Eying the impacts from Earth and space.

Telescopes in the Southern Hemisphere have the best chance of capturing any reflected flashes from Shoemaker-Levy 9. That's because Jupiter is visible for 4 to 5 hours a night there and about 2 to 3 hours in the north.

Paul Chodas and his JPL colleagues Donald Yeomans and Zdenek Sekanina calculate that one of the collisions, on July 19, will occur just as Jupiter's moon Europa undergoes an eclipse. The most optimistic picture has the fireball from this impact illuminating Europa so brightly that observers in Australia and New Zealand may need only a backyard telescope to see this 1-minute event.

Observers should also keep their eye on Io, says Peter J.T. Leonard of the University of Maryland at College Park. Because of the moon's large size and proximity to Jupiter, astronomers should monitor this Jovian satellite each time it's in eclipse between July 15 and July 22.

While calculations indicate that none of the kilometer-size fragments will hit while Io is in eclipse, smaller pieces -- roughly the size of a football field--may be spread throughout the train of Shoemaker-Levy 9 fragments, suggests Leonard. "When you break a cookie, you get some crumbs as well as bigger pieces," he says. Leonard estimates that the meteoric flash from one of these 100-meter "crumbs" may brighten Io in eclipse from complete invisibility to a level an amateur astronomer could see with a 12-inch telescope.

The most favorable sites for viewing the reflected fireballs include Africa, New Zealand, Australia, and Hawaii (see chart, p.27), Chodas says. Some astronomers will fly from one site to another, since no one place offers a vantage point for all 21 known fragments. On average, observers at any one Southern Hemisphere site may see the immediate aftermath of four or five hits, while a northern observer might detect only one or two.

Spectroscopy of the plumes--analyzing the intensity of specific wavelengths of light emitted by particular molecules and atoms -- could reveal the comet's composition and the temperature of the fireballs. But if the explosions occur beneath Jupiter's visible cloud tops, there may be too little light to do such measurements.

When the impact sites rotate into view, some of Earth's largest telescopes will have the best chance of imaging the crash zones and making infrared temperature measurements. Amateur astronomers, however, will have difficulty detecting postimpact changes in the planet's cloud structure. "Unless you already know exactly what Jupiter looks like right before the impact, you're not going to notice any changes," says theorist Mordecai-Mark Mac Low.

Spectroscopic measurements may reveal chemical changes in Jupiter's upper atmosphere, including atoms and molecules dredged up by the fireballs. The cometary impacts may also introduce molecules never before found on Jupiter.

The Galileo spacecraft will have the best vantage point of any telescope in the solar system. Poised at about 246 million kilometers from the planet -- less than one-third the distance from Earth to Jupiter -- Galileo will look directly at the impacts with its high-resolution visible-light camera. Other instruments on the craft may track the entry of the comet fragments and the resulting fireballs at wavelengths ranging from the ultraviolet to the infrared.

Because Galileo's main antenna remains jammed, the craft can transmit to Earth only a few percent of the data it will store on its tape recorder. The slower transmission rate of its low-gain antenna and the limited time available on NASA's network of ground-based receivers means the craft will take weeks to months to return the collision data.

Complicating matters further, the craft received its commands to observe the impacts several weeks ago, when the collision times were known only within a window of about 40 minutes. To compensate for the uncertainty, the craft may begin observations 30 minutes before the time of each expected impact and continue for 30 minutes afterward.

"This is the most godawful experiment design problem I've ever run into," says Torrence Johnson of JPL.

Although the Voyager 2 spacecraft has an even better viewing angle than Galileo, the aging craft is now at the edge of the solar system, about 6.1 billion km from Jupiter. NASA scientists have calculated that it's not worth the effort to try to turn Voyager's camera back on. But the craft's detectors may gather radio and ultraviolet data from the impacts.

Ulysses, a spacecraft designed to study the poles of the sun, will lie about 378 million km from Jupiter next week. Ulysses doesn't carry a camera, but it can use its radio receivers to detect heat from the fireballs--once they rise above the radio noise of Jupiter's ionosphere.

Two Earth-orbiting missions devoted to ultraviolet work will study the aftermath of the collisions. Both the venerable International Ultraviolet Explorer satellite, now in its 17th year of operation, and the Extreme Ultraviolet Explorer craft, launched in 1992, will take spectra of the plumes of material carried aloft by the Jovian explosions.

While the Earth-orbiting Hubble Space Telescope won't see the impacts, it will track the fragments until about 6 hours before they hit Jupiter. And for hours to days after each crash, Hubble and ground-based telescopes will look for atmospheric ripples and seismic waves triggered by the explosions.