Kodiak Island, Alaska ? The U.S. military will test its missile defense system Thursday, the fullest demonstration since a pair of tests grounded the program 18 months ago.

Military officials sought to lower expectations, though. Although a target missile will be fired from Kodiak Island, Alaska, and an interceptor rocket topped with a “kill vehicle” will launch from California’s Vandenberg Air Force Base, both military and industry officials say they are not actually trying to shoot down the missile.

“We are not going to try to hit the target,” said Scott Fancher, head of Boeing Corp.’s ground-based missile defense program. “It is not a primary or secondary test objective to hit the target.”

After a tour of the missile interceptor silos Sunday at Fort Greely, Defense Secretary Donald H. Rumsfeld said that although he wanted to see a “full end-to-end test,” he was patient – and rejected suggestions that the system should try to hit the target this time.

“Why not proceed in an orderly way with the kind of the test expert people (want to do)?” Rumsfeld told reporters. “They do not have to do it to demonstrate to you.”

Critics have long raised doubts about the $43 billion system. Although the interceptors have hit dummy missiles in five out of 10 tests, some outside experts have said the conditions were too controlled and the targets not realistic enough.