Confirming a comet's belated return.

When Comet Swift-Tuttle last visited the inner solar system, its icy glow shared the American skies with the flash of Civil War cannons. More than a century later, astronomers used the 1862 sighting to peg the comet's return for about 1981. They waited. But Swift-Tuttle didn't show.

Finally, Swift-Tuttle has graced the inner solar system again, albeit a decade later than expected. The comet's recent sighting confirms a 1973 prediction by Brian G. Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., that Swift-Tuttle might return in 1992.

The solar family includes many comets whose round-trip journeys are measured in centuries. However, Swift-Tuttle has set a record as the longest-period comet whose return has been predicted and then confirmed by observation.

This comet is the source of the debris that rains down upon Earth every summer during the Perseid meteor shower. Thus, the frozen wanderer's return provides astronomers with a unique opportunity to combine observations of a comet with existing information on its dusty footprint, says Marsden.

In 1973, Marsden calculated that the comet would probably return in 1981, give or take two years. He based this prediction on the 1862 observations and the gravitational effects of the sun and planets.

But Marsden also offered an alternative prediction that took into account the presumed effects of gas outbursts from the comet as well as information from a 1737 sighting of a comet he thought could have been Swift-Tuttle. In this second calculation, Marsden gave the comet an "outside chance" of returning to the inner solar system in 1992 instead of 1981.

Then, early on the morning of Sunday, Sept. 27, comet enthusiast Tsuruhiko Kiuchi of Usuda, Japan, spotted a fuzzy ball among the stars of the Big Dipper. Professional astronomers soon confirmed the object as Comet Swift-Tuttle, back in its old neighborhood for the first time in 130 years.

Swift-Tuttle's orbit shows an unprecedented difference of 11 years between its expected date of return and its actual arrival. In comparison, Comet Halley's schedule is off by only four days per round trip. Marsden and others believe these discrepancies result from forces exerted by outbursts of sun-warmed gas from the comets' innards.

Astronomer Donald Yeomans of the Jet Propulsion Laboratory in Pasadena, Calif., says Swift-Tuttle provides a rare opportunity to study the long-term behavior of a comet. "There's a big controversy about how long these things last," he says. "Do they last tens of returns, thousands of returns, tens of thousands?"

The latest observations indicate that Marsden will have to wait until Dec. 12 to observe his prodigal comet at perihelion--the point in its orbit nearest the sun--instead of his originally projected date of Nov. 25. This slightly skewed calculation does not trouble Marsden much, however. "A 17-day error out of 130 years is good enough for me," he says.