Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: WEDNESDAY, July 20, 1994                   TAG: 9407220084
SECTION: CURRENT                    PAGE: NRV-6   EDITION: NEW RIVER VALLEY 
SOURCE: By PAUL DELLINGER STAFF WRITER
DATELINE: BLACKSBURG                                LENGTH: Medium

PROFESSOR NOT UNHAPPY HE WAS WRONG ON COMET IMPACT

John Broderick's prediction on Comet Shoemaker-Levy 9's impact was made to a former student during a grocery store conversation.

``So I was way off on that,'' said the scientist this week. As it turned out, a better comparison might have been a cherry bomb dropped into a toilet bowl.

One of the 21 comet fragments burning into the massive atmosphere of Jupiter, the solar system's biggest planet, set off a fireball this week more than 600 times as powerful as if all of Earth's nuclear weapons had been detonated at once.

Before the impacts started, astronomers were divided about whether there would be massive explosions or whether Jupiter would swallow the comet fragments without a burp.

The biggest fragments are more than 2 miles wide, and Jupiter measures 88,700 miles at its equator - big enough to line up seven Earths side by side. In fact, 1,300 Earths could fit inside Jupiter if it were hollow, and it would take 318 Earths to equal its mass - more than two-thirds of all the material in the solar system, not counting the sun.

Scientists can be forgiven for not expecting fireworks from a fragile little comet.

There has been speculation that the impacts could stir up something like Jupiter's Great Red Spot, a whirling storm observed for 100 years in Jupiter's southern hemisphere.

``I'd bet against it but, then again, who knows?'' said Brian Dennison, another physics professor at Tech. ``You've got to remember, astronomers have never seen anything like this before.''

The breakup of the comet into 21 pieces made it easier, rather than more difficult, for astronomers to spot because that increased the surface area and cometary ``tails'' from the gases coming off of it. It may have been orbiting Jupiter for 50 years before that, he said, until the last time around when Jupiter's tidal forces broke it up.

``It's rather interesting, seeing predictions and seeing what actually happens,'' he said, ``because we were kind of expecting that it was going to be a fizzle.''

Fictional speculation goes back at least to 1933 when Philip Wylie and Edwin Balmer published their novel, ``When Worlds Collide,'' made into a movie in 1951 - except in that story Earth was the target.

Since then, many scientists have come to accept a theory that it was an impact from space that led to the extinction of the dinosaurs 65 million years ago.

Now they are getting to see the collision effects on a largely gaseous planet, rather than a hard one like Earth. There has been speculation that Jupiter has no solid core, but Dennison said it now seems likely that there is one, possibly made of metallic hydrogen, ``and it may even have a rocky core inside that.''

Dennison said one hope is that the explosions from 100 miles in will churn up material that can be analyzed ``by getting a spectrum of the fireball.... We don't really know what's down there.''

It will also provide information on the strength of cometary bodies and how often they hit planets. ``These impacts really do happen, but infrequently,'' Dennison said. ``It may be millennia before it happens again.''

``I'm not sure they'll bring any gases up from the surface of Jupiter,'' Broderick said. ``I think most of the research will be on the impacts.... It's the first time anybody has seen an impact like this happening,'' he said. ``We appreciate now that these kinds of events may happen more regularly.''

They are more likely to happen to a giant planet like Jupiter than to Earth, he said. Jupiter is hit because its heavier gravity pulls comets off their courses toward the planet, while one might hit Earth ``just because it happened to be in the way, rather than sucking it in,'' he said.

``The stuff on TV, the pyrotechnics, are really fascinating,'' Dennison said, but ``the brightest pictures are infra-red, not optical,'' so the human eye could not see them that clearly.

``I wouldn't rush out and buy a telescope,'' Broderick agreed. He and Dennison have been among those visiting Tech's Martin Observatory for a peek at the comet show, but so far clouds have interfered and they have to watch it on television like everybody else.