Every day, millions of objects, with a combined weight of 20 elephants, hurtle down on us from space. But these “missiles” are just tiny, dust-sized bits of asteroids and comets. Most vaporize before reaching the surface. And the ones that do hit are so tiny that they cause no harm.
Encounters with larger extraterrestrial objects, though, have been more jarring. In February 2013, an asteroid about 66 feet (20 meters) wide, the size of a five-story building, exploded over Chelyabinsk, Russia. It injured 1,600 people. Objects larger than 459 feet (140 meters) across—the height of the Washington Monument—can be still more devastating. One such monstrosity slamming into Earth would release the energy of 60 million tons of TNT.
Unfortunately, more than 2,000 objects of this size or larger occasionally stray near the Earth as (or PHOs) if they come within five million miles (eight million kilometers) of our home.
