Nuclear-Powered Probe Begins Its 2 Billion-Mile Journey

The largest, most expensive, and most controversial interplanetary probe in the history of NASA blasted off yesterday without a hitch.

The mammoth Titan IVB rocket carrying the Cassini spacecraft rocketed through clouds into a moonlit sky well before dawn, and scientists and engineers embraced when Cassini shot out of Earth orbit 40 minutes after liftoff. The first phase of a seven-year, 2 billion-mile journey was now over.

Although the spacecraft's final destination is Saturn, it will go around Venus twice and Earth once for a gravitational acceleration before heading to the ringed planet.

Now that the probe has made it into space without any problems, it is this Earth flyby - which will bring Cassini within 500 miles of Earth in 1999 - that has Cassini opponents worried. With 72 pounds of plutonium aboard Cassini - the most ever flown in space - antinuclear activists fear that if things go wrong the plutonium could be released into Earth's atmosphere.

But there is a less than 1 in a million chance that the probe would reenter Earth's atmosphere and spread plutonium, says the Energy Department's Beverly Cook. The plutonium is needed to power Cassini.

Its $3.4 billion mission won't begin, scientifically, until the spacecraft reaches Saturn in 2004. If all goes well, the two-story robotic explorer will be the first probe to orbit Saturn, doing so 74 times from July 1, 2004 through 2008.

Cassini also will release a probe to land on frigid Titan, Saturn's largest moon and the second-largest moon in the solar system. Scientists say its cold, preserved state could provide clues as to how life evolved on Earth.