Circled by distinctive rings and attended by a coterie of a dozen and a half moons, Saturn has been called one of the most intriguing planetary realms in the solar system. Its largest moon, Titan, boasts organic chemistry that may hold clues to how life formed on the primitive Earth.
Saturn and Titan will be the destination for the Cassini mission, a project under joint development by NASA, the European Space Agency and the Italian Space Agency. The U.S. portion of the mission is managed for NASA by the Jet Propulsion Laboratory.
After arriving at the ringed planet, the Cassini orbiter will release a probe, called Huygens, which will descend to the surface of Titan. The Cassini orbiter will then continue on a mission of at least four years in orbit around Saturn.
Launched in October 1997 on a Titan IV-Centaur rocket from Cape Canaveral, Florida, Cassini will first execute two gravity- assist flybys of Venus, then one each of the Earth and Jupiter to send it on to arrive at Saturn in June 2004.
Upon reaching Saturn, Cassini will swing close to the planet -- to an altitude only one-sixth the diameter of Saturn itself -- to begin the first of some five dozen orbits during the rest of its four-year mission.
In late 2004, Cassini will release the European-built Huygens probe for its descent of up to two and a half hours through Titan's dense atmosphere. The instrument-laden probe will beam its findings to the Cassini orbiter to be stored and finally relayed to Earth.
During the course of the Cassini orbiter's mission, it will execute some three dozen close flybys of particular bodies of interest -- including more than 30 encounters of Titan and at least four of selected icy satellites of greatest interest. In addition, the orbiter will make at least two dozen more distant flybys of the Saturnian moons. Cassini's orbits will also allow it to study Saturn's polar regions in addition to the planet's equatorial zone.
Throughout the mission, costs will be contained and efficiency enhanced by streamlined operations. The Cassini Project uses simplified organizational groups to make decisions; flight controllers will take advantage of high-level building blocks of spacecraft action sequences to carry out mission activities.
"I do not know what to say in a case so surprising, so unlooked for and so novel," Galileo Galilei wrote in 1612. The source of the Italian astronomer's astonishment: Only two years after he discovered them, the rings of Saturn vanished before his eyes.
Not that Galileo, however, recognized the rings for what they were when he first sighted them in 1610. Having recently discovered Jupiter's major moons, he assumed that what he saw next to Saturn were two sizable companions close to the planet. Two years later, however, they abruptly disappeared. In a few more years, they mysteriously returned, larger than ever. Galileo concluded that what he saw were some sort of "arms" that grew and disappeared for unknown reasons.
Nearly half a century later, the Dutch scientist Christiaan Huygens solved the puzzle that vexed Galileo. Thanks to better optics, Huygens was able to pronounce in 1659 that the companions or arms decorating Saturn were in fact a set of rings. The rings were tilted so that, as Saturn orbited the Sun every 29 years, the sheet of rings would occasionally seem to vanish as viewed on-edge from Earth.
While observing Saturn, Huygens also discovered the moon Titan. A few years later, the French-Italian astronomer Jean- Dominique Cassini added several other key Saturn discoveries. Using new telescopes, Cassini discovered Saturn's four other major moons -- Iapetus, Rhea, Tethys, and Dione. In 1675, he discovered that Saturn's rings are split largely into two parts by a narrow gap -- known since as the "Cassini Division."
We now know that Saturn is one of four giant gaseous (and ringed) planets in the solar system, joined by Jupiter, Uranus, and Neptune. Second in size only to Jupiter, Saturn is nearly ten times the diameter of Earth and its volume would enclose more than 750 Earths. Even so, its mass is only 95 times that of Earth; with a density less than that of water, it would float in an ocean if there were one big enough to hold it.
Unlike rocky inner planets such as Earth, Saturn and the other gas giants have no surface on which to land. A spacecraft pilot foolhardy enough to descend into its atmosphere would simply find the surrounding gases becoming denser and denser, the temperature progressively hotter; eventually the craft would be crushed and melted.
A large, modern telescope will reveal Saturn banded in pale yellow and gray; photos from the Voyager 1 and 2 spacecraft that flew by Saturn in the early 1980s showed even more detail in the cloud tops of its upper atmosphere. Its neighbor Jupiter runs toward reds, whereas the more remote Uranus and Neptune are shades of blue.
Why the distinctive colors? The answer, in part, is because of how far each planet is from the Sun. This in turn determines the temperature, which decides which chemicals will be gases, fluids or ices. At Saturn -- some 10 times more distant from the Sun than the Earth is -- the temperature is about -180 C (-290 F). In addition to two primary, colorless gases -- hydrogen and helium -- ammonia is relatively plentiful in the planet's upper atmosphere. We do not understand fully, however, the source of the colors in Saturn's clouds -- an issue that the Cassini mission may well resolve.
Although the best telescopes on Earth show three nested main rings about Saturn, we now know that the ring system is a breathtaking collection of thousands of ringlets. They are not solid but rather are made up of countless unconnected particles, ranging in size from nearly invisible dust to icebergs the size of a house. The spacing and width of the ringlets are orchestrated by gravitational tugs from a retinue of orbiting moons and moonlets, some near ring edges but most far beyond the outermost main rings. Instruments tell us that the rings contain water ice, which may cover rocky particles.
There are ghostly "spokes" in the rings that flicker on and off. What causes them? Scientists believe they may be electrically charged particles, but we do not really know. Where do the subtle colors in Saturn's rings come from? We cannot say; the Cassini mission may well provide the answer.
And what is the origin of the rings themselves? One theory is that they are the shattered debris of moons broken apart by repeated meteorite impacts. Another theory is that the rings are leftover material that never formed into larger bodies when Saturn and its moons condensed. Scientists believe that Saturn's ring system may even serve as a partial model for the disc of gas and dust from which all the planets formed about the early Sun. The Cassini mission will undoubtedly give us important clues.
Saturn has the most extensive system of moons of any planet in the solar system -- ranging in diameter from about 40 kilometers (24 miles) to 5,150 kilometers (3,200 miles), larger than the planet Mercury. Most are icy worlds heavily studded with craters caused by impacts very long ago.
The moon Enceladus, however, poses a mystery. Although covered with water ice like Saturn's other moons, it displays an abnormally smooth surface; there are very few impact craters on the portions seen by Voyager. Has much of the surface of Enceladus recently melted to erase craters? Could the moon also contain ice volcanoes that provide particles for Saturn's most distant faint ring beyond the three main rings?
Saturn's moon Iapetus is equally enigmatic. On one side -- the trailing side in its orbit -- Iapetus is one of the brightest objects in the solar system, while its leading side is one of the darkest. Scientists surmise that the bright side is water ice and the dark side is an organic material of some kind. But how the dark material got there is a mystery. Did it rise up from the inside of the moon, or was it deposited from the outside? The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp.
But by far the most intriguing natural satellite of Saturn is its largest. Titan lies hidden beneath an opaque atmosphere more than fifty percent denser than Earth's. Titan has two major components of Earth's atmosphere -- nitrogen and oxygen -- but the oxygen is likely frozen as water ice within the body of the moon. If Titan received more sunlight, its atmosphere might more nearly resemble that of a primitive Earth.
What fascinates scientists about Titan's atmosphere is that it is filled with a brownish orange haze made of complex organic molecules, falling from the sky to the surface. Thus in many ways it may be a chemical factory like the primordial Earth.
Most scientists agree that conditions on Titan are too cold for life to have evolved -- although the most daring speculate about the possibility of lifeforms in covered lakes of liquid hydrocarbons warmed by the planet's internal heat. Yet even if Titan proves to be lifeless, as expected, understanding chemical interactions on the distant moon may help us understand better the chemistry of the early Earth -- and how we came to be.
The Cassini orbiter weighs a total of 2,150 kilograms (4,750 pounds); after attaching the 350-kilogram Huygens probe and loading propellants, the spacecraft weight at launch is 5,630 kilograms (12,410 pounds). Because of the very dim sunlight at Saturn's orbit, solar arrays are not feasible and plans call for power to be supplied by a set of radioisotope thermoelectric generators, which use heat from the natural decay of plutonium to generate electricity to run Cassini. These power generators are of the same design as those used on the Galileo and Ulysses missions.
Equipment for a total of twelve science experiments is carried onboard the Cassini orbiter. Another six fly on the Huygens Titan probe, which will detach from the orbiter some four to five months after arrival at Saturn.
The Cassini orbiter advances and extends the United States' technology base with several innovations in engineering and information systems. Whereas previous planetary spacecraft used onboard tape recorders, Cassini pioneers a new solid-state data recorder with no moving parts. The recorder will be used in more than twenty other missions both within and outside NASA.
Similarly, the main onboard computer that directs operations of the orbiter uses a novel design drawing on new families of electronic chips. Among them are very high-speed integrated circuit (VHSIC) chips developed under a U.S. government-industry research and development initiative. Also part of the computer are powerful new application-specific integrated circuit (ASIC) parts; each component replaces a hundred or more traditional chips.
Elsewhere on the Cassini orbiter, the power system benefits from an innovative solid-state power switch being developed from the mission. This switch will eliminate rapid fluctuations called transients that usually occur with conventional power switches, with a significantly improved component lifetime.
The Huygens probe, supplied by the European Space Agency, carries a well-equipped robotic laboratory that it will use to scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan.
Released by the Cassini orbiter in late 2004, the Huygens probe will drop into Titan's atmosphere some three weeks later. As the 2.7-meter-diameter (8.9-foot) probe enters the atmosphere it will begin taking measurements in the haze layer above the cloud tops. As it descends -- first on a main parachute and later on a drogue chute for stability -- various instruments will measure the temperature, pressure, density, and energy balance in the atmosphere.
As the Huygens probe breaks through the cloud deck, a camera will capture pictures of the Titan panorama. Instruments will also be used to study properties of Titan's surface remotely -- and perhaps directly, should the probe survive the landing.
Many scientists theorize that Titan may be covered by lakes or oceans of methane or ethane, so the Huygens probe is designed to function even if it lands in liquid. If the battery-powered probe survives its landing, it will relay measurements from Titan's surface until the Cassini orbiter flies beyond the horizon and out of radio contact.
Hundreds of scientists and engineers from 14 European countries and 32 states of the United States make up the team designing, fabricating and flying the Cassini-Huygens spacecraft.
In the United States the mission is managed by NASA's Jet Propulsion Laboratory in Pasadena, California, where the Cassini orbiter is also being designed and assembled.
Development of the Huygens Titan probe is managed by the European Space Technology and Research Center (ESTEC). ESTEC will use a prime contractor in southern France, with equipment supplied by many European countries; the batteries and two scientific instruments will come from the United States.
The Italian Space Agency is contributing the Cassini orbiter's dish-shaped high-gain antenna as well as significant portions of three science instruments.
Communications with Cassini during the mission will be carried out through stations of NASA's Deep Space Network in California, Spain, and Australia. Data from the Huygens probe will be received at an operations complex in Darmstadt, Germany.
At JPL, Richard J. Spehalski is Cassini project manager. Dr. Dennis Matson is Cassini project scientist.