Comet splashdown.

Last summer's comet crash on Jupiter has astronomers and others wondering: Can we keep such a catastrophic collision from happening on Earth?

You probably remember all the excitement last July when a string of comet fragments slammed into the planet Jupiter. Thanks to astronomers Carolyn and Eugene Shoemaker and David Levy, who spotted the comet 16 months before it hit,

everyone on Earth had a chance to see the biggest fireworks show in history.

From scientists equipped with powerful telescopes to kids glued to TV and computer screens, Earthlings around the world watched as fragments of comet Shoemaker-Levy produced Earth-size fireballs on Jupiter. One collision sent a plume of hot gases spewing 1,000 kilometers into space and released 100,000 times the energy of the largest nuclear bomb ever exploded.

A blast like that could ruin your day--if it had happened here on Earth. And scientists say it can happen here. In fact, small chunks of space rocks, called meteoroids, continually crash into Earth's atmosphere. Most burn up before striking the ground (see SW 9/4/92, p. 10). But the recent collisions on Jupiter (of chunks measuring more than a kilometer across) have renewed debate in Congress: Should we spend money to track large space rocks and protect Earth from a similar bombardment?

"I don't think most people really believed [comets could crash into planets] until they saw it happen," says Carolyn Shoemaker, a planetary astronomer at the Lowell Observatory in Flagstaff, Arizona. Shoemaker tracks the paths of comets and asteroids--large, rocky (and, in the case of comets, icy) chunks of space rubble left over from the formation of the solar system. She pays particular attention to those chunks whose orbits intersect Earth's path around the Sun. If a space rock is large enough, it could kill millions of people by colliding with Earth, Shoemaker says.

Two years ago, NASA scientists studied the threat that large space rocks pose to our planet. They recommended building a network of early-warning telescopes to detect incoming objects so that we could somehow deflect the "invaders." But Congress never gave the project the millions of dollars of funding it would require, so the plan never got off the ground.

That may change, astronomers say, as data from the Jupiter comet crash pour in.

CRASH!

Most comets spend their time orbiting the Sun at the distant edges of our solar system. But the icy rocks occasionally get pulled off course by the gravity of passing stars or dust clouds, astronomers say. That's when the comets come hurtling toward the inner solar system--and us.

Fortunately for Earthlings, says Carolyn Shoemaker, comet Shoemaker-Levy struck Jupiter. A giant ball of gas that weighs more than two-and-a-half times all the other planets put together, Jupiter is a big target. It would take 11 Earths, put side by side, to span the width of this massive planet. If the giant were hollow, a thousand Earths would fit inside.

All that mass exerts a tremendous gravitational pull on everything around it. Jupiter's gravitational tug is actually what broke comet Shoemaker-Levy into more than 20 pieces. After tearing the comet apart, Jupiter's gravity captured the chunks and whipped them around the planet. On their next trip around, the fragments crashed.

Because Jupiter is mostly gas, each collision was more like a splash than a crash, says NASA astronomer Cathy Imhoff. By studying light given off by the splattering gases, scientists have discovered some pretty stinky stuff in Jupiter's atmosphere: chemicals such as hydrogen sulfide (a poisonous gas that smells like rotten eggs) and carbon disulfide. Since these sulfur-containing chemicals aren't usually seen on Jupiter's cloud tops, it's possible that the impacts dredged them up from as deep as 20 kilometers before throwing them into space.

The high temperatures generated by the fiery collisions may also have triggered the formation of new substances never detected on Jupiter before.

That kind of bombardment on Earth would cause a lot of trouble, says astronomer Imhoff. "We think a crash like this may have killed off the dinosaurs and other creatures 65 million years ago." If just one of the Shoemaker-Levy fragments had collided with Earth, it would have blown "huge amounts of dirt and dust into the atmosphere, blocking out the sunlight," she says. Without sunlight, there would be no life on Earth.

ON THE LOOKOUT

How do we know if and when we'll be hit by a deadly space rock again? It will happen, says Jurgen Rahae, chief scientist in NASA's Division of Solar System Exploration. It's just a question of time, he says. Rahae helped prepare a report for Congress on the probability of objects from space colliding with Earth.

Large, dangerous collisions, he says, are much less probably than smaller ones. "Very large objects, 10 to 15 kilometers across, like the one that [may have killed the dinosaurs], hit only once in about every 100 million years," he says.

Still, some people say we should be prepared. After comet Shoemaker-Levy's fireworks show, Congress asked NASA to reinvestigate the plan to build six comet-and asteroid-tracking telescopes around the world.

Though there's no need to panic, says Shoemaker, the plan makes sense. If the telescopes spotted a space rock coming our way, there would be time to act, she says. One way: Knock the chunk off course with, say, a nuclear explosion. Though that plan may raise other safety concerns, it might be worth a shot. What do you think?

BRAVE NEW WORLDS

While many eyes and telescopes were focused on Jupiter last summer, some astronomers were excited by another recent find--the first discovery of planets outside our solar system.

Scientists can't actually see these far-off planets. But they can detect evidence of them. Using radiotelescopes--instruments that pick up invisible electromagnetic energy waves emitted by some stars--astronomers at PennSylvania State University and the National Radio Astronomy Observatory at Socorro, New Mexico, detected disturbances around a distant pulsar.

Pulsars are remnants of exploded stars. They spin and emit pulses of energy as regular as the ticking of a clock. Astronomers observing this pulsar noticed irregularities in the "ticking." The interference was periodic and predictable. That led the scientists to conclude that something was orbiting the pulsar, regularly causing the star's pulse to "wobble." Further studies confirmed that at least two never-before-seen planets were causing the disturbance.

Sparked by this discovery, some scientists are saying there must be more planets out there. After all, the Sun at the center of our planetary solar system is just one of 400 billion such stars in our galaxy. And there are billions and billions of galaxies in the universe. Many of those stars, could have planets orbiting them.

Taking that reasoning one step further, astronomer Frank Drake says the odds are that some of those yet-to-be-discovered planets could harbor life. In January, Drake will begin a controversial project. With the world's largest radiotelescope, he'll eavesdrop on nearby stars, searching for clusters of "intelligent" signals--signals made by beings communicating with each other, and maybe even trying to communicate with us.

He may not find anything. But if he does, he says the findings could tell us a lot about the aliens. If those distant beings tuned in to our planet, he adds, they would "hear" chattering on cellular telephones, radio waves, satellites, and more. The strongest signal they'd pick up? "Television," says Drake.

"And they would learn all about us without having to ask a question."