Space travel leading to skylife is vital to human survival, because the question is not whether we will be hit by an asteroid, but when. A planetary culture that does not develop spacefaring is courting suicide. All our history, all our social progress and growing insight will be for nothing if we perish. No risk of this kind, however small it might be argued to be, is worth taking, and no cost to prevent it is too great. No level of risk is acceptable when it comes to all or nothing survival.