Sea-based missile defense is being advocated as an alternative to the Clinton administration's limited land-based national missile defense (NMD), which is in the early stages of testing. Proponents of sea-based NMD (which is only a concept, not a program) argue that such a system can be deployed more quickly and will be less expensive than the Clinton administration's land-based system. Some argue that the Navy Theater Wide (NTW) system--which is being designed to provide midcourse intercept capability against slower, shorter-range theater ballistic missiles--can be upgraded to attack long-range intercontinental ballistic missiles (ICBMs) in their boost phase (when under powered flight at the beginning of their trajectories). Interestingly enough, advocates of sea-based NMD include not only traditional supporters of missile defense but also people who were previously opposed to missile defense.
The claims made about sea-based NMD and boost-phase intercept capability should be viewed skeptically. The current NTW system does not have boost-phase capability against ICBMs. The NTW interceptor cannot be easily and cheaply modified to provide such capability. In fact, a new interceptor would have to be designed and built, and a faster, larger interceptor with boost-phase capability might not be compatible with the vertical launchers on Aegis ships. Moreover, the technological problems associated with sea-based NMD are compounded by operational issues.
Finally, sea-based NMD is really not a "national" missile defense system; it is the first step to a much larger and more expensive global missile defense system. Thus, the claims that sea-based NMD is inexpensive do not ring true. Even if initial costs of sea-based NMD are relatively low (which is a doubtful proposition), the follow-on costs to deploy space-based defenses will be significantly greater--in all likelihood by an order of magnitude or more--and certainly much greater than the cost of a limited land-based system.