The Last Days of the Nuclear Shuttle (1971)

Image credit: NASA.

In July 1969, as the Apollo 11 moon landing brought the Apollo Program to its culmination, Lockheed Missiles and Space Company (LMSC), McDonnell Douglas Astronautics Company (MDAC), and North American Rockwell (NAR) began the Nuclear Flight Systems Definition (NFSD) study on contract to NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Alabama. The NFSD study occurred against a backdrop of great change in the U.S. civilian space program, and its evolution through three phases reflected this.

Cutaway of a typical NERVA nuclear-thermal rocket engine. The explanation of its operation that follows is simplified. Turbopumps push liquid hydrogen propellant from a large insulated propellant tank (not shown) through the reactor core. A reflector focuses neutrons radiated from uranium fuel rods back into the core, causing nuclear fission to occur. Liquid hydrogen picks up heat generated by the fission reaction in the core, helping to prevent the fuel rods from melting. The hot hydrogen then expands outward from the nozzle skirt extension, producing thrust. Liquid hydrogen also cools the nozzle. During engine shutdown, a control drum blocks the reflector, damping nuclear fission in the core. Propellant flow is maintained for a "cool-down" period after fission is halted to prevent fuel rods melting. Note the external and internal radiation shields which reduce crew radiation exposure. Image credit: NASA.

In Phase I of the NFSD study, MSFC charged LMSC, MDAC, and NAR with producing a "detailed analysis [and] conceptual design" of an expendable nuclear-thermal rocket stage equipped with a 200,000-pound-thrust NERVA II engine. The NERVA II was expected to be flight-ready in late 1977. The contractors were directed to study "development requirements of a nuclear propulsion system, including its evolution from a flight test stage to an operational. . .stage."

The Phase I nuclear stage was envisioned as a NERVA II with a single 33-foot-diameter propellant tank sized for launch from Earth atop a two-stage, 33-foot-diameter Saturn V rocket. Its main purpose would be to push piloted spacecraft out of low-Earth orbit (LEO) toward Mars.

Phase II commenced in October 1969, immediately after President Richard Nixon's Space Task Group (STG) endorsed (with reservations) NASA's aggressive Integrated Program Plan (IPP) for future U.S. spaceflight. The IPP was the brainchild of George Mueller's NASA Headquarters Office of Manned Space Flight, which supervised NASA's manned spaceflight centers, including MSFC.

In NFSD Phase II, MSFC directed its contractors to design a reusable nuclear rocket stage equipped with a 75,000-pound-thrust NERVA I engine. The stage, dubbed the Reusable Nuclear Shuttle (RNS), was intended mainly for roundtrip crew and cargo flights between space stations in LEO and lunar orbit. In January 1970, MSFC presented the contractors with an ambitious RNS traffic model calling for 157 Earth-moon flights between 1980 and 1990 by a fleet of 15 RNS vehicles, each toting 50 tons of cargo. Piloted Mars missions, though still considered a part of the IPP, were in NFSD Phase II relegated to secondary importance.

Image credit: NASA.

As in Phase I, the Phase II nuclear stage would reach Earth orbit on top of a Saturn V rocket. Its liquid hydrogen propellant would, on the other hand, climb into orbit in the payload bay of the proposed reusable winged Earth-to-Orbit Shuttle (EOS). NASA envisioned that more than 40% of EOS flights would be devoted to delivery of propellants. The EOS would pump liquid hydrogen/liquid oxygen propellants it carried directly into the tanks of chemical-propulsion Space Tugs. Most propellant carried in the EOS would, however, be liquid hydrogen meant for RNS propulsion. MSFC engineers envisioned that the EOS would cache RNS liquid hydrogen in LEO at an Orbital Propulsion Depot.

Electricity from twin nuclear reactors arranged in a "Y" configuration (right) powers refrigeration systems that keep liquid hydrogen stored in the Orbital Propellant Depot from turning to gas and escaping. Image credit: NASA.

A Reusable Nuclear Shuttle tanks up at the Orbital Propellant Depot using a soft-docking Refueling Adapter. Image credit: NASA.

That same month, NASA Administrator Thomas Paine permanently terminated Saturn V production and canceled Apollo 20 so that its Saturn V could launch the Apollo Applications Program (AAP) Dry Workshop (DWS). In February 1970, NASA gave the AAP DWS the new name Skylab and redesignated AAP as the Skylab Program.

Soon after, MSFC directed LMSC to examine launching the RNS inside the Space Shuttle payload bay, which was expected to measure 15 feet wide by 60 feet long. LMSC's Shuttle-launched "modular" RNS would comprise a NERVA I engine and multiple hydrogen tanks launched separately into LEO and joined together through a labyrinth of pipes. NAR continued work on a single-tank RNS sized for launch on a future heavy-lift rocket, while MDAC divided its study efforts between the two launch options.

Phase II segued into Phase III in May 1970, when MSFC directed the NFSD contractors to assume a 1978 or 1979 NERVA I flight readiness date. The postponement reflected an anticipated Fiscal Year 1971 NERVA funding cut. MSFC also directed the contractors to limit to 150 tons the amount of liquid hydrogen propellant each RNS would carry.

In February 1971, with the NFSD study set to conclude in less than two months, D. J. Osias, an analyst with NASA Headquarters planning contractor Bellcomm, summarized and critiqued reports prepared by the three contractors. He began by examining the ways that the contractors had approached the problem of radiation shielding. "Nuclear propulsion," he wrote, "complicates in-space operations by introducing a radioactive environment."

All the RNS designs included a 3000-pound radiation shield on top of the NERVA I to create a conical radiation "shadow" for crew protection, but also relied on the vehicle's propellants and structure for supplemental shielding. Osias asserted that "in regard to radiation shielding. . .the most optimistic results are being accepted and attention to the problem is diminishing."

He also noted that, as liquid hydrogen was expended as propellant, it would cease to be available to serve as radiation shielding. As the RNS tank or tanks emptied, crew radiation dose would thus steadily increase. To solve this problem, NAR had developed a "stand-pipe" single-tank RNS concept, in which a cylindrical "central column" running the length of the main tank stood between the crew and the NERVA I engine. The central column would remain filled with hydrogen until the surrounding main tank was emptied. MDAC, for its part, had developed a "hybrid" RNS shielding design that included a small hydrogen tank between the bottom of the main tank and the top of the NERVA I engine.

 Image credit: NASA.

The proper way to load cargo onto a Nuclear Shuttle: I = Use Space Tug robot arms to remove cargo module from Space Shuttle payload bay; II = stack cargo module on Space Tug; III = ignite Space Tug chemical rocket motors to rendezvous with Nuclear Shuttle, taking care to approach within the conical shadow created by the NERVA engine's radiation shields; IV = transfer cargo to Nuclear Shuttle. Image credit: NASA.

Crew transfer from the Earth-to-Orbit Shuttle to the Nuclear Shuttle might take place at the Orbital Propellant Depot. In this image, the crew of the Earth-to-Orbit Shuttle takes care to remain within the shadow of the NERVA engine's radiation shields. Image credit: NASA.

Osias postulated a maximum allowable radiation dose for an astronaut from sources other than cosmic rays of between 10 and 25 Roentgen Equivalent Man (REM) per year. Astronauts riding an RNS would, however, receive 10 REM each time its NERVA I engine operated. An astronaut 10 miles behind or to the side of an RNS operating at full power would receive a radiation dose of between 25 and 30 REM per hour. Osias noted that the NFSD contractors had recommended that no piloted spacecraft approach to within 100 miles of an operating NERVA I engine.

Radiation would create other operational problems, Osias wrote. Spacecraft could dock with an RNS by approaching through the cone-shaped radiation shadow that protected its crew. Docking an RNS to a large vehicle that protruded beyond the shadow — for example, a space station or a liquid hydrogen propellant depot — would, however, generate obvious problems. The large vehicle's crew might be exposed to radiation from the NERVA I; more insidious, the large vehicle's structure would reflect radiation back at the RNS, endangering its crew.

The NERVA I engine would emit radiation not only while it was in operation; it would also generate spent nuclear fuel that would emit harmful levels of radiation for decades or centuries. Osias noted that NAR had "repeatedly emphasized [that] maintainability is essential to economic operation of the RNS." A spacewalking repairman who approached to within 400 feet of the side of an RNS 10 days after its tenth (and, going by MSFC's traffic model, final) Earth-moon round-trip would, however, receive one REM per hour from the spent fuel it contained. Maintenance robots might replace the servicing capabilities of astronauts, Osias noted, but such systems would need costly development before they could become available.

Osias also reported that the "NFSD contractors. . .devoted little effort to [studying] emergency operations and malfunctions," adding that "[n]uclear systems, more than chemical propulsion vehicles, have the ability to involve the general population of the [E]arth in a space accident." A NERVA I explosion in LEO, for example, could lead to "random reentry of large pieces of radioactive material" that would probably survive reentry heating and strike Earth's surface. He urged that prevention of "return of the NERVA engine to the [E]arth's surface. . .be a basic rule of nuclear propulsion planning."

In NFSD study Phase II, LMSC estimated that, after just one Earth-moon round-trip, enough spent fuel would have accumulated within a NERVA I engine that it would need to remain in a safe high-altitude disposal orbit for 135 years. By the end of its operational life — after ten Earth-Moon flights — the "most desirable method of disposing of an engine" would, Osias wrote, be to "send the RNS on an unmanned, one-way mission to deep space."

The same month Osias completed his critique of the PFSD contractor studies, veteran New Mexico Senator Clinton Anderson, a close friend of former President Lyndon B. Johnson and a long-time nuclear rocket supporter, called a hearing to highlight the Nixon Administration's plan to slash NERVA funding from $110 million in Fiscal Year 1972 to only $30 million. At the hearing, Acting NASA Administrator Robert Seamans, an STG member, explained that Space Shuttle development had priority over NERVA development because the Space Shuttle was the essential transportation element that would launch into space all other IPP elements, including the RNS. He told Anderson that "NERVA needs the Shuttle, but the Shuttle does not need NERVA."

Six months after the NFSD contractors completed their reports, the Nixon White House unveiled its Fiscal Year 1973 budget request. As many had feared, it contained no funding for continued NERVA development. Anderson was ill and no longer able to adequately defend NERVA. A group of more than 30 pro-NERVA congressmen sought to sway the Nixon Administration, but to no effect. The final NERVA ground tests occurred in June and July 1972, after which the program was terminated, ending nearly 20 years of U.S. nuclear propulsion development.

Sources

"Status of Nuclear Flight System Definition Studies — Case 237," B71 02018 (NASA Contractor Report 116601), D. J. Osias, Bellcomm, Inc., 9 February 1971.

Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA SP-2001-4521, David S. F. Portree, NASA, February 2001.

More Information

Think Big: A 1970 Flight Schedule for NASA's 1969 Integrated Program Plan

Series Development: A 1969 Plan to Merge Shuttle and Saturn V to Spread Out Space Program Cost

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1969)

Humans on Mars in 1995! (1980-1981)

Saturn Ring Observer (2006)

A view impossible from Earth: a fat crescent Saturn throws its shadow across its rings; its rings return the favor. Image credit: NASA.

In 1610, natural philosopher Galileo Galilei became the first human to observe the rings of Saturn. His telescope was, however, insufficiently powerful to permit him to understand what he saw. He wrote that the "planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. . . the middle one (Saturn itself) is about three times the size of the lateral ones." He also referred to the twin objects accompanying Saturn as "ears."

Nearly half a century later, Dutch astronomer Christian Huygens revealed the true nature of Saturn's ears. He wrote in 1655 that the Sun's sixth planet "is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic." Giovanni Cassini observed in 1675 that Saturn's ring is made up of several concentric rings separated by gaps. The most prominent of the gaps, separating the inner B and outer A rings, became known as the Cassini Division.

In 1859, James Clerk Maxwell demonstrated that the rings could not be solid structures; rather, they consist of myriad particles, each orbiting Saturn independently like a tiny moon. James Keeler confirmed Maxwell's theory through telescopic observations in 1895.

Spacecraft exploration of Saturn began with the Pioneer 11 flyby on 1 September 1979. The 259-kilogram robot explorer left Earth on 6 April 1973 and received a gravity-assist boost from Jupiter on 4 December 1974. By passing through the plane of the rings 21,000 kilometers from Saturn, Pioneer 11 acted as a pathfinder for the Voyager 1 and Voyager 2 Saturn flybys.

Voyager 1 flew past the planet a little more than a year later, on 12 November 1980, revealing that Saturn's rings consist of a multitude of ringlets, gaps, and small "shepherd" moons. It also confirmed that the bright B ring is marked by strange ephemeral "spokes." The gaps and ringlets are the result of gravitational interactions with Saturn’s many moons; the spokes, on the other hand, remain mysterious. Voyager 1's twin, Voyager 2, flew past Saturn on 26 August 1981, en route to Uranus and Neptune.

Voyager 2 sweeps past Saturn in this NASA painting.

Saturn's next visitor from Earth did not arrive until almost a full Saturnian year (29.7 Earth years) had passed. On 1 July 2004, after racing through the gap between the F and G rings at more than 88,000 kilometers per hour, the 5600-kilogram, bus-sized Cassini spacecraft fired its main engine for 96 minutes so that Saturn's gravity could capture it into an elliptical orbit. Cassini found that the rings, which average just 10 meters thick and contain particles ranging from one centimeter to 10 meters across, are made up almost entirely of water ice and are surrounded by a thin "atmosphere."

On 1 July 2008, NASA granted Cassini a 27-month mission extension called the Cassini Equinox Mission. Scientists then proposed that the space agency extend Cassini's mission of exploration until 15 September 2017 at a cost of $60 million per year. This would enable observation of seasonal phenomena in the Saturn system — such as anticipated increased ring spoke activity — over half a Saturnian year. NASA announced approval of the extension, dubbed the Cassini Solstice Mission, in February 2010.

Artist's impression of Cassini's arrival in Saturn orbit, 1 July 2004. Image credit: NASA.

Assuming that Cassini remains operational, controllers in 2017 will lower the periapsis (low point) of its orbit so that it dives repeatedly between Saturn's cloud-tops and the inner edge of its rings. Science objectives during these potentially perilous ring-plane crossings will include ring observations.

As one might expect, Cassini has many science priorities besides study of Saturn's rings: to cite just two examples, the Cassini Solstice Mission includes 11 flybys of enigmatic Enceladus, and the primary objective of the 2017 ring-plane crossings is to examine Saturn's magnetosphere. In fact, Cassini planners generally steer clear of the rings because to approach too closely would place Cassini at risk from collision with ring particles.

The daredevil 2017 ring-plane crossings point up this fact; they occur near the end of Cassini's mission, after most science objectives are achieved, precisely because they will place the spacecraft at risk. (At this writing, Cassini is scheduled to dive into Saturn's atmosphere and be destroyed during its 293rd revolution about the planet.)

If JPL engineers Robert Abelson and Thomas Spilker had their way, the next mission to Saturn after Cassini would focus on the rings exclusively. Spilker first proposed the Saturn Ring Observer (SRO) mission concept in 2000. A paper written with Abelson and presented at the 2006 Space Technology and Applications International Forum (STAIF) in Albuquerque, New Mexico in February 2006 fleshed out the conceptual mission.

NASA's Planetary Science Decadal Survey Giant Planets Panel requested a detailed study of the SRO mission concept, which a team under Spilker's direction performed in April 2010. The study focused on new propulsion and power technology. A future post will describe the 2010 SRO study.

In the ring plane: Saturn's largest satellite, cloudy Titan, orbits between Cassini and Saturn's nearly edge-on rings. Image credit: NASA.

Abelson and Spilker's SRO would leave Earth between 2015 and 2020, fly past Venus, Earth (twice), and Jupiter for propellant-saving gravity assists, and reach Saturn in about 2030. Unlike Pioneer 11, the Voyagers, and Cassini, which, out of fear of collisions with ring particles spent as little time as possible near the rings, the SRO orbiter would hop about the B ring, Cassini Division, and A ring for an Earth year. A 981-kilogram propellant supply and an "advanced autonomous collision avoidance system" capable of detecting and dodging threatening ring particles would make this possible.

SRO would launch atop a next-generation heavy-lift rocket capable of placing about 28,000 kilograms on course for Venus. For the first 11 years of its mission — the cruise phase — SRO would consist of a 4648-kilogram lifting-body aeroshell surrounding a 12,227-kilogram cruise stage and the 1823-kilogram orbiter.

Upon arrival at Saturn, SRO would dive through the planet's cloudy atmosphere, reducing its speed by 28 kilometers per second in 15 minutes and allowing the planet's gravity to capture it into a 61,000-by-110,000-kilometer orbit tilted slightly relative to Saturn's equator and the plane of its rings. Its work completed, the aeroshell would separate, exposing the cruise stage and orbiter to space for the first time.

Two hours after aerobraking, the four chemical propulsion rocket motors on the cruise stage would fire for two hours, circularizing SRO's orbit at an altitude of 110,000 kilometers. This would place it near the middle of the B ring. The cruise stage, its propellants exhausted, would then detach, and the orbiter would deploy its eight science instruments and two-meter-wide steerable high-gain radio antenna.

Abelson and Spilker explained that SRO's 129-kilogram instrument suite would be tailored to study "centimeter-scale ring particle interactions," the shepherd moons, the "ring atmosphere," and the electromagnetic environment of the ring system. Data returned from SRO would have application not only to the study to Saturn's rings, they explained, but also to understanding of other planetary ring systems and to protoplanetary disks around other stars.

Intricate rings, intricate shadows. Image credit: NASA.

The SRO mission's nuclear power system would comprise three plutonium-fueled Multi-Mission Radioisotope Thermal Generators (MMRTGs). The orbiter-mounted MMRTGs, which would resemble the single MMRTG on the Curiosity Mars rover, would provide electricity and heat for the cruise stage and orbiter during the flight to Saturn, and for the orbiter and its electricity-hungry instrument suite and high-data-rate communications system in Saturn orbit.

Abelson and Spilker also considered a power system comprising four Sterling Radioisotope Generator units. These would produce less waste heat — handy during aerobraking, when the power system would be unable to radiate heat into space — but would also include turbines that might vibrate and interfere with SRO's sensitive science instruments.

The most novel element of Abelson and Spilker's proposed SRO mission would be the orbiter's intricate maneuvers near Saturn's rings. In its initial circular orbit, the orbiter would circle Saturn once every 10 hours, keeping pace with and studying nearby ring particles but remaining just outside the ring "surface."

Every 2.5 hours, as its slightly inclined orbit about Saturn brought it to one kilometer from the ring surface, it would point its engines toward the ring and fire them for about two seconds. This would move the orbiter an additional 0.4 kilometers away from the ring and would shift the point at which its orbit intersected the ring surface one-quarter of the way around the planet. Other hops would be triggered automatically if the orbiter detected a ring particle or spoke on a collision course.

About once per week, the SRO orbiter would maneuver outward slightly from the planet. Fifty such maneuvers over one Earth year would take it past the Cassini Division to the middle of the A ring, where it would orbit Saturn at a distance of 128,000 kilometers once every 13 hours with hops every 3.25 hours.

Soon after, Abelson and Spilker calculated, the orbiter's propellant supply would become depleted. In all likelihood, the mission would end the first time the SRO intersected the A ring a few hours later, and the spacecraft's battered wreckage would become a permanent (though insignificantly small) part of Saturn's ancient rings.

Sources

"A conceptual Saturn Ring Observer mission using standard radioisotope power systems," T. Spilker and R. Abelson, 2006 Space Technology and Applications International Forum, Albuquerque, New Mexico, 12-16 February 2006.

"Saturn Ring Observer Mission Concept: Closer Than We Thought," T. Spilker, et al., abstract #P23B-1634, American Geophysical Union 2010 Fall Meeting, San Francisco, California, 13-17 December 2010.

More Information

Peeling Away the Layers of Mars (1966)

On the Moons of Mighty Jupiter (1970)

Touring Titan by Blimp & Buoy (1983)

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

What If Galileo Had Fallen to Earth? (1988)

Galileo awaits its chance to fly. Image credit: NASA.

The U.S. Congress authorized new-start funding for the Jupiter Orbiter and Probe (JOP) on 19 July 1977, early in the Administration of President Jimmy Carter. When JOP development began officially on 1 October 1977, at the start of Fiscal Year 1978, NASA planned to launch the new robot explorer in January 1982 on STS-23, the 23rd operational flight of the Space Transportation System (STS). At the time, NASA still maintained the hopeful fiction that the STS could begin a series of six Orbital Test Flights in early 1979 and become operational in May 1980. Until 1986, the STS — the centerpiece of which was the Space Shuttle — was intended to replace all other U.S. launch vehicles.

At liftoff, the Shuttle stack comprised twin reusable Solid Rocket Boosters (SRBs), a reusable piloted Orbiter with a 15-by-60-foot payload bay and three Space Shuttle Main Engines (SSMEs), and an expendable External Tank (ET) containing liquid hydrogen and liquid oxygen propellants for the SSMEs. The STS also included upper stages for boosting spacecraft carried in the Orbiter payload bay to places beyond its maximum orbital altitude. Until the mid-1980s, many in NASA hoped that a reusable Space Tug — perhaps incorporating a propellant-saving aerobrake — would eventually replace the expendable upper stages.

At the start of STS-23 (and, indeed, at the beginning of all STS missions), the three SSMEs mounted on the aft end of Orbiter fuselage and the twin SRBs bolted to the side of the ET would ignite in sequence to push the Shuttle stack off the launch pad. SRB separation would then take place 128 seconds after liftoff at an altitude of about 155,900 feet and a speed of about 4417 feet per second.

The three SSMEs would operate until 510 seconds after liftoff, by which time the Orbiter and its ET would be moving at about 24,310 feet per second at an altitude of 362,600 feet above the Earth. The SSMEs would then shut down and the ET would separate, tumble, and break up as it fell back into dense atmospheric layers over the Indian Ocean.

The Orbiter, meanwhile, would ignite its twin Orbital Maneuvering System (OMS) engines at apogee (the high point in its Earth-centered orbit) to raise its perigee (the low point in its orbit) above 99.99% the Earth's atmosphere. By the time it completed its OMS maneuvers, the STS-23 Shuttle Orbiter would circle the Earth in a 150-nautical-mile-high low-Earth orbit (LEO).

The STS-23 crew would next open the Orbiter payload bay doors and release JOP and its three-stage solid-propellant Interim Upper Stage (IUS). After they maneuvered the Orbiter a safe distance away, the IUS first-stage motor would ignite to begin JOP's two-year direct voyage to Jupiter.

Early days: artist concept of Jupiter Orbiter and Probe. Image credit: NASA.

In February 1978, NASA gave JOP the name Galileo. Largely because of its reliance on the STS, Galileo suffered a series of costly delays, redesigns, and Earth-Jupiter trajectory changes. The first of these was, however, not the fault of the STS. As Galileo's design firmed up, it put on weight, and was soon too heavy for the three-stage IUS to launch directly to Jupiter.

In January 1980, NASA decided to split Galileo into two spacecraft. The first, the Jupiter Orbiter, would leave Earth in February 1984. The second, an interplanetary bus carrying Galileo's Jupiter atmosphere probe, would launch the following month. They would each depart LEO on a three-stage IUS and arrive at Jupiter in late 1986 and early 1987, respectively.

In late 1980, under pressure from Congress, NASA opted to launch the Galileo Orbiter and Probe out of LEO together on a liquid hydrogen/liquid oxygen-fueled Centaur G' upper stage. Centaur, a mainstay of robotic lunar and planetary programs since the 1960s, was expected to provide 50% more thrust than the three-stage IUS. Modifying it so that it could fly safely in the Shuttle Orbiter payload bay would, however, delay Galileo's Earth departure until April 1985. The spacecraft would arrive at Jupiter in 1987.

Another delay resulted when David Stockman, director of President Ronald Reagan's Office of Management and Budget, put Galileo on his "hit list" of Federal government projects to be scrapped in Fiscal Year 1982. The planetary science community campaigned successfully to save Galileo, but NASA lost the Centaur G' and three-stage IUS.

In January 1982, NASA announced that Galileo would depart Earth orbit in April 1985 on a two-stage IUS with a solid-propellant kick stage. The spacecraft would then circle the Sun and fly past Earth for a gravity-assist that would place it on course for Jupiter. The new plan would add three years to Galileo’s flight time, postponing its arrival at Jupiter until 1990.

In July 1982, Congress overruled the Reagan White House when it mandated that NASA launch Galileo from LEO on a Centaur G'. The move would postpone its launch to 20 May 1986; however, because the Centaur could boost Galileo directly to Jupiter, it would reach its goal in 1988, not 1990. NASA designated the STS mission meant to launch Galileo STS-61G.

Artist concept of Galileo on a Centaur G' stage. Image credit: NASA.

There matters rested until 28 January 1986, when, 73 seconds into mission STS-51L, the Orbiter Challenger was destroyed. A joint between two of the cylindrical segments making up the Shuttle stack's right SRB leaked hot gases that rapidly eroded O-ring seals. A torch-like plume formed and impinged on the ET and the lower strut linking the ET to the SRB. The plume breached and weakened the ET's liquid hydrogen tank, causing the strut to separate. Still firing — the SRBs were not designed to be turned off once ignited — the right SRB pivoted on its upper attachment and crushed the ET's liquid oxygen tank. Hydrogen and oxygen mixed and ignited in a giant fireball.

Despite appearances, Challenger did not explode. Instead, the Orbiter began a tumble while moving at about twice the speed of sound in a relatively dense part of Earth's atmosphere. This subjected it to severe aerodynamic loads, causing it to break into several large pieces. The pieces, which included the crew compartment and the tail section with its three SSMEs, emerged from the fireball more or less intact. The mission's main payload, the TDRS-B data relay satellite, remained attached to its two-stage IUS as Challenger's payload bay disintegrated around it.

The pieces arced upward for a time, reaching a maximum altitude of about 50,000 feet, then fell, tumbling, to crash into the Atlantic Ocean within view of the Shuttle launch pads at Kennedy Space Center, Florida. The crew compartment impacted 165 seconds after Challenger broke apart and sank in water about 100 feet deep.

NASA grounded the STS for 32 months. During that period, it put in place new flight rules, abandoned potentially hazardous systems and missions, and, where possible, modified STS systems to help improve crew safety. On 19 June 1986, NASA canceled the Shuttle-launched Centaur G' for reasons of safety. On 26 November 1986, it announced that a two-stage IUS would launch Galileo out of LEO. The Jupiter spacecraft would then perform gravity-assist flybys of Venus and Earth. On 15 March 1988, NASA scheduled Galileo's launch for October 1989, with arrival at Jupiter to follow in December 1995.

One month after NASA unveiled Galileo's newest flight plan, Angus McRonald, an engineer at the Jet Propulsion Laboratory (JPL) in Pasadena, California, completed a brief report on the possible effects on Galileo and its IUS of a Shuttle accident during the 382-second period between SRB separation and SSME cutoff.

McRonald was not specific about the nature of the "fault" that would produce such an accident, though he assumed that the Shuttle Orbiter would become separated from the ET and would tumble out of control. He based his analysis on data provided by NASA Johnson Space Center in Houston, Texas, where the Space Shuttle Program was managed.

The Space Shuttle was by far the largest spacecraft to launch with astronauts on board. It was immensely capable — but with capacity came complexity, making it vulnerable. Image credit: NASA.

McRonald also examined the effects of aerodynamic heating on Galileo's twin electricity-generating Radioisotope Thermoelectric Generators (RTGs). The RTGs would each carry 18 General Purpose Heat Source (GPHS) modules containing four iridium-clad plutonium dioxide pellets each. The GPHS modules were encased in graphite and housed in protective aeroshells, making them unlikely to melt following an accident during Shuttle ascent. In all, Galileo would carry 34.4 pounds of plutonium.

McRonald assumed that both the Shuttle Orbiter and the Galileo/IUS combination would break up when subjected to atmospheric drag deceleration equal to 3.5 times the pull of gravity at Earth's surface. Based on this, he determined that the Orbiter and its Galileo/IUS payload would always break up if a fault leading to "loss of control" occurred after SRB separation.

The Shuttle Orbiter would not break up immediately after loss of control occurred, however. At SRB separation altitude, atmospheric density would be low enough that the spacecraft would be subjected to only about 1% of the drag that tore apart Challenger. McRonald determined that the Shuttle Orbiter would ascend unpowered and tumbling, attain a maximum altitude, and fall back into the atmosphere, where drag would rip it apart.

He calculated that, for a fault that occurred 128 seconds after liftoff — that is, at the time the SRBs separated — the Shuttle Orbiter would break up as it fell back to 101,000 feet of altitude. The Galileo/IUS combination would fall free of the disintegrating Orbiter and break up at 90,000 feet, then the RTGs would fall to Earth without melting. Impact would take place in the Atlantic about 150 miles off the Florida coast.

For an intermediate case — for example, if a fault leading to loss of control occurred 260 seconds after launch at 323,800 feet of altitude and a speed of 7957 feet per second — then the Shuttle Orbiter would break up when it fell back to 123,000 feet. Galileo and its IUS would break up at 116,000 feet, and the RTG cases would melt and release the GPHS modules between 84,000 and 62,000 feet. Impact would occur in the Atlantic about 400 miles from Florida.

A fault that took place within 100 seconds of planned SSME cutoff — for example, one that caused loss of control 420 seconds after launch at 353,700 feet of altitude and at a speed of 20,100 feet per second — would result in an impact far downrange because the Shuttle Orbiter would be accelerating almost parallel to Earth's surface when it occurred. McRonald calculated that Orbiter breakup would take place at 165,000 feet and the Galileo/IUS combination would break up at 155,000 feet.

McRonald found (somewhat surprisingly) that, in such a case, Galileo's RTG cases might already have melted and released their GPHS modules by the time the Jupiter spacecraft and its IUS disintegrated. He estimated that the RTGs would melt between 160,000 and 151,000 feet about the Earth. Impact would occur about 1500 miles from Kennedy Space Center in the Atlantic west of Africa.

Impact points for accidents between 460 seconds and SSME cutoff at 510 seconds would be difficult to predict, McRonald noted. He estimated, however, that loss of control 510 seconds after liftoff would lead to wreckage falling in Africa, about 4600 miles downrange.

McRonald summed up his findings by writing that Galileo's RTG cases would always reach Earth's surface intact if an accident leading to loss of control occurred between 128 and 155 seconds after liftoff. If the accident occurred between 155 and 210 seconds after launch, then Galileo's RTG cases "probably" would not melt. If it occurred 210 seconds after launch or later, then the RTG cases would always melt and release the GPHS modules.

STS flights resumed in September 1988 with the launch of the Orbiter Discovery on mission STS-26. A little more than a year later (18 October 1989), the Shuttle Orbiter Atlantis roared into space at the start of STS-34. A few hours after liftoff, the Galileo/two-stage IUS combination was raised out of the payload bay on an IUS tilt table and released. The IUS first stage ignited a short time later to propel Galileo toward Venus.

Free at last: Galileo and its two-stage IUS shortly after release from the Space Shuttle Orbiter Atlantis, October 1989. Image credit: NASA.

Galileo passed Venus on 10 February 1990, adding nearly 13,000 miles per hour to its speed. It then flew past Earth on 8 December 1990, gaining enough speed to enter the Main Belt of asteroids between Mars and Jupiter, where it encountered the asteroid Gaspra on 29 October 1991.

Galileo's second Earth flyby on 8 December 1992 placed it on course for Jupiter. The spacecraft flew past the Main Belt asteroid Ida on 28 August 1993 and had a front-row seat for the Comet Shoemaker-Levy 9 Jupiter impacts in July 1994.

Flight controllers commanded Galileo to release its Jupiter atmosphere probe on 13 July 1995. The spacecraft relayed data from the probe as it plunged into Jupiter’s atmosphere on 7 December 1995. Galileo fired its main engine the next day to slow down so that the giant planet's gravity could capture it into orbit.

Artist concept of Galileo in communication with its Jupiter atmosphere probe. Blue dots linking the low-gain antenna and Jupiter represent radio signals. After the spacecraft's large main antenna jammed partly open (left), the low-gain antenna, much less powerful, became Galileo's link with Earth. Image credit: NASA.

Galileo spent the next eight years touring the Jupiter system. It performed gravity-assist flybys of the four largest Jovian moons to change its Jupiter-centered orbit. Despite difficulties with its umbrella-like main antenna and its tape recorder, it returned invaluable data on Jupiter, its enormous magnetosphere, and its varied and fascinating family of moons over the course of 35 orbits about the giant planet.

As Galileo neared the end of its propellant supply, NASA decided to dispose of it to prevent it from accidentally crashing on and possibly contaminating Europa, the ice-crusted, tidally warmed ocean moon judged by many to be of high biological potential. On 21 September 2003, the venerable spacecraft dove into Jupiter's turbulent, banded atmosphere and disintegrated.

Sources

Galileo: Uncontrolled STS Orbiter Reentry, JPL D-4896, Angus D. McRonald, Jet Propulsion Laboratory, 15 April 1988.

Mission to Jupiter: A History of the Galileo Project, NASA SP-2007-4231, Michael Meltzer, NASA History Division, 2007.

More Information

A 1974 Plan for a Slow Flyby of Comet Encke

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

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

On the Moons of Mighty Jupiter (1970)

Portrait of Galileo Galilei by Justus Sustermans. Image credit: Wikipedia.

In January 1610, Pisan natural philosopher Galileo Galilei pointed a small refracting (spyglass-type) telescope of his own manufacture at the bright dot of Jupiter. By mid-month, he had discovered all four of the planet's moons now known as the Galilean satellites. In mid-March, he named them the Medicean Stars to honor Grand Duke Cosimo II Medici of Tuscany, who granted Galileo a life-long patronage that July.

Meanwhile, in Germany, Simon Mayr (known as Marius) had turned a telescope toward Jupiter at about the same time Galileo discovered its moons. In 1614, he published a tract in which he stated that he was the first to glimpse the moons of Jupiter, a claim Galileo successfully refuted.

Though Marius was unable to assert priority for their discovery, the names he gave to the moons — the names of four lovers of the god Jupiter — caught on and are still in use today. They are, in order out from the planet, Io, Europa, Ganymede, and Callisto.

By the late 19th century, astronomers were able to determine the approximate masses of the Galilean moons and estimate their sizes and densities. The inner pair, Io and Europa, turned out to be smaller and denser than the outer pair, Ganymede and Callisto. In the 1920s, the satellites were confirmed — not surprisingly — to be synchronous rotators, always keeping the same hemisphere pointed toward Jupiter.

Astronomers noticed that Io, Europa, and Ganymede have resonant orbits: that is, that Europa's orbital period (3.6 Earth days) is twice Io's (1.8 days) and Ganymede's orbital period (7.2 days) is twice Europa's. Callisto, not in a resonating relationship with the other moons, orbits Jupiter in 16.7 days.

Jupiter and Ganymede imaged by the Hubble Space Telescope in April 2007. Image credit: NASA.

By the 1960s, astronomers had begun to discern a few finer details of the Jupiter system, such as Io's lack of surface ice and its orangish color. They had by then also detected eight more moons circling the planet, all much smaller than the four Galilean satellites.

Drawing upon their growing awareness of Earth's magnetosphere (the result of exploration using early Earth-orbiting artificial satellites, such as Explorer 1), theoreticians calculated that the Galileans should all orbit beyond Jupiter's magnetospheric bubble. This meant that they would not be subjected to high-energy particles trapped in the giant planet's equivalent of Earth's Van Allen radiation belts.

In January 1970, M. J. Price and D. J. Spadoni, engineers with the Chicago-based Illinois Institute of Technology Research Institute (IITRI), completed a feasibility study of soft-landing missions to Io, Europa, Ganymede, and Callisto for the NASA Headquarters Office of Space Science and Applications (OSSA) Planetary Programs Division. Their study was one of nearly 100 "Long Range Planning Studies for Solar System Exploration" IITRI performed for NASA OSSA beginning in March 1963. Price and Spadoni discussed the scientific merits of landings on the worlds Galileo discovered, but their study mainly emphasized propulsion systems for reaching them.

When the IITRI engineers conducted their study, only one type of U.S. soft-lander had explored another world: solar-powered, three-legged Surveyor. Of seven Surveyors launched to Earth's moon between March 1966 and January 1968, five touched down successfully. No robotic lunar or planetary mission had lasted for longer than a few months. Missions of longer duration —- for example, of the duration needed to reach the Jupiter system — were considered a daunting technical challenge.

November 1969: Apollo 12 visits and photographs Surveyor 3 on the moon. The robot lander set down on the Ocean of Storms in April 1967. Image credit: NASA.

Price and Spadoni assumed a 1000-pound science payload for their Jupiter moon landers. The payload would, they wrote, include instrument support equipment, such as a radio transmitter for transmitting data to Earth and an unspecified system for generating electricity, a soil sampler for determining surface composition and electrical and thermal conductivity, a seismometer and a heat-flow meter for revealing internal structure, a magnetometer to determine magnetic field strength, a television system for imaging the lander's surroundings, and instruments to monitor atmospheric composition, pressure, and temperature. They noted that the atmospheres of the Galilean moons would necessarily be "very tenuous" since none had yet been detected from Earth.

In addition to returning data concerning the moons, the landers would visually monitor Jupiter. The giant planet rotates in a little less than 10 hours, so any feature in its cloud bands — for example, its swirling Great Red Spot — could be viewed from its moons for no more than five hours at a time.

Viewed from the center of Io's inboard (Jupiter-facing) hemisphere, Jupiter spans 38.4 times the apparent diameter of the Sun or the full moon in Earth's sky. The corresponding figures for Europa, Ganymede, and Callisto are 24.4, 15.2, and 8.6, respectively. Price and Spadoni expected that the Galilean moons, which have nearly circular orbits, would make "extremely stable platforms" for Jupiter observations.

True color image of Jovian satellite Io, the first Galilean moon in order out from Jupiter. Nearly every dot is a volcano. Image credit: NASA.

They also assumed that NASA would have in hand a host of highly capable launch vehicles and propulsion technologies by the time it sought to place automated landers on Io, Europa, Ganymede, and Callisto. They applied these anticipated launchers and propulsion systems to four Jupiter landing mission phases: Earth launch, interplanetary transfer, a retro maneuver to slow the lander so that the target moon's gravity could capture it into orbit, and a "terminal descent" maneuver that would end with a (hopefully) gentle touchdown.

For mission phase one, Earth launch, Price and Spadoni assumed the existence of three launch vehicles. These were, in order of least-to-greatest capability, the Titan IIIF, the Saturn INT-20, and the Saturn V. The first two rockets were largely hypothetical. A liquid hydrogen/liquid oxygen Centaur upper stage could be used to augment all three rockets.

The Titan IIIF booster would resemble the never-flown Titan IIIM designed for the cancelled U.S. Air Force Manned Orbiting Laboratory program. It would include the Titan IIIM's twin 10-foot-diameter solid-rocket boosters (SRBs) and a new-design liquid-propellant "transtage" upper stage.

The Saturn INT-20, a new mid-range addition to the Saturn rocket family, would comprise a 33-foot-diameter S-IC first stage and a 22-foot-diameter S-IVB second stage. The Jupiter moon landing mission Saturn V, with its S-IC first stage, S-II second stage, and S-IVB third stage, would be virtually identical to the Saturn V rockets used to launch Apollo moon missions.

Cracked crust: ocean moon Europa, the second Galilean satellite. Image credit: NASA.

The second phase of the Jupiter moon landing missions, interplanetary transfer, would last the longest and probably be the least eventful. Price and Spadoni looked at two types of transfer: ballistic and low thrust.

The Earth-launch phase of all ballistic transfer missions would conclude with injection of the lander and its retro stage or stages onto an Earth-Jupiter transfer trajectory. The lander/retro combination would coast until it neared Jupiter, where the giant planet's gravity would pull it toward its target Galilean satellite.

Low-thrust transfers would employ a nuclear- or solar-electric propulsion stage. In all but one of the cases Price and Spadoni examined, the Earth-launch phase would end with the electric-propulsion stage, chemical retro stage or stages, and lander on an interplanetary trajectory that would not intersect Jupiter. Thrusters on the electric-propulsion stage would then operate for most or all of the interplanetary transfer, gradually accelerating the lander/retro combination and bending its course toward Jupiter.

Midway through its voyage, the electric-propulsion stage/lander/retro combination would turn end-for-end so that the electric thrusters would face in its direction of travel. It would then gradually slow so that, as it neared Jupiter, the planet's gravity could capture it into a distant orbit. Continued braking thrust would cause the spacecraft to spiral gradually inward toward Jupiter until its path intersected its target Galilean moon.

Price and Spadoni studied four electric-propulsion stages. The first, a solar-electric system with a total mass of about 9000 pounds, would switch on its thrusters after the Titan IIIF/Centaur launch vehicle injected it and a lander/retro combination onto an interplanetary trajectory. Of its mass, between 3100 and 3410 pounds would comprise propellant (probably the element cesium) and between 3130 and 3450 pounds would comprise electricity-generating solar arrays.

Their second electric-propulsion system, also Sun-powered, would achieve an interplanetary trajectory atop a Saturn INT-20/Centaur. Its mass would total between 15,960 and 19,760 pounds, of which propellant would account for between 2890 and 6980 pounds. Solar arrays would account for between 4700 and 8910 pounds of the spacecraft's mass.

Price and Spadoni's third electric-propulsion system, which they dubbed Nuclear-Electric System-A (NES-A), would launch onto an interplanetary trajectory atop a Titan IIIF/Centaur. NES-A would have a mass at electric thruster activation of about 17,000 pounds. Its 7200-pound nuclear power plant would generate 100 kilowatts of electricity for its thrusters.

Their fourth and heaviest electric-propulsion system, 35,000-pound NES-B, would not end its Earth-launch phase on an interplanetary trajectory. Instead, a Titan IIIF launch vehicle would boost the NES-B/lander/retro combination into a 300-nautical-mile-high Earth orbit, where NES-B would activate its thrusters to begin an outward spiral.

After Earth escape, the thrusters would continue to operate to bend the lander/retro combination's course toward Jupiter. NES-B's 10,800-pound nuclear power plant would generate 200 kilowatts of electricity.

Ganymede, the third Galilean satellite out from Jupiter and the largest moon in the Solar System. The dark patch of ancient terrain covering the upper right third of this view of Ganymede is called Galileo Regio. Image credit: NASA.

For the third of their four Jupiter moon mission phases, the retro maneuver, Price and Spadoni investigated space-storable chemical, cryogenic chemical, solid chemical, and nuclear-thermal propulsion systems alone and in combination with electric-propulsion systems. They emphasized exotic high-energy chemical propellant combinations with which NASA had little experience, such as storable oxygen difluoride/diborane and cryogenic fluorine/hydrogen. Operational simplicity led them to favor single-stage retro, though in practice most of their Jupiter moon landing missions would need two retro stages to capture into orbit around their target Galilean moon.

Price and Spadoni found that, for ballistic spacecraft, direct approach to a target satellite could be worrisome; because of Jupiter's powerful gravitational pull, the lander/retro combination would close rapidly on its destination, leaving no margin for error. Lander/retro combinations coupled with electric-propulsion systems, on the other hand, would close with their target much more slowly.

They next paired their candidate retro systems with launch vehicles to arrive at Earth-Jupiter flight times. They cautioned that all of their results should be viewed as approximate and preliminary.

The innermost Galilean, Io, would not be accessible to a lander with a storable-propellant retro system, they found. A lander approaching Io would be greatly accelerated by nearby Jupiter's gravity, so would need too much storable propellant to make capture into Io orbit practical.

A Saturn V/Centaur-launched lander with two-stage storable-propellant retro could, on the other hand, reach Europa orbit or Ganymede orbit from Earth in 600 days. The same combination launched on a Saturn V without the Centaur could reach Ganymede orbit in 800 days or Callisto orbit in 600 days. Finally, a lander with two-stage storable retro launched on a Saturn INT-20/Centaur could reach Callisto orbit in 750 days.

Cryogenic propellants, though difficult to maintain in liquid form for long periods, would provide more propulsive energy than storables. Io orbit would be accessible to a lander with a two-stage cryo retro system launched on a Saturn V/Centaur following a flight lasting 800 days. A lander with two-stage cryo retro launched on a Saturn V/Centaur would need 600 days to reach Europa orbit, while one with two-stage cryo retro launched on a Saturn V without a Centaur could reach Europa orbit in 800 days or Ganymede orbit in 700 days.

Callisto, they found, would be a special case: because the icy moon orbits relatively far from Jupiter, a lander approaching it would not be accelerated much by the giant planet's gravity. Single-stage cryo retro would thus suffice to slow the lander enough for capture into Callisto orbit. A Saturn V/Centaur-launched lander/single-stage cryo retro combination could attain orbit around Callisto after an Earth-Jupiter transfer lasting 600 days; one launched on a Saturn V or a Saturn INT-20/Centaur would need 700 days or 750 days, respectively.

Nuclear retro held considerable promise for trimming trip-times, Price and Spadoni concluded. It would, however, involve some technical challenges. Specifically, its cryogenic liquid hydrogen propellant would have to be kept liquid for long periods and its 200-kilowatt reactor would need to come on line reliably after an interplanetary hibernation lasting no less than 20 months.

Assuming that these challenges could be met, however, a single nuclear-thermal retro stage launched on a Saturn V/Centaur could slow a lander for capture into Io or Europa orbit after an interplanetary journey of 650 days. The same combination launched on a Saturn V could reach Ganymede orbit in 625 days or Callisto orbit in 600 days; launched on a Saturn INT-20/Centaur, the nuclear-thermal retro stage could place a lander into Ganymede orbit in 800 days or Callisto orbit in 650 days.

Price and Spadoni next considered solar-electric propulsion paired with two-stage storable retro. They did not explain why they examined only missions launched on Titan IIIF, Titan IIIF/Centaur, and Saturn INT-20/Centaur rockets: they may have wished to demonstrate that electric propulsion could enable Galilean moon landing missions to be launched on rockets smaller and cheaper than the Saturn V or Saturn V/Centaur.

If that was their intent, then their effort was a failure (at least in the case of solar-electric propulsion). They determined that Io could not be reached by a lander with solar-electric propulsion and storable retro. If launched on a Saturn INT-20/Centaur, the combination could deliver a lander to Europa in 950 days, Ganymede in 800 days, or Callisto in 650 days. If launched on a Titan IIIF, Callisto alone could be reached, and then only after a prohibitively long flight-time of 1600 days.

Finally, they looked at nuclear-electric plus single-stage solid-propellant retro. An NES-A/lander/solid retro combination launched on a Titan IIIF/Centaur would need 1475 days to reach Io orbit, 1125 days to reach Europa orbit, 1300 days to reach Ganymede orbit, and 900 days to reach Callisto orbit. The more powerful NES-B/solid retro launched into 300-nautical-mile Earth orbit on a Titan IIIF could reach Io orbit in 1175 days, Europa or Ganymede orbit in 1050 days, and Callisto orbit in 875 days.

Callisto, the fourth Galilean satellite of Jupiter. Because heavily cratered Callisto orbits relatively far from Jupiter, it is infrequently imaged close-up by passing spacecraft. As a result, the images making up this montage are of widely varying resolutions. Image credit: NASA.

For the fourth and final mission phase, terminal descent, Price and Spadoni invoked a single propulsion system for all missions: a throttleable engine burning nitrogen tetroxide and Aerozine 50, the same hypergolic (ignite-on-contact) propellants used in the Apollo Lunar Module. The terminal-descent propulsion system would ignite first to slow the lander so that its orbit would intersect the surface near the target landing site, then would ignite again for final descent and touchdown.

Price and Spadoni drew on Surveyor experience when they calculated the landed masses of their Galilean moon landers. In addition to the previously described 1000-pound scientific payload, they assumed that each lander would include a landing system (rocket motors, propellant tanks, control systems, landing legs, and structure) with a landed mass of about 500 pounds.

Price and Spadoni's Jupiter moon landing plans were ahead of their time in terms both of societal needs and technological maturity. Even as they completed their study, the heady early days of the Space Age were drawing to a close.

Faced with rapidly declining budgets, NASA Administrator Thomas Paine announced on 13 January 1970, within days of their study's completion, that Saturn V rocket production would be put on indefinite hold. He announced that Apollo 20 would be cancelled and its Saturn V diverted to launch the Apollo Applications Program Orbital Workshop. The program and the workshop were subsequently renamed Skylab.

The Titan IIIF never materialized, though the Titan IV, active in two variants between 1989 and 2005, had some of its features: for example, the 10-foot-diameter solid-rocket boosters. The rocket was used to launch only one interplanetary spacecraft: the 5560-pound Cassini Saturn orbiter left Earth atop a Titan IVB in October 1997. Cassini captured images of Jupiter and its moons as it flew past the planet in December 2000.

U.S. work on nuclear-thermal propulsion was defunded three years after the IITRI engineers finished their study. Neither chemical rocket stages employing exotic propellants nor nuclear-electric propulsion have enjoyed much support in the U.S., although as recently as 2004-2005 NASA attempted to begin development of the nuclear-electric Jupiter Icy Moons Orbiter (JIMO). A part of the Project Prometheus technology development program, JIMO was cancelled after new NASA Administrator Michael Griffin diverted the space agency away from new technologies and sustainable, open-ended piloted exploration and toward Apollo reenactment using repurposed Space Shuttle hardware.

NASA has developed solar-electric thrusters over a span of decades. The Dawn mission, at this writing exploring the asteroid Ceres, provides an excellent example of the technology's potential. To date, however, no solar-electric propulsion system has attained the scale Price and Spadoni envisioned.

New knowledge of the Jupiter satellite system also undermined their plans. In December 1973, less than four years after they completed their work, Pioneer 10 flew close past Jupiter. The doughty 568-pound spinning probe confirmed that a powerful magnetic field encompasses all of the Galilean moons, creating a complex system of flux tubes and radiation belts. Radiation near Io was sufficiently powerful to damage Pioneer 10's electronics.

On the other hand, new knowledge also revealed Jupiter's moons to be fascinating targets for exploration. Voyager 1 flew through the Jupiter satellite system in December 1977, revealing that Io is dotted with active volcanoes and boiling sulfur lakes, while Europa's cracked, icy surface conceals a water ocean. The orbital resonance first noted in the early 20th century is responsible for these wonders: it means that Io is repeatedly and regularly caught in a gravitational tug-of-war between Jupiter, Europa, and Ganymede. This kneads the moon's interior, generating heat. The same process is at work on Europa, though to a lesser degree than on Io.

The Galileo Jupiter orbiter and probe reached Earth orbit on 18 October 1989, on board the Space Shuttle Atlantis. Because the solid-propellant Inertial Upper Stage (IUS) was insufficiently powerful to boost the 5200-pound spacecraft on a direct path to Jupiter, it followed a course more complex than any Price and Spadoni envisioned for their Jupiter moon landers.

The IUS placed Galileo on course for Venus, where a gravity-assist flyby on 10 February 1990 boosted it back to Earth. A gravity-assist Earth flyby on 8 December 1990 boosted Galileo into the Asteroid Belt between Mars and Jupiter; the spacecraft then flew past Earth a second time on 8 December 1992, at last gaining enough energy to reach Jupiter.

On 13 July 1995, Galileo released an unnamed Jupiter atmosphere probe; on 7 December 1995, the probe returned data for nearly an hour as it plummeted through the outermost fringe of the giant planet's atmosphere. Galileo fired its main engine the following day to slow down so that Jupiter's gravity could capture it, then commenced the first of 35 orbits about the planet. Most included at least one Galilean moon close flyby for science and for a course-changing gravity assist.

Galileo's mission ended on 21 September 2003 with an intentional collision with Jupiter. The spacecraft, which by then was running out of propellants, met its end in Jupiter's atmosphere so that it would not accidentally land on and possibly contaminate Europa, considered by many to be a promising place to seek extraterrestrial life.

Artist concept of the Europa Mission spacecraft. Blue lines represent the spacecraft's many planned Europa flybys. Image credit: NASA.

During 2014-2015, support grew within the U.S. Congress for Europa Clipper, a mission to explore Europa's ice-covered ocean. The spacecraft would orbit Jupiter, but would swing past Europa during most of its orbits. Orbiting Jupiter would demand less propulsive energy than would orbiting Europa and would help to safeguard the spacecraft against damage from Jupiter's radiation belts — the spacecraft would dip into the severe radiation environment as it flew by Europa, not soak in it.

NASA approved the Europa Mission, as it became known, in June 2015. The Europa Mission science team held its first meeting on 4 August 2015. At this writing, the spacecraft is expected to carry nine instruments including ice-penetrating radar. NASA plans to launch it on a Space Launch System (SLS) variant in the early 2020s and to include a small Europa lander. It might also include miniature Cubesat spacecraft; these it would be released to pass through water plumes first spotted rising from Europa's south polar region in late 2012.

Sources

Preliminary Feasibility Study of Soft-Lander Missions to the Galilean Satellites of Jupiter, Report No. M-19, M. J. Price and D. J. Spadoni, Astro Sciences Center, IIT Research Institute, January 1970.

Mission to Europa (http://www.nasa.gov/europa — accessed 6 February 2016).

Mission to Jupiter: Galileo (http://www.jpl.nasa.gov/missions/galileo/ — accessed 6 February 2016).

Jupiter Icy Moons Orbiter (http://www2.jpl.nasa.gov/jimo/ — accessed 7 February 2016).

More Information

Touring Titan by Blimp & Buoy (1983)

The Challenge of the Planets, Part Two: High Energy

The Challenge of the Planets, Part Three: Gravity

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