Stephen Hawking. Sarah Lee/ Science Musem/AP Photo

Stephen Hawking, the famed physicist, turns 70 on Sunday, and while he's done pioneering work on the great questions of the universe, he said the one he thinks about most is… women.

"They are a complete mystery," he said, probably tongue in cheek, when New Scientist asked.

Hawking, twice married and a father of three, is almost immobilized by ALS, but still communicates with the aid of a synthesized computer voice. He picks out letters one by one, commanding a cursor by twitching his cheek.

"The way he communicates can seem frustratingly slow to most people but he doesn't let that impede his thinking," said his assistant, Judith Croasdell, in an interview with the AP.

Asked for words of wisdom to mark his birthday, he said he's confident human beings will colonize space, and that neutrinos will not undo Einstein's work by proving to move faster than light.

Answering questions from listeners on Britain's Radio 4, he said, "I believe that we will eventually establish self-sustaining colonies on Mars and other bodies in the Solar System, although probably not within the next 100 years.

"I am optimistic that progress in science and technology will eventually enable humans to spread beyond the Solar System and out into the far reaches of the Universe."

He worries that global warming and nuclear war will undo us if we don't settle other worlds. He's fascinated by variations in the cosmic microwave background, the telltale remnant of the big bang. And he still thinks about how all-consuming black holes really are.

New Scientist asked him what he would do if he were a young physicist just starting out today. His answer: "I would have a new idea that would open up a new field."