Cassini will get another gravity assist from Earth on Aug. 17 and then its on to a Dec. 30, 2000 encounter with Jupiter, whose massive gravity will serve as the final slingshot to send Cassini into orbit around Saturn beginning July 1, 2004. By the time it reaches the ringed planet, the four gravity assists will have provided the helpful equivalent of 75 tons of rocket fuel.
During its visit to Saturn, Cassini will be flying over, under and in between its largest rings and sending a probe to the surface of Saturns intriguing moon, Titan. It will have at least 30 encounters with Titan, one of the few moons in the solar system that scientists believe could support some kind of primitive life, because it's the only moon with its own atmosphere.
The mission begins 19 days before the spacecraft arrives at the planet when Cassini flies by Phoebe, Saturns outermost moon. Of all the moons in the solar system, Phoebe has to be one of the strangest. Colored red, Phoebe circles Saturn in the opposite direction to the 17 other moons, hinting it isnt a moon at all but a comet or asteroid caught by Saturns gravity at some distant point in the past.
Cassini has scheduled encounters with most of the other moons, and for good reason. MiMas is dimpled by an enormous crater named Herschel, the clear sign of an impact that came close to breaking MiMas in two. The moon lapetus has a dark side and a bright side. Enceladus has a bright frozen surface where ice geysers appear to be supplying ice to Saturns rings to replenish them. Dione and Tethys have little companion moons that orbit along with them. Helene is in front of Dione; Calypso and Telesto accompany Tethys on either side. Tethys itself features a canyon called Ithaca Chasma that goes three quarters of the way around the moon. What moonshaking event could possibly have formed this canyon?
In the four years it will explore the Saturnian system, Cassini will visit Titan almost every time it circles the planet. Whats under those thick orange clouds? Why are the clouds even there? On one pass, Cassini will drop a probe built by the European Space Agency and named Huygens. As it uses its two parachutes to descend 90 miles to the surface of Titan, it will analyze the chemistry of Titans atmosphere. Scientists know Titan has a thick atmosphere, which is unusual for a moon. They also think the atmosphere is full of nitrogen and may be similar to early earth. Scientists theorize the surface is an ocean of liquid methane or ethane. Theres a microphone on the probe, to send back the sound of touchdown. Or should we say splashdown?
Thomas OToole is a freelance writer and former editor of Aerospace America magazine who covered space during much of his 21 years at the Washington Post.
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