[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Dangerous Debris

ASTRONOMY, November 1999, pp. 46-51: When Disaster Strikes

Dangerous Debris, pp. 48-49:

Of all threats from space, asteroids and comets are surely the most well
known. Since 1980, when Luis Alvarez, Walter Alvarez, Frank Asaro, and
Helen V. Michel proposed that a giant meteorite impact had caused the
extinction of the dinosaurs 65 million years ago, such impacts have
become fodder for movies, TV shows, plays, and talk shows. The subject's
popularity was enhanced when Comet Shoemaker-Levy 9 smashed into Jupiter
in July 1994, producing easily visible impact sites surrounded by rings
of clouds expanding at supersonic speeds.
The destructive force of an asteroid is daunting. A single bolide six
miles across (about the size of the asteroid that killed the dinosaurs
65 million years ago) would hit Earth with the force of about 100
million megatons of TNT, about 10,000 times greater than the world's
entire arsenal of nuclear weapons. If the object hit an ocean it would
produce tsunamis hundreds of feet high. The debris from such an impact
would carpet hundreds of miles of surrounding terrain with house-sized
boulders, while saturating the atmosphere with dust for years.
Fortunately, the threat from asteroids can be predicted. Astronomers
have estimated that about 2,000 asteroids more than a kilometer in
diameter cross Earth's orbit. Although only about 10 percent of these
have been discovered, a concerted effort could effectively map the
nearby threat, giving humanity warning of any future impact. Three
different projects, one run jointly by NASA and the Air Force and the
other two under the supervision of the Lunar and Planetary Laboratory of
the University of Arizona, today scour the skies for these
Earth-crossing asteroids, finding a handful of new objects each month.
The danger from comet impacts, however, is less predictable. Most comets
come from the Oort Cloud, orbiting the sun at about half a light-year's
distance. Directly observing these objects is currently impossible, so
Jay A. Frogel and Andrew Gould of Ohio State University recently found
another way. The two astronomers surveyed the positions and motions of
most nearby stars, theorizing that if one had recently passed close to
the sun it would have disturbed the Oort Cloud, triggering a downpour of
comets earthward. Since such a deluge would take several hundred
thousand years to reach the inner solar system, Frogel and Gould figured
a recent close approach would predict an impending cloudburst.
Fortunately for our immediate descendants, the scientists discovered
that in the last several million years no star has passed close enough
to disturb the Oort Cloud. Our distant descendants, however, will have
to contend with one star (Gliese 710) that in 1.3 million years will
pass close enough to turn the present drizzle of comets into a
thunderstorm.


Best regards,

Bernd

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------