The head of NASA has warned that it is time to take the threat of an Earth-destroying asteroid seriously.
Jim Bridenstine spoke as the space agency began its planetary defence conference exercises, based around the hypothetical scenario of an asteroid impact.
NASA's administrator expressed concern that similar warnings have historically prompted amusement and said that asteroid collisions were more common than people believed.
In the planetary defence exercises this week, simulating a global response to an Earth-destroying meteorite, NASA's first step will be to precisely measure the object's speed and trajectory.
After that, a decision will have to be made: attempt to prevent the collision by deflecting the asteroid, or evacuate the area it could crash into.
Planetary defence and astronomical experts have warned against humanity destroying large asteroids with nuclear weapons out of the concern that they would simply shatter into smaller dangerous pieces.
A scientific study released in March poured cold water on the film theory that humans could simply blow up an asteroid on a collision course with Earth, and found that blasted away fragments from the asteroid would be likely to reform with it because of gravity.
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The most likely response is to evacuate the area which the asteroid is expected to collide into, especially if the asteroid is smaller than 50m (164ft) across.
Mr Bridenstine noted three devastating asteroid impacts which had occurred over the last century.
The most recent he mentioned occurred in 2013, when a meteor exploded over the Russian city of Chelyabinsk, injuring hundreds of people.
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