Written by Earle Holland
7/31/98
Two Ohio State University astronomers reported in Astrophysical Journal Letters that a new review of the motions of thousands of nearby stars failed to show any rogue stars capable of pulling comets out of their orbits and into the earth's path.
Jay Frogel and Andrew Gould, professor and associate professor of astronomy at Ohio State, were looking for evidence of the so-called "death star" scenario where a passing star might alter the current orbits of comets near our solar system and send them our way.
There is ample evidence both on earth and on other planets, they say, that shows comets and asteroids have impacted with devastating results. Two new movies -- "Deep Impact" and "Armageddon" -- depend on this premise for their drama. Frogel's interest, however, was spurred by geological evidence of such past impacts, he says, and not by the new movies.
He and Gould turned to a relatively new resource to conduct their search -- the HIPPARCOS catalogue. In 1989, the European Space Agency launched the HIPPARCOS satellite with its mission to accurately measure the location and motion of more than 120,000 stars.
Astronomers believe a massive cloud of comets -- the Oort Cloud -- lies as much as 100,000 AUs out from the sun, surrounding our solar system. (An AU is the distance between the earth and the sun -- approximately 93 million miles.) If a star passed through that cloud, its gravitational field might nudge a comet out of orbit and towards the earth.
Frogel and Gould looked in the HIPPARCOS Catalogue specifically for stars with near zero proper motion -- stars that were either coming directly in our direction, or moving directly away. Any star that had already passed would appear to be moving directly away.
"For all intents and purposes, you should just see a star that appeared not to be moving at all," Gould said. The one potential candidate the researchers did find turned out to be a star previously identified by other scientists. They failed also to find evidence of stars that may have already passed nearby.
Gould's analysis of the HIPPARCOS catalogue showed that it should be sensitive enough to detect zero proper motion of any stars brighter than 8th magnitude. Eighth magnitude stars appear about 25 times fainter than those visible to the naked eye.
Gould said that these bright stars are important candidates for the death star scenario. "They're bright either because they are close by or because of their size," he said. The larger the star, the greater it's gravitational effect might be on nearby comets.
"We showed that theoretically, about 96 percent of the possible damaging events (the passing of such stars) should show up in the HIPPARCOS catalogue," Gould said. They had defined a "damaging event" as a star passing within 20,000 Aus of the sun.
Frogel and Gould are cautious with their predictions -- "We can't guarantee that a comet won't hit the earth next year." Their analysis of the catalogue, however, makes it "unlikely that a major (comet) shower will occur in the next half-million years."
Gould said, "The chance that a big enough star to cause significant damage would go through (our region) in the next 10 million years is extremely small."
Frogel said he and Gould are confident about their analysis of the HIPPARCOS catalogue. The next step would be to seek a "death star" candidate among stars that were too faint to be included in HIPPARCOS.
Another satellite -- GAIA -- has been proposed by ESA which would measure the motions of 50 million objects, including stars as faint as 15th magnitude. If approved, GAIA would be launched no sooner than the year 2009.
Some support for this research came from the National Science Foundation.
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