Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters

The Skylab Orbital Workshop as seen by the Skylab 4 crew, the last astronauts to live on the station. Image credit: NASA.

On 14 May 1973, the last Saturn V rocket to fly, designated SA-513, launched the Skylab space station into a 435-kilometer-high orbit about the Earth. Flight controllers soon realized that the 85-ton space laboratory was in trouble. Although they did not know it at the time — Skylab climbed rapidly into dense clouds, so could not be imaged during most of its ascent — 63 seconds after liftoff a design flaw caused its meteoroid shield to rip away. Shield debris jammed one of the workshop's two main electricity-producing solar arrays. The other array remained attached to Skylab's side only at its hinge (forward) end.

Shield debris also pummeled SA-513, tearing at least one hole in the tapered interstage adapter that linked its S-II second stage with the Skylab station. Debris also apparently damaged the system for separating the cylindrical adapter that linked the S-II to the S-IC first stage. The adapter, meant to separate shortly after the spent S-IC, remained stubbornly attached to the S-II all the way to orbit.

Skylab 1 launch, 14 May 1973. Image credit: NASA.

After the S-II's five J-2 engines shut down, forward-facing solid-propellant rockets ignited to push the spent stage away from Skylab. Their plumes blasted open and tore away the loose solar array. Ironically, the jammed array probably survived because it was tied down by meteoroid shield debris.

Without the protection of the reflective meteoroid shield, temperatures within Skylab's 11,303-cubic-foot pressurized volume soon soared, raising fears that its air would become tainted by outgassing from materials on board, film would be ruined, and food and medicines spoiled. Flight controllers soon found to their dismay that maneuvers designed to cool Skylab's interior tended to starve it of electricity, for they turned away from the Sun the four "windmill" solar arrays on the Apollo Telescope Mount (ATM), the beleaguered space laboratory's only functioning sources of power.

NASA immediately began a Skylab salvage effort. Engineers developed deployable sunshields and tools for freeing the stuck main solar array, flight controllers carefully maneuvered Skylab to maximize the amount of electricity the ATM arrays could produce while reducing temperatures on board as much as possible, and the first crew meant to board Skylab (their mission was designated Skylab 2) hurriedly trained to become the world's first orbital repairmen.

Skylab 2 astronauts Joseph Kerwin, Charles "Pete" Conrad, and Paul Weitz. Image credit: NASA.

On 25 May, the Skylab 2 crew of Pete Conrad, Paul Weitz, and Joe Kerwin lifted off in an Apollo Command and Service Module (CSM) atop a Saturn IB rocket. After a failed attempt to pull open the one remaining main solar array with a hook extended from the open CSM hatch, they docked with and entered Skylab, then deployed a sunshield through an experiment airlock. Temperatures began to fall, but the station remained starved for electricity. On 7 June, Conrad and Kerwin succeeded in forcing open the surviving main solar array, saving not only their own 28-day mission but also the planned Skylab 3 and Skylab 4 missions.

The Skylab 3 crew of Alan Bean, Jack Lousma, and Owen Garriott lifted off on 28 July. During their 6 August spacewalk, Lousma and Garriott deployed an improved sunshield. They lived and worked on board the station for 59 days.

The Skylab 4 crew of Jerry Carr, William Pogue, and Ed Gibson boarded the station on 16 November. Carr and Gibson mounted a meteoroid collector on an ATM strut during their spacewalk on 3 February 1974, in the hope that a Space Shuttle crew might retrieve it as early as 1979. When the Skylab 4 crew undocked on 8 February 1974 after a record-breaking 84 days in space, Skylab was expected to remain aloft until 1983, when atmospheric drag would cause it to re-enter Earth's atmosphere. They left Skylab's airlock hatch closed but not latched so that it could provide entry for future visitors.

This pre-launch cutaway illustration of Skylab shows the station as it would have appeared if it had reached Earth orbit undamaged. In addition to two large main solar arrays, it includes the micrometeoroid shield which tore free during Skylab's ascent through the atmosphere. Skylab was the largest single-launch space station ever; astronauts, dressed in brown, look very small inside it. Image credit: NASA.

During the solar-minimum years of the mid-1970s, the Sun was more active than had been anticipated at the time of Skylab's launch. Solar activity heated and expanded Earth's upper atmosphere, subjecting the first U.S. space station to more aerodynamic drag than expected. In March 1977, the NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, asked NASA Headquarters to grant it permission by mid-1977 to begin work on a mission to raise Skylab's orbital altitude so that its lifespan could be extended, giving NASA time to consider future uses for the space station.

That MSFC maintained a strong proprietary interest in Skylab should not be surprising. In November 1965, the Huntsville center had proposed that a space laboratory based on a spent Saturn V S-IVB stage be added to the Apollo Applications Program (AAP), at the time NASA's main post-Apollo piloted program. The spent-stage AAP workshop, a low-cost space station, had much greater potential for supporting long-duration astronaut stays in orbit than did modified Apollo CSM and Lunar Module (LM) spacecraft. NASA Headquarters quickly approved MSFC's plan.

For its first three-and-a-half years, the AAP Workshop was the S-IVB second stage of a Saturn IB rocket and, on its top, a small pressurized module with multiple docking ports. During ascent to Earth orbit, it would act as a normal Saturn IB stage. After its single J-2 rocket motor shut down, the four segments of its streamlined launch shroud would open like the petals of a flower, revealing the docking module. Controllers would then command vents in the stage to open so that residual liquid oxygen/liquid hydrogen propellants could escape into space. Meanwhile, solar arrays would unfold from the inside of two of the four shroud segments to generate electricity.

The AAP spent-stage workshop. At left an AAP CSM docks with one of the docking modules four radial ports through the intermediary of an add-on module. Image credit: NASA.

A crew launched on a second Saturn IB would rendezvous and dock with the spent stage in an AAP CSM, enter the docking module, then enter the cavernous liquid hydrogen tank, the largest of the two S-IVB stage tanks. They would pressurize the tank with gaseous oxygen and nitrogen from tanks in the docking module, then install in the tank furnishings, fabric floors and walls, lights, and experiments transferred from the CSM and docking module. Subsequent AAP Saturn IB/CSM flights would deliver Earth-looking and space-looking science modules for attachment to the docking module, including an array of solar telescopes based on the Apollo LM design.

In July 1969, NASA Administrator Thomas Paine approved plans to shift from the Saturn IB-launched "wet workshop" (as it was colloquially known) to a Saturn V-launched "dry workshop." The latter, more capable than the former, would include neither propellants nor an engine and would reach Earth orbit fully outfitted. In February 1970, the AAP workshop (and, indeed, AAP as a whole) was renamed Skylab. NASA Headquarters made MSFC responsible for Skylab Saturn V and Saturn IB rockets, overall Skylab systems engineering and integration, and most onboard experiment apparatus.

On 10 June 1977, former Skylab Deputy Director John Disher, by then NASA's Director of Advanced Programs, requested that MSFC conduct a preliminary in-house study of the feasibility of reusing Skylab in the Space Shuttle era. At about the same time, NASA Headquarters directed NASA Johnson Space Center (JSC), lead center for the Space Shuttle, to study an early Shuttle mission to either boost Skylab to a higher, longer-lived orbit or cause it to safely reenter over an unpopulated area.

In September 1977, JSC informed NASA Headquarters and MSFC that the earliest it could reboost or deboost Skylab was September 1979, as part of the fifth Orbital Flight Test (OFT) Shuttle mission. At the time, NASA envisioned a total of six OFT missions before the Shuttle was declared operational. NASA Headquarters then gave the go-ahead for MSFC and JSC to begin work toward a September 1979 Skylab reboost/deboost mission.

On 16 November 1977, MSFC engineers J. Murphy, B. Chubb, and H. Gierow presented to NASA Associate Administrator for Space Flight John Yardley results of the study they had begun in June. They were addressing a Skylab expert: before coming to NASA in 1974, Yardley had managed Skylab work at McDonnell Douglas, the prime contractor for the OWS.

The MSFC engineers first described Skylab's condition. They reported that when the Skylab 4 crew returned to Earth, the Orbital Workshop's water system contained 1930 pounds of water (enough to supply three men for 60 days). The water, they said, probably remained potable, but might have developed a bad taste. If it was no longer potable, then it might be used for bathing. In any case, the Skylab water system included resupply points, so a Space Shuttle crew could refill it with fresh water if water transfer equipment were developed.

The oxygen/nitrogen supply remaining on Skylab was probably sufficient to supply three men for 140 days at Skylab's standard operating pressure of five pounds per square inch, the MSFC engineers estimated. The station's ventilation and carbon dioxide-removal systems were almost certainly functional. Even if they were not, their most important components were designed to be replaceable in space.

The MSFC engineers also assessed Skylab's electrical power system. They estimated that the main solar array Conrad and Kerwin had freed could still generate between 1.5 and 2.5 kilowatts of electricity, and that the batteries it had charged, located in Skylab's Airlock Module, were probably still usable. The batteries for the four ATM arrays, located inside the ATM, were, on the other hand, almost certainly frozen. The team recommended that controllers reactivate the main array electrical system from the ground before the first Shuttle visit, and that any effort to revive the ATM electrical system be left for a later time.

More problematic than the electrical system was Skylab's attitude control system, which relied on a trio of Control Moment Gyros (CMGs) to turn Skylab so that, among other things, it could reliably point its solar arrays at the Sun. At the time the Skylab 4 crew departed, one CMG had already failed and another showed signs of impending failure. In addition, Skylab's guidance computer was probably dead after being subjected to "extreme thermal cycling" as Skylab passed between daylight and night. The Orbital Workshop's thruster system, on the other hand, was probably operational with about 30 days of propellant remaining.

Finally, the MSFC team looked at Skylab's cooling system, which had leaked while the astronauts were on board and had probably frozen and ruptured since the last crew returned to Earth. They called "serviceability of [the] cooling system. . .the most questionable area" as far as Skylab's reusability was concerned, but added that "any inflight 'fixes' should be well within the scope of crew capability."

The MSFC engineers then proposed a four-phase plan for reactivating and reusing Skylab. The target date for their first Phase I milestone had already passed by the time they briefed Yardley: though it was already mid-November, they made a point of calling for an October 1977 decision on whether Skylab should be reboosted to a higher orbit, extending its orbital lifetime until about 1990, or deboosted so that it could reenter safely over an unpopulated area.

Assuming that NASA decided to reboost Skylab, then a ground-controlled Skylab reactivation test would occur between June 1978 and March 1979. If the test was successful, then the fifth OFT Space Shuttle mission would rendezvous with Skylab. As already mentioned, in September 1977 JSC estimated that the fifth OFT would fly in September 1979. Two months later, when the MSFC team briefed Yardley, the mission had already slipped to February 1980.

Artist concept of Teleoperator spacecraft. Image credit: NASA.

The MSFC team anticipated that the Space Shuttle crew would conduct an inspection fly-around of Skylab, then would deploy an unmanned Teleoperator spacecraft from the Shuttle Orbiter payload bay. Using a control panel on the Orbiter flight deck, the astronauts would guide the Teleoperator, which would carry an Apollo probe-type docking unit, to a docking with the drogue-type docking unit on the front of Skylab's Multiple Docking Adapter. The Teleoperator would fire its thrusters to raise Skylab's orbit; then, its work completed, it would detach, freeing up Skylab's front docking port for Phase II of MSFC's plan.

Astronauts in a nearby Space Shuttle Orbiter stand by as the Teleoperator ignites its thrusters to raise Skylab's orbit and extend its orbital lifetime. Image credit: NASA.

Phase II would begin in March 1980, when NASA would initiate development of Skylab refurbishment kits, a 10-foot-long Docking Adapter (DA) module, and a 25-kilowatt Power Module (PM). The DA would include at one end an Apollo-type probe docking unit for attaching it to Skylab's front port and at the other end an Apollo-Soyuz-type androgynous unit with which Shuttle Orbiters and the PM could dock.

The first refurbishment kit and the DA would reach Skylab on board a Shuttle Orbiter in January 1982, almost two years after the reboost mission. During the 1982 mission, spacewalking astronauts would fold two of the four ATM solar arrays out of the way to improve clearance for visiting Orbiters and would retrieve the meteoroid experiment the Skylab 4 astronauts had left on the ATM. As time allowed, this and other Phase II crews would perform unspecified "simple passive experiments" on board Skylab and would collect samples of its structure for engineering analysis on Earth.

The third Shuttle visit to Skylab would not take place until August 1983. The astronauts would install additional refurbishment kits and would tackle the daunting job of repairing Skylab's damaged cooling system.

The refurbished Skylab station after the start of Phase III of the NASA MSFC reactivation program. The Power Module, Docking Adapter, and Shuttle-carried Spacelab are clearly visible. Image credit: Junior Miranda.

The MSFC engineers told Yardley that Phase III of the Skylab reactivation program would begin in March 1984 with delivery of the PM and any remaining refurbishment kits. Using the Shuttle Remote Manipulator System robot arm, astronauts would lift the PM from the Orbiter payload bay and turn it 180° so that it protruded forward well beyond the Orbiter's nose. They would then dock one of the PM's three androgynous docking units to an identical unit at the front of the Orbiter payload bay. The Shuttle would use another of the PM's docking units to dock with the DA on Skylab.

Following docking with Skylab, the astronauts would deploy the PM's twin solar arrays and thermal radiators, link the PM to Skylab's systems using cables extended through open hatchways or installed on the hull during spacewalks, and power up the PM's three CMGs to replace Skylab's crippled attitude control system. The Orbiter would then undock from the PM, leaving it attached permanently to Skylab. Shortly thereafter, NASA would declare the revived and expanded Orbital Workshop to be fully habitable.

Phase III would continue with the first in a series of 30-to-90-day missions aboard Skylab. During these, a Shuttle Orbiter carrying a Spacelab module in its cargo bay would remain docked with the Orbital Workshop. The astronauts would work in the Spacelab module, take advantage of Skylab's large pressurized volume to perform "simple experiments" requiring more room than Shuttle and Spacelab could provide (for example, preliminary trials of space construction methods), and begin building up stockpiles of food, film, clothing, and other supplies on the revived station.

Another 30-to-90-day mission would see the astronauts refurbish and use selected Skylab science equipment, install new experiments based on Spacelab experiment designs, and stockpile more supplies. Between these missions, the new and improved Skylab would fly unmanned under control from the ground.

The view from the Sun: all of the solar arrays deployed for Phase III of the Skylab reactivation program are visible in this image by Junior Miranda.

The MSFC engineers told Yardley that the volume available to a crew on board a Shuttle Orbiter without a Spacelab module in its payload bay would total only 1110 cubic feet. Adding a Spacelab would increase that to about 5100 cubic feet. This would, however, amount to less than half the pressurized volume of Skylab. For a mission including a Shuttle Orbiter, Spacelab module, and Skylab, the total volume available to the crew would exceed 16,400 cubic feet.

They were not specific about what Skylab would be used for when Phase IV of their program began in mid-1986, though they did offer several intriguing possibilities. Shuttle Orbiters might, for example, attach modified Spacelab modules and experiment pallets to the third docking port on the PM.

A Shuttle External Tank might be joined to Skylab to serve as a strongback for large-scale space construction experiments using a mobile "space crane." These experiments might include construction of a large space solar power module or a multiple beam antenna.

A new "floor" might be assembled within Skylab, enabling it to house up to nine astronauts. As NASA developed confidence in the revived space laboratory's health, manned missions on board Skylab without a Shuttle Orbiter present might commence, leading to permanent manning and "support [of] major space operations."

The MSFC engineers did not estimate the cost of Phases I and IV of their plan, though they did provide (perhaps optimistic) cost estimates for Phases II and III. Their estimates did not include Space Shuttle transportation and contractor study costs.

In Fiscal Year (FY) 1980, NASA would spend $2 million each on Phases II and III. This would increase to $5 million for Phase II and $3.4 million for Phase III in FY 1981. FY 1982, their plan's peak funding year, would see $4.5 million spent on Phase II and $10.2 million spent on Phase III. In FY 1983, NASA would spend $2.5 million to close out Phase II and $12 million to continue Phase III. The following year it would spend $9.1 million on Phase III. Phase III closeout in FY 1985 would cost $4.5 million. Phase II would cost a total of $14 million, while the more ambitious Phase III would total $41.2 million.

In November 1977, the month the MSFC engineers briefed Yardley on their study, NASA awarded Martin Marietta Corporation a small ($1.75-million) contract to begin development of the Teleoperator. The remote-controlled spacecraft was envisioned as a small space tug made up of modular components.

No decision was taken at that time as to whether the Teleoperator would reboost Skylab to make it available for possible future use or would deorbit it in a controlled manner; that decision would await assessment of Skylab's condition and additional study of potential applications. McDonnell Douglas and Martin Marietta subsequently commenced more detailed and extensive Skylab reuse studies under MSFC supervision with inputs from JSC and NASA Headquarters.

Sources

Skylab 1 Investigation Report, Hearing Before the Subcommittee on Manned Space Flight of the Committee on Science and Astronautics, US House of Representatives, Ninety-Third Congress, First Session, 1 August 1973.

"Skylab Reuse Study Presented to Mr. Yardley by MSFC," 16 November 1977.

Living and Working in Space: A History of Skylab, NASA SP-4208, W. David Compton & Charles D. Benson, 1983, pp. 361-372.

More Information

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

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

What Shuttle Should Have Been: NASA's October 1977 Space Shuttle Flight Manifest

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

Skylab liftoff on a two-stage Saturn V rocket, 14 May 1973. Had the ISS Program gone ahead as planned, its four station launches in 1976, 1978, 1981, and 1983 would have closely resembled this one. Image credit: NASA.

On 6 April 1971, eight engineers in the Advanced Concepts & Missions Division, NASA Headquarters Office of Advanced Research and Technology (OART), completed a blueprint of NASA's future. Their detailed report was strictly internal and of limited circulation.

Had the OART team's plan become more widely known, it would surely have generated controversy. This was because it proposed to end U.S. lunar exploration with Apollo 15 so that the Saturn V rockets earmarked for missions 16, 17, 18, and 19 could be used to launch into Earth orbit a series of four "interim" space stations, each more capable than the last, between early 1976 and late 1983.

Although the OART plan sounds like an Apollo massacre, it would in fact have deprived the U.S. of two piloted Moon missions, not four. By the time the OART team proposed its program, NASA had already cancelled three Apollos. First was Apollo 20, nixed in January 1970 so that its Saturn V rocket could launch 85-ton Skylab, a temporary space station, into low-Earth orbit (LEO).

Next to go were Apollo 15 and Apollo 19 in September 1970. NASA Administrator Thomas Paine scrapped the two lunar landing missions — an H-class walking mission and a J-class rover mission, respectively — to free up funds for NASA's hoped-for 12-man permanent Space Station and the fully reusable winged Space Shuttle intended to deliver its crews, supplies, and experiment equipment. NASA subsequently renumbered its remaining Apollo missions, so the cancelled missions are more commonly known today as Apollo 18 and Apollo 19.

The Interim Space Station (ISS) Program would have played much the same role for NASA in the mid-1970s/early 1980s as Gemini played in the 1960s. Soon after President John F. Kennedy's 21 May 1961 call for an American on the Moon by the end of the 1960s decade, aerospace engineers realized that they needed an experience-building "bridge" program to link simple Mercury suborbital and LEO missions with complex Apollo lunar orbiter and landing missions. Gemini evolved from Mercury — it was initially called "Mercury Mark II" — to fulfill that role.

The Saturn IB stage served as the Apollo Moon rocket's third stage, the Saturn IB rocket's second stage, and the structural basis of the Skylab station. Image credit: NASA.

OART's ISS Program was envisioned as an evolutionary extension of the Skylab Program. Skylab A and its backup, Skylab B, employed 22-foot-diameter Saturn S-IVB rocket stages as their basic structure. The S-IVB was the third stage of the three-stage Saturn V Moon rocket and the second stage of the two-stage Saturn IB rocket. From top to bottom, the stage comprised the ring-shaped Instrument Unit (the "electronic brain" of the Saturn V or Saturn IB rocket of which the S-IVB stage was part), a large tank for low-density liquid hydrogen fuel, a small tank for higher-density liquid oxygen oxidizer, and a restartable J-2 rocket engine.

Through the addition of metal-grid decks, life-support equipment and consumables, lights and air ducts, a film vault, living quarters, and experiment apparatus, the S-IVB hydrogen tank became the Orbital Workshop (OWS), Skylab A's main habitable volume. The empty S-IVB liquid oxygen tank served as a dumpster, and a radiator replaced its J-2 engine.

The OWS hydrogen tank had bolted to its top the Airlock Module (AM), which in turn linked to the Multiple Docking Adapter (MDA) at Skylab A's front. The AM included a surplus Gemini hatch for spacewalks. The MDA included a main axial (front) docking port and a back-up radial port.

Besides the OWS, MDA, and AM, Skylab A included the Apollo Telescope Mount (ATM), an unpressurized compartment containing instruments for viewing the Sun. The ATM, mounted on a truss attached to the side of the MDA, included four electricity-generating solar arrays arranged in "windmill" fashion. These augmented two large solar-array "wings" on Skylab A's sides.

The Skylab space station as envisioned in 1970. Image credit: NASA.

Skylab A was commonly referred to simply as Skylab, since no firm plan existed to actually launch Skylab B. When the OART engineers completed their report, NASA planned to launch Skylab in late 1972; then, over a period of about nine months, the U.S. civilian space agency would launch to the station three crews in Apollo Command and Service Module (CSM) spacecraft atop Saturn IB rockets. The three-man crews would live and work on board Skylab for up to 56 days. While unoccupied — months might pass between one crew's departure and the next crew's arrival — Skylab would operate under ground control.

The OART engineers applied the term "interim" to their eight-and-half-year program because they intended that it should lead from the Skylab Program to a permanent Space Station through "evolutionary, gradual, and step-wise spacecraft systems development." Beginning about three years after the third and final Skylab crew returned to Earth, a new ISS would reach LEO every two and a half years. Each would be staffed continuously for from 360 to 420 days.

NASA planning was in flux at the time the OART team prepared its report, and would remain so even after President Richard Nixon approved development of a semi-reusable Space Shuttle in January 1972. The ISS Program would span most of a decade, and NASA had in its dozen-year history experienced program instability on the scale of months. These factors caused the OART engineers to avoid making assumptions about the nature of NASA's eventual permanent Space Station when they planned their ISS Program.

They went so far as to suggest, in fact, that the Station/Shuttle Program might be delayed or abandoned in favor of some new space goal before the ISS Program ran its course. For planning purposes, however, they adhered to a timeline which saw NASA's permanent Space Station become operational in late 1987, about six years after the date they gave for the Shuttle's maiden flight and a little more than three years after the last ISS crew returned to Earth.

In keeping with the $3.3-billion Fiscal Year 1972 NASA budget Nixon's Office and Management and Budget had sought from Congress in January 1971, the OART engineers optimistically assumed a steady NASA annual funding stream of $3.3 billion throughout the ISS Program. They estimated that each interim station would cost $2 billion, of which about $330 million would be spent on hardware development, $500 million on experiments, and $1.6 billion on spacecraft hardware. Their program would, they calculated, cost on average about $500 million per year, leaving $2.8 billion for other NASA projects, including Station/Shuttle development.

Interestingly, just 13 days after the OART team completed its report, the Soviet Union launched 20-ton Salyut 1, the world's first space station. The Soviets had during the 1969-1970 period made it known publicly — most prominently in an October 1969 speech by Soviet leader Leonid Brezhnev — that they intended to establish Earth-orbiting stations, so it is tempting to suppose that OART's study was at least in part motivated by Soviet statements.

In January 1970, in fact, the U.S. Central Intelligence Agency had completed a report, classified "SECRET," in which it suggested that the Soviets might construct a series of stations, each larger and more capable than the last, culminating, perhaps, in a $5-billion, 150-ton station between 1976 and 1980. The OART engineers did not, however, mention Soviet space plans in their report.

Interim Space Station design. Image credit: NASA.

Like Skylab, the interim stations would reach LEO atop two-stage Saturn V rockets. The first station in the series, designated Interim Space Station-A (ISS-A), would be mainly outfitted for biotechnology research. It would operate in a a 245-nautical-mile (nm) orbit inclined 28.5° relative to Earth's equator. The OART team envisioned that ISS-A would be built from Skylab B. Like the other three stations in its series, ISS-A would lack an ATM.


Based on Skylab experience, the OART engineers calculated that ISS-A would at launch weigh at least 57.25 tons. They then assumed a 30-ton "growth allowance" which could be wholly or partly used during development and assembly. This meant that ISS-A might weigh as much as 87.25 tons at launch.

NASA would launch the first three-man ISS-A crew — indeed, the first crew of the ISS Program — in a modified CSM within a day or two of the station's launch. No more than 16 hours after they reached LEO, the astronauts would pilot their spacecraft to a docking at one of ISS-A's two MDA docking ports.

The CSMs that delivered astronauts to the interim stations would differ significantly from their Apollo/Skylab predecessors. The most obvious change would be a new-design launch vehicle. The OART engineers considered using either the Saturn IB or the Titan-IIIM to launch ISS CSMs before they settled on a hybrid of the two.

Dubbed the SRM-S-IVB, the new rocket's first stage would comprise a cluster of three 10-foot-diameter, seven-segment Titan-IIIM solid-propellant rocket motors. The Titan-IIIM, never flown, had been meant to launch the U.S. Air Force Manned Orbiting Laboratory, which was cancelled in February 1969. As its name implies, the SRM-S-IVB launch vehicle's second stage would be a lightly modified Saturn S-IVB stage.

The SRM-S-IVB would be capable of launching a 28.7-ton payload from Kennedy Space Center, Florida, to a 245-nm orbit at 28.5° of inclination. For comparison, the Saturn IB could launch about 17.5 tons to the same orbit.

The ISS CSM, like its Apollo and Skylab predecessors, would be a two-part spacecraft. The smaller of the two parts was the conical Command Module (CM), a three-man crew capsule with a reentry heat shield on its broad aft end and an active probe docking unit on its nose. It would lower on parachutes to a splashdown at mission's end. The drum-shaped Service Module (SM) had a Service Propulsion System main engine bell protruding from its aft end.

The 6.3-ton ISS CM would closely resemble its Apollo and Skylab counterparts. The ISS SM, on the other hand, would undergo many changes. Because it would need to carry only enough propellants for Earth-orbital rendezvous and docking maneuvers plus an end-of-mission de-orbit burn, OART proposed to replace its propellant tanks, which were sized for a voyage to lunar orbit and back, with smaller tanks derived from those in the Apollo Lunar Module. Because the ISS CSM would fly independently for a total of less than a day, rechargeable batteries in the ISS SM would stand in for the Apollo SM's trio of fuel cells and tanks of fuel-cell reactants.

These changes would free up for conversion into cargo holds four of the six 175-cubic-foot bays clustered around the SM's cylindrical core bay. The four bays would transport a total of about 10 tons of supplies and equipment. Minus cargo, the ISS SM would weigh 8.6 tons.

The Apollo Command and Service Module (CSM) spacecraft would undergo considerable modification for the ISS Program. Image credit: NASA.

Water, oxygen, and nitrogen stored in tanks in the ISS SM cargo bays would pass through umbilicals to nozzles in the ISS CM. The astronauts would attach hoses to the nozzles to transfer the water, oxygen, and nitrogen to storage tanks inside the ISS.

Solid cargo, on the other hand, could only be transferred from the ISS SM to the ISS by spacewalks. The OART team noted that the spacewalking astronauts would have to travel only about 15 feet to reach the ISS SM from the ISS AM.

The astronauts would hinge open panels in the ISS SM's sides and transfer cargo items to the open ISS AM hatch by attaching them to a clothesline-like "endless line" similar, perhaps, to that used on the Moon to convey sample boxes and film from the base of the LM ladder to the LM ascent stage hatchway. Cargo items as large as 3.5 feet wide by 12 feet long could be removed from the ISS SM cargo bays and transferred through the Gemini-type hatch into the ISS AM, the OART team estimated.

Because it would be cast off to burn up in Earth's atmosphere after it performed the deorbit burn, the ISS SM could transport only "up" cargo. "Down" cargo — for example, biological samples and exposed film — would reach Earth within the relatively small volume of the ISS CM. The OART engineers estimated that, by removing all lunar mission equipment and supplies from the ISS CM, enough room would be freed up to enable it to convey to Earth all experiment cargo a three-man crew was likely to generate during a 90-day stint on board an ISS.

Converting the Apollo CSM into the ISS CSM would cost $100 million, the OART engineers estimated. This price-tag would not include the $80-million cost of developing the SRM-S-IVB launcher.

The Saturn S-IVB rocket stage would form the largest habitable component of Skylab and the ISS stations. While spacious with a crew of three, it would become increasingly crowded as the ISS Program evolved. Image credit: NASA.

Astronauts would occupy ISS-A continuously for 360 days. Four three-man crews would live and work on board for 90 days each. During crew rotations, the replacement crew would dock at the vacant MDA port and six men would temporarily inhabit ISS-A.

OART made biotechnology ISS-A's main research emphasis because its crews would need to demonstrate that astronauts could remain fit and competent throughout a 90-day stay in space. In addition, it would seek to advance medicine on Earth through the study of the human organism in novel conditions. Most of the experiments performed in the ISS series would have a similar dual purpose: that is, to advance the cause of spaceflight and to provide tangible benefits to people on Earth.

ISS-A's mission, the OART team explained, would continue and expand the biomedical research program begun on board Skylab. In addition to copies of Skylab experiment apparatus, experiment equipment launched on board ISS-A would include a 1750-pound "Manned Onboard Centrifuge" — a centrifuge large enough to spin a human — and a 1300-pound Integrated Medical and Behavioral Laboratory Measurement System (IMBLMS). The IMBLMS would be linked to operational control systems throughout ISS-A to monitor crew performance. Centrifuge, IMBLMS, and "peripheral equipment" such as a bicycle ergometer, an experiment airlock, and a sound-proofed work area would together cost $72 million.

Astronauts on board the interim stations would work 10 hours per day, six days per week. At any one time, two-thirds of the crew on an ISS would be focused on its experiment programs, while the rest would maintain systems and perform housekeeping chores. For ISS-A, this meant that, during any particular working day, two of the three astronauts on board would focus on experiments while the third served as space handyman.

Forty-five man-hours per week would be spent on IMBLMS experiments and 55 man-hours per week on centrifuge experiments. Other experiments — for example, assessment of techniques for weightless maintenance of life-support equipment intended for more than a year of continuous use — would require a total of 30 man-hours per week.

The OART team estimated that Skylab's six solar arrays and the batteries it carried for storing electricity for the night part of its orbit would produce about six kilowatts of continuous power and have a total mass of 7.5 tons. The ATM arrays and OWS arrays would each produce about half of Skylab's electricity. The team assumed that the ISS-A arrays and batteries would weigh the same as those on Skylab, but would produce between six and 10 kilowatts of continuous electricity. In the absence of an ATM, the ISS-A solar array configuration would necessarily differ from that of Skylab.

Of the stations in its series, ISS-A would most resemble Skylab. Beginning with ISS-B, larger crews and more complex experiment programs would drive evolutionary modifications to the ISS design, though all would retain the basic MDA-AM-OWS layout.

The AM would have undergone little modification from its first flight as part of Skylab to its last as part of ISS-D. Image credit: NASA.

The first three-man ISS-B crew would arrive for a 90-day stint beginning in July 1978, one-and-a-half years after ISS-A's last crew returned to Earth. A second three-man crew would reach the station a month later. The resulting six-man crew would work together for 60 days, then the first three-man crew would return to Earth. A third three-man crew would arrive almost immediately to replace them. Thirty days later, the second ISS-B crew would return to Earth and a fourth crew would replace them. The seventh three-man ISS-B crew would return to Earth in July 1979 and not be replaced, and the eighth and last three-man crew would splash down a month later, about 390 days after ISS-B reached LEO.

ISS-B's main mission would be to perform experimental Earth surveys, which the OART team placed into five multi-part categories. These were: agriculture/forestry/geography; geology/mineralogy; hydrology/water resources; oceanography; and meteorology. The station would revolve around the Earth in an orbit inclined 50° relative to the equator, so that it would pass over the "most populace [sic] and agriculturally productive areas of the Earth."

ISS-B astronauts would spend 90 man-hours per week testing, calibrating, and modifying a $40-million, 4700-pound suite of 19 experiment sensors covering the spectrum from ultraviolet through visible light to infrared and microwave. They would also continue biotechnology experiments; for example, the OART team allotted 70 man-hours per week to continuation of the IMBLMS program begun on board ISS-A.

ISS-B solar arrays and batteries would produce between seven and 15 kilowatts of continuous electricity for experiments and station operations. As with ISS-A, the OART engineers did not specify ISS-B's solar array configuration, though they implied that it would have a collecting area larger than the ISS-A configuration.

The MDA flown as part of Skylab was more more cluttered than it appears in this NASA cutaway. Skylab crews did not feel comfortable within the MDA because it lacked an obvious "up-down" orientation; no doubt the ISS MDAs would have been modified to take this into account. EREP = Earth Resources Experiment Package. ATM = Apollo Telescope Mount.

ISS-C, scheduled for launch in January 1981, and ISS-D, scheduled for launch on NASA's last Saturn V rocket in July 1983, would have many similarities. Each would have a crew complement of nine, making NASA's reliance on the three-man ISS CSM for crew rotation and resupply somewhat problematic. Somewhat surprisingly, though the OART engineers acknowledged that, based on their own NASA flight schedule, the reusable Space Shuttle would have begun flights in late 1981, they elected (for the sake of "simplicity") not to use it for ISS-C and ISS-D crew rotation and resupply.

ISS CSM launches in January, February, and March 1981 would launch ISS-C's initial nine-person crew. Only a month after its third crew arrived, its first crew would complete its 90-day stint on board the station and would return to Earth. NASA would immediately launch a fourth crew to replace them.

ISS-C and ISS-D would each receive 12 three-man crews. Each station would support nine men for 360 of the 420 days it was occupied. Flights to ISS-C and ISS-D would bring to 36 the total number of ISS CSMs and SRM-S-IVB boosters required for the program.

ISS-C astronauts would "evaluate in terms of direct Earth economic benefits the use of the space environment for materials processing and manufacture." Taking advantage of weightlessness and nearly pure vacuum, the astronauts would devote 95 man-hours per week to manufacturing large crystals, exotic composite materials, and biological compounds impossible (or at least very difficult) to create under terrestrial conditions. Manufactured materials and compounds would splash down with returning astronauts as "down" cargo in the ISS-C CMs.

The Saturn V S-IC second stage would have served as a counterweight for the ISS-C artificial-gravity experiment. Image credit: NASA.

ISS-C would also see a 45-day artificial-gravity experiment that would preempt the space exploitation experiments. The OART engineers provided few details of the experiment, though they did explain that the spent S-II second stage of the Saturn V that launched ISS-C into orbit would serve as an artificial-gravity counterweight. Probably cables would have linked the interim station and the spent stage; as the cables were slowly reeled out, thrusters on ISS-C would have fired to spin the assemblage end-over-end and keep the cables under tension. As the cables reached maximum extension, thrusters would have carefully trimmed the spin rate to ensure the desired acceleration — which the crew would feel as gravity — on board the ISS-C station.

The ISS-C/ISS-D solar array configuration would be identical to that of ISS-B; technological advancements would, however, enable their power systems to provide no less than 15 kilowatts of continuous electricity. The ISS-C and ISS-D astronauts would also evaluate Isotope Brayton nuclear power units for use on NASA's permanent Space Station.

The Isotope Brayton units would not reach space attached to ISS-C and ISS-D; rather, they would be launched separately, possibly atop Titan rockets. The OART engineers did not describe how they would rendezvous and dock with ISS-C and ISS-D. The five-ton ISS-C Isotope Brayton unit would generate six kilowatts of electricity; the more advanced six-and-a-half-ton ISS-D unit would produce 15 kilowatts, doubling that station's electrical supply.

Biotechnology experiments would continue during the ISS-C and ISS-D missions. The ISS-C biotechnology program would, of course, include assessment of the effects of spin-induced artificial gravity. With their nine-person crews, the third and fourth stations of the ISS program would be more crowded than their predecessors, offering an opportunity for study of complex human interactions aboard spacecraft.

ISS-D would include three free-flying astronomy modules in addition to astronomy instruments on the station. How the free-flyers would reach LEO was not made clear. The $50-million Cosmic Ray Physics Laboratory would weigh in at a whopping 26,700 pounds. The $125-million, 6195-pound Solar Astronomy Module would include "larger versions" of the Sun-observing instruments in the Skylab ATM. The $130-million, 6000-pound Stellar Astronomy Module would carry a telescope with a three-meter mirror. For comparison, the Hubble Space Telescope primary mirror is 2.4 meters across. Astronauts would regularly collect exposed film from the free-flying modules, though how they would reach them was not explained.

The OART engineers estimated that, by the time the last ISS-D crew returned to Earth, NASA would have accrued the equivalent of more than two years of permanent Space Station biomedical data and operations experience from its four interim stations. This would, they concluded, constitute the ISS Program's chief benefit to U.S. spaceflight; specifically, it would

enable the [permanent] Space Station to start its effective experimental usefulness almost at initial manning. . . [because] most of the human and operational uncertainties of long duration spaceflight would have been removed by. . .results [from the] four earlier interim space station flights.

Sources

Study of an Evolutionary Interim Earth Orbit Program, Memorandum Report MS-1, J. Anderson, L. Alton, R. Arno, J. Deerwester, L. Edsinger, K. Sinclair, W. Tindle, and R. Wood, Advanced Concepts and Missions Division, Office of Advanced Research and Technology, NASA Headquarters, 6 April 1971.

"Intelligence Report: Aims and Costs of the Soviet Space Station Program," SR IR 70-1-S, Directorate of Intelligence, Central Intelligence Agency, January 1970.

More Information

An Alternate Station Shuttle Evolution: The Spirit of '76 (1970)

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

Skylab-Salyut Space Laboratory (1972)

Dreaming a Different Apollo, Part One

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

Image credit: Martin Marietta.

The destruction of the Orbiter Challenger on 28 January 1986, just 73 seconds into the 25th Space Shuttle mission, put an end to many proposals and plans for Shuttle improvement and augmentation. The powerful liquid hydrogen/liquid oxygen-propelled Centaur G' upper stage, routine satellite servicing and refueling in orbit, the nitrogen-gas-propelled Manned Maneuvering Unit, launches from the U.S. West Coast, launches to polar and retrograde orbits, frequent non-astronaut passengers, solar-powered long-duration Spacelab missions, and an eventual flight rate upwards of 50 per year — all of these were abandoned as NASA sometimes reluctantly acknowledged the Shuttle's frailties and foibles.

Among the proposed improvements permanently grounded after the Challenger accident was Martin Marietta's Aft Cargo Carrier (ACC), a cargo canister meant to be bolted over the dome-shaped aft end of the Space Shuttle External Tank (ET). Martin Marietta, prime contractor for the 154-foot-long ET, had begun in-house studies of the ACC at about the time the first Shuttle launched into orbit (STS-1, 12-14 April 1981).

Aft Cargo Carrier and Orbiter payload bay dimensions compared. The entire ACC is 31.9 feet long; the aft shroud is 20.8 feet long. Image credit: Martin Marietta.

By the middle of 1982, Martin Marietta aggressively pitched the ACC concept at aerospace conferences. NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, soon took notice and contracted with the company for ACC engineering and economic feasibility studies. MSFC had since the mid-1970s sought out low-cost ways of incrementally improving the Space Shuttle and evolving NASA piloted programs toward a permanent Space Station (see "More Information" below).

The ACC's position adjacent to the Orbiter's three Space Shuttle Main Engines (SSMEs) and between the powerful twin Solid-Rocket Boosters (SRBs) meant that payloads it carried would be subjected to more heating and acoustic pounding than would those in the Orbiter payload bay. Martin Marietta proposed an ACC "environmental protection system" made up of 707 pounds of thermal insulation and a 2989-pound "acoustical barrier."

Adding these layers would make the ACC shell a little more than a foot thick, reducing the diameter of payloads it could carry to about 25 feet. Even so, this made the ACC payload volume about 10 feet wider than the 15-by-60-foot Orbiter payload bay.

Martin Marietta assumed that, with planned Shuttle performance upgrades, an Orbiter would be able to boost 36.9 tons of payload into a 160-nautical-mile-high orbit inclined 28.5° relative to Earth's equator. An empty ACC would add 8.3 tons to the Shuttle's mass at liftoff, potentially reducing the payload mass the Orbiter and ACC could inject into orbit. If the ACC remained attached until SSME cutoff, then the payload mass the Orbiter and ACC could place into orbit would total 28.7 tons.

Left to right: ACC shroud; ring for mounting cargoes; and ACC skirt with twin solid-propellant rocket motors. Image credit: Martin Marietta.

Martin Marietta had, however, found a way around this problem. The ACC would include an aft shroud and a forward skirt. Discarding the 3.7-ton aft shroud as early as possible during the Shuttle's eight-minute climb to orbit would reduce the payload mass penalty to only about four tons. This meant that the Orbiter payload bay and ACC skirt could together deliver to 160-nautical-mile orbit payloads with a total mass of 33 tons.

The twin SRBs would burn out and fall away from the ET 120 seconds after liftoff at an altitude of about 146,000 feet. The ACC shroud would then detach from the skirt and fall away 35 seconds after SRB separation.

During Orbiter-only Shuttle missions, the Orbiter would shut down its SSMEs and discard the ET before attaining orbital velocity. The ET would reenter the atmosphere and be destroyed over the Indian Ocean. This would, of course, deprive the SSMEs of their source of liquid hydrogen/liquid oxygen propellants: hence, after ET separation, the three engines would amount to "dead weight." The astronauts would then ignite the Orbiter's twin Orbital Maneuvering System (OMS) engines for the first of two orbit-insertion burns.

Orbiter/ACC missions would see Orbiter, ET, ACC skirt, and payloads in a 57-by-160-nautical-mile orbit at SSME cutoff, so that the first orbit-insertion OMS burn would be unnecessary. When the assemblage attained apogee (the highest point in its orbit around the Earth), the astronauts would ignite the OMS engines, increasing its velocity by 183 feet per second. This would raise its perigee (the low point in its orbit around the Earth) and circularize its orbit at an altitude of 160 nautical miles.

Martin Marietta proposed a host of potential ACC payloads. Many would ride on a mounting ring attached to the ACC skirt. "Catch tanks" might collect liquid hydrogen/liquid oxygen propellants left in the ET at SSME shutdown for later use in orbit. A turbine generator might burn leftover propellants to augment the electricity the Orbiter fuel cells would provide.

The ACC skirt might carry a 25-foot-diameter, 20-foot-long space station module. The module might be designed to remain attached to the ET, so that the big tank could become a strong-back for mounting large payloads or, with the addition of an access hatch linking the ET's hydrogen tank with the module, a large enclosed volume for experiments or habitation. Large folded structures — for example, an umbrella-like radio dish antenna more than 50 feet across — might also be deployed from the skirt.

Potential Aft Cargo Carrier payloads. Image credit: Martin Marietta.

Martin Marietta described three example Orbiter/ACC payload manifests and deployment scenarios. Flight 1, a mission with an initial circular 160-nautical-mile orbit at 28.5° of inclination, would see three satellites with identical solid-propellant upper stages launched in the ACC. These were the 4.4-ton Brazilsat/Payload Assist Module (PAM)-D; the 4.4-ton GOES/PAM-D; and the 4.7-ton Telsat/PAM-D. The Orbiter, meanwhile, would carry a 58-foot-long, 14-foot-diameter "large observatory" with a mass of 9.4 tons.

Without the ACC, payload mass for Flight 1 would be limited to what could be carried in the Orbiter payload bay, or about a quarter of the 36.9-ton theoretical maximum for the flight. With the ACC, the Flight 1 payload could total 22.9 tons. Following deployment from the ACC skirt, the satellites would ride their PAM-D stages to their assigned slots in the geostationary orbit (GEO) belt, 22,236 miles above the equator.

The Orbiter crew would then cast off the ET and its attached ACC skirt. A two-ton pair of solid-propellant deorbit rocket motors on the ACC skirt would ignite over the western Pacific Ocean, causing the ET/ACC combination to tumble and reenter the atmosphere. Any parts that survived reentry would splash into the Pacific south of Hawaii.

The astronauts, meanwhile, would maneuver the Orbiter to a 190-nautical-mile-high orbit and deploy the large observatory from the payload bay. They would then ignite the OMS engines to slow the Orbiter and cause it to re-enter Earth's atmosphere. The delta-winged space plane would glide to a runway landing.

Flight 2 would launch the 1.7-ton Tiros-N satellite inside the ACC and the 8.2-ton Atmosphere Monitor satellite at the aft end of the Orbiter payload bay. Because the Orbiter/ET/ACC skirt/payloads assemblage would be required to ascend to an energetically challenging 160-nautical-mile-high, 98.2° near-polar retrograde orbit, Flight 2's payload mass could total at most 11.8 tons.

The Flight 2 crew would first guide their spacecraft to a rendezvous with a two-ton Thermosat payload, which they would captured and stow at the front of the Orbiter payload bay for return to Earth. They would then fire the OMS engines to climb to a 380-nautical-mile orbit, where they would deploy the Atmosphere Monitor.

Next, they would ignite the OMS engines again to climb to a 448-nautical-mile orbit inclined 98.8° to Earth's equator. There they would deploy Tiros-N from the ACC skirt. After discarding the ET/ACC skirt, they would ignite the OMS engines to return Orbiter, crew, and Thermosat to Earth.

Aft Cargo Carrier in flight. Image credit: Martin Marietta.

Flight 3, with an initial 100-nautical-mile orbit at 28.5° of inclination and a payload mass of 26.5 tons, would see the introduction of a new reusable hardware element made possible by the ACC's large-diameter payload envelope: the 15-foot-long, 25-foot-diameter, 17-ton Orbital Transfer Vehicle (OTV). The OTV would be based in space. Visiting Orbiters would supply it with propellants and service its systems as required.

Martin Marietta noted that, by providing a second payload volume, the ACC could enable secret Department of Defense (DOD) payloads to be carried separate from but on the same flight as NASA civilian payloads. The Orbiter payload bay would thus on Flight 3 carry two Department of Defense payloads: the NATO IV/PAM-D satellite and the 35-foot-long, 10-foot-wide, 6.5-ton Synchronous Observation Satellite (SOS).

The OTV would scavenge residual ET propellants to fill its tanks, then would detach from the ACC skirt. The Orbiter crew, meanwhile, would raise the SOS on a tilt-table mounted in the payload bay. The OTV would dock with the SOS, detach it from the tilt-table, boost it to its assigned slot in GEO, and release it. Mission accomplished, the OTV would fire its engines to return to low-Earth orbit for a new mission.

The Orbiter crew, meanwhile, would cast off the ET/ACC skirt and maneuver to a 160-nautical-mile orbit, where they would deploy NATO-IV/PAM-D from the payload bay. The PAM-D stage would boost the satellite to GEO. The astronauts, meanwhile, would fire the Orbiter's OMS engines to re-enter Earth's atmosphere.

Martin Marietta placed great emphasis on the cost savings that would accrue from making the ACC a Shuttle hardware element. First, however, it estimated the costs of developing and using the cargo canister. The company assumed that NASA would give a green light to begin ACC development in late 1983, and that the first ACC would lift off three years later.

The company calculated that ACC development would cost $113 million. Changes to the Shuttle design to accommodate ACCs would cost $78 million, and changes to Kennedy Space Center facilities would cost $35 million.

Martin Marietta quoted NASA when it placed the base cost of a Shuttle flight without an ACC at an optimistic $75 million. The base cost of a Shuttle flight would increase by about $5 million when it included an ACC, the company estimated.

For its cost-savings calculations, the company employed a Shuttle traffic model less optimistic than the one NASA touted. It assumed that 331 Shuttle flights would take place between 1988 and 2000, with 34 flights in 1988 and a steady decline to 20 flights per year in 2000. During the same 12-year period, NASA assumed 26 flights per year in 1988, an upward trend to nearly 60 flights per year by 2000, and a total of 581 flights.

Based on its "low" traffic model, Martin Marietta estimated that NASA might benefit from flying 71 civilian and 35 Department of Defense Shuttle/ACC missions. The company conservatively assumed, however, that NASA would be able to fund only a total of 75 civilian and Department of Defense Orbiter/ACC flights.

Martin Marietta determined that the added payload capacity the ACC could provide would permit the elimination of 40 Orbiter-only Shuttle missions. It placed the cost of 331 Orbiter-only missions at $24.8 billion and the cost of 216 Orbiter-only and 75 Orbiter/ACC missions at $22.2 billion. The ACC would thus save NASA $2.6 billion over 12 years.

Sources

Space Transportation System with Aft Cargo Carrier: A Natural Augmentation of System Capability, Martin Marietta, no date (late 1982).

"External Tank Aft Cargo Carrier," T. Mobley and J. Hughes; paper presented at the Twentieth Space Congress, Cocoa Beach, Florida, 26-28 April 1983.

ACC External Tank Aft Cargo Carrier, Martin Marietta, no date (late 1985).

More Information

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

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

Humans on Mars in 1995! (1980-1981)

An Alternate Station/Shuttle Evolution: The Spirit of '76 (1970)

North American Rockwell concept of a fully reusable Space Shuttle. Image credit: NASA.

NASA Administrator Thomas Paine was optimistic that national enthusiasm for Apollo 11, the first piloted lunar landing, would translate into national support for an expansive future NASA program. He viewed a 12-person Space Station served by a reusable Space Shuttle as the necessary first step toward such a future for the U.S. civilian space agency.

On 22 July 1969 — the day after Apollo 11 astronauts Neil Armstrong and Edwin Aldrin lifted off from the Moon in the Ascent Stage of the Lunar Module Eagle — NASA awarded Phase B Space Station study contracts to McDonnell Douglas Aerospace Company (MDAC) and North American Rockwell (NAR). Each contractor led a team of subcontractors, so in all as many as 30 aerospace companies were involved in executing the Phase B studies. Marshall Space Flight Center in Huntsville, Alabama, directed the MDAC contract and the Manned Spacecraft Center in Houston, Texas, directed NAR.

I described the MDAC study in considerable detail in a March post (see the links at the bottom of this post). NAR's 12-person Space Station was, like the MDAC Station, meant to reach low-Earth orbit atop a two-stage Saturn V rocket in 1975 and operate for up to 10 years. The barrel-shaped 33-foot-diameter, 50-foot-tall Station would be ready for staffing as soon as it deployed automatically in 270-nautical-mile-high orbit.

Plan drawing of North American Rockwell's Phase B Space Station. Image credit: North American Rockwell/NASA/David S. F. Portree.

The Phase B Station would comprise four pressurized living decks. Deck 1 would include a galley and a wardroom with recreation equipment and seating for 12, a sickbay with space medicine research equipment, two "neuter" docking units, and two observation portholes set into one of the round docking port hatches. Deck 2 would include individual staterooms for six astronauts (a large cabin with an office for the Station Commander and smaller cabins for crew members), a personal hygiene compartment including a full-body shower, and the Primary Control Center (PCC).

Deck 3 would resemble Deck 2, except that its large stateroom would be set aside for the Chief Science Investigator and a repair shop would replace the PCC. Deck 4, the laboratory deck, would include experiment equipment for eight major scientific disciplines, an airlock with an extendable boom for exposing experiments to space, a small backup control center, and two docking units.

Image credit: North American Rockwell/NASA.

Image credit: North American Rockwell/NASA.

Image credit: North American Rockwell/NASA.

Image credit: North American Rockwell/NASA.

To enhance crew safety, Decks 1/2 and Decks 3/4 would comprise a pair of independent, redundant living volumes. If a fire broke out and burned out of control on Deck 4, for example, the Station crew would evacuate to the Deck 1/2 volume through an "inter-volume airlock" adjacent to the repair shop on Deck 3. They would seal off the damaged volume and call for help from Earth.

Upper and lower equipment bays atop Deck 4 and below Deck 1, respectively, would each contain a conical "tunnel" airlock, a pressurized "torus" ring for storing supplies and spare parts, and an section open to vacuum containing spherical storage tanks for life-support gases and liquids. The equipment bays would also house propellant tanks supplying the Phase B Station's attitude-control thrusters. Spacewalking astronauts would leave the station through the lower tunnel. Rectangular openings in the Deck 1 floor and the Deck 4 ceiling would enable astronauts to enter the storage rings.

Image credit: North American Rockwell/NASA.

Image credit: North American Rockwell/NASA.

NASA envisioned that the Phase B Space Station design would, with minimal modifications, serve as a "building block" module for advanced space projects. Multiple modules might, for example, be stacked and clustered to form a 100-man Earth-orbiting Space Base. A single module — perhaps cut down to two decks — might serve as a lunar-orbital Space Station or Mars spacecraft crew module.

After considering Brayton-isotope and nuclear-reactor power systems, NAR settled on solar power for its Phase B station. The company explained its decision by noting that a Brayton-isotope power source would provide adequate electricity for the 12-person Station but could not be scaled up to serve a Space Base; by the same token, a reactor capable of supplying a Space Base would be too large and complex to efficiently power a 12-person Station.

A cylindrical "power boom" would carry four rolled-up advanced-design steerable solar arrays. A total of 10,000 square feet of solar cells would generate 25 kilowatts of electricity. The rotating boom would reach space attached to a port atop the upper equipment bay. The upper bay's conical tunnel would lead through a hatch into the hollow power boom.

The major technical challenge of the Phase B Station was, NAR explained, its anticipated long lifetime in orbit. The company invoked detailed on-board subsystems monitoring, subsystems designed for maintainability, easy subsystems accessibility, and a large on-board stockpile of spare parts as solutions to the Station lifetime problem.

Astronauts would reach NAR's orbiting Station on board fully reusable Space Shuttle Orbiters. The Shuttle's mission would be to economically change out Station crews, replenish supplies, and deliver scientific equipment and other cargo. Most of the many Shuttle designs under study by mid-1970 comprised a winged, piloted Booster and a winged, piloted Orbiter; the latter would include a cylindrical payload bay that could be opened to space.

The payload bay would be sized to transport standardized cylindrical modules. Most commonly carried would be the cargo/crew transfer module; other modules would arrive at the Station's five ports outfitted as specialized laboratories, instrument carriers, or free-flyers.

Upon achieving orbit, the Orbiter crew would open the payload bay doors and activate a mechanism that would pivot the module it carried onto a neuter docking port on the Orbiter crew cabin roof. The Orbiter would then dock with the Station using the neuter docking unit on the other end of the module. Astronauts would enter and depart the Station through the module. When time came to leave the Station, the Orbiter would undock from the module, leaving it attached to the Station, or would undock the module from the Station and pivot it back into the payload bay for return to Earth.

A Space Shuttle Orbiter docks with the NAR Phase B Space Station using a module deployed from its payload bay and linked to the docking port atop its crew cabin. Image credit: North American Rockwell.

On 28 July 1970, a little more than a year after NASA awarded the Phase B Station contracts, Administrator Paine resigned effective 15 September. The next day (29 July), NASA instructed MDAC and NAR to examine Stations that could be launched in pieces in Space Shuttle Orbiter payload bays and assembled in space. This marked the continuation of a gradual shift toward a "phased" Station/Shuttle Program.

The phased approach, a response to deep cuts in NASA funding, would postpone Station development until late in the Shuttle development phase, when Shuttle development costs would wind down, or until after the first few Shuttle orbital flights. In either case, the Shuttle would begin operational flights before it began to launch Space Station modules into orbit.

Shortly after the 29 July directive, NAR engineers offered an alternative to the Shuttle-first phased approach. In a brief presentation titled "Spirit of '76," they proposed that NASA postpone Shuttle development and instead in 1976 launch a prototype Phase B Station on a two-stage Saturn V.

The Station-first phased approach was, they argued, superior to the Shuttle-first phased approach because the Shuttle would demand a much greater technological leap than would the Station. This meant that it might hit development roadblocks that would increase its estimated cost and delay its first launch (as indeed did happen). In addition, the Spirit of '76 Station could better address the emerging post-Apollo space priorities of President Richard Nixon. These included international space cooperation and direct benefits to people on Earth.

NAR's Spirit of '76 Station was outwardly very similar to NAR's Phase B Station. Differences included less advanced, smaller solar arrays capable of generating 20 kilowatts of electricity and docking ports of the Apollo passive drogue design. The Spirit of '76 Station would support a smaller crew complement (normally six astronauts — nine during crew rotation) and have a rated lifetime of 72 man-months instead of 10 calendar years. The latter attribute would largely eliminate the technical challenges of building for a long lifetime in orbit.

Like the Phase B Station, the Spirit of '76 Station would circle the Earth in an orbit inclined 55° relative to the equator, causing it to overfly nearly all inhabited regions. Earth observations, NAR claimed, would yield improved weather forecasts that would save the U.S. $2.5 billion per year (how this figure was calculated was not explained). Spirit of '76 crews would also "patrol" for storms, research "weather modification," seek geothermal energy sources and "new sources of dwindling resources," watch out for crop diseases and water pollution, predict earthquakes, and improve "sea food production."

Besides Earth observations, the Spirit of '76 astronauts would conduct "aerospace medicine" experiments. In keeping with the goal of benefits for people on Earth, many of these would aim to discover the healing potential of the "benign space environment" and seek new and improved "medical diagnostic and treatment techniques." Other experiments would assess and develop countermeasures for spaceflight effects on humans.

The Spirit of '76 presentation bears no date, but its projected 1970s NASA flight schedule indicates that it was prepared in August 1970 — that is, after NASA directed the Phase B contractors to study Shuttle-launched Stations (29 July) but before Paine canceled two Apollo missions (2 September). It has Apollo 18, the final piloted Moon mission, leaving Earth in the second quarter of 1974. For reasons not immediately clear, Apollo 19 is not shown on the NAR schedule. As it turned out, Apollo 17 was the last piloted lunar flight; it flew in December 1972.

NAR expected that the Skylab Program, precursor to the Spirit of '76 Program, would take place between Apollo 17 and Apollo 18. The Orbital Workshop, a converted Saturn S-IVB rocket stage, would reach Earth orbit in the last quarter of 1972, and the last of its three crews would return to Earth in mid-1973. In reality, Skylab did not reach orbit until May 1973 and its last crew did not return to Earth until February 1974.

After no NASA piloted flights in 1975, the Spirit of '76 Station would reach Earth orbit early the following year. As its name implies, it would be staffed during the U.S. Bicentennial festivities on 4 July 1976. The orbiting Station would stand as a "source of national pride" as the United States celebrated its 200th birthday.

Unlike Skylab, which would operate unstaffed between crews, the Spirit of '76 Station would be staffed continuously after its first crew arrived early in the second quarter of 1976. Four consecutive three-person crews would launch to the Station for overlapping six-month stays.

Apollo 7 Saturn IB rocket lifts off. Visible below the silver-and-white Command and Service Module (CSM) is the tapered Spacecraft Launch Adapter (SLA). Image credit: NASA.

This artist concept from 1966 shows a CSM turning end-over-end to dock with and extract an Apollo Lunar Module (LM) from the top of a spent S-IVB stage. Note the four partially open SLA segments; these would protect the LM during ascent through Earth's atmosphere. NAR envisioned that Spirit of '76 Cargo Modules would also ride to orbit within the SLA attached to the top of an S-IVB stage. Image credit: NASA.

In the absence of a Shuttle Orbiter, NAR invoked two-stage Saturn IB rockets and modified Apollo Command and Service Module (CSM) spacecraft as its Spirit of '76 crew transports. Cargo would reach the Station inside modules carried within the tapered Saturn Launch Adapter (SLA) which linked the CSM to the top of the Saturn IB S-IVB second stage.

After CSM separation, the SLA's four petal-like segments would fold open, exposing a Cargo Module with an Apollo drogue docking unit on top. The CSM, which carried an active probe docking unit on its nose, would link up with the drogue unit and detach the Cargo Module from the top of the S-IVB stage. The crew would then ignite the CSM's Service Propulsion System main engine to begin maneuvering to Station altitude. They would dock with the Spirit of '76 Station using an Apollo probe unit on the bottom end of the Cargo Module. All four crews would arrive at the Spirit of '76 Station with a Cargo Module. At least one Cargo Module would be outfitted as a instrument carrier: after docking with the Station it would deploy cameras, a radar, and other sensors for Earth observations.

The second crew would arrive three months after the first crew arrived, increasing the Spirit of '76 crew complement to six. Three months later, at the end of their six-month stay in space, the first crew would depart and the third crew would replace them. The fourth crew would replace the second crew three months after that; then, having completed their six-month stint, the third crew would return to Earth three months later, leaving the three astronauts of the fourth crew to finish up the planned experiment program and mothball the Station. Late in the third quarter of 1977 they would undock in their CSM, reenter, and splash down, ending the Spirit of '76 Program.

NAR offered two funding models for the Spirit of '76 Station. Both would require a $2.3-billion Spirit of '76 Station, four Cargo Modules at a cost of $9 million each, and $220 million for experiments.

The first funding model, with a cost of $2.8 billion spread over six years, assumed use of re-purposed or leftover Apollo and Skylab rockets and spacecraft. It would see the CSMs built for Apollo 18 (designated 114) and Apollo 19 (115) diverted from the lunar program. Along with the Skylab backup/rescue CSM (119) and 115A, which was committed to no program, they would be converted into Spirit of '76 Station ferries. Ending Apollo with Apollo 17 would free up two Saturn V rockets (514 and 515, the last remaining of the original Apollo Program buy), one of which would launch the Spirit of '76 Station (the other, presumably, would launch Skylab). The four CSMs would reach Earth orbit on the last remaining Saturn IBs (designated 209, 210, 211, and 212).

NAR's other Spirit of '76 funding model, with a total cost of $3.1 billion, would see lunar missions continue through Apollo 19 in the fourth quarter of 1974. NASA would buy two new CSMs (120 and 121) and convert 119 and 115A for the Spirit of '76 program. To trim costs, 120 and 121, which would launch the third and fourth crews, might include the 119 and 115A Command Modules; NAR envisioned refurbishing them after they returned the first and second crews to Earth. A new two-stage Saturn V (516) for launching the Spirit of '76 Station would cost $260 million including launch operations.

Artist concept of Space Shuttle Orbiter with Saturn V S-IC first stage. Image credit: NASA.

If all of its many technological challenges could be met successfully, the first Space Shuttle would soar into orbit in the first quarter of 1978. Early in its career, it would take the form of a reusable Orbiter launched atop an expendable Saturn V S-IC first stage. NAR suggested that the Spirit of '76 Station might be revived and become a destination for Shuttle Orbiters during this period. Through phased development, NASA would soon replace the S-IC stage with a reusable winged Booster, then a Shuttle-launched modular station would be assembled in Earth orbit sometime in the 1980s.

Ironically, Rockwell International — formerly North American Rockwell — became the Space Shuttle prime contractor. The company that argued that a Space Station should be developed first because Space Shuttle development would be fraught with technical challenges thus became responsible for tackling the challenges of building the Shuttle.

Sources

"Spirit of '76," North American Rockwell Space Division, undated presentation (August 1970).

Space Station Program, North American Rockwell Space Division, Briefing to the European Space Research Organization on Space Station Plans and Programs in Paris, France, 3-5 June 1970.

Astronautics and Aeronautics 1970, NASA SP-4015, pp. 193-194.

Space Stations: A Policy History, J. Logsdon, George Washington University, NASA Contract NAS9-16461, NASA Johnson Space Center, no date (1980), pp. I-16, II-1-5, II-8-10, II-13-15, II-18-33.

More Information

McDonnell Douglas Phase B Space Station (1970)

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

A Forgotten Rocket - The Saturn IB

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

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)