This fact sheet was originally released in 2011, then updated and re-released in June 2015, and again in 2017.
A space-based boost-phase missile defense system is intended to target ballistic missiles in the first few minutes of flight, while the missile’s engines are burning and providing a bright target. 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. 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.
Learn more by downloading the fact sheet.