[meteorite-list] U.S.-European Team Map Asteroids, Some Of Which Could Threaten Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:33 2004
Message-ID: <200107231509.IAA22732_at_zagami.jpl.nasa.gov>

http://www.spacedaily.com/news/asteroid-01g.html

U.S.-European Team Map Asteroids, Some Of Which Could Threaten Earth
SpaceDaily

Ithaca - July 23, 2001
A recent study portrays the paths of asteroids in the inner solar system as
a vast Los Angeles-style traffic system crisscrossed with superhighways
along which are hurtling huge, rocky projectiles. And in the middle of the
highway network, on a possible collision path, is the planet Earth.

The study estimates that an armada of asteroids, 900 strong, all a kilometer
in diameter or larger, present a potential hazard to life on Earth. Some
pass within a few moon distances of Earth every year. "Sometime in the
future, one of these objects could conceivably run into the Earth," warns
astronomy researcher William Bottke at Cornell University.

"One kilometer (about .6 of a mile) in size is thought to be a magic number,
because it has been estimated that these asteroids are capable of wreaking
global devastation if they hit the Earth."

Bottke is lead researcher on a U.S.-French team that has discovered the
spatial and size distribution of a large group of asteroids called NEAs (for
near-Earth asteroids), a vast system of orbiting rocks in inner space,
ranging in size from mere specks to more than 64 kilometers (40 miles) in
diameter.

The astronomers believe the results of their observational and
computer-based study will better quantify the likelihood of future
catastrophic collisions with Earth. The survey also is expected to help
observational astronomers in improving their search for hard-to-find
asteroids that might pose a threat to the planet.

The team's report, "Understanding the Distribution of Near-Earth Asteroids,"
appears in the latest edition (June 23) of the journal Science. The authors,
besides Cornell's Bottke, are astronomers with the Spacewatch group at the
University of Arizona's Lunar and Planetary Laboratory and at the
Observatoire de la Cote d'Azur in Nice, France.

Calculating which, if any, of the 900 asteroids identified in the study
could hit the Earth is tricky, says Bottke. "The problem is that fewer than
half of these Earth-threatening asteroids have been discovered so far.

Of those we have found, we can accurately predict whether they will strike
the Earth over the next hundred years or so, but we can't project out
several thousands of years. So it's possible some of these asteroids
eventually will move onto an Earth-collision trajectory. It's a dangerous
place out there."

The new predictions for the distribution of NEAs in the inner solar system,
say the astronomers, imply that 40 percent of the kilometer-or-larger
asteroids near Earth already have been discovered.

The remaining 60 percent, however, might be more difficult to find, says
Bottke. "Most of these asteroids are too far from Earth to be easily
detected or are located in regions of the sky that are challenging for
astronomers to survey."

The study's authors refer to their survey as a "NEA treasure map" indicating
in which orbits most NEAs spend their time. The researchers say the new
estimate of the number of large asteroids is about half of that predicted by
similar types of analyses reported in the past decade and is slightly larger
than an estimate published recently in the journal Nature.

For many decades there has been good evidence that most of the small chunks
of rocky or iron material that slam into the Earth's atmosphere daily are
chips off old blocks of asteroids.

Most of the asteroids in the solar system revolve around the sun on
independent orbits, corralled between Mars and Jupiter in a formation known
as the main belt.

Occasionally, two of these asteroids -- some of them hundreds of miles in
diameter -- slam into each other at great speed, causing chunks of all sizes
to be blasted off the surfaces.

Most of this material continues to orbit the sun in the main belt. But
sometimes the newly formed asteroids migrate to unstable regions of the
asteroid belt known as resonances, areas where the tiny gravitational kicks
produced by nearby planets such as Mars, Jupiter or Saturn can significantly
change asteroid orbits. In some cases, these changes are enough to swing
asteroids into a possible future collision path with the Earth.

To find the location of these potentially threatening and hard-to-find
projectiles, the researchers used the results of the Spacewatch group's
10-year search for asteroids in the solar system during which it has
discovered about 100 NEAs. The problem is that this tally is only a small
fraction of the predicted number of NEAs.

Using a statistical technique to compensate for the big gaps, Spacewatch
astronomers were able to calculate the total number of NEAs but not their
approximate location.

To obtain the orbits of the undetected NEAs, Spacewatch astronomers combined
their NEA population estimates with theoretical models, produced by the
Cornell and Nice researchers, which show how asteroids in the main belt are
transported to the near-Earth environment.

Other authors of the study were Robert Jedicke of the University of Arizona
and Alessandro Morbidelli, Jean-Marc Petit and Brett Gladman of the
Observatoire de la Cote d'Azur. The study was funded by NASA and the
European Space Agency.
Received on Mon 23 Jul 2001 11:09:57 AM PDT