There may very well be primitive alien life on other planets, believes University of Cambridge astrophysicist Stephen Hawking, but it might not be intelligent enough to send signals to communicate with us. "Primitive life is very common and intelligent life is fairly rare," he said Monday, during a speech at George Washington University celebrating the 50th anniversary of NASA. Hawking also called for further space exploration, saying that if "the human race is to continue for another million years, we will probably have to go where nobody has gone before." That would include a base on the moon, which he'd like to see by 2020, as well as a manned mission to Mars by 2025. —Heather Wax