If mankind is to survive we must leave earth for colonies on other planets within the next 100 years, warns renowned physicist Stephen Hawking. "I see big dangers for mankind. Our only chance of long-term survival is not to remain inward looking on planet Earth but to spread out into space," he said in a Big Think interview. "Our future is in space." If we are the "only intelligent beings in the galaxy, we should make sure we survive." And if we encounter aliens in outer space? Be careful, warns Hawking, because they might not be so friendly.