Apollo-Soyuz II (1974)

Image credit: NASA.

The Apollo-Soyuz Test Project (ASTP) had its origins in talks aimed at developing a common U.S./Soviet docking system for space rescue. The concept of a common docking system was first put forward in 1970; it was assumed at that time, however, that the docking system would be developed for future spacecraft, such as the U.S. Space Station/Space Shuttle, not the U.S. Apollo Command and Service Module (CSM) and Soviet Soyuz spacecraft in operation at the time.

A joint U.S./Soviet space mission served the political aims of both countries, however, so the concept of a near-term docking mission rapidly gained momentum. In May 1972, at the superpower summit meeting held in Moscow, President Richard Nixon and Premier Alexei Kosygin signed an agreement calling for an Apollo-Soyuz docking in July 1975.

NASA and its contractors studied ways of expanding upon ASTP even before it was formally approved; in April 1972, for example, McDonnell Douglas proposed a Skylab-Salyut international space laboratory (see "More Information," below). A year and a half later (September 1973), however, the aerospace trade magazine Aviation Week & Space Technology cited unnamed NASA officials when it reported that, while the Soviets had indicated interest in a 1977 second ASTP flight, the U.S. space agency was "currently unwilling" to divert funds from Space Shuttle development.

Nevertheless, early in 1974 the Flight Operations Directorate (FOD) at NASA Johnson Space Center (JSC) in Houston, Texas, examined whether a second ASTP mission might be feasible in 1977. The 1977 ASTP proposal aimed to fill the expected gap in U.S. piloted space missions between the 1975 ASTP mission and the first Space Shuttle flight.

Cutaway illustration of ASTP Apollo Command Module (lower left), ASTP Docking Module (DM), ASTP Soyuz Orbital Module, and ASTP Soyuz Descent Module (upper right). The three U.S. crewmembers wear brown coveralls. Image credit: NASA.

The brief in-house study focused on mission requirements for which NASA JSC had direct responsibility. FOD assumed that Apollo CSM-119 would serve as the prime 1977 ASTP spacecraft and that the U.S. would again provide the Docking Module (DM) for linking the Apollo CSM with the Soyuz spacecraft. CSM-119 had been configured as the five-seat Skylab rescue CSM; work to modify it to serve as the 1975 ASTP backup spacecraft began as FOD conducted its study, soon after the third and final Skylab crew returned to Earth in February 1974. FOD suggested that, if a backup CSM were deemed necessary for the 1977 ASTP mission, then the incomplete CSM-115 spacecraft should get the job. CSM-115, which resided in storage in California, had been tapped originally for the cancelled Apollo 19 moon landing mission.

FOD also assumed that the ASTP prime crew of Thomas Stafford, Vance Brand, and Deke Slayton would serve as the backup crew for the 1977 ASTP mission, while the 1975 ASTP backup crew of Alan Bean, Ronald Evans, and Jack Lousma would become the 1977 ASTP prime crew. FOD conceded, however, that this assumption was probably not realistic. If new crewmembers were needed, FOD noted, then training them would require 20 months. They would undergo 500 hours of intensive language instruction during their training.

FOD estimated that Rockwell International support for the 1977 ASTP flight would cost $49.6 million, while new experiments, nine new space suits, and "government-furnished equipment" would total $40 million. Completing and modifying CSM-115 for its backup role would cost $25 million. Institutional costs — for example, operating Mission Control and the Command Module Simulator (CMS), printing training manuals and flight documentation, and keeping the cafeteria open after hours — would add up to about $15 million. This would bring the total cost to $104.7 million without the backup CSM and $129.7 million with the backup CSM.

The FOD study identified "two additional major problems" facing the 1977 ASTP mission, both of which involved NASA JSC's Space Shuttle plans. The first was that the CMS had to be removed to make room for planned Space Shuttle simulators. Leaving it in place to support the 1977 ASTP mission would postpone Shuttle simulator availability.

A thornier problem was that 75% of NASA JSC's existing flight controllers (about 100 people) would be required for the 1977 ASTP in the six months leading up to and during the mission. In the same period, NASA planned to conduct "horizontal" Space Shuttle flight tests. These would see a Shuttle Orbiter flown atop a modified 747; later, the aircraft would release the Orbiter for an unpowered glide back to Earth. FOD estimated that NASA JSC would need to hire new flight controllers if it had to support both the 1977 ASTP and the horizontal flight tests. The new controllers would receive training to support Space Shuttle testing while veteran controllers supported the 1977 ASTP.

ASTP Apollo spacecraft and Saturn IB rocket sit atop the "milkstool" on Launch Pad 39B, Kennedy Space Center, Florida. Image Credit: NASA.

ASTP Soyuz 19 spacecraft and Soyuz rocket lift off from Baikonur Cosmodrome in Soviet central Asia. Image credit: NASA.

The ASTP Apollo CSM (CSM-111) lifted off on a Saturn IB rocket on 15 July 1975 with astronauts Thomas Stafford, Vance Brand, and Donald Slayton on board. The ASTP Saturn IB, the last rocket of the Saturn family to fly, lifted off from Launch Complex (LC) 39 Pad B, one of two Saturn V pads at Kennedy Space Center, not the LC 34 and LC 37 pads used for Saturn IB launches in the Apollo lunar program. This was because NASA had judged that maintaining the Saturn IB pads for Skylab and ASTP would be too costly. A "pedestal" (nicknamed the "milkstool") raised the Skylab 2, 3, and 4 and ASTP Saturn IB rockets so that they could use the Pad 39B Saturn V umbilicals and crew access arm.

Once in orbit, the ASTP CSM turned and docked with the DM mounted on top of the Saturn IB's second stage. It then withdrew the DM from the stage and set out in pursuit of the Soyuz 19 spacecraft, which had launched about eight hours before the Apollo CSM with cosmonauts Alexei Leonov and Valeri Kubasov on board. The two craft docked on 17 July and undocked for the final time on July 19. Soyuz 19 landed on 21 July. The ASTP Apollo CSM, the last Apollo spacecraft to fly, splashed down near Hawaii on 24 July 1975 — six years to the day after Apollo 11, the first piloted Moon landing mission, returned to Earth.

Conceptual illustration of proposed Space Shuttle/Salyut docking. Image credit: Junior Miranda.

U.S. Space Shuttle Atlantis docked with the Russian Mir space station, 4 July 1995, as imaged from the Russian Soyuz TM-21 spacecraft. Image credit: NASA.

The proposal for a 1977 ASTP repeat gained little traction. Though talks aimed at a U.S. Space Shuttle docking with a Soviet Salyut space station had resumed in May 1975, no plans for new U.S.-Soviet manned missions existed when the ASTP Apollo splashed down. Shuttle-Salyut negotiators made progress in 1975-1976, but the U.S. deferred signing an agreement until after the results of the November 1976 election were known.

In May 1977, the sides formally agreed that a Shuttle-Salyut mission should occur. In September 1978, however, NASA announced that talks had ended pending results of a comprehensive U.S. government review. Following the December 1979 Soviet invasion of Afghanistan, work toward joint U.S.-Soviet piloted space missions was abandoned on advice from the U.S. Department of State. It would resume a decade later as the Soviet Union underwent radical internal changes that led to its collapse in 1991 and the rebirth of the Soviet space program as the Russian space program.

Sources

"Second ASTP Unlikely," Aviation Week & Space Technology, 3 September 1973, p. 13.

Memorandum for the Record, "information. . . developed in estimating the cost of flying a second Apollo-Soyuz Test Project (ASTP) mission in 1977," NASA Johnson Space Center, 4 April 1974.

Thirty Years Together: A Chronology of U.S.-Soviet Space Cooperation, NASA CR 185707, David S. F. Portree, February 1993.

More Information

Skylab-Salyut Space Laboratory (1971)

"Still Under Active Consideration": Five Proposed Apollo Earth-Orbital Missions for the 1970s (1971)

NASA's 1992 Plan to Land Soyuz Space Station Lifeboats in Australia

SEI Swan Song: International Lunar Resources Exploration Concept (1993)

Testing Shuttle Manipulator Arms During Earth-Orbital Apollo Missions (1971-1972)

In this drawing by NASA engineer Caldwell Johnson, twin human-like Space Shuttle robot arms with human-like hands deploy from the Apollo Command and Service Module (CSM) Scientific Instrument Module (SIM) Bay to grip the derelict Skylab space station. Image credit: NASA/Caldwell Johnson.

Caldwell Johnson, co-holder with Maxime Faget of the Mercury space capsule patent, was chief of the Spacecraft Design Division at the NASA Manned Spacecraft Center (MSC) in Houston, Texas, when he proposed that astronauts test prototype Space Shuttle manipulator arms and end effectors during Apollo Command and Service Module (CSM) missions in Earth orbit. In a February 1971 memorandum to Faget, NASA MSC's director of Engineering and Development, Johnson described the manipulator test mission as a worthwhile alternative to the Earth survey, space rescue, and joint U.S./Soviet CSM missions then under study. 

At the time Johnson proposed the Shuttle manipulator arm test, three of the original 10 planned Apollo lunar landing missions had been cancelled, the second Skylab space station (Skylab B) appeared increasingly unlikely to reach orbit, and the Space Shuttle had not yet been formally approved. NASA managers foresaw that the Apollo and Skylab mission cancellations would leave them with surplus Apollo spacecraft and Saturn rockets after the last mission to Skylab A. They sought low-cost Earth-orbital missions that would put the surplus hardware to good use and fill the multi-year gap in U.S. piloted missions expected to occur in the mid-to-late 1970s.

Johnson envisioned Shuttle manipulators capable of bending and gripping much as do human arms and hands, thus enabling them to hold onto virtually anything. He suggested that a pair of prototype arms be mounted in a CSM Scientific Instrument Module (SIM) Bay, and that the CSM "pretend to be a Shuttle" during rendezvous operations with the derelict Skylab space station. 

The CSM's three-man crew could, he told Faget, use the manipulators to grip and move Skylab. They might also use them to demonstrate a space rescue, capture an "errant satellite," or remove film from SIM Bay cameras and pass it to the astronauts through a special airlock installed in place of the docking unit in the CSM's nose. 

Image credit: NASA/Caldwell Johnson.

Faget enthusiastically received Johnson's proposal (he penned "Yes! This is great" on his copy of the February 1971 memo). The proposal generated less enthusiasm elsewhere, however. 

Undaunted, Johnson proposed in May 1972 that Shuttle manipulator hardware replace Earth resources instruments that had been dropped for lack of funds from the planned U.S.-Soviet Apollo-Soyuz Test Project (ASTP) mission. President Richard Nixon had called on NASA to develop the Space Shuttle just four months before (January 1972). Johnson asked Faget for permission to perform "a brief technical and programmatic feasibility study" of the concept, and Faget gave him permission to prepare a presentation for Aaron Cohen, manager of the newly created Space Shuttle Program Office at MSC. 

In his June 1972 presentation to Cohen, Johnson declared that "[c]argo handling by manipulators is a key element of the Shuttle concept." He noted that CSM-111, the spacecraft tagged for the ASTP mission, would have no SIM Bay in its drum-shaped Service Module (SM), and suggested that a single 28-foot-long Shuttle manipulator arm could be mounted near the Service Propulsion System (SPS) main engine in place of the lunar Apollo S-band high-gain antenna, which would not be required during Earth-orbital missions. 

During ascent to orbit, the manipulator would ride folded beneath the CSM near the ASTP Docking Module (DM) within the streamlined Spacecraft Launch Adapter. During SPS burns, the astronauts would stabilize the manipulator so that acceleration would not damage it by commanding it to grip a handle installed on the SM near the base of the CSM's conical Command Module (CM). 

Johnson had by this time mostly dropped the concept of an all-purpose human hand-like "end effector" for the manipulator; he informed Cohen that the end effector design was "undetermined." The Shuttle manipulator demonstration would take place after CSM-111 had undocked from the Soviet Soyuz spacecraft and moved away to perform independent maneuvers and experiments. 

The astronauts in the CSM would first use a TV camera mounted on the arm's wrist to inspect the CSM and DM, then would use the end effector to manipulate "some device" on the DM. They would then command the end effector to grip a handle on the DM, undock the DM from the CSM, and use the manipulator to redock the DM to the CSM. Finally, they would undock the DM and repeatedly capture it with the manipulator.

Caldwell Johnson's depiction of a prototype Shuttle manipulator arm with a hand-like end effector. The manipulator grasps the Docking Module meant to link U.S. Apollo and Soviet Soyuz spacecraft in Earth orbit during the Apollo-Soyuz Test Project (ASTP) mission. Image credit: NASA/Caldwell Johnson. 

Johnson estimated that new hardware for the ASTP Shuttle manipulator demonstration would add 168 pounds (76.2 kilograms) to the CM and 553 pounds (250.8 kilograms) to the SM. He expected that concept studies and pre-design would be completed in January 1973. Detail design would commence in October 1972 and be completed by 1 July 1973, at which time CSM-111 would undergo modification for the manipulator demonstration.

Johnson envisioned that MSC would build two manipulators in house. The first, for testing and training, would be completed in January 1974. The flight unit would be completed in May 1974, tested and checked out by August 1974, and launched into orbit attached to CSM-111 in July 1975. Johnson optimistically placed the cost of the manipulator arm demonstration at just $25 million. 

CSM-111, the last Apollo spacecraft to fly, reached Earth orbit on schedule on 15 July 1975. By then, Caldwell Johnson had retired from NASA. CSM-111 carried no manipulator arm; the tests Johnson had proposed had been judged to be unnecessary. 

That same month, the U.S. space agency, short on funds, invited Canada to develop and build the Shuttle manipulator arm. The Remote Manipulator System — also called the Canadarm — first reached orbit on board the Space Shuttle Columbia during STS-2, the second flight of the Shuttle program, on 12 November 1981. 

During Space Shuttle mission STS-7 (18-24 June 1983), the crew of the Orbiter Challenger used the Canada-built Remote Manipulator System (RMS) arm to deploy and retrieve a small satellite. In this image, the deployed RMS is bent to form a numeral "7." Image credit: NASA. 

Sources 

Memorandum with attachment, EW/Chief, Spacecraft Design Division, to EA/Director of Engineering and Development, "Flight Demonstration of Shuttle docking and cargo handling techniques and equipment using CSM/Saturn 1-B," NASA Manned Spacecraft Center, 1 February 1971. 

Memorandum with attachment, EW/Chief, Spacecraft Design Division, to PA/Special Assistant to the Manager, "Demonstration of Shuttle manipulators aboard CSM/Soyuz rendezvous and docking mission," NASA Manned Spacecraft Center, 25 May 1972. 

Memorandum with attachment, EW/Chief, Spacecraft Design Division, to LA/Manager, Space Shuttle Program Office, "Proposal to Demonstrate Shuttle-type Manipulator During Apollo/Soyuz Test Project," NASA Manned Spacecraft Center, 28 June 1972. 

More Information

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station Shuttle (1970)

"Still Under Active Consideration": Five Proposed Earth-Orbital Apollo Missions for the 1970s (1971)

Skylab-Salyut Space Laboratory (1972)

Viking on the Moons of Mars (1972)

Phobos, the inner moon of Mars, as imaged by the Viking 1 Orbiter. Viking images of Deimos, Mars's outer moon, can be found at the bottom of this post. Image credit: NASA.

In June 1972, NASA Langley Research Center (LaRC) in Hampton, Virginia, contracted with Martin Marietta Corporation to look at using spacecraft based on the planned Viking Mars Lander and Orbiter designs to explore the martian moons Phobos and Deimos. NASA LaRC managed Project Viking, which aimed to launch two Lander/Orbiter combinations toward Mars in 1975, while Martin Marietta was prime contractor for the Viking Lander. The Jet Propulsion Laboratory (JPL) in Pasadena, California, built the Mariner-derived Viking Orbiter.

Viking was the stepchild of the Voyager Program, first proposed by JPL in 1960. Voyager, which had as its major goal the discovery and detailed study of life on Mars, had suffered from management ineptitude, turf battles, schedule slips, and a comprehensive redesign (and attendant cost increase) resulting from new Mars data gathered during the Mariner IV Mars flyby (July 1965).

The Voyager spacecraft as envisioned at the time of its cancellation in 1967. The lander is packaged in the dark-colored aeroshell at upper right. Image credit: NASA.

In an unusual move, top NASA officials met with leaders of Congress shortly after Voyager's cancellation in August 1967 to seek funding for a replacement. The latter agreed to fund Viking starting in Fiscal Year 1969, which began on 1 October 1968. Like Voyager, Viking would seek life on Mars. Congressional leaders also agreed to fund a pair of Mariner Mars orbiters that would launch in 1971.

Mariner Mars 1971, Viking, and the proposed Viking-based Phobos/Deimos missions were all in part a response to declared Soviet space plans. As NASA astronauts explored Earth's Moon, the Soviet Union proclaimed to the world that they had never meant to accomplish the same feat; that they had in fact opted to explore space using robots because they would not place human lives at risk. They pointed to their robotic Luna lunar sample-returners and Lunokhod 1 lunar rover when they claimed that they would soon dispatch robot orbiters, landers, sample-returners, and rovers throughout the Solar System. 

The Mariner VII spacecraft glimpsed Phobos during its fast Mars flyby in 1969, and the Mariner 9 orbiter returned the first clear images of both martian moons in November 1971, while Martin Marietta's study was underway. Mariner 8, Mariner 9's twin and planned fellow traveler, had fallen into the Atlantic Ocean north of Puerto Rico after the failure of its Centaur upper stage on 9 May 1971. 

Phobos and Deimos were the first non-spherical Solar System bodies humankind examined close up. They revolve about Mars in circular equatorial orbits. Phobos completes one orbit in about 7.5 hours at an altitude of about 5980 kilometers (3715 miles), while Deimos orbits in about 30 hours at 20,070 kilometers (12,470 miles). 

Phobos measures 21 by 25 kilometers (13 by 16 miles) and Deimos is about half as large. Small size means low gravity; Phobos has only about 0.1% as much surface gravity as Earth. Because of this, setting down on Phobos or Deimos would be more like docking than landing.

Viking Orbiter (top) shortly after release of the aeroshell containing the Viking Lander. Image credit: NASA/Don Davis.

A Viking Lander samples the surface of Mars in this artist's concept from March 1974. Image credit: U.S. Geological Survey/Don Davis.

NASA LaRC directed Martin Marietta to assume that its Viking-based Phobos/Deimos missions would depart Earth in the 1979 and 1981 Earth-Mars minimum-energy transfer opportunities. The study report described several Phobos/Deimos spacecraft designs. 

The first, the baseline Phobos/Deimos landing spacecraft, would comprise a heavily modified Viking Lander and a Viking Orbiter with tanks carrying 38% more propellants than the Viking 1975 design (Martin Marietta called this a "38% Stretch Orbiter"). Mass at Earth-orbit departure would total about 3600 kilograms (7940 pounds) during the 1979 minimum-energy Earth-Mars transfer opportunity. The Lander would account for 482 kilograms (1063 pounds) of that mass. 

Upon arrival at Mars, the Orbiter would fire its rocket motor to slow down and place itself and the attached Mars moon Lander into an elliptical, equatorial "capture orbit" about the planet. The spacecraft would then maneuver into an elliptical, 15-hour "observation orbit." The apoapsis (high point) of this orbit would reach the orbit of Deimos, while its periapsis (low point) would dip inside the orbit of Phobos. 

The spacecraft would repeatedly fly past both moons, gathering data at each encounter so that scientists on Earth could decide which moon most warranted in-depth exploration. Controllers would then command the spacecraft to match orbits with the moon selected. 

The Lander would separate from the Orbiter and move toward its target using Viking Lander attitude-control thrusters. It would set down gently on three spidery legs and deploy 82 kilograms (181 pounds) of instruments, including a seismometer, a surface sample auger, and a boom-mounted camera. The Lander would be able to hop across the surface in the weak gravity by briefly firing its thrusters; an alternate mobility scheme would employ spindly umbrella-shaped wheels at the ends of the landing legs. 

Viking Lander-derived Phobos/Deimos rover. Image credit: Martin Marietta/NASA.

Martin Marietta proposed an alternate baseline mission in which the Viking Orbiter would land on the target moon. This more efficient "landed orbiter" scenario could land about 500 kilograms (1100 pounds) of science instruments, the company estimated. Total cost for a baseline Phobos/Deimos landing mission would come to $324 million.

The company targeted its second design, the baseline Phobos/Deimos sample-return spacecraft, for launch in 1981 "to allow more time for additional mission design and hardware development." The sample-return mission would build on experience gained in the 1979 landing mission. Its 3374-kilogram (7438-pound) spacecraft would consist of a 38% Stretch Viking Orbiter with four legs and a 260-kilogram (573-pound) drum-shaped Earth-return vehicle based on a proposed Venus Pioneer spacecraft design. 

The Orbiter would land on the target moon, collect a two-kilogram (4.4-pound) sample, and transfer it to a sample-return capsule inside the Earth-return vehicle. The Earth-return vehicle would then fire its rocket to separate from the landed Orbiter and maneuver into a 1500-by-95,000-kilometer (930-by-59,000-mile) Mars orbit. There it would trim its orbital plane so that the subsequent Mars departure maneuver could place it on course for Earth. 

Near Earth, the saucer-shaped sample-return capsule would separate from the Earth-return vehicle. It would enter Earth's atmosphere at up to 12.8 kilometers (8 miles) per second, slow to subsonic speed, deploy a parachute, and lower to a soft landing. The baseline sample-return mission would cost a total of $446 million. 

Martin Marietta's third design, the baseline combined Phobos/Deimos landing and Mars landing spacecraft, would comprise a minimally modified Viking Lander and a 26% Stretch Viking Orbiter. Total weight at Earth-orbit departure would come to 4150 kilograms (9150 pounds) in 1979. 

For this "Mars + Phobos/Deimos landing" mission, the Orbiter would fire its rocket to place itself and the Viking Lander into an elliptical equatorial capture orbit about Mars requiring 97 hours to complete, then would release the lander. De-orbiting from the capture orbit would impose restrictions on the Lander — it would be able to set down only within a latitude band extending 12° north and 12° south of Mars's equator and would need a beefed-up heat shield to withstand a greater Mars atmosphere entry velocity. 

The Orbiter would then maneuver to a 15-hour observation orbit, match orbits with either Phobos or Deimos, and land bearing 62 kilograms (137 pounds) of science instruments. The baseline combined mission would cost $441 million. 

Martin Marietta also considered "Mars + Phobos/Deimos observation orbit," "Mars + Phobos/Deimos rendezvous," and "Mars + Phobos/Deimos sample-return" missions. Its "Mars +" missions would, the company estimated, be more cost-effective than Phobos/Deimos missions without Mars landings. A separate Phobos/Deimos landing mission would, for example, cost 80% as much as a Mars landing mission, while a "Mars + Phobos/Deimos landing" mission would cost only 14% more than a Mars landing mission. 

Martin Marietta then looked at whether sufficient interest existed in the planetary science community to justify missions to the martian moons. The company found that there were "no active and forceful champions" of Phobos/Deimos exploration. It added, however, that it had 

repeatedly found easily excited curiosity and conjecturing among space scientists about the origin and nature of these tiny bodies. This undercurrent of scientific interest, which has been given impetus by the recent returns of Mariner 9, may be the forerunner of well defined and enthusiastically supported recommendations for exploring the moons of Mars. If this is the case, NASA's decision to conduct this study may prove to be a very timely one. 

Viking 1 left Earth atop a Titan III-E rocket with a Centaur upper stage on 20 August 1975. Viking 2 launched on 9 September 1975. The twin two-part spacecraft entered Mars orbit on 19 June 1976, and 7 August 1976, respectively. The Viking 1 Lander separated from its Orbiter and touched down successfully on 20 July 1976; Viking 2's Lander followed on 3 September 1976. 

While the Landers operated on the surface, the Orbiters imaged Mars and its satellites. On 15 October 1977, the Viking 2 Orbiter passed just 30 kilometers (19 miles) from Deimos (images at bottom of post). 

NASA supported studies of a Viking-derived Moon lander, a Viking 1979 Mars rover mission, and other Viking derivatives, but the U.S. opted not to fund new missions based on Viking technology. Much like the $25-billion Apollo Program, Project Viking — which had cost over $1 billion in 1975 dollars (about $5 billion in 2023 dollars) — ended with its potential barely exploited. 

This occurred because the Soviet Union did not keep its promise to explore the Solar System, because NASA's budget shrank to half its Apollo-era value, and because public interest slumped after the Viking search for life on Mars returned equivocal results. The U.S. would launch no new spacecraft toward Mars until 1992, two decades after Martin Marietta completed its study. 

Deimos as imaged by the Viking 2 Orbiter. Image credit: NASA.

Individual house-sized boulders and craters submerged in dust are visible in this Viking 2 Orbiter close-up of Deimos. Image credit: NASA.

Sources

A Study of System Requirements for Phobos/Deimos Mission, Final Report, Volume I, Summary, Martin Marietta Corporation, June 1972.

On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, Edward Clinton Ezell and Linda Neumann Ezell, NASA, 1984, pp. 83-153.

More Information

The First Voyager (1967)

The Russians are Roving! The Russians are Roving! A 1970 JPL Plan for a 1979 Mars Rover

Bridging the 1970s: Lunar Viking (1970)

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)

Nomad Explorer (1992)

The Nomad Explorer Very Long Traverse Vehicle (VLTV) with its "power cart" in the background (right). A cable links the cart's nuclear reactor to the VLTV. Please note the "periscope" viewing window (below Earth at upper left), robotic arm (one of a pair), and EVA Bell (lower center). Image credit: Madhu Thangavelu

North America and Europe combined have fewer square kilometers of surface area than the Moon: 36.8 million for the two continents versus 37.8 million for Earth's natural satellite. In August-September 1992, at the 43rd Congress of the International Astronautical Federation (IAF) in Washington, DC, Madhu Thangavelu, a research associate at the University of Southern California's Institute of Aerospace Systems Architecture and Technology, argued that explorers operating from a fixed surface base — the traditional advanced lunar exploration scenario — could hope to survey only a small fraction of the lunar surface. Moreover, only after several costly piloted lunar landing missions had investigated candidate sites could a single fixed base site be selected.

At the time Thangavelu presented his paper, the Space Exploration Initiative (SEI), launched by President George H. W. Bush on the 20th anniversary of the first piloted Moon landing (20 July 1989), was nearing its end. Though at SEI's start NASA had proposed a traditional fixed-site permanent lunar base concept, by the time of the 1992 IAF meeting it had shifted its attention to a temporary lunar outpost concept called First Lunar Outpost (FLO). NASA made the change based on recommendations in the May 1991 report of the SEI Synthesis Group (the Stafford Committee).

Thangavelu did not mention FLO in his paper, though he might have noted that it had many of the limitations of the fixed-site base scenario. In its most basic form, FLO would see a series of 45-day piloted lunar missions, each employing one Habitat Lander and one Crew Lander. FLO astronauts would have at their disposal roving vehicles not too different from the jeep-like Apollo rovers. These would permit traverses of at most a few tens of kilometers from their temporary lunar outpost.

Thangavelu suggested that NASA replace the fixed-site lunar base approach with a "roving base" that would, in a single ambitious piloted mission, explore multiple candidate base sites and the terrain between them along an 11,000-kilometer traverse route. He called his roving base Nomad Explorer. 

The chief element of the Nomad Explorer roving base was the 35-tonne Very Long Traverse Vehicle (VLTV), which would measure 16 meters long, 4.5 meters wide, and 10 meters high. The VLTV would roll on four large wheels, each powered independently by a 120-horsepower electric motor. The complex wheels would change shape automatically to accommodate obstacles and ensure a smooth ride. Typically, the VLTV would move at about 20 kilometers per hour, though it could trundle along at up to 30 kilometers per hour if necessary.

The VLTV would provide its three-person crew with 600 cubic meters of pressurized volume. It would include a control cockpit, crew quarters, a meeting room/galley, an airlock, and a hygiene facility. 

Life support water tanks and stacked bags containing lunar dirt on the vehicle's roof would partially shield against solar flare and galactic cosmic radiation. A periscope-like assemblage of mirrors and baffles would provide the driver with an elevated view of the surface while blocking some radiation; along with movable cameras and floodlights, it would augment a more conventional  "windshield" with sloping tinted windows directed toward the surface.

Thangavelu proposed a novel system for providing the Nomad Explorer roving base with electricity — an automated "power cart" bearing a nuclear reactor that would follow about a kilometer behind the VLTV to limit crew radiation exposure. It would supply 50 kilowatts of electricity to the piloted rover either through a long durable cable or through intermittent microwave beaming. An auxiliary fuel cell/solar cell system on the VLTV would provide 10 kilowatts of backup electricity.

The most novel feature of Thangavelu's Nomad Explorer design was, however, the EVA Bell, an accordion-like structure that would extend down from the VLTV's underside. Thangavelu intended that the 48-cubic-meter EVA Bell should eliminate what he considered to be the worst feature of moonwalks: the need for bulky space suits. Space suits, he explained, decreased astronaut mobility and dexterity, caused fatigue, and required excessive time for donning. The EVA Bell would also protect the astronauts from abrasive lunar dust.

In addition to the EVA Bell, the VLTV would include two robot arms that could stand in for or assist space-suited astronauts. These would ride on tracks on the VLTV's exterior, enabling them to reach out from the rover in any direction.

The Nomad Explorer roving base would, of course, require a supporting space transportation infrastructure. Thangavelu envisioned a revived Saturn V rocket which he called the "Saturn V-B." This would launch Autonomous Modular Common Landers (AMCLs) configured for either automated or piloted operation. Though he did not mention it, NASA's proposed FLO launch vehicle, informally dubbed the "Saturn VI," might have stood in for the Saturn V-B with modest uprating or if used in an Earth-Orbit Rendezvous architecture. Uprated, modified FLO Crew and Habitat Landers might have replaced the AMCLs.

An automated AMCL would land the Nomad Explorer roving base at the start of its planned traverse route. Others would land supplies and experiment payloads no more than 3000 kilometers apart along the route. A one-way piloted AMCL would deposit the VLTV crew near the roving base at the starting point of the long traverse, and an automated AMCL bearing a crew Earth-return vehicle would land at the end of the traverse route.

The astronauts would then begin their six-month journey across the Moon's rolling, dusty terrain. Upon reaching the first resupply AMCL, they would use the VLTV's robot arms to transfer supplies it carried to a special port on the VLTV, then would put the EVA Bell into action. First, they would use the VLTV's robot arms to spread a "mat" on the lunar surface. The crew would then use the arms to transfer a site-specific scientific payload from the AMCL to the center of the mat.

Next, the astronauts would position the VLTV so that it straddled the payload. They would extend the EVA Bell, which would lock onto the mat, forming an air-tight seal. The astronauts would fill the EVA Bell with air, then would climb down into it to deploy the payload. After they completed their tasks, they would exit the EVA Bell, pump out its air, and raise it off the mat, exposing the payload to lunar surface conditions.

In addition to scientific instruments, the astronauts would deploy a telecommunications network for future operations as they moved over the lunar surface. Upon reaching the end of their traverse, they would place the Nomad Explorer roving base in "hibernation." They would then board the pre-landed AMCL Earth-return vehicle and blast off for home.

Source

"The Nomad Explorer Assembly Assist Vehicle: An Architecture for Rapid Global Lunar Infrastructure Establishment," IAF-92-0743, Madhu Thangavelu; paper presented at the 43rd Congress of the International Astronautical Federation, 28 August-5 September 1992, Washington, DC.

More Information

Rocket Belts and Rocket Chairs: Lunar Flying Units

Robot Rendezvous at Hadley Rille (1968)

SEI Swan Song: International Lunar Resources Exploration Concept (1993)

A Long Way Home: Abort from Piloted Mars and Venus Missions (1970)

Cislunar abort: the crew of Apollo 13 on the deck of the aircraft carrier Iwo Jima after their safe return to Earth. Pictured are Lunar Module Pilot Fred Haise (left), Commander Jim Lovell, and Command Module Pilot Jack Swigert. Image credit: NASA.

On 13 April 1970, an oxygen tank exploded in the Apollo 13 Command and Service Module Odyssey, badly damaging the spacecraft 200,000 miles from Earth. NASA had no choice but to scrub the planned third Apollo lunar landing and return the Apollo 13 crew to Earth as quickly as possible. 

Astronauts Jim Lovell, Fred Haise, and Jack Swigert used the Lunar Module Aquarius as a backup propulsion system and lifeboat, swung around the Moon, and splashed down safely in Odyssey's Command Module on 17 April, about three and a half days after the explosion.

Amidst the drama of Apollo 13's mission abort, A. A. VanderVeen, a mathematician with NASA planning contractor Bellcomm, drafted a memorandum. In it, he pointed out that the time needed to return to Earth following a malfunction during the outbound leg of a Mars or Venus mission would nearly always be measured in months.

VanderVeen analyzed aborts for a piloted Mars orbiter ("capture") mission launched in 1981. An abort would begin with a rocket burn to slow the Mars spacecraft in its orbit around the Sun and cause it to fall back toward Earth. Because of the great cost of launching propellants off the Earth, he assumed that the spacecraft would carry no propellants dedicated entirely to an abort; it would perform its abort burn using only the propellants which, in a normal mission, would slow the spacecraft by 12,000 feet per second (3658 meters per second) at the end of its Earth-Mars transfer so that the gravity of Mars could capture it into orbit (that is, its Mars orbit insertion propellant).

A 270-day flight to Mars could be divided into three abort phases, VanderVeen found. Phase 1 would span from late in mission day 1 through mission day 60. During this period, the spacecraft would move slowly out from the Sun but remain relatively close to the Earth. A 12,000 feet per second (3658 meters per second) abort burn on mission day 60 would return the crew to Earth in 80 to 110 days.

Phase 2 would span mission days 60 through 180. Earth would pull ahead of the spacecraft during this period. Following the abort burn, the spacecraft would have to dip inside the orbit of Venus to gain speed and complete nearly one full orbit of the Sun in order to catch up with Earth from behind. An abort burn on mission day 180 would return the crew to Earth in 290 days, VanderVeen calculated.

Phase 3 would span mission days 180 through 270. The spacecraft would be playing catch up with Mars during this period. Aborts in Phase 3 would result in Earth returns longer than those in Phase 1 but shorter than those in Phase 2.

Increasing the quantity of abort propellant available would not meaningfully decrease Earth-return time, VanderVeen found. An abort on mission day 100 using only the spacecraft's Mars orbit insertion propellant would yield a 260-day Earth return. Adding enough propellant to change the spacecraft's speed by an additional 1000 feet per second (305 meters per second) would shave only 10 days off that return time. Providing enough propellant to change its speed by 24,000 feet per second (7315 meters per second) — that is, twice the velocity change needed for Mars orbit insertion in a normal mission — would yield a disappointingly lengthy 140-day Earth return.

Abort timing would also affect the speed at which the returning spacecraft would reach Earth. VanderVeen assumed that the Mars crew would reenter Earth's atmosphere directly in an Apollo-derived Earth entry module. The Apollo reentry system was designed to withstand reentry at up to 40,000 feet per second (12,192 meters per second). This would suffice for aborts initiated during the first 130 days of flight. 

For aborts initiated between 130 and 200 days after Earth departure, however, an improved Apollo heat shield or braking rockets designed to slow reentry would be required. Aborts during the remainder of the Earth-Mars transfer would demand "advanced [reentry] systems or a high retro-fire maneuver" requiring much additional propellant.

VanderVeen concluded that, all things considered, successfully aborting a Mars mission after departure from Earth orbit would be extremely challenging. His memorandum's chief recommendation reflected the essential hopelessness of an abort in interplanetary space: he wrote that "all major [spacecraft] systems should be thoroughly checked out very early in the mission while a short return abort opportunity exists."

Source

"Abort from Mars and Venus Missions - Case 103-8," A. A. VanderVeen, Bellcomm, Inc., 15 April 1970.

More Information

North American Aviation's 1965 Plan to Rescue Apollo Astronauts Stranded in Lunar Orbit

A CSM-Only Back-Up Plan for the Apollo 13 Mission to the Moon (1970)

What if a Crew Became Stranded on Board the Skylab Space Station? (1972)