Introduction

NASA's Cassini space probe left the inner solar system for good on August 17th, streaking 725 miles above the South Pacific Ocean at 11:28 p.m. EST 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 gravity assist this evening.

Close approach will occur over a point in the South Pacific and Cassini may be visible to observers on Pitcairn Island, Easter Island or others in the vicinity as it streaks overhead. The Earth flyby will bend Cassini's trajectory and send the spacecraft on to a close encounter with mighty 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.

The Controversy

Activists opposed to the use of nuclear power in space and have 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. In the event of a major launch accident or inadvertent re-entry, critics say, some or all of Cassini's 71 pounds of plutonium 238 could 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 will use the moon as a calibration target and probe the environment in the far reaches of the teardrop-shaped region defined by Earth's magnetic field as it heads on to Jupiter.

Quotes and pictures provided by CBS.com