[meteorite-list] True Tally of Asteroids Probably at High End of Estimates

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:09 2004
Message-ID: <200207231615.JAA05233_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/asteroid_tally_020723.html

True Tally of Asteroids Probably at High End of Estimates
By Robert Roy Britt
space.com
23 July 2002

The continued rapid pace of discovery for large asteroids in relatively
close proximity to Earth suggests there may be more of them than some
scientists have predicted. The speculation, from one of the astronomers who
helps count the rocks, does not imply a significantly increased threat to
Earth, but it does extend a long-running debate over just how many of these
space rocks exist.

Large Near Earth Objects (NEOs) are asteroids bigger than 1 kilometer (0.62
miles) that loosely inhabit the region of the solar system through which
Earth orbits. More than 600 of these large NEOs have been found.

Not a single one is known to be on a collision course with the planet.

However, the big, Sun-orbiting hunks of metal and stone are of great
interest to scientists because of the potential for one being discovered
that could hit us down the road. Such an impact would cause devastation on a
global scale, possibly pushing humans into a Dark Age existence. Experts say
the scenario might be avoided by mounting a mission to deflect or destroy an
asteroid known to be targeting us.

Various research groups from around the world have taken their best shots in
recent years at calculating how many 1-kilometer and larger NEOs are out
there. Estimates have ranged from 700 to 2,000.

Astronomers frequently cite 1,000 as the best estimate these days. Another
popular count, however, is 1,200. Both prognostications are based on
detailed studies and have error margins of plus or minus 100 or 200
potentially deadly giant boulders.

If either of the leading estimates is correct, then just more than half of
all NEOs have been found.

To Brian Marsden, director of the Minor Planet Center where asteroids are
officially catalogued, logic dictates that the pace of discovery ought to be
slowing down, because there are now fewer NEOs that remain to be detected
and hence the chances of finding each one is reduced.

But the pace has not slowed.

"Since the new discoveries are indeed still coming along at about 100 per
year ... the number is probably not 1,000 minus anything, but more likely at
least 1,200 or so," Marsden told SPACE.com.

If Marsden is correct, astronomers would not be overly surprised. But his
logic does represent an unexpectedly simple method of suggesting that the
lowball estimates are probably not correct.

Meanwhile, SPACE.com has learned of a new estimate. It comes from NASA
researcher David Morrison and his colleagues and will be presented in a
forthcoming book titled "Asteroids III" (University of Arizona Press).

Morrison, senior scientist at the NASA Astrobiology Institute at the Ames
Research Center, said via e-mail that it's "pretty clear that the number is
not substantially less than 1,000." His team prefers 1,100, plus or minus
100 -- which, Morrison points out, encompasses the possibility of 1,200
suggested by Marsden.

Alan Harris is a co-author of the chapter in "Asteroids III" that deals with
the impact odds. Harris is also researcher at NASA's Jet Propulsion
Laboratory, and he spends a lot of his time making and analyzing these
estimates. Harris said the discovery rate continues to be high in part
because detection methods have improved. It's very difficult to know, he
said, how strong this effect is versus the possibility that there are simply
more asteroids out there than expected.

"A population of 800 is implausible in the light of present discovery rates
and the number already discovered," Harris said. One thousand is plausible,
he added, as is 1,200. "I have to say though that I would be less surprised
if the real population is greater than 1,200 than that if it is less than
1,000."

Ultimately, the exact number is of little consequence. The important thing
is simply whether or not one of them is headed our way.
Received on Tue 23 Jul 2002 12:15:46 PM PDT