Astronomers await Jupiter fireworks

January 11, 1994|By Frank D. Roylance | Frank D. Roylance,Staff Writer

COLLEGE PARK -- The newly discovered comet Shoemaker-Levy 9 will unleash an explosive power that will dwarf the biggest nuclear war humans could ever ignite on Earth when it collides this summer with the planet Jupiter, scientists say.

The resulting fireballs and flashes will occur 480 million miles from Earth -- too far away to see without a telescope. But scientists gathered this week at College Park to organize observations that will involve nearly every big observatory in the world, the Hubble Space Telescope and other orbiting observatories.

Many amateurs also will be watching the giant, gaseous planet.

Nothing this big has happened on Jupiter for 10,000 years, according to some calculations. And modern astronomers have never seen anything like it.

"It's the first time we've ever seen a body come into a planetary atmosphere . . . with more than a few seconds advance warning," said University of Maryland astronomer Michael I. A'Hearn.

Scientists hope the collision will teach them something about comets, about the atmosphere and interior of Jupiter, and about past and future cometary impacts on Earth.

"This impact will bring in far more energy than you would get in a nuclear war, and almost as much as the impact in the Yucatan Peninsula [of Mexico] that many people think caused the extinction of the dinosaurs" 65 million years ago, said Mr. A'Hearn.

Crashing comets also are believed to have delivered much of Earth's atmosphere and water vapor during the formation of the planet 4 billion years ago.

"We will get a lot of information. The question is, will we be able to interpret the information in such a way as to be able to predict how a similar event would occur on Earth? There are no good physical models," Mr. A'Hearn said.

Eugene and Carolyn Shoemaker and David Levy -- the comet's '' discoverers -- attended yesterday's meeting.

Shoemaker-Levy 9 was first discovered in a photograph taken in March at the Mount Palomar Observatory in Southern California. (The "9" means it was the team's ninth joint cometary discovery.)

Astronomers have since calculated that Shoemaker-Levy 9 probably orbited the sun, like most comets, until it was captured by Jupiter's gravity several decades ago. The comet has been orbiting the giant planet ever since, in a two-year loop made unstable by the sun's gravity.

On its last swing by Jupiter on July 8, 1992, scientists believe, the comet passed so close that Jupiter's gravity shattered it into a train of at least 21 pieces averaging about two miles in diameter.

The comet that smashed into the Yucatan 65 million years ago was an estimated 6 to 12 miles in diameter.

For seven days beginning July 16, the fragments of Shoemaker-Levy 9 will race up from below Jupiter's south pole, and crash one at a time into the southern hemisphere at speeds of about 40 miles per second.

Each one of the fragments will unleash energy equal to 10 million megatons of TNT, said Harvard University astrophysicist George Field.

A megaton is 1 million tons. The largest nuclear weapon ever detonated on Earth was a 58-megaton device set off by the former Soviet Union. Most U.S. weapons tests have been smaller than one megaton.

The impacts and resulting flashes and heat plumes will occur just beyond the Jovian sunrise on the planet's night side, with no direct view from Earth.

However, Paul Chodas of the California Institute of Technology's Jet Propulsion Laboratory, said the impacts will be visible to the Galileo spacecraft now en route to Jupiter.

"Galileo can only take a limited number of photographs," he said. "The challenge for . . . us is to predict the exact time of the impacts." Even 18 hours from impact, the margin of error will still be more than 3 minutes.

Earth-based observers looking for effects on Jupiter's atmosphere will have to wait 30 to 60 minutes for Jupiter's rotation to bring each impact point into view.

They will also watch for impact flashes reflected on Jupiter's moons or on the comet's dusty halo. Others will watch in infrared wavelengths for signs of pressure waves moving through Jupiter's atmosphere, like ripples through a spherical pond. Those waves may in turn reveal the structure of the planet's gaseous interior.

Still others armed with spectrographic instruments will search the impact zones for signs of chemicals like ammonia and water stirred up like paint in a can from deep regions of the Jovian atmosphere.

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