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Cassini Heads For Deep Space

NASA's Cassini space probe is leaving the inner solar system for good after streaking 725 miles above the South Pacific Ocean in a long-awaited - and in some quarters, feared - velocity boosting flyby.

To reach the planet Saturn the nuclear-powered probe was launched toward Venus on Oct. 15, 1997 and used the cloudy planet's gravity in April 1998 and again on June 24, to pump up its velocity sending it back to Earth for a third and final gravity assist.

The Earth flyby served to bend Cassini's trajectory and send the spacecraft on to a close encounter with mighty planet Jupiter in December 2000. If all goes well the most complex and expensive interplanetary spacecraft ever built will brake into orbit around Saturn on July 1, 2004.

Over the next four years, the robotic explorer will study the planet's rings, its turbulent atmosphere and its many moons. Shortly after arrival, the Cassini mother ship will release a European-built probe that will parachute into the murky atmosphere of the enigmatic moon Titan, beaming back digital images of its unseen surface.

So far, the Cassini mission has proceeded without a hitch, program manager Bob Mitchell calls it "the cleanest, most problem-free flight of a spacecraft like this that we've had. It's a very complicated spacecraft but it's just a fantastic piece of engineering."

Artist's rendering of Cassini streaking past Jupiter.

Even so, activists opposed to the use of nuclear power in space had long argued that Cassini posed a threat to planet Earth in the event of a launch accident or an inadvertent re-entry during the flyby.

Cassini is powered by three radioisotope thermoelectric generators (RTG's) that use the heat produced by the decay of plutonium 238 to generate electricity. If a major launch accident or inadvertent re-entry had occurred, critics argued that some or all of Cassini's 71 pounds of plutonium 238 could have be released into the atmosphere.

NASA built the RTG's to withstand launch accidents intact and argued that the odds of an inadvertent re-entry were around one-in-one million. The second Venus flyby was set up so that Cassini would miss Earth by a wide margin in case of a malfunction.

Putting its time near Earth to good use, nine of Cassini's 12 science instruments have used the moon as a calibration target and will probe the environment in the far reaches of the Earth's magnetic field as it heads on to Jupiter.

"There are quite a number of observations planned ot in the magnetotail region some four weeks after closest approach, and that's genuine, unique science," Mitchell says.

He adds, "The combination of the instrumentation we have on board and the fact that we've never flown a spacecraft out to this region before gives us a chance to do some measurements the scientists are quite excited about."