Jubilance As Craft Reaches Saturn

Seven years after launch on a four-planet gravitational bank shot covering more than 2 billion miles, NASA's $3.3 billion nuclear-powered Cassini probe - the most sophisticated robotic spacecraft ever built - has finally reached the solar system's most spectacular target: The ringed planet Saturn.

Kicking off a four-year orbital study of nature's own "lord of the rings," Cassini flew past the strange moon Phoebe June 11, passing within 1,285 miles of the tiny satellite. Seen as a mere speck of light from Earth and a blurry orb from the Voyager 2 probe in 1981, Phoebe was revealed as an icy, heavily cratered world that defies easy explanation, reports CBS News Space Consultant Bill Harwood.

The Phoebe flyby was a priceless opportunity to study what may be a captured chunk of debris left over from the birth of the solar system. But it was just the opening act in one of the most scientifically rich voyages of planetary exploration ever attempted.

"It is really, truly the flagship mission of our time," said Carolyn Porco, chief of Cassini's imaging team and an authority on Saturn's rings. "We are exploring the richest planetary system available to us with this magnificent spacecraft, which is the most sophisticated suite of instrumentation ever taken into the outer solar system. So we are going to discover many things.

"And on top of all of that, Saturn is the most alluring of all the planets, it's the icon among planets and that's where we're going. ... I think that people, from the email I'm getting from the public, are very jazzed about this mission. They do very much have the feeling that they are stowaways on this spacecraft."

If all goes well, Cassini will brake into orbit around Saturn the night of June 30, firing its main engine for a nerve-wracking 96.4 minutes. Another 51-minute rocket firing in late August will raise the low point of Cassini's orbit and set the stage for a true voyage of discovery.

Equipped with state-of-the-art telescopes, an imaging radar system and a battery of other powerful instruments, Cassini will spend at least four years orbiting the sixth planet from the sun, studying its rings in unprecedented detail, making high-resolution movies of its windy atmosphere, charting its magnetic field and mapping a host of icy moons.

Saturn's largest moon, Titan, will get special treatment in January when the European-built Huygens probe now bolted to Cassini's hull makes a parachute descent to the surface through the moon's smoggy atmosphere.

Bigger than the planet Mercury, Titan is the only moon in the solar system with a thick atmosphere, one in which hydrocarbons fall as rain and liquid ethane pools on its ultra-cold surface.

"Imagine a world that's smaller than Mars and bigger than the planet Mercury, where the air is four times denser at its surface than the air in this room and the surface pressure is about the same as you'd experience at the bottom of a neighborhood swimming pool," said Jonathan Lunine, a University of Arizona physicist and a member of the Cassini science team. "On that world, the distant sun is never seen and at high noon, things are no brighter than a partly moonlit night on the Earth.

"What I have described to you is Titan, the second largest moon in the solar system, nearly the largest. It was partly revealed to us by Voyager 1 in 1980. Through its many instruments, Voyager discovered and characterized a dense atmosphere around this cold world. Yet ... Voyager's cameras could not penetrate the organic haze and so we still do not know what awaits Cassini-Huygens at the end of its journey."

"Titan is almost certainly not the home of life today," Lunine said. "But the organic chemical cycles that go on may constitute a chemical laboratory for replaying some of the steps that led to life on Earth. Titan is in some ways the closest analog we have to the Earth's environment before life began and this makes Titan very important."

The Cassini mother ship, meanwhile, will fly through 76 ever-changing orbits of Saturn over the next four years, using the gravity of Titan to warp its trajectory, setting up subsequent encounters. Cassini will make more than 50 close passes by seven of Saturn's 31 known moons.

"Probably the most important thing that our generation can do is to understand the evolution of life in our solar system and throughout the universe," said Charles Elachi, director of the Jet Propulsion Laboratory and leader of the Cassini radar experiment. "In a sense, Cassini will write one of those chapters ... in the book of how life evolved in our universe."

And throughout the voyage, Cassini's instruments will study Saturn itself, its rings and its space environment.

"Saturn, its rings and its moon system hold clues to understanding the origin of our solar system," said Wesley Huntress, NASA's associate administrator for space science at the time of Cassini's launch. "Its rings are a system are not too unlike the early solar nebula out of which our own planets formed. The processes that sustain that ring may yield information on processes of planetary formation and this information in turn can help us understand what happens around other stars as well."

Cassini is the most ambitious - and expensive - interplanetary project ever attempted, eclipsing even the dual Mars Viking orbiters and landers in inflation-adjusted total cost.
Equipped with three nuclear power sources, 12 instruments, multiple radios, a pair of digital data recorders, two primary computers and more than 50 other subcomputers, Cassini is a marvel of late 20th Century engineering.

Standing 22 feet tall and 13 feet wide, the six-ton spacecraft features a 13-foot wide communications and radar antenna, a 40-foot-long magnetometer boom, 22,000 wire connections, 7.5 miles of electrical cabling, 82 radioisotope heater units to keep its internal systems warm, 16 hydrazine thrusters for coarse attitude control, four reaction wheels for finer, gyroscopic attitude control and two main engines (one is a backup) for major course changes.

Said Lunine: "We've never sent a spacecraft with the kind of instrumentation, the variety and number of instruments and the power of the instruments to the outer solar system prior to Cassini.

"It's really a tour de force," he said in an interview. "And it's going to be, if all goes well, an incredibly exciting four years in the Saturn system. I think we'll learn that this is more than just a solar system within a solar system. There are just things unique about the Saturn system - the rings, Titan, other aspects - that will make it an incredibly interesting place to explore. ... Cassini should really knock our socks off."