The NEOP has identified 95% of asteroids of this most dangerous order, Chodas said.

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"The smaller ones may not be seen until a week, or days before impact, " Chodas said.

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Any asteroid as large as a half-mile across would cause a global catastrophe, if it struck the blue planet, Chodas said.

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"We would expect an event of this magnitude to occur once every 100 years, " said Paul Chodas of NASA's Near-Earth Object Program Office, in a statement.

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Paul Chodas says the object must have arrived quite recently or else it would have been easily detected by any of several automated sky surveys that astronomers are conducting.

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"We would expect an event of this magnitude to occur once every 100 years on average, " Paul Chodas of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory said last week.

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"We would expect an event of this magnitude to occur once every 100 years on average, " said Paul Chodas, of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in California.

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