The Pentagon claimed a victory for America's missile defence system last night when a mock warhead was successfully destroyed in space in a test which cost $85m (£45m).

A target missile was launched from Kodiak island, Alaska, yesterday morning. Seventeen minutes later, an interceptor missile left a silo in California, hitting the target above the Pacific Ocean at a speed of 18,000mph.

Military chiefs hailed the test as a "total success" for the defence system, originally known as Star Wars, which has been plagued by political opposition and technical troubles since it began in 1983.

A real interceptor missile has never before successfully destroyed a target missile. In the previous such attempts, in 2004 and 2005, the rockets jammed in their silos. "What we did today is a huge step in terms of our systematic approach to continuing to ... develop a missile defence system for the United States, for our allies, our friends, our deployed forces around the world," said Lieutenant General Henry Oberling, the Pentagon's missile defence chief. He said the system now had a "good chance" of shooting down a real enemy long-range missile. "I feel a lot safer and sleep a lot better at night," he said.

Critics dispute the Bush administration's claim that the system, which has cost almost $100bn to date, is vital to protect against attacks from "rogue states" such as North Korea. They argue that the end of the cold war rendered the scheme obsolete, and the test was unrealistic because the military knew the size, speed, and timing of the missile at which they were aiming.

The Pentagon said the interception was only a secondary aim of yesterday's test: the main point had been to gather and analyse data. But since the test was declared a success before any data had been analysed, opponents suggested it had been partly a public relations exercise.

"It makes you wonder how serious they are about the primary purpose of the test," Philip Coyle, the Pentagon's chief weapons-tester during Bill Clinton's presidency, told Reuters.

Lt Gen Oberling refused to reveal the military's judgment of whether the system in its current form could destroy a North Korean missile aimed at America. In July, Pyongyang unsuccessfully tried to test a Taepodong 2 missile believed to be capable of reaching the north-west US.

"What we saw today was a very realistic trajectory for the threat, for the target, and [for] intercept speeds," Lt Gen Obering said. "We don't know ... what a Taepodong 2 is going to look like or perform."