Humans: It's time to abandon   the earth or face extinction. This prediction isn't science fiction. It's a glimpse of the future, according to famed British scientist Stephen Hawking. "I see great dangers for the human race," he said in an interview with global forum   Big Think. "There have been a number of times in the past when its survival has been a question of touch and go  ." Hawking says humans must  _ our home planet within the next two centuries and expand into space to survive. "I believe that the long-term future of the human race must be in space," he said. "It will be difficult to avoid disaster on our planet Earth in the next hundred years, let alone the next thousand, or million." He also voiced some concerns about the future of life on the earth. In April, he spoke about his fear of aliens   in an interview for the Discovery Channel. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. "Such advanced aliens would perhaps become wanderers, looking to conquer   and colonize whatever planets they can reach." But Hawking says he is still hopeful for the future of humanity. "I'm an optimist," he told Big Think. "We have made remarkable progress in the last hundred years. But if we want to continue beyond the next hundred years, our future is in space." Getting to another planet will prove a challenge, not to mention colonizing it for humanity. University of Michigan astrophysicist   Katherine Freese told Big Think that "the nearest star to Earth is Proxima Centauri which is 4.2 light years away. That means, if you were traveling at the speed of light the whole time, it would take 4.2 years to get there" --- or about 50,000 years using current rocket science. Despite some concerns, Hawking's attitude towards the future of humans is   _  .
objective negative subjective positive