A comet's fiery finale.

Broken into some 20 icy fragments that line up like pearls on a string, Comet Shoemaker-Levy has dazzled astronomers ever since its discovery in late March (SN: 4/10/93, p. 231). But its expected demise next year promises to be even more spectacular: a fiery death in Jupiter's atmosphere that could release as much energy as a billion megatons of TNT.

Researchers say they are now certain that last July, Jupiter's gravity tore apart the comet, which is currently about 49.5 million kilometers from the center of the giant planet. Moreover, the fragments have a velocity just under that needed to escape the planet's tug. So, on or about July 22, 1994, scientists calculate, the fragments of Shoemaker-Levy will likely crash into the Jovian atmosphere at a speed of 60 kilometers per second. If the biggest pieces measure about 10 kilometers across, they may create a cataclysm so powerful it would rival the impact suspected of wiping out the dinosaurs on Earth, reports one of the comet's discoverers, Eugene M. Shoemaker of the U.S. Geological Survey in Flagstaff, Ariz.

Alas, notes Shoemaker, next July's Jovian fireworks won't be seen from Earth, since the comet's fragments are expected to strike the planet's nightside. However, the sites of impact-thought to lie in Jupiter's southern hemisphere - should rotate into view a few hours later.

Zdenek Sekanina, a comet specialist at the Jet Propulsion Laboratory (jPL) in Pasadena, Calif. says dust created by the explosions could temporarily increase the amount of sunlight reflected from Jupiter's atmosphere, an effect perhaps detectable from Earth. He likens the phenomenon to the increase in nighttime brightness reported soon after an unidentified body, now known as the Tunguska object, struck Siberia in 1908.

Writing in the June 10 NATURE, Clark R. Chapman of the Planetary Science Institute in Tucson notes that the amount of sunlight reflected from jupiter's moons may also increase after Comet Shoemaker-Levy's fiery finale. But he adds that Jupiter's cloudy atmosphere might prevent researchers from glimpsing such an increase. Other scientists suggest that the explosion could create a lasting disturbance, similar to the planet's Great Red Spot.

Paul Chodas and Donald K. Yeomans of the JPL calculate that the comet wiII approach within 38,000 kilometers of Jupiter's center of mass -well within the 75,000-kilometer radius of this gaseous planet. In fact, notes Chodas, the fragments will burn up long before they reach that depth. Sekanina estimates that massive fragments will burn up a few kilometers below the visible cloud tops of jupiter, while lighter fragments might disintegrate 100 kilometers or so above.

Using different coordinates for the position of Jupiter, Brian G. Marsden of the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Mass., calculates that the comet will enter the Jovian atmosphere closer to July 25 and that its orbit will take it in within 45,000 kilometers of the planet's center. Marsden agrees that the comet will hit on Jupiter's nightside. Another comet researcher, Andrea Carusi of the Istituto di Astrofisica Spaziale in Rome, Italy, concurs.

Chodas says he is "more than 95 percent certain" that Shoemaker-Levy will plunge into Jupiter. Says Gareth Williams, who works with Marsden at the SAO: "It's very unusual to have such agreement between these groups of researchers: