IT WAS, we wrote on 23 July 1994, the astronomical event of the century: the first time we had ever had advanced notice of a collision between a comet and a planet. And it lived up to its billing. “Only the most naive amateur astronomers could have been disappointed by the fireworks when the fragments of Comet Shoemaker-Levy 9 hit Jupiter this week,” wrote our correspondent Jeff Hecht.
The comet had been discovered by astronomers Carolyn and Eugene Shoemaker and David Levy the year before, orbiting Jupiter on a wildly eccentric course – one that would lead to its spectacular end in the upper atmosphere of the gas giant.
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Better still, from a planetary science point of view, there would be more than one collision to observe. When the comet was spotted, it had already been broken up into pieces by Jupiter’s gravity, thought to have happened in 1992, with the rocks strung out in a line like pearls.
“The fragments hit the far side of Jupiter, so the collisions could not be observed directly from Earth,” we advised anyone worried they might have missed something. “However, they were close to the visible edge of the planet,” Hecht wrote, “and Jupiter’s rapid rotation – its day is only 10 hours long – brought the crash sites into view a few minutes after the fragments hit the Jovian atmosphere.”
Fragment A entered Jupiter’s southern hemisphere on 16 July 1994. Over the next six days, 21 impacts were seen. The largest was on 18 July, when fragment G hit, creating a dark spot over 12,000 kilometres across.
The following week, on 30 July, Hecht catalogued an exhausting week of feverish observation. “During the week-long spectacle, telescopes around the world recorded thousands of pictures at every possible wavelength – visible, infrared and ultraviolet,” he wrote. “The cataclysmic images included fireballs 2000 kilometres across rising above the rim of Jupiter.”
The images were enough to frighten the US Congress into taking the possibility of asteroids and comets heading towards Earth a bit more seriously. “As pieces of comet pelted Jupiter last week,” Hecht reported, “the House Committee on Science, Space, and Technology decided that NASA should be required ‘to catalogue and track any major comets or asteroids that may cross the orbit of the Earth’.” Simon Ings
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