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)

Bridging the Gap Between Space Station and Mars: The IMUSE Strategy (1985)

NASA's sprawling Dual Keel space station design in 1986. Niehoff's Interplanetary Platform would have resembled the crew-tended freeflyer located at top left. Image credit: NASA.

In common with many space advocates past and present, I long for the day when humans set foot on Mars. In addition to being a fascinating place to explore, it is the world most like Earth in our planetary system (though it is still very alien).

We have a lot of work to do to get ready to go to Mars. Before we can plan long-term stays (the most economical kind), we need to determine whether martian gravity, which pulls with only one-third the strength of Earth gravity, is adequate to halt (or at least dramatically curtail) bone loss and other afflictions of microgravity. We also need to determine as best we can whether life exists there.

The level of effort we invest in seeking to ensure that we do not damage a martian biota through careless introduction of Earth microorganisms will say much about us as a species. Two salient facts should be kept in mind as we consider the question of how best to interact with life on Mars: first, Earth and Mars are probably very similar a few kilometers down, where we find abundant chemosynthetic life on Earth (that is, Mars is likely to be, like Earth, warm and wet below the surface); second, life formed early and rapidly on Earth, but it remained unicellular until just about 600 million years ago. Mars life, if it exists, might now be in a process of retreat, a rear-guard action leading, perhaps, to extinction as the planet cools and dries out; alternately, it might be biding its time.

That we know neither whether the human body can withstand Mars conditions for prolonged periods nor whether Mars life (if it exists) can withstand unharmed the microbiota the human body carries with it indicates that, for now, we should take a cautious approach to humans on Mars. That does not mean we should sit forever in low-Earth orbit. On the contrary, it means that we should seek to accomplish intermediate goals which themselves are important and exciting.

Intermediate steps would link where we are (a space station in low-Earth orbit and remote-controlled rovers, landers, and orbiters slowly exploring Mars) with where we logically should be headed (a science base at Mars with a long-term human population — think Antarctica — working closely with teleoperated machines). Achievement of that goal could in turn lead where some of us believe we would like to be (a permanent, self-sustaining Mars colony serving as a jumping-off place for a new branch of humanity).

I like how John Niehoff's Integrated Mars Unmanned Surface Exploration (IMUSE) strategy logically ties together the NASA automated and piloted space programs. This has been attempted many times over the years — below I will mention one such attempt, the joint Jet Propulsion Laboratory (JPL)/NASA Johnson Space Center (JSC) Mars Sample Return (MSR) studies of the 1980s — but it has always run into institutional barriers or tripped over new, typically ill-considered, large-scale moon/Mars initiatives.

Niehoff was the manager of the Space Sciences Department at Science Applications International Corporation (SAIC) when, on 30 July 1985, he presented his IMUSE strategy to the National Academy of Science Space Science Board Major Directions Summer Study. He proposed employing reusable automated spacecraft with designs "deeply rooted" in planned U.S. space station technology to carry out a complex, evolving series of automated Mars Sample Return (MSR) missions between 1996 and 2016.

His work had its origins in the 1984 joint Jet Propulsion Laboratory/NASA Johnson Space Center MSR study and the work of the National Commission on Space (NCOS), a blue ribbon panel appointed by President Ronald Reagan at the insistence of Congress to chart a future for the U.S. in space. Former NASA boss Thomas Paine chaired the NCOS, which included such luminaries as Neil Armstrong, Sally Ride, and Chuck Yeager. Niehoff and SAIC provided both the JPL/JSC MSR study and the NCOS with planning and engineering support.

Niehoff explained that linking MSR with the Space Station Program would integrate it with "other capabilities and objectives of the larger space program." It would also create a bridge between early 1990s Earth-orbital station operations and a piloted Mars landing in the early 2020s.

At the time Niehoff made his presentation, the Space Station Program was just 18 months old. Reagan had used his January 1984 State of the Union Address to launch (in a bureaucratic sense, at least) the manned space laboratory. He gave NASA until 1994 to complete it.

NASA and its contractors studied a range of possible station configurations in 1984-1985. They had in fact begun concerted station planning before the first Space Shuttle launch in 1981. In early 1986, six months after Niehoff's presentation to the Major Directions Summer Study, NASA settled on the ambitious Dual Keel station design. The Dual Keel would provide ample facilities for space construction and satellite servicing and a home base for space tugs that could launch or retrieve spacecraft and satellites.

Niehoff's IMUSE spacecraft — which he dubbed an Interplanetary Platform (IP) — would transport smaller vehicles between Earth and Mars. It would provide them with "keep-alive" solar cell-generated electrical power, thermal control, course-correction propulsion, and other requirements typically provided by an expendable spacecraft bus.

The IP would cut costs over the course of the IMUSE program because it would need to be launched onto its interplanetary path only once. As the IP flew without stopping past Mars or Earth, the smaller vehicles it supported would separate to land on or go into orbit around the planet or would leave the planet to rendezvous and dock with the it.

Had Niehoff's IMUSE proposal gone ahead (and used his first scenario), the Interplanetary Platform would have been en route to its first Mars encounter at the time the Hubble Space Telescope captured these images. Image credit: NASA.

Niehoff described a pair of IMUSE scenarios. In both, the IP would follow SAIC-developed Versatile International Station for Interplanetary Transport (VISIT) cycler orbits, which, he explained, would be "simultaneously resonant with both Earth and Mars." A spacecraft in a VISIT-1 orbit would circle the Sun in 1.25 Earth years, which meant that it would encounter Earth four times in five Earth years and Mars three times in two Mars years. A VISIT-2 orbit, on the other hand, would need 1.5 Earth years to complete. A spacecraft on a VISIT-2 path would encounter Earth twice in three Earth years and Mars five times in four Mars years.

Niehoff's first IMUSE scenario would begin with Earth-orbit departure of one 6340-kilogram IP — possibly pushed by a Space Station-based space tug — in May 1996. During its first Mars encounter (December 1997), the IP would drop off a 400-kilogram "smart rover" capable of complex autonomous operations and a 1110-kilogram communications orbiter for relaying radio signals between Mars and Earth. The rover and orbiter, packed separately in identical 2570-kilogram streamlined aerocapture vehicles, would skim the martian atmosphere to slow down so that Mars's gravity could capture them into orbit.

The rover would then descend to Mars's surface atop a 1170-kilogram "generic lander" capable of precision landing. After rolling off the lander onto the surface, it would employ a variety of scoops, picks, and drills to gather rock, sand, and dust samples.

In April 2001, a second rover and two 4300-kilogram Mars ascent vehicles would rendezvous and dock with the IP as its Sun-centered orbit carried it past Earth for the first time. This would demonstrate "hyperbolic rendezvous" ahead of its use in the piloted Mars program. Hyperbolic rendezvous would occur not in Mars or Earth orbit, but rather in the IP's orbit around the Sun. The technique would save propellants because the IP would not fire rocket motors to capture into and escape from Earth or Mars orbit.

Seven months later (November 2001), the IP would swing by Mars for the second time and drop off the 2001 rover, which would land at a new site on Mars. Ascent vehicle #1, meanwhile, would land near the 1996 rover and ascent vehicle #2 would set down near the 2001 rover.

Earth would not be positioned properly for the IP to make a direct return after the November 2001 Mars encounter, so the IP would orbit the Sun twice and return to Mars for the third time in July 2005. Ascent vehicle #1 would lift off from Mars bearing the 10 kilograms of samples the 1996 rover collected and ascent vehicle #2 would lift off bearing 2001 rover samples. The ascent vehicles would perform hyperbolic rendezvous and dock with the IP as Mars slowly shrank behind the three spacecraft.

In April 2006, the IP would swing by Earth for the second time to drop off the Mars samples it had collected 10 months earlier. A Space Station-based tug would rendezvous and retrieve the samples after they aero-captured into Earth orbit. The IP would also pick up ascent vehicle #3 and two 2000-kilogram automated Mars surface stations.

It would release these during its fourth Mars encounter in April 2009. Ascent vehicle #3 would land close to the still-operational 1996 rover. The surface stations would land at separate sites, bringing to four the number of Mars landing sites explored in the IMUSE program. The stations would conduct life science experiments, test manufacture of propellants from martian resources, and study the effects on spacecraft materials of long exposure to martian surface conditions.

During its third Earth encounter (April 2011), the IP would pick up a "manned precursor payload" consisting of equipment and supplies for the first piloted Mars landing expedition. It would drop off the manned precursor payload in December 2013, during its fifth Mars encounter, and pick up samples from the 1996 rover launched from Mars by ascent vehicle #3. In April 2016, the IP would encounter Earth for the fourth and final time to drop off the samples.

Niehoff's second IMUSE scenario would employ two IPs. These would deliver the same payloads to Mars in the same manner as his first scenario, but would start later and then proceed at an accelerated rate. The first IP would leave Earth in July 1998 and fly past Mars in February 2000, November 2003, August 2007, and May 2011. It would encounter Earth in July 2003, July 2008, and July 2013. IP #2 would leave Earth in April 2001, fly past Mars in November 2001, July 2005, and April 2009, and encounter Earth in April 2006 and April 2011. IMUSE scenario #2 would return the first Mars samples to Earth in April 2006 and drop off the first piloted program precursor payload at Mars in May 2011.

The piloted program, which eventually might employ large cycling spacecraft based on Space Station modules and other hardware to rotate crews to and from a long-term Mars surface outpost, would commence shortly thereafter. Piloted cyclers might travel permanently in VISIT-type orbits, becoming in effect space stations in solar orbit. The NCOS timetable called for a Mars surface outpost to be in place by 2035, 50 years after Niehoff presented his study.

Source

"Integrated Mars Unmanned Surface Exploration (IMUSE), A New Strategy for the Intensive Science Exploration of Mars," J. Niehoff, Science Applications International Corporation; presentation to the Planetary Task Group, Major Directions Summer Study, Space Science Board, National Academy of Science, 30 July 1985.

Pioneering the Space Frontier: The Report of the National Commission on Space, Bantam Books, May 1986.

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

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

Making Rocket Propellants from Martian Air (1978)

The Collins Task Force Says Aim for Mars (1987)

The Russians are Roving! The Russians are Roving! A 1970 JPL Plan for a 1979 Mars Rover

1984: A Proton-K rocket very similar to those that launched Luna 17/Lunokhod 1 in 1970 and Luna 21/Lunokhod 2 in 1973 lifts off from Baikonur Cosmodrome bearing one of a pair of Vega probes destined for Venus and Halley's Comet. Image credit: Lavochkin Association/NASA.

As night fell at Baikonur Cosmodrome in Soviet Kazakhstan on 10 November 1970, a Proton rocket thundered to life and began its climb toward space. Six days later, the rocket's payload, the automated Luna 17 Moon lander, soft-landed on broad, flat Mare Imbrium. A team of six operators in the Soviet Crimea — five main operators plus a spare — remotely drove the Lunokhod 1 rover down ramps lowered from the lander onto the Moon's dusty surface.

The solar-powered (but nuclear-heated) 756-kilogram rover, measuring 1.35 meters tall and 2.15 meters across its tub-shaped equipment compartment, rolled on eight metal wheels with cleats at a top speed of 0.1 kilometers per hour. A hinged, bowl-shaped lid lined with electricity-generating solar cells opened to expose a thermal radiator atop the tub; as night approached, Lunokhod 1's operators commanded it to close the lid to hold in heat and protect its delicate electronics.

Lunokhod 1. Image credit: Lavochkin Association/NASA.

Lunokhod 1 originated in the abortive Soviet piloted Moon program, though this would not be revealed until the late 1980s. Its initial role was to have been to certify as safe the landing site selected for the piloted lunar landing.

The rover would then have stood by until a lander bearing a single cosmonaut arrived. If his lander became damaged during touch-down so that it could not return him to lunar orbit, the Lunokhod operator team on Earth would drive the rover to pick him up for transfer to a waiting, pre-landed backup lander. The United States had, incidentally, in the early 1960s considered launching site-survey rovers to Apollo landing sites, and had studied long-range automated rovers that visiting astronauts could board and drive.

Even before the successful Apollo 11 landing (20 July 1969), the Soviets claimed that they never intended to land cosmonauts on the Moon. This was, of course, untrue, but it found a receptive audience among those who opposed piloted lunar exploration on the basis of cost or who favored the Soviet Union in the Cold War.

Through their official media, the Soviets declared that they had opted instead for robot explorers that cost much less than Apollo and placed no human life at risk. This message was particularly potent in the months following the near-disaster of Apollo 13 (11-17 April 1970).  They told the world that Lunokhod 1 and its cousins, the Luna automated sample returners, presaged a new era of extensive and intensive robotic lunar and planetary exploration.

U.S. space planners took note. In a report called An Exploratory Investigation of a 1979 Mars Roving Vehicle Mission, completed a timely three weeks after Luna 17 landed on Mare Imbrium and Lunokhod 1 began its traverse, a 12-man design team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, described a NASA Mars rover mission in 1979.

Billed as a "logical follow-on" to the Viking landings planned for mid-1976, JPL's 1127-pound rover would include six wire wheels akin to those on the Apollo Lunar Roving Vehicle, which at the time was scheduled to be driven by astronauts on the Moon for the first time in 1971. Mobility would enable "extended" Viking objectives: for example, while Viking would land on a safe, flat plain and seek living organisms only within reach of its three-meter-long robot arm, the 1979 rover could land in a flat area, then enter rugged terrain to seek out biologically promising sites.

Viking 1 launch on a Titan III-E rocket on 20 August 1975. Image: NASA.

The Mars rover would leave Earth between late October and mid-November 1979 on a Titan III-C rocket with a Centaur upper stage — the same rocket/upper stage combination that would launch the Vikings in 1975. It would lift off sealed within a Viking-type lander aeroshell and bioshield cap attached to a Viking-type orbiter. The orbiter's rocket motor would perform a course correction burn 10 days after launch.

Assuming a 3 November 1979 launch, Earth-Mars transfer would need 268 days. During the voyage, a door would open in the top of the aeroshell and the rover's cylindrical electricity-generating Radioisotope Thermal Generators (RTGs) would extend into space on a boom. The plutonium-powered RTGs would continually generate heat; if kept sealed within the aeroshell during the flight to Mars, heat build-up would damage the rover.

Mars arrival would occur in August 1980. The orbiter's rocket motor would slow the spacecraft so that the planet's gravity could capture it into orbit. Two days later, it would tweak its orbit so that it would pass over the rover's primary landing site. The JPL team estimated that its Mars rover could reach sites between 30° north and 30° south latitude.

Cutaway of JPL's proposed 1979 Mars rover packed into its Viking-type aeroshell. The arrow points to the rover's twin RTGs, which are extended beyond the aeroshell on a boom to prevent them from overheating the rover's electronic systems during the flight to Mars. Image credit: JPL/NASA.

Five days after Mars orbit arrival, the rover would cast off its bioshield cap to expose the aeroshell. Shortly before separation from the orbiter, the rover would retract its RTGs. The aeroshell would then separate and fire thrusters to slow down and fall toward Mars.

The JPL engineers described the rover landing sequence in considerable detail. Two hours after separation from the orbiter and 300 seconds before landing (that is, at L minus 300 seconds), the aeroshell would encounter the thin upper atmosphere of Mars. Entry deceleration would peak at about 12 times the force of Earth's gravity.

At L minus 80 seconds, moving at a speed of Mach 2.5, the aeroshell would deploy a compact ballute ("balloon-parachute") 21,000 feet above Mars. Three seconds later, at 19,000 feet and a speed of Mach 2.2, a single parachute would deploy and the ballute would separate.

At L minus 73 seconds, with the rover streaking through the martian sky at Mach 2, the parachute would fill with thin martian air. Six seconds later, the lower aeroshell would separate, exposing the rover's underside and twin landing radars.

JPL's 1979 Mars rover in its landed configuration. Arrows point to the three terminal descent rocket motors. Image credit: JPL/NASA.

Three terminal descent rocket motors on the rover would begin firing at L minus 33 seconds. Three seconds later, at an altitude of 4000 feet and a speed of 300 feet per second, the parachute and upper aeroshell would separate from the rover. The rover would then touch down gently on Mars directly on its wheels.

JPL's rover would comprise a train of three compartments, each with one wheel pair. Flexible connectors would link the compartments. The forward compartment (the "science bay") would include a Viking-type soil sampler arm with an attached soil magnetic properties experiment, a new-design "chisel and claw" arm, four biology experiment packages (the number NASA planned to launch on the Viking landers at the time JPL completed its rover report), a mass spectrometer, a weather station, and a seismometer. The forward compartment wheel hubs would carry one terminal descent rocket motor each, and the front wheel pair would be steerable.

The middle compartment (the "electronics bay") would house the 95-pound dual-purpose (science & rover control) computer.  A telescoping stalk would support a dish-shaped high-gain antenna, a low-gain antenna, a facsimile camera capable of generating a 360° panorama, and a vidicon camera with rangefinder.

The rear compartment (the "power bay") would include the twin externally-mounted RTGs, landing radars on its wheel hubs, and a rear-mounted terminal descent rocket motor. The rear wheel pair would, like the front pair, be steerable.

From some time before Earth launch until its second day on Mars, the three compartments would be squeezed together tightly with their wheels touching. This would enable the rover to fit within the confines of its Viking-type aeroshell.

Controllers on Earth would check out the rover during its first day after touchdown on Mars. On Day 2, they would spread out its compartments, deploy its appendages, and discard the terminal descent motors and landing radars. The JPL design team looked briefly at retaining the terminal descent rockets to enable the rover to "hop" over obstacles, but rejected this capability as being too fraught with risk.

Science operations would commence on Day 3. Mars surface operations would span one Earth year, from August 1980 to August 1981.

Controllers on Earth would guide the rover through its daily program. Operations would occur only during the martian daylight hours, when line-of-sight radio contact with Earth was possible.

Time available for operations during each 24-hour, 39-minute martian day would vary over the rover's one-Earth-year mission, as would radio-signal travel time. On 9 August 1980, for example, a rover at a site on the martian equator would remain in contact with Earth for 10.93 hours, while radio signals would need about 21 minutes to cross the gulf between the planets. In May 1981, Earth and Mars would be far apart — on opposite sides of the Sun — and radio-signal travel time would reach its maximum value of 41 minutes.

Typically, the rover would move from 50 to 100 meters at a time, then halt, image its surroundings, perform one of its science experiments, transmit its data to Earth, and then await new commands. JPL assumed that high-interest science sites would occur on average about 14 kilometers apart along its traverse route, and estimated that early in its mission the rover would travel about 300 meters per day, enabling it to traverse the distance between two science sites in 47 days. Distance traversed would, JPL optimistically assumed, rapidly increase as controllers gained confidence in their remote driving ability: the team estimated that in one Earth year its rover might traverse up to 500 kilometers.

Inspired, perhaps, by Lunokhod 1, the JPL team concluded its study by looking briefly at a lunar variant of its Mars rover design. The team found that the basic design of both rovers could be much the same, though the lunar rover launch vehicle would not need to be as large and powerful (a Titan III/Centaur without strap-on solid-propellant boosters would suffice) and a solid-propellant braking rocket would need to replace the Mars rover's aeroshell, ballute, and parachute because the moon has no atmosphere. In addition, the lunar version could tote an additional 150 pounds of science payload.

As the team's study circulated to a limited JPL audience, Lunokhod 1 continued its slow traverse over dusty Mare Imbrium. The Soviet rover was designed to function for three months, but did not officially cease operations until the 14th anniversary of the launch of Sputnik 1 on 4 October 1971, some 10 months after JPL completed its report (radio contact with Lunokhod 1 was, however, lost on 14 September 1971). During its 10.54-kilometer traverse, it beamed to Earth more than 20,000 images of its surroundings and analyzed lunar surface composition at 25 locations.

Lunokhod 2 included new cameras and instruments and was designed to travel over the lunar surface more quickly than had Lunokhod 1. In this image, the dual-purpose dish-shaped solar array/thermal cover is shown folded open, as it would have been during lunar daytime. Image credit: Lavochkin Association/NASA.

The Soviets followed up Lunokhod 1's success a few weeks after Apollo 17 (7-19 December 1972), the final piloted lunar mission. On 17 January 1973, Luna 21 landed inside rugged Le Monnier Crater bearing the Lunokhod 2 rover. The new rover was, the Soviets stated, superior to its predecessor. It would, for example, scuttle across the lunar surface much more rapidly than could Lunokhod 1.

On 9 May 1973, after traversing some 37.5 kilometers in less than three months, Lunokhod 2 rolled accidentally into a dark-floored crater. Its open bowl-shaped solar array/thermal cover apparently brushed against the crater wall, becoming partly filled with lunar dirt.

When, shortly thereafter, controllers in the Crimea commanded the array/thermal cover to shut at lunar sunset, the dirt fell on Lunokhod 2's thermal radiator. Two weeks later, as the Sun rose again at Le Monnier, controllers commanded the array/thermal cover to hinge open in preparation for a new day of lunar driving.

The dirt-covered radiator could no longer reject heat adequately, so Lunokhod 2 rapidly overheated in the harsh lunar sunlight. The Soviets declared its mission ended on 3 June 1973. Lunokhod 2 was the last rover to operate on another world until Mars Pathfinder's Sojourner mini-rover in 1997.

In March 2010, NASA released high-resolution Lunar Reconnaissance Orbiter (LRO) images of the Moon's surface showing the Lunokhod 1 and Lunokhod 2 rovers and the Luna 17 and Luna 21 landers. In the intervening years, LRO has orbited lower over the Lunokhod landing sites, enabling higher-resolution imaging. LRO images clearly show the extended Luna 17 and Luna 21 ramps and the tracks Lunokhod 1 and Lunokhod 2 left on the lunar surface.

The last journey of Lunokhod 2: the black arrow points to the crater where the rover accidentally collected a load of lunar dirt. Soon afterwards, the rover parked for the lunar night. The white arrows running up the center of the image highlight tracks Lunokhod 2 left as the Sun rose and the dirt on its radiator caused it to overheat. The white arrow near the top of the image points to Lunokhod 2 in its final resting place. This image shows an area of Le Monnier crater roughly 400 meters square. Image credit: NASA.

Proposals for a Viking follow-on robot rover mission would be put forward throughout the 1970s and 1980s, but none would move beyond the stage of proposals and studies. In part, this was because the Soviet Union failed to follow through on its promise (or threat) to launch robot sample returners and rovers to the planets.

Competition with the Soviet Union was rarely mentioned as a motive for robotic exploration after the early 1970s. When it was, it lacked its old punch: for example, it utterly failed to move lawmakers when comet scientists sought to use it as a justification for funding a U.S. mission to Comet Halley during its 1985/1986 apparition.

JPL's proposed 1979 rover bears a passing resemblance to the Mars Science Laboratory (MSL) rover Curiosity launched on 26 November 2011, almost exactly 41 years after Lunokhod 1. Both the JPL 1979 rover design and Curiosity have six wheels, rear-mounted nuclear power sources, stalk-mounted cameras, and front-mounted arms.

The nuclear-powered Mars Science Laboratory Curiosity captured a selfie on 3 February 2013. Image credit: NASA.

Curiosity, however, has a single body, solid wheels, and a more elaborate suspension system. Curiosity is also larger and heavier (about 2000 pounds) and depended on a more complex (and, to many observers, more worrisome) landing system known as the Sky Crane. The new system functioned as advertised, however, gently lowering Curiosity onto its wheels in equatorial Gale crater late in the evening U.S. Pacific Time on 5 August 2012.

Perhaps the most profound difference between the 1979 and 2011 rovers has to do with expectations. JPL engineers in 1970 assumed that their rover might cover half a thousand kilometers in a single Earth year. Curiosity, by contrast, cautiously traversed about 7.9 kilometers during its first 687-day martian year, which ended on 24 June 2014.

Although it has suffered wheel damage, Curiosity continues to climb the foothills of Aeolus Mons, an immense geologic layer-cake that fills much of Gale crater. Curiosity is expected to continue exploring until it suffers a catastrophic failure or until its electricity-generating nuclear source runs down, whichever comes first.

Sources

An Exploratory Investigation of a 1979 Mars Roving Vehicle Mission, JPL Report 760-58, J. Moore, Study Leader, Jet Propulsion Laboratory, 1 December 1970.

Challenge to Apollo: The Soviet Union and the Space Race, 1945-1974, NASA-SP-2000-4408, Asif Siddiqi, NASA, 2000, pp. 532-533, 740-743.

Press Kit, Mars Science Laboratory Landing, NASA, July 2012.

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