Comet Collision
Some of that uncertainty comes from asteroids’ sometimes forgotten cousins, comets. (Comets are made up of ice and dust, while asteroids are made up of rock and metals.)
Hartley 2 came within 11 million miles of Earth on Oct. 20, which was among one of the closest times a comet has gotten to Earth in centuries.
“Comets are especially dangerous because they are coming from farther distances, at higher velocities,” Asphaug said.
Comets zoom through space at almost 100,000 mph and pick up speed due to Earth's gravitational pull, he said. The faster an object moves, the bigger the force it exerts on whatever it happens to hit and the more energy it deposits. For Earth, that means more damage. For humans, it may spell out R.I.P.
To add insult to potential injury, finding comets in the outer solar system is very difficult because these dirty snowballs are extremely dark.
But when comet gets within about 390 million miles from Earth, the sun heats comets' dark surface and starts to warm its icy interior, making it spew out the dust and gas that form its distinctively bright tails.
Assuming astronomers developed the technology to discover Earth-bound comets farther away than Jupiter, scientists might have about 10 years before a comet hit Earth in a worst case scenario, Asphaug said.
But “if there’s a 10-kilometer (6-mile) hunk of ice and rock that’s heading straight toward the Earth," he added, "there aren’t very many options there, except to do the Bruce Willis thing,” and nuke it.
Image courtesy of National Science Foundation