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Impact Explosions May be Smaller, Higher Than Expected



From Lori Stiles, (602) 621-1877
University of Arizona, News Services
Sent July 13, 1994

(Science contact: Willy Benz, UA Steward Observatory
phone: 41 22 755-2611
fax: 41 22 755-3983

Comet Shoemaker-Levy 9 was originally "a rubble pile composed of hundreds, if not thousands, of smaller snowballs" no greater than 1.5 kilometers in diameter. Observers will probably see a smaller explosion higher in Jupiter's atmosphere than they now expect, University of Arizona astronomer Willy Benz reports in the July 14 issue of Nature.

Benz, an associate professor of astronomy at the UA Steward Observatory, and former UA graduate student Erik Asphaug, now with the NASA Ames Research Center, collaborated in modeling the structure of Comet SL9. Benz is in Switzerland to observe the impact from the Geneva Observatory and possibly from the Observatoire de Haute- Provence, France.

"All the predictions and calculations of the impact itself hinge on knowing what is actually impacting Jupiter," Benz said in an email message received yesterday. "We tried to use the closest approach of SL9 with Jupiter in July 1992 to constrain cometary models. What we found was that the usual large, homogeneous dirty snowball model would not work. By this, we mean it did not break-up in 21-plus pieces when it flew by Jupiter.

"We investigated further and found that the model that worked best...the model that best fit observations...was one in which the comet was initially a rubble pile composed of hundreds, if not thousands, of smaller snowballs. The diameter of the parent object depends a bit on the initial rotation of the comet, but is on the order of 1.5 kilometers.

"Our model implies that each of the 21 or so fragments is actually composed of a large number of smaller entities. All these snowball clusters are loosely bound by their own gravity. As a result, we believe the impact will be higher in the atmosphere than anticipated by most numerical simulations." Some simulations assume that each fragment is an ice ball with a diameter of about two kilometers, Benz said. "The parent comet being smaller, we also anticipate a somewhat smaller explosion."

Benz said of the coming impact, "Even if this impact is somewhat less spectacular than the media would like to have it, I believe that potentially much can be learned about the structure of comets and the circulation in Jupiter's atmosphere. And the best of all, we are going to learn all this for a lot less money than (the cost of) sending a spacecraft!"

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