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Comets Get a Galactic Boot

Every few years, it seems, scientists identify a new comet. But why comets suddenly swim into view has long remained a mystery. Now a report in the November Astrophysical Journal suggests that gravity from distant stars in our galaxy appears to knock them from their roost and carry them into Earth's neighborhood.

Most long-period comets--those that orbit the sun every 200 years or more--come from the Oort cloud, a mass of icy cometary material that lies outside our solar system and orbits the sun. Astronomers now believe that these comets are nudged out of the cloud by two types of "gentle galactic tugging," says co-author John Matese, an astrophysicist at the University of Southwestern Louisiana. One type, the z-tide, is exerted by stars and matter within 100 light years of our solar system. The other is the much-weaker radial tide, which is caused by distant matter in the center of the Milky Way.

After measuring the size of deviations in the orbits of 86 comets, Matese claims that the radial tide--despite being roughly 16 times as weak as the z-tide--has dislodged about a third of the comets that originate in the Oort cloud. He also posits that the  solar system's "merry-go-round" movement around the galactic center causes the radial tide to ebb and flow, varying the number of comets to reach our solar system over a cycle of roughly 30 million years.

Not everyone agrees with his interpretation of the data. "It's a nice try, but I'm a very hard person to convince," says Brian Marsden, an astronomer at the Harvard-Smithsonian Center for Astrophysics, who has provided much of the data that Matese used. "I have no problem with the theoretical analysis," he says, but  it is "a miserable, crummy set of data." Better data, he believes, could yield different results.