An experimental Army missile defense system succeeded yesterday in doing what it had failed to do in six previous attempts: destroy a speeding missile with another missile.

Nearly 60 miles over a New Mexico test range, the Army's Theater High-Altitude Area Defense (THAAD) interceptor collided with its intended target in a powerful explosion that produced a large white puff of smoke visible from the ground, according to observers.

"The test went exactly as planned," said a delighted Brig. Gen. Dan Montgomery, who oversees the Army's missile defense effort.

THAAD is designed to protect troops and bases from medium-range ballistic missiles like those being developed by North Korea, Iran and Iraq. But yesterday's demonstration has implications beyond battlefield defense. The same "hit-to-kill" concept is at the core of an even more ambitious antimissile system, which is under development to guard the entire United States and recently got fresh impetus from Congress.

Even with yesterday's success, defense officials were careful to avoid claims that all hurdles are overcome. But relief was widespread, along with a sense that the long-troubled THAAD program may have turned a corner.

"We have confidence now that we can continue through this flight test program," said Robert Snyder, executive director of the Pentagon's Ballistic Missile Defense Organization. "We have no guarantees that between now and the next test there won't be some unknown problem. But of the ones we knew of, we believe we're over that hurdle."

After previous test failures were blamed on poor quality control in producing and assembling the interceptor, Lockheed Martin Corp., the prime contractor, went to considerable lengths during the past year to reexamine its inventory of THAAD prototype missiles. It also removed a senior executive who oversaw the program, put a seasoned troubleshooter in charge of getting through the remaining tests and agreed to work more closely with another defense giant, Raytheon Co. -- already responsible for producing THAAD's ground radar -- to help review details of the interceptor.

In yesterday's test, a Hera missile, meant to simulate an enemy target similar to an Iraqi Scud, lifted off at 5:09 a.m. local time (7:09 a.m. EDT) over the White Sands test range. About seven minutes later, the THAAD interceptor was launched and streaked toward the target, blasting it out of the sky in just over two minutes.

The results buoyed missile defense advocates on Capitol Hill, where Republicans have pushed for accelerated development of such programs.

"Today's successful intercept is the one that we have been waiting for," said Rep. Curt Weldon (R-Pa.), who chairs the research and development panel of the Armed Services Committee.