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Unmanned US Spacecraft Approaches Saturn's Largest Outer Moon - 2004-06-19


An unmanned American spacecraft flew past Phoebe, the largest outer moon of Saturn June 11. The fly-by of the Cassini space probe was a high point in the craft's seven-year journey to Saturn's orbit, which it will reach on June 30, the first craft to visit the planet since the two oyager craft swung past more than 20 years ago.

Friday's Phoebe fly-by was also an exciting moment for the 250-plus scientists on Cassini's international team, which is interested in everything from the origins and composition of Saturn's famous rings, to the atmosphere on Titan, the planet's largest moon.

With his flaming red hair, one might expect Kevin Grazier to be more interested in Mars the "red planet" than in Saturn. Yet for years Mr. Grazier has been one of the Cassini Mission's key scientists and planning engineers.

"We will spend four years in orbit answering many of the questions that the Voyager [missions] raised," he said. "When we had a fly-by like the Voyagers and before that, the Pioneers, they actually raised as many questions as they answered."

Some of the key questions Mr. Grazier wants answered have to do with the planet's beautiful rings.

"How were they formed? How old are they? We think they are fairly young," answered Kevin Grazier. "Some people think they are old. They [believe the rings] are caused by an incomplete accretion or a moon that tried to form and just didn't quite form. That's probably not the case, but it's on the table as one possibility. Another possibility is it is a moon that shattered fairly recently."

Mr. Grazier adds that, if that theory is proved, Saturn's rings may turn out to be a mere one hundred million years old.

"For the age of the solar system, that's fairly young," he said. "But we don't know! We are going there to find out! Of what are they composed? Mostly ice. What else? We don't know. We are going there to find out!"

Mr. Grazier says that knowledge about Saturn's rings and their structure may help scientists to understand other parts of the universe as well and at many levels of scale.

"We have here a large collection of particles orbiting a central gravitational mass," continued Kevin Grazier. "Well, that can be considered also a paradigm or a model for an asteroid belt or a galaxy or even a cluster of galaxies, whenever you have large amounts of objects orbiting a central mass. So by understanding the inner rings, we can scale this up to galaxy-sized phenomena."

The Cassini spacecraft will make 45 approaches to Titan, by far the largest of Saturn's 31 known moons.

"Titan is the second biggest moon in the solar system, larger than the planet Mercury," he said. "And it is the only moon in the solar system with an appreciable atmosphere. The atmosphere is mostly nitrogen. But what else? It has clouds that we have yet to see through, at least not well. The clouds are bright orange composed of hydrocarbons. We think that this atmosphere on Titan is similar to Earth's atmosphere [as it was] 3.8 billion years ago - the atmosphere that gave rise to life on Earth. So, in essence, we think studying Titan is essentially a study of Earth a long time ago and we are looking at a pre-biotic or pre-life Earth. Another phrase you'll hear about Titan is 'the early Earth in a deep freeze.'"

When the Cassini probe actually drops into an orbit around Saturn on June 30, it will begin a research phase of its mission that will last into 2008.

Kevin Grazier:"We will be performing our orbit insertion right over the ring system looking at the rings up close. Because for the rest of the mission we avoid the rings, because they are a collision hazard and can damage the spacecraft. And we will also be looking at Saturn's magnetic field with other instruments. Because again, this is the closest we get. This is our best opportunity to get measurements of the magnetic field up close and personal."

Adam Phillips: What is the emotional tone of the mission right now as you start to hit this climactic moment?

Kevin Grazier:"Right now, we feel like the guy sitting on the bench for seven years saying 'put me in, Coach!' Well, now we're in the game and it's 'show time!'"

Kevin Grazier is a science planning engineer and scientific investigator for the Cassini-Huygens Mission to Saturn and Titan.

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