Other guidance systems -- both on the ground and aboard the rocket -- worked well, said a senior military official.

The official compared the test to finding a house. "You start off with the satellite sensors telling you the state, the early warning radar telling you the ZIP code, while the [radars aboard the rocket] get you to the street address," said the official. "What we failed to do is ring the doorbell."

The official said the infrared sensors guide the kill vehicle in the final seconds of flight. The test -- called integrated flight test 4 -- pitted the experimental national missile defense system against a simulated warhead launched aboard a Minuteman missile from Vandenberg Air Force Base, Calif. The kill vehicle launched from Kwajalein Missile Range in the Pacific.

The previous test vaporized the target with a direct hit. The kill vehicle is the bullet of the weapon system. It destroys a target using the kinetic energy of the force of impact; there are no explosives involved. Had the test worked, the kill vehicle and the target would have hit at about 15,000 miles an hour.

Officials said that initially everything appeared to go well. One objective of the test was to evaluate the kill vehicle's deployment and orientation. The vehicle launched, deployed and, using visual sensors, oriented it correctly, officials said. "It was in 'the basket,'" said the official.

Another goal was to demonstrate the ability of the space-based sensors and ground-based radars to detect and acquire a simulated threat, track the threat and discriminate using the prototype ground-based x-band radar at Kwajalein. This, too, worked well.

Finally, the test demonstrated the Battle Management Command and Control and Communication system. This is the nerve center of the system. It receives the information it gathers from the space-based sensors and from radars and then processes it. "Then it sets up the engagement and provides real-time data communications directed to the weapon," said the official. The system's performance was "nominal" -- space talk for it worked well.

The test seemed to be a success until the last six seconds. "It appears that there was an anomaly or an issue with the infrared sensor packages," the official said. "This seeker has two infrared sensors and one visual light. We have to go figure out what happened to the IR sensors.

"As we're sitting there at 29 minutes, 49 seconds after the target lifted off, there was not the bright flash that we saw on the [previous test, indicating a hit]."

The prime contractor for the system is Boeing. Raytheon built the exoatmospheric vehicle, and TRW built the Battle Management Missile Command and Control and Communication system. The test cost about $100 million. Ballistic Missile Defense Organization officials along with the contractors will examine the data from the test and make corrections as needed. The official said DoD can "absolutely" overcome the technical challenges of the program.

The next test is set for spring.