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King of the Rings; Cassini Spacecraft Changing Scientists' Outlook on Saturn

Posted on: Wednesday, 21 July 2004, 06:00 CDT

When Galileo first turned his telescope to Saturn, he described the world as being a "planet with ears."

He was puzzled that subsequent observations revealed the "ears" changing shape, disappearing, and then returning. What was the mystery that this distant world had that made it so different than Mercury, Venus, Mars and Jupiter, the only other known planets at the time?

This mystery, of course, was the fantastic ring system that orbits the planet.

As Saturn moves through its roughly 30-year orbit, our perspective on the rings changes. At times we see them tilted sharply toward us, revealing their splendor, and at other times we do not see the rings at all, for they are so thin (less than a kilometer) that they cannot be resolved from Earth.

This explains why the "ears" seemed to change through Galileo's crude instrument.

Although three other planets in our solar system have rings -- Jupiter, Uranus and Neptune -- Saturn's are certainly the most dramatic and have been a prime target for study by astronomers.

Even before our first robotic missions arrived at Saturn, telescopic observations had already begun to hint that the rings, about 250,000 km. across, were more dynamic than originally thought, possessing gaps and structure.

The two Voyager spacecraft expanded our understanding, showing us that a series of thousands of individual ringlets, each with its own orbit, formed the ring system of Saturn. These ringlets, in turn, are composed of billions of small rock and ice particles, from snowflakes to good-sized boulders.

In addition, we found that there were ringlets that crossed over each other, small moons embedded within ring gaps, and unusual dark "spokes" that seemed to move around the rings independent of the ringlets' own orbital velocity.

Clearly, there was more here than meets the eye. But Voyagers 1 and 2 had only a short while at Saturn and limited capabilities. We would have to return, this time armed with better equipment and more time.

Enter Cassini-Huygens.

A joint project of the National Aeronautics and Space Administration, the European Space Agency and the Italian Space Agency, this $3.2 billion project is the largest and most complex probe ever built.

Launched in October 1997, the Cassini orbiter and Huygens probe finally achieved orbital insertion at Saturn on June 30.

What follows is an ambitious four-year study of the Saturnian system, with more than 76 orbits of the planet and 45 flybys of the moon Titan.

The objectives of this mission are many, to be sure, but even after just three weeks, we have witnessed and gained a surplus of rich science concerning Saturn's rings.

With instruments that have a resolution 100 times greater than the Voyager probes, Cassini has provided clues to the particulars of ring structure and origin.

By studying the rings with the probe's ultraviolet imaging spectrograph, scientists have already learned that the rings seem to have a natural sorting mechanism. There is more ice in the outer parts of the ring than in the inner.

Further, Cassini has revealed that the size of the ring particles is not random: They tend to get larger moving outward from the planet.

As scientists learn more about the physics of ring dynamics, they can expand those models into theories of star and planet formation throughout the galaxy.

I would be neglectful if I did not mention the showstopper for this mission. On Christmas of this year, the Huygens probe will separate from the Cassini orbiter for a three-week coast to the moon Titan. Experts in many fields are excited to have this opportunity to study this world, as it is surrounded by a thick blanket of nitrogen and is rich in hydrocarbons.

This rich environment of organic chemistry gives scientists a chance to study pre-biotic processes that led to the development of life on Earth.

On Jan. 14, 2005, Huygens is scheduled to arrive at Titan and parachute through the atmosphere, taking pictures and analyzing conditions. If it survives the roughly two-hour descent, it is equipped to conduct further measurements on the surface.

In only its first three weeks, the Cassini-Huygens mission has already provided clues and insights that will rewrite our understanding of our own solar system. It is difficult for us to envision what other fantastic science will be learned in the upcoming months and years.

We are truly fortunate to be living in a time of such discovery, and it is with anticipation and imagination that we wait for that next step in our understanding.

Thomas R. Webber is director of the Heritage Planetarium in Blount County. If you have a question about an astronomical happening, email him at skyguy@blountk12.org.

MISSION HISTORY

* Oct. 15, 1997 Cassini-Huygens launches from Cape Canaveral Air Force Station in Florida

* April 26, 1998 Flies by Venus, picking up a boost from the planets gravity

* June 24, 1999 Passes Venus again, getting another gravity assist

* Aug. 18, 1999 Gets a third celestial push when it flies by Earth

* Dec. 30, 2000 Flies by Jupiter, snapping photos and getting a final boost; with Galileo still orbiting the planet, its the first time two spacecraft have explored the gas giant simultaneously

* June 11, 2004 Flies by Saturns moon Phoebe, uncovering evidence that the moon may be a frozen artifact of a bygone era, 4 billion years ago

UPCOMING EVENTS

* Oct. 26 First close flyby of Titan

* Dec. 25 Huygens probe to separate from Cassini

* Jan. 14, 2005 Huygens to descend into Titans atmosphere (descent begins 5 a.m. EST).

* Aug. 1, 2005 Mimas flyby

* Sept. 23, 2005 Tethys flyby

* Sept. 25, 2005 Hyperion flyby

* Oct. 10, 2005 Dione flyby

* Nov. 25, 2005 Rhea flyby

* Dec. 3, 2007 Epimetheus flyby

Source: NASA

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