A 1974 Plan for a Slow Flyby of Comet Encke

So close: the CONTOUR spacecraft. Image credit: NASA.

Comet Halley is often called "Humankind's Comet" because it has appeared throughout much of recorded human history and because its orbital period of about 76 years is roughly equivalent to a human lifespan. Given the often frustrating nature of spaceflight planning, Comet Encke could be nicknamed "Spaceflight's Comet."

It has made the short list of targets for comet-exploring spacecraft for half a century. With one of the shortest orbital periods of any comet — just 3.3 years — and an inclination relative to the plane of the Solar System of only about 10°, Encke is among the comets most easily accessible to spacecraft. Yet despite being named the target of many proposed comet missions, Encke has never received a visitor from Earth.

Humans came closest to exploring Comet Encke nearly two decades ago. Following its launch on 3 July 2002, NASA's 775-kilogram COmet Nucleus TOUR (CONTOUR) spacecraft moved through a series of elliptical phasing orbits about the Earth designed to position it for a solid-propellant rocket motor burn on 15 August 2002. The burn would have launched it into solar orbit near the Earth. CONTOUR would then have re-encountered Earth in August 2003. The gravity-assist kick it was meant to receive from our planet would have put it on course for a Comet Encke close flyby on 12 November 2003.

Instead, the CONTOUR spacecraft disintegrated during its Earth-departure burn. Observers visually tracked three objects where there should have been one CONTOUR.

The CONTOUR Mishap Investigation Board determined that the most likely cause of the failure was an obvious-seeming design flaw: that the spacecraft's solid-propellant rocket motor, embedded at its center, produced enough heat that it weakened CONTOUR's structure, causing the spacecraft to break apart under acceleration. The Board cautioned, however, that lack of telemetry during the Earth-departure burn left open the possibility of several other causes, including rocket motor casing rupture, meteoroid or human-made space debris collision, or attitude-control failure leading to a destructive tumble.

If engineers and scientists at NASA Goddard Space Flight Center (GSFC) had gotten their way, Comet Encke would have received its first visitor as early as 3 December 1980. In fact, it would have received two visitors at the same time, for they envisioned launching two spacecraft to Comet Encke on a single rocket. The Encke probes, near twins, would have flown by the comet at a relatively slow speed compared with other proposed comet spacecraft; hence, in the November 1974 NASA Technical Note they wrote to describe it, they dubbed their mission a "ballistic slow flyby."

The twin Comet Encke ballistic slow flyby spacecraft stacked within their streamlined Centaur launch shroud. The adapter would join with the top of the Centaur upper stage. Image credit: NASA.

The Comet Encke probes were meant to depart Earth between 16 and 30 August 1980 atop a Titan rocket with a Centaur upper stage. Ironically, given CONTOUR's fate, the GSFC team rejected an additional solid-propellant "kick" rocket motor as too risky. The probes would travel on a curving ballistic path directly from Earth to Encke; hence the term "ballistic" in the mission's description.

Robert Farquhar led the four-person GSFC team. In 1972-1973, he had participated in GSFC's 35-member Cometary Explorer Study Group, which aimed to explore Comet Grigg-Skjellerup in April 1977 and Comet Giacobini-Zinner in February 1979 using a single 450-kilogram spinning spacecraft. The NASA-appointed Comet and Asteroid Science Advisory Committee had endorsed Cometary Explorer as the first step in a logical program of comet exploration leading to a NASA Comet Halley mission in 1985-1986.

Unfortunately, the U.S. civilian space agency, faced with rapidly declining budgets and bearing the heavy burden of Space Shuttle development, had been unable to fund Cometary Explorer. The 1980 Encke slow flyby mission would, it was hoped, put NASA comet exploration back on track to Halley.

Technicians at Cape Canaveral lower the launch shroud over the West German-U.S. Helios B solar probe spacecraft. Image credit: NASA.

Farquhar's team modeled its Comet Encke mission on the German-U.S. Helios A/Helios B Sun probe missions. Helios A left Earth in late 1974 (about a month after the GSFC group published its Technical Note, in fact). The Helios probes were designed to survive an approach to 0.3 times the Earth-Sun distance, which is inside the orbit of the planet Mercury. The Encke probes, for their part, would pass their cometary target as it neared perihelion (the point in its orbit where it was nearest the Sun) at 0.34 times the Earth-Sun distance. The Helios probes would orbit in the plane of the Solar System; the Encke probes would match their target's modest orbital tilt.

The GSFC team's Encke probes, which would spin to create gyroscopic stability, would move apart immediately after they separated from their launch vehicle's Centaur stage. Farquhar's team dubbed them the "tail probe" and the "coma probe." Each would resemble the lower half of a hourglass-shaped Helios spacecraft. Solar cells on their sides would power spacecraft systems and a suite of science instruments.

If necessary, a course-correction rocket burn would take place 10 days after launch. A second burn 50 days after launch would aim the tail probe at a point in the Comet Encke's wan tail about 10,000 kilometers behind the nucleus and would aim the coma probe at a point immediately in front of the nucleus. A third, very modest, course-correction burn was scheduled for Launch +85 days. The two spacecraft would encounter Comet Encke at about Launch +102 days.

Depending on their launch date, the Comet Encke spacecraft would reach their target between 3 December and 8 December 1980 moving at between 7.6 and 9.03 kilometers per second. Comet Encke would reach perihelion on 6 December. The Encke flybys would occur at around 1000 hours Greenwich Mean Time on all days of their arrival window so that the 100-meter dish-shaped antenna at Effelsberg, West Germany — the same antenna used to communicate with the Helios probes — could receive data for as long as possible before the twin probes set below the local horizon.

Image credit: NASA.

Farquhar and his colleagues envisioned that their two probes would carry slightly different science payloads. The 375-kilogram coma probe, which would linger within 1000 kilometers of the sunlit side of the nucleus for nearly 42 minutes, would include a despun platform bearing its radio dish antenna, TV camera, neutral mass spectrometer, UV spectrometer, and Lyman-alpha spectrometer. The 325-kilogram tail probe would include a despun antenna, but would lack the coma probe's despun platform with its four instruments. Both probes would include on their spinning main sections an ion mass spectrometer, a DC magnetometer, an AC magnetometer, an electron analyzer, a plasma analyzer, an electric field detector, a dust detector, and a dust composition instrument.

The GSFC team was not the only group in 1974 that planned a 1980 Comet Encke mission. The GSFC scientists and engineers made a point of comparing their mission plan with its main rivals. They explained that, in their comparison, "the primary evaluation criteria [would] be the science value and realism of attaining mission objectives."

Their plan's leading rival, a mission design advocated mainly by the Jet Propulsion Laboratory and its contractors, was based on solar-electric propulsion. Launch would take place on 17 December 1978 and a Comet Encke flyby would occur on 6 November 1980. The GSFC team noted that the mission's 30-centimeter-diameter solar-electric (ion) propulsion thruster had yet to be developed, let alone tested; nevertheless, it would be expected to operate flawlessly for 690 days.

In addition, the thruster would interfere with the spacecraft's particle-and-fields instruments. Interference would not cease when the thruster was switched off.

Assuming that its untried thruster functioned as hoped, however, the solar-electric spacecraft would pass Comet Encke moving at only four kilometers per second, which constituted an advantage over GSFC's ballistic slow flyby. It would do so, however, more than a month before perihelion, when Comet Encke was still about 0.5 times the Earth-Sun distance from perihelion. At that point in its orbit, the nucleus would be relatively inactive: if past observations were any guide, Comet Encke would have almost no tail.

The ballistic slow flyby's lesser rival was a ballistic fast flyby advocated mainly by NASA Ames Research Center and its contractors. A spin-stabilized spacecraft similar to the Pioneer 10 and Pioneer 11 outer Solar System spacecraft would launch on 18 August 1980 atop a relatively cheap Atlas/Centaur rocket with a solid-propellant kick stage. After a voyage of just 92 days, the spacecraft would whiz past Comet Encke on 18 November 1980 at a blistering 20.1 kilometers per second.

Farquhar's group noted that high-speed impacts with Comet Encke dust particles could easily destroy the ballistic fast flyby spacecraft, and that its camera would likely return only motion-blurred images (assuming that it had time to locate the nucleus or any other important comet features). It would remain within 1000 kilometers of the nucleus for a mere nine minutes.

The GSFC team concluded that, compared with the solar-electric and ballistic fast flybys, the ballistic slow flyby was "superior in every respect." This assertion may well have been correct; the rivalry between the slow flyby, solar-electric, and fast flyby groups split the small community of comet exploration advocates, however, helping to ensure that no spacecraft explored Comet Encke in 1980.

Comet Encke as observed by the MESSENGER Mercury orbiter on 17 November 2013. Encke passed the planet Mercury at a distance of just 3.7 million kilometers and reached perihelion four days later. Image credit: NASA/JHUAPL/Carnegie Institution of Washington.

Sources

Mission Design for a Ballistic Slow Flyby of Comet Encke 1980, NASA Technical Note D-7726, R. Farquhar, D. McCarthy, D. Muhonen, and D. Yeomans, NASA Goddard Space Flight Center, November 1974.

Comet Nucleus Tour CONTOUR Mishap Investigation Board Report, NASA, 31 May 2003.

More Information

Cometary Explorer (1973)

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

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

238,000 miles from home — Earth as viewed by the Apollo 8 astronauts in lunar orbit, Christmas Eve 1968. Image credit: NASA.

The three-man crew of Apollo 8 — Commander Frank Borman, Command Module Pilot James Lovell, and Lunar Module Pilot William Anders — was the first to leave Earth on a giant Saturn V rocket. They departed Cape Kennedy, Florida, on 21 December 1968, and left Earth orbit for the Moon about two and a half hours after launch.

Though its target was the Moon, the Apollo 8 mission included no Lunar Module (LM). The piloted lunar lander had suffered production delays, which was understandable given that no one had previously built a vehicle for landing humans on another world.

NASA's planned mission sequence for piloted Apollo missions had begun with a low-Earth orbit (LEO) test of the Command and Service Module (CSM) during Apollo 7 (11-22 October 1968). This was to have been followed immediately by an LEO test of the CSM and LM, then a CSM/LM test flight in higher Earth orbit. During the fourth mission in the sequence, astronauts would test the CSM and LM in lunar orbit, then the first Apollo lunar landing attempt would take place. NASA designated these five increasingly ambitious missions C, D, E, F, and G.

Putting off the next Apollo flight — the D mission — until the LM was ready might have placed in jeopardy attainment of Apollo's goal of landing a man on the Moon ahead of the Soviet Union and before the end of the 1960s. Because of this, in late summer 1968, NASA began to look at a modified mission sequence.

The C' mission, which would see the Apollo 8 CSM orbit the Moon without an LM, was revealed to the public on 12 November 1968, three weeks after Apollo 7 successfully accomplished the C mission. Apollo 8 would test many CSM elements of the lunar landing mission and the world-wide system of radio dishes and transceivers NASA had created for Apollo lunar mission communications and tracking.

The C' mission had been the subject of intense debate at the highest levels of NASA, for it meant traveling to the Moon without the backup life support and propulsion systems the LM could provide. Intelligence reports that indicated that the Soviet Union might launch a man around the Moon during December 1968 gave C' supporters added credibility. The Soviet mission might steal Apollo's thunder; though it would merely swing around the Moon and fall back to Earth, it would enable the Soviets to claim that they had launched a man to the Moon first.

Eleven hours after launch, the Apollo 8 crew carried out a course correction. This required that they ignite the CSM's Service Propulsion System (SPS) main engine for the first time. Had the SPS not functioned as planned, the crew could have adjusted their course using the CSM's cluster of four Reaction Control System (RCS) thruster quads. The CSM would then have swung around the Moon without entering orbit and fallen back to Earth.

Partial cutaway of Apollo CSM spacecraft. Image credit: NASA.

The 20,500-pound-thrust SPS, an AJ-10-137 rocket engine manufactured by Aerojet, was located at the aft end of the CSM. Other AJ-10 variants had propelled Vanguard, Atlas-Able, and Thor-Able launch vehicles.

The SPS burned hydrazine/UDMH fuel and nitrogen tetroxide oxidizer. Chemically inert helium gas pushed the propellants into the engine's ignition chamber. Hydrazine/UDMH and nitrogen tetroxide are hypergolic propellants; that is, they ignite on contact with each other. The resulting hot gas then vented through a large engine bell, which was designed to swivel to help steer the CSM.

The Apollo 8 SPS performed almost perfectly during the 21 December course correction burn and during a second burn 61 hours after launch. Three hours later, Mission Control in Houston gave Apollo 8 a "go" to enter lunar orbit. The spacecraft passed behind the Moon, out of radio contact with Earth, and the crew ignited the SPS for the third time. It burned for a little more than four minutes, slowing the Apollo 8 CSM enough for the Moon's gravity to capture it into orbit.

The Apollo 8 CSM orbited the Moon 10 times over the next 20 hours. Then, on 25 December 1968, about 89 hours after launch, the crew ignited the SPS behind the Moon to begin the journey home to Earth. The rocket motor performed flawlessly during the critically important burn, which NASA dubbed Trans-Earth Injection (TEI).

Two and a half days later, on 27 December, the CSM split into two parts. The Service Module (SM), which included the SPS, separated from the Command Module (CM), which held the crew. The former burned up in Earth's atmosphere as planned, while the latter, protected by a heat shield, maneuvered in the upper atmosphere to reduce heating and deceleration, deployed parachutes, and splashed safely into the Pacific Ocean.

Four days after Apollo 8's triumphant return, A. Haron and R. Raymond, engineers with Bellcomm, NASA's Washington, DC-based planning contractor, completed a brief study of what might have happened had the SPS not ignited for the TEI burn. Specifically, they looked at how long a crew might survive in lunar orbit following a TEI failure.

Haron and Raymond found that the "first constraint" on the crew's endurance would be depletion of the CSM's supply of lithium hydroxide (LiOH) canisters. The square canisters were used in pairs to remove carbon dioxide exhaled by the crew from the CSM's pure oxygen atmosphere. During Apollo 8, the crew traded a saturated LiOH canister for a new one every 12 hours, so expended two per day.

The Bellcomm engineers calculated that, at that rate, the crew would use up the last of the 16 LiOH canisters launched on board the CSM 96 hours after TEI failure. They would then grow drowsy and become unconscious as carbon dioxide accumulated in the crew cabin. Had TEI failed on Apollo 8, Borman, Lovell, and Anders would probably have suffocated on 29 December.

Haron and Raymond noted, however, that LiOH canisters might be changed less often without harming the crew. They cited a November 1968 Manned Spacecraft Center study that showed that a LiOH canister could absorb carbon dioxide for up to 37 hours. If a stranded Apollo CSM crew began to ration its LiOH canisters immediately after TEI failure, they would be able to stretch their survival time to 148 hours. In that case, the Apollo 8 crew would have survived until New Year's Eve – the day Haron and Raymond completed their study.

By that point, limitations on crew survival other than carbon dioxide buildup would begin to come into play. The CSM fuel cells, manufactured by Allis Chalmers, operated by combining liquid hydrogen and liquid oxygen reactants to produce electricity and water. Electricity from the fuel cells powered the CSM through most of the mission. The crew drank the water, which also was used for cooling the CSM's Environmental Control System (ECS) and electronics. Excess water was dumped overboard.

Haron and Raymond looked briefly at the possibility of switching off two fuel cells to conserve reactants. If this were done, then the remaining fuel cell might operate for up to three weeks after TEI failure. However, a single fuel cell would probably not produce enough electricity to operate all CSM systems vital to the crew's continued survival, some of which were not immediately obvious.

As an example, Bellcomm cited the RCS quads. The astronauts would need to use them to maneuver the CSM to keep its ECS radiators in shadow to conserve cooling water. In addition, the LiOH canister shortage would remain. "The feasibility of extending survival time to as much as three weeks cannot be confirmed at this time," Haron and Raymond wrote.

The Bellcomm study was mainly of academic interest; a crew stranded in orbit around the Moon, 238,000 miles from Earth, could not have been rescued even if they did survive for three weeks. NASA did not have the ability to maintain a rescue Saturn V rocket and CSM on standby.

The space agency would have cause to recall the brief Bellcomm study twice during subsequent Apollo missions. During Apollo 13 (11-17 April 1970), an oxygen tank exploded in the CSM Odyssey, badly damaging its SM.

Because the explosion happened while the mission was en route to the Moon, its crew, commanded by Apollo 8 astronaut James Lovell, was able to use the LM Aquarius as a lifeboat. The astronauts employed the LM descent engine in place of the SPS. The docked spacecraft flew behind the Moon, where the crew fired the descent engine to adjust their course and accelerate toward Earth.

During Apollo 16 (16-27 April 1972), as the CSM Casper orbited the Moon, it suffered a malfunction in the system meant to swivel its SPS engine bell. The LM Orion, which had already undocked in preparation for landing, stood by in lunar orbit until the SPS problem was understood, then landed several hours behind schedule.

Had it been judged necessary, NASA could have scrubbed the Apollo 16 landing. Orion would then have redocked with Casper. The astronauts could have used Orion's descent engine and (if necessary) Casper's RCS quads to perform TEI.

Proceeding with the landing eliminated that option; the descent engine used most of its propellants to land on the Moon, then was left behind on the surface with the rest of the LM descent stage. The LM ascent stage, with its smaller engine, returned to lunar orbit with virtually dry tanks. This left only the SPS available for TEI.

As a precaution, NASA moved up Apollo 16's TEI burn by a day in the hope that, should the SPS misbehave, the crew and engineers on Earth would have adequate time to find a solution and ensure a safe, if delayed, return to Earth. As it turned out, the Apollo 16 SPS performed a flawless TEI burn.

Sources

NASA News Press Kit, Project: Apollo 8, 15 December 1968.

"Consumables Affecting Extended CSM Lifetime in Lunar Orbit," Case 320, A. Haron and R. Raymond, Bellcomm, Inc., 31 December 1968.

Apollo 8: "A Most Fantastic Voyage," Lt. Gen. Sam C. Phillips, National Geographic, May 1969, pp. 593-631.

Apollo 13: "Houston, We've Had a Problem," NASA EP-76, 1970.

NASA Mission Report: Apollo 13, A Successful Failure, 20 May 1970.

How Apollo Flew to the Moon, W. David Woods, Springer Praxis, 2008, pp. 236-238.

More Information

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

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

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

Multiple Asteroid Flyby Missions (1971)

Pioneer 10/11. Image credit: NASA.

Between 1 January 1801, when asteroid 1 Ceres was discovered, and March 1971, when the International Astronomical Union (IAU) held its "Physical Studies of the Minor Planets" colloquium in Tucson, Arizona, astronomers compiled a list of 1748 numbered asteroids. That they bore numbers indicated that they had each been observed more than once so that their orbital paths and positions were known.

Astronomers had spotted many thousands of other asteroids. Often they appeared as annoying streaks on photographic plates intended to capture images of objects deemed more worthy of an astronomer's attention: for example, distant galaxies. The vast majority of those asteroids were not knowingly observed again.

As the 1960s ended and the 1970s began, enthusiasm for asteroids as objects of study began to grow. In part this was because the future looked bright for robotic space exploration. No longer were automated spacecraft seen mainly as precursors for piloted moon missions.

Mars, with its two satellites Phobos and Deimos — believed to be captured asteroids — came in for special attention in NASA's 1970s robotic exploration program. Mariners 8 and 9 were scheduled for launch less than three months after the Tucson colloquium. The twin spacecraft, built by the Jet Propulsion Laboratory (JPL) in Pasadena, California, were planned to be the first Mars orbiters, though there was some concern that the Soviets might get there first.

The twin Viking spacecraft were scheduled to leave Earth for Mars in mid-1975. Each would include a Lander and an Orbiter. The Viking Landers were planned to be the first Mars soft-landers. NASA Langley Research Center (LaRC) in Hampton,Virginia, managed Viking; JPL served as contractor for the Mariner-based Viking Orbiters.

Pioneers 10 and 11 (image at top of post) were on track to become humankind's first emissaries to the Outer Solar System. To reach Jupiter (and, in the case of Pioneer 11, Saturn), they would become the first spacecraft to pass through the Asteroid Belt between Mars and Jupiter. Built by TRW and managed by NASA Ames Research Center (ARC) in Mountain View, California, they were due to launch in March 1972 and April 1973, respectively.

Most of the papers presented at the Tucson colloquium emphasized Earth-based telescope observations of asteroids, not spacecraft exploration, and in fact a consensus emerged by the end of the colloquium that spacecraft exploration of asteroids would be premature. Nevertheless, among those present in Tucson was a small cadre of asteroid mission proponents. LaRC engineers David Brooks and William Hampshire, for example, described missions to multiple Main Belt asteroids.

They looked first at the simplest multiple asteroid flyby mission: one which saw a spacecraft launched on a random date into an elliptical Sun-centered orbit with a perihelion of one Astronomical Unit (AU) and an aphelion within the Asteroid Belt at three AU. An AU is equal to the mean Earth-Sun distance. They assumed that, in order to obtain useful data, the spacecraft would need to pass no more than 15,000 kilometers from an asteroid. Based on these stipulations, they calculated that a randomly launched spacecraft stood virtually no chance of exploring even one asteroid.

They added, however, that a randomly launched spacecraft stood a good chance of passing within 0.1 AU of 10 asteroids on average. While 0.1 AU (about 15 million kilometers) was too great a distance for effective exploration, this finding meant that, if the spacecraft could change its velocity, then it could shape its orbital path to pass within 15,000 kilometers of multiple asteroids.

Large velocity changes would enable exploration flybys of many asteroids, but would require costly new spacecraft development. New asteroid discoveries would also increase the number of possible flyby candidates. Brooks and Hampshire were determined, however, to show what could be accomplished with small velocity changes, spacecraft already in the development pipeline, and the 1748 asteroids numbered as of March 1971.

The Viking Orbiter, they noted, would carry enough propellants to change its velocity by 1.5 kilometers per second. They revealed that NASA ARC had studied a Pioneer 10/11-class spacecraft modified to capture into a highly elliptical Jupiter orbit. A Pioneer 10/11-class spacecraft could change its velocity by only 0.2 kilometers per second, but the hypothetical Pioneer Jupiter orbiter would up this to about one kilometer per second.

The LaRC engineers then provided detailed multiple asteroid flyby sequences for three missions. The missions were: leave Earth in the 1980-1982 period and orbit the Sun in a one-AU-by-three-AU orbit; leave Earth in late 1975 and fly past 1 Ceres; and leave Earth in 1975 and travel through the Asteroid Belt to Jupiter.

In no case would a spacecraft perform maneuvers which together would change its velocity by more than one kilometer per second. In each case, Brooks and Hampshire assumed that the spacecraft would pass 15,000 kilometers from the first asteroid it encountered because its launch vehicle put it there; that is, the propellant cost of exploring the first asteroid in any multiple asteroid sequence would count against the rocket stage that boosted the spacecraft out of Earth orbit, not the spacecraft itself.

Brooks and Hampshire found that launch from Earth into a one-by-three-AU orbit on 14 July 1981 would cause the spacecraft to pass within 0.1 AU of 15 asteroids over the course of 659 days. Unfortunately, nudging the spacecraft's path so that it would pass within 15,000 kilometers of all 15 would require a total velocity change of 41.6 kilometers per second.

Large velocity changes were necessary in part because some flybys occurred close together. The spacecraft would, for example, pass about 0.1 AU from asteroid 149 Medusa on 10 January 1982, just nine days after a 15,000-kilometer flyby of asteroid 1515 Perrotin. A velocity change of several kilometers per second would be required to bend the spacecraft's path to enable it to pass just 15,000 kilometers from 149 Medusa so soon after leaving 1515 Perrotin.

Spacing out asteroid encounters meant that a small spacecraft velocity change immediately after a close asteroid flyby could yield a large spacecraft orbit change. If mission designers opted instead to follow the 1515 Perrotin close flyby with a close flyby of 1674 Groeneveld six months later (13 June 1982), then added a 12 July 1983 close flyby of 561 Ingewelde, the total spacecraft velocity change would amount to just 0.93 kilometers per second.

The LaRC engineers identified seven three-asteroid missions and one four-asteroid mission, all launched on 14 July 1981, that would need total velocity changes of less than one kilometer per second. A multiple flyby mission to 149 Medusa, 870 Manto, and 1720 Neils would require the smallest velocity change - just 0.58 kilometers per second. The four-asteroid mission, which would explore 1515 Perrotin, 1674 Groeneveld, 561 Ingewelde, and 1720 Neils, would need a total velocity change of 0.8 kilometers per second.

Brooks and Hampshire gave less attention to their 1975 Ceres and Jupiter multiple asteroid missions. They determined that a late 1975 launch would enable close flybys of 632 Pyrrha and either 946 Poësia or 947 Monterosa en route to 1 Ceres, at 950 kilometers across the largest asteroid. The 632 Pyrrha-947 Monterosa-1 Ceres flyby sequence would need the lowest total velocity change of any of mission they studied: just 0.24 kilometers per second.

Jupiter-bound spacecraft presented two new problems, Brooks and Hampshire explained. First, they would move fast. For example, a spacecraft bound for Jupiter in 1975 targeted to pass the large (124-by-75-kilometer) asteroid 27 Euterpe would zip past at 18 kilometers per second, making data collection difficult.

In addition, a Jupiter-bound spacecraft would follow a short path through the Asteroid Belt, so would pass few asteroids. Brooks and Hampshire were able to identify only one multiple asteroid flyby opportunity for the 1975 Jupiter mission. The spacecraft would fly first past 666 Desdemona on 19 November 1975, then past 396 Aeolia on 6 April 1976. It would change its velocity by 0.52 kilometers per second. The LaRC engineers did not indicate whether any part of the velocity change would be applied to correcting the spacecraft's course to Jupiter after the 396 Aeolia flyby.

NASA would need two decades to carry out its first multiple asteroid flyby mission, and when it did the mission would resemble none of Brooks and Hampshire's scenarios. The Galileo Jupiter Orbiter grew from the NASA ARC Pioneer Jupiter Orbiter they described at the Tucson colloquium. The mission received new-start funding in 1977. Launch aboard the Space Shuttle was scheduled for January 1982.

Space Shuttle delays, fierce political battles over the type of upper stage that would propel Galileo to Jupiter, costly redesigns to enable it to ride different upper stages (one redesign involved splitting Galileo into two spacecraft), and technical problems with the Centaur G' upper stage delayed Galileo's planned launch to May 1986. In January 1985, NASA Administrator James Beggs added to the Galileo mission the option of a flyby of the large (233-by-193-kilometer) asteroid 29 Amphitrite.

Destruction of the Space Shuttle Challenger on 28 January 1986 caused more delays and cancellation of the Centaur G' stage needed to boost Galileo directly to Jupiter. JPL re-planned the Galileo mission for an October 1989 launch with gravity-assist flybys of Venus and Earth (twice) and Jupiter arrival in December 1995. The new path put 29 Amphitrite far out of reach.

Following its first Earth gravity-assist, Galileo entered the Asteroid Belt and performed the first-ever asteroid flyby: a cruise past 18-by-nine-kilometer 951 Gaspra at a distance of 1604 kilometers on 29 October 1991. Galileo flew past Earth a second time to gain the final gravity-assist speed boost it needed to reach Jupiter; then, on 28 August 1993, it flew by 60-by-19-kilometer 243 Ida at a distance of 2410 kilometers, revealing that it has small moon. Dactyl, as the 1.6-by-1.4-by-1.2-kilometer satellite was named, brought to three the number of asteroids Galileo explored during its circuitous voyage to Jupiter.

Main Belt asteroid 243 Ida and its moon Dactyl, 28 August 1993. Image credit: NASA.

Sources

"Multiple Asteroid Flyby Missions," David Brooks and William Hampshire, NASA SP-267, Physical Studies of the Minor Planets, Proceedings of the 12th Colloquium of the International Astronomical Union held in Tucson, Arizona, 6-10 March 1971, Tom Gehrels, editor, 1972, pp. 527-537.

"Reasons for Not Having an Early Asteroid Mission," Edward Anders, NASA SP-267, Physical Studies of the Minor Planets, Proceedings of the 12th Colloquium of the International Astronomical Union held in Tucson, Arizona, 6-10 March 1971, Tom Gehrels, editor, 1972, pp. 479-485.

Memorandum, Clark Chapman to various, "Notes Concerning the 'Centaur Wars' and Possible Action by the Planetary Science Community," 14 September 1982.

Interoffice Memorandum GLL-JRC-84-189, Jet Propulsion Laboratory, J. R. Casani to W. E. Giberson, "Galileo Asteroid Flyby," 11 September 1984.

Memorandum, T. V. Johnson, W. J. O'Neil, and C. M. Yeates to PSG/IDS, "Galileo Asteroid Encounter," Jet Propulsion Laboratory, 1 October 1984.

Press Release, Public Information Office, Jet Propulsion Laboratory, "NASA Administrator James M. Beggs has approved the addition of an asteroid flyby option to the Galileo mission," 17 January 1985.

Interoffice Memorandum GLL/TCC-87.238, Jet Propulsion Laboratory, T. C. Clarke to Galileo Project Science Distribution list, "Minutes of Galileo PSG Meeting of 11/21/86," 20 April 1987.

Journey Into Space: The First Thirty Years of Space Exploration, Bruce Murray, W. W. Norton, 1989, pp. 180-237.

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Earth-Approaching Asteroids as Targets for Exploration (1978)

The Challenge of the Planets, Part Three: Gravity

Solar Flares and Moondust: The 1962 Proposal for an Interdisciplinary Science Satellite at Earth-Moon L4

Mariner 1 launches atop an Atlas-Agena B rocket, a missile-derived workhorse of the early Space Age. Image credit: NASA.

James Van Allen discovered the two Earth-circling radiation belts that bear his name in 1958. The discovery, based on data from Explorer 1 (the first U.S. satellite), Explorer 3, and Pioneer 3, was the first fundamentally new finding of the Space Age. In addition to its scientific and practical importance, it constituted a prestige victory in the Cold War space race with the Soviet Union. Time magazine put Van Allen on the cover of its 4 May 1959 issue.

The Van Allen Belts are a feature of Earth's magnetosphere. Though fascinating in its own right, the magnetosphere became a source of frustration for scientists eager to study the Sun. This is because Earth's magnetic envelope blocks solar particles, preventing detailed study of flares and other solar phenomena.

Physicist James Van Allen (center) holds aloft the backup Explorer 1 satellite and upper stage with Jet Propulsion Laboratory director William Pickering (left) and Wernher von Braun. Image credit: NASA.

Van Allen chaired the National Academy of Sciences Space Science Board Summer Study in Iowa City, Iowa, between 17 June and 10 August 1962. Iowa City is home to the University of Iowa, where Van Allen was a professor. His role as chair of the two-month study pointed up its intended significance. The Summer Study was meant to chart a course for U.S. space science and to bring together under NASA sponsorship the disparate elements of the nascent space science community. It involved more than 100 scientists from many disciplines.

Among them were Leo Steg, a General Electric scientist whose specialties were missile reentry vehicles and orbital mechanics, and Eugene Shoemaker, a U.S. Geological Survey geologist noted for his study of asteroid impacts, impact and explosion craters, and the cratered lunar surface. Their collaboration on a brief report on the uses of a libration (L) point satellite illustrates the interdisciplinary intent of the Iowa City study. It was also among the earliest proposals to treat the L points as destinations that could be explored and put to good use.

The Earth-Moon system contains five L points. They are features as real as the moon and the Earth. In theory, an object parked at one of these "equilibrium" points will remain there indefinitely. In practice, the Sun's gravity perturbs objects parked at the Earth-Moon L points, making station-keeping necessary. The propulsive energy (and thus propellants) needed to keep station is, however, quite modest.

Steg and Shoemaker examined the possibility of placing a satellite in orbit about the L4 or L5 point of the Earth-Moon system. L4 is located 60° ahead of the Moon along its orbit about the Earth; L5 is 60° behind the moon along its Earth-centered orbit.

Beyond Earth's magnetosphere and nearly always in view of the Sun, either L4 or L5 would, they wrote, "be an excellent location for a satellite whose objective is to perform solar-flare observations." Even if the magnetosphere did not interfere with its observations, a solar-observation satellite in low-Earth orbit would spend up to half its time in Earth's shadow, in the night portion of its orbit, so could not monitor the Sun continuously. The L4 or L5 satellite would be eclipsed by the Earth about as often as the Moon is — that is, for a few hours each year.

Schematic illustration of Earth-Moon system with libration points indicated. Image credit: NASA.

In keeping with his scientific discipline, Shoemaker had a geologic interest in the Earth-Moon L4 and L5 points. It stemmed from a possible breakthrough made behind the Iron Curtain 14 months before the Iowa City meeting. In March-April 1961, Polish astronomer Kazimierz Kordylewski had succeeded in photographing very faint dust clouds at the Earth-Moon L4 and L5 points. He had first observed them in 1956 while peering through a telescope, but had at first been unable to capture them on film. The clouds were thought to be made up of dust knocked off the Moon by large asteroid impacts and captured temporarily at the L4 and L5 points.

Had it flown, Steg and Shoemaker's L point mission would have begun with an Atlas-Agena B rocket launch (image at top of post) from Cape Canaveral on 24 October 1963. After arrival in low-Earth parking orbit, the rocket's Agena upper stage would have restarted to boost a nearly 900-pound satellite toward the Earth-Moon L4 point. The satellite would travel the 246,781-mile path to L4 in about 78 hours.

Steg and Shoemaker envisioned that their satellite would include a rocket engine and propellants with a total mass of 360 pounds for course corrections, injection into an elliptical orbit around the L4 point, and station-keeping. The satellite's 70-pound science payload would include a 30-pound micrometeorite collector/analyzer for study of Kordylewski cloud dust grains, thus permitting examination of possible lunar surface material without a Moon landing. The remaining 40 pounds of instrumentation would be dedicated to solar-flare observations.

Fifty pounds of radio equipment would transmit the L4 satellite's findings to Earth. Steg and Shoemaker noted that their proposed satellite's unique position might enable it to serve as a useful "communication base" for future lunar missions. It might, for example, relay radio signals between Earth and part of the Farside, the lunar hemisphere that is turned always away from the Earth.

Sources

A Review of Space Research: The Report of the Summer Study conducted under the auspices of the Space Science Board of the National Academy of Sciences at the State University of Iowa, Iowa City, Iowa, 17 June-10 August 1962, Publication 1079, National Academy of Sciences – National Research Council, Washington, DC, 1962

"Dust-Cloud Moons of the Earth," J. Wesley Simpson, Physics Today, February 1967, p. 39

More Information

Earth-Approaching Asteroids as Targets for Exploration (1978)

"He Who Controls the Moon Controls the Earth" (1958)

The Farside hemisphere of the Moon with Earth in the background: an image captured on 16 July 2015 by NASA's telescopic EPIC camera on the National Oceanic and Atmospheric Administration's DSCOVR spacecraft at the Sun-Earth L1 point. Image credit: NASA/NOAA.

On 28 January 1958, U.S. Air Force Brigadier General Homer A. Boushey, Deputy Director of U. S. Air Force Research and Development, spoke before the Aero Club of Washington. The weekly news magazine U.S. News & World Report took note and published excerpts from his speech.

Boushey warned the Aero Club of dire consequences should the Soviet Union seize control of the Moon. He presented his speech four months after Soviet engineers had launched 83.6-kilogram Sputnik 1, the first artificial satellite, three months after they had launched the dog Laika on board 508.3-kilogram Sputnik 2, and three weeks after the failure of Vanguard TV-3, the first U.S. attempt to launch a satellite.

When Boushey is described in any detail, he is often portrayed as a strangelovian Cold Warrior. He is, however, better seen as an early U.S. rocketry and spaceflight proponent. He had enrolled in Stanford University to study engineering in 1929, but the Wall Street Crash of that year and consequent Great Depression intervened, so in 1932 he joined the U.S. Army Air Corps. During training at Randolph Field, Texas, he encountered a copy of U.S. rocketry pioneer Robert Goddard's seminal 1919 monograph A Method of Reaching Extreme Altitudes.

Pioneering U.S. rocketeer Homer A. Boushey. 
Image credit: U.S. Air Force.

Goddard had launched the world's first liquid-propellant rocket, named "Nell," on 16 March 1926, in Auburn, Massachusetts. The rocket flew 184 feet, or roughly half the length of a Saturn V Moon rocket. He received funding support for his rocket experiments from the Smithsonian Institution, which published his monograph, and from the wealthy Guggenheim family. The latter's support enabled Goddard to move his experiments to the wide-open spaces of New Mexico in 1930.

Boushey completed his aeronautical engineering degree at Stanford in 1936, and joined the Aircraft Laboratory at Wright Field in Ohio. While there, he corresponded with and visited Goddard. The two men became fast friends; Goddard would become the godparent of one of Boushey's daughters.

In August 1941, Boushey served as the test-pilot for a series of U.S. government-funded rocket-assisted take-off experiments. These employed solid-propellant rocket motors to boost a single-seater Ercoupe airplane off a runway. Theodore Von Kármán of the Guggenheim Aeronautical Laboratory at California Institute of Technology led the rocket development effort. The plane's single propeller was removed for the final test on 23 August; Boushey then took off under rocket thrust alone, making him the first American to pilot an exclusively rocket-powered aircraft.

During the Second World War, Boushey commanded the first U.S. jet-powered fighter group. In the days after the Japanese capitulation, he flew over Hiroshima, allowing him to observe firsthand the devastation nuclear weapons could cause.

The first U.S. rocket plane: Army Air Corps pilot Homer Boushey takes to the air in one of a series of rocket-assisted take-off flight tests. Image credit: U.S. Air Force.

In his January 1958 talk, Boushey acknowledged that there existed in the nascent U.S. space community "divided opinion as to whether or not a manned or unmanned Moon base has any military significance." He then presented arguments in favor of a military lunar base.

The Moon, he explained, is 239,000 miles away, a distance a rocket might cross in about two days. Boushey noted that the Moon is a synchronous rotator, which means that it keeps the same face turned always toward Earth. Telescopes on the moon's Earth-facing Nearside could thus monitor military activities on the revolving Earth as they passed in and out of view. Boushey estimated that objects as small as 100 feet wide might be visible. Conversely, the Farside hemisphere (image at top of post) is always turned away from Earth. Boushey believed that this would make it an ideal location for conduct of secret military operations beyond the reach of prying eyes in Russia.

Earth's Moon, Boushey declared, could also provide "a retaliation base of unequaled advantage." If the U.S. gained control of the Moon, then the Soviets would be unable to attack the United States without suffering "sure and massive destruction." They could either attack the U.S. first and endure a counter-strike from the Moon about 48 hours later, or they could launch missiles at the Moon first. The U.S. military lunar base would, of course, immediately detect the light and heat of the Soviet missiles' rocket exhaust and launch a retaliatory strike.

Boushey then spoke what are probably the most famous words in his speech: "[i]t has been said that 'he who controls the Moon controls the earth.' Our planners must carefully evaluate this statement for, if true - and I, for one, think it is - then the U.S. must control the Moon."

Seventy-five years ago: U.S. aircraft dropped nuclear weapons on the Japanese port cities of Hiroshima (left) and Nagasaki on 6 August and 9 August 1945. To date these remain the only nuclear weapons used in anger. Homer Boushey flew over the ruins of Hiroshima and saw the devastation there for himself. Image credit: Wikipedia.

The excerpts from Boushey's speech in U.S. News & World Report contained no overt mention of the possibility that U.S. missiles might be launched from the Moon preemptively; that is, that the U.S. Moon base might be used to destroy the Soviet Union with little risk of retaliation. Boushey did, however, describe attributes of the Moon that would make a preemptive attack feasible.

The Moon's weak gravitational pull, coupled with its lack of an atmosphere, would permit missiles to be "catapulted" from their siloes, thereby avoiding use of easily detected rocket motors. Hiding the siloes on the Farside would further increase the odds that a U.S. attack would go unnoticed until warheads entered Earth's atmosphere over Soviet territory.

Building and maintaining the U.S. military lunar base would not, Boushey maintained, have to break the bank. He assumed that the Moon would be found to be made of the same elements as the Earth, so that the "possibilities of construction and creation of an artificial environment [would be] virtually unlimited." Electricity from solar panels made on the Moon could be stored using massive lunar-made flywheels which, once spun up by lunar-made electric motors, could spin for weeks in the absence of atmospheric friction. By using the flywheels to turn the electric motors, the latter could become generators for supplying the base with electricity during the two-week lunar night.

Boushey ended his speech by offering an alternative to lunar militarization. He pointed out that on "January 16 [1958] Secretary [of State John Foster] Dulles proposed the formation of an international commission to insure [sic] the use of outer space exclusively for peaceful purposes, and if the Soviet premier is sincere in decrying the production of ever-more-powerful weapons he will jump at the chance. In 10 years," he added, "the opportunity of jointly imposing control may have been lost."

U.S. Secretary of State Dean Rusk signs The Outer Space Treaty at the White House as President Lyndon Baines Johnson (right), British Ambassador to the United States Sir Patrick Dean (center), and Soviet Ambassador to the United States Anatoly Dobrynin (left) look on. Image Credit: United Nations.

Almost exactly nine years after Boushey delivered his speech, on 27 January 1967, the U.S., the Soviet Union, and the United Kingdom signed the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies. The following October, they called upon other nations of the world to sign and ratify what had by then become known as The Outer Space Treaty. Among other far-reaching provisions, it required that the Moon not be put to any military purpose. The Outer Space Treaty, which took effect on 10 October 1967, became the rock upon which the body of international space law is built.

By the time The Outer Space Treaty took effect, Boushey had been retired from the Air Force for a little more than six years. He ended his career at age 52 in July 1961 as Commander of the Air Force Arnold Engineering Development Center in Tennessee. By that time, President Dwight Eisenhower had passed over the military in favor of civilian U.S. space exploration under the aegis of NASA. Despite military support for NASA programs and some brave starts, such as the Dyna-Soar spaceplane and the Manned Orbiting Laboratory, U.S. military spaceflight would be limited mainly to automated surveillance satellites until the Space Shuttle era.

Soon after his retirement, Boushey became an outspoken critic of the escalating war in Indochina. Despite this, President Richard Nixon recognized his key role in U.S. astronautics by inviting him to the 13 August 1969 "Astronauts' Dinner" held in Los Angeles to celebrate the July 1969 triumph of Apollo 11, the first piloted Moon landing.

In 1982, while the Administration of President Ronald Reagan called for expansion and modernization of the U.S. nuclear arsenal, Boushey co-sponsored California's Nuclear Freeze ballot initiative, which passed overwhelmingly. In 1985, he joined other retired U.S. military officers in Moscow to draft an agenda for nuclear arms control. He cited his 1945 flight over Hiroshima when he declared that political leaders did not adequately grasp the destructive power of nuclear weapons. The man who had spoken out for a U.S. military Moon base in 1958 spoke out against nuclear weapons to the end of his days. Boushey died in 2000 on Christmas Day at the age of 91.

Sources

"Who Controls the Moon Controls the Earth," Homer A. Boushey, U.S. News & World Report, 7 February 1958, p. 54.

"Gen. Homer Boushey dies; he was a pioneer in rocket-powered aircraft," The Almanac, 3 January 2000 (http://www.almanacnews.com/morgue/2001/2001_01_03.boushey.html — accessed 2/1/20).

"Homer A. Boushey," Keay Davidson, SFGate.com, 6 January 2000 (http://articles.sfgate.com/2001-01-06/news/17580737_1_air-force-lunar-base-rocket — accessed 2/1/20).

More Information

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

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

NASA's 1992 Plan to Land Soyuz Lifeboats in Australia

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

Sunlight glints off NASA Marshall Space Flight Center's proposed Power Module in this artist concept by Junior Miranda.

According to historians Andrew Dunar and Stephen Waring, writing in their 1999 NASA-funded history Power to Explore: A History of Marshall Space Flight Center, in the 1970s two lines of thought emerged within NASA concerning manned spaceflight's course after the Space Shuttle became operational. On the one hand, there was the "revolutionary" line taken by Johnson Space Center (JSC) in Houston, Texas. On the other was the "evolutionary" line of NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama.

At JSC, many managers assumed that, as soon as the Shuttle became operational, NASA would get a green light to assemble a large, new-design, multipurpose Space Station in low-Earth orbit (LEO). They envisioned that a 1980s President would make a speech much like President John F. Kennedy's 25 May 1961 “moon speech.” Visionary goal thus proclaimed, the funding floodgates would open.

At MSFC, by contrast, many managers expected that NASA budgets would remain tight for the foreseeable future, so that any space technology development that took place would need to be incremental; that is, it would have to begin with existing space hardware and occur in small steps. MSFC's work on the Skylab Orbital Workshop, a temporary LEO space station launched in May 1973 on the last Saturn V rocket to fly, probably helped to shape their outlook.

The 169,950-pound Skylab "cluster," which comprised the Multiple Docking Adapter, the Apollo Telescope Mount (ATM), and the Orbital Workshop, had been conceived originally as an element of the Apollo Applications Program (AAP). As its name implies, AAP had been meant to apply hardware developed for the Apollo lunar program to new tasks. The Skylab Orbital Workshop was a converted Saturn S-IVB stage outfitted with experiment apparatus, crew quarters, and supplies for visiting three-man crews. Three crews were launched to Skylab in 1973-1974; the last orbited the Earth for 84 days.

The Skylab Orbital Workshop floats serenely over the Earth, but this image bears evidence of its nearly disastrous launch and the heroic efforts that saved it. Skylab's reflective meteoroid shield deployed during ascent and peeled away, tangling one of its wing-like solar arrays in debris and loosening the other. A stage separation rocket motor then blasted away the loose array and the tangled array refused to open. Skylab was starved for electricity while temperatures inside it soared, threatening to spoil food, film, and medicines. The first Skylab crew (Charles Conrad, Paul Weitz, and Joseph Kerwin) deployed a sun shield and forced open the jammed array. Skylab went on to host astronauts for a total of 171 days. Image credit: NASA.

NASA built most of a second Skylab, but was unable to secure funding to complete it, launch it into orbit, and launch crews to it. The first Skylab was a success, so MSFC might have expected on that basis to have "earned" funding for the second. The Huntsville Center had, however, learned during the 1960s not to equate success with rewards. It had been responsible for the Saturn V moon rocket, the largest and most powerful launcher ever built. Even as MSFC succeeded in making the mighty Saturn V work, however, it began to suffer funding and staff cuts that by the time Skylab flew would make it a shadow of its former self.

When MSFC engineers looked at the Space Transportation System (STS), as NASA called the Space Shuttle and its stable of expendable upper stages and European-built Spacelab components, they saw not the promise of a big new space station, but rather a system which, once operational, could benefit from evolutionary development. In particular, they noted that Spacelab, which MSFC was assigned to integrate with the Shuttle, could not reach its potential as an orbiting laboratory while the Shuttle Orbiter's planned maximum time in space was only seven days. The Orbiter and its payloads would rely for electricity on the former's fuel cells, which meant that the quantity of fuel-cell reactants the Orbiter could carry would determine their endurance.

The Space Shuttle Orbiter with its Payload Bay doors open to space. A drum-shaped, European-built Spacelab module is shown as a cutaway.  Curved panels raised above the front half of the doors are radiators. The Spacelab module is located near the rear of the Payload Bay to ensure that the Orbiter's center of gravity is placed properly for maneuvers and landing. Image credit: NASA.

In early 1977, with the first STS flight test officially planned for March 1979, MSFC proposed "the first step beyond the baseline STS" — a Power Module (PM) capable of supplying 25 kilowatts of electricity continuously. The PM was partly inspired by joint Department of Energy/NASA Solar Power Satellite studies of the 1970s.

The solar-powered PM was meant to be deployed into LEO from a Shuttle Orbiter payload bay and left in space for up to five years. A succession of Orbiters bearing Spacelab modules and pallets in their payload bays would dock with the PM and use its electricity to remain in orbit for up to 30 days at a stretch.

Alternately, a Shuttle Orbiter could attach a "freeflyer" payload to the orbiting PM and leave it to operate on its own. This appealed to materials scientists, who worried that astronauts' movements on board the Shuttle Orbiter and Spacelab would rattle and ruin their microgravity experiments. Orbiters would periodically dock with the materials science freeflyer/PM combination to retrieve experiment products — for example, large flawless crystals — and replenish raw materials.

In addition to electricity, the PM "building block" would provide thermal and attitude control. The latter would permit a docked Orbiter to conserve its Reaction Control System propellants. Freeflyer payloads meant to be docked with the PM could be built without thermal and attitude control systems, reducing their cost.

Image credit: NASA.

MSFC engineers planned at first to base the PM on the Skylab ATM design. They quickly found, however, that modifying the ATM to meet stringent Orbiter payload bay safety requirements would cost more than a new design. They retained the ATM's octagonal cross-section, however, because they found that it made efficient use of the Orbiter's cylindrical payload bay volume while providing flat surfaces upon which to mount subsystems.

Although it nixed the ATM-based design, MSFC still aimed to lower the PM's cost by using subsystems developed for Skylab, Spacelab, Shuttle, and other programs. These included three Skylab Control Moment Gyros for attitude control and four curved Shuttle Payload Bay door radiators for thermal control. MSFC planned to update and improve Skylab systems used in the PM based on Skylab flight experience. All major PM subsystems would be redesigned for easy replacement by spacewalking astronauts.

The 31,000-pound PM would measure 55 feet long from the framework holding its aft- and side-facing international docking ports to the forward ends of its stowed twin solar arrays. This would leave room in the Shuttle Orbiter's 15-by-60-foot Payload Bay only for a docking tunnel with an international docking port. The tunnel would be bolted to the forward wall of the bay over the hatch linking the bay to the Shuttle Crew Compartment.

This NASA artwork shows a Space Shuttle Orbiter bearing a Spacelab module in its Payload Bay docked with a separately launched Power Module which extends forward over the Orbiter Crew Compartment.

Upon arrival in LEO, the astronauts would open the Shuttle Orbiter's Payload Bay doors and release the five pins that secured the PM in the bay. They would then use the Orbiter's robot arm to lift the PM from the bay and berth its side-facing docking port on the Orbiter docking port. This would position the module so that it extended out over the Crew Compartment.

The astronauts would next extend the PM's twin solar arrays. Fully extended, each wing-like array would measure 131 feet long by 30 feet wide. They would together span a little more than 276 feet. MSFC sized the arrays to generate a total of 59 kilowatts of electricity; that is, 34 kilowatts more than the PM would supply to Spacelab-carrying Orbiters and freeflyers. A portion of this excess would power PM systems, but the majority would charge batteries in the PM so that it could supply a constant 25 kilowatts throughout its roughly 90-minute orbital day-night cycle.

Close-up of Power Module showing international docking ports and curved radiator panels. Image credit: Junior Miranda.

MSFC acknowledged that the big solar arrays would degrade over time; its engineers estimated that over five years they would lose 5% of their generating capacity. Similarly, the PM's batteries would gradually lose their ability to charge and discharge. After five years, a Shuttle Orbiter might be sent up to recover the PM and return it to Earth for refurbishment. Another Orbiter would then launch it back to LEO to continue its duties.

MSFC engineers presented the PM concept to scientists at an MSFC-sponsored solar-terrestrial physics workshop in October 1977. They found broad support for the new capabilities the PM would give to the baseline STS.

Lots of living space: Skylab, Power Module, Spacelab-based add-on supply module, Shuttle Orbiter, and Payload Bay-mounted Spacelab module. Image credit: Junior Miranda.

This view emphasizes the solar arrays on the Power Module and Skylab. The 276-foot span of the Power Module arrays dwarfs the Shuttle and Skylab. The Skylab "wing" array lost during launch in May 1973 is conspicuous by its absence; also notable are two Apollo Telescope Mount "windmill" solar arrays stowed to make way for the Power Module and Orbiter. Image credit: Junior Miranda.

They also proposed that the PM become part of NASA plans to reuse Skylab. MSFC contractor McDonnell Douglas had "interrogated" the abandoned Orbital Workshop's data handling system and found that, nearly four years after its last crew had returned to Earth, reactivation remained feasible. The first step toward Skylab reuse would be for a Space Shuttle to rendezvous with it late in 1979 and boost it to a longer-lived orbit.

The PM would be a late addition to the revitalized Skylab cluster; MSFC did not expect that the new STS element would reach LEO for the first time until 1983, by which time several Shuttle Orbiters would already have visited Skylab. Once added to Skylab, however, the PM would enable Skylab to support as many as six astronauts without a Shuttle Orbiter present. They would perform experiments with large-scale space construction and early space industrialization.

MSFC engineers hoped that the PM might also contribute toward NASA's quest for Skylab's successor. They envisioned that PMs attached to Shuttle Orbiters, free-flyers, and Skylab might lead to PMs attached to Spacelab-derived habitat and laboratory modules during the 1980s: in other words, a new NASA Space Station.

In 1978, the Huntsville center contracted with Lockheed Missiles and Space Company to study PM evolution. MSFC expected that PM development might lead to simultaneous operation of several small specialized "space platforms," each with at least one PM attached. The platforms would not need to be staffed continuously. MSFC argued that several small platforms would best serve scientific and engineering disciplines with conflicting needs, and might cost less than a single large station besides.

In early 1979, NASA Headquarters authorized MSFC to spend $90 million on PM hardware development. The Huntsville center created a PM Project Office in March 1979. At about the same time, however, the space agency abandoned plans to reuse Skylab because the Space Shuttle would not be ready in time to prevent its uncontrolled reentry. Skylab reentered Earth's atmosphere over Australia on 11 July 1979.

JSC, meanwhile, pitched a new-design Space Operations Center (SOC). The space station would include hangars for reusable auxiliary spacecraft and satellite repair, robot arms, habitat and laboratory modules, and truss-mounted solar arrays spanning more than 400 feet. It was conceived primarily as a "space shipyard," a role inspired partly by JSC's 1970s enthusiasm for Solar Power Satellites.

Artist concept of the module cluster of the Space Operations Center (SOC). Most modules are a little less than 60 feet long by 15 feet wide (the length and width of the Space Shuttle Payload Bay). At lower left is a "false Payload Bay" for satellite servicing and spacecraft assembly. Had the SOC been built, this would have included robot arms. A Service Module partly covered with gold thermal blankets is located at upper right and a hexagonal hangar is located below it. The artist has included a Spacelab-derived module near a Shuttle docking port at left. Image credit: NASA.

STS-1, the maiden flight of Columbia, the first Space Shuttle Orbiter, took place in April 1981. James Beggs, President Ronald Reagan's choice for NASA Administrator, was confirmed two months later. Beggs soon sought presidential approval for a Space Station. This move seemed to favor JSC's revolutionary vision. At the same time, however, Beggs informed MSFC that he wanted to buy the new station "by the yard" – that is, as money became available. This approach seemed more in line with MSFC thinking.

In November 1981, NASA Headquarters halted PM, SOC, and other station-related work at MSFC and JSC. According to Dunar and Waring, it did this to take charge of station development and to end MSFC-JSC rivalry. Following Reagan's January 1984 State of the Union Address, in which he called upon NASA to build a Space Station by 1994, JSC's revolutionary vision seemed to win out. JSC was designated "lead center" for Space Station in early February 1984.

Although Reagan authorized NASA to spend only the $8 billion Beggs had told him the Space Station would cost and had specifically called for a space laboratory in his State of the Union Address, the agency's first baseline station design, the "Dual Keel," was an elaborate combination of lab, Earth/space observatory, and shipyard measuring more than 500 feet wide. Like the SOC, it included a small fleet of freeflyers and auxiliary vehicles. It also included a pair of solar-dynamic power systems — a NASA Lewis Research Center innovation — for generating large amounts of electricity.

The Dual-Keel Space Station design unveiled shortly after the January 1986 Challenger accident was dead on arrival, though NASA sought to ensure a future for the design until 1990. Image credit: NASA.

The Dual Keel's complex multipurpose design immediately came in for criticism. Materials scientists, for example, complained that space construction, the comings and goings of auxiliary spacecraft, the whirling turbines of solar-dynamic power systems, the presence of a large crew, and atmospheric drag on such a large structure were bound to spoil the station's microgravity research environment. Congress, meanwhile, accused NASA of low-balling its cost estimate to gain the project's approval.

Congressional cost containment, combined with the 28 January 1986 Challenger accident, concern over the number of assembly and maintenance spacewalks the station would need, and a rapidly expanding U.S.-Russian space partnership (one which would have been unthinkable when Reagan delivered his January 1984 speech), led to a decade-long series of station redesigns. The Space Station shrank and lost many of its proposed capabilities. This untidy evolution yielded the International Space Station (ISS), a U.S.-Russian hybrid with Japanese and European labs and Canadian robotics.

Early days of the International Space Station: from upper left to lower right are visible a Progress freighter, the Service Module with docking node, the FGB, and U.S. Node 1. Image credit: NASA.

Ironically, the first ISS element launched into space amounted to a Power Module. The Russian-built, Russian-launched, U.S.-funded FGB provided the second ISS element to reach space, U.S. Node 1, with electricity and attitude control from December 1998 to July 2000, when they were joined by a mini-space station – the Russian-built, Russian-launched Service Module, which had originally been intended as the "base block" of the Soviet Union's Mir-2 station. At that point, ISS became capable of supporting long-duration crews.

Sources

Guntersville Workshop on Solar-Terrestrial Studies, NASA Conference Publication 2037, "summary papers from a University of Alabama in Huntsville/NASA Workshop conducted 13-17 October 1977, at Lake Guntersville State Park Convention Center, Guntersville, Alabama," NASA George C. Marshall Space Flight Center, 1978.

"The 25 kW Power Module – First step beyond the baseline STS," G, Mordan; paper presented at the American Institute of Aeronautics and Astronautics Conference on Large Space Platforms: Future Needs and Capabilities, held in Los Angeles, California, September 1978.

25 kW Power Module Updated Baseline System, NASA TM-78212, NASA George C. Marshall Space Flight Center, Huntsville, Alabama, December 1978.

Power to Explore: a History of Marshall Space Flight Center, 1960-1990, NASA-SP-4313, Andrew J. Dunar and Stephen P. Waring, NASA History Office, 1999.

More Information

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

McDonnell Douglas Phase B Space Station (1970)

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

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

"The Eagle has wings!" The Apollo 11 Lunar Module Eagle shortly after separating from Apollo 11 Command and Service Module Columbia in lunar orbit, 20 July 1969. Image credit: NASA.

At 3:08 p.m. U.S. Eastern Daylight Time (EDT) on 20 July 1969, out of contact with Earth over the Farside hemisphere of the Moon, the computer that guided the Apollo 11 Lunar Module (LM) Eagle opened valves in its descent propulsion system, causing nitrogen tetroxide oxidizer and aerozine 50 fuel to come together in its Descent Stage rocket engine. The propellants were hypergolic, meaning that they ignited on contact with each other.

The descent engine fired for a little more than 12 minutes. At the beginning of the burn, Eagle, Apollo 11 Commander Neil Armstrong, and Lunar Module Pilot Edwin Aldrin were in a 54-by-66-nautical-mile lunar orbit. At the end of the burn, the 16.5-ton, 23-foot-tall lunar lander and its occupants were in an elliptical orbit with an apoapsis (low point) 50,000 feet above the Moon's Earth-facing Nearside hemisphere.

Apollo 11's target landing site was known officially as Site 2. Selected because it was flat and equatorial, Site 2 was a 10-mile-long east-west-trending ellipse on the Moon's Sea of Tranquility centered at 0° 42' 50" north latitude, 23° 42' 28" east longitude. Eagle descended to 50,000 feet about 260 nautical miles and 12 minutes of flight time east of Site 2, at which time its computer ignited its descent engine again to begin braking and final descent.

As the LM dropped below 7000 feet, its computer fired attitude control thrusters to tip it slowly upright so that it pointed its descent engine and footpads at the Moon. This maneuver also aimed Eagle's twin triangular windows forward so Armstrong and Aldrin could see Site 2 up close for the first time.

The astronauts immediately realized that they had a problem. They should have been above the eastern edge of the Site 2 ellipse, about five miles from their target landing point at the center of the ellipse. In fact, they had already flown past the center of their target ellipse and were descending toward its northwestern edge.

Apollo 11's flight plan called for Armstrong to let the computer do the flying until Eagle was about 500 feet above the Moon and 2000 feet east of the target touchdown point. He would then take manual control and lower Eagle almost vertically to the surface. He quickly realized, however, that Eagle's computer was steering it toward a boulder-strewn impact crater the size of an American football field. This was later identified as West Crater.

His heart rate leaping from 77 to 156 beats per minute, Armstrong assumed manual control early. Gripping his hand controller, he leveled Eagle's descent, then scooted the LM almost horizontally across the black lunar sky at an altitude of several hundred feet.

While Aldrin read off descent and translation rates, the LM computer flashed erroneous alarms and Capcom Charles Duke in Houston warned that Eagle was running low on propellants. Armstrong flew past West Crater and an adjacent smaller crater, then lowered to a safe touchdown just inside the Site 2 ellipse. At 4:18 p.m. EDT, he radioed his immortal words to hundreds of millions of people: "Houston, Tranquility Base here — the Eagle has landed."

The Apollo 11 Lunar Module Eagle on the Moon at Tranquility Base. Note lunar dirt stirred up by astronaut activities on the surface. Image credit: NASA.

Armstrong and Aldrin landed at 0° 41' 15" north, 23° 26' east, roughly four miles west and about three-quarters of a mile north of their planned touchdown point. Mission Control estimated that Eagle's Descent Stage tanks contained only enough propellants for about 25 seconds of flight when the descent engine was shut off at Tranquility Base. After the flight, more detailed analysis yielded an estimate of 45 seconds, demonstrating that the system for measuring available propellants in real time left much to be desired.

Mission rules called for an abort if propellants for fewer than 20 seconds of flight remained in the descent stage propellant tanks. What if, as Armstrong anxiously sought a safe place to land, flight controllers on Earth had mistakenly estimated an even slimmer propellant margin? They might then have done as mission rules dictated and called on Armstrong to abort the Apollo 11 lunar landing.

In June 1966, Charles Teixeira, with the Engineering and Development Directorate at the Manned Spacecraft Center in Houston, completed an Apollo Program Working Paper on the hazards of a landing abort during the 45-second period spanning from 65 to 20 seconds before planned touchdown. He assumed that the LM would be no more than 338 feet above the Moon 65 seconds before planned touchdown and about 100 feet high 20 seconds before planned touchdown.

As soon as an abort was initiated, the LM's Descent Stage engine would shut down. Nearly simultaneously, four explosive bolts linking the descent stage with the Ascent Stage would fire. A fifth pyrotechnic device would drive a guillotine that would cut the wiring umbilical linking the two stages. The Ascent Stage engine would then ignite to propel the astronauts toward lunar orbit. The abandoned Descent Stage, meanwhile, would fall to the lunar surface.

From abort initiation to Ascent Stage ignition, the abort procedure — which, apart from occurring at altitude, would duplicate the normal LM Ascent Stage launch procedure — would last from two to four seconds. During that time, the Ascent Stage would follow the same path as the Descent Stage; that is, it would fall toward the Moon.

Teixeira assumed that, following an abort during the 45-second period from 65 seconds to 20 seconds before planned touchdown, the four-legged Descent Stage would strike the Moon with enough force to rupture its propellant tanks. An abort within 20 seconds of planned touchdown — when the Descent Stage was at or below 100 feet — would leave the tanks intact.

If the tanks ruptured, either of two things might occur. The nitrogen tetroxide and aerozine 50 they spilled might boil and evaporate rapidly in the lunar vacuum. Evaporation would cool, then freeze, the propellants, so they would remain safely separated. Alternately, the propellants would come together. This might occur, Teixeira wrote, if after impact enough of the Descent Stage structure remained intact around the ruptured tanks to contain the propellants as they boiled.

Propellant mixing would cause an explosion that would drive gases and fragments of the Descent Stage outward at several thousand feet per second. Teixeira estimated that gases and debris would envelope the LM Ascent Stage less than one-tenth of a second after the explosion.

The extent of the damage this might cause would depend mainly on how long the abort procedure lasted; that is, on how quickly the ascent engine could ignite. The faster the ascent engine ignited, the farther away the astronauts would be when the Descent Stage impacted and exploded.

For a two-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 32.6 and 20 seconds before planned touchdown. If the two-second abort began between 44 and 20 seconds before planned touchdown, then the Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment.

For a four-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 53.7 and 20 seconds before planned touchdown. The Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment if the four-second abort began between 65 and 20 seconds before planned touchdown; that is, throughout the period Teixeira considered.

After the Landing: The Ascent Stage of the Apollo 11 Lunar Module Eagle as viewed from the Apollo 11 Command and Service Module Columbia during rendezvous in lunar orbit. Image credit: NASA.

Teixeira called the "critical time spans" during which damage would be likely to occur "rather short." He acknowledged that the risk of a Descent Stage explosion during a near-surface abort might not be great enough to justify "elaborate remedial action" — for example, a major redesign of the LM Descent Stage.

He recommended, however, that a Descent Stage propellant dump "at as high a rate as safely possible" become a part of the standard LM landing abort procedure. After due consideration, NASA elected not to follow his advice. Had Armstrong and Aldrin been forced to abort the Apollo 11 landing while above 100 feet of altitude, Teixeira's recommendation might have come back to haunt the U.S. civilian space agency.

Sources

Hazards Associated with a LEM Abort Near the Lunar Surface, NASA Program Apollo Working Paper No. 1203, NASA Manned Spacecraft Center, 24 June 1966.

Apollo 11 Mission Report, NASA SP-238, Mission Evaluation Team, NASA Manned Spacecraft Center, 1971.

Chariots for Apollo: A History of Manned Lunar Spacecraft, NASA SP-4205, The NASA History Series, C. Brooks, J. Grimwood, and L. Swenson, NASA, 1979, pp. 343-344.

More Information

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

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

Jimmy Carter's Space Shuttle

Space Shuttle Mission-62: Discovery awaits the imminent arrival of its crew, August 1994. Image credit: NASA.

In January 1978, President James Carter announced a surprise decision: NASA's Space Shuttle, then under development but plagued by delays, cost overruns, and technical snags, would be redesigned to launch and land without a crew on board. A spacecraft based on the tried-and-true Apollo Command and Service Module would launch astronauts to the Orbiter in space. They would enter the Orbiter through a docking unit in the Orbiter Payload Bay and use it as a mini-space station for scientific experiments and satellite servicing. Mission completed, the astronauts would return to Earth in the expendable Apollo, then the Orbiter would return to Earth for refurbishment and reuse. Carter justified his decision by pointing to the Shuttle's lack of credible crew escape systems and abort modes.

Carter's 1978 decision piqued the ire of spaceflight purists in a way that even the Sortie Lab decision of 1972 had not, for it turned the rationale for the Shuttle completely upside-down. The Shuttle had been conceived originally as crew rotation and resupply vehicle for a Saturn V-launched core Space Station. After President Richard Nixon refused to fund the core Station and scrapped the Saturn V, NASA studied a Shuttle-launched Station until it became clear that no Station would receive Nixon's blessing.

Deprived of its true purpose, the Shuttle Orbiter became a piloted spacecraft meant to replace all existing expendable space launch vehicles. It would, NASA promised, dramatically reduce the cost of spaceflight, ushering in a new age of space development. It would also reduce the cost of satellites by servicing them in orbit, serve as a short-term space laboratory by carrying in its Payload Bay a can-shaped Sortie Lab module, and make space readily accessible to non-astronauts.

The 1978 decision to turn the Shuttle into a robot spacecraft ceased to be controversial on the morning of 28 January 1986, when the Orbiter Challenger was destroyed a little more than a minute into Space Shuttle Mission (SSM) 25. Had astronauts been on board, they would have been unaware of the Solid Rocket Booster malfunction that was the root cause of Challenger's destruction. Had they somehow learned of the malfunction, they would have been unable to intervene and would have been trapped at least until the Shuttle stack's twin Solid Rocket Boosters had spent their propellant and detached. That would have been too long, for Challenger was destroyed as its Solid Rocket Boosters still burned.

As it was, Challenger's five-person crew for SSM-25 watched as the automated spacecraft they had been meant to board in orbit for a two-week stay was torn apart by aerodynamic forces and tumbled in fragments into the Atlantic. The Solid Rocket Boosters emerged still firing from the fireball created when the Shuttle's large, fragile External Tank broke up, spilling its liquid hydrogen and liquid oxygen propellants. The Solid Rocket Boosters each painted a twisting smoke-trail across the blue Florida sky until a Range Safety Officer sent the radio command that destroyed them.

As Challenger disintegrated, the Astronaut Transport Spacecraft (ATS) meant to launch the SSM-25 crew into orbit the following day stood atop a Saturn II expendable rocket on nearby Pad 39B. The ATS was an Apollo Command and Service Module spacecraft redesigned to carry five astronauts. The Saturn II rocket comprised the top two stages of the Saturn V - that is, the 33-foot-diameter S-II and 22-foot-diameter S-IVB. It included six uprated J-2 engines - five in its first stage and one in its second - and six small solid-rocket boosters evenly spaced around its base. Without an ATS on top, the Saturn II could launch a 20-ton payload.

After Challenger, some called for an end to unmanned Orbiter flights. They pointed out that the ATS/Saturn II combination included a sizable cargo volume in the tapered shroud that linked the base of the ATS with the top of the Saturn II S-IVB. They referred to early 1970s NASA and contractor studies that showed that increasing the number of solid-rocket boosters to 10 would permit the Saturn II to launch both the ATS and up to 20 tons of cargo.

President Carter, since his election in November 1984 the first President since Grover Cleveland to serve non-consecutive terms, surprised many by declaring his continued support for the Shuttle. This should perhaps not have come as a surprise, given that it had been Carter who made the 1978 decision to launch and land the Orbiter without a crew. The "come-back President" rightfully pointed to the Challenger accident as the vindication of his 1978 decision, and called for continued unmanned Orbiter flights on the grounds that upgrading the Saturn II would not replace all Shuttle capabilities. It is widely assumed that he also sought to continue the unmanned Orbiter flights to preserve the thousands of jobs the Shuttle Program had created.

In August 1986, Carter signed off on NASA's post-Challenger plan to redesign the SRBs and begin construction of two new Orbiters. This would increase the total number of Orbiters in the Shuttle fleet to four, enabling more downtime for inspections and upgrades between flights. To pay for the new Orbiters, Carter reduced the number of annual Orbiter flights to three from the six planned before Challenger was destroyed. As each new Orbiter came online, one additional flight per year would be added, so the four-orbiter fleet would eventually fly five missions per year.

In the meantime, the Hubble Space Telescope reached orbit in May 1986 atop a Saturn II without an ATS. Repairing its flawed optics became a goal for one of the first post-Challenger Shuttle missions. A Saturn II/Centaur launched the third Radio Relay and Tracking Satellite to geostationary orbit in July 1986, enabling for the first time continuous contact between orbiting spacecraft and flight controllers and researchers on the ground.

A Department of Defense-sponsored ATS solo mission designated SSM-X5 launched in December 1986 with a three-person crew to test polar-orbiting missions. (SSM-X1 through X4 had been Orbiter and ATS test missions in the 1980-1981 period.) Shortly after its return to Earth, new NASA Administrator Sally Ride announced that the Defense Department had opted to forego future Orbiter/ATS flights in favor of ATS solo flights.

The Shuttle Orbiter Enterprise soared into space in September 1987 to start the SSM-26 "Return-To-Flight" mission. Its five-person crew arrived in the SSM-26 ATS two days later. The astronauts spent three weeks on board Enterprise.

Columbia reached orbit in November 1987 to begin SSM-27; after its crew docked their ATS and boarded, they piloted the Orbiter to a rendezvous with the Hubble Space Telescope. Through a series of ambitious spacewalks, the astronauts corrected its faulty optics. They returned to Earth after 10 days in orbit. Columbia landed two days later.

Enterprise reached orbit the next time in May 1988 for SSM-29, but returned to Earth early after the Saturn II rocket bearing the SSM-29 ATS malfunctioned shortly after clearing Pad 39A's lightning mast. The ATS's Launch Escape System activated and pulled its Command Module free of the disintegrating Saturn II rocket. The five astronauts on board were uninjured. They would reach Enterprise to carry out SSM-29R in May 1989. The ATS/Saturn II combination had a flight record going back to the first Apollo Saturn V flight in November 1967, so troubleshooting the J-2 engine malfunction that destroyed the SSM-29 Saturn II and returning the system to flight needed only a few months.

The new Shuttle Orbiter Discovery flew an uncrewed orbital test mission (SSM-X6) in December 1989. In October 1991, the new Orbiter Endurance performed a nearly identical test mission (SSM-X7).

Endurance was the first Orbiter upgraded to permit a 12-week orbital stay and docking with two ATSs at one time. It carried out its first long-duration mission (SSM-60) and received two ATSs between mid-April 1994 and mid-July 1994.

Shortly after Columbia's retirement to the National Air and Space Museum in mid-1995, the new long-duration Orbiter Adventure joined the fleet. It would be the last Orbiter constructed and the last retired; its final mission was SSM-90 in February 2003.

By then, the U.S.-Russian-Chinese-European-Japanese-Brazilian International Space Station had become operational, and NASA and Europe had begun flight tests of the jointly developed Hermes shuttle, which became operational in June 2009. NASA retired the ATS in July 2011, ending 43 years of Apollo and Apollo-derived spacecraft missions.

A note on the Presidents: In this alternate timeline Ronald Reagan defeats James Carter in November 1980, but falls to an assassin's bullet (as he very nearly did) in April 1981. His Vice President, George H. W. Bush, finishes Reagan's term, but Carter narrowly defeats him in November 1984 after Bush's Vice President, Alexander Haig, announces a third-party candidacy that draws votes away from the Republican incumbent. Carter declines the nomination in 1988, in part because of Constitutional questions, and Republican James Thompson of Illinois defeats Carter's second Vice President, New Jersey Democrat Bill Bradley, to win the White House. Thompson's two terms (1989-1997) see the collapse of the Soviet Union and the start of an International Space Base with a crew of 25 people.

Sources

The Unmanned Shuttle Decision: Prudence and the Presidency, John Logsdon, NASA, January 1999, pp. 36-49, 53, 111.

SSM-25 Press Kit, NASA, December 1986.

SSM-27 Press Kit, NASA, November 1987.

Enterprise, Discovery, Endurance, Adventure: NASA's Orbiter Fleet, NASA Facts, December 1996.

Chronology of Space Shuttle/Astronaut Transport Spacecraft Missions, 1980-2011, David S. F. Portree, NASA, 2012, pp. 20-22, 26-28, 33-34, 37-40, 45-55, 61-63, 88-91, A-13.