Hawking's comments were part of a lecture at George Washington University on Monday in honor of NASA's 50th anniversary. He theorized that there are possible answers to whether there is extraterrestrial life.

One option is that there likely isn't life elsewhere. Or maybe there is intelligent life elsewhere, but when it gets smart enough to send signals into space, it also is smart enough to make destructive nuclear weapons.

Hawking said he prefers the third option:

"Primitive life is very common and intelligent life is fairly rare," he then quickly added: "Some would say it has yet to occur on earth."

So should you worry about aliens? Alien abduction claims come from "weirdos" and are unlikely. However, because alien life might not have DNA like us, Hawking warned: "Watch out if you would meet an alien. You could be infected with a disease with which you have no resistance."

The 66-year-old British cosmologist, who suffers from ALS and must speak through a mechanical device, believes "if the human race is to continue for another million years, we will have to boldly go where no one has gone before."

Hawking compared people who don't want to spend money on human space exploration to those who opposed the journey of Christopher Columbus in 1492.

"The discovery of the New World made a profound difference to the old. Just think we wouldn't have had a Big Mac or KFC."