President George W. Bush has made the creation of a layered shield against incoming ballistic missiles a priority for his administration.

Since then-president Ronald Reagan launched what critics dubbed his "Star Wars" initiative in 1984, the United States has spent more than $92 billion on missile defense, not including spending this year, the Pentagon's Missile Defense Agency says.

On Friday, the Air Force general responsible for building the system voiced high confidence it could shoot down any U.S.-bound missile from North Korea. Following are key facts about the project:

Overview

Bush announced on Dec. 17, 2002 the United States would begin operating the initial elements by the end of 2004 to defend against a limited attack from a country like North Korea. Since then, U.S. missile defense spending has risen to nearly $10 billion a year, the Pentagon's single biggest annual outlay to develop a weapons system.

Intercept-test failures and technical glitches have delayed development, although commanders said it has a rudimentary capability against a limited attack if ground-based interceptors are put on alert.

The system

Nine interceptor missiles have been installed in silos at Fort Greely, Alaska, and two at Vandenberg Air Force Base in California. Other components include ship-based interceptor missiles, advanced radar stations, early-warning satellites and a command center in Colorada Springs, Colorado.

A modified Boeing 747 jumbo jet also is being equipped with powerful chemical oxygen-iodine laser to down missiles shortly after liftoff by heating them until they melt.

How it works

The ground- and sea-based interceptors are designed to collide with an incoming missile and pulverize its warhead in the collision. The Pentagon compares the challenge to hitting a speeding bullet with another speeding bullet.

The Missile Defense Agency says it is developing systems to intercept incoming missiles in all phases of flight. Boeing Co is the primary contractor for the ground-based leg. Lockheed Martin Corp, Northrop Grumman Corp and Raytheon Co also have big contracts.

Testing:

In ten intercept tests of the ground-based leg, the interceptor has hit a mock incoming warhead five times. Such testing was suspended after interceptors failed to leave their silos during tests in December 2004 and February 2005 -- failures blamed on quality-control.

Bush policy

The system ultimately will be capable of defending U.S. friends and allies as well as the United States and its deployed forces. Japan, which has been involved in joint research and development, is modifying four destroyers to turn them into ballistic missile defense ships.

(Source: Reuters)