How fast is an asteroid?

Asteroids have an average speed of 25 kilometers per second or roughly 15.5 miles per second. This measurement comes from scientists who monitor asteroids in earth's vicinity.

Astronomers have been observing asteroids for centuries, using everything from the naked eye to advanced equipment like radio telescopes. Today, researchers monitor asteroids in part because of their potential impact with the planet. Scientists believe that up to 90 percent of large asteroids are under surveillance, and that despite the speed of these rocky objects, in the event of a collision, humans would have up to a century to prepare for a large impact.