunnel. 

Finally, an experiment pallet based on the Spacelab pallet designed originally for operation in the Orbiter payload bay would be attached to the exterior of one of the ETs. It would be the first of many experiment payloads that would employ the ETs as stable space platforms. 

The Space Facility would be fully operational after just three Shuttle flights. Attached to the ETs at center right are the Service Module with twin solar arrays and the Habitat Module. An experiment pallet designed originally to conform to the Shuttle payload bay stands out against the ET exterior just left of image center. In this artist's conception other components — a logistics module with black stripes, a small space tug, and the Docking/Service Tunnel to which the Orbiter is docked — are incorrectly depicted. See post text for their correct locations and sizes. Image credit: Martin Marietta/DSFPortree.

By the time the Orbiter departed for the third time, the Space Facility would, Martin Marietta declared, enable "a permanent manned presence in space." The services it offered, the company added, would "significantly complement. . .the basic Shuttle capability." 

Martin Marietta saw no reason to stop there. It proposed that astronauts would eventually outfit the interiors of the Space Facility's ET LH2 tanks with decks and furnishings. NASA might also expand the Space Facility by adding new ETs. These could be converted in orbit into hangars for storing and servicing satellites. The 27.5-foot-diameter (8.4-meter-diameter) LH2 tank would, the company noted, provide ample room for satellites sized for launch in the Orbiter payload bay.

Space Facility expansion: a scheme for outfitting the interior of an ET liquid hydrogen tank as a comfortable habitat housing 16 astronauts. Image credit: Martin Marietta.

Martin Marietta's Space Facility concept died an early death in large part because it was seen to compete with NASA's Space Station plans, which favored trusses and modules sized for launch in the Shuttle payload bay. After January 1984, when President Ronald Reagan called on the space agency to build a Space Station, plans to exploit ETs as habitats, hangars, or platforms stood almost no chance of acceptance.

Sources

"News Digest," Aviation Week & Space Technology, 20 August 1973, p. 25. 

"Shuttle Tanks Undergo Tests at Michoud," Aviation Week & Space Technology, 23 May 1977, p. 49. 

"The Low (Profile) Road to Space Manufacturing," G. O'Neill, Astronautics & Aeronautics, Vol. 16, No. 3, March 1978, pp. 24-32. 

"NASA Studying Shuttle-Derived Launch Vehicles," Aviation Week & Space Technology, 8 March 1982, p. 81.

"NASA Seeks Shuttle Capability Growth," C. Covault, Aviation Week & Space Technology, 23 April 1982, pp. 42-43, 45, 47, 51-52.

"Martin Studies Shuttle Aft Cargo Unit," E. Kolcum, Aviation Week & Space Technology, 12 July 1982, p. 65-66. 

"External Tank Applications in Space," K. Timmons, A. Norton, and F. Williams, Martin Marietta; paper presented at the Unispace Conference in Vienna, Austria, 9-17 August 1982.

"External Tank Depicted as Space Station Element," Aviation Week & Space Technology, 6 September 1982, p. 246.

External Tank ACC Aft Cargo Carrier, Martin Marietta, no date (late 1982).

More Information

S-IVB/IU Applications: The LASS Proposal (1966)

Where to Launch and Land the Space Shuttle? (1971-1972)

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

A Robotic and Piloted Planetary Exploration Program for the 1970s and Early 1980s (1968)

Leaving home: Earth as viewed from Apollo 4 on 9 November 1967. Image credit: NASA.

It was the best of times. It was the worst of times. (With apologies to Charles Dickens.)

For NASA, the year 1967 began with the promise of a bold start for the Apollo Applications Program (AAP), the planned successor to the Apollo lunar program, which would see space station missions in low-Earth orbit and advanced lunar exploration missions. Top NASA officials briefed the press on their ambitious AAP plans on 26 January 1967 (see "More Information" below). 

Barely a day later, fire raged through the crew cabin of the Apollo 1 Command and Service Module (CSM) spacecraft during a test on the launch pad, killing astronauts Gus Grissom, Ed White, and Roger Chaffee. The resulting investigation angered Congress — NASA had failed to report persistent problems in its relations with North American Aviation (NAA), the CSM prime contractor. Affronted legislators, already eager to cut government expenditures because of the soaring cost of U.S. military involvement in Indochina, responded in August-September 1967 by slashing President Lyndon Baines Johnson's Fiscal Year (FY) 1968 NASA budget request by nearly half a billion dollars. 

The cuts mostly affected projects aimed at giving NASA a post-Apollo future; AAP, of course, but also the Voyager robotic Venus/Mars exploration program (see "More Information" below) and advance planning for piloted missions beyond the Moon, including piloted Mars/Venus flybys. Members of the NASA Office of Manned Space Flight (OMSF) Planetary Joint Action Group (JAG) had hoped that major funding for piloted flybys could begin in FY 1969, with the first in a series of piloted flybys — a Mars flyby with sample return — leaving Earth in late 1975 (see "More Information" below).

Even as OMSF had sought piloted flybys, the scientific community had continued its perennial quest for an expanded robotic program. In a February 1967 report to the Johnson White House, the President's Science Advisory Council (PSAC) disparaged piloted flybys and urged a 1970s program that would see robotic spacecraft begin a wide-ranging reconnaissance of the entire Solar System. Scientists were outraged when instead the FY 1968 budget cuts threatened to end U.S. robotic exploration entirely after the twin Mariner '69 Mars flybys.

In October and November 1967, NASA Administrator James Webb spoke out in favor of new robotic planetary missions in the 1970s. He urged members of Congress to take note of Soviet plans for robotic exploration beyond the Moon. Talks began with White House budget officials and Congressional leaders aimed at salvaging a 1970s planetary program from the wreckage of the FY 1968 budget process.

Meanwhile, in Florida, components of AS-501, the first flight-ready Saturn V rocket, came together with an Apollo CSM in the giant Vertical Assembly Building (VAB) at NASA Kennedy Space Center (KSC). Without the three-stage behemoth an Apollo Moon landing was impossible.  The testing and assembly process had begun months before the Apollo 1 fire with the aim of a launch in the first quarter of 1967, but preparation for the automated test mission — which NASA designated Apollo 4 — hit one snag after another. 

Following the fire, NASA subjected the CSM NAA had delivered to KSC for the Apollo 4 mission to enhanced scrutiny. The spacecraft, designated CSM-017, was found to contain more than 1400 wiring errors. Fixing them required months. Welding errors in the NAA-built Saturn V S-II second stage also needed correction. 

Troubled assembly: the Apollo 4 CSM and Saturn V rocket in the Vertical Assembly Building at NASA Kennedy Space Center, Florida. Image credit: NASA.

The giant rocket was at last rolled out to Launch Pad 39A on 26 August 1967, but its troubles were not over, for Apollo 4 was also a test of launch pad hardware and pre-launch procedures. As the launch team struggled to make pad and rocket function together, the press, the public, and the Congress became increasingly impatient.

Apollo 4 lifted off at last on 9 November 1967. Rocket, spacecraft, launch facilities, and world-wide tracking & communications network operated together almost flawlessly.

The Apollo 4 Saturn V and CSM climb toward orbit. Image credit: NASA.

About three hours after insertion into a 190-kilometer-high (118-mile-high) low-Earth orbit, the AS-501 Saturn V S-IVB third stage restarted to boost CSM-017 into an elliptical orbit. It was the first orbital restart of the stage, which would boost Apollo missions out of Earth orbit to the Moon. 

Near orbital apogee CSM-017 separated from the S-IVB. The spacecraft fired its Service Propulsion System (SPS) main engine to increase its altitude to 18,092 kilometers (11,242 miles), then fired it again for 4 minutes and 30 seconds to hurl itself at Earth at a lunar-return speed of 24,911 miles (40,090 kilometers) per hour. 

CSM-017 split into its component modules — Command Module (CM) and Service Module (SM) — then the former reoriented itself with its bowl-shaped heat shield forward so that it could withstand fiery atmosphere reentry. The SM burned up as planned. The CM's heat shield, meanwhile, reached a temperature of nearly 2760 C (5000° F). Crew cabin temperature did not exceed 21 C (70° F). Just eight and a half hours after liftoff, the Apollo 4 CM deployed three parachutes and lowered to a splashdown in the Pacific. 

The unmanned Apollo 4 Command Module (right) bobs in the Pacific Ocean near Hawaii at the end of its eight-and-a-half-hour test flight. One of its three main parachutes remains attached; it would be retrieved for analysis along with the spacecraft. Image credit: NASA.

The trade magazine Aviation Week & Space Technology reported that, ironically, on the very day of NASA's Apollo 4 triumph, NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, had laid off workers as a result of the FY 1968 budget cuts. NASA MSFC was the home of the Saturn family of rockets. 

On 12 December 1967, a little more than a month after Apollo 4, President Lyndon Baines Johnson toured NASA's Michoud Assembly Facility near New Orleans, Louisiana, where Saturn rockets were assembled and tested. His visit was meant to reassure local and state officials and to raise worker morale. Whether he succeeded is open to interpretation. Standing before a partially complete Saturn V S-IC first stage, Johnson told the workers

. . .that man will make space his domain is inevitable. Whether America will lead mankind to that destiny does not depend on your ability, but depends on our vision, our willingness, and our national will and determination. This great pilgrimage of man — like all his adventures — costs money. Christopher Columbus spent more years trying to find money for his voyage than he spent discovering the New World. In the modern world, we can no longer depend on Queen Isabella pawning her jewels. We have to depend on taxes. We must have revenues that only Congress can grant. . . So we will advance in space to the extent that our people and their representatives are prepared for us to advance and are prepared to pay the cost of that advance. We may not always proceed at the pace we desire. I regret — I deeply regret — that there have been reductions and there will be more. There have been interruptions. . . But I do have faith and confidence in the American people.

This background may help to explain why two engineers at Bellcomm, NASA's Washington, DC-based advance planning contractor, responded as they did when NASA invited them in late November-early December 1967, to state their opinions on the course U.S. planetary exploration should take in the 1970s and early 1980s. In a report completed and distributed to relevant NASA facilities on 26 February 1968, J. P. Downs and W. B. Thompson were cautiously optimistic. 

Downs and Thompson explained that their report reflected "the authors' thinking at. . . [a] particular time" and that it was "a reflection of a long term point of view." They assumed that the deep FY 1968 budget cuts were a short-term, temporary setback, not a sign of a long-term trend. In fact, they anticipated an annual NASA budget of between $5 billion and $6 billion by FY 1971 or FY 1972, when, they expected, NASA would start development of a piloted planetary program.

At the same time, the Bellcomm engineers cautioned that "[a]s more information becomes available on technical details and resources, the program may change." They added, however, that "the rationale expressed. . . is expected to remain much as it is now."

Downs and Thompson described a NASA planetary program containing 21 missions to 11 Solar System bodies between the years 1969 and 1984. Missions would occur in three "branches." The first branch would comprise missions to Venus and Mars that would serve as precursors to at least three piloted Mars and Venus missions. Missions in the second branch would explore Mercury, Jupiter, and the other "major planets" (Saturn, Uranus, and Neptune), a task they called "the major challenge to the unmanned program." The third branch would include missions to explore two comets and two asteroids. 

Their program would begin with the twin Mariner '69 Mars flybys already on NASA's schedule and continue in 1970 with a Mariner Venus/Mercury dual flyby mission launched on an Atlas/Centaur rocket. The Atlas/Centaur was already in early 1968 the workhorse of the NASA robotic lunar and planetary program. 

The Venus/Mercury mission, which would form part of both the first and second of Downs and Thompson's three branches, would seek gaps in Venus's cloud cover in the hope of glimpsing its mysterious surface. In addition, as the spacecraft flew past the planet, it would transmit radio signals to Earth through the Venusian atmosphere in an attempt to chart its structure.

Mariner Mars '69 engineering model. Note the large steerable camera "pod" mounted below the hexagonal bus body, the high-gain dish antenna on top, and the four solar arrays. Image credit: NASA.

Space workhorse: an Atlas-Centaur rocket launches the Surveyor 1 lunar lander on 30 May 1966. Image credit: NASA.

During the flyby, Venus would give the spacecraft a gravity assist that would reduce by between 50% and 75% the amount of propulsive energy it would need to reach Mercury. Downs and Thompson explained that the innermost planet is, by dint of its proximity to the Sun, often lost in glare when viewed from Earth and hence mysterious; orbiting close to the Sun also means that its orbital speed is high, making it difficult for spacecraft to reach.

In 1971, NASA would launch on a Titan III-C rocket its first new-design Mars orbiter and surface probe. Downs and Thompson suggested that the new orbiter might be based on the Boeing Lunar Orbiter design. The Titan III-C, a U.S. Air Force rocket, was meant to replace the Saturn IB-Centaur rocket formerly emphasized in NASA planetary mission plans. Use of the Titan III-C in the Downs and Thompson program was a response to a statement by NASA Administrator James Webb that the Saturn IB would be phased out to save money. 

18 June 1965: the first Titan III-C rocket stands on the pad at Launch Complex 40, Cape Canaveral Air Force Station, Florida. Image credit: U.S. Air Force.

Boeing-built Lunar Orbiter spacecraft. Image credit: NASA.

The 159-kilogram (350-pound) battery-powered survivable surface impactor probe would include an atmosphere entry shell, a parachute, a protective impact shell carved from soft, lightweight balsa wood, and 13 pounds of science instruments. These might include a life detection device. Instruments on the entry shell would  chart atmospheric structure as it plummeted toward the surface after separation from the impactor. These data would enable engineers to design heavier, more sophisticated Mars landers. 

NASA would launch in 1972 its first new-design Venus orbiter and atmospheric probe on a Titan III-C. In addition to "a concentrated search over the entire planet for visible access to the surface," the orbiter would employ an imaging radar to chart surface topography. The probe would measure the thermodynamic properties of the atmosphere to enable design of meteorological balloon probes suited to Venusian conditions.

In 1973, NASA would ramp up the pace by launching on three Titan III-Cs a pair of Mars orbiter/impactor probe missions and a second Mariner-derived Venus/Mercury flyby spacecraft. The latter would resemble that launched in 1970 but would add a Venus survivable surface impactor probe. The prime objective of the Mars impactor probes would be to search for life. 

The 600-pound Venus impactor probe would attempt to return data on the planet's harsh surface conditions for at least an hour. The dense Venusian atmosphere would, Downs and Thompson wrote, enable a survivable landing without a parachute.

The following year, NASA would launch its first flyby mission to Jupiter on a Titan III-C augmented with a Centaur upper stage. Dubbed a "galactic Jupiter probe," it would be the first NASA spacecraft designed for an operational lifetime of up to 10 years. It would survey interplanetary particles and fields and aid future spacecraft designers by surveying the interplanetary meteoroid environment with particular emphasis on the Asteroid Belt between Mars and Jupiter. A Jupiter gravity-assist would make it the first spacecraft to escape the gravitational grip of the Sun.

NASA would ramp up the planetary exploration pace in 1975 by launching four rockets — probably Titan III-Cs with Centaur upper stages. An orbiter and surface probe would leave Earth for Mars. Two orbiters with impact lander probes would launch to Venus. The space agency would also launch a clone of the 1974 galactic Jupiter probe mission.

The year 1976 would see NASA's first mission to a comet. After launch on an Atlas/Centaur, a Mariner-derived spacecraft would race past Comet d'Arrest. Downs and Thompson explained that the small size of the comet nucleus and the rapid speed of the flyby would require NASA to develop a sophisticated new tracking system for its comet spacecraft cameras.

In 1977, the first Mariner-derived "Grand Tour" spacecraft would depart Earth on a Titan III-C/Centaur. A series of gravity-assist flybys would speed it across the outer Solar System, enabling it to explore all four planets beyond the Asteroid Belt in the space of a decade. That same year, NASA would launch on two Titan III-C/Centaur rockets a Mars orbiter with an impactor and a Venus orbiter with a pair of impactors. The Venus impactors might be targeted to land on high-elevation surface features; these might, Downs and Thompson suggested, have cooler temperatures than lower elevations, and thus be more likely to support life.

The year 1978 would see launch of NASA's first asteroid mission (a flyby of asteroid Icarus using a Mariner-derived spacecraft launched on a Atlas/Centaur) and the second "Grand Tour" mission (a clone of the 1977 mission). It would also see an significant shift in the character of the U.S. planetary program as astronauts joined the action. 

Thompson was a veteran of the NASA OMSF Planetary JAG piloted flyby studies. The NASA budget seemed unlikely to stretch far enough to support development in time to carry out the Planetary JAG's 1975 piloted Mars flyby mission, so the Bellcomm engineers opted instead to take advantage of an opportunity to launch a piloted Venus/Mars/Venus flyby mission in late 1978. 

The piloted flyby spacecraft and its Earth-orbit departure booster stack would be assembled in Earth orbit using components launched on two-stage Saturn V rockets. After leaving Earth orbit and discarding its boosters, it would follow a free-return heliocentric path that would end at Earth. Only minor course corrections would be required after Earth-orbit departure.

In 1979, the crew of the piloted flyby spacecraft would deploy automated meteorological balloons and impactor probes as they passed Venus for the first time and automated sample returners as they passed Mars. The balloons would drift the Venusian atmosphere for a long period. They would seek evidence of life in cool atmosphere layers. 

Astronauts would examine in a sealed lab the Mars dirt and air the sample returners launched to the flyby spacecraft to determine whether they could be safely returned to laboratories on Earth. The following year (1980) would see the mission carry out its second Venus flyby — a clone of the first — followed a few months later by a direct Earth-atmosphere reentry.

The years 1979 and 1980 would also see the last two Mariner-derived comet/asteroid flyby missions on the Downs and Thompson schedule. The first, the last mission launched on an Atlas/Centaur, would visit asteroid Eros, while the second, launched on a Titan III-C/Centaur, would race past Comet Encke.

A second piloted flyby mission would depart Earth in 1981. During its Venus flybys in that year and in 1983 it would deploy a pair of balloon-borne "several thousand pound" Buoyant Venus Stations of a type proposed by the Martin Company in 1967, as well as an unspecified number of long-duration Venus landers. All would look for life. The Mars flyby in 1982 would see more surface sample collection and observations tailored toward selecting sites for eventual piloted Mars landings.

Downs and Thompson expected that their 1984 piloted planetary mission, the last on their schedule, would probably take the form of a Venus orbiter. A piloted Venus mission would, they wrote, "serve to pace the development of a high energy space storable propulsion system." After proving that it could slow the piloted Venus spacecraft so that Venus's gravity could capture it into orbit and accelerate it out of Venus orbit back toward Earth, the compact, powerful, long-lived rocket stage would propel piloted Mars orbiter and landing missions and boost out of Earth orbit large new-design robotic outer planet and "deep space" spacecraft.

The Bellcomm engineers' report landed on desks across NASA in late February. Their timing could have been better — barely a month ahead of its distribution North Vietnam attacked South Vietnam on the eve of Tet, the Chinese New Year, leading to greatly expanded U.S. involvement in the Vietnam War. The Tet Offensive created new pressure on the Federal purse, helping to ensure (among other things) that NASA's budget slide would continue in FY 1969 and beyond.  

Despite the war and other national challenges, in the period covered by the Downs and Thompson plan NASA managed to fly a dozen planetary missions, of which 11 reached their targets. In large part, these were justified in terms of heading off new Soviet space victories and providing an avenue for the development of new technology with defense implications. 

All the flown missions were directed toward major planets; none would visit asteroids or comets and (of course) none would include astronauts. Italicized initial dates given below are launch years.

• 1969: The Mariner '69 Mars flyby spacecraft were designated Mariner 6 and Mariner 7 after launch; they left Earth atop Atlas/Centaur rockets.
• 1971: The Mariner '71 Mars orbiter spacecraft were designated Mariner 8 and Mariner 9 after launch; Mariner 8's Atlas/Centaur rocket malfunctioned but Mariner 9, the first planetary orbiter, was a great success, mapping all of Mars until late 1972.
• 1972: Pioneer 10, launched on an Atlas/Centaur rocket with a solid-propellant kick stage, became the first spacecraft to traverse the Asteroid Belt;  in 1973, it became the first spacecraft to fly past Jupiter. The gravity-assist kick it received made it the first spacecraft placed on a path to escape the Solar System.
• 1973: Pioneer 11 followed Pioneer 10 through the Asteroid Belt to Jupiter; in 1979 it became the first spacecraft to fly past Saturn.
• 1973: Mariner 10 left Earth on an Atlas/Centaur rocket and flew past Venus in early 1974; later that year it became the first spacecraft to fly past Mercury. It flew past Mercury twice more in 1974-1975.
• 1975: Viking 1 and Viking 2, each of which comprised a lander and a Mariner-derived orbiter, launched atop Titan III-E rockets, arriving in Mars orbit in June 1976 and August 1976, respectively. Viking 1, which touched down on 20 July 1976, was the first successful Mars lander; Viking 2 landed successfully on 3 September 1976. Their life detection experiments yielded equivocal results.
• 1977: The Mariner Jupiter-Saturn '77 spacecraft were renamed Voyager 1 and Voyager 2. They left Earth atop Titan III-E rockets. Voyager 1 flew past Jupiter in 1979 and Saturn in 1980; Voyager 2 flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989.
• 1978: Pioneer Venus Orbiter and Pioneer Venus Multiprobe (PVM) launched atop Atlas/Centaur rockets. Though not designed to survive landing, one PVM small probe continued to operate after striking the surface, becoming the first (so far only) successful U.S. Venus lander.

The Pioneer Venus Multiprobe bus (lower right) is shown deploying three small probes (center) and one large probe (upper left). In reality the large probe was deployed on 16 November 1978 and the small probes were deployed on 20 November 1978. The bus and probes entered the Venusian atmosphere on 9 December 1978. Image credit: NASA.

In their report, Downs and Thompson anticipated that NASA would be given the go-ahead to start a new piloted planetary program in FY 1971 or  FY 1972, and after a fashion they were correct. In January 1972, President Richard Nixon called on Congress to fund the winged Earth-orbital Space Shuttle. 

Originally proposed as a low-cost fully reusable Space Station crew rotation and resupply vehicle, the Shuttle became instead a multi-purpose spacecraft after Nixon refused to fund a Space Station. It would be only semi-reusable, which lowered its development cost but dramatically increased its operations cost. Among its goals was to launch all U.S. robotic planetary spacecraft.

Downs and Thompson's NASA budget prediction — $5-6 billion annually by about FY 1972 — entirely missed the mark. In terms of buying power in an inflationary time, NASA's budget remained at about half that amount throughout the 1970s and early-to-mid 1980s. Funding scarcity adversely impacted both Shuttle development and planetary exploration. 

Shuttle development problems traceable to funding shortfalls, lack of successful new Soviet planetary missions, tight planetary science budgets, and the Challenger accident (28 January 1986) came together to create an 11-year hiatus in new U.S. planetary launches following the 1978 Pioneer launches. The stoppage ended at last with the launch of the Magellan Venus radar mapper on board the Shuttle Orbiter Atlantis on 4 May 1989. 

By the time Magellan flew, NASA had announced that it would cease Shuttle planetary launches after it launched the Galileo Jupiter orbiter and probe and Europe's Ulysses solar polar orbiter in favor of resuming planetary launches on expendable rockets. Galileo launched on board the Orbiter Atlantis on 18 October 1989 and Ulysses launched on board the Orbiter Discovery on 6 October 1990. 

Sources

The first two sentences of this post are based on the first sentence of Charles Dickens' 1859 novel A Tale of Two Cities.

The Space Program in the Post-Apollo Period: A Report of the President's Science Advisory Committee, "Prepared by the Joint Space Panels," The White House, February 1967.

"Science Advisers Urge Balanced Program," Aviation Week & Space Technology, 6 March 1967, pp. 133-137.

"Orbiters Studied for Planetary Missions," W. J. Normyle, Aviation Week & Space Technology, 23 October 1967, pp. 30-32.

"Washington Roundup: NASA Thanks You," Aviation Week & Space Technology, 20 November 1967, p. 25.

"Apollo 4 Closes Gaps to Lunar Mission," W. J. Normyle, Aviation Week & Space Technology, 20 November 1967, p. 26-27.

"NASA Pushes Planetary Program," W. J. Normyle, Aviation Week & Space Technology, 27 November 1967, pp. 16-17.

"Remarks Following an Inspection of NASA's Michoud Assembly Facility Near New Orleans," President Lyndon Baines Johnson, 12 December 1967 (https://www.presidency.ucsb.edu/documents/remarks-following-inspection-nasas-michoud-assembly-facility-near-new-orleans — accessed 30 August 2022).

"A Feasible Planetary Exploration Program Through 1980 — Case 710," J. P. Downs and W. B. Thompson, Bellcomm, Inc., 26 February 1968.

Astronautics & Aeronautics 1967, NASA SP-4008, 1968, pp. 43-45, 246, 248, 255-256, 282-284, 295-296, 314, 320, 323-324, 333, 336-343, 352-353, 373-375.

Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, NASA SP-4206, Roger E. Bilstein, NASA, 1980, pp. 351-360.

More Information

"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Programs

NASA's Planetary Joint Action Group Piloted Mars Flyby Study (1966)

Missions to Comet d'Arrest & Asteroid Eros in the 1970s (1966)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

The First Voyager (1967)

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

Things to Do During a Venus-Mars-Venus Piloted Flyby Mission (1968)

Mars Sample Return Site Selection and Sample Acquisition Study (1980)

The Tharsis hemisphere of Mars. The four large volcanoes are marked by clouds and the western half of Valles Marineris is visible at right. Image credit: NASA.

In 1977-1978, the Jet Propulsion Laboratory (JPL) Mars Program studied a low-cost "minimum" Mars Sample Return (MSR) mission as a potential follow-on to the Viking missions. Late in 1978, JPL Mars Program engineers called upon the NASA-sponsored Mars Science Working Group (MSWG) for aid in defining science requirements to help guide MSR spacecraft design and operations planning. 

The MSWG, chaired by JPL's Arden Albee, included scientists from JPL, NASA, the U.S. Geological Survey (USGS), universities, and aerospace contractors. Many had participated in the MSWG's July 1977 Mars 1984 study, which proposed a long-range rover, an orbiter, and a penetrator network as a post-Viking/pre-MSR mission (see "More Information" below).

The MSWG scientists divided into Site Selection and Sample Acquisition teams. The teams held two joint workshops and produced 10 detailed reports before the middle of 1979. Edited by JPL's Neil Nickle, they did not see print until November 1980. 

Publication was delayed in part because Mars planning at JPL slowed markedly in early 1979. It would not begin to emerge from the doldrums again until the following year, after President Jimmy Carter's NASA Administrator, theoretical physicist Robert Frosch, created the Solar System Exploration Committee in an (ultimately successful) effort to revitalize the space agency's flagging robotic exploration program.

Because the MSWG reports were based on limited data, they may appear archaic to some readers. Nevertheless, they remain important, for they capture snapshots of the state of Mars science as the busy first era of robotic Mars exploration ended and the long gap began between the Viking missions, which reached Mars in 1976, and Mars Pathfinder and Mars Global Surveyor, which arrived at the planet in 1997.

The first MSWG report, which looked at polar landing sites for the minimum MSR mission, was authored by J. Cutts, K. Blasius, W. Roberts, and K. Pang of the Planetary Science Institute (PSI) of Science Applications, Inc., and A. Howard of the University of Virginia (UV). They submitted their report to JPL on 30 April 1979.

The PSI/UV team began by pointing out that humans had already explored Mars's poles for more than a decade. Mariner 7 had begun close-up martian polar exploration by imaging the entire southern ice cap at low resolution during its August 1967 flyby. Mariner 9 imaged both caps from Mars orbit during 1971-1972, and the Viking 2 orbiter began high-resolution polar imaging in 1976.

In many respects, polar MSR sites constituted a special case, the PSI/UV team wrote. Whereas missions to the other MSR sites would focus mainly on rock samples, the polar MSR mission would acquire meter-long core samples of ice or dust and ice. Rock samples would be "an unplanned bonus."

The north pole of Mars. Image credit: NASA.

The five scientists looked at two MSR sites near Mars's north pole (image at top of post). Site A, at 86.5° north (N), 105° west (W), included wide "featureless" expanses of undulating perennial ice underlain by layered deposits. Core samples of perennial ice might provide data on ice cap formation processes and time scale, martian climate history, and organic compounds trapped in the ice. They would establish "ground truth" for interpreting polar data from orbital spacecraft.

They assumed that a landing might safely occur anywhere within a target ellipse 25 kilometers wide by 40 kilometers long, and calculated that a lander that set down in the ellipse would stand at least a 99% chance of landing on perennial ice. For this reason, no mobility (that is, no rover) would be required at Site A.

The second polar site, Site B (84.5° N, 105° W), included perennial ice and "partially defrosted" terraced troughs. The latter, the PSI/UV scientists explained, would "form windows through the layered deposits and cross-sections through martian history." The 25-kilometer-by-40-kilometer Site B target ellipse would also overlap the edge of the permanent ice cap. 

Selecting such a varied area would, they warned, reduce the probability of landing on perennial ice to between 60% and 90%. If, however, the Site B mission included a short-range (about 10 kilometers) rover, then the probability of sampling more than one terrain and of sampling perennial ice would increase to greater than 90%.

In discussing the engineering problems of a polar MSR mission, the PSI/UV team cited Purdue University's 1976-1977 Mars Polar Ice Sample Return study (see "More Information," below), but otherwise left engineering to the engineers. Potential problems identified included acquisition and preservation of ice and permafrost cores, mechanical operations at extremely low temperatures, and water and carbon dioxide frost accumulation and evaporation that might impede a rover.

As a "next logical step" toward a polar MSR mission, the PSI/UV scientists recommended establishment of a science working group with "substantial participation by earth scientists involved with studies of terrestrial sedimentary records[,] particularly those pertaining to climate change." They did not recommend an MSR precursor mission; that is, they judged that the Viking missions had provided data adequate for planning a minimum MSR mission to Mars's north pole.

Arizona State University (ASU) geologists R. Greeley, A. Ward, A. Peterfreund, D. Snyder, and M. Womer submitted the second of the 10 MSWG reports to JPL in March 1979. Their quest for a young volcanic MSR site was hampered, they explained, by a dearth of high-resolution (better than 50 meters per pixel) orbital images. Nevertheless, they located six candidate sites that looked to be volcanic and had few craters, signifying youth. (Planetary scientists count craters to estimate terrain age; the more densely craters pock a landscape, the older it is likely to be.)

Arsia Mons (right of center) is southernmost of the four great Tharsis volcanoes. ASU's "Arsia Mons West" MSR site is located near the center of the left (west) half of the image. Image credit: NASA.

The ASU geologists picked Arsia Mons West, located at 8.5° south (S), 132.5° W, 500 kilometers from Arsia Mons, the southernmost of the four great Tharsis volcanoes, because the site appeared to be both very young and relatively homogenous geologically. The latter, they explained, was a desirable quality because it would facilitate interpretation of sample data. 

The Arsia Mons West site, which had been imaged by the Viking orbiters at 34-meters-per-pixel resolution, included eight overlapping lava flows. The flows measured from eight to 35 kilometers wide and averaged 51 meters thick.

The ASU team found room for two target ellipses 80 kilometers long by 50 kilometers wide on either side of a five-kilometer crater at their site's center. They calculated that a rover with a 14-kilometer range would have a "complete guarantee" of reaching an outcrop of young volcanic rock.

At JPL's request, the ASU geologists also assessed Viking 1's Chryse Planitia landing site as a potential MSR landing site. The volcanic rocks were old at Chryse, a smooth-floored basin at the confluence of several large flood-carved channels. Based on the in-situ evidence provided by Viking 1 lander images, it was clear that no mobility would be needed to acquire a rock sample. 

The ASU team noted, however, that the "value of a returned sample [would be] severely diminished because it may be impossible to determine if the material represents local [lava] flows. . .[or] if it has been deposited from the floods that eroded the channels." The ASU team added that "[w]ithout mobility of at least 200 to 300 kilometers, the [Chryse Planitia] site [would be a] a poor choice to answer basic scientific questions about Mars." For neither site did they recommend an MSR precursor mission.

A Young-Lavas Landing Site Northwest of the Volcano Apollinaris Patera and a Landing Site on the Ancient Terrain Southeast of the Schiaparelli Basin, had a single author: Brown University geologist P. Mouginis-Mark. He argued for mobility at his young Elysium Lavas (5° S, 190° W) and Ancient Terrain (8° S, 336° W) minimum MSR sites. The former, 150 kilometers from the Apollinaris Patera volcano, comprised rolling plains with scattered volcanic domes and shields, stratovolcanoes, and fresh impact craters. He identified a ridge running through the center of the 80-by-50-kilometer target ellipse as the feature most likely to yield a "good sample" (that is, a well-preserved volcanic rock representative of the site).

Mouginis-Mark calculated that without mobility the probability of obtaining a good sample would be nil, while the probability of landing on a sand dune and obtaining no sample at all would be as high as 22%. The probability of obtaining a good sample would increase to 91%, however, if the mission included a rover with a round-trip range of 20 kilometers.

The smooth-floored crater Schiaparelli (top of image, just right of center).  Mouginis-Mark's "Ancient Terrain" MSR site is located near the center of the image. Image credit: NASA.

Mobility would be even more important at Mouginis-Mark's heavily-cratered Ancient Terrain site, located 150 kilometers from the 400-kilometer-diameter crater Schiaparelli. The site, which dated from the Noachian, the earliest identified era of martian geological history, included highly eroded large craters buried under ejecta from Schiaparelli's violent formation. 

Mouginis-Mark expected that a good sample might be found on the rim of a fresh crater more than two kilometers across, five of which occurred in Ancient Terrain target ellipse. He calculated that a rover round-trip range of 50 kilometers would be needed to achieve a 90% probability of acquiring a good sample.

For their contribution, USGS geologists H. Masursky, A. Dial, M. Strobell, G. Schaber, and M. Carr recycled four sites that they had studied in 1977-1978 for a proposed Viking follow-on long-range rover mission. Masursky and Dial were co-authors of the Viking '79 traverse study in 1974, while Carr led the Viking orbiter imaging team (and thus was involved in capturing the high-resolution images the minimum MSR Site Selection Team used in preparing its reports).

The USGS sites represented two martian terrain types. Tyrrhena Terra and Iapgyia Terra included ancient cratered terrain similar to that at Mouginis-Mark's Schiaparelli site, which is perhaps unsurprising given that such terrain covers more than 60% of Mars. The sites contained a jumble of overlapping craters and an intercrater mantle of old lava flows.

Samples collected in Tyrrhena and Iapgyia would permit age-dating of the oldest martian crustal material, the USGS geologists wrote. This would enable calibration of the crater counts used for dating martian terrains. In addition, data from the samples could "be compared to comparable analyses made of ancient lunar crustal materials returned by Apollo 16 and [to] ancient terrestrial rocks in order to make interplanetary comparisons of [how rocks are formed], physical and chemical properties, and age."

Of the two sites, Tyrrhena was "superior as a potential sample site in all respects," the USGS team wrote. They proposed that the minimum MSR lander set down where the old lava flows appeared to be thin, near a six-kilometer-diameter crater - one large enough, they judged, to have excavated ancient crust buried beneath the flows. They calculated that a landing ellipse 30 kilometers long and a rover with a 10-kilometer round-trip range would reach only old lava samples. 

Obtaining an ancient crustal rock sample ("the primary science objective"), on the other hand, would demand a five-kilometer landing ellipse and a 14-kilometer round-trip rover. Achieving such landing accuracy implied that the minimum MSR lander would be capable of automated guidance and precision maneuvers during descent.

The other two USGS sites, Candor Chasma and Hebes Chasma, were both part of Valles Marineris, Mars's great equatorial canyon system. "These sites," the USGS team wrote, would "offer a unique opportunity to sample rock layers and their interbedded soils that would reveal the petrochemical history, age dates[,] and the history of environmental changes that may correlate with episodes of channel formation" on Mars. They might also yield organic material ("if the present red anorganic climate did not exist at times in the past") and a record of "the history of solar variations."

Martian Canyonlands: Candor Chasma. Image credit: NASA.

At Candor, their preferred site, parallel rock layers were exposed in the sloping sides of a 1.3-kilometer-tall mesa standing at the bottom of the four-kilometer-deep canyon. If the MSR lander could set down within a five-kilometer landing ellipse atop the mesa, then a seven-kilometer round-trip traverse would permit sampling of some of the layers. Recalling their 1977-1978 study, which assumed a more capable (and more costly) rover, they noted that a "much longer traverse — more than 200 km — would allow the full thickness of rock layers (~4 km) in the canyon walls to be sampled."

The MSWG's fifth report, the first of the six prepared by members of the MSWG Sample Acquisition Team, looked at the availability of rocks on Mars with emphasis on the equatorial Central Latitude Belt, which spanned between 30° N and 30° S. The report's author, University of Houston geologist E. King, explained that celestial mechanics and MSR lander engineering constraints would probably dictate that the Belt contain the first MSR landing site.

The twin Viking landers had had trouble collecting small rocks on Mars, King noted. This had led some to suggest that what looked like rocks at the Viking sites were in fact soft "clods" of martian dirt. If correct, then this hypothesis would mean that rocks were rare on Mars, which would in turn eliminate the primary motivation for an MSR mission; that is, to collect rocks.

King reported that his "evaluation of all of the presently available relevant data" had eliminated this concern "completely" for large parts of Mars, including for the Central Latitude Belt. Especially encouraging were data from the Viking orbiter Infrared Thermal Mapping (IRTM) experiment, which mapped thermal inertia (that is, how long it takes a given surface to become cool at night). Rocky surfaces need longer to cool down than do dusty surfaces. 

Viking IRTM data indicated that much of the Central Latitude Belt has thermal inertias as high as 12. "It is very difficult to construct a reasonable model of the martian surface that has a thermal inertia of more than about 3 that does not have a substantial percentage of the surface area covered with rocks," King wrote.

He attributed the Vikings' inability to collect small rocks to inadequacies in the Viking sampler design. After it scooped a sample containing small rocks, controllers on Earth commanded the sampler to turn upside-down and shake for up to two minutes to sieve out dust. King noted that shaking the sampler caused its lid to flap open as much as an inch. This would allow any pebbles it contained to escape. 

He advocated collecting rock samples in the form of drilled cores, since drilling could penetrate past any weathered rock rinds. Drilling could also collect uniform cylindrical samples that could be handled easily and stored efficiently in the MSR spacecraft.

King was ambivalent about the need for mobility in an MSR mission; he wrote that, if the objective of the mission were to collect fresh igneous rocks, and if the MSR landing site were similar to the Viking landing sites, then little mobility would be necessary. He added that, while it might be prudent to "build in some additional mobility as a margin of safety and to afford additional possibilities for sample collection. . .such provisions [had to be] traded off against lander science and returned sample weight."

USGS geologist H. Moore wrote the sixth MSWG report, which constituted a tour of the landscape within view of the Viking 1 and Viking 2 lander cameras. Viking 2 landed in Utopia Planitia, near the large impact crater Mie, a region more northerly than Viking 1's site in Chryse Planitia. Like King, Moore wrote that Viking 1 rocks were varied (there were 30 types) and tended to be smaller than Viking 2 rocks. The Viking 2 rock population, for its part, appeared to be dominated by ejecta from Mie. 

Moore then described hypothetical rover traverses at the two sites. In each, the rover would visit 17 sampling stations, traverse about 100 meters, and range up to 20 meters from its lander.

The boulder named "Big Joe" at the Viking 1 landing site in Chryse Planitia. Image credit: NASA.

At the Viking 1 site, the rover would collect samples of cloddy soil, crunchy "duricrust" material, an active dune, and drift material, as well as 10-centimeter-long cores from bedrock outcrops, layered rocks, dark and light rocks, a pink rock, rocks formed by asteroid impacts, and gray-hued "Big Joe" (the largest rock near the lander). The rover at the Viking 2 site would collect samples of "inter-rock drift" material, a "drift dunelet," thick crust near a rock, and small rocks, along with cores from a coarsely pitted rock, planar and rounded rocks, a banded rock, the "massive" and pitted ends of one angular rock, and a ventifact (a rock scratched and carved by wind-blown dust and sand).

Moore estimated that the rover would spend between six and eight days traversing and collecting for each station. Each traverse would thus last from 102 to 136 days. The total mass of samples collected on each traverse would total about two kilograms.

The seventh MSWG report sought to estimate the number of crystalline rocks — that is, volcanic rocks such as basalt — at the Viking landing sites and to plan traverses that would adequately sample them. Its authors, R. Arvidson, E. Guinness, S. Lee, and E. Strickland, geologists in the Department of Earth and Planetary Sciences at Washington University in St. Louis, Missouri, argued that any rock larger than about 10 centimeters in diameter at the Viking sites was a good candidate for being crystalline.

Such rocks, they added, cover 9% of the Viking 1 site and 17% of the Viking 2 site. The former, they wrote, included bedrock exposures and at least four soil types, while the latter included two soil types and no bedrock. They pointed out that, while a sampler arm could probably reach a crystalline rock at either site, it would not be able to sample all of the available materials. For that reason, they proposed that MSR landers at the Viking sites should each deploy a "mini-rover."

The Viking 1 site was "such an interesting place," the Washington University team wrote, that they had planned for it a 40-meter traverse with seven sampling stations (with an option to extend to 50 meters and 10 stations). The basic traverse would collect 10-centimeter core samples from three rocks and four soil samples. The extended traverse would sample two more rocks, including Big Joe, and would gather a total of five soil samples, including very red soil from atop Big Joe.

The Viking 2 site, by contrast, featured minimal variety, so the Washington University team's traverse there would cover only 25 meters and seven stations. The mini-rover would collect four soil samples and core samples from three rocks.

N. Nickle of JPL's Flight Projects Planning Office authored the eighth MSWG report, which was titled Requirements for Monitoring Samples. The report was published originally as a JPL Interoffice Memorandum dated 20 October 1978. Nickle wrote that the "scientific integrity of the returned Martian samples is of prime importance." "Scientific integrity," he explained, meant "the preservation of the physical and chemical state of the acquired samples."

To maintain the scientific integrity of the samples collected during the minimum MSR mission, Nickle recommended that they be kept 20° C cooler than the estimated minimum temperature they had experienced on Mars, and that they be sealed within a container with martian air at typical martian surface pressure. In addition, he recommended that the samples be exposed to no more galactic cosmic and solar radiation than they had been on Mars, and to no magnetic field stronger than Earth's natural field.

The minimum MSR mission sought to control cost in part by avoiding science instrumentation not required for sample collection. In the MSWG's ninth report, J. Warner of NASA's Johnson Space Center (JSC) in Houston, Texas, looked at low-mass, low-power MSR science instruments designed to "provide adequate information to select samples." 

His candidate instrument suite included a steerable imager, a reflectance spectrometer, a chemical analyzer on a boom, a boom-mounted densitometer, and a tool for measuring hardness (this might, Warner suggested, be made a function of the sample scoop; the Viking arm and claw had been used to scratch and chip at rocks to judge their hardness).

Warner also prepared the tenth and last report of the Site Selection and Sample Acquisition Study, which he titled A Returned Martian Sample. In it, he looked at the form the minimum MSR sample should take. He looked at two different landing site types: a Viking-like site "laden with a variety of rocks and soils" and a hypothetical "smooth plains site."

The JSC geologist cited Moore's report when he wrote that, at a Viking-like site, an adequate sample could be "obtained on a traverse of a few hundred meters that never leaves the field of view of the lander." He estimated that an atmosphere sample, a soil core, nine rock cores, four small rock fragments, two duricrust samples, and six scoops of soil would adequately represent a Viking-like site. Together these samples would have a mass of 4.1 kilograms.

An eight-month, 15-station traverse could adequately sample a rock-poor smooth plains site, Warner wrote. The rover would range widely over the smooth terrain. Sampling stations would occur at "obstructions" (for example, craters). The rover would drill two or three rock cores and collect one rock fragment at each station, scoop soil at every other station, and collect duricrust at every fifth station. Adding a soil core and an atmosphere sample would bring the total sample mass to 5.7 kilograms if two rock cores were collected and 6.9 kilograms if three cores were collected.

Sources

Mars Sample Return: Site Selection and Sample Acquisition Study, JPL Publication 80-59, Neil Nickle, editor, NASA Jet Propulsion Laboratory, 1 November 1980.

Detailed Reports of the Mars Sample Return Site Selection and Sample Acquisition Study, JPL 715-23, Volumes I-X, Mars Science Working Group Mars Sample Return Study Effort, NASA Jet Propulsion Laboratory, November 1980. 

More Information

Mars Polar Ice Sample Return (1976-1978)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Safeguarding the Earth from Martians: The Antaeus Report (1978-1981)

The 1980s Lunar Revival: Lunar Oxygen (1983)

Climbing toward reusability: liftoff of the Space Shuttle Orbiter Columbia at the start of mission STS-2 (12-15 November 1981). Image credit: NASA.

At the heart of the Space Transportation System (STS) was the Space Shuttle. The first Space Shuttle mission, STS-1 (12-14 April 1981), was the first two-person Orbiter Flight Test (OFT) mission and the first flight of the Shuttle Orbiter Columbia. The second OFT, STS-2 (12-15 November 1981), had to be cut to two days in orbit from a planned five following the failure of one of Columbia's three electricity-producing fuel cells. Nevertheless, STS-2, the first reflight of a reusable spacecraft and the first flight of the Canada-built Remote Manipulator System (RMS) robot arm, was viewed as a success.

When Columbia glided to a landing for the second time, the form the STS would eventually take was still poorly defined. It would remain so at least until the destruction of the Shuttle Orbiter Challenger (28 January 1986) at the start of the STS-51L, the 25th flight of the Shuttle Program. The loss of Challenger and her seven-member crew marked the end of the optimistic first phase of the Space Shuttle Program.

Before that, however, the STS seemed ripe for augmentation. It would, of course, include expendable rocket stages attached to satellites carried to low-Earth orbit (LEO) in the Shuttle Orbiter Payload Bay; these "upper stages" were meant mainly to boost payloads from LEO to geosynchronous orbit (GEO), but could also launch robotic spacecraft from LEO on interplanetary trajectories. In addition, the STS would include Spacelab, a European-built system of Payload Bay-mounted laboratory modules and scientific instrument pallets. Development of upper stages and Spacelab had commenced in the 1970s, shortly after Space Shuttle development began.

Many saw the stages and Spacelab as interim steps toward more complex and competent STS elements. The former, it was expected, would lead to a reusable Orbital Transfer Vehicle (OTV); the latter, to a Space Station assembled in LEO from Orbiter-launched components. The OTV and its "little brother," the Orbital Maneuvering Vehicle (OMV), were typically seen as auxiliary vehicles based at the Space Station.

NASA Johnson Space Center (JSC) in Houston, Texas, anticipated that the Space Station would support ambitious space construction projects; for example, large communications platforms in GEO. In 1979, inspired in part by its involvement in joint Department of Energy/NASA Solar Power Satellite studies, JSC studied a Space Station concept it called the Space Operations Center (SOC). After an initial flurry of planning activity, Shuttle delays put the SOC on the back burner; immediately after STS-1, however, JSC efforts to promote the assembly base in LEO kicked into high gear.

The Space Operation Center (SOC), a Shuttle-launched low-Earth orbit (LEO) space station, as it was envisioned in 1982. Intended as an assembly and repair base, the SOC would have included a "surrogate" Shuttle Orbiter Payload Bay (lower center) for satellite servicing and a hexagonal hangar (center right) for storing and servicing the Orbital Transfer Vehicle (OTV) and Orbital Maneuvering Vehicle (OMV). Image credit: NASA. 

A remote-controlled Orbital Maneuvering Vehicle (OMV), a small "space tug," closes in on the Gamma Ray Observatory (GRO) in this illustration from 1986. After its launch in 1991, GRO was renamed the Compton GRO. The reusable modular OMV, intended as an auxiliary vehicle for extending Shuttle Orbiter and Space Station capabilities, was cancelled in 1987 in the aftermath of the Challenger accident. Image credit: TRW/NASA.

JSC is widely known as the home base of the astronauts and site of the Mission Control Center. Less well known, perhaps, is its long-time contribution to lunar and planetary science. The Lunar Receiving Laboratory (LRL), built in 1967, was used for analysis and storage of lunar geologic samples beginning with Apollo 11 in July 1969. JSC also played a key role in organizing the annual Lunar Science Conference (LSC), the first of which was held in Houston in January 1970.

At the time, JSC was called the Manned Spacecraft Center (MSC). It was renamed in 1973 after the death of President Lyndon Baines Johnson. The LSC was renamed the Lunar and Planetary Science Conference (LPSC) in 1978.

Michael Duke was on hand when the Apollo 11 samples arrived at the LRL; he was Lunar Sample Principal Investigator for the mission. In July 1969, he had been working for the U.S. Geological Survey (USGS) Branch of Astrogeology for six years. In 1970, he left USGS Astro (as ambitious people are wont to do) to become Lunar Sample Curator at NASA MSC, a post he held until 1977, when he became Chief of the NASA JSC Planetary and Earth Sciences Division.

Shortly after STS-2, Duke and another scientist in his division, Wendell Mendell, became concerned that developing the SOC and other proposed STS elements might mean reduced funding for NASA science programs. Space science at NASA was already hurting; the new Administration of President Ronald Reagan had made deep cuts. Rather than oppose new STS elements, Duke and Mendell sought ways that the SOC, OTV, and other proposed hardware could advance scientific exploration. Specifically, they sought to make the case for a base on the Moon.

Their efforts in some ways paralleled those of lunar scientists at the dawn of the Apollo Program, when lunar science barely existed as a field of study. Much like those early pioneers, Duke and Mendell sought to find and bring together individuals and organizations to build a constituency. Initially, they found prospective lunar base allies through informal, low-profile contacts. By late 1982, however, it was time to go public.

This they did by organizing three public special sessions at the 14th LPSC, which was held at NASA JSC in March 1983. The lunar sessions were titled "Return to the Moon" and "Future Lunar Programs." The third session, "Prospects for Planetary Exploration," sought to tie their lunar base efforts to the interests of the broader Solar System exploration community.

In their introduction to the lunar special sessions abstract volume, Duke and Mendell explained that "very little vision is required to see the [STS] reaching to the Moon." They argued that "the lunar option requires decisions today — but not dramatic ones." They pitched a Fiscal Year (FY) 1985 start for the lunar base program, but took pains to stress that the lunar base would need little or no new dedicated funding before FY 1991 or FY 1992.

In fact, they expected that lunar capability would grow more or less naturally from the STS in the late 1990s, several years after the SOC, OTV, and OMV were in place to support GEO missions. The amount of energy required to put a satellite into GEO is, after all, very nearly the same as that needed to put a payload into low-lunar orbit (LLO).

The lunar special sessions abstract volume included 22 abstracts by more than 30 authors and co-authors. The abstracts covered topics ranging from philosophy, law, and economics to geology, physiology, and energy. Of particular interest was an abstract by Hubert Davis, Senior Vice President of Houston-based Eagle Engineering, Incorporated (EEI). In it, he proposed to extend the STS to the Moon. His aim: to make the STS more economical by mining, refining, and using as rocket propellant oxygen chemically locked in lunar dirt.

EEI had been established by retired NASA JSC engineers. Davis, a co-founder of the company, began his career as an aircraft maintenance engineer in the U.S. Air Force during the Korean War. Inspired by President John F. Kennedy's May 1961 call for a man on the Moon, he went to work for the MSC Power & Propulsion Division in March 1962. He oversaw the test program for Lunar Module 5; better known as Eagle, it bore Apollo 11 astronauts Neil Armstrong and Edwin "Buzz" Aldrin to the lunar surface on 20 July 1969.

As Apollo ended, Davis transferred to the MSC Special Projects Office, where he studied new STS elements — cheap solid-propellant STS upper stages and a Shuttle-derived heavy-lift launcher — as well as Solar Power Satellites manufactured from lunar materials. He took early retirement in 1978 after being made the JSC Engineering Directorate representative to the Space Shuttle Program Office; in a 2009 NASA JSC oral history interview, Davis explained that he had left the space agency at age 48 because he found the Shuttle oversight job to be boring.

The industry magazine Aviation Week & Space Technology made his presentation the centerpiece of its coverage of the LPSC lunar base special sessions. Two months following the special sessions Davis published an expanded version of his abstract and presentation as an EEI report called Lunar Oxygen Impact Upon STS Effectiveness.

Davis acknowledged that his study was incomplete and preliminary. He did not, for example, examine how his oxygen mining and refining facility would be established on the Moon. That the study was preliminary explained why it was incomplete; if it could not provide an early indication that lunar oxygen would enhance STS capabilities, Davis argued, then there would be little point in conducting a more detailed study of the concept.

To illustrate how lunar-produced liquid oxygen might enhance the STS, Davis used as an example STS-40, which was scheduled to place the Galileo spacecraft into LEO on 30 May 1986. Galileo would reach LEO attached to an expendable Centaur G' upper stage which, following release from the Orbiter payload bay, would launch it out of LEO on a direct trajectory to Jupiter.

The Galileo spacecraft was expected to weigh 2510 kilograms and the Centaur G' without propellants, 2650 kilograms. Support equipment for maintaining Galileo and Centaur G' in the payload bay would weigh 470 kilograms and 3640 kilograms, respectively. Filling the Centaur G' large tank with low-density liquid hydrogen fuel would add 3310 kilograms to STS-40's payload weight; filling the Centaur G' small tank with dense liquid oxygen oxidizer would increase payload weight by a whopping 16,570 kilograms. STS-40 payload weight thus came to 29,480 kilograms, with liquid oxygen making up 56% of the total.

The Galileo Jupiter spacecraft and an expendable Centaur G' upper stage move away from the Shuttle Orbiter that deployed them in low-Earth orbit. The large-diameter forward section of the stage (center), to which Galileo is attached, contains low-density liquid hydrogen fuel; the small-diameter aft section, to which are attached two rocket motors, high-density liquid oxygen oxidizer. Image credit: NASA.

Davis then imagined a 2510-kilogram Shuttle-launched payload in the first decade of the 21st century, after the STS had been extended to the Moon. A reusable OTV would replace the Centaur G'. Permanently basing the OTV at the SOC meant that it would not add to Shuttle payload weight.

The Shuttle Orbiter would carry a 660-kilogram tank containing 3310 kilograms of liquid hydrogen for fueling the OTV in LEO. The propellant dump in LEO near the SOC would provide the OTV with lunar liquid oxygen. Support equipment in the payload bay for the spacecraft and hydrogen tank would bring the total Shuttle payload weight to just 7280 kilograms, or about one quarter of the STS-40 total.

A chemical process called hydrogen reduction of ilmenite formed the basis of the lunar oxygen STS infrastructure. Ilmenite (chemical formula FeTiO3), a titanium ore, is a mineral common in the basaltic rocks, dirt, and dust that form the dark-hued lunar plains known as maria (Latin for "seas").

Davis focused on ilmenite rather than lunar polar ice — which can provide both hydrogen and oxygen — because in 1983 no one knew that water ice exists at the lunar poles. Though the lunar polar ice hypothesis was by then more than 20 years old, the first evidence that it might be correct would not be found until 1994, when Clementine became the first spacecraft to explore the Moon from lunar polar orbit.

Mining robots would continuously gather ilmenite-rich lunar dirt at a rate of 28 metric tons per hour and deliver it to a separation facility. The dirt would first be sieved to remove large dirt particles, clods, and rocks. The resulting fine-grained dirt and dust would then be heated and subjected to an electrostatic process that would separate the ilmenite at a rate of 2.27 metric tons per hour.

The ilmenite would be moved to the hydrogen-reduction unit, where it would be exposed to hydrogen gas at 700° Celsius (C) and 2.7 Earth atmospheres of pressure. The hydrogen would bind with and free the oxygen bonded to the iron. This would yield water vapor at a rate of 0.26 metric tons per hour, which would be cooled, condensed, and subjected to electrolysis, splitting it into oxygen and hydrogen.

The oxygen would be chilled until it condensed into dense liquid, then stored in spherical tanks. The hydrogen would be returned to the reduction unit for reuse and the powdery titanium oxide and iron left over from the reduction process — about 90% of the original mass of the lunar dirt — would be stacked out of the way for possible future use.

The facility would use a little more than six megawatts of electricity continuously; this might be reduced if waste heat from the refining process could be exploited effectively. Davis estimated that his mining and refining facility could produce 150 metric tons of liquid oxygen per month.

The Aft Cargo Carrier (ACC) — the blue short cylinder and truncated cone attached at left to the bottom of the orange Space Shuttle External Tank (ET) — would augment the 4.57-by-18.28-meter Shuttle Orbiter Payload Bay (the blue cylinder within the Orbiter outlined by dashed lines). The 9.72-meter-long, 8.38-meter-wide ACC would, among other things, facilitate launch of OTV components and spherical tanks filled with liquid hydrogen. Image credit: Martin Marietta.

This painting by Eagle Engineering artist Pat Rawlings displays elements of Davis's proposed lunar oxygen STS infrastructure in LEO. The close proximity of the elements is schematic, not realistic. At lower right, a remote-controlled OMV detaches a spherical tank filled with liquid hydrogen from an ET/ACC. The LEO propellant dump is at lower left. Small in the distance above it, silhouetted against the Earth, is a Shuttle Orbiter. A version of the SOC is depicted to the right of the Orbiter. In the foreground, a small OTV prepares to leave Earth orbit; liquid hydrogen tanks bound for lunar orbit ride on its bowl-shaped rigid aerobrake heat shield.

Davis described his lunar oxygen STS infrastructure in operation. A Space Shuttle would launch liquid hydrogen for the LEO propellant dump in a spherical tank inside an Aft Cargo Carrier (ACC) fitted over the dome-shaped end of its External Tank (ET).

Normally, the ET would separate from the Orbiter as it neared orbital velocity, tumble and break up in the upper atmosphere, and fall into the Indian Ocean. When the ACC was attached, however, the Shuttle Orbiter would boost the ET/ACC combination to LEO and separate. A remote-controlled OMV based at the SOC would then detach the hydrogen tank from the ACC and move it to the propellant dump, where refrigeration and high-tech insulation would ensure that no hydrogen was lost to boil-off.

Zero liquid hydrogen boil-off was, Davis explained, critical to making his lunar oxygen STS infrastructure viable. He wrote that the Centaur G' stage was expected to lose about 3% of its liquid hydrogen to boil-off per day. A similar boil-off rate at any point in his lunar oxygen STS infrastructure would be "intolerable."

Davis assumed two types of modular OTVs, each of which could be tailored to carry out several types of missions. The OTVs, clusters of spherical propellant tanks linked by struts, would perform roundtrip missions between LEO and GEO and between LEO and an LLO SOC and propellant dump. The OTVs could operate with or without a pressurized module containing a crew.

The smaller OTV, which would burn 25 metric tons of liquid hydrogen and liquid oxygen during a voyage from LEO to LLO and back, would include a rigid aerobrake heat shield 18 meters wide. The heat shield, which would include thermal protection tiles akin to those attached to the Space Shuttle Orbiter, would enable the OTV to use atmospheric drag to capture into LEO with minimal propellant expenditure. The smaller OTV could transport nearly 43 metric tons of liquid hydrogen from the LEO propellant dump to its counterpart in LLO.

A single-stage Lunar Module lander based on the smaller OTV design would burn 28 metric tons of liquid hydrogen/liquid oxygen propellants to travel from LLO to the lunar surface and back. It would be capable of lifting 41 metric tons of liquid oxygen from the lunar surface to the LLO propellant dump.

Davis used Lunar Module landing gear as an example of how hardware in his lunar oxygen STS infrastructure would need to be optimized to reduce mass. The Apollo Lunar Module's four landing legs and foot pads accounted for 3.3% of its landed weight; the small OTV-based Lunar Module would exploit new materials and improved understanding of the lunar surface to reduce the figure to 2%.

On the Moon: at lower left, a robotic front-end loader scoops lunar dirt; behind it, another deposits dirt in a hopper at the start of the lunar oxygen refining process. Cables strung on poles link the lunar oxygen refinery to a nuclear reactor just over the horizon. Lunar oxygen is liquified and poured into a tank at center right; the filled tank will be added to the stack of tanks in the background at center top. Conveyor belts transport tailings to a storage area at upper left, just beyond the Lunar Module launch & landing pad, and to the open pit mine at center left. Image credit: Pat Rawlings/Eagle Engineering, Incorporated/NASA.

In lunar orbit: in the foreground, a large OTV with a stowed white ballute heat shield prepares to depart LLO for Earth orbit carrying a cargo of lunar oxygen. In the background, the LLO propellant dump orbits close by the lunar SOC. Meanwhile, a Lunar Module bearing lunar liquid oxygen moves in to dock with the propellant dump. Image credit: Pat Rawlings/Eagle Engineering, Incorporated/NASA.

In its basic form, the larger OTV would carry a propellant load of 33 metric tons. Two such OTVs could be combined to form an OTV with a propellant load of 78 tons. The latter configuration would be capable of transporting more than 200 metric tons of lunar oxygen from LLO to the LEO propellant dump. This would, he calculated, require an aerobrake heat shield about 115 meters wide; that is, wider than an American football field with end zones.

One might be forgiven for asking why such a large lunar oxygen cargo was necessary; that is, why Davis did not propose transporting it to LEO using several smaller OTVs spaced out over time. He explained that minimum-energy opportunities for travel from the LLO propellant dump to LEO would occur less than once per month. They would be infrequent because the OTV could only depart LLO as its orbital plane coincided with that of the LEO propellant dump. To do otherwise would demand plane-change maneuvers that would contribute toward making the lunar oxygen STS infrastructure uneconomical.

During aerobraking, the OTV would pass through Earth's upper atmosphere at an altitude of between 50 and 100 kilometers so that atmospheric drag could reduce its speed. The OTV would then climb back into space toward an apogee (orbit high point) near SOC altitude (about 400 kilometers). At apogee, it would ignite its engines to raise its perigee (orbit low point) out of Earth's atmosphere. For the perigee-raise maneuver, Davis budgeted only enough propellants to change OTV speed by 100 meters per second. He suggested that, if further analysis showed this to be insufficient, then an SOC-based OMV might retrieve the OTV and lunar oxygen payload at apogee.

In neither the small OTV nor the large OTV case could aerobrake heat shield mass exceed 3.5% of OTV mass at the time of Earth atmosphere entry. Davis focused on heat shield mass reduction because other OTV systems were already optimized, OTV propellants had been trimmed to the bare minimum required, and reducing the liquid oxygen cargo would defeat the purpose of the exercise. He conceded that cutting aerobrake heat shield mass so dramatically might constitute a significant technical challenge; most OTV studies, he explained, had assumed that the heat shield would make up at least 10% of OTV mass at Earth atmosphere entry.

Ballute in action. This is representative of ballutes in general; the inflatable heat shield is shown here attached to a single-engine cylindrical tug, not the two-engine large OTV Davis described, and aerobraking events take place at higher altitudes than in his scenario. Image credit: NASA.

To reduce the mass of the large OTV heat shield, Davis suggested that it might take the form of an expendable fabric ballute ("balloon-parachute"). The OTV would point its twin engines in its direction of motion as the donut-shaped ballute inflated; the engines would then operate in "idle" mode to create a relatively cool gas barrier between the ballute surface and the high-temperature plasma generated in front of the ballute by Earth atmosphere reentry at lunar-return speed (3.2 kilometers per second).

Davis used computer models to attempt to determine the Mass Payback Ratio (MPR) of his proposed lunar oxygen STS infrastructure. An MPR of 1 would mean that the mass of resources (mainly propellants) expended to exploit lunar oxygen would equal the mass of the lunar oxygen supplied to LEO. NASA would thus gain nothing from putting lunar oxygen to work in the STS. If, on the other hand, the mass of the lunar oxygen delivered to LEO exceeded the mass of the resources needed to exploit it, then more detailed study might be justified.

Davis cited a computer model that included 25 roundtrip OTV flights between LEO and LLO and 103 roundtrip Lunar Module flights between LLO and the lunar surface. He wrote that, in exchange for 983 metric tons of liquid hydrogen, hydrogen tanks, and OTV attitude-control system propellant dispatched to the Moon, 2414 metric tons of lunar liquid oxygen would arrive in the LEO. He judged that this quantity could support 90 OTV flights between LEO and GEO over a period of about five years.

This indicated a preliminary MPR of 2.45, which, Davis wrote, justified additional study. He anticipated, however, that it probably would not provide enough margin to maintain a positive MPR if the mass of hardware and propellants required to establish and maintain the lunar oxygen STS infrastructure were taken into consideration.

Davis did not provide weight estimates for the LEO propellant dump, the LLO propellant dump and LLO SOC, and the Lunar Modules. Neither did he estimate the weight of the Earth-launched liquid hydrogen and liquid oxygen propellants needed to initiate the lunar oxygen STS infrastructure, nor the weight of Earth-launched liquid hydrogen needed to fly resupply and crew rotation missions after lunar oxygen became available. He assumed that the OTVs and LEO SOC would be built for LEO and GEO operations even if NASA did not return to the Moon, so disregarded their weight in his model.

He did, however, provide a weight estimate for the lunar surface mining and refining facility. Mining robots, a habitat for housing 10 facility caretakers, refining equipment, storage tanks, a nuclear reactor for generating electricity, radiator panels, and other equipment would have a combined weight of 437 metric tons. Adding this to the 983 tons of hydrogen, tanks, and attitude-control propellant would lead to an MPR of only 1.7.

If, somehow, the MPR remained sufficiently favorable after more detailed technical studies yielded credible complete weight estimates, then complex economic analyses would follow. These would, Davis explained, be based on real-world dollars and would take into account societal factors such as "affordability."

Davis conceded that extending the STS to the Moon probably could not be justified solely on the basis of economics. He argued that lunar resources should nevertheless be developed. He cited a January 1982 Los Alamos National Laboratory (LANL) proposal for an international research laboratory on the Moon; it promised wide-ranging scientific, economic, political, and defense benefits. With a nod to the political language of the 1980s United States, Davis declared that the "vitality of Free World commerce and physical security would be greatly increased by the presence of. . .resources in space."

Sources

Space Operations Center presentation materials, NASA Johnson Space Center, 18 January 1982.

"NASA Conference to Highlight Return to the Moon," NASA News Release 83-007, Steve Nesbitt, no date (March 1983).

"Economic Benefits of Lunar Base Cited," E. Bulban, Aviation Week & Space Technology, 18 April 1983, pp. 132-133, 135-137.

Fourteenth Lunar and Planetary Science Conference Special Sessions Abstracts — Return to the Moon — March 16, 1983, Future Lunar Program — March 17, 1983, LPI Contribution 500, Lunar and Planetary Institute, 1983.

Lunar Oxygen Impact Upon STS Effectiveness, Report No. 8363, Hubert Davis, Eagle Engineering, Inc., May 1983.

"Return to the Moon," Andrew Chaikin, Sky & Telescope, June 1983, p. 493.

NASA Johnson Space Center Oral History Project Edited Oral History Transcript: Hubert P. Davis, 28 July 2009 (https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/DavisHP/DavisHP_7-28-09.htm — accessed 20 March 2020).

More Information

Harold Urey and the Moon (1961)

Apollo Science and Sites: The Sonett Report (1963)

Electricity from Space: The 1970s DOE/NASA Solar Power Satellite Studies

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

"A Vision of the Future": Military Uses of the Moon and Asteroids (1983)

Mission to the Mantle: Michael Duke's Moonrise (1999-2009)