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Aha! some of you might be saying -- What about Shoemaker-Levy 9? Meanwhile, the rest of you are probably asking, what the heck is Shoemaker-Levy 9? It is -- or at least was -- a comet. It's not there anymore. And that's the whole point.
Traditionally, comets are named for their discoverers, and this comet was discovered by Carolyn and Eugene M. Shoemaker and David Levy. They had found a bunch of other comets as well, which was why this was number 9. (We'll call it SL9 for short.) They discovered it in 1993, and soon realized that SL9 wasn't orbiting the Sun -- it was orbiting Jupiter. Working back from what they knew about its orbit, they figured out that Jupiter had probably captured the comet sometime in the 1960s or 1970s.
But there were other startling discoveries. SL9 wasn't one big comet -- it was lots of little ones. The scientists figured out that, in 1992, a year before they discovered it, SL-9 got so close to Jupiter that the planet's gravity tore the comet to pieces, breaking it up into 21 large fragments and lots of smaller ones too small to see from Earth -- all of them lined up almost like beads on a string.
But the wildest news was yet to come. Working SL9's orbit forward, they realized that some or all of the comet fragments were going to crash into Jupiter in July 1994!
Astronomers around the world pointed their telescopes at Jupiter. Space-borne instruments like the Hubble Space telescope and the Galileo space-probe watched as well. Between July 16 and 22, scientists observed 21 impacts on the planet in the most spectacular fireworks show in history -- a display that demonstrated very clearly just how real the threat of large impacts was.
So, the question is, if one rock could break into 21 pieces and smash into Jupiter over the course of a week, why couldn't one rock split into two and smash into Earth a month or two apart, just the way it happens in the movies?
The answer is that the various pieces of SL9 were orbiting Jupiter, not the Sun. The pieces were traveling in a stretched-out circle around the planet, and the orbit was unstable. Jupiter was pulling the comet closer with every pass. Pull that orbit in so close that the orbital path actually goes through the planet, and, one after another, each fragment in that orbit will crash into the surface. If the comet fragments had been orbiting the Sun, Jupiter would have swept past them, probably only suffering one or two impacts at most -- but drastically altering the orbits of the other fragments.
SL9 got pulled into Jupiter's orbit because Jupiter is so big, and because it has such a strong gravity field that reaches so far out into space. One estimate is that Jupiter gets hit by one-kilometer-sized rocks two thousand to eight thousand times more frequently than Earth.1 Jupiter is also between Earth and the outer reaches of the Solar System, where comets come from. Any comet will have to pass Jupiter's orbit before it gets to us, and it will be much more likely to have its orbit affected by Jupiter's gravity than by Earth's. There just isn't any likely way for a comet to get pulled into Earth's orbit the way SL9 was pulled into Jupiter's orbit.
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