Space-Based Missile Defense: Why It Would Reduce U.S. Security
June 2011 Fact Sheet
A space-based boost-phase missile defense system would target ballistic missiles during launch, while the boosters were burning. Proponents argue that by engaging a missile during boost phase, space based interceptors (SBIs) could avoid the crippling problems that plague interceptors designed to engage warheads during midcourse phase, when the warhead is above the atmosphere. And a boosting missile is a more attractive target than a warhead because it is large, easy to detect (given its large plume), and vulnerable to attack (as it is not hardened).
However, such a system would require many hundreds of orbiting interceptors to defend against one or two missiles, and it would have serious inherent vulnerabilities that would render it ineffective.
The fact sheet linked above provides a more detailed discussion.
