George Landwehr von Pragenau's Quest for a Stronger, Safer Space Shuttle

The Space Shuttle Challenger and its booster system moments before they were destroyed. The plume of flame emerging from the side of its malfunctioning SRB is clearly visible. Image credit: NASA.

The Space Shuttle Orbiter Challenger was minding its own business on 28 January 1986, working hard to get its seven-member crew and its large satellite payload to low-Earth orbit, when its booster stack betrayed it and everything began to go badly wrong. First, hot gas within its right Solid Rocket Booster (SRB) began to burn through a seal meant to contain it. Soon, a fiery plume gushed from the side of the SRB, robbing it of thrust, and reached out menacingly toward the side of the brown External Tank (ET) and the strut linking the lower end of the SRB to the ET (image at top of post). The plume broke though the ET's foam insulation and aluminum skin, then the strut pulled free of the weakened ET.

Challenger fought back as the ET began to leak liquid hydrogen fuel. It swiveled (the aerospace term is "gimballed") the three Space Shuttle Main Engines (SSMEs) in its tail as it struggled to stay on course. The plume from the SRB, meanwhile, glowed brighter as it began to burn hydrogen leaking from the ET. At the same time, the SRB began to rotate around the single strut left holding it to the ET. That strut was located not far from the Orbiter's gray nose, near the conical top of the errant SRB.

Throughout these events, Challenger's last crew remained oblivious to the technological drama taking place around them. This was just as well, since they had no way to escape what was about to happen to them.

When Challenger at last lost its struggle against its own booster stack, significant events were separated by tenths or hundredths of seconds. Immediately after the right SRB's lower strut came free, the entire Shuttle stack lurched right. Mike Smith, in Challenger's pilot seat, had time for a startled "Uh-oh" less than a second after the lurch. The ET's dome-shaped bottom then fell away, freeing all the hydrogen fuel it contained. The right SRB's pointed nose slammed into and crushed the top of the ET, freeing liquid oxygen oxidizer. The escaped hydrogen blossomed into a fireball that encompassed Orbiter, rapidly disintegrating ET, and SRBs.

Yet the Orbiter Challenger did not explode. Instead, it broke free of what was left of the ET and began a tumble. The aerodynamic pressures the Orbiter experienced as its nose pointed away from its direction of flight were more than sufficient to snap it into several large pieces: the crew cabin, the satellite payload, the wings, and the SSME cluster emerged from the fireball more or less intact. The SRBs, still firing, flew out of the fireball, tracing random trails across the blue Florida sky until a range safety officer commanded them to self-destruct. The Orbiter's wreckage, meanwhile, plummeted into the Atlantic within sight of the Florida coast.

NASA recovered the bodies of the crew and portions of the wreckage, including the section of the right SRB that had leaked hot gas. The wreckage was turned over to accident investigators.

This 1975 NASA illustration depicts the basic components of the Space Shuttle system. The Orbiter includes three Space Shuttle Main Engines (left). Two Solid Rocket Boosters, one of which is mostly hidden behind the External Tank, provide thrust during liftoff and the early part of ascent. The tank includes (from right to left) a small tank for dense liquid oxygen, a drum-shaped structural support ring/tank separator below the Orbiter's nose, and a large tank for low-density liquid hydrogen.

During a Shuttle launch, the three SSMEs ignited first. This caused the twin SRBs, the bases of which were mounted to the launch pad by explosive bolts, to flex along their entire length away from the SSMEs, then straighten out again just as they ignited. O-ring seals between the cylindrical segments making up the SRBs often became unseated during flexure, then had to reseat to contain hot gases after SRB ignition. Accident investigators concluded that failure of one of those seals doomed Challenger. Even more damning, they found that partial seal failures followed by hot exhaust leaks had occurred on pre-Challenger flights — and had been disregarded by NASA managers.

After Challenger, NASA and its contractors redesigned the SRB joints and seals, added crew pressure suits and a limited crew escape capability, and banned potentially unsafe practices and payloads from Shuttle missions. Yet the U.S. civilian space agency might have gone much farther when it sought to enhance Space Shuttle safety after Challenger.

Even before the accident, NASA had at its disposal redesign proposals that could have made the Shuttle stack stronger and safer. In 1982, for example, George Landwehr von Pragenau, a veteran engineer at NASA's Marshall Space Flight Center, filed a patent application — granted in 1984 — for a Shuttle stack design that would have made the Challenger accident impossible.

Born and educated in Austria, von Pragenau joined the von Braun rocket team in Huntsville, Alabama, in 1957. He became a U.S. citizen in 1963. He specialized in rocket stability and flight effects on rocket behavior. He had, for example, been part of the team that found the cause of the "pogo" oscillations that crippled Apollo 6, the second unmanned Saturn V-launched Apollo test mission (4 April 1968).

In the conventional Shuttle stack, von Pragenau explained, SRB thrust was transmitted through the forward SRB attachment points to a reinforced intertank ring between the ET's top-mounted liquid oxygen tank and its liquid hydrogen tank.  He considered this "indirect routing" of thrust loads to be perilously complex. SSME thrust loads, for their part, passed through the Orbiter to its twin aft ET attachment points on the large, fragile liquid hydrogen tank.

By the time he filed his 1982 patent application, von Pragenau had spent almost a decade thinking about how the Shuttle stack might be rearranged to reduce weight and aerodynamic drag, increase stability, simplify thrust paths, and provide greater structural strength. His 1984 patent was, in fact, not his first aimed at Shuttle improvement.

Von Pragenau's 1974 alternative Shuttle stack. Image credit: U.S. Patent Office.

In 1974, von Pragenau had filed a patent — granted the following year — in which he proposed a more slender, more vertically oriented Shuttle stack; that is, one that would mimic conventional rocket designs in which stages are stacked one atop the other. He linked the twin SRBs side by side. Moving the tank for dense liquid oxygen from the ET's nose to its tail placed its concentrated mass nearer the base of the stack, improving in-flight stability. He then mounted the SRBs to the Orbiter's belly and perched the ET atop the SRB/Orbiter combination. SRB and SSME thrust loads were conveyed through struts to meet at the ET's flat, reinforced base.

Von Pragenau's 1982 Shuttle stack design was in some ways a less radical departure from the existing Shuttle design than was his 1974 design. He left the SRBs, ET, and Orbiter in their normal positions relative to each other, but made other significant changes. As in his 1975 patent, he moved the liquid oxygen tank from the ET's nose to its tail and brought the SRBs closer together to improve stability. The liquid oxygen tank became skinny, cylindrical, and almost as long as the Orbiter and SRBs attached to it. The liquid hydrogen tank, fat with low-density fuel, von Pragenau mounted atop the oxygen tank, partially overhanging the Orbiter and SRBs.

Von Pragenau's 1982 Shuttle stack redesign. The numeral "15" points to the rigid thrust structure framework. "34," "35," "36" are Solid Rocket Booster attachment fixtures. These would link to slide rails ("31" and "32") that would run the length of the liquid oxygen tank ("20"). "19" is the liquid hydrogen tank. Image credit: U.S. Patent Office. 

Von Pragenau could not tolerate flexing SRBs. He proposed to mount a slide rail on either side of the liquid oxygen tank. Three attachment fixtures on each SRB would link to the slide rails, helping to ensure rigidity. When the SRBs depleted their propellant, pyrotechnic bolts would fire, freeing them to slide backwards down the rails and fall neatly away from the Orbiter/ET stack.

The most important feature of von Pragenau's redesign was a rigid framework – a thrust structure – that would link the bottom of the SRBs just above their rocket nozzles. In addition to holding the SRBs rigidly in place, the thrust structure would transmit SRB thrust loads to the bottom of the ET oxygen tank, which would rest atop the center of the thrust structure. When the spent SRBs slid away from the Orbiter/ET stack, they would take the thrust structure with them.

Side view of Von Pragenau's 1982 Shuttle stack concept. Image credit: U.S. Patent Office.

Von Pragenau's concepts apparently exerted little influence on NASA's post-Challenger recovery effort. A likely explanation is that neither of his proposals — if they were known to decision-makers at all — was deemed affordable. In addition to extensive changes in manufacturing tooling, both proposals would have required modifications to the Vehicle Assembly Building, the twin Complex 39 Shuttle pads at Kennedy Space Center (KSC), and even the barge that delivers ETs to KSC. Instead of beefing up the existing Shuttle, NASA studied designs for new shuttles which, for lack of funding, remained firmly in the low-cost realm of CAD drawings, conference papers, and conceptual artwork.

On 1 February 2003, the Space Shuttle claimed another crew. The oldest Orbiter, Columbia, was heavier than her sisters Atlantis, Discovery, and Endeavour, which limited the amount of cargo she could deliver to the International Space Station (ISS). For this reason, NASA largely relegated to Columbia the few remaining non-ISS missions — for example, Hubble Space Telescope servicing.

As they began Earth-atmosphere reentry at 8:44 a.m. Eastern Standard Time after a nearly 16-day life sciences mission, the seven STS-107 astronauts on board Columbia were unaware that, during ascent, a piece of ice-impregnated insulating foam nearly a meter wide had broken free from the ET and impacted their spacecraft's left wing. Ice and foam had broken free from ETs before, but the damage they caused was, after cursory examination, deemed acceptable by Shuttle Program managers. This time, however, the impact opened a hole up to 10 inches wide in the Orbiter's left wing leading edge.

Hot plasma generated during reentry entered the hole and began to destroy Columbia's left wing from the inside out. Observers along the Orbiter's flight path, which cut across the southern tier of U.S. states, reported unusual flashes. Meanwhile, members of the STS-107 crew on Columbia's Flight Deck observed and recorded on video flashes visible outside their windows. In the recovered video, the astronauts appear to realize that the flashes were unusual but show no signs of panic.

Much as Challenger had before it, Columbia fought bravely against the forces destroying it. Onboard computers took account of increased drag on the left side of the Orbiter and sought to compensate to keep it on the flight path. At 8:59 a.m. Eastern Standard Time, however, Columbia tumbled and disintegrated over northeast Texas.

Both of von Pragenau's design concepts placed all or part of the ET above the Orbiter, so one might argue that they would not have prevented a failure resembling that which killed the STS-107 crew. On the other hand, one can be forgiven for speculating that a U.S. civilian space agency provided with the means after Challenger to rebuild the Shuttle system to make it safer might also have evolved an organizational culture more prone to investigating and less prone to tolerating recurring flight anomalies.

Von Pragenau retired from NASA in 1991 after more than 30 years of service. He remained involved in engineering efforts at NASA Marshall Space Flight Center during his retirement. He died two years after the Space Shuttle's final flight (STS-135, 8-24 July 2011), on 11 July 2013, at the age of 86.

Sources

Patent No. 4,452,412, Space Shuttle with Rail System and Aft Thrust Structure Securing Solid Rocket Boosters to External Tank, George L. von Pragenau, NASA Marshall Space Flight Center, 15 September 1982 (filed), 5 June 1984 (granted).

Patent No. 3,866,863, Space Vehicle, George L. von Pragenau, NASA Marshall Space Flight Center, 21 March 1974 (filed), 18 February 1975 (granted).

Hampton Cove Funeral Home Obituaries: George Landwehr von Pragenau (http://www.hamptoncovefuneralhome.com/fh/obituaries/obituary.cfm?o_id=2150841&fh_id=13813 — accessed 27 October 2016).

NASA History: Columbia Accident Investigation Board (http://history.nasa.gov/columbia/CAIB.html — accessed 29 October 2016).

NASA History: Challenger STS 51-L Accident (https://www.hq.nasa.gov/office/pao/History/sts51l.html — accessed 29 October 2016).

More Information

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

What if a Space Shuttle Orbiter Had to Ditch? (1975)

What If Galileo Had Fallen to Earth? (1988)

Naming the Space Station (1988)

The 1982 "Space Platform" was the Station design President Ronald Reagan displayed to leaders of prospective Space Station International Partner nations in 1984. Image credit: NASA.

On 12 April 1988, James Odom, NASA's Associate Administrator for Space Station, sent out a memorandum with two attachments to a long distribution list. Recipients included NASA Administrator James Fletcher, Deputy, Associate, and Assistant Administrators at NASA Headquarters, the NASA Inspector General, the NASA Chief Scientist, the nine NASA field center Directors, Public Affairs Officers across the agency, the six field center Space Station Project Offices, and representatives of the Space Station International Partners (Canada, the European Space Agency, and Japan). The memo's subject line summed up its purpose succinctly: it was called "Naming the Space Station."

Odom explained that the Space Station President Ronald Reagan had called upon NASA to build in his State of the Union Address of 25 January 1984 was about to enter its "development phase," so the time was ripe to decide on a name for it. He attached a NASA Management Instruction laying out guidelines for naming NASA projects and, more interestingly, a list of 16 names suggested by a Space Station Name Committee he had appointed. Each name included a brief rationale.

The naming rules were straightforward. Candidate names were to be simple and easily pronounced, not refer to living persons, neither duplicate nor closely resemble other NASA or non-NASA space program names, be translatable into the languages of the International Partners, and have neither ambiguous nor offensive meanings in the International Partner languages. In addition, acronyms were to be avoided. The naming process was not to be revealed to the public; if, however, members of the public happened to submit names that followed the rules, the Name Committee would consider them.

NASA's 1985 "Power Tower" Space Station configuration included hangars for space tugs and satellite servicing, a laboratory, a free-flyer providing an enhanced microgravity environment, and zenith- and nadir-pointing truss-type platforms for instruments. Image credit: NASA.

Several of the Name Committee's 16 names sought a return to the Greco-Roman mythology naming tradition of the Mercury, Gemini, and Apollo programs. Hercules was, its rationale explained, an appropriate name for the Space Station because the mythical hero was "a symbol of extraordinary strength. . . who won immortality by performing 12 labors." Minerva was the Roman goddess of wisdom and learning; Aurora, the Roman goddess of the dawn. Jupiter, the greatest Roman god, was a candidate name; the Name Committee explained, however, that it was meant to refer to the Solar System's largest planet, and that it was "symbolic of mankind's greatest adventure, the exploration of space."

Pegasus was suggested because the Space Station would "dwell among the stars" like "the winged horse who ascended into Heaven." Olympia referred to the sacred grove where ancient Greece held its Olympic Games; the name was meant to invoke the spirit of international cooperation.

Another proposed 1985 NASA Space Station design provided ample room for hangars and attached payloads, as well as a simplified track arrangement for the Station's mobile platform-mounted robot arm. The Station's electricity-generating solar arrays, steerable only by moving the entire Station, cover its zenith-facing side. Image credit: NASA.

Two names, Earth-Star and Starlight, referred to the Station's likely bright star-like appearance in the skies of Earth, while another, Skybase, was said to build on "the tradition of Skylab," the first U.S. space station, which had reached orbit in May 1973. Landmark was suggested because the Space Station Program would constitute a "landmark" in the history of the NASA spaceflight. Pilgrim referred to outer space settlement, and the rationale for Prospector declared that the Space Station would "serve as a base for exploration of space for natural deposits."

This last Station function must have come as a surprise to many on Odom's distribution list, for the Space Station was meant to serve as a laboratory; before 1988 it had lost essentially all of its transportation node capabilities, so had little hope of ever playing a significant role in the development of space resources. NASA planners, meanwhile, had moved toward the concept of a separate transportation node space station. They believed (with good reason) that separating transportation and laboratory functions would result in two stations with optimized designs.

Other names — Freedom, Independence, Liberty, and Unity — advertised U.S. political and social values, and thus followed Soviet convention. ("Soyuz," for example, means "Union," which refers to the multi-modular nature of the Soyuz spacecraft, but also to the Union of Soviet Socialist Republics.) Liberty also referred to breaking "the bond of Earth's gravitational pull," Unity to international cooperation, and Independence to “man’s first permanent step to be 'independent' of Earth."

Freedom, the Name Committee explained, was appropriate because the Space Station would "provide scientific and technological 'freedom' to explore avenues of research," as well as freedom "from the confines of gravity." In addition, freedom was "a political value central to all of the Space Station's international partners."

In his memorandum, Odom explained that names submitted to the Space Station Name Committee would be given to the NASA Administrator, who would make the final selection. In the end, though, the list of candidate names landed on President Reagan's desk. On 18 July 1988, NASA announced that he had selected the name Space Station Freedom.

Space Station Freedom — sometimes referred to as Space Station "Fred" due to its much-reduced size and capabilities — in 1991. Image credit: NASA.

Despite Odom's assertion that it would soon enter development, Space Station Freedom underwent repeated redesigns to reduce cost and complexity and narrowly dodged cancellation. In mid-1993, new President William Clinton ordered a comprehensive review of the U.S. space station program, a management overhaul, and a redesign based on one of three options, designated A, B, and C. In November 1993, Clinton selected Option A, a pruned-back version of Freedom that became known as Alpha. (One might speculate, none too seriously, that had Option B been selected, the station would have been called Beta.)

During his single term in office, President George H. W. Bush, Clinton's predecessor, had concluded agreements first with Soviet leader Mikhail Gorbachev and then with Russian President Boris Yeltsin that called for, among other things, use of the Soyuz spacecraft as a Space Station Freedom lifeboat. Bush had stopped short of giving the Russians a central role in the NASA station; nevertheless, he laid groundwork for things to come.

President Clinton's decision to accept the Russian invitation to combine Alpha and Mir-2 had its critics. Nevertheless, the move created a geopolitical rationale that ensured broad Congressional support for the station for the first time. The combined space station would keep Russian rocketeers occupied so that they would not peddle their missile-manufacturing services internationally. In terms of space operations, the merger amounted to what was probably the first instance of Earth-orbital barter: the Russian segment would provide orbit-maintenance propulsion and early staffing in exchange for abundant electricity from the U.S. segment's enormous truss-mounted steerable solar arrays.

The International Space Station in August 2005. Image credit: NASA.

For a time, the combined U.S.-Russian station was jokingly referred to as "Ralpha," for "Russian Alpha," which at least had the advantage of being distinctive. Ultimately, however, NASA and its partners settled on the prosaic name International Space Station (ISS).

Following (mainly) Russian tradition, individual ISS modules have received names. The Mir-2 core module, a small independent space station NASA referred to as the "Service Module," was named Zvezda ("Star"), while the first U.S. component, Node 1 — a module designed as an attachment point for other modules — was dubbed Unity for reasons similar to those given by the 1988 Name Committee. Other names include Zarya ("Dawn") for the FGB propulsion and storage module, the first ISS component launched; Destiny for the U.S. Lab Module; Kibo ("Hope") for the Japanese lab; and Columbus for the European lab.

The International Space Station in 2011. Image credit: NASA.

Source

Memorandum with attachments, S/Associate Administrator for Space Station to Distribution, "Naming the Space Station," 12 April 1988.

More Information

NASA's 1992 Plan to Land Soyuz Space Station Lifeboats in Australia

Skylab-Salyut Space Laboratory (1972)

He Who Controls the Moon Controls the Earth (1958)

Catching Some Comet Dust: Giotto II (1985)

Giotto 1 liftoff. Image credit: European Space Agency.

On the overcast morning of 2 July 1985, the eleventh Ariane 1 rocket to fly lifted off from the Centre Spatial Guyanais in Kourou, French Guiana, an outpost of the European Community located a few degrees north of the equator on the northeast coast of South America. The last Ariane 1 to fly, it bore aloft Giotto, the first European Space Agency (ESA) interplanetary spacecraft. Giotto's destination was Comet Halley.

A "dirty snowball" containing materials left over from the birth of the Solar System 4.6 billion years ago, Halley needs about 76 years to revolve around the Sun once. Its elliptical orbit takes it from the cold emptiness beyond Neptune to the space between the orbits of Venus and Mercury. Halley travels around the Sun in a retrograde orbit, meaning that it orbits "backwards" relative to the eight planets and most other objects making up the Solar System.

Comet Halley has passed through the inner Solar System 30 times since its first verified recorded apparition in 240 B.C. In 837 A.D., it passed just 5.1 million kilometers from Earth; during that apparition, its dust tail must have spanned nearly half the sky, and its bright coma — the roughly spherical dust and gas cloud surrounding its icy nucleus — may have appeared as large as the full Moon.

Shortly after its bright apparition in the year 1301, Italian artist Giotto di Bondone was inspired to add Comet Halley to his painting The Adoration of the Magi. The Giotto spacecraft was named for him.

Comet Halley appears near the top of Giotto di Bondone's The Adoration of the Magi. 

Throughout most of its known apparitions, Comet Halley was not understood to be one comet repeatedly passing through the inner Solar System. Not until 1705 did English polymath Edmond Halley determine that comets seen in 1531, 1607, and 1682 were probably one comet orbiting the Sun. He predicted that, if his hypothesis was correct, the comet should reappear in 1758 (which it subsequently did).

The Ariane 1's third stage injected 980-kilogram Giotto into a 198.5-by-36,000-kilometer orbit about the Earth. Thirty-two hours after launch, as it completed its third orbit, flight controllers in Darmstadt in the Federal Republic of Germany commanded drum-shaped Giotto to ignite its French-built Mage solid-propellant rocket motor. The motor burned 374 kilograms of propellant in 55 seconds to inject the spinning 2.85-meter-tall, 1.85-meter-diameter spacecraft into an orbit about the Sun.
 
Two months before Giotto's launch, Americans P. Tsou (Jet Propulsion Laboratory), D. Brownlee (University of Washington), and A. Albee (California Institute of Tech) proposed in a paper in the Journal of the British Interplanetary Society that a second Giotto mission be launched to fly close by one of 13 candidate comets between 1988 and 1994. They proposed that the new spacecraft, which they dubbed Giotto II, might launch on an Ariane 3 or in the payload bay of a Space Shuttle. Giotto II's "free-return" trajectory would take it as close as 80 kilometers from the target comet's nucleus, then would return it to Earth.

Near the comet, Giotto II would expose sample collectors to the dusty cometary environment. Near Earth, it would eject a sample-return capsule based on the proven General Electric (GE) Satellite Recovery Vehicle (SRV) design. The capsule would enter Earth's atmosphere to deliver its precious cargo of comet dust to eager scientists.
 
Tsou, Brownlee, and Albee pointed out that the Mage solid-propellant motor had not been required to boost Giotto into interplanetary space; that is, that the Ariane 1 could have done the job itself. Giotto was, however, based on a British Aerospace-built Geos magnetospheric satellite design, which included the Mage motor. Re-testing the design without the motor would have cost time and money, so ESA elected to retain it for Giotto. After noting that the GE SRV could fit comfortably in the space reserved for the Mage, they proposed that, in Giotto II, the reentry capsule should replace the motor.

Giotto included on its aft end a "Whipple bumper" — named for its inventor, planetary astronomer Fred Whipple — to protect it from hypervelocity dust impacts. During approach to Comet Halley, the spacecraft turned the bumper toward its direction of flight. The bumper comprised a one-millimeter-thick aluminum shield plate designed to break up, vaporize, and slow impactors, a 25-centimeter empty space, and a 12-millimeter-thick Kevlar sheet to halt the partially vaporized, partially fragmented impactors that penetrated the aluminum shield.
 
In the case of Comet Halley, dust was expected to impact the bumper at up to 68 kilometers per second. Tsou, Brownlee, and Albee noted that the 13 candidate Giotto II target comets were all less dusty and would have lower dust impact velocities than Halley. Because of this, Giotto II would need less shielding than Giotto.

Comet dust would, nevertheless, create challenges for Giotto II. Tsou, Brownlee, and Albee devoted much of their paper to a description of how the spacecraft might successfully capture dust for return to Earth. One proposed capture system, based on the Whipple bumper design, would use a shield made from ultrapure material to vaporize and slow impacting dust particles. The vapor from the impactor and the impacted part of the bumper would then be captured as it condensed. Scientists would disregard the bumper material when they analyzed the condensate.
 
Tsou, Brownlee, and Albee also noted that thermal blankets from the Solar Maximum Mission (SMM) satellite, launched into Earth orbit on 14 February 1980, had demonstrated that intact capture of high-velocity particles was possible. The multilayer Kapton/Mylar blankets, which were returned to Earth on board the Space Shuttle Orbiter Challenger at the end of mission STS 41-C (6-13 April 1984), were found to have collected hundreds of intact meteoroids and human-made orbital debris particles.

The three scientists described preliminary experiments in which gas guns were used to fire meteoroid and glass fragments at "underdense materials," such as polymer foams and fiber felts. The experiments suggested that such materials could capture at least partially intact comet dust particles.

Giotto's encounter with Comet Halley spanned 13-14 March 1986. At closest approach the spacecraft passed just 596 kilometers from Halley's nucleus. The comet's 15-by-eight-by-eight-kilometer heart turned out to be extremely dark, with powerful jets of dust and gas blasting outward into space.

Artist's concept: Giotto at Halley. Image credit: European Space Agency.

Halley's hot heart as imaged by ESA's Giotto spacecraft. 

The intrepid probe suffered damage from dust impacts — one large particle sheered off more than half a kilogram of its structure — but most of its instruments continued to operate after the Comet Halley flyby. ESA thus decided to steer Giotto toward another comet.

On 2 July 1990, five years to the day after its launch, Giotto flew past Earth at a distance of 16,300 kilometers, becoming the first interplanetary spacecraft to receive a gravity-assist boost from its homeworld. The gravity-assist flyby put it on course for Comet Grigg-Skjellurup, which it passed at a distance of 200 kilometers on 10 July 1992.
 
After determining that Giotto had less than seven kilograms of hydrazine propellant left on board, ESA turned it off on 23 July 1992. The inert spacecraft flew past Earth a second time at a distance of 219,000 kilometers on 1 July 1999.
 
By that time, a comet coma sample return mission was underway with two of the Giotto II proposers playing central roles. In late 1995, Stardust became the fourth mission selected for NASA's Discovery Program of low-cost robotic missions. Brownlee and Tsou, respectively Stardust Principal Investigator and Deputy Principal Investigator, designed the mission's sample capture system.

Artist's concept of the NASA Stardust spacecraft at Wild 2. Image credit: NASA.

The 380-kilogram Stardust spacecraft left Earth on a free-return trajectory on 7 February 1999, and flew past Comet Wild 2 (one of the 13 Giotto II candidates) at a distance of about 200 kilometers on 2 January 2004. Stardust captured dust particles in aerogel, a silica-based material of extremely low density that was invented in the 1930s. Tsou, Brownlee, and Albee had apparently been unaware of aerogel when they proposed Giotto II in 1985.

Stardust returned to Earth on 15 January 2006. Its sample capsule streaked through the pre-dawn sky over the U.S. West Coast before parachuting to a landing on a salt pan in Utah.

When opened on 17 January 2006 at NASA Johnson Space Center, in the same lab that received the Apollo Moon rocks, Stardust's 132 aerogel capture cells were found to contain thousands of intact dust grains captured from Wild 2. Subsequent analysis indicated that some probably formed close to other stars before our Solar System was born.
 
Sources

"Comet Coma Sample Return via Giotto II," P. Tsou, D. Brownlee, and A. Albee, Journal of the British Interplanetary Society, Volume 38, May 1985, pp. 232-239.

"ESA Remembers the Night of the Comet," European Space Agency, 11 March 2011 (https://www.esa.int/Science_Exploration/Space_Science/Rosetta/ESA_remembers_the_night_of_the_comet — accessed 26 August 2016).

"Stardust: NASA's Comet Sample Return Mission," NASA Jet Propulsion Laboratory (https://stardust.jpl.nasa.gov/home/index.html — accessed 26 August 2016).

More Information

A 1974 Plan for a Slow Flyby of Comet Encke

Cometary Explorer (1973)

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

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)

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.

More Information

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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)

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

Image credit: Martin Marietta.

The destruction of the Orbiter Challenger on 28 January 1986, just 73 seconds into the 25th Space Shuttle mission, put an end to many proposals and plans for Shuttle improvement and augmentation. The powerful liquid hydrogen/liquid oxygen-propelled Centaur G' upper stage, routine satellite servicing and refueling in orbit, the nitrogen-gas-propelled Manned Maneuvering Unit, launches from the U.S. West Coast, launches to polar and retrograde orbits, frequent non-astronaut passengers, solar-powered long-duration Spacelab missions, and an eventual flight rate upwards of 50 per year — all of these were abandoned as NASA sometimes reluctantly acknowledged the Shuttle's frailties and foibles.

Among the proposed improvements permanently grounded after the Challenger accident was Martin Marietta's Aft Cargo Carrier (ACC), a cargo canister meant to be bolted over the dome-shaped aft end of the Space Shuttle External Tank (ET). Martin Marietta, prime contractor for the 154-foot-long ET, had begun in-house studies of the ACC at about the time the first Shuttle launched into orbit (STS-1, 12-14 April 1981).

Aft Cargo Carrier and Orbiter payload bay dimensions compared. The entire ACC is 31.9 feet long; the aft shroud is 20.8 feet long. Image credit: Martin Marietta.

By the middle of 1982, Martin Marietta aggressively pitched the ACC concept at aerospace conferences. NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, soon took notice and contracted with the company for ACC engineering and economic feasibility studies. MSFC had since the mid-1970s sought out low-cost ways of incrementally improving the Space Shuttle and evolving NASA piloted programs toward a permanent Space Station (see "More Information" below).

The ACC's position adjacent to the Orbiter's three Space Shuttle Main Engines (SSMEs) and between the powerful twin Solid-Rocket Boosters (SRBs) meant that payloads it carried would be subjected to more heating and acoustic pounding than would those in the Orbiter payload bay. Martin Marietta proposed an ACC "environmental protection system" made up of 707 pounds of thermal insulation and a 2989-pound "acoustical barrier."

Adding these layers would make the ACC shell a little more than a foot thick, reducing the diameter of payloads it could carry to about 25 feet. Even so, this made the ACC payload volume about 10 feet wider than the 15-by-60-foot Orbiter payload bay.

Martin Marietta assumed that, with planned Shuttle performance upgrades, an Orbiter would be able to boost 36.9 tons of payload into a 160-nautical-mile-high orbit inclined 28.5° relative to Earth's equator. An empty ACC would add 8.3 tons to the Shuttle's mass at liftoff, potentially reducing the payload mass the Orbiter and ACC could inject into orbit. If the ACC remained attached until SSME cutoff, then the payload mass the Orbiter and ACC could place into orbit would total 28.7 tons.

Left to right: ACC shroud; ring for mounting cargoes; and ACC skirt with twin solid-propellant rocket motors. Image credit: Martin Marietta.

Martin Marietta had, however, found a way around this problem. The ACC would include an aft shroud and a forward skirt. Discarding the 3.7-ton aft shroud as early as possible during the Shuttle's eight-minute climb to orbit would reduce the payload mass penalty to only about four tons. This meant that the Orbiter payload bay and ACC skirt could together deliver to 160-nautical-mile orbit payloads with a total mass of 33 tons.

The twin SRBs would burn out and fall away from the ET 120 seconds after liftoff at an altitude of about 146,000 feet. The ACC shroud would then detach from the skirt and fall away 35 seconds after SRB separation.

During Orbiter-only Shuttle missions, the Orbiter would shut down its SSMEs and discard the ET before attaining orbital velocity. The ET would reenter the atmosphere and be destroyed over the Indian Ocean. This would, of course, deprive the SSMEs of their source of liquid hydrogen/liquid oxygen propellants: hence, after ET separation, the three engines would amount to "dead weight." The astronauts would then ignite the Orbiter's twin Orbital Maneuvering System (OMS) engines for the first of two orbit-insertion burns.

Orbiter/ACC missions would see Orbiter, ET, ACC skirt, and payloads in a 57-by-160-nautical-mile orbit at SSME cutoff, so that the first orbit-insertion OMS burn would be unnecessary. When the assemblage attained apogee (the highest point in its orbit around the Earth), the astronauts would ignite the OMS engines, increasing its velocity by 183 feet per second. This would raise its perigee (the low point in its orbit around the Earth) and circularize its orbit at an altitude of 160 nautical miles.

Martin Marietta proposed a host of potential ACC payloads. Many would ride on a mounting ring attached to the ACC skirt. "Catch tanks" might collect liquid hydrogen/liquid oxygen propellants left in the ET at SSME shutdown for later use in orbit. A turbine generator might burn leftover propellants to augment the electricity the Orbiter fuel cells would provide.

The ACC skirt might carry a 25-foot-diameter, 20-foot-long space station module. The module might be designed to remain attached to the ET, so that the big tank could become a strong-back for mounting large payloads or, with the addition of an access hatch linking the ET's hydrogen tank with the module, a large enclosed volume for experiments or habitation. Large folded structures — for example, an umbrella-like radio dish antenna more than 50 feet across — might also be deployed from the skirt.

Potential Aft Cargo Carrier payloads. Image credit: Martin Marietta.

Martin Marietta described three example Orbiter/ACC payload manifests and deployment scenarios. Flight 1, a mission with an initial circular 160-nautical-mile orbit at 28.5° of inclination, would see three satellites with identical solid-propellant upper stages launched in the ACC. These were the 4.4-ton Brazilsat/Payload Assist Module (PAM)-D; the 4.4-ton GOES/PAM-D; and the 4.7-ton Telsat/PAM-D. The Orbiter, meanwhile, would carry a 58-foot-long, 14-foot-diameter "large observatory" with a mass of 9.4 tons.

Without the ACC, payload mass for Flight 1 would be limited to what could be carried in the Orbiter payload bay, or about a quarter of the 36.9-ton theoretical maximum for the flight. With the ACC, the Flight 1 payload could total 22.9 tons. Following deployment from the ACC skirt, the satellites would ride their PAM-D stages to their assigned slots in the geostationary orbit (GEO) belt, 22,236 miles above the equator.

The Orbiter crew would then cast off the ET and its attached ACC skirt. A two-ton pair of solid-propellant deorbit rocket motors on the ACC skirt would ignite over the western Pacific Ocean, causing the ET/ACC combination to tumble and reenter the atmosphere. Any parts that survived reentry would splash into the Pacific south of Hawaii.

The astronauts, meanwhile, would maneuver the Orbiter to a 190-nautical-mile-high orbit and deploy the large observatory from the payload bay. They would then ignite the OMS engines to slow the Orbiter and cause it to re-enter Earth's atmosphere. The delta-winged space plane would glide to a runway landing.

Flight 2 would launch the 1.7-ton Tiros-N satellite inside the ACC and the 8.2-ton Atmosphere Monitor satellite at the aft end of the Orbiter payload bay. Because the Orbiter/ET/ACC skirt/payloads assemblage would be required to ascend to an energetically challenging 160-nautical-mile-high, 98.2° near-polar retrograde orbit, Flight 2's payload mass could total at most 11.8 tons.

The Flight 2 crew would first guide their spacecraft to a rendezvous with a two-ton Thermosat payload, which they would captured and stow at the front of the Orbiter payload bay for return to Earth. They would then fire the OMS engines to climb to a 380-nautical-mile orbit, where they would deploy the Atmosphere Monitor.

Next, they would ignite the OMS engines again to climb to a 448-nautical-mile orbit inclined 98.8° to Earth's equator. There they would deploy Tiros-N from the ACC skirt. After discarding the ET/ACC skirt, they would ignite the OMS engines to return Orbiter, crew, and Thermosat to Earth.

Aft Cargo Carrier in flight. Image credit: Martin Marietta.

Flight 3, with an initial 100-nautical-mile orbit at 28.5° of inclination and a payload mass of 26.5 tons, would see the introduction of a new reusable hardware element made possible by the ACC's large-diameter payload envelope: the 15-foot-long, 25-foot-diameter, 17-ton Orbital Transfer Vehicle (OTV). The OTV would be based in space. Visiting Orbiters would supply it with propellants and service its systems as required.

Martin Marietta noted that, by providing a second payload volume, the ACC could enable secret Department of Defense (DOD) payloads to be carried separate from but on the same flight as NASA civilian payloads. The Orbiter payload bay would thus on Flight 3 carry two Department of Defense payloads: the NATO IV/PAM-D satellite and the 35-foot-long, 10-foot-wide, 6.5-ton Synchronous Observation Satellite (SOS).

The OTV would scavenge residual ET propellants to fill its tanks, then would detach from the ACC skirt. The Orbiter crew, meanwhile, would raise the SOS on a tilt-table mounted in the payload bay. The OTV would dock with the SOS, detach it from the tilt-table, boost it to its assigned slot in GEO, and release it. Mission accomplished, the OTV would fire its engines to return to low-Earth orbit for a new mission.

The Orbiter crew, meanwhile, would cast off the ET/ACC skirt and maneuver to a 160-nautical-mile orbit, where they would deploy NATO-IV/PAM-D from the payload bay. The PAM-D stage would boost the satellite to GEO. The astronauts, meanwhile, would fire the Orbiter's OMS engines to re-enter Earth's atmosphere.

Martin Marietta placed great emphasis on the cost savings that would accrue from making the ACC a Shuttle hardware element. First, however, it estimated the costs of developing and using the cargo canister. The company assumed that NASA would give a green light to begin ACC development in late 1983, and that the first ACC would lift off three years later.

The company calculated that ACC development would cost $113 million. Changes to the Shuttle design to accommodate ACCs would cost $78 million, and changes to Kennedy Space Center facilities would cost $35 million.

Martin Marietta quoted NASA when it placed the base cost of a Shuttle flight without an ACC at an optimistic $75 million. The base cost of a Shuttle flight would increase by about $5 million when it included an ACC, the company estimated.

For its cost-savings calculations, the company employed a Shuttle traffic model less optimistic than the one NASA touted. It assumed that 331 Shuttle flights would take place between 1988 and 2000, with 34 flights in 1988 and a steady decline to 20 flights per year in 2000. During the same 12-year period, NASA assumed 26 flights per year in 1988, an upward trend to nearly 60 flights per year by 2000, and a total of 581 flights.

Based on its "low" traffic model, Martin Marietta estimated that NASA might benefit from flying 71 civilian and 35 Department of Defense Shuttle/ACC missions. The company conservatively assumed, however, that NASA would be able to fund only a total of 75 civilian and Department of Defense Orbiter/ACC flights.

Martin Marietta determined that the added payload capacity the ACC could provide would permit the elimination of 40 Orbiter-only Shuttle missions. It placed the cost of 331 Orbiter-only missions at $24.8 billion and the cost of 216 Orbiter-only and 75 Orbiter/ACC missions at $22.2 billion. The ACC would thus save NASA $2.6 billion over 12 years.

Sources

Space Transportation System with Aft Cargo Carrier: A Natural Augmentation of System Capability, Martin Marietta, no date (late 1982).

"External Tank Aft Cargo Carrier," T. Mobley and J. Hughes; paper presented at the Twentieth Space Congress, Cocoa Beach, Florida, 26-28 April 1983.

ACC External Tank Aft Cargo Carrier, Martin Marietta, no date (late 1985).

More Information

Evolution vs. Revolution: The 1970s Battle for NASA's Future

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

Humans on Mars in 1995! (1980-1981)

Touring Titan By Blimp & Buoy (1983)

Image credit: NASA.

The planet Saturn needs a little more than 29 years to circle the Sun once. At its mean orbital distance, 1.43 billion kilometers from our star's warming fires, it receives about 1% as much solar energy as does Earth. The planet was known to ancient peoples the world over, but its most distinctive feature – its bright and complex ring system – remained undiscovered until after the invention of the telescope.

Galileo Galilei, famous for his telescopic discovery of Jupiter's four largest moons, spotted Saturn's rings in 1609-1610. Though perhaps the most advanced in the world at the time, his telescope was too crude to enable him to determine their nature.

A half-century later, Christian Huygens announced that the "appendages" that had defied Galileo's analysis were in fact a ring that encircled the planet without touching it. Huygens also discovered Titan, Saturn's largest moon, and determined that it circles the ringed planet in about 16 days.

Little new was learned of Titan until 1944. In that year, planetary astronomer Gerard Kuiper discovered that it has an atmosphere containing methane.

Data from the Voyager 1 spacecraft, which flew past Titan at a distance of about 4000 kilometers on 12 November 1980, showed that 98% of its atmosphere is nitrogen, and that its surface atmospheric pressure is roughly half again as great as Earth's at sea-level. Titan's surface temperature averages about 94 Kelvin (-179° Celsius, -290° Fahrenheit) and the low-density moon's surface gravitational pull is just 14% of Earth’s. The surface of the 5150-kilometer-diameter satellite remained mysterious; it lay hidden beneath a high-altitude haze layer and dense orange clouds.

Titan as observed by Voyager 1, November 1980. The haze layer above the dense orange cloud deck is just visible, as is the ephemeral polar hood. Image credit: NASA.

In 1983, the NASA Advisory Council's Solar System Exploration Committee (SSEC) released the first part of its report Planetary Exploration Through the Year 2000. The SSEC, chartered in 1980 by NASA Administrator Robert Frosch at the recommendation of NASA Associate Administrator for Space Science Thomas Mutch, aimed to develop missions to carry out the scientific strategy put forward by the National Academy of Sciences Committee on Planetary and Lunar Exploration (COMPLEX).

The SSEC report described a "core program" of planetary missions for the remainder of the 20th century. The four "initial" missions of the core program were a Venus Radar Mapper, the Comet Rendezvous/Asteroid Flyby (CRAF) mission, a Mars Geoscience/Climatology Orbiter, and — reflecting the many questions the Voyager 1 flyby had raised — a Titan Probe/Radar Mapper.

The last of these would see a Saturn flyby or orbiter spacecraft drop a short-lived instrument capsule into Titan's dense atmosphere and probe the hidden surface using an imaging radar. The SSEC hoped that the Titan Probe/Radar Mapper would leave Earth between 1988 and 1992 and return data from Saturn and Titan between 1995 and 1997.

Even as the SSEC published its core program, it commenced work on a new report outlining an "augmented program" of planetary exploration; that is, a collection of candidate missions that might follow and expand upon its core program. As part of its new study, it convened a workshop in Snowmass, Colorado, in the summer of 1983. On 2 August 1983, Science Applications Incorporated (SAI) briefed workshop participants on a six-month study of advanced Titan missions it had completed a month earlier for NASA's Solar System Exploration Division.

SAI's presentation began with an overview of the scientific rationale underlying its mission proposals. The study team told the SSEC workshop that "the most important characteristic of Titan is the chemical evolution that has occurred and is still occurring in its atmosphere." For example, carbon monoxide and hydrogen cyanide found by Voyager 1 in trace amounts in Titan's atmosphere had the potential to evolve into nucleotide bases and amino acids, critical building blocks of terrestrial life.

Scientists suspected that Titan's atmospheric chemistry offered clues to the nature of its surface, though they split over what those clues meant. Some believed that Titan was awash in an ocean — or at least large lakes — of liquid ethane or methane. In that model, ethane or methane behaved on Titan much as water behaves on Earth.

Others believed that organic goop from the orange clouds drizzled down and accumulated to a depth of several kilometers on its solid ice surface. In places, perhaps, exotic ice volcanoes poked through the goop layer and belched methane into Titan's dense atmosphere, providing raw material for more chemical evolution.

SAI proposed eight spacecraft systems for its Titan missions. These were: the non-imaging Titan orbiter; the imaging Titan orbiter; the Titan flyby bus; the combined haze probe/penetrator probe; the sounding rocket; and the large and small buoyant stations. The orbiter and flyby bus would operate outside of Titan's atmosphere. The other systems would operate within it.

Whether imaging or non-imaging, an orbiter would be an essential element of all SAI's proposed Titan mission concepts. In addition to collecting valuable scientific data, it would provide the crucial radio-relay link between the Titan atmosphere/surface systems and mission controllers and scientists on Earth.

Based on the proposed Saturn orbiter/Titan probe spacecraft design, the orbiter would circle Titan in a 1000-kilometer-high circular polar orbit requiring 3.93 hours to complete. This would enable it to link a system floating in Titan's atmosphere near its equator with controllers and scientists on Earth about half the time. The orbiter might reduce its required propellant load by employing aerocapture; that is, by skimming through Titan's upper atmosphere to slow down so that the cloudy moon's gravity could capture it into orbit.

Of SAI's eight Titan exploration systems, only the flyby bus would carry no scientific instruments.  The flyby bus, which would be based on Galileo Jupiter orbiter and Pioneer Venus hardware, would leave Earth about a year after the Titan orbiter. Its mission would end as it flew past Titan and released a cluster of atmosphere and surface probes.

The simplest system in SAI's Titan exploration arsenal was the combined haze/penetrator probe, the design of which was based on a proposed Mars penetrator. A solid-propellant rocket motor would blast the haze/penetrator probe from a launch tube on the orbiter and slow it so that it would fall into Titan's atmosphere. An umbrella-like fabric decelerator would then deploy, slowing the probe to a speed of Mach 1 by the time it fell to within 265 kilometers of Titan's surface. It would then begin to collect data on the hazy uppermost atmosphere.

The penetrator would then separate and descend to a hard landing (or a splashdown) on Titan's surface. The haze probe, meanwhile, would descend for 23 minutes to an altitude of 100 kilometers, at which point the orbiter would pass below its horizon. This would break the radio link with Earth and end the haze probe's mission.

The penetrator would be more long-lived; it would collect and store Titan surface data for transmission to the orbiter when it rose above the horizon again. If Titan's surface were confirmed to be covered by an exotic ocean before the orbiter left Earth, then the penetrator might be fitted out as a floating sonar buoy.

This image from the Huygens probe shows Titan's misty, icy surface from a height of five kilometers. Image credit: ESA/NASA.

SAI's most novel and picturesque Titan exploration systems were its large and small buoyant stations. The small stations, instrument-laden gondolas suspended from balloons, would be delivered into Titan's atmosphere by the flyby bus packed into 1.25-meter-diameter aeroshells based on the Galileo Jupiter atmosphere probe design. The large stations, packed into aeroshells twice as large, would take the form of either larger balloons or powered blimps. The small buoyant stations would operate between 100 and 10 kilometers above Titan, while large buoyant stations would operate between 10 kilometers above Titan and Titan's surface.

SAI provided few details about its proposed sounding rocket, which it envisioned would explore the same level of Titan's atmosphere as the haze probe. During descent, at an altitude of about 100 kilometers, the solid-propellant rocket would detach from the a buoyant station, ignite its motor, and ascend into the haze layer.

The company looked at several methods for launching its Titan missions from Earth. These included an advanced Nuclear-Electric Propulsion (NEP) system, though most relied instead on one or more Centaur G' chemical rocket stages.

In keeping with U.S. space policy in 1983, all the Earth-departure methods assumed that the Titan mission spacecraft would reach Earth orbit packed into the payload bays of Space Shuttle Orbiters. Reliance on the Shuttle imposed severe penalties on the Titan missions, SAI found. These included minimal science payloads and trip times of up to eight years with multiple Venus, Earth, and Jupiter gravity-assist flybys.

SAI sought to circumvent these penalties by assuming that NASA would become capable of On-Orbit Assembly (OOA) and in-space liquid oxygen/liquid hydrogen refueling by the time the Titan missions were ready to depart Earth. These operations might take place at an Earth-orbiting space station, SAI suggested.

SAI then described five Titan exploration mission concepts which combined its eight systems in what it called "mix 'n match" fashion. Concept #1, a minimal mission, included only a Titan orbiter with a limited Titan atmosphere probe complement. The company explained that the 1978 Pioneer Venus mission — which included separately launched Orbiter and Multiprobe spacecraft — had inspired Concepts #2, #3, and #4, all of which included a Titan orbiter and a separate flyby bus. Concept #5 relied on NEP in place of chemical-propellant rocket stages.

The company described in some detail its Concept #4 mission; with 28 experiments, it was SAI's most ambitious in terms of science return. A Centaur G' stage loaded with propellants in Earth orbit coupled with a Star-48 solid-propellant rocket motor would boost Concept #4's 1885-kilogram imaging orbiter toward Saturn in July 1999, and a pair of Centaur G' stages filled in Earth orbit with liquid oxygen and liquid hydrogen would launch its 2730-kilogram flyby bus a year later. SAI calculated that these stage configurations combined with Titan aerocapture for the orbiter would permit direct Earth-to-Saturn flights with no planetary gravity-assists.

In January 2004, after a flight time of 4.5 years, the imaging orbiter would aerocapture into Titan orbit. Over the next eight months, it would deploy three haze probes without penetrators and bring to bear on Titan's mysteries an impressive array of cloud-penetrating sensors.

In September 2004, after a 4.2-year flight, the flyby bus would speed past Titan and dispense one large buoyant station (a blimp) and three small buoyant stations (probably spherical balloons). The buoyant stations would enter Titan's atmosphere, decelerate, and deploy their gas envelopes as they slowly fell on parachutes. Kept aloft by heat from radioisotope thermal generators, they would each operate for at least two months. The large buoyant station might fly close enough to Titan's surface to lower an instrument package on a tether, permitting the first direct sampling of Titan's surface materials.

SAI placed the cost of its Concept #4 mission at $1.586 billion in 1984 dollars. This included a 30% contingency fund, but did not include launch costs. Adding in the cost of 2.5 $100-million Shuttle launches, three $45-million Centaur G' stages, one $5-million Star 48 motor, and OOA (the cost of which SAI optimistically placed at $10 million per Titan-bound spacecraft) yielded a total mission cost of $1.99 billion.

Artist's concept from 1988 of the Mariner Mark II Cassini Saturn orbiter releasing the Huygens probe above Titan's orange clouds. Image credit: NASA.

In its 1986 final report, the SSEC ranked SAI's advanced Titan mission proposals below Mars sample return and comet nucleus sample return on its list of desirable augmentation missions. Meanwhile, the 1983 core program's Titan probe/Radar Mapper mission shifted emphasis to take in the entire Saturn system. This helped to move it closer to reality.

Reflecting this new broader focus, the Saturn orbiter/Titan probe mission was named for Giovanni Cassini, discoverer of Saturn's "second-tier" moons Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini detected the broadest division in Saturn's rings, which is also named for him.

NASA and the European Space Agency (ESA) jointly studied Cassini, and ESA agreed to build the Titan probe, which was named Huygens. The U.S. Congress approved new-start funding for Cassini in 1989.

Initially Cassini was meant to be one of the first Mariner Mark II spacecraft, along with the Comet Rendezvous/Asteroid Flyby (CRAF) spacecraft. Mariner Mark II was intended to be a standardized (and thus inexpensive) spacecraft bus for advanced interplanetary missions. Congress scrapped CRAF in 1992 after it went over budget and diverted its remaining funds to Cassini, marking the end of the Mariner Mark II cost-cutting experiment.

Image credit: NASA/JPL.

Following the January 1986 Challenger Shuttle disaster, NASA cancelled Centaur G' and moved planetary spacecraft off the Shuttle launch manifest. The bus-sized Cassini spacecraft left Earth on a Titan IVB/Centaur expendable rocket in October 1997 and, after gravity-assist flybys of Venus, Earth, and Jupiter, arrived in Saturn orbit in July 2004.

The Huygens probe entered Titan's dense atmosphere in January 2005 and floated on a parachute to a rough landing. Its six instruments included an imaging system, which revealed an enigmatic surface covered with rounded water ice "pebbles."

The following year, scientists using Cassini's radar discovered ethane lakes large and small in Titan's north polar region. By early 2008, several lines of evidence pointed to a global water ocean perhaps 100 kilometers beneath the water-ice crust of Titan.

Radar swaths from the Cassini Saturn orbiter reveal Titan's north polar "land of lakes" in this false-color image. The largest, Kraken Mare, is roughly the size of Earth's Persian Gulf. Image credit: NASA.

In late 2005, scientists using Cassini's imaging system found evidence that another world orbiting Saturn besides Titan has biological potential: bright white Enceladus, which William Herschel discovered in 1789. They detected numerous geysers near the 500-kilometer-diameter moon's south pole. Driven by tidal flexing and possibly other processes that generate heat, these shoot water laced with salt, silica particles, and organic chemicals into space.

After Cassini flew past Enceladus 20 times at a distance of less than 5000 kilometers — eight of those flybys were within 100 kilometers — scientists in September 2015 announced that a global ocean up to 31 kilometers deep underlies its icy surface. During its last close Enceladus flyby on 28 October 2015, Cassini will fly past at a distance of 49 kilometers. Cassini's 22nd and last planned Enceladus flyby is scheduled for 19 December 2015 at a distance of 4999 kilometers.

In May 2008, Cassini completed its primary mission and began its first extended mission (the Equinox Mission). In February 2010, NASA agreed to extend Cassini's mission until September 2017 to enable it to observe Titan's north polar region at mid-summer. Assuming that the spacecraft survives to complete its new extended mission (the Solstice Mission), it will have carried out more than 125 Titan flybys since reaching Saturn orbit.

Sources

Titan Exploration with Advanced Systems: A Study of Future Mission Concepts, Report No. SAI-83/1151, Science Applications Incorporated; presentation to the SSEC Summer Study in Snowmass, Colorado, 2 August 1983.

Planetary Exploration Through Year 2000: A Core Program, Solar System Exploration Committee, NASA Advisory Council, 1983.

Planetary Exploration Through Year 2000: An Augmented Program, Solar System Exploration Committee, NASA Advisory Council, 1986.

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