Titan blimps (1983)

In 1983, NASA contractor Science Applications, Inc. proposed a half-dozen missions to Saturn's mysterious moon Titan. Space historian and Beyond Apollo blogger David S. F. Portree describes the blimps, buoys, and air-launched sounding rockets of SAI's unflown Titan exploration program. 

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, but its most distinctive feature - its bright and complex ring system - remained undiscovered until 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 the most advanced in the world at the time, his telescope was too crude to allow him to determine their nature. A half-century later, Christian Huygens announced that the "appendages" Galileo had glimpsed 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 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 November 12, 1980, showed that 98% of Titan's atmosphere is nitrogen, and that its surface atmospheric pressure is roughly half again as great as Earth's at sea-level. Its surface temperature averages about 94 Kelvin (-179° Celsius, -290° Fahrenheit) and its surface gravitational pull is just 14% of Earth's. The 5150-kilometer-diameter moon's surface remained mysterious; it lay hidden beneath a high-altitude haze layer and dense orange clouds.

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, a Comet Rendezvous/Asteroid Flyby (CRAF), a Mars Geoscience/Climatology Orbiter, and - reflecting the many questions the Voyager 1 flyby had raised - a Titan Probe/Radar Mapper. This mission would see a Saturn flyby or orbiter spacecraft drop a short-lived instrument capsule into the moon's cloudy atmosphere and explore its hidden surface using an imaging radar.

By the time the SSEC completed its 1983 report, the Saturn-orbiting version of the Titan Probe/Radar Mapper mission had been named Cassini. The choice of name reflected the expansion of the mission's objectives to take in the entire Saturn system. Seventeenth-century astronomer Jean Dominique Cassini had discovered Saturn's "second-tier" moons Rhea, Dione, Tethys, and Iapetus, and had spotted the Cassini Division, the most prominent gap in the planet's ring system.

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. Science Applications, Inc. (SAI), briefed workshop participants in August 1983 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 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 critical radio relay link between the Titan atmosphere/surface systems and mission controllers and scientists on Earth. Based on the Cassini 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. Based on Galileo Jupiter orbiter and Pioneer Venus hardware, the flyby bus 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 design. 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.

SAI's most novel and picturesque Titan exploration systems were its large and small buoyant stations. 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 small stations would take the form of instrument-laden gondolas suspended from balloons. The large stations, packed into aeroshells twice as large, would be either large 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 of altitude 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 large 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 mission 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 liquid oxygen/liquid hydrogen refueling in time for its missions to leave 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 refueled 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 refueled in Earth orbit 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 (balloons). The buoyant stations would enter Titan's atmosphere, decelerate, and deploy their gas envelopes as they slowly fell on parachutes. They would each operate for at least two months, kept aloft by heat from radioisotope thermal generators. The large buoyant station might move close enough to Titan's surface to lower an instrument package on a tether, permitting the first direct sampling of the large moon's surface.

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.

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, work toward making Cassini a reality continued. The U.S. Congress approved new-start funding for the Saturn orbiter/Titan probe 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.

Following the January 1986 *Challenger *Shuttle disaster, NASA cancelled Centaur G' and moved planetary spacecraft off the Shuttle manifest. The bus-sized Cassini spacecraft instead left Earth on a Titan IVB/Centaur expendable rocket in October 1997 and, after gravity-assist swingbys of Venus, Earth, and Jupiter, arrived in Saturn orbit in July 2004. The European-built Huygens probe entered Titan's atmosphere in January 2005 and floated on a parachute to a rough landing, revealing an icy surface. The following year, scientists using Cassini's radar discovered lakes large and small in Titan's north polar region.

In May 2008, Cassini completed its primary mission and began its first extended mission (the Equinox Mission).