Could the Space Voyages in the Film and Novel "2001: A Space Odyssey" Really Happen? (Part 1)

An advanced LANTR Moon shuttle departs low-Earth orbit. Image credit: Pat Rawlings/NASA.

The film and novel 2001: A Space Odyssey changed my life. I know, that sounds overwrought, but it's true. I was six years old in April 1968, when the classic collaboration between science fiction author Arthur C. Clarke and film director Stanley Kubrick hit movie screens around the world. By that point in my young life, I had been reading for three years. I knew that I liked science — especially geology — so a "science fiction" film sounded intriguing.

By the time the movie drew to a close, I had become a spaceflight fan and a science fiction buff. I remain so afflicted today. (I expect that the existence of this blog makes the "spaceflight fan" part kind of obvious.)

The film 2001 is enigmatic, with mostly banal dialog and an ending that left many who saw it in its first run feeling confused and even cheated. Clarke's novel fills in gaps, but its narrative differs from the cinematic narrative. For example, in the movie 2001, Jupiter is the spaceship Discovery's destination; in the book 2001, the natural wonders of the Saturn system as understood in 1968 are used to good effect. None of this ambiguity troubled me; in fact, the mysteries stoked my young imagination.

Arthur C. Clarke (left) and Stanley Kubrick on the Aries-1B Moon shuttle passenger cabin set. Image credit: The Stanley Kubrick Archives.

This background explains why a pair of technical papers caught my attention as the year 2001 approached and passed. Nuclear propulsion engineers at NASA's Lewis Research Center (now Glenn Research Center) in Cleveland, Ohio, authored both papers. They described ways that the piloted spaceflight capabilities portrayed in the film and book 2001 might be made reality using technology and techniques that are either already in existence or are plausibly foreseeable.

In the first paper, first published in 1997 and subsequently lightly revised, Stanley Borowski and Leonard Dudzinski looked at how a 24-hour voyage to a lunar surface base might be accomplished using Nuclear Thermal Rocket (NTR) propulsion and liquid oxygen (LOX) mined from the Moon. For comparison, Apollo spacecraft needed more than three days to travel from Cape Kennedy to lunar orbit.

The second paper is much more ambitious, but also more speculative. It offers a design and operational scenario for a nuclear-fusion-propulsion spacecraft named Discovery II which could reach Jupiter orbit in just four months and Saturn orbit in seven. I will discuss it in my next two posts.

The film and book 2001 both begin with a band of man-apes who are having a tough time of it. They grub in the dust for bits of vegetation beside competing quadruped herbivores and huddle together at night listening to screeching big cats, for whom they make easy prey.

The book focuses on a hominid named Moonwatcher. Some time after an intellect-boosting encounter with an alien black monolith, he grasps the related concepts of tool-use and hunting. Soon his entire band wields bone clubs. They hunt the unsuspecting herbivores, drive off the big cats, and make war on a technologically backward rival band.

After murdering the rival band's leader, Moonwatcher of the film 2001 hurls his club triumphantly at the sky, where it becomes an Earth-orbiting satellite bearing nuclear warheads. In a heartbeat we leap over three million years of human evolution and technology advancement.

A more hopeful sign of advanced technology appears — a gleaming white space plane in Pan American Airlines livery. On board is Dr. Heywood Floyd, a high-level bureaucrat on a mission for the United States Astronautics Agency (USAA). National security, the novel 2001 explains, requires that he fly with only a pilot, co-pilot, and stewardess for company.

USAA is evidently a NASA successor organization. One can speculate that, in the 2001 timeline, a well-funded NASA worked with large commercial entities and handed off certain of its roles as, with NASA aid, those commercial entities succeeded in proving themselves capable of providing necessary spaceflight services. Along the way, NASA handed off aeronautical research (the first "A" in the acronym NASA) and became largely focused on advanced spaceflight development and scientific exploration. This prompted a name change.

The Pan Am space plane deposits Floyd at a wheel-shaped artificial-gravity international space station. It is the fifth in a series, so is called Space Station V. There Floyd confronts members of a rival band — a group of Soviet scientists on their way home from the Moon — and transfers to a near-spherical Aries-1B Moon shuttle to begin his journey to the U.S. base in 150-mile-wide Clavius crater.

Aries-1B Moon shuttle. Image credit: Turner Entertainment/Metro Goldwyn Mayer.

In the book 2001, the 30-passenger Moon shuttle's "low-thrust plasma jet" propulsion system operates for "more than 15 minutes" to begin the 25-hour voyage to the Moon. Floyd, alone on board with a pilot, co-pilot, steward, stewardess, and two engineers, hears the "whistling" of the "electrified" plasma jets. He feels the acceleration they impart to the Moon shuttle as a "mild" pressure pushing him down into his seat.

In their 1997 paper, Borowski and Dudzinski dubbed their Moon shuttle propulsion system LANTR, which stands for "LOX-Augmented Nuclear Thermal Rocket." They envisioned that LANTR propulsion would form a critical component of a cislunar transportation infrastructure that ultimately would include multiple interdependent vehicles and a Lunar Oxygen (LUNOX) mining and refining base near the Apollo 17 landing site at Taurus-Littrow on the southeast edge of Mare Serenitatis.

LUNOX, the NASA Lewis nuclear propulsion engineers explained, was likely to become the first large-scale space commodity. The "orange soil" Apollo 17 explorer Harrison Schmitt kicked up on the flank of Shorty Crater — which, it turns out, occurs at many places on the Moon — would, they estimated, make a rich feedstock for LUNOX refining, with every 25 tons of the volcanic glass-rich dirt collected and processed yielding a ton of LUNOX. For comparison, about 327 tons of more typical lunar surface material would need to be mined and refined to produce a ton of LUNOX.

The LANTR architecture, based on a NASA Lewis Nuclear Thermal Rocket (NTR) architecture developed for the abortive Space Exploration Initiative (1989-1993) of President George H. W. Bush, would evolve over time. It would not at first use LUNOX, reach the Moon in a day, or include reusable vehicles.

Borowski and Dudzinski sought to reduce the cost of their Earth-to-low-Earth-orbit (LEO) launches by exploiting then-existing Space Shuttle hardware and facilities. A pair of Shuttle-Derived Launch Vehicles (for example, Shuttle-C), each capable of placing a 66-ton payload into 407-kilometer-high LEO, would suffice to launch an expendable "two-tank" NTR stage, expendable piloted lunar spacecraft, and cargo.

Shuttle-C in its most basic form: an expendable cargo canister with a two-engine Shuttle boat-tail replaces the Space Shuttle Orbiter. Image credit: NASA.

The first Shuttle-C payload, the 24-meter-long "core stage," would comprise a pair of NTR engines, attitude control and docking systems, and a 7.6-meter-diameter, 17.5-meter-long insulated, meteoroid-shielded tank with 49.3 tons of cryogenic liquid hydrogen (LH2) inside. The engines would serve as both electricity generators and rocket motors. Because they would have two roles, or "modes," Borowski and Dudzinski dubbed them Bimodal Nuclear Thermal Rocket (BNTR) engines.

The BNTR engine's basic design would resemble that of NTR engines going back to the 1950s. LH2 would serve double duty as nuclear-fission reactor coolant and rocket propellant. After passing through and cooling the reactor, the hydrogen, now hot and gaseous, would vent into space through a bell-shaped nozzle to produce thrust.

The second Shuttle-C payload would comprise a 4.6-meter-diameter, nine-meter-long tank with nine tons of LH2, an adapter for linking with the core stage, and a conical crew capsule with four astronauts on board. It would also include a second spacecraft: a 44-ton LH2/LOX chemical-propulsion Lunar Landing Vehicle (LLV) with a five-ton crew cabin and nine tons of cargo bound for the lunar surface.

The two Shuttle-C payloads would dock in LEO, forming what Borowski and Dudzinski called a Lunar Transfer Vehicle (LTV). With the LLV attached, it would measure 46 meters in length. Its twin BNTR engines would heat and expel LH2 for 47.5 minutes to place the LTV/LLV combination on course for an Earth-moon voyage lasting 84 hours.

At the end of this cislunar journey, the BNTR engines would fire a second time so that the Moon's gravity could capture the LTV/LLV combination into 300-kilometer-high orbit. The crew would board the LLV and descend to the lunar surface with their nine tons of cargo, which would include equipment for mining, refining, and storing LUNOX, as well as scientific gear and lunar base components.

The crew would spend 45 days on the Moon living out of the LLV. They would then pilot the LLV back to lunar orbit, transfer to the LTV capsule, cast off the spent LLV, and fire the BNTR engine pair to depart lunar orbit for an 84-hour journey to Earth.

Near Earth, the crew capsule would detach from the LTV and reenter the atmosphere directly. The rest of the LTV would swing by Earth and fire its BNTR engines briefly to boost itself into a Sun-centered disposal orbit. In total, the LTV BNTR engines would operate for 61.4 minutes during a 54-day round-trip lunar mission.

Borowski and Dudzinski also described one-way cargo missions derived from their piloted architecture. Twenty-five tons of additional cargo would replace the crew cabin and propellants for boosting the LLV back to lunar orbit, bringing total cargo delivered to the Moon's surface to 34 tons.

LUNOX production would ramp up with each successive expendable LTV/LLV lunar mission. In lockstep with the increasing supply of LUNOX, NASA would upgrade the cislunar transportation system so that, after an unspecified number of flights, it would evolve into Borowski and Dudzinski's reusable LANTR architecture. The LANTR architecture would, they explained, support routine weekly 24-hour Earth-Moon "commuter" flights.

By then, LUNOX production would amount to 10,878 tons per year. Of this, reusable Earth-bound LANTR LTVs would use 4888 tons, while reusable LLVs for transporting LUNOX, crews, and cargoes between the LUNOX surface base and a lunar orbit propellant depot would expend 5990 tons.

The NASA Lewis engineers assumed that 11 solar-powered, teleoperated LUNOX plants operating 35% of the time (this is, for 70% of each two-week lunar daylight period) could each strip-mine and refine 25,000 tons of orange soil to produce about 1000 tons of LUNOX per year. They estimated that the orange soil area near the Apollo 17 landing site might yield up to 700 million tons of LUNOX; that is, enough to support weekly 24-hour commuter flights for the next 60,000 years.

LANTR would see the basic all-LH2 BNTR engine augmented with a system for introducing LOX into the supersonic hot hydrogen exhaust flow "downstream" of the reactor. The LOX would enable the hydrogen to burn much as it does in a conventional LOX/LH2 chemical rocket engine, dramatically increasing LANTR thrust. This, Borowski and Dudzinski wrote, would offer "big engine" performance from "smaller, more affordable, easier to test NTR engines."

To trim development cost, the LANTR LTV would structurally closely resemble the all-LH2 LTV already described. At 7.5 meters long, the LANTR LTV's forward section would measure 1.5 meters shorter than its all-LH2 counterpart. The aft section, the core stage, would be outwardly identical to its all-LH2 predecessor. As with the all-LH2 LTV, a pair of Shuttle-Cs would launch the fore and aft sections of the LANTR LTV, which would then rendezvous and dock automatically in LEO.

The LANTR LTV would then dock automatically with a propellant depot in LEO. There it would fill its large tank with 45.5 tons of LH2 and its small tank with 112.3 tons of LOX, which is much denser than LH2. The propellant depot's LOX and LH2 would all be produced on Earth and boosted into LEO on Shuttle-derived launch vehicles.

Meanwhile, a Space Shuttle or a next-generation reusable piloted spacecraft would deliver to the International Space Station (ISS) 20 passengers bound for the LUNOX production base on the Moon. Accommodations on board the ISS are, of course, not spacious, so the new arrivals would immediately move into a 15-ton, 4.6-meter-diameter, eight-meter-long cylindrical Passenger Transport Module (PTM) docked with the station. Even in its most advanced form, Borowski and Dudzinski wrote, their Earth-Moon transportation system would be "spartan" compared with Heywood Floyd's Moon shuttle; it would, for example, not employ stewards.

The 20 Moon voyagers would remain inside the PTM throughout their 24-hour Earth-Moon journey, so would see little change in their immediate surroundings from the time they boarded it until they entered the lunar surface base. The PTM would, however, interface with three vehicles besides the ISS during each lunar flight.

As the time for LEO departure approached, the PTM would undock from the ISS and move away using its own chemical-propellant attitude-control thrusters. It would rendezvous with a LANTR LTV standing by near the LEO propellant depot at a "safe distance" from the ISS: that is, far enough away that radiation from its BNTR engines could neither harm the ISS crew nor reflect off ISS structure and harm the astronauts in the PTM. The PTM would approach and dock tail-first with the LANTR LTV, forming a 195.6-ton LANTR "commuter shuttle."

The commuter shuttle would climb away from LEO quickly (image at top of post). Acceleration to 24-hour Earth-Moon transfer velocity would need only 21.2 minutes, or less than half the duration of the all-LH2 LTV burn required to achieve an 84-hour Earth-Moon transfer. During the climb away from LEO, the 20 passengers would, much like Dr. Floyd in the book 2001, feel only a "mild" pressure — to be precise, just 0.23 Earth gravities of acceleration at BNTR start-up, when the commuter shuttle was fully loaded with propellants, and 0.46 gravities just before BNTR engine shutdown, when about half its propellants were expended.

Twenty-four hours after LEO departure, the BNTR engines would fire again to slow the commuter shuttle so that the Moon's gravity could capture it into lunar orbit. It would rendezvous with a lunar-orbit propellant depot containing LH2 brought from Earth and LUNOX.

The PTM would undock from the commuter shuttle and link up with a waiting lunar surface-based reusable LLV, the second vehicle with which it would interface during its trip to the Moon. The skeletal four-engine LLV would weigh 10.9 tons without propellants or cargo and 59.5 tons loaded with 33.6 tons of propellants and the 15-ton PTM.

In lunar orbital night: at the end of a 24-hour Earth-Moon crossing, the PTM (lower left) has separated from the LANTR LTV (lower right) near the Moon-orbiting propellant depot (barely visible above the LANTR LTV). A reusable LLV now moves into position to grapple the PTM and begin descent to the LUNOX production base on the Moon's surface. Image credit: Pat Rawlings/NASA.

The LLV would descend to the LUNOX base on four plumes of burning Earth hydrogen and LUNOX. After touchdown, a wheeled flatbed — the third vehicle with which the passenger module would interface — would move into position beneath the PTM and detach it from the overhead LLV framework. The PTM/flatbed combination would then roll over the lunar surface from the landing field to an airlock leading into a surface habitat. After linkup with the habitat, the 20 passengers would exit the PTM to begin their duties on the Moon.

In addition to moving passengers and cargo between Earth and Moon and back again, the LANTR architecture would, as already indicated, move LUNOX and Earth-produced LH2. Four times per week a reusable tanker module with an empty weight of five tons loaded with 25 tons of LUNOX would ride a flatbed to a waiting automated LLV and then ascend to the Moon-orbiting propellant depot. After pumping its LUNOX cargo into the propellant depot's tanks, it would return to the LUNOX base.

Welcome to the Moon: in the lower right corner, a PTM rides a six-wheeled flatbed to an airlock leading into a large inflatable lunar surface habitat. Meanwhile, at upper center-right, a tanker LLV lifts off on a mission to transport LUNOX to the lunar-orbital propellant depot. Image credit: Pat Rawlings/NASA. 

Borowski and Dudzinski assumed a fleet of four LANTR LTVs. Each would carry out 13 Earth-Moon round-trips per year, for a total of 52 commuter flights (that is, one per week). Each LANTR LTV would in its BNTR engines hold enough fissile material to permit 44 Earth-Moon round-trips, giving it an operational lifetime of 3.3 years.

A LANTR LTV near end-of-life would perform a one-way all-cargo mission before disposal into a Sun-centered orbit. One-way cargo would include Earth-produced LH2 for the propellant depot in lunar orbit. With about 23 tons of surplus LH2 in its "core stage" tank, a one-way LOX load of only 66 tons, and a potential cargo weight of about 80 tons, the LANTR LTV might deliver more than 100 tons of LH2 to lunar orbit during its final mission.

When Borowski and Dudzinski wrote their paper in 1997, existence of lunar polar ice in permanently shadowed craters, first predicted in 1961, remained uncertain. Data from a 1994 joint experiment using the Clementine lunar orbiter and NASA's Deep Space Network antennas had hinted strongly at the existence of hydrogen at the poles, but alternate explanations for the hydrogen signal existed, and an experiment employing the giant Arecibo radio telescope in Puerto Rico could find no trace of lunar hydrogen.

Robotic spacecraft in lunar polar orbit have since confirmed that a large quantity of water ice exists at the Moon's poles — in the billions of tons. Provided that mining equipment can be designed to operate in the very cold, very dark environment of the permanently shadowed craters, the existence of water ice means that both oxygen and hydrogen await us on the Moon in potentially easily processed form.

In theory, water ice-rich feedstock need only be heated to separate out the water, which would then be split into hydrogen and oxygen using electrolysis. Though this would seem to render Borowski and Dudzinski's LUNOX mining scenario irrelevant, their LANTR-based transportation system might burn LOX and LH2 derived from lunar polar ice as easily as it could Earth LH2 and LUNOX.

Early drafts of the 2001: A Space Odyssey screenplay — there were many — are replete with informative dialog. Though actors spoke some of the dialog during filming, most was replaced with classical music and sound effects in the final film.

In a late 1965/early 1966 draft of the script, the Aries-1B moon shuttle pilot and co-pilot speculate about the purpose of Heywood Floyd's unprecedented single-passenger lunar flight. The pilot remarks that the Moon shuttle will return to Earth orbit without passengers because Clavius Base is under quarantine. Only Floyd would be permitted to leave the Moon shuttle and no one at the base  would be permitted to board it.

The co-pilot points out that Moon shuttle tickets cost $10,000 one-way, so Floyd's mission will cost USAA and U.S. taxpayers about $600,000 — that is, the same as a round-trip flight with 30 passengers on board going each way. Alas, Borowski and Dudzinski provided no estimate of the cost of reaching the Moon using their proposed infrastructure.

In the book 2001, Floyd disembarks from the Aries-1B and stops for a glass of lunar sherry — made from Moon-grown algae — in the Clavius Base Administrator's office. He then attends a briefing in which he hears the latest news about the find that motivated his secretive single-passenger Moon flight. A black monolith found beneath the floor of Tycho crater has, he learns, nothing to do with the Chinese expedition of 1998. It was, geologists from Clavius Base have determined, deliberately buried three million years ago. It is the first evidence of intelligent life off the Earth.

After a moonbus ride across the rugged southern Lunar Highlands to Tycho, Floyd witnesses the beginning of a slow lunar dawn. He dons an advanced space suit — doing so takes only a few minutes, and it barely restricts his movements — then descends into the pit excavated around the monolith. Meanwhile, the Sun rises slowly over the lip of the excavation, shining its light on the monolith for the first time in three million years.

In the film 2001, Floyd joins other bureaucrats for a group photo in front of the monolith. Set against the brooding monolith, which seems to soak up all light, this very human ritual is so mundane as to be comical. As the photographer gestures repeatedly for them to move closer together — a critical part of the group-photo ritual — the monolith interrupts by emitting a powerful electronic scream.

Floyd and the others stumble around in pain and confusion as their suit radios receive the signal and blast it into their helmets. Only later do they realize that, by exposing the monolith to the Sun, they have tripped an ancient alarm.

Sources

"2001: A Space Odyssey" Revisited — The Feasibility of 24 Hour Commuter Flights to the Moon Using NTR Propulsion with LUNOX Afterburners," AIAA-97-2956, Stanley Borowski and Leonard Dudzinski; paper presented at the 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit in Seattle, Washington, 6-9 July 1997.

"2001: A Space Odyssey" Revisited — The Feasibility of 24 Hour Commuter Flights to the Moon Using NTR Propulsion with LUNOX Afterburners, NASA/TM-1998-208830/REV2, Stanley Borowski and Leonard Dudzinski, NASA Glenn Research Center, June 2003.

Kubrick, Stanley, and Arthur C. Clarke, 2001: A Space Odyssey, directed by Stanley Kubrick, Metro Goldwyn Mayer, April 1968.

2001: A Space Odyssey, Screenplay by Stanley Kubrick and Arthur C. Clarke, pp. b35-b36a, 12/1965.

2001: A Space Odyssey, Arthur C. Clarke, New York: New American Library, October 1999.

The Making of Stanley Kubrick's 2001: A Space Odyssey, Piers Bizony, Taschen, 2014, p. 58-59.

2001Italia.it: A Blog Devoted to 2001: A Space Odyssey (http://www.2001italia.it/ — accessed 12 June 2016).

More Information

Could the Space Voyages in the Film and Novel "2001: A Space Odyssey" Really Happen? (Part 2)

Could the Space Voyages in the Film and Novel "2001: A Space Odyssey" Really Happen? (Part 3)

The Last Days of the Nuclear Shuttle (1971)

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

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)

New Horizons II (2004-2005)

Artist concept of the New Horizons spacecraft in the Pluto system. From bottom left to top right: New Horizons, Pluto, and Charon. Image credit: NASA.

The New Horizons II (NH II) mission was originally conceived in mid-2002 as a backup for the New Horizons (NH) mission to Pluto, its moons, and other bodies of the Kuiper Belt. The brainchild of Southwest Research Institute (SwRI) scientist and NH Principal Investigator Alan Stern, NH II was meant to ensure that, should NH fail, NASA could still satisfy the stated desires of the planetary science community.

In their 2002-2003 Decadal Survey of future goals for planetary exploration, planetary scientists rated Kuiper Belt Object (KBO) exploration as their highest scientific priority. Nevertheless, the planetary science community, NASA, and the Congress have many competing priorities, so the NH II mission was judged to be a reach too far.

NASA had approved the NH mission proposal in November 2001. The compact 478-kilogram spacecraft was scheduled to launch atop a hefty Atlas V 551 rocket in January-February 2006. A Jupiter gravity-assist flyby in March 2007 would accelerate it toward Pluto with a flight time of only about eight years. If all went well, NH would bring to bear on Pluto and its satellites a suite of seven science instruments in July 2015. NH would then fly past one or more additional Kuiper Belt Objects (KBOs) in the 2016-2020 period.

For a time in 2004-2005, however, it appeared that it would leave Earth with a minimal supply of plutonium in its electricity-generating Radioisotope Thermal Generator (RTG). (In the image at the top of this post, the RTG is the black vaned cylinder at lower left.) The plutonium shortage stemmed from a security breakdown at the Department of Energy (DOE) laboratory that produced the plutonium New Horizons needed. Without an RTG fully loaded with plutonium, it was unlikely that NH could operate for long enough to reach any KBO beyond Pluto.

The shortage caused SwRI to propose a modified version of NH II. In its purest form, the new NH II would aim exclusively to explore one or more KBOs. It would leave Earth at least a year after NH, hopefully enabling it to launch with a topped-off RTG.

To cut costs, NH II would be a "clone" of New Horizons. SwRI estimated that, by avoiding new development and by drawing on the experience it had gained from NH, the NH II mission would cost only $472 million; that is, at least $200 million less than NH.

SwRI found that NH II could launch to one or more of the hundreds of KBOs known in 2004-2005 any time that a launch window for Jupiter opened (that is, every 13 months). The March 2008 and April 2009 launch opportunities were especially attractive, however, because they would permit a Uranus flyby in the 2014-2017 period en route to the target KBO without dramatically increasing mission duration. This would make NH II only the second spacecraft to explore the Sun's seventh planet; the first was Voyager 2 in January 1986.

This Hubble Space Telescope image shows Uranus approaching equinox in 2005 and at the time of equinox in 2007. The image also shows bright clouds, bands, and other signs of atmospheric activity absent when Voyager 2 flew through the Uranus system during southern hemisphere summer in 1986. Image credit: NASA/Space Telescope Science Institute.

All of the planets except Uranus rotate on an axis more or less perpendicular to the plane of their orbit around the Sun. Earth, for example, is tilted at 23.44° relative to the plane of its orbit. Uranus is tipped on its side relative to the other planets, meaning that its rotational axis is nearly parallel to the plane of its orbit.

Uranus has at least 27 moons, of which five (Miranda, Ariel, Umbriel, Titania, and Oberon) range from 450 to 1600 kilometers in diameter. It also has a system of at least 11 rings. The rings and moons revolve around Uranus in the plane of its equator, which means that the entire Uranus system appears to pivot around the Sun on its side. Uranus needs a little more than 84 years to circle the Sun once.

When Voyager 2 flew past Uranus, the planet's south pole was pointed toward the Sun; that is, its southern hemisphere was near the middle of a 21-year summer. Its northern hemisphere was pointed away from the Sun, so was locked in dark winter. The same applied to its moons; their southern hemispheres were fully lit and their northern hemispheres were cloaked in cold darkness. This meant that Voyager 2 could not image their northern hemispheres. The Uranian equator would be turned more toward the Sun when NH II flew past, so the spacecraft would be able to observe the Uranus system in its entirety.

Uranus appeared bland to Voyager 2, and the visible parts of its largest moons showed many intriguing features but no signs of present-day activity. In 1998, however, the Hubble Space Telescope revealed about 20 bright clouds in the Uranian atmosphere, and more bright clouds have since been observed. In addition, astronomers have spotted glowing aurorae at its magnetic poles, which do not match its rotational poles.

Small worlds similar to the Uranian moons in size and mass have turned out to be surprisingly active. Saturn's 500-kilometer-diameter moon Enceladus, to cite the best-known example, has squirting from warm areas at its south pole jets of water vapor laden with salt and organic compounds.

Binary Kuiper Belt Object 1999 TC36, Image credit: NASA/Space Telescope Science Institute.

After flying past Uranus, NH II would zoom onward to its primary destination. If launched from Earth in March 2008, the spacecraft could zip past the binary KBO 1999 TC36 as early as September 2020. Launch in April 2009 could lead to a flyby no later than April 2023. 1999 TC36, currently orbiting the Sun at about 31 times the Earth-Sun distance, comprises two close KBOs, one about 285 kilometers across and the other about 265 kilometers in diameter. Circling the close pair is a moon about 140 kilometers wide.

NH II might instead be directed toward a flyby of 2002 UX25, a roughly 680-kilometer-diameter KBO with a 205-kilometer satellite. If launch took place in March 2008, the flyby could occur as early as July 2022. Earth departure in early May 2009 would yield a 2002 UX25 flyby in July 2023. 2002 UX25 currently orbits the Sun at about 41 times the Earth-Sun distance. With a fully fueled RTG, additional KBO flybys after the 1999 TC36 or 2002 UX25 flyby would be possible.

In late 2004, as the plutonium shortage became apparent, the NH team appealed to Congress for funds for an NH II mission study. NASA's Fiscal Year 2005 budget appropriation called for such a study, though Congress declined to fund it. Nevertheless, in early 2005 NASA Headquarters tasked NASA Goddard Space Flight Center in suburban Washington, DC, with an independent study of the NH II concept.

The DOE subsequently was able to resolve its security problems and provide a full load of plutonium for the NH RTG, so NASA dropped the NH II concept. NH left Earth on 19 January 2006, flew past Jupiter on 28 February 2007, and flew through the Pluto system in mid-July 2015.

Seven months after NH left Earth, Pluto's classification as a planet, long tenuous, was updated to take into account new knowledge of the outer Solar System. The NH flyby made Pluto both the first KBO to be discovered and the first to be visited. NH flew past a second KBO, provisionally designated 2014 MU69, in January 2019. In November 2019, the 35-kilometer-long KBO was named Arrokoth.

Sources

New Horizons 2: A Journey to New Frontiers, presentation materials, A. Stern, Southwest Research Institute, 10 June 2005.

New Horizons II Mission Design, presentation materials, Y. Guo, 16 June 2004.

“New Horizons II: Doubling UP in the Outer Solar System,” L. David, Space.com (no longer online), 17 June 2004.

“New Horizons Set to Launch with Minimum Amount of Plutonium,” B. Berger, Space News, 4 October 2004.

More Information

The Challenge of the Planets, Part Three: Gravity

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

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

The next in line: from its vantage point in orbit of Saturn, the sixth planet, the Cassini spacecraft catches a glimpse of Uranus, the seventh (blue dot, upper left). Image credit: NASA.

The four largest and most massive satellites of Jupiter are, in order out from the planet, Io, Europa, Ganymede, and Callisto. Io and Europa form a pair of roughly the same size, as do Ganymede and Callisto. Io has a diameter of 3636 kilometers, while Europa, the smallest of the four, is 3138 kilometers in diameter. Ganymede, the largest moon in the Solar System, measures 5262 kilometers across. Callisto, Jupiter's outermost large moon, is 4810 kilometers in diameter.

The presence of four large, massive moons enabled the Galileo spacecraft to carry out a complex tour of the Jupiter system between December 1995 and September 2003. Over the course of 35 revolutions around the giant planet, Galileo used gravity-assist flybys of the four moons to change its orbit.

By contrast, Saturn and Neptune each have only one large, massive moon. Saturn's moon Titan, the second-largest moon in the Solar System, measures 5152 kilometers in diameter, while Neptune's moon Triton is just 2706 kilometers across. The Cassini Saturn Orbiter, at this writing exploring the Saturn system, must rely on Titan for most of its gravity assists, which means that it must rely more often than did Galileo on its finite supply of rocket propellants to make orbital changes. A Neptune orbiter, with only Triton available for significant gravity assists, would face a similar challenge.

The four largest and most massive moons of Uranus are puny compared with Io, Europa, Ganymede, Callisto, Titan, and Triton. Titania, the largest, measures just 1578 kilometers in diameter. The others are: Ariel (1158 kilometers across), innermost of the four moons; Umbriel, 1169 kilometers wide; and Oberon, (1522 kilometers), outermost of the four. Titania orbits between Umbriel and Oberon.

To scale: the five largest moons of Uranus in order (left to right) out from the planet. Image credit: NASA.

Though often derided as small and dull, the reality is that the Uranian satellites are little known. Voyager 2, the only spacecraft to visit Uranus, imaged no more than 40% of any Uranian moon as it flew through the system in January 1986. Furthermore, the Cassini Saturn tour has revealed that even small outer Solar System satellites can be surprising: Enceladus, for example, just 505 kilometers wide and by all rights cold and dead, is hot enough inside that it blasts salty water into space from parallel cracks ("tiger stripes") at its south pole at more than 2000 kilometers per hour.

In a paper published in the Journal of Spacecraft and Rockets shortly before Galileo concluded its Jupiter satellite tour, Andrew Heaton of NASA Marshall Space Flight Center and James Longuski of Purdue University demonstrated that the Uranus system could support a complex Galileo-style tour. This was, they acknowledged, "contrary to intuition. . .because the Uranian satellites are much less massive than those of Jupiter."

A Galileo-style tour would be possible, they explained, because "the key to a significant gravity assist is not the absolute size of the satellite, but the ratio of its mass to its primary, and the mass ratios of the Uranian satellites to Uranus are similar to those of the Jovian satellites to Jupiter." Titania and Oberon form a large outer pair similar to Ganymede and Callisto, they noted, while Ariel and Umbriel form a small inner pair equivalent to Io and Europa. The "Uranian system is nearly a smaller replica of the Jovian system," Heaton and Longuski wrote.

To perform their calculations, they relied on "Tisserand graphs" developed at Purdue University in the late 1990s. Their mathematical tool was named for 19th-century mathematician Felix Tisserand, who had calculated the effects of planetary gravity on the motion of comets. Tisserand followed in the footsteps of Anders Johan Lexell, who in the early 1770s had sought to explain the sudden appearance and subsequent disappearance of a previously unknown comet. In 1770, Comet Lexell flew past the Earth at a distance of 2.3 million kilometers.

A previous post detailed how, in the early 1960s, Michael Minovitch used his own graphs and University of California, Los Angeles and Jet Propulsion Laboratory computers to calculate dozens of gravity-assist trajectories (see "For More Information" at the end of this post). His work laid the groundwork for many planetary missions, including the Mariner 10 Venus-Mercury flybys and Voyager 2's Jupiter-Saturn-Uranus-Neptune "Grand Tour." Minovitch did not, however, calculate satellite system tours; presumably this was because in the early 1960s so little was known of outer Solar System moons.

Heaton and Longuski described a three-phase, 811-day Uranian system tour. After launch from Earth in March 2008 and a gravity-assist flyby of Jupiter in September 2009, the Uranus tour spacecraft would fire its main rocket engine to capture into an elliptical Uranus orbit on Valentine's Day in 2018. This would mark the start of the first Uranus tour phase, which would be devoted to matching the plane of the Uranian equator, ring system, and moon orbits.

Images of Uranus and its ring system taken over four years using the Hubble Space Telescope in Earth orbit. South is to the left. The 2003 and 2005 images show features in the planets clouds; a southern hemisphere polar hood and, in 2005, a northern hemisphere storm. In the 2007 image, the planet's rings and equator are edge on to the Sun.

Uranus is tipped on its side relative to the other planets in the Solar System, and its moons have equatorial orbits. Heaton and Longuski wrote that the Uranian system would appear edge-on to the Sun in 2007, then would tilt gradually until the planet and its moons pointed their north poles at the Sun in 2028.

The Uranus tour spacecraft would capture into an initial orbit tilted 13.6° relative to the planet's equator and system plane. It would fly past Titania in May 2019 at a distance of 316 kilometers, allowing the largest Uranian satellite to "crank" its orbital plane. A total of nine similar Titania flybys over 261 days would place the spacecraft into the same plane as the Uranian equator, rings, and moons.

The second phase of the Uranus tour, the energy-reduction phase, would see the spacecraft reduce the size of its orbit, thus shortening its orbital period, while at the same time conducting a thorough exploration of the four largest Uranian moons. This would begin 287 days after the spacecraft captured into Uranus orbit with a flyby of Oberon at a distance of 414 kilometers and would proceed through eight Ariel flybys, five Umbriel flybys, three Titania flybys, and four additional Oberon flybys over the course of the next 395 days.

The spacecraft would pass nearest any world in the Uranian system during this phase. At the start of its 14th revolution about Uranus, almost exactly one Earth year (364.3 days) after arriving at the planet, it would pass just 54 kilometers over Umbriel's icy landscapes.

The southern hemisphere of Miranda: a mosaic of images from the January 1986 Voyager 2 Uranus flyby. Image credit: NASA.

Heaton and Longuski did not include the enigmatic moon Miranda on their list of close flybys because it orbits close to Uranus and, with a diameter of just 480 kilometers (only a little smaller than surprising Enceladus) is less than half the size of Ariel, the smallest moon they employed for gravity assists. Close proximity to Uranus and low mass would mean that Miranda's gravity could contribute little to shaping the Uranus tour.

Miranda has some of the most intriguing known surface features on the Uranian satellites — for example, Verona Rupes, a five-kilometer-high fault scarp that begins near the edge of the lighted area visible to Voyager 2. Presumably the Uranus tour spacecraft would image Miranda whenever its tour route took it relatively close by.

The third and final phase of the tour would commence 691 days after Uranus arrival with a 151-kilometer Umbriel flyby. The somewhat arbitrary goal of the third phase would be to place the Uranus tour spacecraft into orbit around Ariel. Through three additional Umbriel flybys and four Titania flybys over 120 days the spacecraft would nearly match Ariel's orbit about Uranus, reducing its maximum velocity relative to its target to slightly less than one kilometer per second. The Uranus tour spacecraft would then briefly fire its rocket motor to slip into orbit about Ariel.

Source

"Feasibility of a Galileo-Style Tour of the Uranian Satellites," A. Heaton and J. Longuski, Journal of Spacecraft and Rockets, Volume 40, Number 4, July-August 2003, pp. 591-596.

More Information

The Challenge of the Planets, Part Three: Gravity

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)

Touring Titan By Blimp & Buoy (1983)