Ten or twenty years ago, interplanetary space probes were built like battleships: big, rugged, bristling with instruments—and costing a boatload of money. Although nasa has been phasing out such missions, only in October did it finally launch its last: the Cassini probe to Saturn.
By 2004, if all goes well, Cassini will park itself in orbit around Saturn and loop around and around, taking readings and snapping close-ups of the planet, its gossamer rings, and its 18 moons—the same sort of work its cousin Galileo is now doing at Jupiter (see story next page). Like Galileo, Cassini is two probes in one. While the main craft orbits Saturn, a second probe, the European-built Huygens, will detach and fall into the atmosphere of Titan, Saturn’s largest moon. Titan is a world unto itself: nearly as big as Mars, it has an atmosphere that astronomers think is laced with organic chemicals—the building blocks of life. There are only a few solid bodies in the solar system with thick atmospheres—Earth, Venus, and Titan, says planetary scientist Jonathan Lunine of the University of Arizona in Tucson. And Titan is the best model for the Earth prior to the time when life began.
Cassini also resembles Galileo in that it carries radioactive plutonium—72 pounds of the poisonous stuff, which provides heat to power the probe beyond Mars. The risk of that much plutonium being accidentally released into the atmosphere, either at launch or when Cassini flies by Earth for a gravity assist in August 1999, drew a great deal of protest, as did the 1989 launch of Galileo. But in a perfect replay, Cassini headed off into deep space without a serious hitch.