An experimental Army missile defense system succeeded yesterday for the second time in hitting a high-flying, fast-moving target, boosting Pentagon hopes that after years of failures, the antimissile weapon can work.

More than 50 miles over a New Mexico test range, the Army's Theater High-Altitude Area Defense (THAAD) interceptor collided with a mock enemy missile in a burst of brilliant light that defense officials said could be seen as far away as Tucson.

Yesterday's intercept, which occurred in the vacuum of space, was more challenging in some ways than the last test in June, which took place in the upper atmosphere. THAAD's ability to go after enemy missiles at such high altitudes is intended to give U.S. troops time for follow-up shots if the first one misses.

The Pentagon's excited response to the predawn experiment, which lasted a little more than eight minutes, bordered on the hyperbolic. "Today was probably one of the watershed events in the technological history of our country," Air Force Lt. Gen. Ronald T. Kadish, who oversees the Defense Department's missile defense efforts, told reporters.

But critics of the $15.4 billion program, which is several billion dollars over initial projections and years behind schedule, cautioned that two successful intercepts under tightly controlled conditions -- and after six consecutive failures in four years -- hardly is proof that all hurdles have been overcome.

"Today's THAAD test was akin to getting a hit in slow-pitch softball," said Tom Z. Collina, an arms control specialist with the Union of Concerned Scientists. "Technological readiness for national missile defense means being able to hit a dozen major-league curveballs simultaneously."

THAAD is designed to protect troops and bases from medium-range ballistic missiles like those being developed by North Korea, Iran and Iraq. But the system's test results have implications beyond battlefield defense. The same "hit-to-kill" concept is at the core of an even more ambitious antimissile system -- a successor to the Reagan-era Strategic Defense Initiative, also known as "Star Wars" -- which is under development to guard the entire United States.

After THAAD's previous failures were blamed on poor quality control in producing and assembling the interceptor, Lockheed Martin Corp., the prime contractor, went to considerable lengths to improve management of the program. The Pentagon had said it would wait until three successful intercepts before advancing the project beyond the experimental phase into early production. But buoyed by the two successes so far, Kadish left open the possibility yesterday of moving ahead without waiting for a third hit.