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If you get asteroids about a kilometer in size, those are large enough and carry enough energy into our system to disrupt transportation, communication, the food chains, and that can be a really bad day on Earth.

"Cosmic Coincidence: Asteroid Careens by as Meteor Delivers ‘Buzz Cut’ to Earth". www.pbs.org. February 15, 2013.
If you get asteroids about a kilometer in size, those are large enough and carry enough energy into our system to disrupt transportation, communication, the food chains, and that can be a really bad day on Earth.