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In the intervening two decades, astronomical hardware has improved markedly, giving scientists the chance to study comets in unprecedented detail. They want to know precisely what these bodies are made of, since comets are believed to be the only objects that have remained unaltered since the solar system was born, 4.5 billion years ago. The planets and the asteroids have been heated and cooled, smashed apart and re-formed, but the comets, lingering on the solar system's periphery, have stayed relatively pristine.

So goes the theory, at least, and early studies of Hale-Bopp's gases bear this out. "We've found a type of hydrogen cyanide that's otherwise seen only in interstellar space," says Owen. "We saw it in Hyakutake too, but we thought it could be a fluke." Astronomers have found other gases they suspected would be there, including ammonia, methane, alcohol, formaldehyde and other organic compounds. Says Michael Mumma, an astronomer at NASA's Goddard Space Flight Center: "It's been suggested that both the building blocks of life and the water in our oceans fell to Earth on comets. Our observations of Hale-Bopp may help settle that question."

Astronomers also believe a comet impact is probably what did in the dinosaurs 65 million years ago. That object was perhaps 10 miles across. At double the size, Hale-Bopp packs a lot more potential energy. Luckily for civilization, Hale-Bopp will miss Earth by 120 million miles.

That should reassure worriers, but it's too bad for sky watchers. Hale-Bopp will be brighter than Hyakutake was, but it's also 15 times as far away. "If Hale-Bopp came as close as Hyakutake did," says Harvard's Green, wistfully, "it would be incredible. You'd even be able to see it easily in the daytime." Along with the rest of us, he's going to have to settle for what will merely be the best celestial show in decades.

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