Four of the system's nine major tests have been failures, and the five successes have been achieved under tightly controlled conditions.
Meanwhile, key components are absent, including a high-resolution, high-power, sea-based radar and two big satellite constellations.
President George W. Bush said two years ago he wanted the system up and running by September 2004. The Pentagon's Missile Defence Agency later said the intention was to make it operational by the end of 2004. Now Missile Defence Agency spokesman Rick Lehner says there is no firm timetable for activating it.
Creation of a missile defence system has been a goal of many US conservatives dating back to a space-based plan developed under President Ronald Reagan two decades ago. Bush touted his version during his re-election campaign.
The current approach, to shield America and its allies from missile attack by nations like North Korea, is based on the concept of using one missile to shoot down another before it can reach its target.
"What we have here is a developmental system that is well along," Defence Secretary Donald Rumsfeld told a recent briefing. "And at some point soon, it will have a modest capability" and can be "perfected and improved" over time.
Proponents argue even a rudimentary missile defence capability is better than none at all. But the latest failure presented another hurdle to the Pentagon's idea of deploying the system piece by piece rather than waiting for every element to be fully developed.
The December 15 test was the system's first in two years. The previous one, on December 12, 2002, also was a failure, with the interceptor not separating from its booster rocket and missing its target by hundreds of miles (km).
The next test could come in March or April, Lehner said, although the cause of the December 15 failure remains unknown.