Two decades ago, President Ronald Reagan memorably asked what the United States could do to stop a nuclear missile hurtling toward an American city, and was memorably told by an Air Force general, "Sir, we could do absolutely nothing." Aghast, Reagan set in motion a series of technology projects that, while plagued by snafus, may soon grant America at least some protection from nuclear destruction. Reagan's instinct--that it was irresponsible for government to do nothing about a threat to millions of lives--was the right one. A few weeks ago, researchers at the Harvard-Smithsonian Center for Astrophysics briefly believed that an...