An artist's concept of Galileo nearing its close encounter with Io. Jupiter is seen in the background. Image: NASA TV/Spaceflight Now

NASA's Galileo orbiter will dart past Jupiter's moon Io on Thursday in the veteran spacecraft's last and closest flyby of any of the giant planet's four major moons.

Io's volcanoes have presented many surprises since they were first seen in 1979 by NASA's Voyager spacecraft and especially during the six years that Galileo has been orbiting Jupiter. Scientists hope this week's encounter will reveal how several regions of Io have changed over the years.

"Galileo's days are numbered now, so it's especially exciting to visit Io one last time," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "An orbital mission like Galileo gives you the advantage of getting to examine interesting places repeatedly over a period of time. That's been great for studying Io, since it keeps changing so much."

The Galileo flight team at JPL aimed the orbiter to skim just 100 kilometers (62 miles) above Io's multicolored surface at 14:09 Universal Time (9:09 a.m. EST) on Jan. 17. "The reason we're going so close is to put Galileo on a ballistic trajectory for impact into Jupiter in September 2003," Theilig said.

Galileo has operated in orbit more than three times longer than its originally planned mission. The resilient spacecraft has survived about three and a half times as much exposure to radiation from Jupiter's radiation belts as it was designed to withstand. In its 33 loops around Jupiter, it has flown near Io six times previously and near the other three of Jupiter's planet-sized moons -- Europa, Ganymede and Callisto -- a total of 27 times.

The tour has relied on expert navigators to calculate several moves in advance, using each moon's gravity to help adjust the spacecraft's trajectory toward its various encounters.

However, the propellant supply needed for steering the spacecraft and keeping its antenna pointed toward Earth is now nearly exhausted. To avoid even a slim chance that Galileo could crash into Europa after its mission ends, NASA has decided to send it to a controlled demise in the crushing pressure of Jupiter's dense atmosphere. Galileo had earlier found evidence that Europa has a deep ocean of melted saltwater under its frozen surface, heightening interest in keeping Europa pristine for later studies of its potential for harboring extraterrestrial life.

Before its final plunge, Galileo will make the first close flyby of Amalthea, a small, inner moon of Jupiter, in November 2002.

This week, Galileo will make direct measurements of the charged particles and magnetic environment around Io. Also, its camera and instruments for infrared and thermal imaging have been programmed to make observations during the flyby. As much of the data as possible will be transmitted to Earth from the spacecraft's tape recorder in coming months, Theilig said.

Io, like Earth's Moon, always keeps the same side facing inward toward its planet. On Thursday, Galileo will be in position for its best-ever look at the Jupiter-facing side of Io. "We're hoping to see areas we haven't seen well since Voyager imaged them back in 1979," said JPL's Dr. Torrence Johnson, Galileo project scientist. "We'd like to know more about rates of change for volcanic features on Io." New observations are also planned for a previously inactive volcano that unexpectedly lofted a tall plume last summer.

On this swing through the inner portion of the Jovian system, Galileo will also examine storms on Jupiter itself and the Io torus, a doughnut-shaped band of charged particles encircling Jupiter at Io's distance from the planet.

A sporadic malfunction has affected performance of Galileo's camera since mid-2000, apparently due to radiation damage to an electronic component. The camera worked flawlessly during the most recent Io encounter in October 2001, but each time Galileo swings as close to Jupiter as Io's orbit, odds increase for more serious damage to the spacecraft from exposure to the planet's radiation belts.

Io is the innermost of Jupiter's four large moons. Heat from tidal flexing powered by Jupiter's gravitational pull makes it the most volcanically active world in the solar system, with an estimated 200 to 300 volcanoes rapidly resurfacing it.

Galileo left Earth aboard the space shuttle Atlantis in 1989. JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.