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

The Skylab 2 Apollo CSM and Saturn IB launcher stand ready atop the "milk stool" on Pad 39B at NASA's Kennedy Space Center, May 1973. Image credit: NASA.

From its conception in 1959 until President John F. Kennedy's 25 May 1961 call to put a man on the Moon, Apollo was seen mainly as an Earth-orbital spacecraft. NASA intended to use Apollo in the second and third phases of its planned 1960s piloted space program. The first phase, characterized by suborbital flights lasting minutes and sorties into Earth orbit lasting at most a few days, would be accomplished by brave pioneers in missile-launched single-seater Mercury capsules.

In the second phase, three astronauts would live and work on board Apollo spacecraft for ever-longer periods. They would use a pressurized Mission Module (MM) launched attached to their spacecraft as a small space station. The third phase would see Apollo spacecraft transport crews to and from an Earth-orbiting space station. Cargo bound for the station would ride in the MM. An Apollo circumlunar mission — a flight around the Moon without capture into lunar orbit — was an option, but was considered unlikely before 1970.

Simplified cutaway of the General Electric D-2 Apollo, perhaps the best known of the pre-Moon program Apollo designs. Colored lines represent separation planes: orange is the spacecraft/launch vehicle separation plane; red is the abort separation plane (two "pusher" solid-propellant abort rockets are visible on the outside of the Service Module); green is the shroud/Service Module separation plane; and blue is the Mission Module/Command Module separation plane. In the event of a launch abort, the part of the Service Module to the right of the red line would remain attached to the launch vehicle. During reentry, the spacecraft would first split along the green line, then the Command Module would separate from the Mission Module along the blue line. The shroud covering the Command Module during flight would remain with the Mission Module. The Command Module would lower on parachutes and perform a land landing while the Service Module and Mission Module/shroud would both burn up. Image credit: General Electric/DSFPortree.

Following studies that lasted six months, in mid-May 1961 General Electric (GE), The Martin Company, and Convair submitted Apollo spacecraft designs suited to NASA's three-phase plan. In the event, none of the designs left the drawing board; after Apollo became NASA's lunar landing mission spacecraft, the agency funded new studies and selected North American Aviation (NAA) as its Apollo spacecraft contractor.

Initially, NASA intended to land NAA's Apollo on the Moon atop a descent stage with landing legs. In July 1962, however, after more than a year of sometimes acrimonious debate, the space agency selected Lunar Orbit Rendezvous (LOR) as its lunar landing mission mode. NAA's Command and Service Module (CSM) spacecraft became the LOR mission's Moon-orbiting mother ship, and to Grumman's bug-like Lunar Module (LM) went the honor of landing on the Moon.

Before the Lunar Module. Image credit: NASA.

As flown, the CSM, which measured a little more than 11 meters long, comprised the conical Command Module (CM) and the drum-shaped Service Module (SM). The MM of the May 1961 GE, Martin, and Convair designs was judged unnecessary for lunar landing missions. In fact, at first some sources perceived the LM to be the MM's replacement.

The CM's nose carried a probe docking unit, and at the aft end of the SM was mounted the Service Propulsion System (SPS) main engine. The SPS remained sized for CSM launches from the lunar surface, which meant that it was more powerful than necessary for CSM insertion into and escape from lunar orbit.

Technical details of the Apollo Command and Service Module (CSM) spacecraft configured for lunar missions. Image credit: NASA.

The CM also included a pressurized crew compartment, crew couches, flight controls, a compact guidance computer, rendezvous aids, a bowl-shaped heat shield for Earth atmosphere reentry, reentry batteries, and parachutes for descent to a gentle splashdown at sea.

The SM, which was discarded before atmosphere reentry, included propellant tanks, fuel cells for making electricity and water, fuel cell reactants (liquid oxygen and liquid hydrogen), four attitude-control thruster quads, radiators for discarding excess heat generated by on board systems, a high-gain radio antenna, and room for a Scientific Instrument Module (SIM) Bay. An umbilical beneath a streamlined housing linked CM and SM.

Almost all piloted Apollo Earth-orbital missions were launched atop two-stage Saturn IB rockets. The sole exception was Apollo 9 (3-13 May 1969), which used NASA's fourth Saturn V. All Apollo lunar missions left Earth on three-stage Saturn V rockets.

Apollo 7 and Apollo 9 were test flights, so their CSMs operated exclusively in low-Earth orbit. This image shows the CM of the Apollo 9 CSM Gumdrop as viewed from the LM Spider in May 1969. Apart from thruster quads and antennas, very little of Gumdrop's SM is visible. No other Apollo spacecraft would operate only in low-Earth orbit until the Skylab 2 CSM flew in May 1973. Image credit: NASA.

The United States began to abandon the technology of piloted lunar exploration by late 1967, nearly a year before the first astronauts reached Earth orbit in an Apollo CSM (Apollo 7, 11-22 October 1968). Abandonment of the Moon began with deep cuts in the Apollo Applications Program (AAP), the planned successor to the Apollo lunar program. Ambitious two-week stays on the Moon were among the first AAP missions to feel the budget-cutters' blades.

In early 1970, NASA brought together the parts of AAP that survived — several space station-related Earth-orbital missions — to form the Skylab Program, which was expected to include at least one and possibly two temporary Skylab Orbital Workshops. The first, Skylab A, was meant to receive at least three Apollo CSMs, each bearing a three-man crew, over a period of about nine months.

By late 1970, with just two Apollo Moon landings (Apollo 11 and Apollo 12) and the Apollo 13 accident under its belt, NASA cancelled three lunar landing missions. Apollo 20, the planned final Apollo lunar mission, was cancelled in early 1970 to free up its Saturn V rocket to launch Skylab A. Apollo 15, the planned fourth and last walking mission, was cancelled in September 1970, as was Apollo 19. NASA Administrator Thomas Paine dropped the missions at least partly in an attempt to to gain President Richard Nixon's support for a large permanent space station. The space agency renumbered the surviving missions so that Apollo lunar exploration would end with Apollo 17.

On 27 August 1971, Philip Culbertson, director of the Advanced Manned Missions Program Office at NASA Headquarters in Washington, DC, dispatched a letter to Rene Berglund, Manager of the Space Station Project Office at NASA's Manned Spacecraft Center (MSC) in Houston, Texas. In it, he outlined five Earth-orbital CSM missions for the 1970s that were "still under active consideration" at NASA Headquarters.

Culbertson explained that his letter was meant to "emphasize the importance" of statements he had made in a telephone conversation with Berglund on 19 August. Based on his letter, Culbertson had phoned Berglund in an effort to impress on him the seriousness of NASA's budget situation.

Space Base: a large permanent Space Station, c. 1980. The nuclear-powered station, shown here passing over Australia and New Guinea, would have had a crew of from 50 to 100 persons. Image credit: NASA.

Berglund and his predecessor at MSC, Edward Olling, had throughout the 1960s remained staunch advocates of a large permanent Earth-orbiting space station. MSC Director Robert Gilruth was also a station supporter. They regarded AAP as at best a not-too-necessary rehearsal for a space station; they saw it at worst as a waste of time and money. They anticipated that before the mid-1970s AAP would draw to a close, freeing up funds for a real space station.

By mid-1971, however, it was increasingly obvious that a permanent space station was of interest neither to Nixon's White House nor the Congress. In fact, a reusable space station logistics resupply and crew rotation vehicle — a Space Shuttle — was by then emerging as the preferred post-Apollo program. The space station — if it were built at all — would have to wait until the Shuttle could launch its modules and bring them together in Earth orbit.

Culbertson referred to an unspecified new contract MSC had awarded CSM contractor North American. He told Berglund that, in "the early stages of your contract. . .you should concentrate on defining the CSM modifications required to support each of the [five] missions and possibly more important defining the effort at North American which would hold open as many as possible of the options until the end of the [Fiscal Year] 1973 budget cycle." Fiscal Year 1973 would conclude on 1 October 1973.

Culbertson's five missions were all to some degree station-related. The first and simplest was an "independent CSM mission for earth observations." Earth observation by astronauts was often mentioned as a space station justification. The mission's CSM would probably include a SIM Bay fitted out with remote-sensing instruments and cameras. At the end of the mission, an astronaut would spacewalk to the SIM Bay to retrieve film for return to Earth in the CM.

A SIM Bay was part of the final three Apollo lunar CSMs. The image above shows the Apollo 15 CSM Endeavour in lunar orbit with its rectangular SIM Bay (upper center) open to space. Image credit: NASA.

The second mission on Culbertson's list was an Apollo space station flight that would have been almost unimaginable at the time Kennedy diverted Apollo to the Moon. It would see a CSM dock in Earth orbit with a Soviet Salyut station.

Salyut 1, the world's first space station, had reached Earth orbit on 19 April 1971. The 15.8-meter-long station remained aloft as Culbertson wrote his letter, but had not been manned since the Soyuz 11 crew of Georgi Dobrovolski, Viktor Patseyev, and Vladislav Volkov had undocked on 29 June 1971, after nearly 24 days in space (at the time, a new world record for human space endurance). The three cosmonauts had suffocated during reentry when a malfunctioning valve caused their capsule to lose pressure, so the Soviet Union halted all piloted missions while the Soyuz spacecraft was put through a significant redesign.

The third Earth-orbital CSM mission on Culbertson's list combined the first two missions. The CSM crew would turn SIM Bay instruments toward Earth before or after a visit to a Salyut.

Culbertson's fourth CSM mission would see CSM-119 dock first with a Salyut for a brief time, then undock and rendezvous with the dormant Skylab A Orbital Workshop. After docking with and reviving Skylab A, CSM-119's crew would live and work on board for an unspecified period.

Image credit: NASA.

NASA planned that, during the three CSM missions to Skylab A in the basic Skylab Program, CSM-119 would stand by as a rescue vehicle capable of carrying five astronauts (Commander, Pilot, and the three rescued Skylab A crewmen). The Salyut-Skylab A mission, which would include no rescue CSM, was planned to begin 18 months after Skylab A reached orbit, or about nine months after the third Skylab A mission returned to Earth.

The fifth and final Earth-orbital CSM mission was really two (or possibly three) CSM missions. A pair of "90 day" CSMs would dock with the Skylab B station while a rescue CSM modified to carry five astronauts stood by. NASA had funded partial assembly of Skylab B so that it would have a backup in the pipeline in case Skylab A failed. Reflecting uncertainty about the availability of Saturn rockets and CSMs, Culbertson gave no date for the Skylab B launch.

Of the five missions Culbertson declared to be on the table in August 1971, none flew. In January 1972, Nixon called on Congress to fund Space Shuttle development, and Congress agreed. Shuttle costs and continued NASA budget cuts pushed even the least complex and cheapest of Culbertson's five missions off the table.

For a short time, his second mission looked to be within reach. Formal joint U.S./U.S.S.R. planning for an Apollo docking with a Salyut was under way when Culbertson wrote his letter. In early April 1972, however, shortly before finalizing its agreement with NASA to conduct a joint Apollo-Salyut mission, the Soviet Union declared the concept to be impractical and offered instead a docking with a Soyuz.

NASA was disappointed to lose an opportunity for an early post-Skylab space station visit; the Nixon White House, on the other hand, saw the mission as a poster child for its policy of detente with the Soviet Union, so any sort of piloted docking mission would do. At the superpower summit in Moscow on 24 May 1972, Nixon and Soviet Premier Alexei Kosygin signed the agreement creating the Apollo-Soyuz Test Project (ASTP).

Skylab A, re-designated Skylab 1 (but more commonly called simply Skylab), reached orbit on 14 May 1973 on a two-stage Saturn V. It suffered damage during ascent, but NASA and its contractors pulled it back from the brink.

Skylab in a photograph taken by the second crew to live on board. Signs of damage the Orbital Workshop suffered during ascent to low-Earth orbit are obvious: one solar array wing is missing (left) and a hastily improvised solar shield stands in for the reflective meteoroid shield that would have protected Skylab's crew volume from the Sun. Image credit: NASA.

The three CSM missions to Skylab spanned 25 May-22 June 1973, 28 July-25 September 1973, and 16 November 1973-8 February 1974, respectively. Leaks in attitude control thrusters on the second CSM to dock with Skylab caused NASA to ready CSM-119 for flight, going so far as to roll it and its Saturn IB rocket out to the launch pad; the leaks stopped by themselves, however, so the rescue CSM remained earthbound.

In August 1973, with Skylab functioning well in Earth-orbit, NASA began to mothball its backup. Several plans were floated for putting Skylab B to use in Earth orbit. In December 1976, however, NASA turned the second Skylab over to the newly opened Smithsonian National Air and Space Museum on the National Mall in Washington, DC.

Apollo CSM-111 was the ASTP prime spacecraft, while CSM-119 was refitted to serve as its backup. In the event, the backup was not needed. CSM-111, officially designated "Apollo" (but sometimes informally called Apollo 18), docked with Soyuz 19 on 17 July 1975. CSM-111 did not include a SIM Bay. The last CSM to reach space undocked on 19 July and, after a period during which its crew performed experiments in the CM, splashed down in the Pacific Ocean near Hawaii on 24 July 1975, six years to the day after Apollo 11, the first Moon landing mission, returned to Earth.

Artist concept of the Apollo-Soyuz docking in Earth orbit, 17 July 1975. Image credit: NASA.

Sources

A Summary of NASA Manned Spacecraft Center Advanced Earth Orbital Missions Space Station Activity from 1962 to 1969, Maxime Faget and Edward Olling, NASA Manned Spacecraft Center, February 1969.

Letter, Philip E. Culbertson to Rene A. Berglund, 27 August 1971.

Skylab News Reference, NASA Office of Public Affairs, March 1973, pp. IV-6 - IV-8.

Living and Working in Space: A History of Skylab, NASA SP-4298, W. David Compton and Charles Benson, NASA, 1983.

Thirty Years Together: A Chronology of U.S.-Soviet Space Cooperation, NASA CR 185707, David S. F. Portree, February 1993, pp. 9-26 (http://ntrs.nasa.gov/search.jsp?R=19930010786 — accessed 10 May 2017).

Mir Hardware Heritage, NASA RP 1357, David S. F. Portree, March 1995, pp. 33-35, 65-72 (http://history.nasa.gov/SP-4225/documentation/mhh/mhh.htm — accessed 10 May 2017).

"Skylab B: Unflown Missions, Lost Opportunities," Thomas Frieling, Quest, Volume 5, Number 4, 1996.

More Information

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter

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

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

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

Skylab-Salyut Space Laboratory (1972)

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

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

Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)

The authors of the dual-stopover study did not design a spacecraft. The 6.4-year cycle of mission opportunities they identified repeats endlessly, however, so the NASA image above, which shows a present-day design for a piloted Mars spacecraft, can be pressed into service to illustrate this post. With relatively minor changes, this spacecraft might orbit both Mars and Venus during a single mission. 

The piloted flyby missions NASA studied in the 1960s often included close encounters with both Mars and Venus. The October 1966 NASA Planetary Joint Action Group report Planetary Exploration Utilizing a Manned Flight System, for example, emphasized a piloted Mars flyby mission departing Earth orbit during the September 1975 free-return opportunity, but also noted an opportunity to launch a Earth-Venus-Mars-Venus-Earth flyby in February 1977 and an Earth-Venus-Mars-Earth flyby in December 1978.

Piloted flybys in the 1970s were intended to clear a path to piloted "stopover" missions in the 1980s. Stopovers — a category which included Mars and Venus orbiters and Mars landings — almost always emphasized a single objective. That is, each mission would travel to a single world, then return to Earth. The closest stopovers came to visiting more than a one planet was when a Mars stopover mission performed a Venus "swingby" to bend its course, slow its approach to Earth to enable a safe direct Earth-atmosphere reentry, or accelerate toward Mars.

During a Venus swingby, a Mars stopover spacecraft might explore the cloudy planet much as piloted Venus flybys were meant to do. That is, it might drop off probes insulated and armored against Venusian temperatures and pressures and scan the hidden Venusian surface with radar.

That a piloted spacecraft might stop at both Mars and Venus during a single mission was unthinkable. It was widely accepted that such a mission would demand enormous quantities of propellants, all of which would need to be launched into Earth orbit atop costly heavy-lift rockets.

In a brief September 1969 NASA Technical Memorandum, E. Willis and J. Padrutt, mathematicians at NASA Lewis Research Center (LeRC) in Cleveland, Ohio, sought to overturn the prevailing view of what would be possible during stopover missions. Lead author Willis was no stranger to NASA piloted Mars mission planning: he had designed interplanetary trajectories at LeRC at least since early 1963.

Willis and Padrutt's mission design would see a piloted spacecraft depart a circular low-Earth orbit and capture into an loosely bound high-apoapsis (that is, orbit high point) elliptical orbit around Mars or Venus. It would then transfer to a loosely bound high-apoapsis elliptical orbit around Mars (if the first stopover were at Venus) or Venus (if the first stopover were at Mars). From there, the spacecraft would transfer back to Earth, where the crew would reenter the atmosphere directly in a small capsule. Its usefulness ended, the dual-stopover spacecraft would, meanwhile, swing past Earth into a disposal orbit around the Sun.

The mission plan was designed to reduce the amount of energy required to move between worlds, thus conserving propellant. The piloted dual-stopover spacecraft would travel between planets only when an opportunity for a minimum-energy transfer occurred; that is, only when the planets moved into positions relative to each other necessary for a minimum-energy transfer. Loosely bound orbits would reduce energy needed to capture into and escape from orbit. Direct reentry into Earth's atmosphere would ideally require only enough energy to deflect the capsule's course so that it would intercept Earth after it separated from the dual-stopover spacecraft.

The LeRC mathematicians calculated the total "propulsive effort" necessary to carry out the seven dual-stopover missions in the 1979-1986 cycle. They measured propulsive effort in terms of the total velocity change firing the dual-stopover spacecraft's rocket motor or motors would produce. Propulsive effort would expend precious propellants, so most of the time small velocity changes were to be preferred over large ones.

They explained that they had discovered a repeating 6.4-year cycle of seven potentially useful dual-stopover mission opportunities. The seven opportunities varied only slightly from one 6.4-year cycle to the next. The first, fourth, and sixth opportunities would begin with an Earth-Mars transfer, while the second, third, fifth, and seventh would begin with an Earth-Venus transfer. In most cases, the minimum propulsive effort needed to perform Earth-Venus-Mars-Earth dual-stopovers would be less than that needed for Earth-Mars-Venus-Earth dual-stopovers. In their paper, Willis and Padrutt emphasized the 6.4-year cycle that would begin in late 1979.

A hand-drawn illustration from Willis and Padrutt's NASA Technical Memorandum outlines the dual-stopover mission beginning in late 1979. 1 = departure from circular low-Earth orbit on a minimum-energy path to Mars. 2 = Arrival in high-apoapsis elliptical Mars orbit at the beginning of a 78-day stopover during which Mars's position changes as it orbits the Sun. 3 = Mars departure on a minimum-energy path to Venus. 4 = Arrival in high-apoapsis elliptical Venus orbit at the beginning of a 177-day stopover during which Venus's position changes as it orbits the Sun. 5 = Venus departure on a minimum-energy path to Earth. 6 = Arrival at Earth. Image credit: NASA.

A spacecraft launched during the late 1979 dual-stopover mission opportunity would spend 78 days at Mars and 177 days at Venus. During each stopover, the planet would orbit the Sun, eventually reaching the correct position to enable the spacecraft to make a minimum-energy transfer to its next destination planet. The Earth-Mars, Mars-Venus, and Venus-Earth tranfer legs of its voyage would together require 638 days. Adding the time spent at Mars and Venus to the time spent between worlds would yield a mission duration of 894 days — that is, slightly less than two and a half years. Total propulsive effort would amount to 9.382 kilometers per second (kps).

The second opportunity of the 6.4-year cycle would occur in the first half of 1980. The dual-stopover spacecraft would depart Earth on a minimum-energy path to Venus. It would spend 180 days at Venus, 10 days at Mars, and 669 days between worlds, for a total mission duration of 860 days (two and a third years). This made it the shortest dual-stopover mission of the seven-mission cycle.

Because short missions limited the time available for hardware breakdowns and the crew medical problems, they were to be preferred to long ones. Willis and Padrutt acknowledged, however, that the opportunity's short stopover at Mars would provide little time for exploration. Total propulsive effort would amount to 8.738 kps.

The third opportunity would occur in late 1981. The dual-stopover spacecraft would leave Earth for Venus, where it would spend about 265 days. It would stop over for 133 days at Mars, and spend 629 days between worlds, yielding a total mission duration of 1027 days (nearly three years). Total effort would equal 8.7 kps.

The fourth opportunity would occur at the end of 1981. The dual-stopover spacecraft would leave Earth for Mars. It would spend about 274 days at Mars, 340 days at Venus, and 680 days between planets, for a total duration of about 1294 days (a little more than three and a half years). Total propulsive effort would equal 9.252 kps.

The fifth opportunity would occur in the first half of 1983. The dual-stopover spacecraft would leave Earth for elliptical Venus orbit, where it would spend just 10 days. It would spend 601 days at Mars and 619 days between worlds, yielding a mission duration of 1230 days (a little less than three and a half years). Total propulsive effort would total 8.896 kps. The short stopover at Venus might make the opportunity undesirable; on the other hand, the mission's Mars stopover would be the lengthiest in the 6.4-year cycle, enabling a long period of exploration.

The sixth opportunity would see the dual-stopover spacecraft depart Earth for elliptical Mars orbit in early 1984. The spacecraft would spend 200 days at Mars, 250 days at Venus, and 639 days between planets, for a total mission duration of 1089 days (a little less than three years). Total propulsive effort would amount to 9.339 kps.

The seventh and last opportunity of the 6.4-year cycle would occur in mid-1985. The dual-stopover spacecraft would spend 767 days in elliptical Venus orbit before voyaging to Mars for a 78-day stopover. It would spend a total of 599 days between worlds — the shortest travel time of the seven opportunities. The long Venus stopover would, however, result in a mission duration of 1444 days (about four years), making it the lengthiest of the seven dual-stopover missions. Total propulsive effort would amount to 9.321 kps.

The 6.4-year-cycle Willis and Padrutt studied in detail would end just before the first dual-stopover opportunity of the next 6.4-year cycle. That opportunity, very similar to the late 1979 Earth-Mars-Venus-Earth opportunity, would occur in the first half of 1986.

Willis and Padrutt compared the total propulsive effort necessary to accomplish four of the dual-stopover missions in the 1979-1986 period with that needed to carry out four Mars stopover/Venus swingby missions. They sought to reduce dual-stopover mission duration, however, so permitted increased propulsive effort. This would enable shorter stays at planets and shorter transfers between planets. The Mars stopover/Venus swingby missions — all of which would include a 30-day Mars stopover — were assumed to leave Earth on approximately the same dates as the dual-stopover missions.

They found that the first dual-stopover mission, the December 1979 Earth-Mars-Venus-Earth mission, would need a total propulsive effort of about 13 kps to reduce its duration to 700 days. A Mars stopover/Venus swingby mission launched at about the same time could be performed in 700 days with a total propulsive effort of only eight kps. The same missions could be carried out in 575 days with propulsive efforts of 20 kps and a little less than 11 kps, respectively. These numbers indicated that the first opportunity in the 6.4-year dual-stopover cycle was not a favorable one for dual-stopover missions of reduced duration.

Dual-stopover missions launched in the other three opportunities compared more favorably with Mars stopover/Venus swingby missions. The fourth mission of the 1980s dual-stopover cycle — another Earth-Mars-Venus-Earth mission — could be shortened to 700 days if a total propulsive effort of about 12 kps were permitted, while a 700-day Mars stopover/Venus swingby mission departing Earth at about the same time would need a propulsive effort of about 10 kps.

The sixth dual-stopover mission (Earth-Mars-Venus-Earth) could be accomplished in just 625 days with a total propulsive effort of a little more than 10 kps. Willis and Padrutt calculated that a 625-day Mars stopover/Venus swingby mission launched at the same time would actually need a greater total propulsive effort: a little less than 12 kps.

The seventh dual-stopover mission in the cycle — an Earth-Venus-Mars-Earth mission — could be shortened to 675 days with a total velocity change of about 10 kps. A 675-day Venus swingby/Mars stopover mission launched at the same time would need a velocity change of eight kps.

Willis and Padrutt conceded that the minimum propulsive effort required to carry out a dual-stopover mission would almost always exceed that of a single mission that traveled from Earth to either Venus or Mars and back to Earth. They noted, however, that the minimum propulsive effort of a separately launched Earth-Venus-Earth stopover mission and a separately launched Earth-Mars-Earth stopover mission combined would always exceed that of a single dual-stopover mission. The two separate missions would together need a minimum propulsive effort of at least 17 kps; that is, nearly double the minimum propulsive effort of a typical dual-stopover mission.

Sources

Round Trip Trajectories With Stopovers At Both Mars and Venus, NASA TM X-52680, E. Willis and J. Padrutt, NASA Lewis Research Center, September 1969.

Planetary Exploration Utilizing a Manned Flight System, NASA Office of Manned Space Flight, 3 October 1966.

More Information

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

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

Humans on Mars in 1995! (1980-1981)

Geosynchronous Drift: Krafft Ehricke's Destination Mankind Apollo Mission (1972)

Apollo 17 launch, 7 December 1972. Image credit: NASA.

In May 1972, Krafft Ehricke, Executive Advisor in the Space Division of North American Rockwell Corporation, proposed that the last piloted lunar landing mission, Apollo 17, scheduled for the end of 1972, be postponed until the U.S. Bicentennial in July 1976 and dispatched to a new destination: a geosynchronous orbit (GSO) 22,300 miles above the Earth. An object in a GSO requires one day to complete one revolution of the Earth. Since Earth revolves in one day, an object in equatorial GSO appears to hang over one spot on the equator.

"The mission into geosynchronous orbit," Ehricke declared, would provide "additional return on America's investment in Apollo" by dramatizing "the usefulness of manned orbital activities." He added that his proposal, which he dubbed Destination Mankind, "would inspire many, as did the lunar missions before it, but in a different, perhaps more direct manner, because of its greater relevance to some of the most pressing problems of our time."

Ehricke's emphasis on practical benefits over lunar exploration reflected a significant shift in the public perception of spaceflight — one which had gained momentum throughout the 1960s. President Richard Nixon had articulated this shift in his "Statement About the Future of the United States Space Program" on 7 March 1970. The 37th President stated that he believed that the U.S. space program should proceed at a measured pace (not on "a crash timetable") and should be devoted to scientific exploration (mainly using interplanetary robots, but with man on Mars as a "longer-range goal").

In addition, NASA should emphasize international cooperation, cost reduction, and, crucially, "practical application — turning the lessons we learned in space to the early benefit of life on Earth." Nixon declared that results of space research should be "used to the maximum advantage of the human community." He listed among the practical applications of spaceflight "surveying crops, locating mineral deposits, and measuring water resources."

Ehricke described a representative 12-day Destination Mankind mission. Reaching GSO would require about as much propulsive energy as reaching lunar orbit, he noted. The three-stage Destination Mankind Apollo Saturn V rocket would lift off from Launch Complex 39 at Kennedy Space Center, Florida, at about 8:30 p.m. local time. Following first and second stage operation, the S-IVB third stage would fire briefly to place itself, the Apollo Command and Service Module (CSM), and a Payload Module (PM) into 100-nautical-mile parking orbit. Ehricke did not describe the PM design.

One orbital revolution (about 90 minutes) later, the S-IVB would ignite again to perform Transynchronous Injection (TSI). After S-IVB shutdown, the astronauts would separate their CSM and turn it 180° to dock with the PM, which would be attached to the top of the S-IVB in place of the Apollo Lunar Module (LM). They would then extract the PM, maneuver away from the S-IVB, and settle in for the 5.2-hour coast to GSO.

Deep Space Climate Observatory (DSCOVR) image of Africa, the Middle East, India, Europe, and adjacent seas and oceans. Cairo, close to the northern limit of the Destination Mankind Afro-Eurasian Station, is located near the center of the image. DSCOVR images Earth every two hours from Sun-Earth L1, not from geosynchronous orbit. Image credit: NASA.

The Destination Mankind CSM would ignite its Service Propulsion System (SPS) main engine to enter a GSO at 31° east longitude. This would place it over the equatorial nation of Uganda — if the CSM entered an equatorial GSO. The mission's GSO would, however, be inclined 28.5° relative to Earth's equator, so the CSM would oscillate between 28.5° south latitude (over South Africa's east coast) and 28.5° north latitude (southwest of Cairo) and back every 24 hours. The CSM would reach its southern limit at 10 a.m. local time and its northern limit at 10 p.m. local time. This 57°-long stretch of the 31° east longitude line would, Ehricke explained, constitute Destination Mankind's "Afro-Eurasian Station."

Destination Mankind mission objectives would fall into three general areas: science, technology, and public relations. Science objectives would draw upon an Apollo Geosynchronous Scientific Experiment Package (AGSEP) carried in the PM. The crew might assess the astronomical value of a GSO observatory, perform high-energy particle experiments, and observe and image the Earth. At the Afro-Eurasian Station, the astronauts could view Africa, Europe, the Middle East, Central Asia, and India. Earth imaging and observation might be conducted in collaboration with observers at "ground truth" sites on land and on ships at sea.

Ehricke emphasized the technology objectives of his Destination Mankind mission. He was particularly enamored of a solar illumination experiment that would see a circular reflector assembled by spacewalking astronauts. The experiment would provide reference data for design and operation of future space-based reflectors, he explained. He calculated that a 100-meter reflector in GSO could light Earth's surface one-tenth as brightly as a full Moon in a selected area. This level of illumination, though "subvisual," would be useful for night meteorology and surveillance of border and coastal areas, Ehricke wrote.

The astronauts would also erect "Manstar," a 500-to-700-foot-diameter reflective balloon visible over a wide area of Earth's surface as a modestly bright star. Ehricke called Manstar "a visible manifestation for all mankind of the potential value of space."

Ehricke called public relations "Public Exposure." Destination Mankind astronauts would become television stars. They would describe their Earth observations — "especially aspects useful and of interest to regional populations" — via TV broadcasts from GSO. Their spacewalks would also make for good TV fare, Ehricke judged.

Apollo 17 Command Module Pilot Ronald Evans retrieves film and data cassettes from the Scientific Instrument Module Bay built into the side of the Apollo 17 CSM America. His 17 December 1972 spacewalk was the last performed beyond low-Earth orbit. Ehricke's Destination Mankind mission would have included several spacewalks in GSO, where none has yet occurred. Image credit: NASA.

DSCOVR image of North America, South America, and Central America with adjacent oceans and seas. New Orleans, near the northern limit of the Destination Mankind Panamerican-Pacific Station, is located near the center of the image. Image credit: NASA.

The Destination Mankind CSM and PM would remain at the Afro-Eurasian Station for an unspecified period (perhaps two days), then the astronauts would fire the CSM's SPS to climb to a slightly higher orbit and begin a two-day "drift" westward across the Atlantic to their Panamerican-Pacific Station. Upon reaching their new station, located at 90° west longitude, the crew would fire the SPS to lower their orbit and halt their drift.

The CSM and PM would oscillate between 28.5° south (over the Pacific off northern Chile) and 28.5° north (over the Gulf of Mexico south of New Orleans), again reaching the southern limit at 10 a.m. local time and the northern limit at 10 p.m. local time. Equatorial crossing would occur above the Galapagos Islands. The astronauts would spend their time much as they did at the Afro-Eurasian Station, then would fire the SPS again to drift westward across the Pacific.

DSCOVR image of Australia, east Asia, east Africa, the Middle East, India, and adjacent bodies of water. The Destination Mankind Australo-Asian Station's southern limit would occur over the Indian Ocean off the coast of Perth, Australia, in the lower half of the image, just right of center. Image credit: NASA.

The last stop on the Destination Mankind crew's world tour would be the 98° east longitude line, which Ehricke dubbed the Australo-Asian Station. They would reach the north point in their south-north oscillation over southern China and the south point over the east Indian Ocean west of Perth. Near the end of their stay at the Australo-Asian Station, they would discard the PM.

The Destination Mankind crew would return to Earth from the Australo-Asian Station. Using the SPS, they would perform a Trans-Earth Injection burn as their CSM crossed the equator near Sumatra moving north at 4 p.m. local time. Fall to Earth would last 5.2 hours, and splashdown would occur in the Pacific west of Hawaii at just after 6 a.m. local time.

Sources

"Destination Mankind: Proposal for a Saturn V - Apollo Mission into Geosynchronous Orbit," K. Ehricke, North American Rockwell, 10 May 1972.

The American Presidency Project, "Statement About the Future of the United States Space Program," Richard Nixon, 7 March 1970 (http://www.presidency.ucsb.edu/ws/index.php?pid=2903&st=Future+of+the+United+States+Space+Program&st1= — accessed 14 April 2017).

More Information

"A Continuing Aspect of Human Endeavor": Bellcomm's January 1968 Lunar Exploration Program

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

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

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

Floaters, Armored Landers, Radar Orbiters, and Drop Sondes: Automated Probes For Piloted Venus Flybys (1967-1968)

Venus as imaged by the European Space Agency's Venus Express spacecraft. Image credit: ESA.

Venera 4 left Baikonur Cosmodrome in Soviet Central Asia early in the morning of 12 June 1967. The first two stages of its three-stage Molniya-M launch vehicle placed the 1106-kilogram automated spacecraft into a 173-by-212-kilometer parking orbit about the Earth, then the launcher's third stage boosted Venera 4 out of orbit onto a fast path Sunward toward the cloudy planet Venus.

Two days later, after launch on an Atlas-Agena D rocket from the Eastern Test Range-12 launch pad at Cape Kennedy, Florida, 244.8-kilogram Mariner 5 followed Venera 4 toward Venus. Mariner 5 had been built as the backup for Mariner IV, which flew successfully past Mars in July 1965. Hardware modifications for its new mission included a reflective solar shield, smaller solar panels, and deletion of the visual-spectrum TV system in favor of instruments better suited to exploring Venus's hidden surface.

When Mariner 5 and Venera 4 left Earth, the nature of Venus's surface was only beginning to be understood. Though the Mariner II Venus flyby (14 December 1962) had measured a surface temperature of at least 800° Fahrenheit (F) over the entire planet, some planetary scientists still held out hope for surface water. They believed that Venus's atmosphere was made up mostly of nitrogen, with traces of oxygen and water vapor. They supposed that, even if Venus was in general hotter than Earth, its polar regions had to be cooler than its equator and mid-latitudes — perhaps cool enough to provide a home for Venusian life. They also suggested that living things — most likely, microorganisms — might float high above the surface of Venus in cool moist cloud layers.

Venera 4 reached Venus on a collision course, as planned, on 18 October 1967. Shortly before entering the atmosphere at a blazing speed of 10.7 kilometers per second, it split into a bus spacecraft and a one-meter-wide, cauldron-shaped atmosphere-entry capsule. Both parts had been sterilized to prevent contamination of Venus with Earth microbes. The capsule was designed to float if it splashed down in water.

Venera 4-type Venus landing capsule. Image credit: NASA.

Radio signals from Venus ceased suddenly as the Venera 4 bus was destroyed as planned high in the Venusian atmosphere; then, after a brief pause, signals from the Venera 4 capsule reached antennas in the Soviet Union. After a steep atmosphere entry, during which it decelerated at 350 Earth gravities, the capsule lowered on a single parachute for 94 minutes. It transmitted data on atmospheric composition, pressure, and temperature as it fell toward the surface. Twenty-five kilometers above Venus, at a pressure 20 times greater than Earth sea-level pressure and a temperature of more than 500° F, transmission abruptly ceased. Venera 4 confirmed that Venus's atmosphere is more than 90% carbon dioxide.

Mariner 5 flew by Venus the next day at a distance of 4100 kilometers. For nearly 16 hours it performed an automatic encounter sequence and stored data it collected on its tape recorder. On 20 October 1967, it began to play back data to Earth. The U.S. spacecraft found no radiation belts akin to the Van Allen Belts that girdle Earth; this was not surprising, since it also measured a magnetic field only 1% as strong as Earth's.

As it flew behind Venus, Mariner 5 sent and received a steady stream of radio signals. The signals faded rapidly as they passed through the dense Venusian atmosphere, yielding temperature and pressure profiles before they were cut off — became occulted — by the solid body of the planet. The occultation experiment revealed that, at the point where it contacts the surface, Venus's atmosphere has a temperature of almost 1000° F. The planet's surface atmospheric pressure, it showed, is from 75 to 100 times greater than Earth sea-level pressure.

As Venera 4 and Mariner 5 explored Venus, D. Cassidy, C. Davis, and M. Skeer, engineers at Bellcomm, NASA's Washington, DC-based Apollo planning contractor, put the finishing touches on a report for the Office of Manned Space Flight at NASA Headquarters. In it, they described automated Venus probes meant to be released from piloted Venus/Mars flyby spacecraft. They based their plans on a sequence of piloted Mars and Venus flyby missions outlined in the October 1966 report of NASA's Planetary Joint Action Group (JAG).

In the Planetary JAG's plan, NASA's piloted flyby program would begin with a Mars flyby mission in 1975. The second mission in the program, the 1977 Triple Planet Flyby, would depart Earth in February 1977, almost a decade after the Venera 4 and Mariner 5 missions. The piloted flyby spacecraft would fly past Venus in June 1977, pass Mars in December 1977, explore Venus again in August 1978, and return to Earth in December 1978. The third and final Planetary JAG piloted flyby mission, the 1978 Dual Planet Flyby, would leave Earth in December 1978, pass Venus in May 1979, pass Mars in January 1980, and return to Earth in September 1980.

Cassidy, Davis, and Skeer presented a progressive plan of Venus exploration, with preliminary reconnaissance during the first Venus flyby and increasingly in-depth studies during the next two. Most of the Venus probes they proposed were designed to float in the planet's atmosphere, though they also described armored Venus landers, impactors, and large orbiters.

1977 Venus-Mars-Venus piloted flyby mission first (dayside) Venus encounter geometry. Image credit: Bellcomm/NASA.

The June 1977 Venus flyby would see a piloted flyby spacecraft pass the planet at a distance of 680 kilometers moving at 11.8 kilometers per second. Periapsis (the point of closest approach to the planet) would occur over a point just north of the equator in the middle of the dayside hemisphere. The astronauts on board the flyby spacecraft would seek to learn about Venus's surface structure using a cloud-penetrating mapping radar and a reflecting telescope with a one-meter-diameter mirror.

The Triple Planet Flyby crew would also release a total of 15 automated probes with a combined mass of 27,200 pounds. These would include six 200-pound Drop Sonde/Atmospheric Probes (DSAPs); four 2075-pound Meteorological Balloon Probes; two 700-pound Venus Landers; two 700-pound Photo-RF Probes; and one 8000-pound Orbiter. The crew would release all of the DSAPs, two Meteorological Balloons, one Lander, one Photo-RF Probe, and the Orbiter during approach to Venus. The other four probes (one Photo-RF probe, two Meteorological Balloons, and one Lander) they would release as the flyby spacecraft moved away from Venus and began its journey to Mars.

The DSAPs would be the first released, separating from the piloted flyby spacecraft between 10 and 16 hours before periapsis passage. Following a fiery entry into the Venusian atmosphere, they would transmit temperature, density, and composition data as they fell toward the surface, much as had Venera 4.

The Bellcomm team recommended targeting one DSAP to the "sub-solar region" (that is, the middle of the dayside), one to the "anti-solar" region (the middle of the nightside), one to the terminator (the line between day and night) near the equator, one to the "mid-light" region (mid-latitude on the dayside), and one to the "mid-dark" region (mid-latitude on the nightside). Because it would enter Venus's atmosphere at the steepest angle of the six DSAPs, the terminator-equator DSAP would need to withstand deceleration equal to 200 Earth gravities.

Following release from the flyby spacecraft, the large Orbiter would fire its rocket motors to place itself into a low near-polar orbit about Venus. It would pass over both the sub- and anti-solar regions during the piloted flyby, then would continue to orbit and explore the planet after the flyby, transmitting its findings directly to Earth. Using radar and a multispectral scanner, it would map Venus's entire surface in about 120 Earth days. Controllers on Earth would also track its orbital motion to chart any Venusian gravity anomalies.

Venus Meteorological Balloon deployment sequence. Image credit: Bellcomm/NASA.

The four Meteorological Balloons would communicate with Earth via the Orbiter, not the flyby spacecraft; the Bellcomm team explained that this would help to reduce the crew's burden of labor during the hectic flyby. The Orbiter would track the Meteorological Balloons for weeks to chart circulation patterns in the Venusian atmosphere at various locations and altitudes.

The Bellcomm team targeted the twin "survivable type" Landers to Venus's north pole and mid-light regions. The former would enter the atmosphere steeply about three hours before flyby spacecraft periapsis, experiencing up to 500 Earth gravities of deceleration. Both Landers would descend through Venus's atmosphere for up to an hour. After they impacted on the surface, they would transmit meteorological and surface composition data for up to an hour.

The first Photo-RF Probe would enter the dense atmosphere over the sub-solar region one hour before flyby spacecraft periapsis. The second would enter over the mid-light Lander site 15 minutes after flyby spacecraft periapsis passage. The Bellcomm engineers explained that the Photo-RF probes, which they likened to the Block III Ranger moon probes, would transmit only while the flyby spacecraft was close enough to accommodate their one-million-bit-per-second data rate. They would each transmit one wide-angle image from their downward-pointing cameras every 10 seconds for up to an hour as they plummeted toward destructive impact on the surface.

1977 Venus-Mars-Venus piloted flyby mission second (nightside) Venus encounter geometry. Image credit: Bellcomm/NASA.

The 1977 Triple Planet Flyby mission's second Venus pass in August 1978, 14 months after the first, would build on knowledge gained in the first pass, enabling a greater emphasis on Venus surface exploration. The flyby spacecraft would reach periapsis 700 kilometers above a point near the equator at the center of Venus's nightside. In addition to performing observations using flyby spacecraft instruments, the astronauts would aim five Lander Probes and five Photo-RF probes at interesting surface features discovered during their first Venus flyby and by the Orbiter they had left behind.

Bellcomm recommended that the third Venus flyby of the series, the 1978 Dual Planet Flyby mission's May 1979 flyby, should emphasize "the search for life and extended surface operations." The astronauts would release 19,000 pounds of probes including a pair of 3100-pound Buoyant Venus Devices (BVDs), twin 3400-pound Near Surface Floaters (NSFs), and a 6000-pound Orbiter. Moving at 14.1 kilometers per second, the flyby spacecraft would attain periapsis 1170 kilometers above a point on the terminator near Venus's north pole.

1978 Venus-Mars piloted flyby mission Venus encounter geometry. Image credit: Bellcomm/NASA.

As they drifted in the cool atmospheric layer some believed existed between 125,000 and 215,000 feet above the Venusian surface, the 82-foot-diameter BVDs would filter "very large quantities" of atmospheric gas in the hope of capturing high-flying Venusian "aerosol life." So hopeful were the Bellcomm planners that life might be found on or above Venus that they set aside 180 pounds of each BVD's 230-pound science payload for biology experiments.

Meanwhile, the 30-foot-diameter NSFs would image the gloomy surface from an altitude of a few hundred feet using floodlights and flares to light the scene as required. The Bellcomm engineers recommended that one NSF seek life in the relatively cool polar region. The other NSF might explore a site on the equator.

Near Surface Floater in sample collection mode. Image credit: Bellcomm/NASA.

The BVDs and NSFs would transmit their data to the flyby spacecraft at a high bit rate as it passed periapsis. The astronauts would examine images from the polar NSF in the hope of finding a biologically interesting site to sample. If the NSF drifted over such a site, the crew would quickly command it to drop a claw-like anchor and lower a biological sampling device to the surface on a cable. After the flyby, control of the Floaters would pass to Earth, with radio signals relayed through the Orbiter at a reduced bit rate.

The Meteorological Balloons deployed during the 1977 Triple Planet Flyby mission and the 1978 Dual Planet Flyby mission Floaters would share many features. All would include "superpressure" balloons filled with hydrogen. They would, however, be made of different materials because of their different operating temperatures. For those floating within 65,000 feet of the surface, the Bellcomm engineers proposed "super-alloy steel fiber weave (impregnated with silicon polymer filler)." Such fabric had been tested on Earth at temperatures of up to 1200° F, they explained. Kapton and Mylar films would probably be adequate at higher altitudes where the Venusian atmosphere would be cooler.

The Bellcomm engineers expected that one day astronauts might explore the Venusian atmosphere in person. They wrote that "the [manned] exploration mode could well employ a class of propeller driven cruising vehicles. . .employing nuclear power," and suggested that the NSF probes might constitute "a first step in achieving this design."

In August 1967, the U.S. Congress, eager to rein in spending in the face of increased expenditures in Vietnam, cut all funds for piloted planetary mission planning and most funds for robotic missions from NASA's Fiscal Year 1968 budget. NASA went to bat for its automated planetary program in September 1967, and succeeded in convincing lawmakers to fund automated Mars missions in the 1969, 1971, and 1973 Mars transfer opportunities.

The agency did not, however, try to save piloted flybys. By the time the Bellcomm team submitted its Venus probe report, the piloted flyby concept was all but defunct. Planning for piloted planetary missions continued at a low level during 1968, enjoyed a resurgence in 1969-1970, and ceased almost entirely by the beginning of 1972 as NASA's piloted spaceflight program focused most of its future-directed energies on the Earth-orbital, semi-reusable Space Shuttle.

Robotic Venus exploration continued, however; in fact, the Soviet Union made Venus its favorite target for planetary exploration. Each new mission confirmed that early optimism about Venusian biology was unfounded. Veneras 5 through 8 were near-copies of Venera 4. In December 1970, Venera 7 crash-landed, yet managed to transmit data to Earth, making it the first spacecraft to return data from the surface of another planet.

The Venera 9 through 14 landers were of a more complex and capable design. Venera 9 returned the first images of the surface of Venus in October 1975; these were also the first images returned from the surface of another planet. Veneras 15 and 16 included no landers; instead, they radar-mapped much of Venus's northern hemisphere between October 1983 and July 1984. The Vega 1 and 2 missions passed by Venus en route to Comet Halley in June 1985; each released a balloon and a lander.

NASA's Mariner 10 spacecraft flew past Venus in February 1974. In addition to collecting data, it used a Venus gravity assist to shape its orbit so that it flew past the planet Mercury three times in 1974-1975. Other spacecraft have explored Venus while using its gravity and momentum to speed them toward some other destination; after the Vega twins, the next spacecraft to do so was the Galileo Jupiter orbiter, which flew by Venus in February 1990.

Pioneer Venus 1 captured into Venus orbit in May 1978 and explored the planet until August 1992, when its orbit at last decayed and it burned up in the atmosphere. It mapped most of the planet's surface using a low-resolution imaging radar. In November 1978, Pioneer Venus 2 released one large and three small Venus atmosphere probes. Although not designed to survive landing, one of the small probes reached the surface intact and continued to transmit for more than an hour.

By the time Pioneer Venus 1 burned up, the Magellan spacecraft was in near-polar orbit around Venus. Launched from the cargo bay of the Shuttle Orbiter Atlantis in early May 1989, the spacecraft reached Venus in August 1990. Using a high-resolution imaging radar, Magellan imaged nearly the entire surface of the planet in unprecedented detail by September 1992, enabling detailed geological mapping. After a series of Venus gravity, radio science, and aerobraking experiments, Magellan descended into the Venusian atmosphere and burned up on 13 October 1994.

Artist impression of the Venus Express spacecraft in orbit over the double vortex at Venus's south pole. Image credit: European Space Agency.

The European Space Agency's Venus Express spacecraft reached Venus polar orbit in May 2006. Venus Express was launched on a Russian rocket from Baikonur Cosmodrome in the Republic of Kazakhstan in November 2005.

In November 2007, scientists participating in the mission reported results from the 500-day Venus Express primary mission in the journal Nature. In addition to evidence for water oceans in the ancient past, they presented images of a strange double vortex in the atmosphere over the planet's south pole. In August 2011, they reported that Venus has an ozone layer.

Venus Express ceased transmitting data to Earth in November 2014 as it ran low on fuel. It is thought to have entered the Venusian atmosphere and burned up in January-February 2015. Scientists studying Venus Express data announced in June 2015 that they had found new evidence for present-day volcanism on Venus.

Sources

"Preliminary Considerations of Venus Exploration via Manned Flyby," TR-67-730-1, D. Cassidy, C. Davis, and M. Skeer, Bellcomm, 30 November 1967.

"Experiment Payloads for Manned Encounter Missions to Mars and Venus," W. Thompson, et al., Bellcomm, 21 February 1968.

Venus Space Probes, Novosti Press Agency Publishing House, 1979.

NASA Facts: Mariner Spacecraft, Planetary Trailblazers, NF-39, NASA, February 1968.

The Voyage of Mariner 10, NASA SP-424, NASA, 1978.

Pioneer Venus, NASA SP-461, NASA, 1983.

Science and Space, Novosti Press Agency Publishing House, Moscow, 1985.

Soviet Space Programs 1980-1985, Nicholas L. Johnson, American Astronautical Society/Univelt, 1987, pp. 179-189.

"Magellan Loss of Contact Caps Venus Mission," NASA Release 94-170, D. Isbell and J. Doyle, NASA/JPL, 12 October 1994.

The Face of Venus: The Magellan Radar Mapping Mission, NASA SP-520, L. Roth & S. Wall, NASA, June 1995.

ESA Venus Express (http://www.esa.int/Our_Activities/Space_Science/Venus_Express — accessed 30 January 2020).

More Information

The Challenge of the Planets, Part Three: Gravity

Centaurs, Soviets, and Seltzer Seas: Mariner II's Venusian Adventure (1962)

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

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

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

One of many contractor proposals for piloted flyby spacecraft put forward in the mid-1960s — this one by Douglas Aircraft Company, c. 1966. The small-diameter section is a modified Apollo Command and Service Module (CSM) spacecraft. Automated Mars probes depart the probe compartment in the large-diameter section. Image credit: Douglas Aircraft Company/San Diego Air & Space Museum (http://sandiegoairandspace.org/).

From 1962 to 1967, NASA and its contractors studied piloted Mars/Venus flybys as a possible interim step between Apollo lunar missions in the 1960s and piloted Mars landing missions in the 1980s. Many of the conceptual flyby spacecraft designs were based on planned or proposed Apollo and Apollo Applications Program technology.

Starting in February 1967, the flyby concept fell into disfavor following criticism by the President's Science Advisory Committee (PSAC). President Lyndon Johnson's PSAC, which had previously supported the piloted flyby concept, declared that piloted flybys made unwise use of astronauts, and that NASA should reassess its plans for using of humans and robots in space. NASA substituted the word "encounter" for "flyby" and continued to task Bellcomm, its Washington, DC-based Apollo planning contractor, with studies of various aspects of piloted flyby missions.

In August 1967, however, Congress eliminated all funds for piloted flyby studies and other advanced mission planning from the Fiscal Year 1968 NASA budget. The lethal AS-204/Apollo 1 fire (27 January 1967) was a key factor in the decision to cut funding designed to give NASA a post-Apollo future. Writing in the aftermath of these cuts, Bellcomm cautioned that its February 1968 report on experiments and observations to be conducted during a 1977 Venus-Mars-Venus encounter mission "should be considered as illustrative of feasibility rather than a plan for the future."

The four-man piloted flyby spacecraft would leave Earth orbit on 23 January 1977. During an Earth-to-Venus transfer spanning mission days one through 148, the spacecraft would pass asteroid 1566 Icarus at a distance of 4.46 million miles (11 May 1977). The astronauts would use the spacecraft's one-meter telescope to measure the asteroid's albedo (reflectivity). At opportune times throughout the mission, they would conduct other astronomical observations, including studies of fluctuations in the radiation from quasars (now known to be the active cores of galaxies), zodiacal light (sunlight reflected off interplanetary dust), faint stars, the planet Mercury, and galaxy redshifts (evidence for an expanding universe).

A piloted flyby spacecraft — this time a 1967 NASA Manned Spacecraft Center design — releases a probe as it flies past the sunlit side of Venus. Visible on the spacecraft are a rectangular radar antenna for probing through the planet's dense clouds; a one-meter optical telescope; a large dish antenna for transmissions to Earth; and a small dish antenna for receiving probe data. Image credit: NASA.

On 16 June 1977, the piloted flyby spacecraft would release a 2.88-ton orbiter for relaying to Earth radio signals from the probes it would release during its first Venus flyby. The orbiter would fire rocket motors to slow down so that Venus's gravity could capture it into a 4000-kilometer-high circular orbit.

The piloted flyby spacecraft would zip past Venus for the first time on mission day 149 (21 June 1977), releasing 10 automated probes. These would include four "rough" landers, four bomb-shaped "photo sinker" probes, and two meteorological balloon probes, each containing six balloons with small instrument packages. The automated landers would be designed to survive the planet's heat and pressure for one hour after touchdown, while the sinkers would drop through the thick Venusian atmosphere for about 30 minutes and be destroyed on impact with the surface. The balloon probes would drift among the hot clouds of Venus for one month.

The flyby astronauts, meanwhile, would study Venus using their telescope and a cloud-penetrating radar. Closest approach would occur in sunlight 680 kilometers above the southern hemisphere, at which time the astronauts would fire the flyby spacecraft's rocket motors briefly to help to bend its course toward Mars.

Flight from Venus to Mars would span mission days 150 through 344. The astronauts would measure the albedo of Mars-crossing asteroid 132 Aethra from a distance of 35.9 million miles on 5 December 1977, and would study radio emissions from Jupiter in collaboration with radio astronomers on Earth. The crew would release three 2.36-ton Mars Surface Sample Return (MSSR) landers on 30 December 1977, five days before closest Mars approach.

On 3 January 1978 (mission day 345), the flyby spacecraft would pass 3960 kilometers above the martian night hemisphere at a speed of 5.6 kilometers per second. As they approached the planet, the astronauts would photograph the martian moons Deimos and Phobos.

The MSSR landers would touch down between two and four hours before flyby spacecraft closest approach. Each would deploy a drill to collect a subsurface sample and an aerosol filter to gather airborne dust. Mortars would launch other collection devices at least 100 feet to sample beyond the zone contaminated by the MSSR probe landing rockets. Each lander would then load its samples into a "rendezvous rocket" and launch it to the passing flyby spacecraft. Geophysics and exobiology experiments on the MSSR landers would then radio data to Earth for up to two years.

The Mars-to-Venus leg of the mission would span days 346 through 573. The astronauts would use the flyby spacecraft's biology laboratory to analyze the Mars samples the MSSR landers collected. They would also measure the albedo of three asteroids: 1192 Prisma, in the Main Belt between Mars and Jupiter, at a distance of 49.5 million miles (14 April 1978); 887 Alinda at 11.5 million miles (25 April 1978); and 1566 Icarus (again) at 62.3 million miles (5 August 1978). On 15 August 1978, the flyby spacecraft would release a second Venus radio-relay orbiter.

The flyby spacecraft would pass by Venus for the second time on mission day 574 (20 August 1978), releasing the same types and number of probes as during the first Venus flyby. They would be directed to targets identified using data obtained from the first flyby. Closest approach would occur in darkness over Venus's southern hemisphere at an altitude of 700 kilometers.

The Venus-to-Earth leg would span mission days 575 through 716. The astronauts would reenter Earth's atmosphere with their cargo of samples and data in a modified Apollo Command Module on 9 January 1979.

Sources

"Experiment Payloads for Manned Encounter Missions to Mars and Venus," W. Thompson, et al., Bellcomm, 21 February 1968.

The Space Program in the Post-Apollo Period, President's Science Advisory Committee, The White House, February 1967, p. 18.

"Science Advisors Urge Balanced Program," Aviation Week & Space Technology, 6 March 1967, p. 135.

Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA-SP-2001-4521, David S. F. Portree, NASA History Division, NASA Headquarters, February 2001, pp. 23-32 (https://history.nasa.gov/monograph21/Chapter%204.pdf - Accessed 28 March 2017).

More Information

EMPIRE Building: Ford Aeronutric's 1962 Plan for Piloted Mars/Venus Flybys

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

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

NASA Johnson's Plan to PEP Up Shuttle/Spacelab (1981)

Early artist concept of a Space Shuttle Orbiter with a "Sortie Lab" at the front of its Payload Bay. The Sortie Lab pressurized module is shown as a cutaway illustration. At this point in its history, the Sortie Lab was expected to be manufactured by a U.S. aerospace contractor. The Sortie Lab depicted is dedicated at least partly to astronomy, as evidenced by the large telescope attached to the aft end of its pressurized module. Image credit: NASA.

On 29 November 1972, NASA Administrator James Fletcher abolished the Space Station Task Force formed in early 1969 by his predecessor, Thomas Paine, and formed the Sortie Lab Task Force. The "Sortie Lab," a concept that emerged during Phase B Space Station planning in 1970, was envisioned as a pressurized laboratory module which would be carried in the Shuttle Orbiter's Payload Bay.

Fletcher's move acknowledged that the Space Shuttle, conceived originally as a vehicle for transporting crews and cargoes between Earth and an Earth-orbiting Space Station at low cost, would need to become a Space Station — or, at least, an interim space laboratory that could demonstrate that a Space Station would be a desirable new NASA goal after the Space Shuttle became operational.

Strapped for funds and encouraged by President Richard Nixon to use spaceflight as a vehicle for international cooperation, NASA asked the European Space Research Organization (ESRO), a predecessor of the European Space Agency (ESA), to provide the Sortie Lab in exchange for European astronaut flights on board the Shuttle. In August 1973, ESRO and European aerospace contractors agreed to build the Sortie Lab, which became known as Spacelab.

Cutaway illustration of a drum-shaped, ESA-built Spacelab module (center) with a pair of U-shaped Spacelab pallets (left). A bent tunnel with an airlock on top for spacewalks (note space-suited astronaut atop pallet at left) links Spacelab with the Shuttle Orbiter Mid-Deck, the main living space for the crew. Above that is the Flight Deck, the Orbiter cockpit. Image credit: NASA.

Spacelab would provide scientists with ample pressurized volume in which to conduct research, but it would rely on limited resources — for example, electricity — provided by the Shuttle Orbiter. Orbiter electricity came from a trio of liquid oxygen/liquid hydrogen fuel cells that in early 1981 were expected to generate 21 kilowatts continuously for just seven days. Of this, 14 kilowatts were required for Orbiter systems. The Orbiter could thus supply only seven kilowatts to Spacelab. Of those seven kilowatts, between two and five kilowatts would be needed for basic Spacelab systems, leaving a paltry two to five kilowatts for Spacelab experiments.

In 1978, NASA Johnson Space Center (JSC) in Houston, Texas, launched the Orbital Service Module Systems Analysis Study, which looked into ways that the Space Shuttle Orbiter could be augmented to enable it to better support Spacelab research. An early product of the study was the Power Extension Package (PEP) concept.

Stowed PEP components in the Space Shuttle Orbiter Payload Bay, between the front of a Spacelab module (right) and the rear of the Orbiter crew cabin. Image credit: NASA.

The PEP deployed in orbit. PEP displays and controls were meant to be located on the Shuttle Orbiter Flight Deck. Image credit: NASA.

The PEP concept was linked with NASA's extensive efforts in cooperation with the U.S. Department of Energy to justify the construction of enormous Earth-orbiting Solar Power Satellites (SPSs). It was portrayed as an experience-building experimental test-bed for SPS technology in the Von Karman Lecture JSC director Christopher Kraft presented to the 15th meeting of the American Institute of Aeronautics and Astronautics in July 1979. The PEP may also have been conceived as a rival for NASA Marshall Space Flight Center's Power Module (see "More Information" below).

The PEP Project Office (PEPPO) at JSC pitched the PEP in a brief report published one month before the first Space Shuttle flight (STS-1, 12-14 April 1981). The PEPPO envisioned the PEP as a "kit" that could be installed easily in the Shuttle Orbiter Payload Bay over the tunnel that would link the Orbiter Mid-Deck with the Spacelab pressurized module.

One hour after launch from Earth, an astronaut on the Orbiter Flight Deck would use the Canada-built Remote Manipulator System (RMS) robot arm to grapple the PEP's Array Deployment Assembly (ADA) and extend it out over the Orbiter's side. The ADA would then unroll a pair of lightweight solar array wings that together would measure more than 100 feet wide. PEP deployment would require about 30 minutes.

The PEP arrays would track the Sun automatically no matter how the Orbiter became oriented, so almost no astronaut intervention would be needed after they were deployed. The RMS and arrays would be sufficiently sturdy to remain deployed during Orbiter attitude-control maneuvers, but the crew would need to stow them before Orbital Maneuvering System burns lest the acceleration cause damage.

The twin arrays would generate a total of 26 kilowatts of electricity. A cable built into the RMS would carry the electricity from the ADA to the PEP's Power Regulation and Control Assembly (PRCA) in the Payload Bay. The PRCA would then distribute it to the Orbiter's electrical system.

The three Orbiter fuel cells would "idle" while the PEP arrays were in sunlight. Each would generate one kilowatt of electricity, bringing the total available on board to 29 kilowatts. Fifteen kilowatts would be available for Spacelab, of which between 10 and 13 kilowatts could be devoted to experiments.

Keeping the Spacelab electricity supply constant throughout each 90-minute orbit of the Earth would require that Orbiter fuel cell output ramp up rapidly from three to 29 kilowatts as the PEP arrays passed into darkness over Earth's night side. To achieve this output, each fuel cell would need to exceed its normal maximum by nearly three kilowatts. The fuel cells would then return to their idle state as the PEP arrays passed again into sunlight. Although it would almost certainly place unusual demands on the Orbiter fuel cells, the PEPPO judged this approach to be "feasible."

The PEPPO estimated that a PEP could extend Shuttle/Spacelab endurance in Earth orbit by four days (that is, to a total of 11 days). If other Orbiter resources (for example, life support consumables) could be augmented, then mission duration might be stretched to 45 days.

The PEPPO explained that it jointly managed PEP solar cell development with NASA's Lewis Research Center. Industry involvement in the PEP project was, it added, already "extensive," with several companies working on small NASA contracts or funding PEP-related work themselves. It estimated that the PEP could power a Spacelab module in Earth orbit as early as 1985 for a total development cost of only $150 million.

Spacelab 1 in Columbia's Payload Bay during STS-9 as viewed from the Flight Deck windows. Cables linking the Orbiter to Spacelab 1 are visible at lower right. Image credit: NASA.

The first Spacelab, appropriately designated Spacelab 1, reached orbit in the Payload Bay of the Orbiter Columbia on 28 November 1983, as part of the ninth Shuttle mission. Columbia's crew for mission STS-9 included ESA's Ulf Merbold, the first non-U.S. astronaut to reach space on board a U.S. spacecraft. Merbold was part of a six-man crew that also included Gemini, Apollo, and Shuttle veteran John Young, Skylab 3 veteran Owen Garriott, and spaceflight rookies Brewster Shaw, Robert Parker, and Byron Lichtenberg. Columbia landed at Edwards Air Force Base, California, on 8 December, ending a busy 10-day mission.

Columbia's fuel cells powered Spacelab 1, and all of the 27 Spacelab missions that followed relied on Orbiter fuel cells for their electricity. PEP work had ended in late 1981 as NASA Headquarters took charge of and terminated Shuttle augmentation and Space Station development efforts across the agency. It did this in part to clear the decks as it began formally to seek approval for a Space Station, which it billed as the "next logical step" after the Space Shuttle. President Ronald Reagan called on Congress to approve new-start funding for a Space Station during his annual State of the Union address in January 1984, less than two months after STS-9. 

Sources

Power Extension Package (PEP) Concept Summary, JSC-AT4-81-081, NASA Johnson Space Center, PEP Project Office, March 1981.

The Solar Power Satellite Concept, NASA JSC 14898, Christopher C. Kraft; Von Karman Lecture, 15th Annual Meeting of the American Institute of Astronautics and Aeronautics, July 1979.

"Spacelab joined diverse scientists and disciplines on 28 Shuttle missions," Science@NASA, 15 March 1999 (https://science.nasa.gov/science-news/science-at-nasa/1999/msad15mar99_1/ - accessed 25 March 2017).

More Information

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

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

Lunar GAS (1987)

During the STS-91 (2-12 June 1998) mission to the Russian Mir space station, the Space Shuttle Orbiter Discovery carried four pairs of GAS canisters along its Payload Bay walls. The red arrow points to one pair. Image credit: NASA.

NASA's Get Away Special (GAS) Program (officially the Small Self-Contained Payloads Program) was conceived in 1976 as a way of providing researchers with low-cost opportunities to fly experiments in the Space Shuttle Orbiter's 15-foot-by-60-foot payload bay. The first operational GAS canister, with a suite of 10 experiments developed by students at Utah State University, Weber State University, and the University of California at Davis, reached low-Earth orbit (LEO) during mission STS-4 (27 June-4 July 1982) on board the Orbiter Columbia. By 17 March 2005, when NASA terminated the GAS Program in the aftermath of the 1 February 2003 Columbia disaster, nearly 170 GAS canisters had flown in low-Earth orbit (LEO).

If four engineers at the Jet Propulsion Laboratory (JPL) in Pasadena, California, had had their way, a GAS payload might have traveled far beyond LEO. In May 1987, the team proposed that an advanced-design small spacecraft be launched on board a Space Shuttle inside an Extended GAS canister and ejected into Earth orbit. The spacecraft, called Lunar GAS (LGAS), would then use electric-propulsion thrusters to spiral outward to the moon.

Close-up of two of the STS-91 GAS canisters in Discovery's Payload Bay. Image credit: NASA.

LGAS anticipated the small, relatively cheap spacecraft of NASA's 1990s Discovery Program, the first mission of which, Near Earth Asteroid Rendezvous (NEAR), departed Earth in 1995. The Discovery Program, a significant break from the large-spacecraft paradigm that characterized much U.S. planetary mission development in the 1970s and 1980s, got its start in 1991-1992 as Defense Department space technology developed for President Ronald Reagan's Strategic Defense Initiative "missile shield" trickled into the civilian space sector. The Discovery Program would become an intermediate evolutionary step leading toward the present-day Cubesat revolution.

The LGAS mission would begin up to three months before planned Space Shuttle launch with the insertion of the 149-kilogram spacecraft into its Extended GAS canister. The spacecraft would at that point enter the routine GAS payload processing flow and no one would see it again until it left its canister in LEO.

The Shuttle Orbiter bearing the LGAS spacecraft would lift off from Kennedy Space Center in Florida and enter an orbit inclined 28.5° relative to Earth's equator. The astronauts would then open its payload bay doors, exposing the closed Extended GAS canister bearing LGAS to space.

NASA required that GAS experiments place minimal demands on Shuttle expendables and astronaut time. The JPL team insisted that, despite its complexity, the LGAS mission could meet this requirement. A few hours after launch, one astronaut would flip a single switch on the Shuttle flight deck to open the motorized Extended GAS canister lid, then would flip two more to release a latch and activate a spring ejection mechanism.

Simplified schematic of the LGAS spacecraft following deployment from its GAS canister. Image credit: JPL/NASA.

The barrel-shaped LGAS spacecraft would leave the Extended GAS canister moving at one meter per second; then, as it moved away from the Shuttle Orbiter, it would automatically extend its twin accordion-fold solar-array wings and its science boom. The slender advanced-design rectangular solar arrays would each have a mass of about 15 kilograms. Their combined 7.25 square meters of collecting area would generate 1.467 kilowatts of electricity at mission start.

Two small chemical-propellant thrusters would turn the spacecraft to point its solar arrays and spin axis toward the Sun, then would spin its barrel-shaped body end over end at up to five revolutions per minute to create gyroscopic stability. After it had moved a safe distance away from the Shuttle, the LGAS spacecraft would switch on one of its twin electric thrusters. Mounted on opposite sides of the spacecraft body, these would take turns thrusting parallel to its spin axis. Fueled from a round tank containing 36 kilograms of compressed xenon gas, the thrusters would each be designed to withstand 3500 start/stop cycles and to operate for a total of 4500 hours (187.5 days).

LGAS spacecraft electric-propulsion thrust and coast arcs during escape from Earth orbit. Image: JPL/NASA.

The LGAS spacecraft's orbit about the Earth would for mission operations purposes be divided into four 90° arcs, the JPL engineers explained. In the first arc, one thruster would point opposite the LGAS spacecraft's direction of motion so that when it operated it would accelerate the spacecraft. In the second arc, which would occur in Earth's shadow, both thrusters would point perpendicular to the spacecraft's direction of motion; this would mean that they could not contribute to accelerating the spacecraft, so they would not operate.

In the third arc, the second thruster would point opposite the LGAS spacecraft's direction of motion, so it would switch on to take its turn accelerating the spacecraft. In the fourth arc, which would see the spacecraft pass between the Earth and the Sun, the thrusters would again point perpendicular to its direction of motion, so would not operate.

Overcoming drag from Earth's atmosphere would require about one-third of the LGAS spacecraft's thrust early in the departure spiral, the team calculated, but drag would taper off quickly as the spacecraft raised its orbital altitude by up to 20 kilometers per day. Starting about three months after launch from the Shuttle, the LGAS spacecraft would spend between 100 and 150 days inside the Earth-girdling Van Allen Belts. High-energy particles in the Belts would gradually degrade the twin wing arrays, reducing their electricity output.

Image credit: JPL/NASA.

About 600 days after launch, the LGAS spacecraft would reach a point about 130,000 kilometers away from the Earth. It would then turn off its thrusters and coast in a lazy 15-day "linking orbit" that would deposit it into a loosely bound 40,000-kilometer circular lunar polar orbit.

The xenon-fueled thrusters would then resume alternating operation with their 90° thrust arcs centered over the moon's polar regions; this time, however, the thrusters would point in the spacecraft’s direction of motion when they operated, gradually slowing the LGAS spacecraft so that it would spiral in toward the moon.

The spacecraft would achieve a 100-kilometer-high, two-hour lunar polar orbit about two years after it departed its Extended GAS canister. In its orbit over the moon's poles, the moon would rotate beneath it about once per month, enabling it to eventually overfly the entire lunar surface. Irregularities in the moon's gravity field would mean that the electric thrusters would need to adjust the spacecraft's orbit about every 60 days.

The LGAS spacecraft would have room for only one science instrument: a 15-kilogram gamma-ray spectrometer (GRS) for charting the composition of the moon's crust. The JPL engineers proposed that the unflown Apollo 18 GRS be mounted on the LGAS science boom. Lunar-orbital science operations would continue for about one year.

Sources

"Lunar Get Away Special (GAS) Spacecraft," AIAA-87-1051, K. T. Nock, G. Aston, R. P. Salazar, and P. M. Stella; paper presented at the 19th AIAA/DGLR/JSASS International Electric Propulsion Conference in Colorado Springs, Colorado, 11-13 May 1987.

"Getaway Special," Wikipedia (https://en.wikipedia.org/wiki/Getaway_Special — accessed 18 March 2017).

More Information

On the Moons of Mighty Jupiter (1970)

Cometary Explorer (1973)

Catching Some Comet Dust: Giotto II (1985)

Chronology: Space Station 1.0

Japanese astronaut Aki Hoshide, a member the International Space Station (ISS) Expedition 32 crew, captures a self-portrait during a 5 September 2012 spacewalk. Reflected in his faceplate are U.S., Japanese, and European components of the ISS silhouetted against the Earth and, above his reflected right hand, NASA astronaut Sunita Williams. The brilliant Sun glaring past Hoshide's shoulder and the camera artifacts it creates make an already fascinating image particularly striking. Image credit: NASA.

My blog is only accidentally chronological in arrangement; because of this, occasionally I feel the need to compile a chronological listing of posts on a given topic as an aid to reader understanding. This is one of those times, and the topic this time around is space stations. Enjoy!

One-Man Space Station (August 1960)

Space Station Gemini (December 1962)

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter (November 1963)

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

"A True Gateway": Robert Gilruth's June 1968 Space Station Presentation

McDonnell Douglas Phase B Space Station (June 1970)

From Monolithic to Modular: NASA Establishes a Baseline Configuration for a Shuttle-Launched Space Station (July 1970)

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

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

Skylab-Salyut Space Laboratory (June 1972)

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

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

Evolution vs. Revolution: The 1970s Struggle for NASA's Future (1978)

Bridging the Gap Between Space Station and Mars: The IMUSE Strategy (July 1985)

Naming the Space Station (1988)

The 1991 Plan to Turn Space Shuttle Columbia Into a Low-Cost Space Station (July-September 1991)

NASA's 1992 Plan to Land Soyuz Lifeboats in Australia (November 1992)