It took a planetary cataclysm to make David Levy a household name.

Much of the world was watching in July 1994 when 21 chunks of the Shoemaker-Levy 9 comet, which Levy helped to discover, slammed into Jupiter, producing one of science's greatest public relations events in living memory.

"Nature held a science experiment in space," Levy said. "As interesting as the comet impacts were, the way the community handled them and the interest they generated were even more interesting. For a week, everyone was a scientist."

Levy said the impact of Shoemaker-Levy 9 was one of the best boosts for science in recent generations. "It was as if all of the astronomers of the Earth were working as one team," he said. "And it wasn't kept from the non-astronomy field. Anyone with a computer or a television could see the results instantly."

He enjoys taking his science to students because "it's one of the biggest problems we have right now. It's 'bad news' to be interested in science . ... I want to do something to change that."

In the wake of Shoemaker-Levy 9, there was a wave of concern that a similar fate might happen to the Earth. Levy isn't concerned; he's certain. "It's not likely, it's definite," he said. "A mile-wide comet hits the Earth every 100,000 years or so, and causes a global reaction. To have something like that happen so often makes us want to know a little bit more about it. But we don't know when the next one is going to hit. It probably won't be for a very long time, but it could be tomorrow."

When it comes, he said, there will probably be very little warning. It was about two months after Levy discovered the clustered comet in March 1993 until he realized what would happen the following summer. "In this case we had 21 events, each one of which had planet-shaking importance," he said.

Levy supports the theory that a large comet struck the Earth and ended the age of the dinosaurs. His theory is supported, if not proven, by the effects of Shoemaker-Levy 9 on Jupiter's atmosphere. It also lends credence to Levy's theory that ancient comets brought water and organic material to a previously barren Earth. "The comets hit Jupiter and they left water," he said. "We got to see that happen."

Surprisingly, Levy never took a course in astronomy. An amateur scientist who learned astronomy in his backyard, he completed college with a master's degree in English. His interest was sparked in 1960, he recalled, when he was riding his bicycle to a sixth-grade picnic. "I fell off and broke my arm," he said. "A friend gave me a book on the planets as a get-well present. That's when I decided I wanted to be an astronomer."

Comet-hunting might sound glamorous, but Levy admits he spends most of his time just staring at the sky. "Think of it as a sport -- the world's slowest sport," he said. "You try to get the discovery before someone else does."

He began his quest in his backyard at age 15. It was 1965. He didn't discover his first comet until 1984. "That's what makes most people leave the field," he admitted. "It can be 19 years before you find anything. But then I figured out better how to do it, and they've been coming a little more quickly for me." So far he has discovered eight comets from his backyard in Arizona and 13 through his Palomar Mountain partnership with Eugene and Carolyn Shoemaker. He has written numerous books and articles on astronomy and related sciences.

It was during his research for a biography on Clyde Tombaugh, the astronomer who discovered Pluto, that Levy found evidence of a nova that Tombaugh had never reported. While searching for more evidence to confirm the nova, Levy soon realized that the same star had exploded at least 10 times in 60 years. Although not as widespread as the interest in Shoemaker-Levy 9, the discovery energized the scientific community and remains one of Levy's most satisfying moments.