Missile Defense Agency prepares to deploy interceptor weapons.

The Missile Defense Agency is pressing ahead with plans to begin deploying a controversial and expensive system to protect the United States and allies against ballistic missiles.

In November, the agency was scheduled to place a sixth ground-based, mid-course interceptor missile into its underground silo at Fort Greely, Alaska. Two more are to be installed in December at Vandenberg Air Force Base, Calif.

The next flight test of an interceptor is set to take place early this month. The objective of that test will be a successful flight of both the booster and kill vehicle, said the agency's director, Air Force Lt. Gen. Henry "Trey" Obering III.

The interceptors are part of an integrated sensors, ground and sea-based radars and an advanced command-and control, battle-management and communication system designed to detect and track a target warhead and launch an interceptor to destroy it before it can reach a target in any of the 50 United States, Obering explained.

Initially, the system will have only limited capability. By the end of 2005, 18 interceptors--not the 20 that were originally scheduled--will be in place.

Two of the interceptors, however, were delayed by accidents in 2003 at Pratt & Whitney's propellant mixing facility in San Jose, Calif. The accidents also delayed work on a number of other MDA programs, including the Terminal High Altitude Area Defense missile and the Standard Missile-3 sea-based interceptor now in development for use aboard Navy Aegis-class warships.

In October, the agency received the first of five SM-3 missiles to be delivered this year. The existing SPY-1 radar systems aboard several Aegis-class cruisers and destroyers are being modified to give them the ability to detect and track ballistic missiles. By 2008, the Navy plans to have 18 ships with this capability.

Obering urged patience. "We're giving birth to a new competency," he said, and that takes time.

A missile defense system is necessary, to protect the United States and its friends against a growing ballistic missile threat, he said. More than 30 nations have such weapons in their arsenals. The number one ballistic-missile threat comes from North Korea, followed by some country in the Middle East, Obering said. He noted that the United States has been monitoring Iran's nuclear development program with growing concern.

The agency's first priority will be to protect the United States, South Korea and Japan. The Japanese government decided in 2003 to acquire a missile-defense system.

Eventually, the United States intends to extend protection to other allies. The United States already is working to help Israel to improve its Arrow anti-ballistic missile system. In August, the two countries completed the second in a series of joint test exercises at the Point Mugu Sea Range in California. In 2003, the United States signed a missile defense memorandum of understanding with the United Kingdom, agreeing to upgrade the early-warning radar facility at the Royal Air Force base in Fylingdales, Yorkshire.

Developing the system has been "a significant challenge, but I'll tell you we are very proud of the progress we've made," Obering said. "Yes, it's been in expensive undertaking."

The 2005 defense budget includes $10 billion for missile defense, a $1 billion increase over 2004.

Although the system may not be 100 percent effective at first, "if it can convince another country that it is not in its interest to launch a missile against us, then it is worth doing," Obering said. "We feel confident that this system will provide us with a more-than-rudimentary defense capability."

To decide not to complete this system would be a mistake, Obering said. "If I were America's enemy and you told me you weren't going to build a missile-defense system, where would I hit you? You just told me where you're vulnerable."