If a collision with an asteroid is going to finish us off, it will have to be a lot larger than anyone thought, according to a controversial new study of the impact that wiped out the dinosaurs. Virtually everyone agrees that the asteroid that hit Chicxulub in Mexico 65 million years ago killed the dinosaurs, but how it did so is unclear. A long-standing theory is that clouds of dust hung in the upper atmosphere for months, blocking sunlight and stopping plants growing. But no one is sure that this is really the reason, and finding out is critical for assessing the risk asteroids pose to humanity.
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