|Image credit: NASA|
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 5150-kilometer-diameter satellite's surface 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|
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.
This last would see a Saturn flyby or orbiter spacecraft drop a short-lived instrument capsule into Titan's dense atmosphere and probe its hidden surface using an imaging radar. The SSEC hoped that it 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. Science Applications Incorporated (SAI) briefed workshop participants on 2 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 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. 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. 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 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 its 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. 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|
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|
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|
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.
Cassini/Huygens was, of course, meant to be followed at some point by more intensive follow-on Titan exploration as described in the 1986 SSEC report. Many alternative missions have been proposed and studied, but to date none has gone very far. Partly this is because NASA has focused much of its robotic program on Mars, a world which is relatively easy and inexpensive to reach and which remains of high biological potential and considerable geological interest.
In addition, Titan now has competition in the Saturn system in the form of remarkable little Enceladus. Several proposed missions have focused on Titan and Enceladus together; others have focused on Enceladus alone or Titan alone in an effort to prevent sticker shock.
Titan and Enceladus might both have to wait, however, because NASA has begun the long-awaited development of a mission to survey Jupiter's moon Europa, the prototypical ice-covered global-ocean world. Assuming that current White House and Congressional support for the Europa multiple-flyby mission persists, it might leave Earth as early as 2022. Experience gained by exploring Europa through as many as 45 flybys will almost certainly be applicable to the in-depth exploration of other moons of the outer Solar System, including Titan and Enceladus.
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
The Challenge of the Planets, Part Two: High Energy
The Challenge of the Planets, Part Three: Gravity
What Shuttle Should Have Been: NASA's October 1977 Space Shuttle Flight Manifest
The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)