An unarmed intercontinental ballistic missile was launched early Wednesday on a test flight that began with its silo's 105-ton door being blasted open, officials said.

The Minuteman 3 missile's dummy warhead traveled some 4,200 miles in about 20 minutes, hitting a predetermined target at the Kwajalein Atoll in the western chain of the Marshall Islands.

The launch at 12:14 a.m. from the Central Coast base marked the first time in years that the Air Force conducted a closed-door missile test. In previous launches, the 576th Flight Test Squadron performed the tests with the concrete door already open to cut costs.

The test was also the first flight using only global positioning satellites for navigation instead of radio transponders.

The Air Force returned to closed-door testing because such launches provide more accurate data for tracking the missile, according Lt. Col. Stephen L. Davis, the unit's commander.

The environment is also a factor. During open-door launches, the silo is protected by a temporary plastic cover. But heavy rains in 2005 caused the silo to become water-logged, damaging the weapon.

Vandenberg routinely tests Minuteman 3 missiles by firing them at the target range in the Pacific Ocean.