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Astronomers Prepare for Comet SL9 Crash


From the "JPL Universe"
July 15, 1994

Astronomers Preparing for the Crash


It has been called the event of a thousand lifetimes. In other words, something we--or many future generations--probably never will see again.

The problem is, we may not even get to see this one.

When the 20-odd fragments--each following their own path--of Comet Shoemaker-Levy 9 (SL9) collide with Jupiter July 16-22, their impacts could release more energy into the planet's atmosphere than that of the world's combined nuclear arsenals, minus the radiation (according to one theory). But because the impacts will occur on the night side of Jupiter, the explosions will not be directly observable from Earth.

Yet, scientists around the world are hoping they'll gain new insight into the composition of comets, as well as the atmosphere of Jupiter, according to Dr. Donald Yeomans, senior research scientist in JPL's Solar Systems Dynamics Group in Section 314. He explained the event's significance to a packed von Karman Auditorium July 11. "We're not expecting any visually impressive phenomena, nor do we expect any changes on Earth.

"What we do expect--besides the once-in-a-lifetime event--is a large standing army of excited astronomers, as well as a change in how we're beginning to view comets."

Until recently, scientists believed comets to be dirty ice balls, a few kilometers in size. But, Yeomans said, "The Hubble Space Telescope indicates that SL9 is a not a monolithic iceball but a concentration of loosely bound fragments. If SL9 is typical of comets, perhaps cometary interiors are very fragile structures, composed of separate pieces--a sort of rubble pile.

"We're trying to learn how large the pieces are, which will determine the kind of reactions we can expect upon impact, as well as shed light on the question of what comprises comets," Yeomans said.

"This particular comet orbited the sun for about 4 1/2 billion years," he told the audience, "only to be captured by Jupiter a few decades ago, then torn apart by an exceptionally close approach to the planet in 1992.

"This raises an interesting question: We know how fragile SL9 is, but given the ease with which it became fragmented, how did it live for 4 1/2 billion years?" he wondered.

That long life is not the only thing that about Shoemaker-Levy 9 that strikes researchers as unusual. According to Dr. Paul Chodas, Yeomans' colleague in Section 314 and architect of the software that predicts the times of impact, "We've never had the opportunity to study the impact of a large solar system body on a planet.

"Also unusual is the fact that this comet is orbiting Jupiter," Chodas added. "Comets usually orbit the sun.

"When we trace the motion of comets backward in time, we find that a few of them have been temporarily captured by Jupiter, but SL9 is the first comet to be actually observed orbiting the planet."

Chodas also pointed out that SL9 came closer to Jupiter than any other comet. In July 1992, it made the closest known approach--113,000 kilometers (70,000 miles) from Jupiter's center. "The difference in the gravitational pull between the near and far sides caused the comet to break apart," he explained.

"Although other comets have broken apart, none has broken into so many pieces as SL9," Chodas added.

While Chodas and Yeomans know the hour of that close pass, they cannot say precisely where SL9 came from before that. "We think that for the past two decades it has been orbiting Jupiter in a loosely bound path," Chodas said.

The pair will be stationed on-Lab for the week of impacts, predicting impact times and locations while astronomers worldwide attempt to observe various effects of the impacts, such as flashes reflected off the inner satellites of Jupiter.

Chodas said professional astronomers from such far-flung spots as Arizona, Texas, Chile, Australia and Japan have been observing and tracking the comet for a year, sending back information on the comet's position, which is used to refine impact predictions. "However," he said, "the pieces of the comet are faint, and are getting harder to see. As it approaches Jupiter, the planet's light interferes with the observations."

The comet was discovered by Eugene and Carolyn Shoemaker and David Levy. It was first detected on a photograph taken March 24, 1993, from Caltech's Palomar Observatory in California.

Most of the bigger pieces of the comet have been estimated by Dr. Zdenek Sekanina [senior research scientist, Division 32], along with Chodas and Yeomans, and to be 3 km (1.8 miles), while the smaller ones are guessed to be about 0.5 km (1,600 feet).

"We think the nucleus of the original comet [before it broke up] was about 10 km (6 miles) across," Chodas said, "based on the positions of the fragments as they have been observed over the last year."

The comet is accelerating tremendously as it approaches Jupiter, Chodas also noted. On June 27--20 days before impact--the comet's speed was about 4 km per second (9,400 mph). But when the fragments strike the planet, they will be traveling at 60 km per second (135,000 mph).

Fragments of the comet have been named by Sekanina, Chodas and Yeomans using the letters A through W, with A hitting first on July 16. Southern California astronomers who hope to get a peek at the action may have to wait until July 20 at roughly 10:24 p.m., when fragment R will hit, followed by fragment V's impact on July 21, at roughly 9:15 p.m. (See table below for predicted impact times for each fragment.)

Powerful telescopes are not the only means by which scientists hope to view the event and its results. JPL's Galileo spacecraft will be about 240 million kilometers (150 million miles) from Jupiter, as it makes it way to its Dec. 7, 1995, rendezvous. The spacecraft has a direct view of the impact sites and should be able to observe impact flashes.

The pictures will go on Galileo's tape recorder and will be returned to Earth over several months.

Voyager 2 will observe impact signatures via planetary radio astronomy and ultraviolet spectrometer. Ulysses will use infrared radio and plasma wave experiments to observe radio emissions and radio bursts from the planet during the fragments' entries.

In answer to a journalist's question on how the impact will affect the general public, Yeomans said that besides being able to witness something that happens approximately once every thousand years, the event's greatest significance is really intellectual. "It's an amazing opportunity for scientists to learn about comets, Jupiter and the solar-system formation process."

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