[meteorite-list] Earth's Asteroid Risk Reduced

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:48 2004
Message-ID: <200311141720.JAA22133_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/031110/031110-14.html

Earth's asteroid risk reduced

Warmth of nearby space rocks gives clue to their size.

TOM CLARKE
Nature Science Update
14 November 2003

A new survey revises down the likelihood of a massive
asteroid hitting the Earth by 20-30%. We're only due to
collide with rocks larger than one kilometre across roughly
once every 600,000 years, it concludes[1].

"There was a lot of error in our previous estimates," says
astronomer Alan Harris of the German space agency, DLR.
"It's all because near-Earth asteroids are somewhat
brighter than we thought".

Near-Earth asteroids, or NEAs, are too small and too
far away to measure directly, so astronomers approximate
their size from how much light they reflect. But reflectiveness
varies among asteroids of the same dimensions, thanks to
different rock types or dust coatings, says Harris.

So instead his team used infrared detectors on the powerful Keck
telescope at Mauna Kea in Hawaii to calculate the warmth of 20
NEAs - or how much energy each absorbs.

Objects either reflect or absorb the light that reaches them. So
subtracting an asteroid's warmth from the total light that falls on it
from the Sun gives a better measure of how reflective it is, and hence
how large, the researchers argue.

Applying the results of the sample to the 2,200 known NEAs,
suggests that around 1,090 are more than a kilometre across.
Previous estimates put the number between 1,200 and 1,300.

The analysis doesn't change the chance of an asteroid hitting the
Earth, points out astronomer Iwan Williams of Queen Mary
University of London, UK. "But assuming that there are fewer large
asteroids, the damage will be less," he says.

References

    1. Delbó, M., Harris, A. W., Binzel, R. P., Pravec, P. & Davies,
       J. K. Keck observations of near-Earth asteroids in the
       thermal infrared. Icarus, 166, 116 - 130,
       doi:10.1016/j.icarus.2003.07.002 (2003). |Article|
Received on Fri 14 Nov 2003 12:20:31 PM PST