What is the chance of Earth being hit by a comet or asteroid?
Not much in our lifetimes — perhaps 1 in 10,000 — but over thousands or millions of years, major impacts become pretty likely. Ancient craters on Earth’s surface prove that large objects have hit Earth in the past, and there’s no reason to think this won’t continue in the future. The chance of an impact depends on the size of the object: the bigger the comet or asteroid, the smaller the chance, since there are many more small objects out there than large ones. Tons of debris — much of it in pieces smaller than grains of sand — strike Earth’s atmosphere and burn up every day. These are the “shooting stars” commonly seen at night. Some larger rocks survive their fiery descent to the surface; you can see some of these “meteorites” displayed in museums. The truly dangerous objects, those large enough to cause regional or global catastrophe when they hit, may appear once every few hundred thousand years. Therefore, the chance that such an object will hit us in any given year is roughly
