Ohio State University
Contact: Jay Frogel, (614) 292-5651; firstname.lastname@example.org
Andrew Gould, (614) 292-1892; email@example.com
Written by Earle Holland, (614) 292-8384; firstname.lastname@example.org
July 31, 1998
COMETARY IMPACT WITH EARTH UNLIKELY IN THE NEXT 500,000 YEARS
COLUMBUS, Ohio -- Contrary to Hollywood's latest predictions, it is highly
unlikely that a comet will rain death and destruction on the earth during
the next half-million years, according to a new study.
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
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
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
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.