A black hole hitting Earth sounds like the ultimate doomsday scenario. But it probably won't hurt much, say researchers who created computer simulations to see what would happen if a puny primordial black hole, born just after the big bang, struck our planet. If they exist, such black holes constitute some of the galaxy's dark matter; they're much smaller than the black holes we know about, having the masses of asteroids but the diameters of atomic nuclei. As the scientists will report in The Astrophysical Journal, the hypothetical black hole would zip through Earth in about a minute, barely shaking the world's surface  the way a very weak earthquake would. Larger primordial black holes would shake the ground more but are thought to be much rarer. Don't stay up waiting for those tremors, however: Even collisions with the smallest and most common primordial black holes should happen no more than once every few million years. That's good news for everyone, except those who'd like to see whether these exotic objects really exist.
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