Cassini Probe Nears Saturn

Cassini, considered the most ambitious planetary space exploration project ever mounted, is close to its target. It is a NASA probe that's been traveling through space for the last seven years, and is now finally poised to give scientists an unprecedented view of one of out most distant and fascinating planets.

"We're about to start on a delicious smorgasbord of scientific opportunities before us," said Dr. Dennis Matson, a Cassini project scientist told CBS News Correspondent Drew Levinson.

The Cassini spacecraft has been listening to natural radio signals from Saturn, the most reliable method of determining a day's length. Cassini's transmissions show a complete rotation takes 10 hours, 45 minutes and 45 seconds, plus or minus 36 seconds, NASA said in a statement Monday.

That's about six minutes longer than measurements performed by the Voyager 1 and 2 spacecraft that flew by Saturn in 1980 and 1981. Observations made in France in 1997 also differed from the Voyager findings.

Cassini, due to enter Saturn's orbit Wednesday night, gathered radio data from April 29, 2003, to June 10, 2004. It will be about 12-thousand-five hundred miles above Saturn's cloud tops, closer than any other spacecraft in history.

"We all agree that the radio rotation period of Saturn is longer today than it was during the Voyager flyby in 1980," said Michael D. Desch, a member of the Cassini team and a scientist at NASA's Goddard Space Flight Center.

But scientists doubt the planet is rotating more slowly. They are looking instead for something deep inside Saturn that would cause variability in the radio pulse.

"I don't think any of us could conceive of any process that would cause the rotation of the entire planet to actually slow down," said Don Gurnett, a Cassini scientist who works at the University of Iowa.

Gurnett said there appears to be "some kind of slippage between the deep interior of the planet and the magnetic field, which controls the charged particles responsible for the radio emission."

Inside the Cassini is a probe called the Huygens, which will parachute through the atmosphere and land on and explore Saturn's largest Moon: Titan.

The seven-year voyage is just about over, now the next phase is set to begin.

For the next four years the Cassini Space Probe will check out Saturn from top to bottom.

"We're going to end up a four year tour with a bang, look down on Saturn and get a fabulous view," said Dr. Jeremy Jones, the navigation team chief.

One Scientist likened Cassini and Huygens to time machines - taking us back to examine a world that may resemble the Earth four and a half billion years ago.

In all, Cassini is expected to complete 77 orbits of Saturn over the next four years, requiring 157 trajectory-nudging rocket firings.
The gravity of Titan will be used for major course changes, with 45 planned flybys. Seven close flybys of smaller, icy moons also are planned.

But first, Cassini must execute the Saturn Orbit Insertion maneuver, or SOI.

To achieve orbit around Saturn, the 12,600-pound Cassini must reduce its velocity by about 1,400 mph using a rocket engine that only produces 100 pounds of push. As a result, the engine must fire for 96.4 minutes to put Cassini into the desired orbit.

If the engine shuts down early, the computer will switch to a spare. But the end result must be roughly the same - 96 minutes of braking - or Cassini might not be able to achieve its long-awaited mission.