It is Tuesday's approach that frightens the activists. Should Cassini pass too close to Earth and burn up in the atmosphere, they warn, radioactive plutonium in the generators that provide the craft's electricity could cause millions of cancer deaths. Most scientists and doctors scoff at such claims. Any plutonium vaporized in an accident, they explain, would be so diluted in the atmosphere that it would pose no real threat to most people. Still, activists say, had Cassini been equipped with solar panels for electricity, all danger could have been averted. But Saturn receives only a hundredth of the sunlight Earth does, and the solar panels needed to supply Cassini at that distance would have to be far too large for such a mission. Other than plutonium generators, says physicist James Van Allen, discoverer of Earth's radiation belts, "there is no practical source of electrical power for spacecraft that go to the outer planets." Controllers at the Jet Propulsion Laboratory report that Cassini, having already flown more than a billion miles, is in excellent shape. All systems are operating well, and the craft is on course for a flyby of home.
Closing in at 42,500 m.p.h., one of the largest and most complex spacecraft ever built will pass only 725 miles from Earth Tuesday on its way to a 2004 rendezvous with Saturn, its spectacular rings and its giant moon, Titan. The ship is Cassini, and while it's an object of pride for space scientists, it's an object of fear for antinuclear activists. Weighing in at around six tons at its launch in October 1997, Cassini lacked the rocket power to fly directly out to Saturn, which is on average 800 million miles from Earth. Instead it headed inward, swooping twice around Venus for "gravity assists" to increase its speed. Its encounter with Earth will boost its velocity further, and a flyby of Jupiter in 2000 will give the ship the final kick it needs to reach Saturn.