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Once in a Blue Moon

Astronomers witnessed a rare event when they watched comet Shoemaker-Levy 9 (SL9) smash into Jupiter. Gene Shoemaker explained that a comet 1.5 kilometers wide should strike Jupiter about every 100 years. But the odds that a comet the size of Shoemaker-Levy 9 — about 8 kilometers across — would break up and hit Jupiter are once every 2,000 years!

Most comets reside in the Kuiper belt, a region beyond the planet Pluto, or in the Oort cloud, located even farther away in the distant outskirts of the solar system. But they don't spend all their time there. Their wide, looping orbits bring them into our solar system and around the Sun. Sometimes, our Sun and planets can be unkind to these visitors. Their gravitational muscle can break off a piece of a comet that wanders too close to them. Comets, after all, are delicate objects; they are loosely held together chunks of rock and ice.

But SL9 was different from the normal comet visitor. During a voyage to the solar system, the comet was "captured" by Jupiter after it traveled within the massive planet's gravitational grasp. SL9 became one of only two comets known to have orbited a planet instead of the Sun. And it was the only comet that astronomers witnessed orbiting a planet. Then SL9 made its closest pass by the planet and was ripped apart by tidal forces. Some astronomers estimate that the comet had been orbiting Jupiter for 20 years before the breakup. More than 20 comet chunks, lined up in a row, began looping around Jupiter in an unstable orbit. Soon, astronomers calculated that SL9 was heading straight for Jupiter. The predicted collision occurred in July 1994.

Comet SL9 fragments distancing themselves over time.
Comet SL9 fragments drifting
apart over time

How about Earth?

Is Earth a target for wayward comets or asteroids? Scientists have determined that it is possible but unlikely. Duncan Steel, a member of Spaceguard, estimates that if an object were to slam into Earth, the size would range from very large objects (larger than 10 kilometers) to very small ones (30 meters or smaller). The odds of a very large object hitting Earth is once in 100 million to 1 billion years, and would cause total mass extinction!

The odds of a very small object striking Earth are once every 1 to 100 years. Such an impact would create less widespread destruction, like the one that occurred in the Siberian town of Tunguska on June 30, 1908. Astronomers aren't sure whether the object, which was about 30 to 60 meters across, was an asteroid or a comet. The object streaked across the sky, exploding about 5 kilometers above the remote Siberian town. The explosion was more powerful than the Hiroshima and Nagasaki nuclear bombs.

Most trees were incinerated just below the blast site, and forests were flattened as far as 30 kilometers away. A shock wave produced by the explosion blew carpenters off a building about 200 kilometers away. The closest surviving observers on record were some reindeer herders asleep in their tents about 80 kilometers from the blast site. The explosion blew the herders and their tents into the air. All of the reindeer herders survived, but many of their reindeer weren't as lucky. About 1,500 of them were killed.