27 June 2015

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 manned 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 manned 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 manned 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 heat shield that ablates (that is, chars and erodes away, carrying away heat). The reentering 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-powered electricity sources, 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 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 GĂ©rard 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 results 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 mid-point 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 improved 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 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 unmanned and manned 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 superhot 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. 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 manned 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.


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

15 June 2015

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 manned 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 men 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 unmanned Saturn-Icarus 3 Interceptor, not the first manned moon-landing mission. Image credit: NASA.

Professor Sandorff's students proposed to hijack Project Apollo, delaying NASA's first manned lunar landing by about three years. They would take over the first nine Saturn V rockets earmarked for the moon program, commence construction in April 1967 of a third Launch Complex 39 launch pad (Pad 39C), and add 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 unmanned 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 would 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 unmanned 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 remain bolted together throughout its flight.

The first Project Icarus Saturn V (Saturn-Icarus 1) would lift off from Cape Kennedy on 7 April 1968, 73 days before the asteroid was due to collide with Earth. Its payload, Interceptor 1, would reach 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 detect Icarus for the first time.

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

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

Saturn-Icarus 5 would leave Earth near dawn on the U.S. East Coast on 14 June 1968, and Interceptor 5 would reach Icarus 2.26 million kilometers (1.4 million miles) out from Earth, 22 hours before expected impact. By then, the asteroid would appear as a modest star in the pre-dawn sky near the constellation Orion.

Saturn-Icarus 6 would lift off a few hours after Saturn-Icarus 5. When Interceptor 6 reached it, Icarus would be 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 spot the asteroid. The modified AGC would then use 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 detect the asteroid and trigger the nuclear device, which would explode 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 excavate on Icarus a bowl-shaped crater up to 300 meters (1000 feet) wide. The effect the explosions would have on Icarus' course was, of course, not known with precision; the students calculated that each blast would alter the asteroid's 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 target the largest fragments. Data from each Interceptor as it approached Icarus and from Earth-based optical telescopes and radars would be 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 stand 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 provide pure science data.

The first IMS would leave Earth atop an Atlas-Agena rocket on 27 February 1968. It would pass between 115 and 220 kilometers (70 and 135 miles) from Icarus at the time of the first nuclear explosion. This would place 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 analyze the small fragments and hot gases to determine the asteroid's composition. A 23-kilogram (50-pound) foam-honeycomb "bumper" would shield IMS-1 during passage through the debris cloud.

No IMS would monitor 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 lift 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 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.

Icarus' closest approach 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.


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)

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

Fun with Killer Asteroids

12 June 2015

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 but colors are accurate. 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 several more Kuiper Belt Objects after the Pluto flyby.

In 2006, representatives of the International Astronomical Union (IAU) voted to adopt a new definition of "planet" and create a new class of Solar System body: the "dwarf planet." Pluto and the largest Main Belt asteroid, Ceres, became dwarf planets. Many saw this as a demotion for Pluto and a promotion for Ceres. The move was controversial, not least because the IAU definitions are ambiguous. It seems likely that at some point they will undergo revision.

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.


"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

03 June 2015

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 repressurized 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 out-do 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 repressurized 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.


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)