According to a “newly released study”:http://cboblog.cbo.gov/?p=211 — see the “full report”:http://www.cbo.gov/ftpdocs/100xx/doc10013/02-27-MissileDefense.pdf — the -existing- Ground-Based Midcourse Defense (GMD) system, as it will exist in 2012 or so, would not be able to defend itself against attack from Iran. First, there’s the radars:

To intercept U.S.-bound missiles from Iran, the GMD Block 3.0 system will rely on tracking from radars in Fylingdales, England, and Thule, Greenland. However, it will not be capable of defending those radar sites against Iranian missiles, which makes the system vulnerable to attacks in which multiple missiles first target the radars and then target the United States. That vulnerability could be removed by using local defenses to protect the radars — say, Aegis ballistic missile defense ships or terminal-phase systems such as the Terminal High-Altitude Area Defense or Medium Extended Air Defense System.

And then there’s the main interceptor base itself at Ft. Greely, Alaska. Unless I misunderstand Fig. 3-1 on page 28, it’s not covered from attack by a hypothetical ICBM coming over the pole.

Not that it has to be; no such threat has yet emerged, and presumably won’t by 2012, either. (Right?) But it’s still a very interesting little quirk.

The assumptions of the study are given in a convenient appendix, for the curious.