If North Korea ever unleashed nuclear-armed missiles against America, the defense of U.S. cities and towns would depend to no small degree on something called a divert thruster.

These small rocket motors would be counted on to keep U.S. anti-missile interceptors on target as they hurtled through space toward the incoming warheads.

If the thrusters malfunctioned – and they have a record of performance problems – an interceptor could veer off-course, allowing a warhead to slip through. The consequences could be catastrophic.

So a lot was at stake when the Pentagon’s Missile Defense Agency conducted the first flight test of a new and supposedly improved version of the thruster on Jan. 28.

An interceptor launched from Vandenberg Air Force Base in California was to make a close fly-by of a mock enemy warhead high above the Pacific. The interceptor’s four attached thrusters would provide precision steering.

The missile agency issued a news release that day touting a “successful flight test.” The agency’s lead contractors were no less effusive. Aerojet Rocketdyne Inc., maker of the thrusters, said the new model “successfully performed its mission-critical role.”

Raytheon Co., which assembles the interceptors, said the “successful mission proved the effectiveness of a recent redesign of the … thrusters, which provides the control necessary for lethal impact with incoming threats.”

In fact, the test was not a success, the Los Angeles Times has learned. One of the thrusters malfunctioned, causing the interceptor to fly far off-course, according to Pentagon scientists.

The mishap raises fresh questions about the Ground-based Midcourse Defense system, the network of ground-launched interceptors that is supposed to protect the United States against a nuclear attack.