‘Squashed Comets’ Could Be A Bang Or A Bust

Like mountain-sized slugs from a celestial machine gun, fragments of a shattered comet will ratta-tat-tat into Jupiter starting next Saturday. It could be the show of a lifetime - or a dud.

Comet Shoemaker-Levy 9 is a string of space rock or ice speeding in line toward Jupiter at about 130,000 miles per hour. The largest fragment is almost two miles in diameter.

When they hit, the comet shards will explode with the force of about 200,000 megatons of TNT. Each is expected to trigger a fireball more than 1,500 miles across and rising up more than 600 miles above the Jovian clouds.

“That’s many times larger than any nuclear bomb ever exploded on earth,” said Lucy McFadden of the University of Maryland, one of an army of astronomers who will watch the show.

Unfortunately, most of the fireworks will be shielded from direct view by telescopes on Earth. The impacts will occur just beyond Jupiter’s western horizon, as viewed from Earth.

Even so, just about every major telescope on Earth, along with an array of instruments in space, will be aimed at Jupiter on July 16 for what most astronomers believe will be the most spectacular space collision they’ll ever witness.

“This is the first time in history we’ve been able to predict a major impact and then prepare to observe it scientifically,” said Gene Shoemaker, a U.S. Geological Survey scientist who is co-discoverer of the comet. “We’re seeing an event that happens once in a millennium. There is about one chance in 10,000 that we would find something that would hit during a time period of a century or so.

“I’ve dreamed my whole life of an impact on the moon,” he said. “It never occurred to me that I would have an opportunity to see an impact on Jupiter.”

Shoemaker, his wife Carolyn and David H. Levy discovered the comet on March 24, 1993 while conducting their monthly photo scan of the heavens from an outdated telescope on Palomar Mountain, Calif.

“We still do it the old fashioned way,” said Shoemaker. “We’re right out in the open (observation) dome, guiding the telescope by hand.”

The team has conducted sky searches for years and made previous comet discoveries.

Shoemaker said they found this one on a night when the weather was marginal and when the photo film was thought to be of poor quality because of an exposure accident. They took the pictures and hoped.

The comet was first sighted by Carolyn Shoemaker, who was analyzing the film with a flicker-device that picks up celestial movement. What she saw appeared to be an elongated point of light.

“It was clearly the strangest object we had ever seen,” said Shoemaker. “Carolyn called it `a squashed comet’ when she first looked at it. Carolyn never had any doubt. She was sure it was a comet.”

Soon, some of the most sophisticated astronomy instruments in the world, including the orbiting Hubble Space Telescope, were focused on the “squashed comet.”

By May, the Hubble and other instruments had confirmed that Shoemaker-Levy 9 was actually 21 pieces, streaking along in a game of follow-the-leader.

Backtracking the orbit, experts determined that the comet apparently passed near Jupiter in July, 1992 and was shattered by powerful gravitational forces of the giant planet. The pieces lined up like freight cars and then swung far back into space, as if taking a running start toward a dive to oblivion.

Now they are bearing down on Jupiter at a speed of more than 37 miles per second.

The first rock hits the upper clouds of Jupiter at 3:54 p.m. EDT Saturday. Pieces keep flying in at irregular intervals for the next five days, with the last one hitting at about 3:53 a.m. EDT Friday.

Just what will be visible from Earth 480 million miles from the action - is an open question. Some astronomers say the whole thing could be a dud, producing little effect.

The speed and mass of Shoemaker-Levy 9 is expected to create explosions that send waves of light bouncing off of Jupiter’s moons that will be visible by telescope from Earth. The fireballs are expected to be as red and as bright for about 45 seconds as the sunlight reflected from Jupiter.

But the experts admit that they don’t know for sure what will happen when the shards of Shoemaker-Levy 9 meet up with Jupiter.

“The worst case would be if they all just disappear and there is no effect,” said Shoemaker. “But I will really be astonished if we don’t see something.”