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Close Encounters of the Stellar Kind

WASHINGTON, D.C.--Astronomers have honed their ability to forecast the motions of nearby stars--some of which, they have found, may pass close enough to our solar system to nudge distant comets toward Earth. But the likelihood of seeing a civilization stopper sent our way is slight: The new analysis, described here last week at a meeting of the American Astronomical Society, suggests that only rarely will a star come close enough to the sun to possibly send new comets streaking toward Earth.

Researchers think a swarm of perhaps trillions of comets, called the Oort Cloud, envelops our solar system out to a distance of about 1 light-year. Even small gravitational changes can perturb the orbits of these icy bodies, because their ties to the remote sun are weak. Nearby stars could act as such triggers, because they gradually change their relative positions in the Milky Way over millions of years. If a star drifts within 1 or 2 light-years of the Oort Cloud, some scientists have theorized, it might provoke a million-year-long shower of up to 20 times more comets than the few per century that now invade the inner solar system. But until now, astronomers have lacked the techniques needed to gauge the trajectories of stars with enough precision to predict such events.

To reach that precision, astronomer Lawrence Molnar of the University of Iowa in Iowa City devised a way to account for the Milky Way's "Coriolis force"--a subtle force arising from the galaxy's rotation that pulls stars on curved paths with respect to each other. He also computed, more thoroughly than before, how a star's mass, speed, and distance from the sun all dictate how much it will stir up the Oort Cloud. Finally, as a test case, Molnar used the Very Long Baseline Array of radio telescopes to pinpoint the present position and recent motions of a massive triple-star system called Algol, which drifted by the sun 4 million years ago. Algol's closest approach was 13 light-years, he found--more than twice the distance previously calculated and not close enough to nudge any comets.

Molnar says the enhanced accuracy of his tracking technique, combined with an extensive new catalog of the locations and velocities of nearby stars from the European Space Agency's Hipparcos satellite, will help astronomers search for stars that might pack a greater Oort Cloud punch. "This is a very promising approach," says planetary scientist Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. However, Marsden predicts, "the chances are rather small that we will find a nearby star both massive and close enough to have a significant impact."