An artist's concept of Galileo flying over Io. Image: NASA TV/Spaceflight Now

NASA's Galileo spacecraft will buzz the north pole of Jupiter's moon Io early next week to get unprecedented magnetic measurements and examine the site of a dramatic recent volcanic eruption.

The durable robot will skim about 200 kilometers (124 miles) above Io's surface at 12:59 a.m. EDT (0459 GMT) on Aug. 6. A few seconds later, Galileo will speed over an area that was belching a giant plume of volcanic gases seven months ago. The spacecraft will be flying at a lower altitude than the top of the plume, creating the possibility that Galileo will fly right through a volcanic plume for the first time.

"Plumes in the polar regions of Io appear to be infrequent and short-lived, so we don't know whether this one will still be there or not," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The Galileo flight team at JPL chose the polar path of this flyby because magnetic readings above the pole might reveal whether Io generates a surrounding magnetic field of its own, like the magnetic field around Earth. Determining that would add to understanding of Io's hot interior, said Dr. Torrence Johnson, Galileo project scientist at JPL.

"All of our previous magnetic measurements at Io have been on equatorial passes, and from those we can't tell whether the field at Io is induced by Jupiter's strong magnetic field or produced by Io itself," Johnson said. Polar measurements may give enough additional information to distinguish between those two possibilities.

As a side benefit, the path will take Galileo directly over a dynamic volcano named Tvashtar. In November 1999, Galileo imaged an active "curtain of fire" eruption at Tvashtar. The volcano was hurling magma more than 1.5 kilometers (1 mile) high. The eruption had become much less violent and had shifted to a different part of Tvashtar by Galileo's next flyby, three months later. Then, a tenuous gas plume from Tvashtar was discovered from images taken by Galileo and by NASA's Cassini spacecraft within a few days of when Cassini passed Jupiter on its way toward Saturn on Dec. 30, 2000. The plume rose as much as 385 kilometers (239 miles) high. Where sulfurous material from it fell back to the surface, it created a red ring about 1,400 kilometers (870 miles) in diameter.

If the plume is still active and the same size, Galileo will fly through about the top quarter of it. The project's scientists and engineers estimate that at that altitude the plume material is very thin gas without particles large enough to penetrate the spacecraft.

Whether the plume is active or not, Galileo will look for changes in the Tvashtar region. "We'll be trying to figure out just where the plume erupted from," Johnson said.

As Galileo passes Io, it will be out of communication with Earth. NASA's Deep Space Network, which provides the communication link for interplanetary spacecraft, has a large antenna temporarily out of service in Spain, the only one of the network's three sites that will have Jupiter in view above it during the flyby. The antenna is being upgraded to help handle an increased number of missions that will need communications in 2003 and 2004. Confirmation of Galileo's status during the flyby will not be received until several hours afterwards. Images and other data from the flyby will be returned gradually over the following two months.

Io is the innermost of Jupiter's four largest 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 has been orbiting Jupiter in elongated loops since 1995. Four of its previous 31 close flybys of Jupiter's moons have been by Io. It will swing near Io twice more after next week, once in October and again in January, then near the small inner moon Amalthea once before a mission-ending plunge into the crushing pressure of Jupiter's atmosphere in 2003. Galileo's mission was originally scheduled to end in 1997, but has been extended three times to take advantage of the spacecraft's durability.

The orbiter has survived more than three times the cumulative radiation exposure it was designed to withstand. Some electronic components have been affected by the radiation, and each loop near Jupiter increases the odds of more serious damage from exposure to the radiation belts around the planet, Theilig said. Engineers have developed some new strategies for attempting to minimize the loss of images caused by an intermittent problem that has affected Galileo's camera since last summer.

JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.