Officials prepared Thursday to install a ground-based interceptor in Alaska's Interior — the first component of a national defense system designed to shoot down enemy missiles.

Crews at Fort Greely (search) prepared to lower the 55-foot-long, three-stage interceptor into one of six silos built behind a double perimeter fence reinforced by heavy barbed wire. The installation was scheduled for Thursday afternoon.

"We're coming to the end of an era where we have not been able to defend our country against long-range ballmissile attacks," said Major Gen. John Holly (search), who heads the ground-based missile defense program for the Pentagon's Missile Defense Agency (search).

Five additional interceptors will be installed at the 700-acre complex — another four at Vandenberg Air Force Base (search) in California — by the end of the year. Ten more will be installed at Fort Greely by late 2005, launching the Bush administration's multibillion dollar system.

Missile defense is an essential part of President Bush's national security policy. It hasn't been as politically divisive as President Reagan's more elaborate "Star Wars" program (search), but Democrats complain the administration is spending billions of dollars to deploy interceptors without conducting adequate tests to see if they will even work.

According to Missile Defense Agency officials, the interceptors will be linked to a vast network of satellites, radars, computers and command centers. In an attack, satellites would alert the U.S. Northern Command in Colorado, triggering a response by interceptors topped with optical sensors while a complex radar system would track incoming enemy missiles.

As illustrated by a video simulation produced by the agency, the interceptor would zero in on the warhead more than 100 miles over the Pacific Ocean, destroying it at speeds faster than 15,000 mph.

Critics disagree.

The interceptors have not proven their reliability, hitting targets only five times in eight tests, said Philip Coyle (search), former assistant secretary of operational test and evaluation at the Pentagon. He said they failed even when using advanced information "an enemy would never give us," including when they were launched.

"It's not something you want to depend on in real battle," said Coyle, now a senior adviser at the Center for Defense Information, a Washington think tank. "It's also misleading to say we don't have any defense now. If troops in North Korea saw that country building a missile, they would blow it up on the ground. They would never wait to see if it was launched."