Using Space to Save Earth   Veteran science journalist William E Burrows offers a bold new mission for the U.S. space program: to protect the Earth from the ever-growing number of perils that threaten our way of life--and even our very survival.   We are living through one of the most dangerous times in human history. Nuclear, chemical, and biological weapons technology are proliferating, and missile technology is falling into more and more hands. Extreme natural disasters like hurricanes, floods, fires, and earthquakes, are becoming increasingly costly -- not only in dollars, but in lives -- as population expands. Environmental crises threaten to provoke massive famines and widespread social collapse. Asteroids the size of battleships streak within striking distance of the earth every year.   One strategy offers the best hope of protecting us from all of these dangers -- a revitalized national space program that coordinates efforts in global defense, in environmental protection, in communications, and in military security. The Survival Imperative offers an impassioned argument for this bold initiative.   To demonstrate the urgency of his cause, Burrows presents a vivid scenario: an impact by a moderately large asteroid that triggers a series of nuclear exchanges, environmental devastation, and the slow disintegration of civilization. And he examines the existing space program from the heady days of the Moon landing through the political compromises that have characterized the history of NASA in the 35 years following our retreat from the Moon.   Most of all, Burrows warns that the primary obstacle to achieving a true planetary defense program is not financial or scientific, but social--an unwillingness to acknowledge the urgency of the crisis, and to take the political risks needed to address it. The question, says Burrows, is not whether we can do it, but whether we will act before it's too late.