By Jim Wolf WASHINGTON (Reuters) - The U.S. military shot down a target ballistic missile over the Pacific on Friday in the widest test of its emerging antimissile shield in 18 months, the Defense Department announced. The Pentagon's Missile Defense Agency said it had successfully completed an important exercise involving the launch of an improved ground-based interceptor missile designed to protect the United States against a limited long-range ballistic missile attack. The test results will help improve the performance of a multibillion-dollar shield against the type of long-range ballistic missile that could be used to attack a U.S. city with a weapon of mass destruction, the agency said in a statement. Officially, the $85 million test was designed to collect data rather than shoot down the target. It was the first involving a live target since interceptor rockets failed to leave their silos during tests in December 2004 and February 2005. It was also the first since the ground-based system, which is part of a layered shield that includes naval and aerial components, was activated to guard against ballistic missiles test-fired on July 4 and 5 by North Korea. Boeing Co. is prime contractor for the ground-based mid-course defense. Major subcontractors include Lockheed Martin Corp., Northrop Grumman Corp. and Raytheon Co.. In the exercise, a target missile was launched from Kodiak, Alaska. And for the first time, the ground-based interceptor missile was launched from Vandenberg Air Force Base in central California. Previous launches have been from the Kwajalein Atoll in the Marshall Islands. Â© Reuters 2006. All Rights Reserved.