Thursday, August 16, 2007

Doomsday?

Comet Swift-Tuttle, the parent of the Perseid meteor shower of August, was "discovered" during its apparition of 1862. It was one of the brightest comets of the 19th century.

As comets travel their elliptical courses, they shed part of their substance. Eventually the tracks of comets become dirty spaces, littered with icy grit and bits of stone. Once each year, the Earth intersects the orbit of Swift-Tuttle and sweeps up debris. We see these tiny bits of comet stuff, heated to incandescence by atmospheric friction, as "shooting stars."

In 1867, the Italian astronomer Giovanni Schiaparelli announced that he had calculated the orbit of Swift-Tuttle and that it was coincident with the tracks of the Perseids. It was the first time that a recurring meteor shower was convincingly linked to a periodic comet.

It was predicted that Swift-Tuttle would return in the early 1980s, after a long, slow sojourn in the outer reaches of the solar system. It did not appear on schedule. Brian Marsden, of the Harvard-Smithsonian Astrophysical Observatory, thought the calculations might be wrong. Ten years wrong. He calculated a return in 1992, and he was right. This time around, however, the comet did not achieve naked-eye visibility, which is why you probably don't remember it.

Keep in mind that the orbits of the Earth and the comet intersect. In 1992, the comet arrived at the intersection in December, four months after the Earth had passed by. That's why Swift-Tuttle wasn't as bright as on its 19th-century visit, when the encounter was closer.

Brian Marsden has another interesting prediction. When Swift-Tuttle makes its next appearance in 2126, he calculates a nearer rendezvous with Earth, even a very slim chance of collision. When the Earth reaches the appointed spot in mid-August of that year, it may not find just a sprinkling of cometary dust but the progenitor itself. Mark your calendars.