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).

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