One less thing to worry about: astronomers have decided that we are much less likely to get wiped out by a big asteroid than previously thought. The odds are only about 1 in 5,000 that an asteroid big enough to wipe out civilization will hit the Earth in the next 100 years, a team at Princeton University reported, which is far lower than previous estimates of 1 in 1,500.

Research on asteroids that have hit the Earth in the past, like the one that wiped out the dinosaurs 65 million years ago, shows that a collision with a large asteroid at least half a mile in diameter could kill a quarter of the world?s population. The same research has shown that these large asteroids strike the planet regularly, every 100 million years or so.

Zeljko Ivezic and his Princeton team used data from the Sloan Digital Sky Survey to estimate that the solar system contains about 700,000 asteroids big enough to do major damage if they hit the Earth. This is about one-third the number of earlier estimates that put the number at about 2 million.

Asteroids with a surface of carbon are dark, like lumps of coal, while rocky asteroids are much brighter. ?You don?t know precisely the size of an object you are looking at unless you know what type it is,? Ivezic says. The Sloan survey looks at the color of objects, so astronomers can distinguish between carbon and rock.

?Our estimate for the chance of a big impact contains some of the same uncertainties as previous estimates, but it is clear that we should feel somewhat safer than we did before we had the Sloan survey data,? Ivezic says.

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