The $40 billion Ground-based Midcourse Defense system was developed and deployed to intercept incoming ballistic missiles and consists of ground-based interceptor missiles, kill vehicles and radar located in Alaska and California.

But with a failed intercept test July 5, where a kill vehicle failed to separate from its booster, the Missile Defense Agency has had only eight successful hit-to-kill intercepts out of 17 since 1999.

In July, Vice Adm. James D. Syring, director of the Missile Defense Agency, told the Senate Appropriations Defense Subcommittee the capability enhanced-1, or CE-1, interceptor missile launched properly but failed to intercept the target.

He noted that a review was launched to determine the cause of the failure but added that the test fulfilled its secondary objectives.

Syring made the case that the testing process should continue while the Defense Department proceeds with its plan to add 14 new interceptors to the system by 2017, at a cost of $75 million per missile.

“We have obtained three now out of four intercepts with the version that we just flew in July,” Syring added.

Expansion of the system, Syring stressed, hinges on a successful intercept by the next-generation kill vehicle, the CE-2. However, a test of the CE-2 kill vehicle also failed in December 2010, as did another CE-2 test in January 2010. The last successful missile defense test was back in December 2008.

The test failures have caused some lawmakers and experts to question the viability of the missile defense system

Indeed, Richard J. Durbin, D-Ill., chairman of the Senate Appropriations Defense Subcommittee, asked Syring in July whether half of the 30 deployed intercepters have obsolete parts and another 10 have been taken off operational status because of a known design flaw.

Syring said the system has undergone extensive upgrading and that the CE-1 kill vehicle, which failed its test in early July, was one of those that had been upgraded.

“I won’t stipulate the number due to classification,” Syring told Durbin, “but there are a number of GBIs that are available to the war fighter, but in a lesser readiness condition, but still usable by the war fighter.”