A lucky anniversary

Posted by Rich Talcott
on Thursday, July 12, 2007
 
Several dark spots mar Jupiter’s atmosphere
after Comet Shoemaker-Levy 9 smashed into
it between July 16 and 22, 1994. NASA/Hubble
Space Telescope Science Team

Where were you 13 years ago? If you're like me, you were eagerly anticipating a once-in-a-lifetime event — with no clue as to how it would play out. Astronomers and backyard observers around the globe had their sights set on Jupiter. More than 20 fragments of a rogue comet known as Shoemaker-Levy 9 were steaming toward the giant planet at about 120,000 mph (200,000 km/h). The "great comet crash" would soon be here.

The story started about 100 years earlier, when Jupiter's mighty gravity captured a single comet. Then, in 1992, the now-orbiting comet skimmed just 12,000 miles (20,000 km) above the jovian cloud tops. Tidal forces from the close passage split the comet into a couple dozen pieces. Still, the comet lurked in the shadows, undiscovered here on Earth. That changed in March 1993, when astronomer Carolyn Shoemaker noticed a "squashed comet" on an image taken by Eugene Shoemaker and David Levy. Soon, astronomers noticed the multiple fragments and deduced its approaching destiny.

Astronomers didn't know what to expect, but predictions sprouted like dandelions in spring. The best guesses were that the collisions with Jupiter would release an energy equivalent to somewhere between millions and trillions of tons of TNT. But would there be any observational effects? The consensus was yes, but they would be subtle in optical light and hard to detect. Astronomy's then associate editor Dave Eicher summed up the prevailing view when he wrote: "Any visible effects such as disturbances in the planet's cloud tops will be visible only to keen-eyed observers who can detect the differences in jovian clouds from the previous day or hours."

Shortly after the first comet fragment hit Jupiter July 16, we knew Dave (and everyone else) was wrong. Each of the major pieces left behind an obvious dark spot that lasted for months. We were all glued to our eyepieces. I still remember targeting Jupiter with my 4-inch scope in September, and showing the planet to people who had never observed it before. They had no trouble seeing the spots. It was one of the few times novices could see and feel the excitement we all feel when we look through a telescope.

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