[Spaceflight History Blog] The Challenge of the Planets

[Orionid]

The Challenge of the Planets, Part One: Ports of Call
14 March 2015 David S. F. Portree


Image credit: NASA

In the 1950s, those few individuals who made it their business to think seriously about interplanetary travel predicted that the immense gulfs between the planets would be crossed only through enormous efforts. To be sure, chemical-propellant rocket engines akin to those already in use on missiles at the time could probably accomplish round-trip journeys to the moon and reach Mars and Venus; for worlds beyond these near neighbors, however, new technology and techniques almost certainly would be needed.
One proposed technique would effectively mimic the old European and Chinese voyages of exploration, which saw sailing ships seek repair and resupply at exotic seaports and remote islands as they made their way to distant destinations. When applied to the Solar System, this technique would see space travelers use the planets as "ports-of-call" where they could refuel, resupply, and wait while planets aligned to permit a minimum-energy transfer to their next destination.

This is as good a place as any to ask (and answer) an obvious question: why did 1950s planners feel obligated to include crews on board their interplanetary spacecraft? In those days, it was widely assumed that spacecraft would need continuous repair to remain functional for long enough to reach another planet. The harsh vacuum and temperature variations in space, combined with micrometeoroids and radiation, meant that robot spacecraft would likely malfunction and, with the nearest repairmen millions of miles away, rapidly degrade and fail. In addition, automation, radio communications, and sensor capabilities remained severely limited.

A voyage to Mercury employing the ports-of-call technique would begin with a trip from Earth to Venus. Every 19 months, the two planets align so that a minimum-energy crossing becomes possible. After a propulsive escape from Earth orbit and four months spent coasting along a curving path about the Sun, the spacecraft would intersect Venus. Its crew would ignite rockets to slow the spacecraft so that Venus's gravity could capture it into orbit, then they would rendezvous with a space station to refill its propellant tanks and take on fresh supplies.

Venus-Mercury minimum-energy transfer opportunities occur about every five months. If the crew were unlucky, they might reach Venus orbit just as an opportunity for a minimum-energy transfer to Mercury ended. In that case, they would need to wait five months for Venus and Mercury to align again. The flight to Mercury would begin with propulsive escape from Venus orbit. The spacecraft would then spend three months coasting along a curving Sun-centered path. When it intersected Mercury, its crew would fire its rocket motors so that the little planet's gravity could capture it into orbit.

If they were the first humans to reach Mercury, then they would look for valuable resources – rocket propellants, to begin with – and perhaps establish the nucleus of a permanent base. Then, when Mercury and Venus lined up again, they would retrace their steps to Venus and Earth.

If a spacecraft's destination lay in the other direction – that is, beyond Mars, in the outer Solar System – then the challenges of interplanetary voyaging became much more daunting. Because Jupiter, Saturn, Uranus, Neptune, and Pluto orbit far from the Sun, their years are long, so opportunities to begin minimum-energy transfers between them occur infrequently. Because their orbits are far apart, travel between one cold outer world and the next using minimum-energy transfers can require years or even decades.

A spaceship from Earth bound for Uranus, for example, would have to wait for an opportunity to begin a minimum-energy transfer to Mars (they occur every 26 months). About six months after Earth departure, its curving Sun-centered orbital path would intersect Mars. The crew would then fire their spacecraft's rocket motors to slow down so Mars's gravity could capture it into orbit.

At a Mars-orbiting space station – perhaps on Phobos, the innermost martian moon – they would ready their spacecraft to take advantage of the next Mars-Jupiter minimum-energy transfer opportunity (they happen every 28 months). After a journey of about three years, the intrepid Uranus-bound crew would fire rocket motors to capture into orbit around banded Jupiter, where they would wait for the next minimum-energy Jupiter-Saturn transfer opportunity (they occur every 20 years). While they cooled their heels they might explore the giant world's sprawling family of moons. Thirteen were known in the 1950s, and almost nothing was known about any of them.

The minimum-energy journey from Jupiter to ringed Saturn would last 10 years. While they waited in the Saturn system for a Saturn-Uranus minimum-energy transfer opportunity (they occur about every 54 years), the crew might refuel their spacecraft at the large moon Titan, for much of the 20th century the only planetary satellite known to have an atmosphere. Astronomers in the 1950s thought that Titan's air envelope was made of methane, which, when liquefied, constitutes a reasonably efficient rocket fuel.

The journey from Saturn to Uranus would last 27 years. Hence, even if the wait time at every stop along the way were of the shortest duration possible, the one-way trip from Earth to Uranus would last at least 40 years.

Of course, these examples are somewhat disingenuous, for no one really expected that space travelers would restrict themselves to minimum-energy transfers. They would, it was assumed, exploit the ability to refuel at each planetary port-of-call to take on extra propellants and apply extra energy to each leg of their spacecraft’s interplanetary trek. By so doing, voyage duration and the time between minimum-energy transfer opportunities could be reduced. Even so, round-trip voyages to worlds past Jupiter were likely to test the endurance of even the hardiest crews.

Confronted with these cold facts, many 1950s space writers felt certain that spacecraft exploration of the Solar System would not begin until at least the 21st century. Patrick Moore, for example, wrote in 1955 that none of his readers would live to see Mars and Venus up close, and that no spacecraft would reach Jupiter or Saturn for generations. Though he cautioned against excessive pessimism, Moore declared that spacecraft might never reach Uranus, Neptune, and Pluto: he wrote, for example, that "we need not waste time working out the possibility of a journey to Uranus," adding that the tilted world would be "left to roll along in its icy solitude, remote, unwelcoming, and lonely beyond our understanding."

Even as these somewhat florid words saw print, however, propulsion engineers were hard at work developing new fast ways of reaching the planets. Part Two of this post will look at some of the spacecraft designs they proposed. Part Three will then describe discoveries that undermined their plans and threw open the entire Solar System to scientific exploration.

Sources

The Exploration of Space, Arthur C. Clarke, Harper & Bros., New York, 1951, pp. 137-162

Space Travel, Kenneth Gatland and Anthony Kunesch, Philosophical Library, New York, 1953, pp. 173-175

Guide to the Planets, Patrick Moore, Eyre & Spottiswoode, London, 1955; pp. 141, 195

The Exploration of the Solar System, Felix Godwin, Plenum Press, New York, 1960; pp. 152-161

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Source: The Challenge of the Planets, Part One: Ports of Call

[Orionid]

The Challenge of the Planets, Part Two: High Energy
16 March 2015 David S. F. Portree


JPL's nuclear-electric "Space Cruiser" could in theory reach Pluto from Earth orbit in slightly more than three years. Image credit: NASA/JPL

President John F. Kennedy did not call only for a piloted lunar landing by 1970 in his 25 May 1961 "Urgent National Needs" speech before a joint session of the U.S. Congress. Among other things, he sought new money to expand Federal research into nuclear rocketry, which, he explained, might one day enable Americans to reach "the very ends of the solar system."

Today we know that Americans can reach the "ends" of the Solar System without resorting to nuclear rockets. When President Kennedy gave his speech, however, it was widely assumed that "high-energy" propulsion - which for most researchers meant nuclear rockets - would be desirable for round-trip journeys to Mars and Venus and an outright necessity for voyages beyond those next-door worlds.

In his speech, President Kennedy referred specifically to the joint NASA-Atomic Energy Commission (AEC) ROVER nuclear-thermal rocket program. As the term implies, a nuclear-thermal rocket employs a nuclear reactor to heat a propellant (typically liquid hydrogen) and expel it through a nozzle to generate thrust.

ROVER had begun under U.S. Air Force/AEC auspices in 1955. AEC and the Air Force selected the Kiwi reactor design for nuclear-thermal rocket ground testing in 1957, then the latter relinquished its role in ROVER to the newly created NASA in 1958. As President Kennedy gave his speech, U.S. aerospace companies competed for the contract to build NERVA, the first flight-capable nuclear-thermal rocket engine.

Nuclear-thermal propulsion is not the only form of nuclear-powered high-energy propulsion. Another is nuclear-electric propulsion, which can take many forms. This post examines only the form known widely as ion drive.

An ion thruster electrically charges a propellant and expels it at nearly the speed of light using an electric or magnetic field. Because doing these things requires a large amount of electricity, only a small amount of propellant can be ionized and expelled. This means in turn that an ion thruster permits only very gradual acceleration; one can, however, in theory operate an ion thruster for months or years, enabling it to push a spacecraft to high velocities.

American rocket pioneer Robert Goddard first wrote of electric rocket propulsion in his laboratory notebooks in 1906. By 1916, he conducted experiments with “electrified jets.” He described his work in some detail in a report in 1920.

Interest remained minimal, but picked up in the 1940s. The list of ion-drive experimenters and theorists reads like a "Who's Who" of early space research: L. Shepherd and A. V. Cleaver in Britain, L. Spitzer and H. Tsien in the United States, and E. Sanger in West Germany all contributed to the development of ion drive before 1955.

In 1954, Ernst Stuhlinger, a member of the German rocket team the U.S. Army brought to the United States at the end of the Second World War, began small-scale research into ion-drive spacecraft while developing missiles for the Army Ballistic Missile Agency (ABMA) at Redstone Arsenal in Huntsville, Alabama. His first design, poetically nicknamed the "cosmic butterfly," relied on banks of dish-shaped solar concentrators for electricity, but he soon switched to nuclear-electric designs. These had a reactor heating a working fluid which drove an electricity-generating turbine. The fluid then circulated through a radiator to shed waste heat before returning to the reactor to repeat the cycle.

Stuhlinger became a NASA employee in 1960 when the ABMA team at Redstone Arsenal became the nucleus for Marshall Space Flight Center (MSFC). In March 1962, barely 10 months after Kennedy's speech, the American Rocket Society hosted its second Electric Propulsion Conference in Berkeley, California. Stuhlinger was conference chairman. About 500 engineers heard 74 technical papers on a wide range of electric-propulsion topics, making it perhaps the largest professional gathering ever devoted solely to electric propulsion.

Among the papers were several on ion propulsion research at the Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL had formed its electric-propulsion group in 1959 and commenced in-depth studies the following year.

One JPL study team compared different forms of "high-energy" propulsion to determine which, if any, could perform 15 robotic space missions of interest to scientists. The missions were: flybys of Venus, Mars, Mercury, Jupiter, Saturn, and Pluto; Venus, Mars, Mercury, Jupiter, and Saturn orbiters; a probe in solar orbit at about 10% of the Earth-Sun distance of 93 million miles; and "extra-ecliptic" missions to orbits tilted 15°, 30°, and 45° with respect to the plane of the ecliptic. In keeping with their robotic payloads, all were one-way missions.

The five-person JPL comparison study team found that a three-stage, seven-million-pound chemical-propellant Nova rocket capable of placing 300,000 pounds of hardware - including a hefty chemical-propellant Earth-orbit departure stage - into 300-mile-high Earth orbit with a meaningful scientific instrument payload could achieve just eight of the 15 missions: specifically, the Venus, Mars, Mercury, Jupiter, and Saturn flybys; the Venus and Mars orbiters; and the 15° extra-ecliptic mission. A chemical/nuclear-thermal hybrid comprising a Saturn S-I first stage, a 79,000-pound Kiwi-derived nuclear-thermal second stage, and a 79,000-pound Kiwi-derived nuclear-thermal stage with interplanetary payload could carry out the Nova missions plus the 30° extra-ecliptic mission.

A 1500-kilowatt ion system starting from Earth orbit could achieve all 15 missions. The JPL team told the Berkeley meeting that an unspecified chemical-propellant booster rocket would launch the 45,000-pound ion system into a 300-mile-high orbit as a unit. There the reactor and ion thrusters would activate and the slow-accelerating ion system would begin gradually to gain speed and climb toward Earth-escape and its required interplanetary trajectory.

For several of the missions to more distant targets - for example, the Saturn flyby - the ion system had enough time to accelerate so that it could reach its goal hundreds of days ahead of the Nova and chemical/nuclear-thermal hybrid systems. It could also provide its instrument payload and long-range telecommunications system with ample electricity, boosting data return. A smaller ion system (600-kilowatts, 20,000 pounds) that could be launched atop NASA's planned Saturn C-1 booster rocket could accomplish all but the extra-ecliptic 45° mission.

Missiles & Rockets magazine devoted a two-page article to the JPL comparison study. It headlined its report "Electric Tops for High-Energy Trips," which must have been gratifying for many long-time ion-drive supporters.

Many technical problems remained, however. The five JPL engineers who performed the comparison study optimistically assumed that for every kilowatt of electricity its 1500-kilowatt system applied to generating thrust, only 13 pounds of hardware - reactor, turbo-generator, radiator, structure, wiring - would be required. In 1962, a ratio of about 70 pounds of hardware per kilowatt of thrust with a maximum generating capacity of only 30 kilowatts was considered much more realistic.

They also assumed that its electricity-generating system and its ion-drive system could operate more or less indefinitely despite the presence of moving parts operating at high temperatures. The whirling turbo-generator, for example, would for some missions need to operate non-stop at a temperature of about 2000° Fahrenheit for years. A one-year operating time was considered a bold aspiration in 1962.

The five engineers did not specify the precise form their ion-drive spacecraft would take, but it would probably have resembled the design depicted at the top of this post. A trio of JPL engineers produced it during the 1960-1962 period, while the five-person JPL team conducted its comparison study.

The automated, 20,000-pound "space cruiser," as the three engineers dubbed their creation, would include a radiator surface area of roughly 2000 square feet, making it a large target for micrometeoroid strikes. In 1962, little was yet known of the quantity of micrometeoroids in interplanetary space, so no one could judge accurately the likelihood that such a radiator might be punctured, nor the mass required for effective puncture-resistant radiator tubes, redundant cooling loops, or "make-up" cooling fluid.

The five-person team only briefly mentioned the potentially profound effects of ion-drive power and propulsion systems on other spacecraft systems. The turbo-generator, for example, would impart torque to the spacecraft, creating a requirement for a spin-nulling attitude-control system - for example, a momentum wheel and chemical-propellant thrusters (the momentum wheel is visible near the center of the truss in the image above).

The turbine, flow of coolant through the radiator, and momentum wheel would, it was expected, cause vibration that could interfere with scientific instruments. In addition, ion drive systems would of necessity generate powerful magnetic and electric fields that might make difficult many desirable scientific measurements.

The space cruiser engineers sought to reduce radiation effects by placing its reactor at its front (upper right in the illustration above) and its science instruments at its rear. Unfortunately, this put the instruments among the space cruiser's ion thrusters, where intense electric and magnetic fields would occur.

The space cruiser designers looked at a thermionic power system that would use electrons from its reactor to produce electricity directly and would include neither moving parts nor high-temperature systems. They did not favor it because it was new technology. In addition, the thermionic system's nuclear reactor would need cooling fluid, a circulating pump, and a radiator, so in terms of vibration and micrometeoroid damage would offer only a little improvement over the better-understood turbo-generator design.

Close on the heels of the ARS Electric Propulsion Conference in Berkeley, NASA Headquarters opted to concentrate electric propulsion research at the NASA Lewis Research Center in Cleveland, Ohio. The move was probably intended to eliminate costly redundant research programs and keep JPL and MSFC focused on their Apollo Program-related tasks. Research did not stop entirely at NASA MSFC and JPL, however. Stuhlinger, for example, continued to produce designs for piloted ion-drive spacecraft.

Ironically, while the nearly 500 electric-propulsion engineers met near San Francisco, a young mathematician working alone near Los Angeles was busy eliminating any immediate need for ion drive or any other kind of high-energy propulsion system for planetary exploration. The third part of this three-part series of posts will examine his work and its profound impacts on planetary exploration.

Sources

"Electric Tops for High Energy Trips," Missiles & Rockets, 2 April 1962, pp. 34-35

Nuclear Electric Spacecraft for Unmanned Planetary and Interplanetary Missions, JPL Technical Report No. 32-281, D. Spencer, L. Jaffe, J. Lucas, O. Merrill, and J. Shafer, Jet Propulsion Laboratory, 25 April 1962

The Electric Space Cruiser for High-Energy Missions, JPL Technical Report No. 32-404, R. Beale, E. Speiser, and J. Womack, Jet Propulsion Laboratory, 8 June 1963

"Electric Spacecraft – Progress 1962," D. Langmuir, Astronautics, June 1962, pp. 20-25

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Source: The Challenge of the Planets, Part Two: High Energy

[Orionid]

The Challenge of the Planets, Part Three: Gravity
22 March 2015 David S. F. Portree



It is strange that Lexell's Comet is not better remembered. Discovered by ace comet-hunter Charles Messier on the night of 14 June 1770, it passed Earth just two weeks later at a distance of only 2.3 million kilometers, closer than any other comet in recorded history. On the evening of 1 July 1770, its nucleus shown as brightly as Jupiter at its brightest, and its silvery coma was five times larger than the full moon.

Lexell's Comet then drew close to the Sun - that is, it reached perihelion - and was lost in the glare. Messier saw it next in the pre-dawn sky on 4 August. Having moved away from Earth and the Sun, it had become small and faint. Messier observed the comet with difficulty before dawn on 3 October 1770, then lost sight of it.

Comets are today named for their discoverer or discoverers, but in the 18th century it was the mathematicians who computed their orbits who got all the credit. Comet Halley is, for example, named for Edmond Halley, who computed its orbit and determined that what had seemed like a series of individual comets was in fact a single comet that returned again and again. Partly this was because in Comet Halley's case no one knows who discovered it; records of the comet's apparitions extend back at least to 240 BCE, but it almost certainly was noticed in Earth's skies much earlier.

Lexell's Comet was named for Anders Johan Lexell, who determined that it completed one elliptical orbit around the Sun in 5.6 years. This was for the time a remarkably short period for a comet, raising questions as to why it had not been observed before. Lexell hypothesized that the comet had previously had a large orbit with a perihelion close to Jupiter's orbit, but then had passed Jupiter at a distance of about 3.2 million kilometers in 1867. The giant planet had, he wrote, slowed it and deposited it into its new short-period orbit.

Lexell's Comet was due to reach perihelion again in 1776, but this occurred on the far side of the Sun as viewed from Earth and so was not observed. Astronomers eagerly awaited its next perihelion in 1781 or 1782, but nothing was seen. Again, Lexell offered an explanation: in 1779, as it neared the point in its new orbit where it was farthest from the Sun - its aphelion - the comet had again intersected Jupiter. This time, it has sped up and entered an unknown but probably long-period orbit. It might even have escaped the Sun's gravitational grip entirely. In any case, Lexell's Comet has not been seen since and is officially designated "lost."

The light-show of 1 July 1770 should have ensured that no one forgot Lexell's Comet, but both its close pass by Earth and its orbit changes soon faded from memory. If they had not, then Michael Minovitch's mathematical research in 1961-1964 might not have shaken the interplanetary mission planning world the way they did.

Minovitch, in 1961 a 25-year-old graduate student at the University of California - Los Angeles (UCLA), began his research while working a summer job at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He calculated that a flyby spacecraft which passed behind a planet as it orbited the Sun would in effect be towed by the planet's gravity, increasing its speed. As the spacecraft departed the planet's vicinity, it would keep that speed. Conversely, a flyby spacecraft that passed ahead of a planet would be slowed. Minovitch viewed this as a new form of propulsion; he called the effect the planet had on the spacecraft "gravity thrust."

Minovitch determined that a spacecraft could use gravity thrust flybys to travel from world to world indefinitely without use of rocket propulsion. It could even return to the vicinity of Earth, enter a close solar orbit, or escape the Solar System entirely. In all, he calculated about 200 different planetary-flyby sequences using charts he devised and computers at JPL and UCLA.

Many engineers who learned of Minovitch's results assumed at first that they violated fundamental physical law. It seemed that the flyby spacecraft would get something for nothing. This was, of course, incorrect: when the spacecraft was slowed, the planet gained a very tiny amount of momentum; when the spacecraft was accelerated, the planet lost a very tiny amount of momentum. Nature thus balanced its books. Minovitch, for his part, was not very skilled at first at explaining his discoveries; he seems to have understood the clean elegance of numbers far better than he did the fuzzy vagaries of human beings.

Nevertheless, he had his champions. The most important was Maxwell Hunter, who met Minovitch at the American Astronautical Society's Symposium on the Exploration of Mars (6-7 June 1963) and quickly recognized the significance of his work. Before joining the professional staff of the National Aeronautics and Space Council (NASC) in January 1962, Hunter had worked at Douglas Aircraft for 18 years. He ended his career there as Chief Engineer for Space Systems. As part of the NASC, he was well placed to promote Minovitch's discoveries; the advisory body, chaired by Vice President Lyndon Baines Johnson, provided advice directly to President John F. Kennedy.

Hunter described Minovitch's "unconventional trajectories" in a report to NASC Executive Secretary Edward Welsh in September 1963. The report became the basis for a prominent article in the May 1964 issue of the important trade publication Astronautics & Aeronautics. Hunter permitted Minovitch to review a draft before the article went to publication.

In June 1964, a month after Hunter's article made the spaceflight world aware of Minovitch's labors, JPL began planning what became Mariner Venus/Mercury 1973, the first planetary mission to employ one of the trajectories Minovitch had calculated. The MVM '73 spacecraft would fly past Venus to slow down and enter a Sun-centered orbit that would take it past Mercury. The flight past Venus was labelled a "gravity-assist flyby" - Minovitch's "gravity-thrust" moniker never caught on.

At nearly the same time, high-energy propulsion systems, which had been deemed essential for travel to worlds beyond Venus and Mars, rapidly began to lose support. As described in the previous post in this "Challenge of the Planets" series, the leader among these systems was electric (ion) propulsion.

In 1962, JPL engineers had prepared a preliminary design for an automated 10-ton nuclear-electric "space cruiser" and proudly presented it at a conference attended by about 500 other electric-propulsion engineers. It was received with great enthusiasm. The system was still early in its development, but the JPL engineers expected that, with sufficient funding, they might develop it for interplanetary spaceflights in the 1970s.

By late 1964, however, such brute-force high-energy systems were increasingly seen as needlessly complex and costly (at least as far as the preliminary reconnaissance of the Solar System was concerned). NASA could instead use a relatively small booster rocket to place on an interplanetary trajectory a package comprising a small chemical-propellant propulsion system for course corrections, star-trackers for precise spacecraft position and trajectory determination, a cold-gas thruster system for turning the spacecraft, science instruments, a computer, an electricity-generating isotopic system or solar arrays, and a radio. By 1962 standards, such a package hardly qualified as a spacecraft, yet it remains the basic form of our proudest interplanetary flyby and orbiter spacecraft to this day.

Electric-propulsion supporters were loathe to give up their labors. In addition to developing small station-keeping electric-propulsion systems for Earth-orbiting satellites, they sought planetary exploration niches where electric propulsion could outshine gravity-assist trajectories.

Ironically, given the adventures of Lexell's Comet, the most significant niche they identified was comet rendezvous. Before the end of the 1960s, the 1985-1986 Comet Halley apparition became a particularly important target for electric-propulsion supporters. Their efforts to explore Comet Halley using electric propulsion will be described in forthcoming posts.

In the years that followed Mariner 10 (as MVM '73 came to be known), more of Minovitch's gravity-assist trajectories were put to use. Though often mistakenly attributed to JPL's Gary Flandro, among Minovitch's trajectories was the Jupiter-Saturn-Uranus-Neptune path of Voyager 2. (Flandro's oft-cited "grand tour" paper saw print in mid-1966, nearly five years after Minovitch began his research; in it Flandro gave credit where it was due by citing two of Minovitch's JPL internal reports.)

The Voyager 2 sequence of flybys has been touted as a once-in-176-years opportunity to visit all the outer Solar System planets during a single mission; Minovitch, however, was quick to point out that this claim is spurious. Jupiter, Saturn, Uranus, and Neptune are each massive enough to bend a passing spacecraft's path and accelerate it toward any other point in the Solar System at any time.

Voyager 2, with a mass at launch of about 726 kilograms, left Earth on 20 August 1977 atop a Titan IIIE rocket. It flew within 564,000 kilometers of Jupiter's trailing side on 9 July 1979; within 102,000 kilometers of Saturn's trailing side on 25 August 1981; about 82,000 kilometers from Uranus's trailing side on 24 January 1986; and within 5000 kilometers of Neptune on 25 August 1989. In all, its primary mission spanned just over 12 years.

The intrepid spacecraft then began its Interstellar Mission, which continues to this day. At this writing, Voyager 2 is more than 19 billion kilometers from the Sun; unless humans catch up to it and reverently bring it home, it will in centuries to come depart the Solar System entirely and wander among the stars.

Minovitch calculated Venus-Earth gravity-assist trajectories; these came in handy beginning with the loss of the Space Shuttle Orbiter Challenger (28 January 1986) and subsequent cancellation of the Shuttle-launched Centaur G-prime upper stage. The accident and stage cancellation grounded the Galileo Jupiter Orbiter and Probe mission, which had been set to launch to Earth orbit in May 1986 in a Space Shuttle payload bay then boost directly to Jupiter on a Centaur-G-prime.

The Space Shuttle resumed flights in September 1988. Galileo was launched in the payload bay of the Orbiter Atlantis (18 October 1989) and boosted from Earth orbit using a solid-propellant Inertial Upper Stage that was incapable of sending it directly to Jupiter.

Instead, Galileo flew by Venus (10 February 1990), Earth (8 December 1990), and Earth again (8 December 1992) before it built up enough speed to begin the trek to Jupiter. Galileo reached Jupiter on 7 December 1995. Over the course of 35 Jupiter-centered orbits, it explored the four largest Jovian moons using gravity-assist flybys to speed up and slow down. A final gravity-assist series caused it to orbit nearly 26 million kilometers from Jupiter and then perform a pre-planned death-dive into its atmosphere on 21 September 2003.

Current operational missions that used or will use gravity-assist flybys include (in no particular order) Voyager 1 (which flew by Jupiter and Saturn), the Cassini Saturn Orbiter (which carried out a Venus-Venus-Earth-Jupiter sequence of gravity-assist flybys), the MESSENGER Mercury orbiter (Earth-Venus-Venus-Mercury-Mercury-Mercury), the Rosetta comet-rendezvous spacecraft and Philae lander (Earth-Mars-Earth-Earth), the Juno Jupiter orbiter (Earth), and the New Horizons Pluto flyby spacecraft (Jupiter). Even the Dawn Vesta/Ceres mission, which relies on solar-electric propulsion, used a gravity-assist Mars flyby on 4 February 2009 to gain speed and reach the Asteroid Belt between Mars and Jupiter.

Sources

"Gravity Propulsion Research at UCLA and JPL, 1962-1964," R. Dowling, W. Kossmann, M. Minovitch, and T. Ridenmoure, History of Rocketry and Astronautics, AAS History Series Volume 20, J. Hunley, editor, 1997, pp. 27-106

Comets: A Chronological History of Observation, Science, Myth, and Folklore, D. Yeomans, John Wiley & Sons, New York, 1991, pp. 157-160

The Voyager Neptune Travel Guide, C. Kohlhase, editor, NASA JPL, June 1989, pp. 103-106

"Fast Reconnaissance Missions to the Outer Solar System Utilizing Energy Derived from the Gravitational Field of Jupiter," G. Flandro, Astronautica Acta, Volume 12, Number 4, 1966, pp. 329-337

"Utilizing Large Planetary Perturbations for the Design of Deep Space, Solar Probe, and Out-of-Ecliptic Trajectories," JPL Technical Report No. 32-849, M. Minovitch, December 1965

"Future Unmanned Exploration of the Solar System," M. Hunter, Astronautics & Aeronautics, May 1964, pp. 16-26

"Determination and Characteristics of Ballistic Interplanetary Trajectories Under the Influence of Multiple Planetary Attractions," JPL Technical Report No. 32-464, M. Minovitch, October 1963

Future Unmanned Exploration of the Solar System, M. Hunter, Report to the Executive Secretary, National Aeronautics & Space Council, September 1963

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Source: The Challenge of the Planets, Part Three: Gravity