COLUMN ONE : The Impact of Cosmic Collisions : When a comet hits Jupiter this week, scientists will get their first chance to see the sort of cataclysmic crash that may have sparked life--and death--on Earth eons ago.

TIMES SCIENCE WRITER

Eugene Shoemaker, who has spent his life scanning the heavens for starry messengers, thinks we are all the children of comets.

The genial Arizona astronomer embraces the theory that organic chemicals sown by comets that struck Earth eons ago may have been the key components enabling life to root in the planet's barren crust.

Then, in periodic impacts so violent that thousands of species died, comets pounded life into its most modern form, indiscriminately wiping out some promising organisms and allowing others to thrive. By some expert estimates, as many as 5 million comets may have bombarded Earth in its first billion years.

At least that's the theory.

No one has actually seen a comet hit anything.

This week, humanity will get its first glimpse of the cataclysmic collisions that helped shape the planets and perhaps the evolution of life on Earth.

An unusual comet, Shoemaker-Levy 9, will strike Jupiter just before 1 p.m. Saturday with the force of up to a million nuclear weapons. The comet was discovered by Shoemaker, his wife, Carolyn, also an astronomer at Lowell Observatory in Flagstaff, and David H. Levy, a colleague at Palomar Observatory near San Diego.

What they hope to see is a glimpse into the Earth's catastrophic past and a troubling look at the hazards it will almost certainly face in the future. Some astronomers believe that the threat of a cosmic collision involving Earth is higher now than at any time in recent history.

So Shoemaker-Levy 9 is the focus of the largest coordinated astronomical effort in history. Telescopes around the world--and space probes around the solar system--will be straining to capture an image of the impacts.

Jupiter's powerful gravitational field already has ripped the comet into a string of 21 fragments--some a mile in diameter--hurtling toward the planet's far side like immense buckshot, according to Hubble Space Telescope scientist Harold Weaver. The bombardment should last five days.

The impending collision has roused public interest as scientists reassess the role that comets may have played in past planetary disasters, while astronomers are becoming more wary of the hundreds of asteroids and comets known to cross Earth's path today.

To a handful of dedicated astronomers, the shooting stars that burn through Earth's atmosphere every night are more than a clear evening's romantic counterpoint. They are tokens of a threat that could wipe out almost all life on the planet--as celestial impacts may have done several times in the past.

Protected by the umbrella of the atmosphere, Earth speeds through a cosmic rain of comets, asteroids, meteors, dust and other rubble left over from the creation of the solar system. There may be 100 billion comets in all, most clustered in a loose shell of debris called the Oort Cloud about 1 1/2 light years from the sun. Each comet, with a nucleus up to 30 miles in diameter, is embedded in a cloud of gas and dust, moving in a highly elliptical orbit around the sun.

Unlike asteroids, which are simply very large rocks, comets can develop spectacular--and characteristic--fiery plumes as they near the center of the solar system. Even if a comet's core is small, its tail can be immense.

One comet, observed in the last century, had a tail that stretched the distance from Earth to Mars. Asteroids tend to cluster in a predictable band between Mars and Jupiter; comets swoop in toward the sun from any angle.

Although small asteroids are thought to strike our planet every century or so, a disastrous encounter with an object the size of just one of the fragments speeding toward Jupiter might take place on Earth only once every few hundred thousand years. Larger objects can be expected every few million years.

The Earth is under a constant shower of smaller cosmic bombs that do not make it all the way through the planet's protective atmosphere.

Previously classified data from military early warning satellites shows that every year up to eight large meteors explode in the upper atmosphere with the force of a small atomic bomb. The explosions have escaped detection by civilian astronomers. They had no effect on the Earth's surface.

But in 1908, a massive fireball hurtled to within four miles of the Earth and exploded, leveling a swath of forest the size of Rhode Island in the Tunguska region of northern Siberia. The blast is thought to have involved an object about 200 yards in diameter.

No one knows what caused the explosion--a comet, an asteroid, a black hole or anti-matter--but it was strong enough to knock a man out of his chair 60 miles away.

Witnesses estimated that the object had a tail of fire 540 miles long.

In 1978, an explosion equal to 100 kilotons of TNT--believed to have been an asteroid impact--occurred in the Pacific. The blast was so powerful it was first mistaken for a clandestine nuclear test.

In 1989 and 1991, asteroids narrowly missed the Earth. The January, 1991, episode was the closer call. An asteroid 30 feet in diameter passed between the Earth and its moon. New research indicates that some asteroids known to stray across Earth's orbit on occasion may be the cores of extinct comets.

In 1992, a 2,200-pound meteorite broke up and rained debris over a heavily populated area of Mbale, Uganda. Chunks of rock smashed through a train station and crashed into a cotton factory and at least one home. A boy was struck by a fragment, the first person known to have survived being hit on the head by an extraterrestrial object, according to the science journal Nature.

Last August, the comet Swift-Tuttle, its orbit perturbed by Jupiter and other planets, passed close enough to Earth to cause one of the most intense meteor storms in a century as the planet brushed its tail. NASA was worried enough to postpone the launch of a space shuttle until the danger passed and took precautions to safeguard its orbiting Hubble telescope from collisions.

In recent years, geochemists and paleontologists have uncovered the buried evidence of Earth's violent encounters with other celestial bodies.

Last year, a team led by UCLA geochemist John Wasson announced that it had detected evidence of a 2-billion-ton celestial object that crashed into Southeast Asia about 770,000 years ago with the force of 1,000 hydrogen bombs.

There are about 150 known impact craters on Earth. The largest--on Mexico's Yucatan Peninsula--is more than 110 miles across. It is the imprint of a comet or asteroid that hit Earth 65 million years ago at the same time dinosaurs apparently vanished.

The object that caused the crater is believed to have been about six miles in diameter--about three times the size of the largest fragment speeding toward Jupiter and 12 times the size of the object that Wasson thinks devastated Southeast Asia.

Researchers offer tantalizing geological evidence that collisions with comets could have caused more than just the dinosaurs' demise. Cosmic collision may have played a role in half a dozen other major extinctions.

Scientists say the regularity of such extinction--every 35 million years or so--suggests the timing of a comet's orbit. David Raup, a paleontologist at the University of Chicago, has concluded that about 60% of species extinctions may have been caused by asteroids, comets and other bodies.

Although comets may have been responsible for eliminating many species, astronomers at the University of Illinois have detected evidence that the amino acids crucial to the development of life on Earth can exist in space, lending support to the idea that comets also seeded the planet with the building blocks of essential proteins.

Whether any more such apocalyptic events await the Earth is unknown, but some European astronomers argue that the impact hazard facing Earth today is unusually severe.

The planet, they say, is passing through an unusual cloud of small comets, asteroids and other debris from a giant comet that broke up about 20,000 years ago as it approached Earth's orbit--just as the Shoemaker-Levy comet is believed to have shattered two years ago on its approach to Jupiter.

The annual Taurid meteor shower is believed to be the most visible remnant of that giant comet. But there may be at least 10 large asteroid-sized fragments of the comet that also cross Earth's path regularly, researchers said.

Since 1973, astronomers have identified more than 200 sizable objects--asteroids or extinct comets that range up to 24 miles in diameter--that cross Earth's orbit, posing some danger of collision. They discover new ones at the rate of a few dozen every year.

In 1992, astronomers briefly thought that the Swift-Tuttle comet--responsible for the annual Perseid meteor shower--would collide with Earth in the 21st Century. When they double-checked their calculations, they realized they were wrong.

"What this all means is that, as far as we can see, the Chinese curse has come true: We do live in interesting times. The impact hazard is much higher at present," said Duncan Steel at the Anglo-Australian Observatory in Coonabarabran, Australia.

The prospect of an impending collision lent new urgency to a lobbying campaign for a missile system to defend the planet against any asteroid or comet that strayed too close. Congressional reports were commissioned. Some astronomers proposed a $50-million "Safeguard" system of telescopes and sensors to pinpoint any large object threatening Earth. Researchers at the Los Alamos National Laboratory in New Mexico suggested spending $100 million a year to develop interceptors to deflect an asteroid or comet. Others suggested hurling nuclear bombs.

Many scientists and policy-makers greeted the ideas with derision. No funds have been allocated, but NASA has taken the threat seriously enough to propose launching a satellite in 1996 to seek clues about the nature and origin of near-Earth objects.

In the very long run, some astronomers suggest, the hazard will only get worse.

The solar system is expected to pass close enough to other star systems in the next 45,000 years that the nearby stars could nudge as many as 100,000 comets out of the Oort cloud and toward our inner solar system, according to research published by the Royal Astronomical Society.

Of course, the distances are so vast that any risk to Earth may not actually materialize for another 20 million years.

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Since before the first telescope, people have brooded over the appearance of comets in the sky, greeting each new apparition as a harbinger of disaster.

Even today, after decades of scientific observation, comets are a mystery compounded by mystery.

Set in motion billions of years ago, they careen unpredictably from the farthest reaches of the solar system, past the planets and around the sun like pinballs.

"There are streams of cometary debris in the solar system," and Earth is continually exposed to the risks they present, said Mark Bailey, of the Liverpool John Moores University in England.

No one is certain what a comet is made of. It may be rock, a loosely compacted snowball of ice and gravel, or a confection of gases that resembles nothing quite so much as frozen car exhaust.

The best evidence suggests that they are so fragile a person could pull one apart. Yet a comet of sufficient size traveling fast enough can lay waste to a planet with one blow.

When the European Space Agency maneuvered a probe alongside Halley's Comet in 1986--mankind's closest intentional encounter with any comet--space scientists caught a fleeting glimpse of an irregular, coal-black object shaped like an immense potato. That encounter raised as many questions about a comet's composition as scientists had hoped it would answer.

What scientists do know is that celestial collisions are as common--at least in astronomical terms--as freeway fender-benders.

Impact craters scar all the inner planets, including Earth. The solar system's largest crater--more than 1,500 miles across and 7.5 miles deep--is no further away than the moon. It was discovered near the moon's south pole this year.

Traditionally, most scientists believed comets were too small to cause such monumental craters. But recently, some European astronomers proposed the existence of so-called "great" comets more than 180 miles in diameter that could be responsible for the impacts previously attributed to large meteors or asteroids.

When such a comet disintegrates, it could shower the inner solar system with a million lethal fragments, each more than half a mile in diameter.

"The bottom line is that great comets exist--with masses thousands of times the mass of Halley's Comet," Bailey says. "The hazard posed by comets has to be considered in the same breath as the asteroid hazard."

Jupiter plays a crucial role in steering the course of comets throughout the solar system.

Its immense gravity can hurl comets and asteroids onto new courses that take them into the inner solar system--and closer to Earth.

Researchers now suspect that comets like Shoemaker-Levy, which Jupiter pulled apart into a distinctive "string of pearls," may not be as rare as initially believed.

Astronomers at the University of Hawaii and the Jet Propulsion Laboratory in Pasadena calculate that comets may break up as often as once a century--relatively frequently in the lifetime of the solar system.

A chain of identical craters 21 miles long on the surface of the moon is believed to be evidence of such a breakup. Inspired by Shoemaker-Levy, astronomers suggest that it was caused when a comet passed too close to Earth, shattered and struck the moon.

There also are similar chains of identically sized craters on two of Jupiter's moons, Callisto and Ganymede.

Sometimes the giant planet captures passing comets--as evidently is the case with Shoemaker-Levy 9. Astronomers even speculate that four of Jupiter's 16 moons are comet fragments.

Astronomers anxiously awaiting Saturday's collision caution that they aren't sure whether the comet will end in a bang or a whimper. By then, Jupiter's immense tidal forces may well reduce the fragments to harmless shoals of flying gravel.

But scientists hope to witness the kind of collision that is thought to have wiped out the dinosaurs 65 million years ago. Even if the bombardment is that violent, it will have little effect on Jupiter because the planet is so large.

"This is the first time in history we've been able to predict a major impact and then prepare to observe it directly," Shoemaker said.

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