The U.S. Galileo spacecraft will take its final swing past Jupiter's colorful moon Io Thursday, its last and closest pass by any Jovian moon since its arrival at the giant gas planet six years ago. The flyby is designed to set the planetary probe on a course to its destruction.

Since 1996, the itinerant Galileo space probe has circled Jupiter 33 times. It has also flown six times near Io, the biggest and innermost of the four planet-sized moons, and near the other three - Europa, Ganymede, and Callisto - a combined total of 27 times.

Now the Galileo mission is winding down. The fuel needed to steer it is almost exhausted. So engineers at the U.S. space agency's Jet Propulsion Laboratory in Pasadena, California are putting it on a trajectory to plunge into Jupiter to its demise in September 2003.

To do this, they are commanding it to swing just 100 kilometers past Io. Galileo project manager Eilene Theilig says the controlled trajectory will ensure that Galileo does not go where it is not welcome. "So just like an airplane that's low on fuel, we want to make sure we can put Galileo in a safe place before we lose control," she says. "This is important because we don't want to inadvertently hit Europa, a moon that Galileo discovered had water beneath the surface. We don't want to take the risk of contaminating that moon."

The space agency will take advantage of Galileo's proximity to Io to measure the charged particles and magnetism around it and take a final look at its surface features.

Scientists are especially interested in the moon's volcanoes. Io is the most volcanically active body in the solar system, with an estimated 200 to 300 volcanoes spewing lava as high as 350 kilometers or more. Eilene Theilig says the great heat is caused by the moon's flexing induced by Jupiter's huge gravitational pull. "There are large lava flows that we haven't seen form on the Earth or any of the other planets in over a billion years," she says. "Sulfur is a significant component in the volcanism. It might not be very pleasant to stand on the surface. It's very colorful, however. It's yellow and red and green and brown, all different varieties of sulfur and some frozen sulfur dioxide fields."

Galileo has proven to be durable. It has operated more than three times longer than originally planned, surviving exposure to Jupiter's radiation belt longer than expected.

The radiation has apparently taken some toll, for Galileo's camera has malfunctioned occasionally for the past year-and-a-half. But scientists hope it works well for the Io flyby and the first close pass to the small inner moon of Jupiter, Amalthea, in November.