Unleashed viruses, environmental disaster, gray goo--astronomer Sir Martin Rees calculates that civilization has only a 50-50 chance of making it to the 22nd century. Death and destruction are not exactly foreign themes in cosmology. Black holes can rip apart stars; unseen dark energy hurtles galaxies away from one another. So maybe it's not surprising that Sir Martin Rees, Britain's Astronomer Royal, sees mayhem down on Earth. He warns that civilization has only an even chance of making it to the end of this century. The 62-year-old University of Cambridge astrophysicist and cosmologist feels so strongly about his grim prognostication that...