Galileo encounters intense dust storm.

Just 2 months before its long-awaited rendezvous with Jupiter, the Galileo spacecraft continues to plow through the most intense interplanetary dust storm ever measured. Scientists first learned of the storm on July 28, when the craft relayed data from its dust detector, says Carol Polanskey of NASA's Jet Propulsion Laboratory in Pasadena, Calif. The storm has a higher density of dust than the two other storms encountered by Galileo, last December and March.

The amount of dust found during the current storm has varied enormously from day to day. But at the storm's peak a few weeks ago, Galileo detected 20,000 dust particles per day, much more than the typical interplanetary rate of one particle every 3 days, notes Eberhard Grun, principal investigator of the dust detector and a researcher at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.

Estimates of the size and velocity of the dust particles vary. According to one model, they have a diameter of about one-tenth of a micrometer--roughly the size of smoke particles--and a velocity of 40 kilometers per second. But 1992 data from the Ulysses craft suggest that the particles are one-tenth that size and four to five times that speed. Even at the higher speed, the particles are too small to damage the craft, scientists note.

Polanskey says the dust appears to emanate from Jupiter's vicinity. "The [dust detector] spins with the craft, and it only sees the dust when it points at Jupiter," she says. Researchers suggest that the dust may originate from a combination of three sources: Jupiter's two faint dust rings, volcanic eruptions on Jupiter's moon Io, and debris left over from the collision of Comet Shoemaker-Levy 9 with Jupiter last year. However, the Ulysses craft detected significant amounts of dust near Jupiter in 1992, before the comet fragmented.

Sometime after Dec. 7, when Galileo begins a 2-year tour of Jupiter and its moons, scientists will probably acquire data that can pinpoint the source of the dust storms. For instance, whenever Io and Jupiter lie on opposite sides of the craft, Galileo will be able to determine whether the dust comes from the volcanically active moon or the giant planet.

In the meantime, notes Polanskey, Galileo may encounter an even greater storm next month, when the craft enters Jupiter's magnetosphere. In this huge region surrounding Jupiter, the planet's magnetic field overwhelms the sun's.