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 his 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 make estimates of 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, incidentally, orbits Jupiter in 16.7 days.

By the 1960s, astronomers had begun to discern finer details of the Jupiter system, such as Io's lack of surface ice and its orangish color. They had 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 all orbited beyond Jupiter's magnetospheric bubble, so 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-lander 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 had discovered, but their study mainly emphasized propulsion systems for reaching them.

Io (Jupiter I). Image: NASA.

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 had touched down successfully. In addition, no robotic lunar or planetary mission had lasted for longer than a few months. Missions of longer duration - for example, of the duration require to reach Jupiter's moons - were considered a daunting challenge.

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

In addition to returning data on 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 (planet-facing) hemisphere, Jupiter has 38.4 times the apparent diameter of the Sun or 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 constitute "extremely stable platforms" for Jupiter observations.

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 ending 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 were hypothetical. A liquid-propellant Centaur upper stage could augment all three rockets.

Europa (Jupiter II). Image: NASA.

Titan IIIF would closely resemble the never-flown Titan IIIM designed for the cancelled U.S. Air Force Manned Orbiting Laboratory program. In addition to the Titan IIIM's twin 10-foot-diameter, seven-segment solid-rocket boosters (SRBs), the Titan IIIF would incorporate a liquid-propellant "transtage" upper stage.

The Saturn INT-20, a proposed new 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 Saturn V, with an S-IC first stage, an S-II second stage, and an S-IVB third stage, would be virtually identical to the Apollo Saturn V.

The second phase of the Jupiter moon-landing missions, interplanetary transfer, would be the longest and potentially 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 case 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 yet 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.

Partway through its voyage, the electric-propulsion stage/lander/retro combination would turn end for end so that the electric thrusters faced 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 it intersected its target Galilean.

Ganymede (Jupiter III). Ganymede (Jupiter III).

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 its Titan IIIF/Centaur launch vehicle had injected it and a lander/retro combination onto an interplanetary trajectory. Of its mass, between 3100 and 3410 pounds would comprise propellant (probably 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 about 15,960 and 19,760 pounds, of which propellant would account for between 2890 and 6980 pounds. Between 4700 and 8910 pounds would comprise solar arrays.

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 powerplant 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 it would activate its thrusters and spiral outward until it escaped Earth's gravity. The thrusters would then continue to operate to bend the lander/retro combination's course toward Jupiter. NES-B's 10,800-pound nuclear powerplant would generate 200 kilowatts of electricity.

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 the electric-propulsion systems. They emphasized exotic high-energy chemical propellant combinations with which NASA had 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.

They 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.

Price and Spadoni 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.

Callisto (Jupiter IV). Callisto (Jupiter IV).

The innermost Galilean, Io, would not be accessible to a lander with a storable-propellant retro system, they found. A lander approaching the innermost Galilean would be greatly accelerated by nearby Jupiter's gravity, so would need too much 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 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 after a flight time of 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 dispatched to 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 of 600 days; one launched on the Saturn V or the Saturn INT-20/Centaur would need 700 days or 750 days, respectively.

Nuclear retro held considerable promise for reducing 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 activate 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 brake a lander 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 relatively small, relatively cheap launch vehicles.

If that was their intent, then, at least in the case of solar-electric propulsion, their effort was a failure. 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-high 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.

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 moon's 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 landed masses for 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 cancelled the Saturn V rocket on January 13, 1970, within days of their study's completion.

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 seven-segment solid-rocket boosters. The rocket was used to launch only one interplanetary spacecraft: the 5560-pound Cassini-Huygens Saturn orbiter left Earth atop a Titan IVB in October 1997. Cassini captured images of Jupiter and its moons (for example, the image at the top of this post, which shows Jupiter and Ganymede) as it flew past the planet in December 2000.

Jupiter Icy Moons Orbiter (JIMO), a proposed nuclear-electric robot explorer. Image: NASA.

U.S. work on nuclear-thermal propulsion ended 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., though 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 suffered cancellation after new NASA Administrator Mike Griffin diverted the space agency away from new technologies and sustainable, open-ended piloted exploration and toward Apollo reenactment using re-purposed Space Shuttle hardware. NASA has developed solar-electric thrusters over a span of decades and has used them for interplanetary missions - for example, Dawn, currently exploring the asteroid Vesta - but to date none 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. Radiation near Io was, in fact, sufficiently powerful to damage Pioneer 10's electronics.

Other new knowledge, on the other hand, 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 apparently conceals a water ocean. The orbital resonance first noted in the early 20th century is responsible: 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.

Preparing Galileo for flight. Image: NASA. Preparing Galileo for flight. Image: NASA.

The Galileo Jupiter orbiter and probe reached Earth orbit on October 18, 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 had envisioned for their Jupiter moon landers. The IUS placed Galileo on course for Venus, where a gravity-assist flyby on February 10, 1990, boosted it back to Earth. A gravity-assist Earth flyby on December 8, 1990, boosted *Galileo *into the Asteroid Belt between Mars and Jupiter; the spacecraft then flew past Earth a second time on December 8, 1992, at last gaining enough energy to reach Jupiter.

On July 13, 1995, Galileo released an unnamed Jupiter atmosphere probe; on December 7, 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 hypergolic-propellant 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 course-changing gravity assists. Galileo's mission ended on September 21, 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.

The LinkedIn app for Windows Phone looks slick with the Metro UI. Image: LinkedIn

At present, no concrete plans exist to intentionally land on the moons Galileo first glimpsed 402 years ago. Automated landings on Europa have, however, received some attention over the past three decades because of its potential as a home for life. In the early 2000s, as part of efforts to identify advanced technologies needed for future ambitious piloted space expeditions, NASA engineers outlined a mission to land humans on Callisto in about 2040. At about the same time, International Space University students described a manned mission to Jupiter's moon Europa.

Reference:

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