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

Wernher von Braun in his office at NASA Marshall Space Flight Center. Image credit: NASA.

At the 7th International Astronautical Congress, held in Rome in September 1956, Italian aviation and rocketry pioneer Gaetano Crocco described a piloted space mission in which a spacecraft would conduct a reconnaissance flyby of Mars, swing past Venus to bend its course toward Earth, and, one year to the day after departing Earth orbit, reenter Earth's atmosphere. After Earth-orbit departure, the spacecraft would need no additional propulsion. Crocco told the assembled delegates that an opportunity to commence such a mission would next occur in June 1971.

A little less than six years later, in May 1962, the Future Projects Office (FPO) at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, awarded manned Mars mission study contracts worth $250,000 each to General Dynamics, Lockheed, and the Aeronutronic Division of Ford Motor Company. General Dynamics was instructed to study Mars orbital missions, Lockheed to look at Mars flyby and orbital missions, and Aeronutronic to study dual-planet (Mars-Venus) flybys. The combined study effort was known as EMPIRE, an evocative (if somewhat tortured) acronym that stood for Early Manned Planetary-Interplanetary Roundtrip Expeditions.

EMPIRE took place against the backdrop of the Apollo lunar program. One year before its start, in a speech before a special joint session of Congress, President John F. Kennedy had put NASA on course for the Moon. He had given the U.S. civilian space agency, which had been founded less three years earlier, until the end of the 1960s to achieve his goal. It was hoped, however, that an American could land on the Moon as early as 1967, during Kennedy's second term in office.

As EMPIRE began, NASA had nearly completed the contentious 14-month process of choosing the fastest, most reliable, and cheapest way of placing men on the Moon. The Lunar Orbit Rendezvous mode, which would rely on MSFC's Saturn C-5 rocket, was selected in July 1962, before EMPIRE reached its conclusion. C-5 was soon renamed Saturn V.

MSFC was fertile ground for NASA's first major manned planetary mission study. The Huntsville Center's director was Wernher von Braun, a famous advocate of piloted flight to the Moon and Mars. Von Braun's efforts in the 1950s to popularize spaceflight had helped to prime the American public for the 1960s Space Race with the Soviet Union.

EMPIRE was, however, not merely the whim of a long-time Moon and Mars fan; it was also part of a strategy to protect and promote MSFC. The Huntsville Center's specialty was rocket development. Its engineers hoped to develop rockets larger than the Saturn V, which they dubbed Nova and Supernova. Von Braun knew that, unless a post-Apollo program was in place as MSFC's Apollo responsibilities wound down in 1966 and 1967, his center would suffer deep staff and funding cuts, not develop new big rockets.

The Saturn family of rockets was expected to lead to larger Nova and Supernova rockets by the end of the 1960s. In the illustration above, dating from early 1962, the three-stage C-1 and four-stage C-5 are anemic predecessors to the Apollo Saturn IB and Saturn V designs. The Apollo-Saturn rockets later adopted features of the proposed (but never flown) Nova depicted; in particular, the Nova rocket's 22-foot-diameter third stage. Image credit: NASA.

Besides calling for a man on the Moon, President Kennedy's May 1961 speech had requested new funding for nuclear propulsion. MSFC was involved in joint NASA/Atomic Energy Commission (AEC) nuclear rocket development through the Reactor In Flight Test (RIFT) program. The precise form the RIFT mission would take changed rapidly; for a time, it aimed to launch a 33-foot-diameter rocket stage with a NERVA nuclear engine on a Saturn V in 1967. Another goal of EMPIRE was, thus, to create a justification for NERVA and RIFT.

In the introduction to their December 1962 EMPIRE final report, Aeronutronic's engineers praised NASA for its "forward thinking approach." They added that

[b]y attacking the areas of interest at this early date[,] it will be possible to obtain a clearer picture of the requirements for early manned planetary and interplanetary flight. Thus the nation's resources, and the NASA and other United States space programs[,] can be oriented toward long range goals at an early date. [EMPIRE] represents an unusually early attack on this type of analysis.

Aeronutronic looked at two interplanetary trajectories for its dual-planet piloted flyby mission. The first was dubbed the Crocco trajectory because it was based on the scenario in Crocco's 1956 paper. The second was the symmetric trajectory, which would see the flyby spacecraft swing around the Sun one-and-a-half times. It would fly past Mars halfway through its mission and would cross the orbit of Venus once before the Mars flyby and once after. It would swing past Venus during one of the orbit crossings.

Interplanetary trajectory types Ford Aeronutronic considered for its Mars/Venus flyby mission. Image credit: Ford Aeronutronic/NASA.

Aeronutronic engineers found that Crocco had been mistaken when he wrote that an opportunity to start an Earth-Mars-Venus-Earth voyage would occur in June 1971. They determined that a dual-planet Crocco mission could depart Earth in mid-to-late August 1971. A symmetric Earth-departure opportunity would occur sooner: between 16 July and 19 August 1970.

The Crocco trajectory would require more propulsive energy than the symmetric trajectory, Aeronutronic found. The amount of energy required would depend on the moment during the launch opportunity that the spacecraft left Earth orbit. Aeronutronic, however, selected single representative values for preliminary design purposes.

Assuming a departure from a 300-kilometer-high circular Earth orbit, a Crocco spacecraft launched in August 1971 would need to increase its speed by 11.95 kilometers per second to achieve the desired dual-planet flyby trajectory. A symmetric mission launched in the July-August 1970 window would, by contrast, need a velocity increase of only 5.3 kilometers per second.

Low departure velocity meant reduced propellant requirements. In short, a symmetric mission would have a much lower mass than a Crocco mission. A low-mass spacecraft would need fewer launches to place its components and propellants into Earth orbit. Fewer launches would mean lower mission cost and less chance that a launch vehicle would fail and destroy its payload, perhaps placing the assembly campaign in Earth orbit in jeopardy. It would also mean less complex orbital assembly (or perhaps no assembly at all, if a Nova rocket were used), further trimming mission risk.

Aeronutronic found that no dual-planet Crocco mission could last only 365 days; for design purposes, the study team assumed that a mission launched in the August 1971 opportunity would last 396 days. Though longer than Crocco had estimated, this was much less than the 611-day representative duration Aeronutronic selected for the July-August 1970 symmetric opportunity.

Aeronutronic's engineers determined that radiation and meteoroid shielding and life support masses, which would increase with trip time, would not approach the mass of the propellant required to launch the Crocco mission out of Earth orbit; they thus accepted the symmetric mission's greater duration.

The symmetric mission would have a low Earth-departure speed, but would pay for it by having a high Earth-atmosphere reentry speed. As a general rule, the faster a spacecraft is moving as it plummets into Earth's atmosphere, the more reentry heating it will undergo. Greater heating calls for more massive thermal control systems; basically, active cooling employing coolant loops and radiators and a thick heat shield that ablates (that is, chars and erodes away, carrying away heat). The re-entering spacecraft might also use a braking rocket to slow itself before reentry.

The Aeronutronic engineers did not consider the difference between the Crocco and symmetric Earth-atmosphere reentry speeds to be significant. For a symmetric mission launched between 16 July and 19 August 1970, reentry speed would top out at 15.8 kilometers per second; the corresponding figure for the August 1971 Crocco opportunity was 13.5 kilometers per second. The propellant mass and Earth-to-orbit launch vehicles saved as a result of the Crocco mission's somewhat slower Earth atmosphere-reentry speed were minimal compared with the mass and launchers saved by the symmetric mission's dramatically reduced Earth-departure velocity.

Dual-planet flyby spacecraft size comparisons. Note comparison with Mercury-Atlas rocket and spacecraft (lower right). Mercury was the only piloted U.S. spacecraft that had flown at the time Ford Aeronutronic completed its study. Image credit: Ford Aeronutronic/NASA.

Aeronutronic calculated that, based on the mission design values that it had selected, the nuclear Crocco dual-planet flyby spacecraft would have a fully assembled, fully fueled mass of 1121.5 tons, while an equivalent nuclear symmetric spacecraft would have a mass of only 188 tons. This would place the latter within the planned payload capacity of a single Nova rocket, eliminating Earth-orbit assembly entirely.

The Aeronutronic engineers did not bother to determine the mass of a chemical Crocco spacecraft because they knew that it would be even greater than that of the nuclear Crocco spacecraft. They calculated, however, that the chemical symmetric spacecraft would have a mass of 350.5 tons (nearly twice that of the nuclear symmetric spacecraft) or 929.5 tons (nearly as much as the nuclear Crocco spacecraft) depending on the thrust capability of its rocket motor. The company thus selected the nuclear symmetric mission for more detailed study.

Aeronutronic envisioned a 156-foot-long, 33-foot-wide symmetric dual-planet flyby spacecraft with a single 18,300-pound NERVA engine capable of generating 50,000 pounds of thrust. A tungsten shield at the front of the drum-shaped NERVA reactor would create a radiation shadow that would encompass the crew modules during Earth-orbit departure.

The engine would need to operate for nearly 48 minutes to boost the spacecraft's speed by 5.3 kilometers per second and place it onto its symmetric trajectory. Aeronutronic identified the lengthy burn time as a possible show-stopper; available reactor materials, it explained, could not withstand such prolonged operation. Increasing thrust would reduce required burn time, but the company was not confident that a nuclear engine with more than 50,000 pounds of thrust could be developed in time for the July-August 1970 symmetric launch opportunity.

The nuclear symmetric spacecraft became Ford Aeronutronic's dual-planet flyby mission baseline design. Image credit: Ford Aeronutronic/NASA.

Two tank clusters would supply liquid hydrogen propellant to the NERVA engine. The first-stage cluster, which would contain 56.2 tons of propellant, would comprise a core propellant tank to which the NERVA engine would be mounted and six detachable "perimeter" tanks. After expending the first-stage propellant and discarding the perimeter tanks, the spacecraft would have a mass of 127.7 tons.

Eight second-stage tanks would then supply a total of 38.3 tons of liquid hydrogen to the NERVA engine. After it expended its second-stage propellant, the NERVA engine would detach along with the empty first-stage core tank.

The empty second-stage tanks, with a total mass of 5.2 tons, would be retained to shield the Aeronutronic flyby spacecraft's cylindrical central core from meteoroids. After NERVA engine/first-stage propellant separation, the spacecraft would have a mass of 76.7 tons and would measure about 78 feet long.

From aft to front, the core would comprise twin SNAP-8 nuclear reactors for generating electricity, the spacecraft's navigational stable platform, a small compartment for weightless experimentation, and a 20-ton command center/solar flare radiation shelter clad in 50 centimeters of polyethylene plastic. Four tanks containing a total of 10.9 tons of chemical trajectory-correction propellants would surround the command center/shelter, providing additional radiation shielding. A two-stage reentry braking propulsion module ("retro-pack") and the Earth-atmosphere reentry vehicle would be attached to the command center/shelter at the front of the spacecraft.

Earth-orbit departure completed, the six-man crew would reconfigure their spacecraft for the interplanetary voyage. The twin cylindrical living modules, each with an empty mass of 4.5 tons, would extend on hollow telescoping arms, and one SNAP-8 would deploy a radiator panel and begin generating electricity (the other would be held in reserve in case the first failed).

The crew would then spin their spacecraft about its long axis at a rate of three revolutions per minute to create acceleration in the living modules which they would feel as gravity. Sixteen-meter-diameter dish antennas would unfurl from the aft end of both living modules to ensure continuous radio communication with Earth. Aeronutronic noted that, when the spacecraft was farthest from Earth, one-way radio-signal trip time would reach 22 minutes.

The nuclear symmetric spacecraft would undergo configuration changes as it carried out its mission, casting off parts that were no longer useful and deploying power, communications, and artificial-gravity systems. The spacecraft would spend most of the flight in the configuration labeled "on-orbit spacecraft." Please click on image to enlarge. Image credit: Ford Aeronutronic/NASA.

Based on experience with nuclear submarine crews, Aeronutronic allocated 750 cubic feet of living space to each astronaut, except inside the command center/radiation shelter, where it judged that 50 cubic feet per man would suffice. The twin living modules and command center/shelter would each have an independent life support system designed to recycle all air and water. The company estimated that life support mass would total 10.9 tons, of which food would make up nearly five tons.

The six-man crew would follow a complex schedule designed to combat boredom while providing adequate rest and recreation. Except for sleep, crew activities would occur in two-hour blocks. The crew would include a Commanding Officer, an Executive Officer, a Flight Surgeon, and three astronauts identified simply as "crew members." All six would take it in turns to serve as duty officer in the command center/shelter, maintenance and repair crewmember, and scientific activity crewmember. The Commanding Officer and Flight Surgeon would not, however, take part in hazardous repairs (for example, those involving spacewalks).

Symmetric Mars-Venus piloted flyby trajectory: 1 = Earth launch; 2 = Venus orbit crossing (possible flyby); 2* = second Venus orbit crossing (possible flyby); 3 = Mars orbit crossing; 3* = second Mars orbit crossing; 4 = Mars flyby/Earth position during Mars flyby; 5 = Earth return. Image credit: Ford Aeronutronic/NASA.

A symmetric dual-planet flyby mission departing Earth in the 16 July-19 August 1970 opportunity would fly first past Venus between 97 and 102 days after launch from Earth orbit. Departure at the start of the launch opportunity would yield the shortest trip time on each leg of the interplanetary voyage (and shortest total trip time — 611 days); departure at the end of the opportunity would yield the longest trip time on each leg (and longest total — 631 days). Earth-departure near the start of the opportunity would yield a Venus flyby distance of about 4890 miles; the corresponding figure for the end of the opportunity would be 7520 miles.

As the spacecraft approached Venus, the crew would fire the trajectory-correction motors, changing their speed by about 650 feet per second to ensure an on-target flyby. The spacecraft would carry enough course-correction propellants to change its velocity by a total of 2000 feet per second.

Citing a November 1961 report planetary astronomer Gérard de Vaucouleurs had prepared for the U.S. Air Force, Aeronutronic allotted 1000 pounds for scientific equipment for studying Venus and Mars. The company declined, however, to define a scientific payload for its spacecraft, arguing that the EMPIRE mission's science objectives at Mars and Venus would be shaped by data from NASA robotic spacecraft launched between 1962 and 1968.

The Mars flyby would occur between 191 and 199 days after the Venus flyby, at the midpoint of the symmetric mission. During Mars approach, the crew would perform a second trajectory-correction burn, changing their spacecraft's speed by 330 feet per second. Mars flyby distance would range from about 2740 miles for an Earth departure at the beginning of the July-August 1970 opportunity to 4220 miles for a departure at its end.

An EMPIRE ground rule was that the contractors should use Apollo hardware and techniques wherever feasible, though this was not strictly enforced. The conical Apollo Command Module (CM) with its bowl-shaped ablative heat shield was considered a prime candidate for use as EMPIRE's Earth-reentry vehicle, but Aeronutronic rejected it.

The company opted instead for a 14.75-ton lifting-body with a pointed nose and a two-stage chemical-propellant retro-pack. Aeronutronic had determined that the lifting-body shape was more tolerant of reentry errors — that is, that it would be less likely to burn up or skip off the atmosphere — than the Apollo capsule. The lifting-body would, Aeronutronic noted, also offer a broader choice of land landing or sea splashdown sites because of its enhanced ability to maneuver in the atmosphere.

Ford Aeronutronic's preferred Earth-return reentry vehicle configuration. Image credit: Ford Aeronutronic/NASA.

Landing/splashdown site flexibility would become especially useful if an abort during Earth departure became necessary. Aeronutronic found that the retro-pack, intended to slow the reentry vehicle as it neared Earth at the end of the mission, could return the crew to Earth no later than 16.7 hours after Earth-orbital launch provided that an abort was initiated before the flyby spacecraft passed beyond about 12,000 miles from Earth.

If, however, the mission unfolded as planned, the astronauts would return to Earth between 312 and 343 days after passing Mars. Prior to reentry, they would perform the final trajectory-correction burn of the mission, changing their spacecraft's speed by about 360 feet per second. Shortly thereafter, the astronauts would strap into the Earth-reentry lifting-body and fire small solid-propellant rocket motors to separate it from the flyby spacecraft. The abandoned flyby spacecraft would subsequently swing past Earth and enter a disposal orbit around the Sun.

After properly orienting the Earth-reentry vehicle, the crew would ignite the two stages of its nose-mounted retro-pack in succession, steering the lifting-body toward its reentry corridor and reducing its reentry velocity by 2.8 kilometers per second to 13 kilometers per second. The stages would detach in turn as they expended their propellants.

During reentry, the hottest part of the vehicle would be its small pointed nose, which Aeronutronic expected would be actively cooled to prevent melting. A blowtorch-like plume of superheated plasma would trail from the nose beneath the lifting-body, but would not contact its concave underside.

The astronauts, who would recline facing away from the nose, would experience deceleration equal to 10 Earth gravities, which they would feel against their backs. After deceleration and descent into the lower atmosphere, the lifting-body would deploy parachutes and descend more or less vertically.

Aeronutronic provided a detailed development schedule and cost estimate for its nuclear symmetric dual-planet flyby mission. The company placed the mission's cost at $12.6 billion, or about half the projected Apollo Program cost. Cost savings would be due in large part to experience gained and hardware developed during Apollo. Peak funding year, with expenditures totaling about $3.5 billion, would be 1966.

Aeronutronic estimated that development of a 50,000-pound-thrust NERVA engine capable operating continuously for 60 minutes would need to begin on 1 January 1963 (that is, 10 days after the company completed its final report for MSFC FPO), as would development of the Earth-reentry lifting body. NERVA engines would be tested at the Nuclear Rocket Development Station (NRDS) at Jackass Flats, Nevada, beginning in mid 1965. Earth-reentry vehicles would be drop-tested over the dry lake bed at Edwards Air Force Base in California beginning in mid-1966 and would be flight-tested with and without a crew using Saturn C-1 boosters beginning a year after that.

Flyby spacecraft development would begin in early 1964, shortly before crew selection and the start of Nova development. Beginning in mid-1967, Saturn C-5 rockets would launch three symmetric dual-planet flyby spacecraft without Earth-reentry vehicles into Earth orbit for testing.

A total of 13 Nova rockets would be required for ground and flight testing between mid-1966 and late 1969. Of these, the first four would be Nova development flights and the last three would launch complete manned symmetric dual-planet flyby spacecraft into Earth orbit for testing and crew training. The fourteenth Nova, designated N9, would launch the mission spacecraft and crew into 300-kilometer-high Earth orbit on 15 July 1970, the day before the symmetric launch opportunity was set to begin.

In a paper presented 10 months after Ford Aeronutronic delivered its final report to MSFC FPO, Franklin Dixon declared that development of an improved NERVA engine should have begun no later than July 1963 if it was to become available in time for the 16 July-19 August 1970 symmetric dual-planet flyby launch opportunity. Dixon had not participated in EMPIRE; on 1 July 1963, Ford had placed Aeronutronic under its Philco division, and Dixon had become Philco Aeronutronic's Manager for Advanced Space Systems.

Cutaway illustration of NERVA nuclear-thermal rocket engine. Image credit: NASA.

NERVA development lagged behind the schedule Dixon said was necessary in part because of Kiwi-B nuclear rocket engine failures in December 1961, September 1962, and November 1962. As the name implies, the Kiwi series engines were not meant for flight. In fact, though they each included a nozzle and blasted super-hot hydrogen gas into the air, they were considered nuclear reactors, not nuclear rocket engines. 

The three failures were similar in nature: liquid hydrogen moving through the hot reactor core caused vibration which broke and eroded the reactor's uranium fuel rods. Hot gaseous hydrogen exhaust then blew uranium fragments out through the engine nozzle. The melting fragments sparkled as they sprayed into the open air.

The Kiwi-B failures set off a duel between the President's Science Advisory Council and the Bureau of the Budget on the one hand and NASA and the Atomic Energy Commission, championed by New Mexico Senator Clinton Anderson, on the other. Democrat Anderson's state contained Los Alamos National Laboratory, which led the AEC side of the nuclear rocket program.

President Kennedy visited the NRDS in early December 1962 to size up the situation. On 12 December 1962, two weeks before Ford Aeronutronic completed its EMPIRE study, he indefinitely postponed RIFT. Citing fiscal restraint, Kennedy's successor, Lyndon Johnson, cancelled RIFT altogether in December 1963, and made NERVA a wholly ground-based research and development effort.

In December 1967, the NRX-A6 NERVA ground-test engine operated for 60 minutes without a hitch; that is, for longer than would have been required to boost Aeronutronic's dual-planet flyby spacecraft out of Earth orbit and onto its symmetric trajectory. Nevertheless, President Richard Nixon cancelled NERVA in 1972 after program expenditures totaling $1.4 billion.

The Nova rocket experienced a less protracted and costly demise. By June 1964, von Braun called publicly for piloted planetary flyby missions using Saturn V rockets and Apollo-derived hardware, thus acknowledging that the Lunar Orbit Rendezvous decision of July 1962 had all but doomed large rockets like Nova.

Later in 1964, the Bureau of the Budget declared Nova rocket development to be of low priority, and called for NASA's post-Apollo program to be confined to Earth orbit and based on hardware developed for Apollo. The MSFC FPO would publish a feasibility study of an Apollo-derived piloted flyby mission in early 1965.

Sources

EMPIRE: A Study of Early Manned Interplanetary Expeditions, NASA Contractor Report 51709, Aeronutronic Division, Ford Motor Company, 21 December 1962.

"The EMPIRE Dual Planet Flyby Mission," Franklin P. Dixon, Aeronutronic Division, Philco Corporation; paper presented at the Engineering Problems of Manned Interplanetary Exploration conference, 30 September-1 October 1963.

"EMPIRE: Early Manned Planetary-Interplanetary Roundtrip Expeditions Part I: Aeronutronic and General Dynamics Studies," Frederick I. Ordway III, Mitchell R. Sharpe, and Ronald C. Wakeford, Journal of the British Interplanetary Society, May 1993, pp. 179-190.

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)

Flyby's Last Gasp: North American Rockwell's S-IIB Interplanetary Booster (1968)

MIT Saves the World: Project Icarus (1967)

Image credit: NASA.

Walter Baade used the 48-inch reflecting telescope at Palomar Observatory in southern California to capture humankind's first image of asteroid 1566 Icarus on 26 June 1949. Icarus showed up as a nondescript streak set against the myriad stars on a glass photographic plate. Icarus, it was soon found, is unusual because its elliptical orbit takes it from the inner edge of the Main Asteroid Belt between the orbits of Mars and Jupiter to well within Mercury's orbit. Icarus needs 1.12 years to circle the Sun once. 

Every nine, 19, or 28 years, always during the month of June, Icarus and Earth reach their point of closest approach, during which they typically pass each other at a relative velocity of about 29 kilometers (18 miles) per second. Baade detected Icarus during one of these close encounters.

MIT Professor Paul Sandorff taught the Interdepartmental Student Project in Systems Engineering in the Spring 1967 term at the Massachusetts Institute of Technology (MIT). At the beginning of the course, he told his students that, on 19 June 1968, Icarus and Earth would pass each other at a distance of 6.4 million kilometers (four million miles) — that is, about 15 times the Earth-Moon distance.

Sandorff then asked his students to suppose that, rather than miss Earth on that date, Icarus would instead strike the Atlantic Ocean east of Bermuda with the explosive force of 500,000 megatons of TNT. Debris flung into the atmosphere would cool the planet to some unknown degree and a 30-meter (100-foot) wave would inundate MIT. Sandorff gave his class until 27 May 1967 to develop a plan to avert the catastrophe.

In 1967, the physical characteristics of Icarus were poorly known. For purposes of their study, Sandorff's students assumed that it measured 1280 meters (4200 feet) in diameter and had an average density of 3.5 grams per centimeter, yielding a mass of 4.4 billion tons. For comparison, Earth has an average density of 5.5 grams per cubic centimeter.

They acknowledged, however, that, given its orbit, which resembles that of a short-period comet, Icarus might be a defunct comet nucleus. In that case, its density and mass would likely be considerably less. They also assumed that Icarus is a solid body; that is, that it is not made up of small pieces held together loosely by weak mutual gravitational attraction.

In March 1967, the MIT students visited Cape Kennedy, Florida, to size up U.S. space capabilities. At the time, the first piloted flight of the Apollo Command and Service Module (CSM) spacecraft had been postponed indefinitely following the Apollo 1 fire (27 January 1967) and the Saturn V Moon rocket had yet to fly. Apollo 4, the successful first Saturn V test flight, would not occur until 9 November 1967.

Nevertheless, the students wrote in their final report that "the awesome reality" of the giant structures NASA had built to launch astronauts to the Moon had "completely erased" any doubts they might have had about using Apollo/Saturn technology in their project. The structures included the Vertical Assembly Building (VAB), in which Apollo spacecraft and three-stage Saturn V Moon rockets were stacked together, and the twin Launch Complex 39 Saturn V launch pads (Pads 39A and 39B). One cannot help but wonder what their fall-back alternative might have been had they found the Apollo infrastructure wanting.

Apollo 11 liftoff on 16 July 1969. If Project Icarus had been necessary, the Apollo 11 Saturn V would have launched the automated Saturn-Icarus 3 Interceptor, not the first piloted Moon-landing mission. Image credit: NASA.

Professor Sandorff's students proposed to hijack Project Apollo, delaying NASA's first piloted lunar landing by about three years. They would have taken over the first nine Saturn V rockets earmarked for the Moon program, commenced construction in April 1967 of a third Launch Complex 39 launch pad (Pad 39C), and added a Saturn V assembly high bay to the VAB, bringing the total to four. NASA had planned to build Pad 39C, going so far as to build a road to the proposed pad site with appropriate signage (image at top of post), until funding cuts made an ambitious post-Apollo piloted space program increasingly unlikely.

Three of the nine Saturn V rockets would have been used for unmanned flight tests. The remainder would each have launched toward Icarus one heavily modified automated Apollo CSM bearing an 20,000-kilogram (44,000-pound) nuclear warhead with the destructive yield of 100 million tons of TNT.

Though the MIT students did not mention it, a 100-megaton warhead was never a component of the U.S. nuclear arsenal. Given the secrecy surrounding nuclear weapons during the Cold War, they probably could not have known that no 100-megaton warhead had ever been tested.

The most powerful nuclear bomb ever, the Soviet Union's 50-megaton "Tsar Bomba," exploded on 30 October 1961, triggering seismic sensors around the globe. Fifty megatons was about half its theoretical yield. The Soviet Union built only a single Tsar Bomba and the U.S. did not deign to match the Soviet feat.

Even had Tsar Bomba warheads been readily available, the Soviet super-bomb was likely so heavy that a Saturn V could not launch it to Icarus. It weighed as much as 27,000 kilograms (60,000 pounds).

The Apollo 14 Saturn V rocket rolls out of the immense VAB at Kennedy Space Center. Had Project Icarus been necessary, the rocket would have launched the automated Saturn-Icarus 6 Interceptor on 14 June 1968. Image credit: NASA.

The Icarus CSM — which the MIT students dubbed the Interceptor — would comprise three modules: a drum-shaped propulsion module corresponding to the Apollo Service Module (SM), with attitude-control thrusters and a Service Propulsion System (SPS) main engine; a drum-shaped payload module based on the SM's structural design but containing the 100-megaton nuclear device; and a stripped-down conical Command Module (CM) containing Icarus detection sensors and an MIT-designed Apollo Guidance Computer (AGC) modified for automated operation. Unlike the two-module Apollo CSM, the three modules of the Interceptor would have remained bolted together throughout its flight.

The first Project Icarus Saturn V (Saturn-Icarus 1) would have lifted off from Cape Kennedy on 7 April 1968, 73 days before the asteroid was due to collide with Earth. Its payload, Interceptor 1, would have reached Icarus 60 days later, when the asteroid was 13 days and 32.2 million kilometers (20 million miles) from Earth. At about the time Interceptor 1 was due to reach its target, the MIT Lincoln Laboratory's Haystack radar would have detected Icarus for the first time.

Saturn-Icarus 2 would have launched on 22 April 1968, 58 days before Icarus was due to strike. Interceptor 2 would have reached its target 25 million kilometers (15.5 million miles) and 10 days out from Earth.

Saturn-Icarus 3 would have lifted off on 6 May 1968, 44 days before Icarus was due to arrive, and its Interceptor would have reached Icarus one week and 17.7 million kilometers (11 million miles) from Earth. Saturn-Icarus 4 would have lifted off on 17 May 1968, 33 days before Icarus arrival, and Interceptor 4 would have reached the asteroid 28 days later, when Earth and Icarus were 12.4 million kilometers (7.7 million miles) apart.

Saturn-Icarus 5 would have left Earth near dawn on the U.S. East Coast on 14 June 1968, and Interceptor 5 would have reached Icarus 2.26 million kilometers (1.4 million miles) out from Earth, 22 hours before expected impact. By then, the asteroid would have appeared as a modest star in the predawn sky near the bright stars of the constellation Orion.

Saturn-Icarus 6 would have lifted off a few hours after Saturn-Icarus 5. When Interceptor 6 reached it, Icarus would have been about 20 hours and 2 million kilometers (1.25 million miles) from impact.

As each Interceptor closed to within 400,000 kilometers (250,000 miles) of Icarus, an optical sensor in its nose would have spotted the asteroid. The modified AGC would then have used the SPS and thrusters in the propulsion module to adjust the Interceptor's course to ensure a successful interception.

Apollo astronauts grew fond of the simple but capable MIT-developed Apollo Guidance Computer (AGC). For Project Icarus, MIT would have added an extra layer of automation so that the AGC could guide the unmanned Interceptor spacecraft to their target. Image credit: Wikipedia.

When an Interceptor closed to a distance of 170 meters (550 feet) from Icarus, a radar would have detected the asteroid and triggered the nuclear device, which would have exploded at a distance of from 15 to 30 meters (50 to 100 feet). If the students' assumptions about the asteroid's mass and density were correct, then each 100-megaton near-surface nuclear blast would have excavated on Icarus a bowl-shaped crater up to 300 meters (1000 feet) wide. The effect the explosions would have had on the asteroid's course was, of course, not known with precision; the students calculated that each blast would alter its velocity by between 8 and 290 meters (26 and 950 feet) per second.

The MIT students acknowledged that Icarus might shatter; in that event, subsequent Interceptors would have targeted the largest fragments. Data from each Interceptor as it approached Icarus and from Earth-based optical telescopes and radars would have been used to target subsequent Interceptors as required. Conversely, if fewer than six explosions were sufficient to deflect or pulverize the asteroid, then the remaining Saturn V rockets and Interceptors would have stood down.

The Project Icarus Intercept Monitoring Satellite (IMS) would have resembled NASA's Mariner 2 Venus flyby spacecraft. Image credit: NASA.

All but one of the Interceptors would be joined at Icarus by a separately launched 245-kilogram (540-pound) Intercept Monitoring Satellite (IMS) based on the Mariner 2 design. Mariner 2, the first successful interplanetary probe, had flown past Venus on 14 December 1962. In addition to data immediately useful for Project Icarus, the IMS would have provide pure science data.

The first IMS would have left Earth atop an Atlas-Agena rocket on 27 February 1968. It would have passed between 115 and 220 kilometers (70 and 135 miles) from Icarus at the time of the first nuclear explosion. This would have placed it outside of the zone of large high-velocity debris from the explosion, but within the zone of plasma, dust, and small debris. IMS-1 would have analyzed the small fragments and hot gases to determine the asteroid's composition. A 23-kilogram (50-pound) foam-honeycomb "bumper" would have shielded IMS-1 during passage through the debris cloud.

No IMS would have monitored the fifth interception (if it were judged necessary) unless the sixth interception had already been called off. The IMS for monitoring the sixth (or fifth) interception would have lifted off on 6 June 1968, between the Saturn-Icarus 4 and 5 launches.

Professor Sandorff's class estimated that Project Icarus would cost $7.5 billion. It would, they calculated, stand a 1.5% chance of only fragmenting the asteroid. If this happened, then Icarus might cause more damage to Earth than if it were permitted to impact intact. The probability that Project Icarus would reduce the damage Icarus would cause was, however, 86%, and the probability that it would succeed in preventing any part of the asteroid from reaching Earth was 71%.

During the June 1968 close approach, Icarus became the first asteroid to be detected using Earth-based radar. By analyzing data gathered over the decades during subsequent close approaches, scientists have found that Icarus is roughly spherical, rotates rapidly (about once every 2.25 hours), is probably a light-colored S-type asteroid made mostly of stony materials, and measures about 1400 meters (4600 feet) across. Its density is probably only about 2.5 grams per cubic centimeter.

The closest approach of Icarus to Earth since 19 June 1968 is taking place as I write this. On 16 June 2015, the asteroid will pass by Earth at a distance of about eight million kilometers (five million miles). It will zip through the northern-hemisphere constellations Ursa Major and Canes Venatici over the course of the day, though it will be too faint to view with unaided eyes. Closest approach to Earth will take place at 1539 UTC (11:39 AM U.S. Eastern Daylight Time). Icarus will not pass so close to Earth again until June 2090.

Sources

Project Icarus, MIT Report No. 13, Louis A. Kleiman, editor, The MIT Press, 1968.

Tsar Bomba: King of Bombs (http://www.tsarbomba.org/ — accessed 15 July 2015).

International Astronomical Union — Near Earth Asteroids: A Chronology of Milestones 1800-2200 (http://www.iau.org/public/themes/neo/nea/ — accessed 15 July 2015).

More Information

Earth Approaching Asteroids as Targets for Exploration (1978)

Multiple Asteroid Flyby Missions (1971)

To Mars By Way of Eros (1966)

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)

Pluto, Doorway to the Stars (1962)

Pluto and its five moons as observed by the Hubble Space Telescope. Charon is roughly half as large as Pluto; the other moons are much smaller. Image credit: NASA.

In just about a month, on 14 July 2015, New Horizons will fly by Pluto at a nominal distance of only 10,000 kilometers moving at a velocity of about 14 kilometers per second. At that speed and distance, the piano-sized 478-kilogram probe will briefly return images of Pluto in which objects as small as 50 meters wide might be visible.

Pluto was discovered in 1930, during Lowell Observatory's hunt for a planet beyond Neptune. The observatory, founded in 1894 by wealthy Bostonian Percival Lowell to find proof of intelligent life on Mars, had begun its search for a trans-Neptunian planet in 1906.

The search for Planet X (as Lowell dubbed his hypothetical world) was at least partly motivated by the growing disdain with which Lowell's Mars theories were greeted by professional astronomers. Lowell was eager that his observatory should be seen to be credible; discovery of a new planet would, he felt, restore and cement its eroded credibility.

Lowell employed a bevy of young women as "computers" to attempt to determine the position of Planet X based on the motion of the planet Neptune, which did not orbit the Sun precisely as expected. By assuming that Pluto had a mass six times as great as Earth, Lowell and his assistants narrowed the region of the sky where they expected to find Planet X to a portion of the constellation Gemini.

Percival Lowell did not live to see a trans-Neptunian world found (he died in 1916). Following his death, the search for Planet X stalled while his observatory and his widow feuded over the money he had bequeathed to endow Lowell Observatory in perpetuity. The search did not resume in earnest until 1929. When it did, it was meant to survey the entire ecliptic, the invisible line in the sky along which the planets move. The ecliptic corresponds to the plane of the Earth's orbit about the Sun.

On 18 February 1930, 23-year-old Lowell Observatory astronomer Clyde Tombaugh discovered that a tiny dot of light on photographic plates he had made on 23 January and 29 January 1930 had changed position slightly against the background stars. The small movement signified that the object Tombaugh found was moving slowly and thus was far from the Sun. The tiny dot in Gemini, near Lowell's predicted position for Planet X, was subsequently found on plates dating back to before Lowell's death.

Lowell Observatory revealed Tombaugh's find to the world on 18 March 1930, on what would have been Percival Lowell's 75th birthday. It named the object Pluto, for the god of the cold, dark Roman underworld. The observatory staff selected the name in part because its first two letters matched Percival Lowell's initials. Newspapers around the world hailed the discovery of the Solar System's ninth planet.

Pluto was a puzzler, however. An object six times Earth's mass was expected to show a disk when observed using large telescopes, but Pluto did not. Furthermore, the planet had a bizarre tilted orbit that partly overlapped that of Neptune.

As astronomers continued their observations of Pluto, they revised estimates of its size downward. By 1960, some astronomers thought that it was about the size of Earth; others thought it might be as small as Mercury. This only deepened the mystery surrounding the planet, for if it was to account for the observed discrepancies in Neptune's orbit, then it had to be several times as massive as Earth. Some astronomers proposed the existence of another, larger planet beyond Pluto. One scientist proposed a much more novel explanation.

Dr. Robert Forward, a physicist at Hughes Aircraft Company, drew attention to Pluto's unusual characteristics in an article he published in Missiles & Rockets magazine on 2 April 1962. He did not speculate about what those characteristics might mean. That task he handed off to author George Peterson Field.

Field was in fact Forward's pen name. Safely hidden from professional ridicule behind the protective cloak of his nom de plume, the newly minted Ph.D. physicist could freely speculate in a "science fact" article in the December 1962 issue of Galaxy science fiction magazine that Pluto was a gift from a "Galactic Federation."

He began by calculating that a body about the size of Mercury but with six times the mass of Earth would be so dense that it would have to be made of the collapsed matter found only in certain dwarf stars. Such an object could not exist naturally; unrestrained by the massive gravity of a dwarf star, it should have exploded long ago. Therefore, he asserted, Pluto must be artificial.

He suggested that Pluto was in fact a "gravity catapult." He wrote that "it would have to be whirling in space like a gigantic, fat smoke ring, constantly turning from inside out." A spacecraft that approached the ring's center moving in the direction of its spin would be dragged through "under terrific acceleration" and ejected from the other side.

If the acceleration the ultradense smoke ring gave the spacecraft were about 1000 times the acceleration Earth's gravity imparts to objects (that is, 1000 gravities), then the ring would boost the spacecraft to nearly the speed of light in about one minute. The passengers and crew would, however, feel nothing as their spacecraft accelerated, for the gravitational force from the roiling ring would act on every atom of their bodies and their ship uniformly. The ring would slow by a small amount as it accelerated the spacecraft.

He wrote that a "network of these devices in orbit around interesting stars" would provide "an advanced race" with an "energetically economical" means of star travel. The rings in the network would "cartwheel slowly" so that over time they would point at many possible destination stars.

A spacecraft a ring accelerated could, upon arriving at another star in the network, enter that star's ring moving against the ring's roiling motion. This would decelerate the spacecraft very rapidly and increase the ring's rate of motion by a tiny amount. In effect, the spacecraft would pay back the network for the acceleration it borrowed when it began its journey.

He ended his article by noting that such a device could be shot through space by a larger gravity catapult and braked "by pushing against a massive planet," such as Neptune. This, he added, might account for Pluto's odd orbit with respect to the eighth planet. He speculated that, at some time in the past, the Galactic Federation had noted the rise of humans and had launched Pluto toward Sol as "a coming out present."

Artist's impression of New Horizons at Pluto, 14 July 2015. Charon is visible at upper right. Details on Pluto and Charon are speculative. Image: NASA.

Forward's concept is so imaginative and appealing that it ought to be true. New data on Pluto soon ruled it out, however. In 1977, James Christy of the U.S. Naval Observatory Western Station, located just a few kilometers from Lowell Observatory in Flagstaff, Arizona, found Pluto's moon Charon. The discovery of a body orbiting Pluto enabled astronomers to calculate its mass accurately for the first time.

Pluto, it turned out, has only about one-quarter of 1% of Earth's mass. Subsequently, it was found to have a diameter of only about 2350 kilometers, making it only two-thirds as large as Earth's moon. Shortly after the turn of the 21st century, Pluto was found to have four more moons, all much smaller than Charon.

Though Pluto did not turn out to be a link in a galactic transportation network, it did turn out to be a link to something big. Pluto was the first member body of the Kuiper Belt to be found. The Kuiper Belt, a part of the Solar System long theorized but only confirmed beginning in 1992, is the "third realm" of bodies orbiting the Sun after the Sun-hugging realm of the rocky planets and the realm of the giant planets. It is far bigger than the first two realms combined.

As New Horizons closes in on Pluto, we know of more than 1000 bodies in trans-Neptunian space. Astronomers estimate that more than 100 times that number might exist. Assuming that New Horizons continues to operate as planned, mission planners expect to direct it past at least one more Kuiper Belt Object after the Pluto flyby.

If Pluto is so small that it cannot account for the discrepancies in Neptune's orbit, then what does? In August 1989, the Voyager 2 spacecraft flew past Neptune. By carefully tracking the robot spacecraft, celestial dynamicists refined their estimate of Neptune's mass. When they did, the observed discrepancies in its orbital motion vanished. There was thus never a need to find a Planet X. Error had led to coincidence, and the result was early knowledge of mysterious Pluto.

Source

"Pluto — the Gateway to the Stars," Robert L. Forward. Missiles and Rockets, 2 April 1962, pp. 26-28.

"Pluto, Doorway to the Stars," George Peterson Field, Galaxy Magazine, December 1962, pp. 78-82.

More Information

Galileo-style Uranus Tour (2003)

Pluto: An Alternate History

The Spacewalks That Never Were: Gemini Extravehicular Planning Group (1965)

3 June 1965: Astronaut Ed White floats outside Gemini IV.

On 18 March 1965 at 0700 UTC, the Soviet Union's Voskhod 2 spacecraft lifted off from Baikonur Cosmodrome in Soviet Central Asia bearing rookie cosmonauts Pavel Belyayev and Alexei Leonov. As soon as Voskhod 2 entered a 167-by-475-kilometer orbit inclined 64.8° relative to Earth's equator, Belyayev assisted Leonov with preparations for the mission's main objective: to accomplish humankind's first-ever spacewalk.

The 5682-kilogram spacecraft carried a 1.2-meter-diameter inflatable airlock called Volga mounted over the inward-opening crew hatch of its 2.3-meter-diameter spherical reentry module. The airlock was necessary because Voskhod 2's electronics were air-cooled, so would overheat if its cramped cabin were depressurized. Following inflation — a process that lasted seven minutes — Volga extended 2.5 meters out from Voskhod 2's silvery hull.

At 0828 UTC, as the spacecraft neared the end of its first orbit, Leonov entered Volga and Belyayev sealed the Voskhod 2 hatch behind him. Belyayev then depressurized Volga, and Leonov opened its 65-centimeter-wide inward-opening outer hatch. At 0834 UTC, over northern Africa, the 30-year-old cosmonaut pulled himself through the hatch, kicked off the hatch rim, and floated away until he reached the end of his 5.35-meter-long safety tether and rebounded.

Leonov wore a white backpack containing enough oxygen for 45 minutes outside Voskhod 2. The oxygen entered his white Berkut space suit — a modified Vostok SK-1 intravehicular suit — then vented into space, carrying away exhaled carbon dioxide, heat, and moisture.

The first spacewalker: cosmonaut Alexei Leonov. Image credit: TASS.

History's first spacewalker experimented with positioning himself using his tether, reporting after his flight that it gave him tight control over his movements. Then, at 0847 UTC, over Siberia, Leonov reentered Volga and closed the outer hatch behind him. Belyayev re-pressurized the airlock and opened the Voskhod 2 hatch so that Leonov could remove his backpack and return to his couch.

After the cosmonauts resealed the hatch, Belyayev fired explosive bolts that separated Volga from Voskhod 2. The spacecraft landed in the Soviet Union on 19 March after 17 Earth orbits. The Soviets declared that world's first spacewalk had been "easy."

NASA took notice. The U.S. civilian space agency had planned its first extravehicular activity (EVA) for Gemini IV, the second of 10 planned piloted Gemini missions. The Gemini IV EVA astronaut would not leave his spacecraft; instead, he would open his hatch (each Gemini astronaut had one) and stand up in the cockpit. This would test the G4C EVA suit and the life-support umbilical linking it to the Gemini spacecraft's life support system.

Cutaway of Gemini spacecraft. Image credit: NASA.

The first full-exit EVA would take place on Gemini V, then EVAs would become progressively more complex with each new mission. After Leonov's easy spacewalk, however, NASA decided that Gemini IV spacewalker Ed White should try to outdo his Soviet predecessor.

Gemini IV's two-stage Titan II launch vehicle boosted it into a 283-by-161-kilometer, 94-minute orbit on 3 June 1965. Gemini IV separated from the Titan II second stage, then command pilot James McDivitt sought to rendezvous with it. The flight plan called for him to pilot Gemini IV to within seven meters of the stage during the mission's first orbit. Near the end of the second orbit, about three hours after launch, White would leave the cockpit and, using a Hand-Held Maneuvering Unit (HHMU), attempt to reach the spent stage.

Unfortunately, rendezvous proved to be more difficult than anticipated. The spent stage vented propellants, causing it to tumble. This subjected it to increased atmospheric drag, causing it to move away from Gemini IV. McDivitt set out in pursuit, but found his efforts thwarted by poor visibility, inability to accurately judge distance (Gemini IV included no rendezvous radar), and an incomplete grasp of orbital mechanics. With Gemini IV's propellant supply dwindling, McDivitt called off the rendezvous.

EVA preparation needed more time than expected, then White's hatch refused to unlatch, so the first U.S. EVA did not begin until Gemini IV's third orbit. After shoving back the stiff hatch, White pushed out of the cockpit. He successfully tested the HHMU, which contained only enough compressed oxygen propellant for 20 seconds of maneuvering (image at top of post).

White then evaluated his umbilical. He found it to be useful for controlling his distance from Gemini IV and for pulling himself back to the spacecraft, but he was unable to demonstrate the precision maneuvering Leonov had reported. At one point, in fact, he accidentally collided with and smeared McDivitt's cockpit window.

White's life-support umbilical was covered in a thin layer of gold to protect it from the fierce sunlight of low-Earth orbit. If the umbilical had for any reason ceased to supply him with oxygen, his chest-mounted Ventilation Control Module (VCM) could have supplied him with enough to return safely to his seat.

As with Leonov's Berkut, oxygen passing through White's 10.7-kilogram G4C suit flushed exhaled carbon dioxide, heat, and moisture into space. America's first spacewalker reported later that he was more comfortable during his EVA than at any other time during the Gemini IV flight.

With Gemini IV moving rapidly toward night, White reluctantly returned to the cockpit. There he found that internal pressure had caused his suit to balloon slightly. During the five-minute struggle to squeeze back into his narrow seat and close his balky hatch, heat from White's exertions overcame the G4C’s cooling capacity. His visor fogged slightly and sweat blinded him until he could remove his helmet in the re-pressurized cockpit and wipe his eyes.

NASA judged White's 20-minute EVA to have been a resounding success. EVA, it seemed, presented few challenges. NASA management was, on the other hand, alarmed by McDivitt's inability to rendezvous with the Titan II second stage. Rendezvous was a critical part of NASA's Lunar Orbit Rendezvous plan for landing a man on the Moon by 1970.

By the end of June, NASA top brass were considering cancelling the progressively more challenging EVAs scheduled for Gemini missions V, VI, and VII so that engineers, flight controllers, and astronauts could concentrate on rendezvous.

In July 1965, NASA made decisions critical to Gemini EVA planning. On 2 July, the Gemini Program Office (GPO) at the Manned Spacecraft Center (MSC) in Houston, Texas, formed the Gemini Extravehicular Planning Group (GEPG) to revise EVA objectives for Gemini missions VIII, IX, X, XI, and XII. On 12 July, NASA Headquarters directed the GPO to postpone the next U.S. spacewalk until Gemini VIII. The GEPG submitted its recommendations to Gemini Program Manager Charles Mathews on 19 July.

The GEPG based its recommendations on several assumptions. First, of course, was that the EVA objectives planned for Gemini VIII would be attainable without the gradual development of skills that would have occurred during the Gemini V, VI, and VII EVAs.

Gemini Agena Target Vehicle (docking adapter at right). Image credit: NASA.

In addition, the GEPG assumed that NASA would beat the rendezvous and docking problem. Gemini missions VIII through XII would each include a docking with a Gemini Agena Target Vehicle (GATV), an Agena-D upper stage modified to serve as a Gemini docking target and auxiliary propulsion stage. The GATV, launched on an Atlas rocket, would include a latch-equipped docking adapter sized to accept the Gemini spacecraft's blunt nose. During the Gemini VIII, IX, X, XI, and XII EVAs, the Gemini would remain docked to the GATV.

The GEPG also recommended that EVA equipment too large for cockpit storage be stowed on the aft-facing concave surface of the Adapter Section, the aft-most and widest part of the Gemini spacecraft, as well as on the GATV. On Gemini VIII, oversize equipment would include an HHMU with 10 times as much compressed oxygen as White's HHMU. The Gemini VIII EVA astronaut would evaluate the Adapter Section stowage concept, then test the HHMU.

The GEPG noted that oxygen flow through White's space suit had kept him cool and dry "except when [he] was working at a high exertion level." On Gemini VIII and subsequent missions, an Extravehicular Life Support System (ELSS) would replace the VCM. The ELSS could be used with a backpack-mounted oxygen supply that would permit hour-long EVAs without an umbilical. The GEPG recommended that the Gemini VIII EVA astronaut test the ELSS chest-pack to ensure that it could cool even a hard-working spacewalker adequately.

Before returning to the cockpit, he would also "inspect the Agena for engineering analysis," test a space hand tool, and evaluate a lightweight safety tether and a backup "suit exhaust" EVA propulsion system. By clambering over the two spacecraft, he would evaluate transfer between two vehicles, a skill of potential use in the Apollo Program if astronauts found themselves compelled in the event of docking problems to move by EVA between the Apollo Command and Service Module (CSM) and Lunar Module (LM).

The many EVA tasks planned for Gemini VIII through XII would require EVAs of greater duration than White's, so the Gemini VIII spacewalker would also evaluate EVA operations during orbital night, which would last for about half of each orbit.

U.S. Air Force Modular Maneuvering Unit. Image credit: NASA.

Gemini IX would see the first use of the U.S. Air Force Modular Maneuvering Unit (MMU), a hydrogen-peroxide-fueled "rocket pack" that would reach orbit stowed in the Adapter Section. The Gemini IX EVA astronaut would back up to the MMU, connect his ELSS to its integral oxygen supply, then grip t-shaped hand controllers and fly away from Gemini IX. The MMU's hot-gas thrusters would require that the astronaut's G4C suit be modified to include protective multilayer metal-fabric and foil leg coverings.

The GEPG noted that MMU development was proceeding to schedule, but added that NASA and the Air Force had yet to agree on the MMU's purpose or on whether it could fly without a safety tether linking it to the Gemini spacecraft. These questions were, it added, "beyond the scope of the present planning study."

The Gemini X EVA astronaut's tasks would focus on his spacecraft and the space environment. He would release "dense smoke" ahead of Gemini X and film its flow over the spacecraft's surfaces, photograph Gemini thrusters firing during day and night, gauge static charge on Gemini X and its GATV using a hand-held electroscope, measure hull temperature, and collect samples of contaminants (for example, the greasy contaminant that tended to cloud Gemini cockpit windows).

The GEPG also recommended two tether dynamics experiments for Gemini X. The spacewalker would simulate an untethered EVA using a "long slack tether," then would link his spacecraft and an inoperative Agena using a "towline." After the EVA, Gemini X would attempt to pull the Agena through space in an "evaluation of dynamics of orbital tow."

Gemini XI would see a dramatic increase in EVA complexity. The spacecraft would intercept the 10.5-ton Pegasus 3 satellite, which was due to be launched into low-Earth orbit on a Saturn I rocket soon after the GEPG submitted its report. Like its predecessors, Pegasus 3 was designed to assess the likelihood that spacecraft in low-Earth orbit would suffer meteoroid impact damage. To do this, it unfolded a pair of 4.3-meter-wide-by-29-meter-long "wings" containing a total of 400 meteoroid-detection panels.

Artist concept of Pegasus satellite. Image credit: NASA.

The GEPG reported that discussions with NASA Headquarters and NASA Marshall Space Flight Center had already led to Pegasus 3 modifications for Gemini rendezvous and EVA mission. Pegasus 1, launched 16 February 1965, had achieved an elliptical 510-by-726-kilometer orbit, while Pegasus 2, launched 25 May 1965, had entered a 502-by-740-kilometer orbit. When launched on 30 July 1965, Pegasus 3 entered a near-circular 535-by-567-kilometer orbit. This made it a more readily accessible rendezvous target for Gemini spacecraft.

In addition, sixteen of Pegasus 3's meteoroid-detection panels had been replaced with removable aluminum meteoroid-capture panels and panels containing thermal control test surfaces. After rendezvous with the giant satellite, the Gemini XI spacewalker would use an HHMU to jet over and remove the panels for return to Earth. The GEPG stated that "[d]etermination of the method of accomplishing this task. . .must still be accomplished."

Gemini XII would see the second flight of the MMU rocket pack. If the Gemini IX MMU test was performed using a tether, then consideration would be given to untethered flight during Gemini XII. The mission would also rendezvous with the 2300-kilogram Missile Defense Alarm System (MIDAS) II satellite, which had failed two days after it reached orbit on 24 May 1960. The EVA astronaut would inspect and photograph MIDAS II in an effort to determine the cause of its failure.

The GEPG suggested alternate missions for Gemini XI and XII that would see one or both missions meet up with Apollo spacecraft in orbit. A Gemini might, for example, rendezvous with the SA-204 Apollo CSM, which in July 1965 was scheduled to be launched in September 1966. SA-204 was planned to be the first manned Apollo CSM flight, but it would be flown unmanned if either of the two suborbital test flights scheduled to precede it failed. The EVA astronaut would transfer to and enter the unmanned CSM, check out its systems, and return to the Gemini.

If Gemini XII were postponed until February 1967, then it might rendezvous with the unmanned LM planned for launch on mission SA-206. The spacewalker would enter the spindly LM, check out its systems, and jet back to Gemini XII.

NASA accepted many of the GEPG's recommendations. As it began preparations to implement them, it conducted Gemini missions V, VI, and VII. After a rough start, Gemini V (Gordon Cooper and Charles Conrad, 21-29 August 1965) successfully conducted an improvised "phantom rendezvous" with a point in space and remained in orbit for eight days. Gemini VII (Frank Borman and James Lovell, 4-18 December 1965) stayed aloft for 14 days, demonstrating that astronauts could survive in space long enough to reach and return from the Moon.

Gemini VI (Wally Schirra and Tom Stafford, 15-16 December 1965) had been scheduled to launch on 25 October 1965, but NASA postponed the mission after its GATV was destroyed during ascent to orbit. The agency decided that Gemini VI should instead pay a visit to the long-duration Gemini VII crew.

On 12 December, the Gemini VI Titan II booster ignited, then shut down before it could rise off its launch pad. Command Pilot Schirra opted not to trigger a perilous pad abort, saving the mission. On 15 December, Gemini VI at last lifted off and performed rendezvous and proximity operations with Gemini VII. As 1965 ended, NASA looked ahead to dockings and spacewalks in 1966.

Gemini VIII (Neil Armstrong and David Scott, 16-17 March 1966) became the first manned spacecraft to perform a docking — and the first Gemini mission with a successful GATV — but then suffered a thruster malfunction that sent the docked vehicles spinning out of control. The astronauts made an emergency landing, so Scott was unable to perform the planned first spacewalk since Gemini IV.

The Gemini VIII Reentry Module floats in the Pacific south of Japan after an emergency reentry which prevented Pilot David Scott (in left seat) from performing NASA's second spacewalk. Command Pilot Neil Armstrong sits in the right-hand seat. Image credit: NASA.

Despite this, NASA proceeded with Gemini IX (Tom Stafford and Eugene Cernan, 1-11 June 1966) as if the Gemini VIII EVA had succeeded. Cernan, the agency announced, would move to the aft end of the Gemini IX Adapter Section, don the Astronaut Maneuvering Unit (AMU) — as the MMU had been renamed — and fly up to 45 meters from the spacecraft.

Cernan's spacewalk was a near-disaster. He quickly overheated, fogging his faceplate. He found that handholds, loop-shaped foot restraints, and velcro patches on Gemini IX's exterior gave him scant help in controlling his movements. He estimated after the flight that 50% of his energy had been devoted to fighting the internal pressure of his modified G4C suit so that he could hold position.

Nearly blinded by sweat, he tore his suit's outer thermal layers as he moved over Gemini's IX's hull. Through heroic efforts, and with his pulse racing at 195 beats per minute, he managed to reach and don the AMU before Stafford ordered him to abandon the EVA and return to Gemini IX's cockpit.

NASA began hurriedly to revise its ambitious EVA plans. Gemini X (John Young and Michael Collins, 18-21 July 1966) started with a low-key EVA during which Collins performed astronomical ultraviolet photography while standing in the cockpit. During his second EVA, which began just 90 minutes after the first, he used an HHMU to move to the derelict Gemini VIII GATV.

Michael Collins in the Gemini X cockpit after his difficult spacewalk. Image credit: NASA.

His clumsy movements caused the GATV to gyrate, making it difficult for Young to keep Gemini X close by. Young called off the EVA, which was to have lasted 90 minutes, just 39 minutes after Collins left the cockpit. 

Gemini XI (Charles Conrad and Richard Gordon, 12-15 September 1966) was, if anything, even worse. Gordon quickly overheated as he fought without adequate handholds to attach a tether to the Gemini XI GATV. Conrad called off the scheduled 107-minute spacewalk after 38 minutes. In his post-flight debrief, Gordon reported that "a little simple task that I had done many times in training to the tune of about 30 seconds lasted about 30 minutes."

No Gemini performed a rendezvous with Pegasus 3. The meteoroid and thermal control test surface panels that the GEPG had hoped a spacewalker would recover during Gemini XI were destroyed when the satellite reentered Earth's atmosphere on 4 August 1969.

NASA kept the AMU on the manifest of Gemini XII (James Lovell and Edwin Aldrin, 11-15 November 1966), going so far as to install it on the spacecraft on 17 September 1966. On 23 September, however, as the significance of Gordon's EVA troubles hit home, NASA Headquarters ordered the hot-gas rocket pack removed.

In the pool: Buzz Aldrin trains for his Gemini XII spacewalks. Image credit: NASA.

Desperate for a successful EVA, the agency revised Aldrin's training regimen and EVA plan. He spent extra time rehearsing his spacewalk while submerged in a swimming pool wearing weights that made him neutrally buoyant. His three EVAs had a relaxed pace and were spread out over three days. He had at his disposal a variety of new handholds, footholds, and other restraint devices. NASA also limited his EVAs to relatively simple tasks, such as testing space tools while firmly restrained.

Until the late 1980s, the Soviet Union and Alexei Leonov maintained the fiction that his historic spacewalk had been "easy." After the fall of the Soviet Union in 1991, it was revealed that Leonov's Berkut suit had ballooned in the vacuum of space. He was unable to reach a camera switch on his thigh, so could not photograph Voskhod 2 as planned.

After about 10 minutes outside, Leonov began his return to Voskhod 2. Unable to adequately control his movements, he entered Volga head first (not feet first, as planned), so had to flip in the airlock to shut its hatch behind him.

After becoming trapped sideways in the fabric airlock, he flirted with dysbarism ("the bends") by lowering his suit's internal pressure so that he could free himself, complete his flip, and seal the hatch. His exertions overwhelmed Berkut's air-flow cooling system, causing his core body temperature to rise 1.8° C in 20 minutes.

Leonov's EVA would be the last Soviet spacewalk until the Soyuz 4-Soyuz 5 docking mission of 14-18 January 1969. When Yevgeni Khrunov and Alexei Yeliseyev performed history's first two-person EVA on 16 January 1969, Soviet space suit designers and EVA planners drew upon NASA's Gemini EVA experience. Khrunov and Yeliseyev wore Yastreb space suits with cable-and-pulley systems and metal parts to prevent ballooning and improve mobility. Their 37-minute external transfer from Soyuz 5 to Soyuz 4 took place without significant incident.

Sources

Memorandum with attachment, GS/Chairman, Gemini Extravehicular Planning Group, to Manager, Gemini Program, Report of Gemini Extravehicular Planning Study, 19 July 1965.

"The First Egress of Man into Space," A. A. Leonov; paper presented at the 16th International Astronautics Congress in Athens, Greece, 13-18 September 1965.

Walking to Olympus: An EVA Chronology, Monographs in Aerospace History Series #7, David S. F. Portree and Robert C. Trevino, NASA History Office, October 1997 (http://history.nasa.gov/monograph7.pdf — accessed 3 June 2015).

More Information

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

Chrysler's Transportation and Work Station Capsule (1965)

Dreaming a Different Apollo, Part One: Shameless Wishful Thinking

Skylab 1 Orbital Workshop atop a Saturn V rocket (foreground) and Skylab 2 Saturn IB rocket (background). Image credit: NASA.

Apollo didn't die; it was killed. The Apollo Program might have continued for many years, evolving constantly to achieve new goals at relatively low cost. Instead, programs designed to give Apollo a future beyond the first lunar landing began to feel the brunt of cuts even before Neil Armstrong set foot on the Moon. By the time Apollo drew to its premature conclusion — the last mission to use Apollo hardware was the joint U.S.-Soviet Apollo-Soyuz Test Project (ASTP) of July 1975 — NASA was busy building a wholly new space program based on the Space Shuttle. Throwing out the Apollo investment and starting over with Shuttle was incredibly wasteful both in terms of learned capabilities and money.

Apollo as we knew it included over its seven-year series of flights a total of seven major hardware elements. They were: the Saturn V rocket, available in three-stage and two-stage varieties; the two-stage Saturn IB rocket; the Apollo Command and Service Module (CSM) spacecraft; the Apollo Lunar Module (LM) Moon lander; the jeep-like Lunar Roving Vehicle (LRV); the Skylab Orbital Workshop, a temporary space station; and the ASTP Docking Module (DM).

Apollo missions 1, 2, and 3 either did not fly (in the case of Apollo 1, which killed astronauts Gus Grissom, Edward White, and Roger Chaffee on 27 January 1967) or were cancelled (in the case of Apollo 2 and Apollo 3). Flown missions began with Apollo 4, the first test of the Saturn V rocket (9 November 1967). It carried no crew. Apollo 5 was a Saturn IB-launched LM test without a crew. Apollo 6 was the second Saturn V rocket test, again without astronauts on board.

All subsequent Apollo and Apollo follow-on missions save one (Skylab 1) were launched bearing three-man crews. Apollo 7 (11-22 October 1968), the first piloted Apollo, was a Saturn IB-launched CSM-only mission in low-Earth orbit. It accomplished the mission originally planned for Apollo 1. Apollo 8 (21-27 December 1968) was a Saturn V-launched lunar-orbital CSM-only mission motivated in part by rumors of a Soviet piloted circumlunar flight, Apollo 9 was a Saturn V-launched, Earth-orbital CSM/LM test, and Apollo 10 was a lunar-orbital dress rehearsal for Apollo 11 (16-24 July 1969), which carried out the first piloted lunar landing.

NASA gave alphanumeric designations to the Apollo missions; Apollo 8 was, for example, designated C-prime. Apollo 11 was the first and only G-class mission. The Apollo 11 moonwalk lasted a little over two hours and the crew remained on the Moon for only 22 hours. Though momentous (and the signal to most people that Apollo could end), Apollo 11 was really a full-up engineering test of the Apollo lunar mission system from Earth launch to Earth splashdown and post-mission quarantine. It paved the way for the H-class missions: Apollo 12 (H-1) which, after a pinpoint landing near the unmanned Surveyor III lander, included a 32-hour surface stay and two moonwalks; Apollo 13 (H-2), the "successful failure" (as NASA called it) which through adversity hinted at Apollo's untapped potential; and Apollo 14 (H-3), which included the longest lunar surface traverse on foot of the Apollo Program.

NASA originally planned for Apollo 15 to be H-4, but upgraded it to J-1 after NASA Administrator Thomas Paine, in an ill-advised attempt at horse-trading with the Nixon White House, cancelled one H mission and one J mission. J missions included LMs with longer landing hover times, lunar surface stays of about three days, improved space suits supporting up to four moonwalks, and the electric-powered LRV. Individual moonwalk duration was stretched to almost eight hours, in part because of suit improvements, but also because riding the LRV reduced astronaut metabolic rates; seated, they used less oxygen and cooling water than when on foot.

Apollo 17 Lunar Module Challenger at Taurus-Littrow, December 1972. Image credit: NASA.

Apollo 16 was called J-2 and Apollo 17 in December 1972 was J-3. The last piloted Moon mission of the 20th century, Apollo 17 was the final flight of the LM, the LRV, and the three-stage Saturn V.

Six months after it abandoned the Moon, NASA launched Skylab I, the first and only Skylab Orbital Workshop, unmanned atop the first and only two-stage Saturn V to fly. Three Saturn IB rockets each launched a CSM bearing three men to Skylab I for stays of up to 84 days. They lifted off from a makeshift raised platform ("the milkstool") on Saturn V Pad 39B. The last mission, Skylab IV, returned to Earth on 8 February 1974.

Eighteen months after Skylab, the last Saturn IB to fly launched the last CSM to fly into low-Earth orbit for a meet-up with a Soviet Soyuz spacecraft. The last CSM was named only "Apollo." The first and only DM, an airlock that enabled crews to move safely between the incompatible atmospheres of the Apollo and Soyuz spacecraft, rode inside the tapered shroud that linked the bottom of the CSM to the top of the Saturn IB's S-IVB second stage.

Upon reaching Earth orbit, the ASTP Apollo spacecraft turned end for end, docked with the DM, detached it from the S-IVB, and began maneuvers that led to the first international docking in space. On 24 July 1975, six years to the day after Apollo 11 returned from the Moon, the ASTP Apollo CSM parachuted to a splashdown in the Pacific.

Though Apollo hardware remained, none of it reached space. A second Skylab workshop was placed on display in the National Air and Space Museum in Washington, DC. Two Saturn Vs, one of which might have launched the second Skylab, and an assortment of Saturn IB rockets, CSMs, and LMs in various states of completion were parceled out to NASA centers and museums for display or were scrapped.

President Lyndon Baines Johnson, a NASA supporter (in 1958, as Senate Majority Leader, he had been instrumental in its creation), predicted Apollo's premature end. In 1967, Congress slashed to just $122 million the $450 million he requested to start the Apollo Applications Program (AAP). AAP — which would rapidly shrink to become the Skylab Program — had been intended to exploit Apollo hardware and operational experience to accomplish new lunar and Earth-orbital missions. In a somber address on the future of the Apollo Program to NASA Marshall Space Flight Center employees in December 1967, Johnson mused that, "the way the American people are, now that they have all this capability, instead of taking advantage of it, they'll probably just piss it all away."

What if Johnson had got it wrong? What if, somehow, Americans cared more about space exploration and so sought to wring from their $24-billion Apollo investment everything they could?

The Soviet Union for many years numbered its Soyuz missions consecutively regardless of changes in spacecraft purpose and design. If Apollo had been allowed to survive and thrive, perhaps the United States would have adopted a similar numbering policy, ultimately yielding impressively high alphanumeric mission designation numbers. What follows is an unabashed exercise in alternate history speculation (and, above all, shameless wishful thinking). It is based on actual NASA and contractor plans and is written as though the events it recounts actually occurred.

A word of caution: in order to simplify an already complex timeline, I have ignored the possibility of accidents. Spaceflight is risky; yet in this alternate history all missions occur exactly as planned. The likelihood that every mission described below would come off as planned, with no mishaps or outright disasters, would in fact be very small.

1971-1972

Because no one sought to kill Apollo, NASA boss Paine felt no urge to trade away two Apollo missions in the vain hope that Nixon would support his plans for a large Earth-orbital space station. This meant that Apollo 15 remained H-4. The first J mission (J-1) was Apollo 16 and Apollo 17 was J-2.


Apollo Earth-orbital space station flights began in late 1971. Apollo 18 was the unmanned launch of the first two-stage Saturn V bearing a temporary Earth-orbiting space station. In keeping with NASA's old penchant for program names from Greek and Roman mythology, the station was dubbed Olympus 1. The Olympus name had a heritage in the world of space station planning going back to the early 1960s.

The Apollo-derived Olympus station resembled the Skylab Orbital Workshop of our timeline, but lacked its side-mounted Apollo Telescope Mount and "windmill" solar arrays. It also included more internal decks.

Within days, Apollo 19, the first K-class Earth-orbital CSM, lifted off on a Saturn IB from Launch Complex 34 bound for Olympus 1 with three astronauts on board. K-class CSMs included batteries in place of fuel cells, an electricity umbilical for linking to the Olympus station power system, a retractable main engine bell to make more room in the S-IVB shroud, extra storage compartments in the Command Module (CM) capsule, an option to install up to two extra crew couches, a pair of small steerable dish antennas in place of lunar Apollo's large four-dish system, and smaller main-engine propellant tanks. It also included modifications that enabled it to remain semi-dormant attached to an Olympus station for up to six months (for example, heaters to prevent fluids from freezing in its tanks and propellant lines).

Apollo 19 remained docked to Olympus 1's axial ("front") docking port while its crew worked on board the station for 28 days – twice as long as any U.S. space mission before it. They returned to Earth on Christmas Eve 1971. The Apollo 20 (K-2) crew, launched on 23 January 1972, subsequently demolished Apollo 19's new record by living on board Olympus 1 for 56 days.

Apollo 21 (I-1), a Saturn V-launched mission to lunar polar orbit, marked the start of a new phase of Apollo lunar exploration. Two astronauts orbited the Moon for 28 days in a CSM with an attached Lunar Observation Module (LOM) in place of an LM. From mid-March to mid-April 1972, the astronauts charted the Moon's surface in great detail to enable scientists and engineers to select future Apollo landing sites and traverse routes.

Apollo 22 (K-3), launched in June 1972, delivered a three-man crew to Olympus 1 for a 112-day stay, doubling Apollo 20's stay-time. Ninety days into their mission, the two-man Apollo 23 (K-4) CSM docked at Olympus 1's single radial ("side") docking port for 10 days. One of the Apollo 23 astronauts was a medical doctor; he conducted health evaluations of the Apollo 22 astronauts. If any member of the Apollo 22 crew had been found to be unhealthy, then all would have returned to Earth in either their own CSM or with the Apollo 23 crew in its CSM, which included three spare couches (the empty Science Pilot couch and two couches located against the Apollo 23 CM’s aft bulkhead).

As it turned out, the Apollo 22 astronauts were in good shape and high spirits, so NASA authorized continuation of their mission to its full planned duration. Before returning to Earth, the Apollo 22 crew used their CSM's main engine to boost Olympus 1 to a higher orbit, postponing its reentry by up to 10 years.

NASA referred to the Apollo 22 astronauts as the third Olympus 1 resident crew and the Apollo 23 astronauts as the first Olympus 1 visitor crew. The full alphanumeric designations for Apollos 22 and 23 were O-1/K-3/R-3 and O-1/K-4/V-1, respectively. Most people did not pay attention to those designations, however, being satisfied to call the missions by their Apollo numbers.

NASA ordered 15 Saturn V rockets for the Apollo Program. In 1968, NASA Deputy Administrator for Manned Space Flight George Mueller asked NASA Administrator James Webb for permission to order more Saturn V rockets for AAP. With budgets for post-Apollo space programs already under fierce attack, Webb rejected Mueller’s request.

In our alternate timeline, Webb's answer was different. Apollo 24 (J-3) (October 1972) used the last Saturn V of the original Apollo buy. This fact aroused only passing interest, however, since in our alternate timeline no one seriously considered halting the Saturn V assembly lines. Apollo 25 (J-4) launched atop the first new-buy Saturn V, the 16th Saturn V to be built.

Two months after the Apollo 24 LM ascent stage lifted off from the lunar surface, the Apollo 25 LM landed about a kilometer away from the derelict Apollo 24 LM descent stage. The Apollo LM descent engine kicked up potentially damaging dust during landing, so the Apollo 25 astronauts inspected Apollo 24's descent stage, LRV, and ALSEP experiments to determine whether a one-kilometer landing separation distance was adequate.

The Apollo 25 crew carried out other technology experiments. They deployed an experimental solar array designed to withstand the cold of the two-week lunar night and a small battery-driven remote-controlled rover. Controllers on Earth drove the small rover several hundred meters in preparation for longer remote-controlled traverses to come.

1973

Apollo 26 (O-2) (January 1973) was the Saturn V launch of the Olympus 2 space station. It lifted off from Pad 39C, a new Complex 39 launch pad north of the existing 39A and 39B pads at Kennedy Space Center (KSC), Florida. 39C was designed for both Saturn V and Saturn IB launches, putting NASA on track to retiring the Complex 34 Saturn IB pad located south of Kennedy Space Center, within the boundaries of Cape Canaveral Air Force Station.

Soon after Olympus 2 reached orbit, the last Saturn IB to use Complex 34 launched Apollo 27 (O-2/K-5/R-1). Its epic mission: to stretch the world spaceflight endurance record to 224 days. Over the course of the Apollo 27 mission, NASA launched four unmanned Saturn IB rockets with Centaur upper stages. Though not given Apollo numbers, the flights are often referred to unofficially as Apollo GEO A, Apollo GEO B, Apollo GEO C, and Apollo GEO D. Two lifted off from Pad 39C and two from newly upgraded Pad 39A.

Each boosted into geostationary orbit one Radio/TV Relay Satellite (RTRS); three operational satellites and a spare. Olympus 2 thus became the first space station capable of uninterrupted voice, data, and TV contact with Mission Control at the Johnson Space Center in Houston, Texas, and Payload Control at the Marshall Space Flight Center in Huntsville, Alabama.

The Saturn IB-launched Apollo 28 CSM lifted off from Pad 39C 45 days into the Apollo 27 crew's stint on board Olympus 2. The six-day, three-person mission, designated O-2/K-6/V-1, included the first female U.S. astronaut. Apollo 29 (O-2/K-7/V-2), another six-day, three-person mission, reached Olympus 2 110 days into the Apollo 27 mission. It included the first non-American to fly on a U.S. spacecraft.

Apollo 30 (O-2/K-8/V-3), a 10-day, two-person mission nearly identical to Apollo 23, reached Olympus 2 190 days into the Apollo 27 mission. The Apollo 27 astronauts proved to be in good health, so NASA authorized them to continue their mission to its full planned duration. The Apollo 30 crew returned to Earth in Apollo 27's CSM, leaving behind their fresh CSM for the long-duration astronauts. The Apollo 27 crew used the Apollo 30 CSM's main engine to boost Olympus 2 to a higher orbit with an estimated lifetime of more than a decade.

Just before the Apollo 27 crew ended their record-setting stay in space in July 1973 — a record that would hold for more than a decade — the unmanned Apollo 31 Saturn V launched a pair of modified RTRS satellites (one operational and one spare) into a loose orbit around the quasi-stable Earth-Moon L2 point, 33,000 miles beyond the Moon. When NASA launched Apollo 34 (J-5) to the Moon's Farside hemisphere, out of sight of Earth, the satellites provided continuous radio, data, and TV communication with both the CSM while it orbited over the Farside hemisphere and the LM parked on the Farside surface.

The Apollo 32 (O-3) Saturn V launched Olympus 3 — intended to be the first "long-life" space station — from Pad 39A (December 1973). Olympus 3 included three equally spaced radial docking ports, expanded solar arrays, an uprated life support system, a "greenhouse" plant growth chamber, improved internal lighting, an observation cupola, and guest living quarters.

1974

The next month, the three-man Apollo 33 (O-3/K-9/R-1) crew lifted off from Pad 39C to begin a 180-stay on board Olympus 3. Starting with Apollo 33, 180 days became the standard duration for Olympus station missions. The Apollo 27 crew had remained on board Olympus 2 for 224 days so that NASA could have in place a "cushion" of biomedical knowledge in the event that a 180-day mission had to be extended; for example, if a resident crew's CSM proved faulty when time came to return to Earth and a rescue mission had to be mounted.

Apollo 34 (J-5) (February 1974) was, as indicated above, the first piloted mission to the Moon's hidden Farside. The last of the J-class lunar landing missions, its crew included the first woman on the Moon.

Olympus 3 could support visiting crews for longer periods, permitting Apollo 35 (O-3/K-10/V-1) to be the first three-person, 10-day visitor mission. It delivered the first Cargo Carrier (CC-1) to Olympus 3 60 days into the Apollo 33 mission. Drum-shaped CC-1 rode to orbit inside the segmented shroud between the top of the Saturn IB's S-IVB second stage and the bottom of the Apollo 35 CSM's engine bell.

After S-IVB shutdown, the Apollo 35 crew separated their CSM from the shroud, which peeled back in four parts and separated from the stage. They then turned their CSM end-for-end to dock with CC-1's "outboard" docking port and detached the carrier from the S-IVB.


Image credit: NASA/David S. F. Portree.

The Apollo 35 CSM docked with one of Olympus 3's three radial ports using CC-1’s "inboard" docking port. Its crew then entered the station through CC-1's meter-wide central tunnel. When their visit with the Apollo 33 crew drew to an end, they undocked their CSM from CC-1, leaving the carrier attached to Olympus 3 so that it could serve as a "pantry" or "walk-in closet."

Apollo 36 (O-3/K-11/V-2) was another 10-day, three-person visitor mission to Olympus 3. Its crew included an African-American mission Commander who had flown first as Command Module Pilot on Apollo 24. The Apollo 36 CSM docked with CC-1's outboard port 120 days into Apollo 33. When time came to return to Earth, they undocked CC-1's inboard port from Olympus 3. Following their deorbit burn, they undocked their CSM from CC-1's outboard port and performed a small separation maneuver. CC-1, packed with trash, burned up in Earth's atmosphere, and the Apollo 36 CM capsule splashed down in the Pacific.

The Apollo 33 resident crew undocked from Olympus 3 and returned to Earth, and two weeks later the Apollo 37 (O-3/K-12/R-2) CSM arrived with Olympus 3's second resident crew and, on its nose, a hefty telescope module. The crew gingerly docked the telescope module to the radial port on the side of Olympus 3 opposite the radial port used for Cargo Carriers, then undocked their CSM from the telescope module's outboard port and redocked with Olympus 3's axial port. Olympus 3 thus became the world's first multi-modular space station.

Attention then shifted back to the lunar track of the ongoing Apollo Program. Apollo 38 (L-1A) (August 1974) saw an unmanned uprated Saturn V-B rocket launch directly to the lunar surface an LM-derived Lunar Cargo Carrier (LCC-1) bearing a nuclear-powered Dual-Mode Lunar Rover (DMLR). The piloted Apollo 40 (L-1B) mission saw the first Augmented CSM (ACSM) and the first Augmented Lunar Module (ALM) launched to lunar orbit on a Saturn V-B. The Apollo 40 ACSM remained in continuous contact with Earth over the Moon's Farside hemisphere through the RTRS satellites at Earth-moon L2.

The ALM descended to a landing within about a kilometer of LCC-1. The astronauts deployed the DMLR and drove it on five traverses during their one-week stay on the Moon. They then reconfigured it for Earth-guided operation. After the DMLR retreated to a safe distance under Earth control, the Apollo 40 ALM ascent stage ignited to return the crew to the orbiting ACSM and, subsequently, to Earth.

In October 1974, a month after the Apollo 40 astronauts left the Moon, DMLR began a 500-kilometer overland trek to the next planned Apollo landing site. As it moved slowly over the rugged surface, it imaged its surroundings, took magnetometer readings, and occasionally stopped to collect an intriguing rock or scoop of dirt. A pair of spotlights permitted limited lunar night-time driving. Assuming that the DMLR reaches its goal, the next ALM crew, set to land next to a pre-landed LCC in July 1976, will retrieve its samples for return to Earth, reconfigure it for astronaut driving, use it to explore their landing site, and then reconfigure it again for Earth-guided operation.

Image credit: NASA.

Sandwiched between Apollo 38 and Apollo 40 was Saturn IB-launched Apollo 39 (O-3/K-13/V-3), a routine 10-day visitor mission to Olympus 3 bearing Cargo Carrier-2. Apollo 39 docked CC-2's inboard port with one of Olympus 3's two unoccupied radial docking ports.

1975

The Apollo 41 (O-3/K-14/R-3) CSM docked with the third Olympus 3 radial port bearing the station's third resident crew in early January 1975. The start of their mission overlapped the end of the Apollo 37 resident crew's 180-day stay in space. The handover marked the start of Olympus 3's continuous occupation, which lasted until the station was safely deorbited in July 1979.

Apollo 42 (O-3/K-15/V-4), another 10-day visitor mission to Olympus 3, docked at the CC-2 outboard port in March 1975 and, when they returned to Earth, deorbited CC-2 over the Pacific Ocean. Apollo 43 (O-3/K-16/R-4) in June 1975, the second 180-day mission resident crew of 1975, delivered CC-3. A week after the Apollo 43 crew arrived, Apollo 41 undocked and returned to Earth. The Apollo 43 crew undocked from CC-3 and redocked at the radial port Apollo 41 had vacated during July.

Apollo 44 (0-3/K-17/R-4) docked with Olympus 3 on 19 December 1975. On their way to Olympus 3, they performed a rendezvous with Olympus 1 to assess its condition. Apollo 43's return to Earth on 31 December 1975 rounded out NASA's 1975 piloted spaceflight schedule.

On our alternate timeline, NASA's Apollo-based piloted space program is hitting its stride. Earth-orbital operations are becoming routine; lunar-surface operations are continuing to evolve and advance.

On our own timeline, Apollo has drawn to its ill-considered close. Apollo would attract general public notice twice before the first Space Shuttle flight in April 1981: in September 1977, when funding cuts compelled NASA to shut off the science instruments the six Apollo lunar landing crews left behind on the Moon; and in July 1979, when Skylab reentered Earth's atmosphere less than a week ahead of Apollo 11's 10th anniversary, pelting Australia with debris.

More Information

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

Dreaming a Different Apollo, Part Two: Naming Names

Jimmy Carter's Space Shuttle

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

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

The next in line: from its vantage point in orbit of Saturn, the sixth planet, the Cassini spacecraft catches a glimpse of Uranus, the seventh (blue dot, upper left). Image credit: NASA.

The four largest and most massive satellites of Jupiter are, in order out from the planet, Io, Europa, Ganymede, and Callisto. Io and Europa form a pair of roughly the same size, as do Ganymede and Callisto. Io has a diameter of 3636 kilometers, while Europa, the smallest of the four, is 3138 kilometers in diameter. Ganymede, the largest moon in the Solar System, measures 5262 kilometers across. Callisto, Jupiter's outermost large moon, is 4810 kilometers in diameter.

The presence of four large, massive moons enabled the Galileo spacecraft to carry out a complex tour of the Jupiter system between December 1995 and September 2003. Over the course of 35 revolutions around the giant planet, Galileo used gravity-assist flybys of the four moons to change its orbit.

By contrast, Saturn and Neptune each have only one large, massive moon. Saturn's moon Titan, the second-largest moon in the Solar System, measures 5152 kilometers in diameter, while Neptune's moon Triton is just 2706 kilometers across. The Cassini Saturn Orbiter, at this writing exploring the Saturn system, must rely on Titan for most of its gravity assists, which means that it must rely more often than did Galileo on its finite supply of rocket propellants to make orbital changes. A Neptune orbiter, with only Triton available for significant gravity assists, would face a similar challenge.

The four largest and most massive moons of Uranus are puny compared with Io, Europa, Ganymede, Callisto, Titan, and Triton. Titania, the largest, measures just 1578 kilometers in diameter. The others are: Ariel (1158 kilometers across), innermost of the four moons; Umbriel, 1169 kilometers wide; and Oberon, (1522 kilometers), outermost of the four. Titania orbits between Umbriel and Oberon.

To scale: the five largest moons of Uranus in order (left to right) out from the planet. Image credit: NASA.

Though often derided as small and dull, the reality is that the Uranian satellites are little known. Voyager 2, the only spacecraft to visit Uranus, imaged no more than 40% of any Uranian moon as it flew through the system in January 1986. Furthermore, the Cassini Saturn tour has revealed that even small outer Solar System satellites can be surprising: Enceladus, for example, just 505 kilometers wide and by all rights cold and dead, is hot enough inside that it blasts salty water into space from parallel cracks ("tiger stripes") at its south pole at more than 2000 kilometers per hour.

In a paper published in the Journal of Spacecraft and Rockets shortly before Galileo concluded its Jupiter satellite tour, Andrew Heaton of NASA Marshall Space Flight Center and James Longuski of Purdue University demonstrated that the Uranus system could support a complex Galileo-style tour. This was, they acknowledged, "contrary to intuition. . .because the Uranian satellites are much less massive than those of Jupiter."

A Galileo-style tour would be possible, they explained, because "the key to a significant gravity assist is not the absolute size of the satellite, but the ratio of its mass to its primary, and the mass ratios of the Uranian satellites to Uranus are similar to those of the Jovian satellites to Jupiter." Titania and Oberon form a large outer pair similar to Ganymede and Callisto, they noted, while Ariel and Umbriel form a small inner pair equivalent to Io and Europa. The "Uranian system is nearly a smaller replica of the Jovian system," Heaton and Longuski wrote.

To perform their calculations, they relied on "Tisserand graphs" developed at Purdue University in the late 1990s. Their mathematical tool was named for 19th-century mathematician Felix Tisserand, who had calculated the effects of planetary gravity on the motion of comets. Tisserand followed in the footsteps of Anders Johan Lexell, who in the early 1770s had sought to explain the sudden appearance and subsequent disappearance of a previously unknown comet. In 1770, Comet Lexell flew past the Earth at a distance of 2.3 million kilometers.

A previous post detailed how, in the early 1960s, Michael Minovitch used his own graphs and University of California, Los Angeles and Jet Propulsion Laboratory computers to calculate dozens of gravity-assist trajectories (see "For More Information" at the end of this post). His work laid the groundwork for many planetary missions, including the Mariner 10 Venus-Mercury flybys and Voyager 2's Jupiter-Saturn-Uranus-Neptune "Grand Tour." Minovitch did not, however, calculate satellite system tours; presumably this was because in the early 1960s so little was known of outer Solar System moons.

Heaton and Longuski described a three-phase, 811-day Uranian system tour. After launch from Earth in March 2008 and a gravity-assist flyby of Jupiter in September 2009, the Uranus tour spacecraft would fire its main rocket engine to capture into an elliptical Uranus orbit on Valentine's Day in 2018. This would mark the start of the first Uranus tour phase, which would be devoted to matching the plane of the Uranian equator, ring system, and moon orbits.

Images of Uranus and its ring system taken over four years using the Hubble Space Telescope in Earth orbit. South is to the left. The 2003 and 2005 images show features in the planets clouds; a southern hemisphere polar hood and, in 2005, a northern hemisphere storm. In the 2007 image, the planet's rings and equator are edge on to the Sun.

Uranus is tipped on its side relative to the other planets in the Solar System, and its moons have equatorial orbits. Heaton and Longuski wrote that the Uranian system would appear edge-on to the Sun in 2007, then would tilt gradually until the planet and its moons pointed their north poles at the Sun in 2028.

The Uranus tour spacecraft would capture into an initial orbit tilted 13.6° relative to the planet's equator and system plane. It would fly past Titania in May 2019 at a distance of 316 kilometers, allowing the largest Uranian satellite to "crank" its orbital plane. A total of nine similar Titania flybys over 261 days would place the spacecraft into the same plane as the Uranian equator, rings, and moons.

The second phase of the Uranus tour, the energy-reduction phase, would see the spacecraft reduce the size of its orbit, thus shortening its orbital period, while at the same time conducting a thorough exploration of the four largest Uranian moons. This would begin 287 days after the spacecraft captured into Uranus orbit with a flyby of Oberon at a distance of 414 kilometers and would proceed through eight Ariel flybys, five Umbriel flybys, three Titania flybys, and four additional Oberon flybys over the course of the next 395 days.

The spacecraft would pass nearest any world in the Uranian system during this phase. At the start of its 14th revolution about Uranus, almost exactly one Earth year (364.3 days) after arriving at the planet, it would pass just 54 kilometers over Umbriel's icy landscapes.

The southern hemisphere of Miranda: a mosaic of images from the January 1986 Voyager 2 Uranus flyby. Image credit: NASA.

Heaton and Longuski did not include the enigmatic moon Miranda on their list of close flybys because it orbits close to Uranus and, with a diameter of just 480 kilometers (only a little smaller than surprising Enceladus) is less than half the size of Ariel, the smallest moon they employed for gravity assists. Close proximity to Uranus and low mass would mean that Miranda's gravity could contribute little to shaping the Uranus tour.

Miranda has some of the most intriguing known surface features on the Uranian satellites — for example, Verona Rupes, a five-kilometer-high fault scarp that begins near the edge of the lighted area visible to Voyager 2. Presumably the Uranus tour spacecraft would image Miranda whenever its tour route took it relatively close by.

The third and final phase of the tour would commence 691 days after Uranus arrival with a 151-kilometer Umbriel flyby. The somewhat arbitrary goal of the third phase would be to place the Uranus tour spacecraft into orbit around Ariel. Through three additional Umbriel flybys and four Titania flybys over 120 days the spacecraft would nearly match Ariel's orbit about Uranus, reducing its maximum velocity relative to its target to slightly less than one kilometer per second. The Uranus tour spacecraft would then briefly fire its rocket motor to slip into orbit about Ariel.

Source

"Feasibility of a Galileo-Style Tour of the Uranian Satellites," A. Heaton and J. Longuski, Journal of Spacecraft and Rockets, Volume 40, Number 4, July-August 2003, pp. 591-596.

More Information

The Challenge of the Planets, Part Three: Gravity

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)

Touring Titan By Blimp & Buoy (1983)