Cosmic Rain : JUPITER ENCOUNTERS A STRING OF COMETS

Researched by ROBERT LEE HOTZ / Los Angeles Times

The planets of the solar system do not move through a void. We are surrounded by objects large and small--comets, asteroids and the debris from their ancient collisions. For the first time, scientists in July will be able to find out firsthand what happens when a comet slams into a planet. For five days starting July 16, an unusual stream of 21 comet fragments--each the size of a mountain--will slam into Jupiter, offering a unique opportunity to study the kinds of cataclysmic impacts that have scarred the planets and helped shape life on Earth.

THE COLLISION

Using supercomputers, scientists have developed several scenarios for what might happen when the Comet Shoemaker-Levy 9 hits Jupiter:

Meteor Shower: If the comet continues to fragment as it plows deeper into Jupiter's gravitational field, it could break into a "flying sand bank" and shower Jupiter's upper atmosphere with thousands of tiny meteors.

Shock Wave: In its plunge toward Jupiter, the comet could generate a shock wave powerful enough to keep the fragments together some distance into the planet's dense atmosphere, generating a flash that may be visible to a nearby space probe.

Depth charge: The shock wave may help the comet penetrate as deep as 15 miles into the atmosphere, generating a fireball that would fountain back up through the channel created by the comet's passage.

Big Bang: In the most spectacular scenario, the comet could penetrate more than 200 miles into Jupiter's atmosphere. The enormous pressure of the atmosphere would help generate an explosion with an energy of 20 million megatons.

HOW COLLISIONS MAY HAVE AFFECTED EARTH

Lunar rocks brought back by Apollo astronauts suggest that the Moon is the result of cataclysmic collision between the Earth and a Mars-sized planetoid more than 4.5 billion years ago. The impact sent a cascade of molten spray into space, scientists theorize. Although most of the debris fell back to Earth, some of it stayed aloft and formed a Saturn-like ring around the planet, which eventually coalesced into the Moon.

END OF THE DINOSAURS

The Chicxulub crater--more than 110 miles across--recently discovered in the Yucatan may be the imprint of a mountain-sized comet or asteroid that blindsided Earth 65 million years ago, causing the demise of the dinosaurs and two-thirds of all species then in existence.

IN MODERN TIMES

In 1908, a celestial fireball trailing a plume 540 miles long exploded in midair over northern Siberia, leveling more than 1,200 square miles of forest in what is considered the most devastating impact in recent history. The object responsible for the "Tunguska Event" could have been a comet, an asteroid, anti-matter, or even a black hole. No trace was ever found.

EYEWITNESS ACCOUNTS

1) In a nearby nomad camp, the reindeer herd was burned to ashes.

2) A herdsman lay unconscious for two days, his entire herd of reindeer killed.

3) One nomad later said: "God in his displeasure with us tore the sky apart."

4) In a nomad's camp, all the tents were knocked down and one elderly man died of shock.

5) A hunter's shirt was on fire: "The sky has split apart. When the fire appeared, it became so hot that one couldn't stand it."

A Comet's Trail

Until it was snared by Jupiter, the comet Shoemaker-Levy 9 may have been circling the solar system since creation. The planet's immense tidal forces broke the comet into a "string of pearls" more than 600,000 miles long. If this diagram were drawn to scale, the length of the comet's orbit would be 5 1/2 feet long.

Disruption at close approach to Jupiter on July 8, 1992

Comet discovered March 25, 1993

Farthest point in orbit--31 million miles--July 16, 1993

Impacts on Jupiter July 16-22, 1994

GAILIEO'S BOX SEAT

NASA's Gailileo space probe might get the only direct view of the impacts on Jupiter's far side. It will be about 150 million miles from the planet when the bombardment begins. Galileo was already no its way to Jupiter when the comet was discovered. It is scheduled to begin a two-year orbit of the red planet in December, 1995.

Evolution of the Brightest Region: These images taken by the Hubble space telescope show the continued fragmentation of some parts of the comet that will decrease the power of its impact into Jupiter's atmosphere. Most fragments have apparently been stable for at least a year.

Taking Measure: Jupiter is about 300 times larger than the Earth, but rotates three times faster.

Pluto: 3,672 million miles from sun

Neptune: 2,795 million miles from sun

Uranus: 1,787 million miles from sun

Saturn: 877 million miles from sun

Jupiter: 483.6 million miles from sun

Mars: 141.6 million miles from sun

Earth: 92.9 million miles from sun

Venus: 67.2 million miles from sun

Mercury: 35.9 million miles from sun

Sources: NASA, Sky & Telescope magazine, Lowell Observatory, Los Alamos National Laboratory, Jet Propulsion Laboratory.

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