Illustration of Galileo at Jupiter

NASA's Galileo spacecraft is plowing through the most intense interplanetary dust storm ever measured as it closes in on Jupiter after a six-year journey to reach the giant planet, scientists report.

This is the latest and greatest of several large dust storms encountered by Galileo since December 1994, when the spacecraft was still almost 110 million miles (about 175 million kilometers) from Jupiter. The current storm has lasted more than three weeks. The spacecraft, launched in October 1989, is now about 39 million miles (63 million kilometers) from the planet. Galileo will enter orbit around Jupiter on December 7, 1995.

During the current dust storm, Galileo has counted up to 20,000 particles per day, compared to the normal interplanetary rate of about one particle every three days, said Dr. Eberhard Grun, principal investigator on the spacecraft's dust detector experiment.

The particles, scientists say, are apparently emanating from somewhere in the Jovian system and may be the product of volcanoes on Jupiter's moon, Io, or could be coming from Jupiter's faint two-ring system. The dust particles, probably no larger than those found in cigarette smoke, may also be leftover material from Comet Shoemaker- Levy, which impacted Jupiter last year.

Scientists believe the particles are electrically charged and then accelerated by Jupiter's powerful magnetic field. They have calculated that the dust is speeding through interplanetary space at between nearly 90,000 and 450,000 miles per hour (40 and 200 kilometers per second), depending on particle size. Even at such high speeds, these tiny particles pose no danger to the Galileo spacecraft, scientists say.

Galileo's dust detector, one of 10 instruments on the spacecraft, is about the size of a large kitchen colander. It counts particle impacts and observes their direction and energy. From these measurements scientists can estimate particle size and speed.

Grun, a scientist at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany, also has dust detectors aboard the Ulysses spacecraft that flew by Jupiter in 1992 on its way to study the Sun, and on the Cassini spacecraft scheduled for launch to Saturn in 1997. His team first discovered dust emanating from Jupiter using the Ulysses instrument in 1992.

The Galileo instrument first observed dust coming from Jupiter in June 1994. Although both Ulysses and Galileo were able to show that the dust storms seem to come from Jupiter, the intensity and timing of Galileo's recent storms differ from those detected by Ulysses.

Chances of understanding the nature of these dust storms are improving since, after the onset of the current storm, Galileo flight engineers commanded the spacecraft to collect and transmit dust data as often as three times a day, according to Dr. Carol Polanskey, team chief for the dust instrument subsystem at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. The normal collection rate had been twice per week.

In addition, the instrument was reprogrammed in July 1994, "to take advantage of the knowledge gained from the Ulysses experience and just in time to observe the start of the series of storms Galileo has seen," said Polanskey. The reprogramming also endowed the instrument with new data compression and other improvements, she added.

"This puts us in an excellent position to view the dust phenomena as Galileo moves toward Jupiter," she said. "We're looking forward to determining the source of the dust storms once we get into the Jovian system."

When Galileo arrives at Jupiter this December, it will receive and relay the data from the atmospheric probe, which was targeted toward Jupiter and separated from the main spacecraft in July. Galileo will then begin a two-year, 11-orbit survey of Jupiter, its satellites, magnetosphere and the dust environment.

JPL manages the Galileo project for NASA's Office of Space Science. Germany is a scientific and engineering partner in Galileo. JPL also manages the U.S. portion of the Ulysses project, a joint effort of NASA and the European Space Agency.

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