Touring Titan By Blimp & Buoy (1983)

Image credit: NASA.

The planet Saturn needs a little more than 29 years to circle the Sun once. At its mean orbital distance, 1.43 billion kilometers from our star's warming fires, it receives about 1% as much solar energy as does Earth. The planet was known to ancient peoples the world over, but its most distinctive feature – its bright and complex ring system – remained undiscovered until after the invention of the telescope.

Galileo Galilei, famous for his telescopic discovery of Jupiter's four largest moons, spotted Saturn's rings in 1609-1610. Though perhaps the most advanced in the world at the time, his telescope was too crude to enable him to determine their nature.

A half-century later, Christian Huygens announced that the "appendages" that had defied Galileo's analysis were in fact a ring that encircled the planet without touching it. Huygens also discovered Titan, Saturn's largest moon, and determined that it circles the ringed planet in about 16 days.

Little new was learned of Titan until 1944. In that year, planetary astronomer Gerard Kuiper discovered that it has an atmosphere containing methane.

Data from the Voyager 1 spacecraft, which flew past Titan at a distance of about 4000 kilometers on 12 November 1980, showed that 98% of its atmosphere is nitrogen, and that its surface atmospheric pressure is roughly half again as great as Earth's at sea-level. Titan's surface temperature averages about 94 Kelvin (-179° Celsius, -290° Fahrenheit) and the low-density moon's surface gravitational pull is just 14% of Earth’s. The surface of the 5150-kilometer-diameter satellite remained mysterious; it lay hidden beneath a high-altitude haze layer and dense orange clouds.

Titan as observed by Voyager 1, November 1980. The haze layer above the dense orange cloud deck is just visible, as is the ephemeral polar hood. Image credit: NASA.

In 1983, the NASA Advisory Council's Solar System Exploration Committee (SSEC) released the first part of its report Planetary Exploration Through the Year 2000. The SSEC, chartered in 1980 by NASA Administrator Robert Frosch at the recommendation of NASA Associate Administrator for Space Science Thomas Mutch, aimed to develop missions to carry out the scientific strategy put forward by the National Academy of Sciences Committee on Planetary and Lunar Exploration (COMPLEX).

The SSEC report described a "core program" of planetary missions for the remainder of the 20th century. The four "initial" missions of the core program were a Venus Radar Mapper, the Comet Rendezvous/Asteroid Flyby (CRAF) mission, a Mars Geoscience/Climatology Orbiter, and — reflecting the many questions the Voyager 1 flyby had raised — a Titan Probe/Radar Mapper.

The last of these would see a Saturn flyby or orbiter spacecraft drop a short-lived instrument capsule into Titan's dense atmosphere and probe the hidden surface using an imaging radar. The SSEC hoped that the Titan Probe/Radar Mapper would leave Earth between 1988 and 1992 and return data from Saturn and Titan between 1995 and 1997.

Even as the SSEC published its core program, it commenced work on a new report outlining an "augmented program" of planetary exploration; that is, a collection of candidate missions that might follow and expand upon its core program. As part of its new study, it convened a workshop in Snowmass, Colorado, in the summer of 1983. On 2 August 1983, Science Applications Incorporated (SAI) briefed workshop participants on a six-month study of advanced Titan missions it had completed a month earlier for NASA's Solar System Exploration Division.

SAI's presentation began with an overview of the scientific rationale underlying its mission proposals. The study team told the SSEC workshop that "the most important characteristic of Titan is the chemical evolution that has occurred and is still occurring in its atmosphere." For example, carbon monoxide and hydrogen cyanide found by Voyager 1 in trace amounts in Titan's atmosphere had the potential to evolve into nucleotide bases and amino acids, critical building blocks of terrestrial life.

Scientists suspected that Titan's atmospheric chemistry offered clues to the nature of its surface, though they split over what those clues meant. Some believed that Titan was awash in an ocean — or at least large lakes — of liquid ethane or methane. In that model, ethane or methane behaved on Titan much as water behaves on Earth.

Others believed that organic goop from the orange clouds drizzled down and accumulated to a depth of several kilometers on its solid ice surface. In places, perhaps, exotic ice volcanoes poked through the goop layer and belched methane into Titan's dense atmosphere, providing raw material for more chemical evolution.

SAI proposed eight spacecraft systems for its Titan missions. These were: the non-imaging Titan orbiter; the imaging Titan orbiter; the Titan flyby bus; the combined haze probe/penetrator probe; the sounding rocket; and the large and small buoyant stations. The orbiter and flyby bus would operate outside of Titan's atmosphere. The other systems would operate within it.

Whether imaging or non-imaging, an orbiter would be an essential element of all SAI's proposed Titan mission concepts. In addition to collecting valuable scientific data, it would provide the crucial radio-relay link between the Titan atmosphere/surface systems and mission controllers and scientists on Earth.

Based on the proposed Saturn orbiter/Titan probe spacecraft design, the orbiter would circle Titan in a 1000-kilometer-high circular polar orbit requiring 3.93 hours to complete. This would enable it to link a system floating in Titan's atmosphere near its equator with controllers and scientists on Earth about half the time. The orbiter might reduce its required propellant load by employing aerocapture; that is, by skimming through Titan's upper atmosphere to slow down so that the cloudy moon's gravity could capture it into orbit.

Of SAI's eight Titan exploration systems, only the flyby bus would carry no scientific instruments.  The flyby bus, which would be based on Galileo Jupiter orbiter and Pioneer Venus hardware, would leave Earth about a year after the Titan orbiter. Its mission would end as it flew past Titan and released a cluster of atmosphere and surface probes.

The simplest system in SAI's Titan exploration arsenal was the combined haze/penetrator probe, the design of which was based on a proposed Mars penetrator. A solid-propellant rocket motor would blast the haze/penetrator probe from a launch tube on the orbiter and slow it so that it would fall into Titan's atmosphere. An umbrella-like fabric decelerator would then deploy, slowing the probe to a speed of Mach 1 by the time it fell to within 265 kilometers of Titan's surface. It would then begin to collect data on the hazy uppermost atmosphere.

The penetrator would then separate and descend to a hard landing (or a splashdown) on Titan's surface. The haze probe, meanwhile, would descend for 23 minutes to an altitude of 100 kilometers, at which point the orbiter would pass below its horizon. This would break the radio link with Earth and end the haze probe's mission.

The penetrator would be more long-lived; it would collect and store Titan surface data for transmission to the orbiter when it rose above the horizon again. If Titan's surface were confirmed to be covered by an exotic ocean before the orbiter left Earth, then the penetrator might be fitted out as a floating sonar buoy.

This image from the Huygens probe shows Titan's misty, icy surface from a height of five kilometers. Image credit: ESA/NASA.

SAI's most novel and picturesque Titan exploration systems were its large and small buoyant stations. The small stations, instrument-laden gondolas suspended from balloons, would be delivered into Titan's atmosphere by the flyby bus packed into 1.25-meter-diameter aeroshells based on the Galileo Jupiter atmosphere probe design. The large stations, packed into aeroshells twice as large, would take the form of either larger balloons or powered blimps. The small buoyant stations would operate between 100 and 10 kilometers above Titan, while large buoyant stations would operate between 10 kilometers above Titan and Titan's surface.

SAI provided few details about its proposed sounding rocket, which it envisioned would explore the same level of Titan's atmosphere as the haze probe. During descent, at an altitude of about 100 kilometers, the solid-propellant rocket would detach from the a buoyant station, ignite its motor, and ascend into the haze layer.

The company looked at several methods for launching its Titan missions from Earth. These included an advanced Nuclear-Electric Propulsion (NEP) system, though most relied instead on one or more Centaur G' chemical rocket stages.

In keeping with U.S. space policy in 1983, all the Earth-departure methods assumed that the Titan mission spacecraft would reach Earth orbit packed into the payload bays of Space Shuttle Orbiters. Reliance on the Shuttle imposed severe penalties on the Titan missions, SAI found. These included minimal science payloads and trip times of up to eight years with multiple Venus, Earth, and Jupiter gravity-assist flybys.

SAI sought to circumvent these penalties by assuming that NASA would become capable of On-Orbit Assembly (OOA) and in-space liquid oxygen/liquid hydrogen refueling by the time the Titan missions were ready to depart Earth. These operations might take place at an Earth-orbiting space station, SAI suggested.

SAI then described five Titan exploration mission concepts which combined its eight systems in what it called "mix 'n match" fashion. Concept #1, a minimal mission, included only a Titan orbiter with a limited Titan atmosphere probe complement. The company explained that the 1978 Pioneer Venus mission — which included separately launched Orbiter and Multiprobe spacecraft — had inspired Concepts #2, #3, and #4, all of which included a Titan orbiter and a separate flyby bus. Concept #5 relied on NEP in place of chemical-propellant rocket stages.

The company described in some detail its Concept #4 mission; with 28 experiments, it was SAI's most ambitious in terms of science return. A Centaur G' stage loaded with propellants in Earth orbit coupled with a Star-48 solid-propellant rocket motor would boost Concept #4's 1885-kilogram imaging orbiter toward Saturn in July 1999, and a pair of Centaur G' stages filled in Earth orbit with liquid oxygen and liquid hydrogen would launch its 2730-kilogram flyby bus a year later. SAI calculated that these stage configurations combined with Titan aerocapture for the orbiter would permit direct Earth-to-Saturn flights with no planetary gravity-assists.

In January 2004, after a flight time of 4.5 years, the imaging orbiter would aerocapture into Titan orbit. Over the next eight months, it would deploy three haze probes without penetrators and bring to bear on Titan's mysteries an impressive array of cloud-penetrating sensors.

In September 2004, after a 4.2-year flight, the flyby bus would speed past Titan and dispense one large buoyant station (a blimp) and three small buoyant stations (probably spherical balloons). The buoyant stations would enter Titan's atmosphere, decelerate, and deploy their gas envelopes as they slowly fell on parachutes. Kept aloft by heat from radioisotope thermal generators, they would each operate for at least two months. The large buoyant station might fly close enough to Titan's surface to lower an instrument package on a tether, permitting the first direct sampling of Titan's surface materials.

SAI placed the cost of its Concept #4 mission at $1.586 billion in 1984 dollars. This included a 30% contingency fund, but did not include launch costs. Adding in the cost of 2.5 $100-million Shuttle launches, three $45-million Centaur G' stages, one $5-million Star 48 motor, and OOA (the cost of which SAI optimistically placed at $10 million per Titan-bound spacecraft) yielded a total mission cost of $1.99 billion.

Artist's concept from 1988 of the Mariner Mark II Cassini Saturn orbiter releasing the Huygens probe above Titan's orange clouds. Image credit: NASA.

In its 1986 final report, the SSEC ranked SAI's advanced Titan mission proposals below Mars sample return and comet nucleus sample return on its list of desirable augmentation missions. Meanwhile, the 1983 core program's Titan probe/Radar Mapper mission shifted emphasis to take in the entire Saturn system. This helped to move it closer to reality.

Reflecting this new broader focus, the Saturn orbiter/Titan probe mission was named for Giovanni Cassini, discoverer of Saturn's "second-tier" moons Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini detected the broadest division in Saturn's rings, which is also named for him.

NASA and the European Space Agency (ESA) jointly studied Cassini, and ESA agreed to build the Titan probe, which was named Huygens. The U.S. Congress approved new-start funding for Cassini in 1989.

Initially Cassini was meant to be one of the first Mariner Mark II spacecraft, along with the Comet Rendezvous/Asteroid Flyby (CRAF) spacecraft. Mariner Mark II was intended to be a standardized (and thus inexpensive) spacecraft bus for advanced interplanetary missions. Congress scrapped CRAF in 1992 after it went over budget and diverted its remaining funds to Cassini, marking the end of the Mariner Mark II cost-cutting experiment.

Image credit: NASA/JPL.

Following the January 1986 Challenger Shuttle disaster, NASA cancelled Centaur G' and moved planetary spacecraft off the Shuttle launch manifest. The bus-sized Cassini spacecraft left Earth on a Titan IVB/Centaur expendable rocket in October 1997 and, after gravity-assist flybys of Venus, Earth, and Jupiter, arrived in Saturn orbit in July 2004.

The Huygens probe entered Titan's dense atmosphere in January 2005 and floated on a parachute to a rough landing. Its six instruments included an imaging system, which revealed an enigmatic surface covered with rounded water ice "pebbles."

The following year, scientists using Cassini's radar discovered ethane lakes large and small in Titan's north polar region. By early 2008, several lines of evidence pointed to a global water ocean perhaps 100 kilometers beneath the water-ice crust of Titan.

Radar swaths from the Cassini Saturn orbiter reveal Titan's north polar "land of lakes" in this false-color image. The largest, Kraken Mare, is roughly the size of Earth's Persian Gulf. Image credit: NASA.

In late 2005, scientists using Cassini's imaging system found evidence that another world orbiting Saturn besides Titan has biological potential: bright white Enceladus, which William Herschel discovered in 1789. They detected numerous geysers near the 500-kilometer-diameter moon's south pole. Driven by tidal flexing and possibly other processes that generate heat, these shoot water laced with salt, silica particles, and organic chemicals into space.

After Cassini flew past Enceladus 20 times at a distance of less than 5000 kilometers — eight of those flybys were within 100 kilometers — scientists in September 2015 announced that a global ocean up to 31 kilometers deep underlies its icy surface. During its last close Enceladus flyby on 28 October 2015, Cassini will fly past at a distance of 49 kilometers. Cassini's 22nd and last planned Enceladus flyby is scheduled for 19 December 2015 at a distance of 4999 kilometers.

In May 2008, Cassini completed its primary mission and began its first extended mission (the Equinox Mission). In February 2010, NASA agreed to extend Cassini's mission until September 2017 to enable it to observe Titan's north polar region at mid-summer. Assuming that the spacecraft survives to complete its new extended mission (the Solstice Mission), it will have carried out more than 125 Titan flybys since reaching Saturn orbit.

Sources

Titan Exploration with Advanced Systems: A Study of Future Mission Concepts, Report No. SAI-83/1151, Science Applications Incorporated; presentation to the SSEC Summer Study in Snowmass, Colorado, 2 August 1983.

Planetary Exploration Through Year 2000: A Core Program, Solar System Exploration Committee, NASA Advisory Council, 1983.

Planetary Exploration Through Year 2000: An Augmented Program, Solar System Exploration Committee, NASA Advisory Council, 1986.

More Information

The Challenge of the Planets, Part Two: High Energy

The Challenge of the Planets, Part Three: Gravity

The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)

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

Edwin Aldrin, Apollo 11 Lunar Module Pilot, stands by the first U.S. flag astronauts planted on the moon, 21 July 1969. Visible in the background is one of the landing legs of the Lunar Module (LM) Eagle, the LM's long, dark shadow, and the flat, cratered-pocked Sea of Tranquility. Image credit: NASA.

On 5 August and 13 August 1970, outgoing NASA Administrator Thomas Paine dispatched letters on the future of the U.S. lunar program to the Lunar and Planetary Missions Board (LPMB) and the Space Science Board (SSB) of the National Academy of Sciences National Research Council. In his letters, he outlined three options for curtailing Project Apollo. Of these options, the first (Option I) would cancel one Apollo mission, while the others would nix two. 

The options he described were in part aimed at avoiding a delay in the Skylab Program, which constituted an important step toward Paine's favorite mid-1970s NASA goal: a 12-man Earth-orbiting Space Station that would be staffed and resupplied using a fully reusable Space Shuttle. Paine also sought to generate alarm in the space science community, which he apparently believed would place pressure on President Richard Nixon. Members of the LPMB and the SSB held an urgent two-day meeting (15-16 August 1970) in Woods Hole, Massachusetts, to develop a response to Paine's letters. 

NASA Administrator Thomas Paine. Image credit: NASA.

By the time they met, NASA had flown three piloted lunar landing missions: Apollo 11 (16-24 July 1969), which landed off-target on Mare Tranquillitatis; Apollo 12 (14-24 November 1969), which landed close by the derelict Surveyor 3 automated lander on Oceanus Procellarum, thus demonstrating the pinpoint landing capability essential for lunar geologic traverse planning; and perilous Apollo 13 (11-17 April 1970), which suffered an oxygen tank explosion in its Command and Service Module (CSM) that scrubbed its planned landing at Fra Mauro. Of these, Apollo 11 and Apollo 12 were mainly engineering missions intended to prove the Apollo system, while Apollo 13 had been intended as the first truly science-focused mission.

Paine had already canceled one Apollo mission, Apollo 20, in January 1970, so that its Saturn V rocket could launch into low-Earth orbit Skylab A, a Saturn IB S-IVB stage converted into a temporary space station. That left six Moon landings before the program concluded with Apollo 19.

The program meant to extend piloted lunar exploration deep into the 1970s, the Apollo Applications Program (AAP), had taken repeated funding hits since 1966, and so had abandoned its lunar ambitions. It became the strictly Earth-orbital Skylab Program in February 1970. Some concepts proposed for AAP lunar missions — for example, three-day lunar-surface stays and a jeep-like roving vehicle — would find their way into Apollo before its end, but when Apollo ended, so would end piloted lunar exploration.

Space Science Board Chair Charles Townes. Image Credit: National Academy of Sciences.

With the goal of a man on the Moon by 1970 successfully attained, pressure had begun to build to cancel some or all of the remaining Apollo lunar missions. In the aftermath of the Apollo 13 accident, some policy-makers — and even managers within NASA — questioned the wisdom of continuing to place astronauts at risk. Apollo 11 had humbled the Soviets on the technological prestige front of the Cold War; future landings could do little to enhance prestige, they argued, but a single lost crew could erase much of what the U.S. had gained by being first on the Moon.

In addition, President Richard Nixon's Office of Management and Budget was eager to rein in Federal expenditures. By mid-1970, the United States was spending roughly the entire $25-billion cost of the Apollo Program every three months to wage war in Indochina. Public interest in U.S. spaceflight had faded rapidly after Apollo 11. Though NASA's budget had decreased to only about $3.7 billion in Fiscal Year 1970 — down from a little over $5 billion in 1966 — the agency still constituted a highly visible and thus highly vulnerable target for new cuts.

This had become evident during Fiscal Year 1971 budget deliberations. Despite Paine's strident protests, the Nixon White House had on 2 February 1970 submitted to Congress a NASA funding request of only $3.33 billion, of which $110 million was devoted to Station/Shuttle. The House of Representatives added $80 million to Station/Shuttle in committee.

An amendment debated on the House floor on 23 April 1970 then sought to cut Station/Shuttle entirely; the amendment's supporters argued that the program was a foot in the door for an expensive piloted Mars mission. The amendment failed (per House rules) in a tie vote — that is, by the narrowest possible margin.

The Senate trimmed Station/Shuttle funding back to $110 million in committee. Repeated amendments on the Senate floor sought to delete all Station/Shuttle funds. Though in the end Station/Shuttle kept its $110 million, NASA's budget suffered other cuts. In early July 1970, House and Senate conferees settled on a NASA budget of $3.27 billion for Fiscal Year 1971.

On 28 July 1970, Paine tendered his resignation effective 15 September 1970. Nixon accepted it with little comment. He denied that his decision to resign had anything to do with the Fiscal Year 1971 NASA budget. 

In their joint response to Paine, dated 24 August 1970, LPMB chair John Findlay and SSB chair (and Nobel Laureate) Charles Townes reminded Paine that past scientific advisory boards — including one Townes had chaired in January 1969, which prepared a report for President-elect Nixon — had advised that NASA should continue piloted lunar exploration throughout the 1970s, and that from 10 to 15 piloted Moon landings should be flown. They cited this when they refused to consider cutting more than one Apollo mission. The SSB had, incidentally, in its January 1969 report expressly opposed Paine's large Earth-orbiting station.

Apollo, they told the outgoing NASA Administrator, was of the greatest scientific importance. They explained that "the Apollo missions do not simply represent the study of a specific small planet but rather form the keystone for a near term understanding of planetary evolution." They then wrote that

[w]e respect the serious fiscal and programmatic constraints. . .However, it should be recognized that any reduction in the number of missions will seriously threaten the ability of the total Apollo program to answer first-order scientific questions. We are on the very beginning of a learning curve, and it is clear that the loss of one mission will have much greater than a proportional effect on the instrumented experiments and, more critically, on the design and execution of the geology experiments involving the astronauts.

Findlay and Townes explained that at Woods Hole the LPMB and SSB had jointly considered their own trio of options for Apollo's future, all of which were different from Paine's. Option I was to fly missions 14, 15, 16, and 17 about six months apart, fly missions to the Skylab A Orbital Workshop over a period of about 20 months, and then carry out Apollo missions 18 and 19 six months apart.

Missions 14 and 15 would be H-class walking missions similar to Apollo 12 and Apollo 13; the remaining Apollos would be J-class missions. The latter would include a Lunar Module (LM) capable of increased lunar surface stay time, a rover, improved lunar surface experiments, remote sensors on the CSM in lunar orbit, and a CSM-released lunar subsatellite.

The long gap between Apollo 17 and Apollo 18 would permit lunar scientists to digest data from the previous missions and to design new experiments for the final mission pair. Findlay and Townes noted, however, that the gap might also make Apollo 18 and Apollo 19 vulnerable to budget cuts. Paine's Option I had cut Apollo 15 and flown the remaining lunar missions before Skylab A.

The LPMB/SSB Option II was to cut Apollo 15, fly 14, 16, 17, 18, and 19 about six months apart, and then fly the Skylab A missions. Their Option III was to cut Apollo 15, fly 14, 16, 17, 18, and 19 five months apart, and then fly Skylab A. Paine's Options II and III had both omitted 15 and 19.

Regions of the Moon surveyed using instruments on board the Apollo 15, Apollo 16, and Apollo 17 Command and Service Modules in lunar orbit. Flying the J-class Apollo 18 and Apollo 19 missions would have nearly doubled surface coverage. Image credit: NASA.

As might be expected, the LPMB and SSB favored their Option I, which cut no missions. If, on the other hand, "retreat from Option I proves unavoidable," they recommended their Option III. This would, they explained, sacrifice Apollo 15 to save Apollo 19, which, they explained, would include 20% of the Apollo program's extravehicular time and cover 25% of the total area planned to be included in the Apollo traverses. In addition, by reducing the time between launches, they hoped to limit the costly delay in Skylab A's launch.

They conceded that most of the experiments planned for Apollo could be carried out even if both Apollo 15 and Apollo 19 were cut. However, an automated station in the passive seismic network would be lost, surface samples would not be obtained from two geologically significant locations, and several experiments would be flown only once, so would have no backup. They concluded by reiterating that the cuts Paine envisioned could prevent lunar scientists from answering first-order questions about the Moon, and added that "the consequences of such failure for the future of [NASA] and, we believe, for large-scale science in this country are incalculable."

In his reply to Townes and Findlay, dated 1 September 1970, Paine declared that he had selected his Option II as originally proposed (that is, elimination of both Apollo 15 and Apollo 19). He explained that Option I was not feasible because earlier budget cuts had forced a change from four-month to six-month gaps between Apollo Moon flights. This might be reduced to five months "at some added cost," he wrote.

Even with the gaps between flights reduced, however, a delay of seven or eight months in the launch of Skylab A would occur, "requiring a high, non-productive expenditure to retain the [Skylab] teams beyond the scheduled launch date." Paine did not address the LPMB/SSB suggestion that Apollo 18 and Apollo 19 fly after Skylab A.

Cutting Apollo 15 and Apollo 19, along with closing down Apollo operations in mid-1972 and terminating Saturn V after the Skylab A launch in late 1972 would, Paine explained, produce "substantial saving over the next four years." The immediate savings from cutting Apollo missions 15 and 19 would amount to only $40 million; if NASA opted to fly both, however, an additional $760 million would need to be spent by the time Apollo 19 returned to Earth.

Paine argued that his cuts placed NASA "in a better position to keep our total program costs down while still pressing forward with our future plans for scientific and application programs and an integrated, low cost space transportation system." Paine referred, of course, to the large Earth-orbiting Space Station and the reusable Space Shuttle he favored.

Paine invoked Apollo 13, then argued that selecting the minimum Apollo program option would enhance safety. Rather than arguing that fewer missions meant fewer chances for failure, as might be expected, he maintained that making cuts up front would preserve "momentum and morale," keeping the NASA/industry team focused and thus reducing risk to crews. He asserted that "rather than risk the integrity of the entire program by cutting out a mission at a time in response to budgetary constraints, we feel we must now take a stand on what constitutes the minimum viable program and then carry it out effectively."

The following day (2 September 1970), Paine held a press conference during which he announced his Apollo program cuts. It was one of his final public acts as NASA Administrator.

Apollo 14 (31 January-9 February 1971), the last H-class mission, landed at Fra Mauro. Alan Shepard and Ed Mitchell pushed the limits of walking astronauts by attempting to climb to the rim of Cone Crater, where geologists hoped that they could sample ancient rocks from deep inside the Moon.

Apollo 16, the first J-class flight, was renumbered Apollo 15 and launched on 26 July 1971. The Apollo 15 LM Falcon, bearing astronauts David Scott and James Irwin, landed at Hadley-Apennine, on the mountainous fringe of Mare Imbrium. They conducted three Lunar Roving Vehicle (LRV) traverses. Meanwhile, on board the CSM Endeavour in lunar orbit, Al Worden released a subsatellite and turned remote sensors and cameras toward the lunar surface. Apollo 15 splashed down in the Pacific Ocean on 7 August.

Apollo 16 (16-27 April 1972) saw John Young and Charlie Duke land at Descartes in the heavily cratered Lunar Highlands. As they deployed their LRV from the side of the LM Orion, Ken Mattingly ejected the panel covering sensors and cameras on board the orbiting CSM Casper.

Apollo 17 Commander Eugene Cernan salutes the sixth and last U.S. flag American astronauts planted on the Moon. Visible behind him are the third and final Lunar Rover to reach the Moon (directly behind Cernan), the Lunar Module Challenger (left), and mountains surrounding Apollo 17's complex Taurus-Littrow landing site. Image credit: NASA.

The last Apollo lunar mission, Apollo 17 (7-19 December 1972), touched down at Taurus-Littrow, on the edge of Mare Serenitatis, nearly six months after Paine's mid-1972 Apollo end date. Eugene Cernan and the only professional geologist to reach the Moon, Harrison Schmitt, used the LM Challenger as their surface exploration base while Ron Evans surveyed the Moon from the orbiting CSM America.

The last Saturn V rocket to fly launched Skylab on 14 May 1973, again about six months after Paine's planned date. Three crews docked with and worked aboard Skylab between May 1973 and February 1974. A second Skylab, Skylab B, was built, but was not launched even though a Saturn V for launching it and Saturn IB rockets, Apollo CSMs, and astronauts for staffing it were available. Skylab B would become an exhibit in the National Air and Space Museum.

Nixon opted to replace Apollo and Skylab with the partially reusable Space Shuttle (but no Space Station). He had in fact never supported Paine's Station/Shuttle plans. Nixon liked to be seen with astronauts, a trait which by and large defined the extent of his interest in NASA; partly because of this, U.S. space policy drifted and was often confused and contradictory during much of his time in office.

Nixon postponed announcement of his Space Shuttle decision until the Presidential election year of 1972. By then, he had nominated and had confirmed James Fletcher as NASA's fourth Administrator. Fletcher read Nixon's Shuttle announcement to reporters on 5 January 1972, in the place where Shuttle Orbiters would be built: California, a state critical to Nixon's reelection bid. The Space Shuttle, Nixon promised, would generate thousands of aerospace jobs.

NASA Administrator James Fletcher (left) and Nixon pose with a model of an early version of the semi-reusable Space Shuttle stack, January 1972. Image credit: NASA.

Sources

Letter, Charles Townes, Chairman, National Academy of Sciences Space Science Board, and John Findlay, Chairman, Lunar and Planetary Missions Board, to Thomas Paine, NASA Administrator, 24 August 1970.

Letter, Thomas Paine, NASA Administrator, to John Findlay, Chairman, Lunar and Planetary Missions Board, 1 September 1970.

Chapter 3, The Space Shuttle Decision, NASA SP-4221, Thomas A. Heppenheimer, NASA, 1999.

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

McDonnell Douglas Phase B Space Station (1970)

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

Skylab-Salyut Space Laboratory (1972)

The 1991 Plan to Turn Space Shuttle Columbia Into a Low-Cost Space Station

This NASA artwork from 1972 portrays the sheer volume of the Skylab Orbital Workshop, the first U.S. space station.

The first U.S. space station was Skylab, which NASA carefully dubbed an "Orbital Workshop" in order to distinguish it from the "real" space station it hoped to launch into low-Earth orbit by the mid-1970s. Skylab — a converted Saturn S-IVB rocket stage with a pressurized volume of more than 12,500 cubic feet — was launched on the last Saturn V heavy-lift rocket to fly. Three three-man crews lived and worked on board the 22-foot-diameter single-launch station for a total of 171 days between 26 May 1973 and 8 February 1974.

Nearly three years earlier, budget cuts had halted Saturn V production, so NASA had been forced to abandon plans for a single-launch, 33-foot-diameter core station. The Space Shuttle, originally intended as a cost-saving fully reusable space station crew and cargo transport, was subsequently tapped to serve also as the sole launch vehicle for a multi-modular space station built up over the course of many flights. This meant that the Shuttle Orbiter payload bay dimensions (15 feet in diameter by 60 feet long) and maximum payload mass (in theory, up to 32.5 tons) would dictate the size and mass of station modules and other components.

NASA's single-launch core station (left) would throughout its life receive independently maneuverable add-on modules delivered by fully reusable Shuttle Orbiters. This 1970 illustration depicts one such module outfitted to transport astronauts and cargo from the Shuttle payload bay to the core station main docking port and back. The modules would also be outfitted as special-purpose labs that would link up with round ports scattered over the station's hull. Image credit: McDonnell Douglas.

Launching the space station in the Shuttle payload bay meant also that NASA could not begin to assemble it until after Space Shuttle development and flight testing were completed. When the last crew left Skylab, the Shuttle's orbital maiden flight was officially set for early 1978. Operational flights were to start by 1980. Some hoped that an early Shuttle flight might boost Skylab to a higher orbit, postponing its eventual reentry and perhaps permitting it to be outfitted as a temporary interim space station in the early 1980s.

In the event, the first mission of the partially reusable Shuttle, STS-1, did not lift off until 12 April 1981, nearly two years after Skylab reentered and broke up over Australia (11 July 1979). The Orbiter Columbia remained aloft for two days before gliding to a landing on the dry lake-bed at Edwards Air Force Base (EAFB), California.

By then, engineers at NASA's Johnson Space Center had been at work for more than two years on a design for a Shuttle-launched station they dubbed the Space Operations Center (SOC). The SOC included a laboratory for experiments in microgravity, but was conceived mainly as a construction site for large structures, a servicing center for satellites, and a home port for a small fleet of space tugs. It was intended, in fact, to serve as a space shipyard, where would be assembled spacecraft for voyages beyond low-Earth orbit and large space structures such as Solar Power Satellites.

The 1982 Space Operations Center design became the point of departure for NASA station studies after the creation of the Space Station Task Force. Visible is a skeletal "false Shuttle payload bay" for satellite servicing and a hexagonal space tug hangar. Image credit: NASA.

On 20 May 1982, a little more than a year after STS-1 and a little more than a month before STS-4 (27 June-4 July 1982), NASA Administrator James Beggs established the NASA-wide Space Station Task Force. President Ronald Reagan was on hand at EAFB Runway 22 that U.S. Independence Day to welcome home the STS-4 crew. Some within NASA hoped that he would use the occasion to declare his support for a permanent Earth-orbiting space station, "the next logical step" after the Shuttle. Instead, Reagan declared that STS-4 was the final Shuttle test flight. With its next flight, STS-5, the Space Shuttle would be considered operational.

Reagan withheld his support for a further 18 months, until the beginning of the 1984 election year, when endorsing a space station — which was bound to create thousands of jobs — could provide maximum political advantage. During his 25 January 1984 State of the Union Address, he echoed President John F. Kennedy's May 1961 "Urgent National Needs" speech by calling on the U.S. civilian space agency "to develop a permanently manned space station and to do it within a decade." Reagan made mention only of the station's role as a laboratory. The station would, he said, "permit quantum leaps in our research in science, communications and in metals and life-saving medicines that can only be manufactured in space."

The Reagan White House disdained a space shipyard for two reasons. First, it was a relatively complicated design that could not be built for $8 billion spent over 10 years, the maximum price Administration budget watchdogs were willing to pay for a space station. The second reason was related to the first: a shipyard in space implied that things would be built there, and that in turn implied a commitment to new expenditures in the future.

Despite this clear message, NASA did not abandon its plans for a shipyard in orbit. In August 1984, the space agency released a "reference configuration" intended to guide aerospace companies bidding on Space Station Program contracts. Called the "Power Tower," it included a 400-foot-long single main truss where SOC-type space construction equipment might eventually be mounted. In NASA artwork depicting the station, featureless boxes stood in for unspecified large user payloads and hoped-for shipyard elements.

The August 1984 Power Tower station configuration was the Space Operations Center with trusses added. The small spacecraft with twin solar arrays at upper right is a self-propelled free-flyer bearing experiments likely to be interfered with by other station activities (for example, astronaut movement). Image credit: NASA.

NASA envisioned that spacewalking astronauts would bolt together the Power Tower truss in orbit piece by piece. During Shuttle mission STS-61B (26 November-3 December 1985), in fact, spacewalkers successfully tested two truss-assembly methods in the payload bay of the Orbiter Atlantis.

From the Power Tower evolved the "Dual Keel" in late 1985. In May 1986, NASA released its Space Station "Baseline Configuration," a Dual-Keel station measuring 503 feet wide and 361 feet tall. The new design included about twice as many truss elements as the Power Tower, providing ample room for both space-facing and Earth-facing user payloads and eventual addition of space construction facilities. Assembly in orbit was to begin in 1992 and to be completed by Reagan's 1994 deadline.

The Baseline Configuration was dead on arrival, however, because of the 28 January 1986 loss of the Shuttle Orbiter Challenger and its seven-member crew. By March 1986, NASA and its contractors had begun to scale back the station. At first it shrank but retained its Dual-Keel shape. After that, in the "revised baseline configuration" of 1987, it lost its keel trusses, becoming only a single truss with solar arrays at either end and laboratory and habitat modules at its center. NASA made sure, however, that the design included "hooks" and "scars" that would enable eventual expansion to the Dual-Keel design.

NASA's ambitious Dual-Keel Baseline Configuration of May 1986 was dead on arrival. Image credit: NASA.

President Reagan christened the Space Station Freedom in 1988; a gesture which for some rang hollow (they had hoped he might support a moon and Mars program that would give Freedom a long-term direction). The following year, with the Station expected to be over-budget, overweight, under-powered, and too demanding to build, NASA entirely abandoned the Dual Keel configuration. At the same time, planners proposed that NASA make plans to build an advanced "transportation node" space station in the early 21st century. This proposed separation of functions was an acknowledgment that the jolts and vibrations one could expect on board an orbital shipyard would wreak havoc with microgravity laboratory experiments.

The year 1990 saw new problems. Persistent hydrogen fuel leaks grounded the three-orbiter Shuttle fleet for nearly half the year, renewing doubts about the Shuttle's ability to reliably launch, assemble, resupply, and staff Freedom. Against this background, news emerged of a dispute within NASA over estimates of the number of spacewalks required to build and maintain the Space Station. The row triggered congressional hearings in May 1990.

In a report released on 20 July 1990, former astronaut and spacewalker William Fisher and JSC robotics engineer Charles Price, co-chairs of the Space Station Freedom External Maintenance Task Team, declared that Freedom would need four two-man spacewalks per week during its assembly and 6,000 hours of maintenance spacewalks per year after its completion. This amounted to 75% more spacewalks than the official NASA estimate, which was already considered excessive. Fisher called the spacewalk requirement "the greatest challenge facing the Space Station."

In November 1990, with new budget cuts in the offing, NASA began yet another Freedom redesign. At about the same time, Space Industries Incorporated (SII), a small engineering firm for which Maxime Faget, co-designer of the Mercury capsule, worked as Technical Advisor, began to examine a radical new approach to solving Freedom's persistent problems. SII performed its Orbiter-Derived Station (ODS) study on contract to Rockwell International, prime contractor for the Shuttle Orbiter.

SII noted that the U.S. House of Representatives Committee on Science, Space, and Technology wanted a "permanently manned Space Station, that meets our International Agreements, retains a capability for evolution, and has minimum annual and aggregate cost." At the same time, it explained, scientists and engineers of the space technology development and microgravity and life sciences research communities wanted NASA to provide an orbiting laboratory "without spending the entire available budget on the laboratory rather than on the experiments."

To satisfy these needs, SII proposed to draw upon Space Shuttle design heritage and operational experience. Specifically, the company proposed that NASA launch in 1996 an unmanned "stripped-down" Orbiter — one without wings, tail, landing gear, body flap, forward reaction control thrusters, and reentry thermal protection — to serve as Freedom's largest single element.

Orbiter Derived Station in Man Tended Configuration after Mission Build-1. Image credit: SII/NASA.

Removing systems with a total mass of 45,600 pounds would boost the Orbiter's payload capacity to 81,930 pounds, permitting it to transport a 56.5-foot-long pressurized module permanently mounted in its payload bay and four pairs of rolled-up 120-foot-long solar arrays under streamlined housings along its sides. The pressurized module would include a single docking port on top and a short tunnel linking it to the stripped-down Orbiter's two-deck crew compartment. In effect, SII's ODS launch approach would briefly restore the heavy-lift capability lost when the U.S. abandoned the Saturn V rocket.

What follows is a synthesis of information from two SII documents concerning the ODS. The first, a set of presentation slides, is not dated, though individual slides in the presentation carry July 1991 dates. The second document is SII's final report to Rockwell International dated September 1991. When the documents differ in significant ways, this is noted.

Copying NASA parlance, SII referred to the launch of the stripped-down Orbiter as Mission Build-1 (MB-1). Upon achieving a 220-nautical-mile-high orbit inclined 28.5° relative to Earth's equator, the ODS would turn its payload bay doors toward Earth, open them to expose the pressurized module and door-mounted radiators, and unroll its solar arrays to generate up to 120 kilowatts of electricity. At that point, the ODS would achieve Man-Tended Configuration (MTC). MTC meant that the station could be staffed while a Shuttle Orbiter was docked with it. According to SII, NASA's Freedom would not achieve MTC until MB-6, and its solar arrays would not generate 120 kilowatts of electrical power until MB-10.

Orbiter Derived Station (top) and Shuttle Orbital Maneuvering System propulsion pod design differences. Image credit: SII/NASA.

During a normal Space Shuttle mission, the twin 6,000-pound-thrust Orbital Maneuvering System (OMS) engines would ignite twice to complete orbital insertion after the Orbiter's three Space Shuttle Main Engines (SSMEs) shut down and and its External Tank separated. The OMS-1 burn would put the Orbiter into an elliptical orbit; then, at apogee (the high point of its orbit about the Earth), the OMS-2 burn would raise its perigee (the low point in its Earth orbit) to make its orbit circular. Subsequently, the OMS engines would be used to perform major maneuvers and would slow the Orbiter at the end of its mission so that it would reenter the atmosphere. The OMS engines would burn hypergolic (ignite-on-contact) hydrazine/nitric acid propellants.

SII proposed changes to the stripped-down Orbiter's OMS pods to increase reliability and enable long-duration use. A hydrazine monopropellant system would replace the baseline Orbiter bi-propellant system. The SSMEs would insert the stripped-down Orbiter directly into its initial elliptical orbit, then two sets of four 500-pound-thrust OMS engines — one set per OMS pod — would each draw on a pair of propellant tanks to perform the OMS-2 orbit-circularization burn at apogee. The roughly 13,000 pounds of propellant remaining after the OMS-2 burn would be sufficient to resist atmospheric drag and supply OMS pod attitude-control thrusters for two years.

SII suggested that the OMS tanks be refilled in orbit after they exhausted their initial load of hydrazine, but provided no details as to how this might be accomplished. Alternately, the company suggested, a new propulsion module might be docked with the ODS after the modified OMS pods ran out of propellant.

With MB-1 complete, SII's ODS would provide 11,000 cubic feet of pressurized volume containing 58 standardized payload racks. NASA’s Freedom, by comparison, would have no pressurized volume at all until the addition of the U.S. Lab during MB-6, and would not exceed 10,000 cubic feet of pressurized volume until MB-13. The U.S. Hab and Lab modules would together hold only 48 racks.

In SII's July 1991 ODS design, the large module launched in the stripped-down Orbiter payload bay on MB-1 included only Hab module functions, and MB-2 in 1997 would see a piloted Shuttle Orbiter deliver the U.S. Lab module. In its September 1991 final report, SII combined Lab and Hab in the stripped-down Orbiter payload bay and substituted a 47.5-foot-long "core module" for the Lab on MB-2. The cylindrical core would include eight docking ports on its sides and one at either end.

One of the core module end ports would be docked permanently with the port on the Hab/Lab module. Visiting Shuttle Orbiters would dock with the Earth-facing port at the core module's other end. Addition of the core module would increase ODS pressurized volume to 15,000 cubic feet. NASA's Freedom station would not exceed 15,000 cubic feet of volume until MB-16.

SII envisioned that ODS assembly flights would be interspersed with utilization flights beginning immediately after MB-1. The first ODS utilization mission would occur in 1996, and three would take place in 1997.

In addition to permitting early research on board the ODS, some utilization flights after MB-2 would deliver supplies and equipment in a drum-shaped Logistics/Life Support Module (LLSM). Astronauts would dock the LLSM to a core module side port using the visiting Orbiter's Canada-built Remote Manipulator System (RMS). Spent LLSMs would be returned to Earth for refurbishment and reuse. SII placed the ODS toilet and shower in the LLSM, arguing that servicing waste and water systems on the ground would be preferable to doing so in orbit.

SII noted that its Station would need very few assembly and maintenance spacewalks. It would, nevertheless, include a modified Shuttle Orbiter airlock attached to one of its core module side ports. The airlock would reach the ODS during an unspecified utilization flight after MB-2. Because ODS assembly would be relatively simple and assembly spacewalks minimal, SII assumed that the Station could do without its own Canada-built RMS. The company did not address how deletion of the Station RMS would affect U.S.-Canada relations.

The second assembly mission of 1997, MB-3, would see arrival of an Orbiter bearing in its payload bay an eight-man Assured Crew Return Vehicle (ACRV), a space station lifeboat. With the docking of the ACRV at a core module side port, the ODS could be staffed by eight astronauts with no Orbiter present. NASA called the ability to maintain a full crew with no Orbiter present "Permanent Manned Configuration" (PMC). NASA's Freedom Station would not achieve PMC until MB-16.

Orbiter Derived Station in Assembly Complete configuration after Mission Build-6 in late 1998. The image displays an RMS robot arm, though SII stated that the stripped-down Orbiter would not carry one. Image credit: SII/NASA.

The year 1998 would see three assembly flights, all international in character. In his January 1984 State of the Union speech, Reagan had invited U.S. allies to lend a hand in building NASA's space station in exchange for opportunities to reap its rewards. In addition to Canada, Japan and Europe had answered the call.

MB-4 would see an Orbiter deliver the pressurized part of the Japanese Experiment Module (JEM). Astronauts would use the Orbiter's RMS to dock it to a core module side port. During MB-5, astronauts would use the visiting Orbiter RMS to add the European Space Agency's Columbus laboratory module. With that, the SII's ODS would achieve its maximum pressurized volume: 24,000 cubic feet, or about 8,000 cubic feet more than planned for NASA's Freedom Station. MB-6 would add Logistics and unpressurized Exposure components to complete the JEM.

SII recommended that the core module's Earth-facing port be designed to rotate so that visiting Orbiters could optimally position themselves for assembly missions. During MB-5, for example, the visiting Orbiter's nose would face in the ODS's direction of flight so that its RMS could place the Columbus module at its designated core module side port. During MB-4 and MB-6, it would face in the opposite direction so that JEM components could be added.

MB-6, which would take place near the end of 1998, would mark the end of ODS assembly. By then, SII's station would have hosted seven utilization flights. For comparison, NASA's Freedom Space Station would host no utilization flights until 1998, when three would take place. Freedom would not reach "Assembly Complete" until 2000.

SII proposed ways that the baseline ODS might be upgraded. The company noted that, beginning with MB-10, NASA's Freedom would provide experimenters with more electricity (180 kilowatts) than would the ODS. If this power level were judged to be necessary for ODS operations, then a 60-kilowatt "power kit" could be added during a utilization flight. The company suggested that the kit's rolled solar arrays be attached to a special port installed in the stripped-down Orbiter's nose behind a streamlined faring.

The ODS included no provision for space-facing experiments; all of its modules were expected to be mounted on its Earth-facing payload bay side. This reflected the science and technology community's desire for a microgravity lab and the fact that highly capable automated space-facing satellites (for example, the Hubble Space Telescope) were available. If, however, space-facing experiments were desired on board the ODS, then it could be launched with a docking port on the Orbiter's space-facing belly. A tunnel through the ODS payload bay floor would link the port to the Hab/Lab module.

Probably the company's most controversial proposal was to accelerate ODS assembly by stripping down Columbia, NASA's oldest Orbiter. SII noted that Columbia was the heaviest Orbiter, so had the least payload capacity. It assumed that NASA would want to replace Columbia with a new, less heavy Orbiter, thus increasing the Shuttle fleet's overall lift capacity. SII called this "disposing of the worst and and replacing it with the best." Some components stripped from Columbia could, it suggested, be reused in the new Orbiter to save money.

By the time SII submitted its final report, NASA's latest Freedom configuration had been public for three months. The new design included truss segments launched pre-assembled, smaller U.S. modules, and other changes meant to reduce the number of spacewalks and assembly flights required to build and maintain it. The station would, however, lose yet more capability (notably in the area of electrical power, which was reduced to about 60 kilowatts at PMC). The April 1991 redesign set the stage for Freedom's near-cancellation in June 1993 (it survived by a single vote in the U.S. House of Representatives) and, beginning later that year, its revival as the International Space Station.

This rendition of Space Station Freedom in its 1991 configuration contains several interesting features. The overall station design is obscured by shadows, denoting the uncertainty surrounding the station's future form. Only the international pressurized modules — the JEM and Columbus labs — are visible. Beginning with the the May 1986 Dual Keel, these modules changed very little in NASA artwork because the International Partners insisted that the U.S. adhere to its agreements. The U.S. modules, in the meantime, decreased in number and shrank to a fraction of their planned former size. A Shuttle Orbiter is displayed, but not attached to Freedom; placing it too close to the station would show plainly the 1991 station's small size relative to earlier designs. The moon and Mars are visible above Freedom; in 1991, NASA still paid lip-service to carrying out President George H. W. Bush's abortive Space Exploration Initiative (SEI), which aimed to launch humans to those worlds. Freedom was meant to play a role in furthering SEI's goals, though the precise nature of that role was not clear. Image credit: NASA.

Sources

The Space Shuttle at Work, NASA SP-432/EP-156, H. Allaway, NASA, 1979, pp. 64-72.

Aboard the Space Shuttle, NASA EP-169, F. Steinberg, NASA, 1980.

Space Station, NASA EP-211, D. Anderton, NASA, no date (1984).

Space Station: The Next Logical Step, NASA EP-213, W. Froehlich, NASA, no date (1985).

Space Station: Leadership for the Future, NASA PAM-509, F. Martin & T. Finn, NASA, August 1987.

Space Station: A Step Into the Future, NASA PAM-510, A. Stofan, NASA, November 1987.

Space Station Freedom Reference Guide, Boeing, 1988.

Space Station Freedom: A Foothold on the Future, NASA NP-107, L. David, NASA, October 1988.

"Freedom Spacewalks 'unacceptable': NASA," Flight International, 1-7 August 1990, p. 18.

"Freedom failure threatens NASA's future," T. Furniss, Flight International, 29 May-4 June 1991, p. 34.

"Operation Scale-Down," T. Furniss, Flight International, 29 May-4 June 1991, pp. 76-78.

Shuttle Derived Space Station Freedom, Space Industries International, Inc./Rockwell International Space Systems Division, presentation materials, n.d. (July 1991).

Expanded Orbiter Missions Final Report: Orbiter Derived Space Station Freedom Concept, prepared by Space Industries, Inc. (SII), Webster, Texas, for Rockwell International, Inc., Downey, California, September 1991.

"House Retains Space Station in a Close Vote," C. Krauss, International New York Times, 24 June 1993 (http://www.nytimes.com/1993/06/24/us/house-retains-space-station-in-a-close-vote.html - accessed 16 October 2015).

International Space Station, Boeing, May 1994.

More Information

McDonnell Douglas Phase B Space Station (1970)

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

Skylab-Salyut Space Laboratory (1972)

Evolution vs. Revolution: The 1970s Battle for NASA's Future

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

Making Rocket Propellants from Martian Air (1978)

Water frost on Utopia Planitia as imaged by the Viking 2 lander. The horizon appears tilted because Viking 2 alighted with one of its three foot pads on a large rock. Image credit: NASA.

In the late 1970s, through the initiative of its director, Bruce Murray, the Jet Propulsion Laboratory (JPL) studied a range of possible Mars missions, including Mars Sample Return (MSR). Murray and others at the Pasadena, California-based lab were aware that funds for new Mars missions would be hard to come by; the U.S. economy was under strain and NASA, JPL's main customer, was devoting most of its resources to developing the Space Shuttle.

In addition, equivocal data from the astrobiology experiments on the twin Vikings, the first successful Mars landers, had been interpreted as negative, helping to damp public enthusiasm for the Red Planet. Would-be Mars explorers reasoned that, if an MSR mission would stand a chance of being accepted, then they would need to find technologies and techniques that could dramatically cut its anticipated cost.

In July-August 1978, two years after the Vikings landed and looked for life on Mars, three engineers at JPL — Robert Ash, a visiting faculty fellow from Old Dominion University in Virginia, and JPL staffers William Dowler and Giulio Varsi — reported on a small study they had conducted of one such cost-saving technology: specifically, making MSR Earth-return rocket propellants from martian resources. Using Earth-return propellants made on Mars would greatly reduce the mass of the MSR spacecraft at launch from Earth, permitting it to be launched on a small, relatively cheap launch vehicle.

Earlier researchers had proposed using Mars resources to make rocket propellants, but Ash, Dowler, and Varsi were the first to base their study on data collected by spacecraft on and in orbit of Mars. The Viking landers had confirmed that martian air is made up almost entirely of carbon dioxide, and had found that the planet's rusty red dirt contains an appreciable amount of water. The Viking 2 lander, at rest on the northern plain of Utopia Planitia, had imaged water frost on the surface in winter (image at top of post). In addition, the twin Viking orbiters had imaged water ice clouds high in the atmosphere and polygonal terrain resembling that found in near-polar permafrost regions on Earth.

Ash, Dowler, and Varsi examined three propellant combinations that would exploit resources the Vikings had found on Mars. The first, carbon monoxide fuel and oxygen oxidizer, could be produced by splitting ubiquitous martian atmospheric carbon dioxide. They rejected this combination, however; while easy to produce, it could yield only mediocre performance.

Hydrogen/oxygen, on the other hand, was a high-performance propellant combination with more than three times the propulsive energy of carbon monoxide/oxygen. It could be produced by collecting and electrolyzing (splitting) martian water, but Ash, Dowler, and Varsi rejected the combination because a heavy, electricity-hungry cooling system would be needed to keep the hydrogen in usable liquid form. This requirement would, they estimated, negate the mass-savings of making Earth-return propellants on Mars.

The third combination they examined was methane/oxygen, which could be produced on Mars using a process discovered in 1897 by Nobel Prize-winning chemist Paul Sabatier. Combining a small amount of hydrogen brought from Earth with martian atmospheric carbon dioxide in the presence of a nickel or ruthenium catalyst would yield methane and water.

The methane would be pumped to the MSR Earth-return rocket stage fuel tank and the water would be split using electricity to produce oxygen and hydrogen. The oxygen would be pumped to the MSR Earth-return oxidizer tank and the hydrogen would be reacted with more martian atmospheric carbon dioxide to produce more methane and water.

Ash, Dowler, and Varsi favored methane/oxygen because it would provide 80% of hydrogen/oxygen's propulsive energy, and because methane remains in liquid form at typical martian surface temperatures. They estimated that launching a one-kilogram Mars sample directly to Earth (that is, with no stop in Mars orbit to rendezvous with and transfer the sample to a Earth-fueled Earth Return Vehicle) would require manufacture of 3780 kilograms of methane/oxygen, and calculated that a Mars surface stay-time of at least 400 days would be necessary to allow sufficient time to manufacture adequate quantities of propellants.

The 1978 JPL study would inspire many other mission designers to tap resources the twin Vikings had confirmed exist on Mars. At the 1982 AIAA/AAS Astrodynamics conference, for example, Science Applications Incorporated engineers presented a paper on use of Mars resources to make propellants for automated rocket-propelled ballistic hoppers and propeller-driven airplanes. The Mars base scenario developed at the second The Case for Mars Conference (1984) relied heavily on extraction of resources from the martian atmosphere for both life-support consumables and rocket propellants.

Conceptual design of a large system for extracting propellants and life-support consumables from martian air. Image credit: C. Emmart/Boulder Center for Science and Policy.

"Feasibility of Rocket Propellant Production on Mars," R. L. Ash, W. L. Dowler, and G. Varsi, Acta Astronautica, Vol. 5, July-August 1978, pp. 705-724.

"In Situ Propellant Production: The Key to Global Surface Exploration of Mars?" AIAA-82-1477, S. Hoffman, J. Niehoff, M. Stancati; paper presented at the AIAA/AAS Astrodynamics Conference in San Diego, California, 9-11 August 1982.

The Case for Mars: Concept Development of a Mars Research Station, JPL Publication 86-28, NASA Jet Propulsion Laboratory, 15 April 1986.

More Information

A Forgotten Pioneer of Mars Resource Utilization (1962-1963)

Gumdrops on Mars (1966)

Astronaut Sally Ride's Mission to Mars (1987)

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

Apollo 15 Command and Service Module Endeavour in lunar orbit. The drum-shaped portion is the Service Module and the conical portion is the Command Module. Note the Service Propulsion System rocket engine bell at upper left and the extended probe docking unit at lower right. Image credit: NASA.

North American Aviation (NAA) became the prime contractor for the Apollo Command and Service Module (CSM) spacecraft on 28 November 1961. In July of the following year, the company received the unwelcome news that its spacecraft would not land on the Moon. NASA had favored the Lunar-Orbit Rendezvous (LOR) mode for carrying out Apollo landings over Direct-Ascent or Earth-Orbit Rendezvous, both of which would have seen the CSM reach the lunar surface.

LOR made the CSM a lunar orbiter and spawned a new spacecraft: the Lunar Excursion Module (LEM) lander. The LEM, later redesignated the Lunar Module (LM - pronounced "lem"), would transport two astronauts from the CSM in lunar orbit to a landing site on the Moon's surface and back again. The LEM comprised a descent stage with landing legs and a throttleable rocket engine and an ascent stage with a pressurized crew cabin, flight controls, a rocket engine, and a concave drogue docking unit on its roof.

LOR meant that NASA needed to develop the technologies and techniques of rendezvous and docking in lunar orbit. The LEM ascent stage would use the descent stage as a launch pad and climb to a low lunar orbit. The CSM would then move in, extend the active probe docking unit on its nose, and dock with the passive drogue on the LEM.

After the LEM crew transferred back to the CSM, the ascent stage would be cast off. The CSM would subsequently ignite its large Service Propulsion System (SPS) main engine to escape lunar orbit and begin the fall back to Earth.

This image of the Apollo 16 Lunar Module Orion shows clearly the separation plane between the descent and ascent stages. The former has legs, a ladder, and is covered with black paint and gold-colored multilayer blankets for thermal control; the latter is silver and black and has four attitude-control thruster quads (two are readily visible), a crew hatch (square with rounded corners), and a pair of triangular windows. Image credit: NASA. 

In December 1965, NAA's engineers briefed the NASA Headquarters Office of Manned Space Flight (OMSF) and Bellcomm, the space agency's Apollo planning contractor, on results of a preliminary feasibility study of a one-person CSM mission to rescue Apollo astronauts stranded in lunar orbit. The NAA engineers did not describe specific lunar-orbit rescue scenarios, though the CSM modifications they outlined offer clues about the types of rescue missions they envisioned.

The most important piece of rescue hardware they proposed was a special docking adapter ring installed on the rescue CSM's nose. Either an active probe or an active drogue could be mounted on the ring, so the rescue CSM could dock with either a LEM or a CSM. The lone rescue CSM astronaut could reconfigure the docking unit during the flight from the Earth to the Moon; this would permit adaptation to changing circumstances in lunar orbit.

NAA anticipated that a lunar-orbit rescue might require spacewalks, so provided the rescue CSM pilot with a tether and a life-support umbilical extension, a cold gas-propelled hand-held maneuvering device, and a protective "meteoroid garment" of the type Apollo astronauts would wear over their suits on the lunar surface. In addition, the rescue CSM would carry an Expandable Structures Space Rescue System (ESSRS) device. ESSRS was an inflatable "pole" meant to serve as a handrail for astronauts spacewalking between two spacecraft.

Other rescue CSM modifications would include new crew couches to accommodate up to four astronauts, a fourth umbilical so that all could link their suits to the rescue CSM's life support system, added breathing oxygen, a dish-shaped LEM docking radar antenna on an extendable boom, and new rendezvous and docking computer software. Modifications and additions would add a total of 445 pounds to the rescue CSM's weight. Removal of science equipment and other systems not required to rescue and return to Earth a crew stranded in lunar orbit would, however, reduce the rescue CSM's weight by 415 pounds, for a net weight gain of only 30 pounds.

Rescue CSMs would be advanced Block II spacecraft of the type earmarked for Apollo lunar missions. In late 1965, NAA expected to build a total of six Block I and Block II CSMs per year beginning in late 1966. Block I CSMs would be used in Apollo testing and Apollo Extension System (AES) Earth-orbital missions. AES, a proposed program intended to apply Apollo hardware to new missions, became a predecessor to the Apollo Applications Program, which subsequently evolved into the Earth-orbital Skylab Program. In the event, only Block II CSMs carried astronauts; work on Block I CSMs ceased following the deadly AS-204 (Apollo 1) fire of 27 January 1967.

NAA offered two plans for providing rescue CSMs for the Apollo Program. The first, Rescue Vehicle Program "A," would see CSM-110 and CSM-113 converted into rescue CSMs; that is, diverted from lunar exploration missions. They would be flight-ready in early 1969 and mid-1969, respectively. Starting in mid-1970, one of the lunar CSMs NAA produced annually would be built as a rescue CSM; the first of these would be designated CSM-119.

Rescue Vehicle Program "B" would see NAA produce nine CSMs per year. The company's representatives told NASA that this approach would guarantee "non-interference with basic Apollo or AES." The first rescue CSM of Program "B," designated CSM R-1, would be ready for flight at the end of 1968, between AES CSM-109 and lunar CSM-110. Program "B" rescue CSMs R-2, R-3, and R-4 would be completed in mid-1969, early 1970, and late 1970, respectively.

NAA assumed that during every Apollo lunar mission a rescue CSM would stand by atop a three-stage Saturn V rocket on one of the two Launch Complex (LC) 39 pads at Kennedy Space Center (KSC), Florida. The lunar mission would launch from the other LC 39 pad.

The rescue CSM Saturn V would be outwardly nearly identical to the lunar mission Saturn V. The rescue rocket would, however, carry no LEM in the tapered Spacecraft Launch Adapter shroud that would link the aft end of the rescue CSM to the ring-shaped Instrument Unit atop the Saturn V S-IVB third stage. In addition, the Boost Protective Cover that protected the conical Command Module during the first part of ascent would need to be modified slightly to make room for the special docking ring.

On the launch pad, the Saturn V rocket bearing the rescue CSM would have appeared nearly identical to one bearing a lunar landing mission CSM Saturn V. The Boost Protective Cover, visible near the top of the image, would have had a slightly more bulbous nose. Internally, the most significant difference would have been the lack of a Lunar Module within the segmented Spacecraft Launch Adapter, the white tapered housing linking the bottom of the CSM to the ring-shaped Instrument Unit on top of the Saturn V S-IVB third stage. Image credit: NASA. 

The rescue CSM and Saturn V would stand by on the launch pad until the Apollo lunar landing mission CSM safely departed lunar orbit and began its fall back to Earth, then would be rolled back to KSC's cavernous Vertical Assembly Building for storage until the next Apollo lunar mission. A single rescue CSM could be prepared for flight three times and and mothballed twice; this meant that it could stand by during three lunar missions, then would need to be replaced.

NAA did not explain what would be done with unused rescue CSMs; presumably they would be scrapped, though perhaps some systems could be salvaged for use in other CSMs. Neither did the company explain what would happen to unused rescue Saturn V rockets.

The company assumed that in most cases the rescue CSM would launch immediately after NASA learned that a crew had become stranded in lunar orbit. Because it would not wait, in most cases it would not be able to rely on Earth launch geometry to help it to match orbits and carry out a rendezvous with the stranded spacecraft.

NAA determined that launching the rescue CSM immediately could create complications. It might, for example, increase the rescue mission's duration. NAA calculated that the time needed to reach a spacecraft stranded in lunar orbit and return to Earth could in fact exceed the Block II CSM's anticipated 240-hour (10-day) operational lifetime by up to 52 hours in the worst case. NAA recommended that NASA delay the rescue CSM's launch until launch geometry could ensure that its mission duration would not exceed 10 days.

When the rescue CSM reached the Moon's vicinity, it would ignite its SPS main engine to place itself into an elliptical "catch up" lunar orbit. At apolune (lunar orbit high point), the pilot could ignite the SPS again to line up the rescue CSM's orbital plane with that of the stranded CSM. At perilune (lunar orbit low point), the pilot would fire the SPS a third time to lower the rescue CSM's apolune, circularizing its orbit and placing it near the stranded spacecraft.

NAA estimated that Rescue Vehicle Program "A" would add a total of $86 million to the cost of the Apollo Program per year. An 18-month program of development and testing would cost $50 million, $6 million would pay for modifications to two Apollo lunar CSMs, and four new rescue CSMs would cost $38 million each. The company provided no cost estimate for its Rescue Vehicle Program "B."

The NAA engineers did not discuss how astronauts stranded in lunar orbit might eke out their limited supplies of consumables — for example, breathing oxygen — while they awaited rescue. This would be particularly worrisome in the case of a LEM stranded in lunar orbit by a catastrophic CSM failure, for at the time of the NAA study the LEM was expected to keep two astronauts alive for at most one or two days. Neither did they assess the risks of a one-person CSM mission to lunar orbit, nor the technical problems of running two lunar missions simultaneously.

Perhaps because of these difficulties, NASA chose not to make preparations for astronaut rescue in lunar orbit. This did not stop Bellcomm from considering the problems of lunar orbit survival three years later, in December 1968, shortly after the Apollo 8 CSM became the first piloted spacecraft to return from lunar orbit (see link under "More Information" below).

Source

4-Man Apollo Rescue Mission, AS65-36, M. W. Jack Bell, et al., North American Aviation, November 1965; presentation at NASA Headquarters, 13 December 1965.

More Information

What If a Crew Became Stranded On Board the Skylab Space Station (1972)

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

Space Race: The Notorious 1962 Proposal to Launch an Astronaut on a One-Way Trip to the Moon

If an Apollo Lunar Module Crashed on the Moon, Could NASA Investigate the Cause? (1967)

All alone in the gray: the Apollo 17 Lunar Module Challenger photographed by its crew from a distance of about two miles. Image credit: NASA.

The early piloted Apollo missions were a rapid series of test flights. Apollo 7 (11-22 October 1968), the first manned Apollo, saw a Command and Service Module (CSM) spacecraft and its three-man crew put through their paces in low-Earth orbit. Apollo 8 (21-27 December 1968), originally planned as a test of the CSM and the Lunar Module (LM) in high-Earth orbit, might have been postponed because the LM was not yet ready; instead, Apollo 7's success and the perceived threat to American prestige of a Soviet manned circumlunar mission induced NASA managers to make it a lunar-orbital CSM test and a trial run for the Apollo tracking and communications network.

Apollo 9 tested the CSM, LM, and the Apollo space suit in low-Earth orbit (3-13 March 1969). Apollo 10 (18-26 May 1969) tested the CSM and LM in lunar orbit and rehearsed the Apollo lunar descent procedure down to an altitude of 50,000 feet.

Apollo 11 (16-24 July 1969), the first lunar landing attempt, was also a test flight, though it is seldom seen that way today. In an effort to make that first landing as easy as possible, engineers directed the Apollo 11 LM Eagle to the northern Sea of Tranquility, one of the flattest stretches of lunar equatorial terrain scientists could find. It was, however, also a U.S. victory in the Cold War with the Soviet Union and the first time humans had explored an alien world first-hand. Scientists and engineers fought a running battle over the degree to which scientific exploration should play a role in Apollo 11, and President Richard Nixon telephoned moonwalkers Neil Armstrong and Edwin "Buzz" Aldrin to read a celebratory speech as they stood next to the U.S. flag.

Eagle landed downrange of its planned landing site. Its overworked computer might have flown it into boulder-filled West Crater had it not been for the quick thinking of former X-15 rocket plane test-pilot Armstrong. Apollo 12 (14-24 November 1969) thus became a test of the Apollo system's ability to make a pinpoint landing. The ability to reach a predetermined spot on the moon was important to scientists planning Apollo geologic traverses. It also helped to ensure safety. The Apollo 12 LM Intrepid landed on the Ocean of Storms, another flat plain, just 600 feet from its target, the derelict Surveyor 3 lander, which had preceded it to the site on 20 April 1967.

Any Apollo mission might have failed catastrophically far from Earth, a point driven home by the explosion on board the CSM Odyssey during Apollo 13 (11-17 April 1970). Hollywood scriptwriters notwithstanding, failure was an option during Apollo missions. Apollo pushed the limits of 1960s technology to do extraordinary things.

The Apollo Program had, in fact, claimed lives before the first Apollo spacecraft left Earth: the AS-204 (Apollo 1) fire killed Gus Grissom, Ed White, and Roger Chaffee during a launch pad training exercise on 27 January 1967, barely a month before their planned launch. Because the Apollo 1 fire occurred on the ground, engineers could take apart the AS-204 CSM piece by piece to try to trace the fire's cause. Even so, they never conclusively identified its ignition source.

A December 1964 report by R. Moore of the Project RAND think-tank anticipated that accidents taking place on the moon would be even more difficult to analyze. Moore proposed that NASA continue the Ranger lunar probe series to enable photography of lunar crash sites. The last four Rangers each carried a battery of six television cameras intended to return images to Earth as the spacecraft plummeted toward destructive impact.

If, for example, Eagle had crashed in West Crater, then NASA would have dispatched a Ranger to image the site. Ranger seemed well suited to aiding accident investigators: Ranger 7, which struck the Ocean of Storms on 31 July 1964, had imaged features as small as 18 inches wide in its final seconds before impact.

Ranger 7, 8, and 9 were designed for close-up photography of the lunar surface. Image credit: NASA.

NASA did not act on Moore's proposal, but the concept of Apollo accident site investigations was not forgotten (or, just as likely, it was discovered again). In November 1967, C. Byrne and W. Piotrowski of Bellcomm, NASA's Washington, DC-based Apollo planning contractor, wrote a memorandum in which they looked at whether a Command Module Pilot (CMP) whose moonwalking colleagues had suffered a fatal mishap on the moon might assist investigators by photographing the accident site from the CSM in lunar orbit.

They began by acknowledging that telemetry could provide valuable accident data: they added, however, that "certain types of failure can be imagined which would not permit enough data to be transmitted to support a diagnosis." In those cases, they wrote, observation from lunar orbit might be the only way to collect data that could guide engineers in their efforts to redesign the Apollo system to avoid similar accidents.

Byrne and Piotrowski then looked at the image resolution necessary to make useful observations of an accident site on the moon. To locate and identify an intact LM, which measured a little more than 20 feet tall, images showing details as small as 10 feet across would be needed. Eight-foot resolution would be needed to determine the status of the LM's 12-foot-tall ascent stage; for example, if it had lifted off from the descent stage and then crashed on the surface. Four-foot resolution would suffice to determine whether the LM had tipped over.

The ability to resolve features as small as a yard across would enable engineers to assess landing site roughness and slope. Two-foot resolution would, they estimated, be adequate to discern astronaut bodies on the surface. One-foot resolution would reveal whether the LM landing gear had failed, "hazardous sinkage" had occurred, the LM ascent stage crew cabin lay open to vacuum, or an explosion in the LM had scattered "litter" around the landing site.

Byrne and Piotrowski then took stock of the cameras and telescopes expected to be on board the CSM during a normal lunar mission and their performance if the CSM were orbiting 80 nautical miles (n mi), 40 n mi, or 10 n mi above the accident site. They suggested that CSM propellants budgeted for rescue of astronauts on board an LM ascent stage that attained only a low orbit could be used to lower the CSM's altitude for accident site observations.

The CSM's scanning telescope would, despite its name, not magnify objects, so would be of "no value" as a diagnostic tool, Byrne and Piotrowski judged. The sextant, on the other hand, could magnify objects 28 times. The Bellcomm engineers found that the sextant would offer 8.6-foot resolution at an orbital altitude of 80 n mi, 4.3-foot resolution at 40 n mi, and 1.1-foot resolution at 10 n mi. (Apollo CMPs did in fact use the sextant to spot LMs — or at least the shadows they cast — on the lunar surface.)

The sextant was, however, designed to superimpose a pair of star images, could not be used to photograph objects, and, with a field of view only 1.8° wide, would require a highly skilled operator to spot an LM at all. This would be the case especially at lower altitudes, when the CSM would be moving fastest relative to the surface. Byrne and Piotrowski estimated that an astronaut searching the surface with the sextant at an altitude of 10 n mi would at most have 10 seconds in which to find and observe an accident site.

Apollo 12 Command Module Pilot Richard Gordon trains with cameras and lenses in a Command Module simulator before his November 1969 flight to the moon. Image credit: NASA.

Byrne and Piotrowski wrote that NASA planned to include among the Apollo CSM experiment equipment a Swedish-built Hasselblad 500EL camera with 80-millimeter (mm) f/2.8 and 250-mm f/5.6 lenses. Used with S0-243 film and the 250-mm lens, the Hasselblad 500EL could in theory take photos of the lunar surface with a resolution of 13 feet at 80 n mi of altitude, 6.5 feet at 40 n mi, and 1.6 feet at 10 n mi.

Other constraints would, however, conspire to reduce camera performance. In particular, there was the problem of image motion compensation. Experience gained through Earth photography during the Gemini V mission (21-29 August 1965) showed that astronaut movements were jerky, not smooth, when tracking and photographing targets. Jerky tracking would cause image "smear," reducing resolution.

Byrne and Piotrowski recommended that the CMP mount the Hasselblad 500EL securely in a new-design clamp or bracket at either the CSM hatch window or one of the side windows after he located the LM site. He would then fire the CSM's Reaction Control System thrusters to roll the spacecraft and keep the surface target in his camera's field of view as the CSM passed over it. This form of image motion compensation was unlikely to be perfect; for one thing, roll rate would be affected by factors beyond the CMP's control, such as the distribution and movement of liquid propellants in the CSM's tanks.

As with the sextant, time-over-target would pose a constraint. The Bellcomm engineers assumed that the CMP would need at least 30 seconds to locate the LM on the moon, 15 seconds to prepare the camera and roll the CSM, and 15 seconds for photography.

For a CSM at an altitude of 80 n mi, an LM on the lunar surface would remain in view for two minutes and 24 seconds. This was ample for photography, but at that altitude resolution would be inadequate — no better than 10 feet. At 40 n mi of altitude, the CMP could keep the LM in view for 90 seconds. At 30 n mi, he would have about 60 seconds — the minimum necessary — to find and photograph his target. Byrne and Pietrowski thus selected 40 n mi as the optimum altitude for accident site photography.

The Bellcomm engineers looked at adding a special cartridge of high-contrast film and a 500-mm f/8 lens for the Hasselblad 500EL, and at replacing the Hasselblad 500EL with the Zeiss Contarex Special 35-mm camera and 200-mm f/4 and 300-mm f/4 lenses. These had already reached space on board Gemini V. They noted that both cameras would yield a resolution of about one yard at an altitude of 40 n mi with a secure mounting bracket and adequate image motion compensation. In the end, they favored the Hasselblad 500EL with 500-mm f/8 lens and high-contrast film because it would be about eight pounds lighter than the Zeiss camera.

Byrne and Piotrowski noted that the camera system and techniques they proposed would have uses other than accident site investigation. They might, for example, be used to photograph the landing site after a successful LM landing. This would, among other things, enable scientists to precisely locate the post-deployment position of the Advanced Lunar Scientific Experiment Package, a suite of instruments the moonwalkers would deploy some distance away from the LM. Images of the landing site might also assist geologists in understanding the context of the samples the moonwalking astronauts would return to Earth.

Sources

"A Suggestion for Extension of the NASA Ranger Project in Support of Manned Space Flight," Memorandum RM-4353-NASA, R. C. Moore, The RAND Corporation, December 1964.

"Diagnostic Observation of Lunar Surface Accidents – Case 340," C. Byrne & W. Piotrowski, Bellcomm, Inc., 7 November 1967.

More Information

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

What if an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

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