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                                                  Diana Steele
                                                  University of Chicago
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Jupiter-Comet Collision Will End In a Bang

PITTSBURGH -- When a comet collides with Jupiter in mid-July, astronomers will see a galactic light show and not an interplanetary dud as previously believed, according to computer simulations performed at the Pittsburgh Supercomputing Center.

Initial simulations had shown the comet -- known as Shoemaker-Levy 9 -- would die with a whimper, being softly caught by the largest planet in the solar system. But studies on Pittsburgh's CRAY C90 supercomputer by a University of Chicago astrophysicist indicate the now fragmented comet will explode after penetrating about 60 kilometers into the planet's atmosphere. The explosion will create a plume of superheated debris, shooting hundreds of kilometers above Jupiter's layered clouds.

"I was trying to predict the results of the impact so that observers could plan for the event," says astrophysicist Mordecai-Mark Mac Low. "I looked at things like how bright the flash of the explosion would be and how much material from beneath the Jovian clouds would be lifted above where it can be observed."

Astronomers not only will glean more understanding of the Jovian atmosphere by studying the plume, but they also will see what happens during a large planetary impact. For instance, scientists believe an asteroid struck Earth 65 million years ago, blanketing the planet with dust, smoke and ash, which blocked the Sun for months and wiped out the dinosaurs. The concern is that a similar impact will occur during the next 100 million years.

"It's the first time we've been able to predict a large planetary impact and then observe it," Mac Low says, "and it probably will be the only time in our lifetimes that an impact this large occurs."

The huge impact, comparable to thousands of hydrogen bombs, will occur even though the Shoemaker-Levy 9 comet no longer is intact: It got ripped into 21 fragments by Jupiter's gravity in July 1992 when it passed near the planet. The comet train, whose largest fragment is believed to be one kilometer in diameter, will smash into Jupiter between July 16 and 21.

Mac Low, along with Kevin Zahnle of NASA Ames Research Center, ran three types of computer simulations on Pittsburgh's CRAY C90: (1) the comet as it enters Jupiter's atmosphere until it explodes; (2) the initial fireball from the explosion; and (3) what happens in the atmosphere after the initial fireball. The simulations predicted that the flash from the explosion will last about a minute, with about as much brightness as the sunlit side of Jupiter. This is crucial because the comet will collide with the planet's back side, meaning the fireball will be visible to either NASA's spacecraft Galileo, which is on its way to Jupiter, or Earth-based telescopes as a reflection off one of Jupiter's moons.

Mac Low and Zahnle's simulations used a computational grid that's 10 times finer than simulations predicting the explosion will be contained in Jupiter's atmosphere. This suggests their results more closely approximate reality. To check their premise, the two researchers ran their computer code at lower resolution and got results similar to the other simulations.

At low resolution you get one result," Mac Low says, "and at high resolution you get another, and as you continue increasing resolution the result stays the same." The high degree of detail in Mac Low's study -- made possible by the C90 -- has stirred optimism among astronomers that they will have a good show to watch. Mac Low and Zahnle formally announced their findings in May at the American Geophysical Union meeting in Baltimore.

A beneficial side-effect of Mac Low's study is his finding that the numerical simulations agree well with an analytical model called the pancake model. This model assumes that once the aerodynamic force from the comet's impact into the atmosphere overcomes its material strength, the comet flattens like a pancake, which greatly increases drag -- essentially stopping the comet in its tracks. If these results prove to be realistic -- and this summer's event could help determine that -- the pancake model can be used to predict what will happen from comet and asteroid impacts on Earth and other planets.

"One of the scientific issues we're hoping to get a handle on in terms of Earth impactors," Mac Low says, "is how big a rock do we need to worry about. One of the motives for modeling this impact is to see if we can do reasonably accurate predictions. If we can, we can start talking about how well the Earth's atmosphere protects us."

The Pittsburgh Supercomputing Center, a joint project of Carnegie Mellon University and the University of Pittsburgh together with Westinghouse Electric Corp., was established in 1986 by a grant from NSF with support from the Commonwealth of Pennsylvania. To date, more than 8,600 scientists and engineers at more than 570 universities and research centers in 49 states and the District of Columbia have used the center's facilities.

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Related article, with graphics, from Projects in Scientific Computing, PSC research report.

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