"I don't think the human race will survive the next thousand years unless we spread into space. There are too many accidents that can befall life on a single planet," Hawking told Britain's Daily Telegraph newspaper.

Hawking, Lucasian professor of mathematics at Cambridge University in England, said Armageddon threatened not in the form of a Cold War-style nuclear holocaust but could arrive in a more insidious and invisible form.

"In the long term, I am more worried about biology. Nuclear weapons need large facilities, but genetic engineering can be done in a small lab. You can't regulate every lab in the world," he said.

Investigators have not pinned down who is behind the U.S. anthrax attacks, but fears are growing they could be retaliation for U.S. military strikes against Afghanistan, which followed last month's suicide attacks on New York and Washington.

Hawking, a leading theoretical physicists who hit the best-seller lists with his book "A Brief History of Time", said the chances of humanity pulling through looked good.

"I am an optimist. We will reach out to the stars," he said.