Earth's asteroid risk reduced

Earth's asteroid risk reduced
Warmth of nearby space rocks gives clue to their size.
14 November 2003

By tom Clarke
Source

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 concludes1.

"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|


© Nature News Service / Macmillan Magazines Ltd 2003