A group of physicists today said the Bush administration's plan to protect the United States from ballistic missiles by destroying them in the boost phase of flight is impractical because the interceptor rockets would have to be far larger and faster than any now in development. The administration is asking Congress for $626 million in fiscal year 2004 to fund research into boost-phase missile defense programs. The plan is to develop ground- and space-based kinetic-energy interceptors and a high-energy laser...