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SKY WATCH : A Cosmic Collision : Beginning July 16, pieces of Comet Shoemaker-Levy 9 will slam into Jupiter.

SPECIAL TO THE TIMES

Astronomers are calling it the most cataclysmic cosmic event of this century--more unusual even than the appearance of Halley’s Comet. And you don’t have to leave the San Fernando Valley to catch it.

For about a week beginning July 16, the scattered pieces of Comet Shoemaker-Levy 9 will slam into Jupiter with more force than dozens of atomic bombs. Although the collisions will occur on the dark side of the solar system’s largest planet, some scientists expect the impacts to briefly affect Jupiter’s appearance, as seen from Earth.

THE HISTORY

Comets are essentially dirty icebergs traveling through space, heating up as they pass close to the sun--an effect that releases comet dust into tails that are stretched out by the solar wind. Most comets are believed to originate from a cloud of comets at the edge of the solar system and to somehow get knocked into orbit around the sun.

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Jupiter is a ball of hydrogen and helium gas about 1,000 times more massive than Earth. It rotates about every 10 hours, making it the fastest spinning planet in the solar system. Jupiter’s Great Red Spot, which could engulf two Earths, is a cyclonic storm that has been raging for centuries.

On July 8, 1992, Comet Shoemaker-Levy 9 passed close enough to Jupiter for gravity to break it into a string of 21 fragments, most with comet tails of their own. By July 16 of this year, the distance between these fragments could span more than 4 million miles, with particles ranging in size from that of the Sherman Oaks Fashion Square to the city of San Fernando.

The comet was discovered by observers at Palomar Observatory near San Diego on March 25, 1993. Like all comets, it was named after those who spotted it first--professional astronomers Carolyn and Eugene Shoemaker and amateur astronomer David Levy. The “9" signifies that this is the ninth comet discovered by this team.

Analysis of the comet’s orbit revealed two startling facts: that it was the first known comet to orbit a planet (Jupiter) rather than the sun, and that the comet’s icy fragments will collide with Jupiter during the week of July 16 to 22.

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THE IMPACTS

Never before have earthbound stargazers had the opportunity to witness the effects of a comet smashing into a planet.

And although the spacecraft Galileo will be positioned perfectly to send us photos of the comet shelling the planet’s atmosphere, scientists have no clear idea of what we will see from Earth.

In the most dramatic scenario, the exploding fragments could illuminate the Jovian moons caught in the shadow of Jupiter. The most likely candidate is Europa, which will be behind Jupiter when fragment K--the particles are given letter names--slams into the planet on July 19. Other scientists predict that the impacts will briefly illuminate Jupiter’s faint ring.

A more likely effect will be atmospheric turbulence on Jupiter. Some scientists expect the impacts to briefly disrupt the planet’s cloud belt, perhaps creating miniature spots south of its Equator. Another prediction is that shock waves could ripple across the entire planet.

Theoretically, the event could also be a dud: Some scientists predict that Jupiter’s atmosphere will quietly swallow up the comets like a whale scooping up shrimp.

Although scientists may not agree on the outcome, there are three things on which they can agree: First, earthbound observers will have to wait at least 30 minutes after each collision before the impact site rotates into view. Next, observers shouldn’t expect to see a dramatic explosion, but rather subtle changes in the appearance of the planet. And finally, it will take knowledge of how Jupiter looks now to appreciate the changes.

HOW TO PREPARE

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Get to know Jupiter.

The cheapest way is to buy the July issue of “Sky & Telescope” magazine at a newsstand, or a $5 guide to the comet / Jupiter impact published by the nonprofit Planetary Society in Pasadena. Familiarize yourself with Jupiter’s cloud belts in the southern hemisphere (about 40 to 50 degrees south latitude), where the impacts will occur.

Then head to Griffith Observatory in Griffith Park to view the action.

A more expensive option--but without the long lines expected at the observatory--is to use your own telescope to study Jupiter in the weeks before and during the impact. Fortunately, Jupiter is bright enough to be seen from your back yard. Turn off indoor lights and take about 30 minutes to let your eyes adjust. The darker the site, the better the view through the telescope.

If you are going to purchase a telescope, get the right one. It should have at least a four-inch aperture. The bigger the aperture, the more you will see. Consult a local telescope store and avoid toy-store varieties. Do not, however, buy a telescope just for the comet / Jupiter impact: If it’s a dud, you’ll feel ripped off. Although this is a once-in-a-lifetime event, a telescope is a purchase best made for a lifetime of stargazing.

Where and When

What: Griffith Observatory’s comet collision activities. Free to the public, the observatory’s 12-inch telescope will be focused on Jupiter throughout the week of July 16-22, with a live video feed of Jupiter from the telescope available for viewing on a screen inside the museum. Additional telescopes will be on the lawn July 16. During this week, the observatory’s 7:30 p.m. planetarium show will feature a 10-minute segment on the collision.

Location: 2800 E. Observatory Road, Los Angeles.

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Hours: Observatory museum is open 12:30 p.m. to 10 p.m. daily. Telescope viewing dusk to 9:45 p.m., except for July 16, 20 and 21, when it will be open until midnight. Planetarium show times are 1:30, 3 and 7:30 p.m. Monday through Friday; 1:30, 3, 4:30 and 7:30 p.m. Saturday and Sunday.

Price: Observatory entry is free. Planetarium show is $4 for adults, $3 seniors 65 and older, $2 children 5-12. Children under 5 are admitted only to the 1:30 p.m. show on weekends, for free with a paid adult admission.

Call: (213) 664-1181 for general information; (213) 663-8171 for weekly sky report.

Also: To purchase the Planetary Society’s guide to the collision, “Once In A Thousand Lifetimes,” call (818) 793-5100. The price is $5.


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