During the predawn hours on Monday, a spacecraft named Cassini is scheduled to embark on a six-year voyage to Saturn, whose ringed profile has come to symbolize outer space.
As it thunders skyward atop a Titan IVB booster, the craft will represent an ambitious follow-up to the highly successful Pioneer and Voyager flybys of the planet between 1979 and 1981. It already has become the most controversial planetary mission to date, as proponents and critics square off over Cassini's use of plutonium oxide to supply it with electricity. The orbiter will carry the largest amount of nuclear material ever to go into space (see story, right).
Planetary scientists, tantalized by data from Pioneer and Voyager, designed the Cassini mission to study Saturn, its ring system, and its moons in detail over a four-year period.
"Cassini finds itself in a particularly potent scientific position today," says Larry Soderblom, a Cassini scientist with the United States Geological Survey in Flagstaff, Ariz. "Its timing could not be more perfect to address some of the most important questions of the day, namely, questions as to our origins - origins of life, origins of planets, origins of solar systems."
With its rings of ice and dust and its 18 known moons, Saturn is believed to hold clues to understanding a range of astrophysical actions, from the birth of solar systems to the dynamics of black holes - particularly their accretion disks, which form around the holes as their gravity draws in dust and gas.
Cassini also carries the European-built Huygens probe, which will plunge into the atmosphere of Saturn's largest moon, Titan. Larger than Mercury, Titan has a nitrogen-laden envelope of gases thought to resemble Earth's atmosphere just before organic life emerged some 4 billion years ago. Although Titan is thought to be too cold for life to have begun, it represents what one researcher calls "an organic chemist's dream" for studying the conditions thought to have existed on the early Earth.
Cassini's four-year period of observations begin July 1, 2004, when it is scheduled to arrive at Saturn. Yet even when it opens new frontiers of knowledge, Cassini also will close an era when missions to other planets carried billion-dollar price tags and took a decade or more to build and launch.
"Their contribution to our understanding of the solar system has been monumental," says Wesley Huntress, associate administrator for space science at the National Aeronautics and Space Administration, referring to missions ranging from Voyager, whose two probes are hurtling beyond the very edge of the solar system, to Galileo, now orbiting Jupiter.
"But we can't afford to do space science this way anymore," he adds, describing NASA's new emphasis on "diversifying risks" by spreading space-science objectives over a larger number of less expensive missions and a more diverse fleet of rockets.
Compared with the $155 million Mars Global Surveyor, Cassini's price tag is steep - $3.4 billion for a mission that from conception to final radio silence will take nearly 30 years to complete.
Hundreds of scientists and engineers in the US and Europe have devoted large portions of their careers to Cassini, working with a once-in-a-lifetime sense of urgency. Because no rocket is powerful enough to send Cassini straight to Saturn, mission planners will use the gravity of Venus, Earth, and Jupiter to give the craft the energy it needs to reach the ringed planet.
Cassini has only one launch period if it is to reach its full research objectives. Each delay would lengthen the travel time, consuming greater portions of the craft's limited fuel and design life just getting to Saturn. Delays much beyond 2001 would kill the mission, NASA officials say, since by 2010 Saturn's rings would be nearly edge-on to the sun, making observations very difficult. Moreover, the planets needed to hustle Cassini along will not be correctly aligned for another 175 years.
The ticking of the celestial clock aside, scientists working with Cassini have been urged on by intriguing data - taken from the ground as well as from the Hubble Space Telescope - gleaned since Pioneer and the two Voyager missions.
In 1989, signals bounced off Titan from a radio telescope at Goldstone, Calif., and picked up by the Very Large Array system at Socorro, N.M., gave the first hints that Titan's surface is not covered by a global sea of methane, as previously thought, but that it has solid areas as well. Using special filters, Hubble and the Keck telescope on Hawaii's Mauna Kea have peered dimly through thin spots in Titan's clouds to bolster the notion of solid regions.
The Huygens probe is designed to withstand a hard landing, and should be able to return images and other data from its landing site - wet or dry - during the brief period it is expected to survive after touching down.
In addition, the landing "gives us an opportunity to assess the organic chemistry of the lower atmosphere and surface," says Jonathan Lunine, a planetary scientist at the University of Arizona at Tucson. "We might find prebiotic 'experiments' under way, or the chemical remains of prebiotic reactions."
Titan's atmosphere may also exhibit a modest greenhouse effect on the moon, he says. And by studying craters and other formations on Titan, scientists may be able to test theories that in its early years, the sun was much fainter than it is today.
Larry Esposito, a planetary scientist at the University of Colorado at Boulder credited with discovering one of Saturn's concentric rings, holds that observations in 1995, when Saturn's rings appeared edge-on from Earth, yielded "exciting new data about the rings' makeup" that Cassini will enrich.