WASHINGTON, May 30 (Reuters) - The Pentagon prepared on Tuesday for a first-ever missile defense test involving a simulated attack by an intercontinental ballistic missile, like the one North Korea seeks to develop, in what experts saw as a high-stakes moment for the U.S. program.
The U.S. military sought to manage expectations ahead of the test, which it acknowledged could go either way, saying it would gain vital data regardless of whether its Ground-based Midcourse Defense (GMD) interceptor hit its target.
It also sought to reassure the public that America's defenses were layered, meaning it had multiple opportunities to strike down a missile headed toward the United States.
"We improve and learn from each test, regardless of the outcome. That's the reason we conduct them," said Pentagon spokesman Captain Jeff Davis.
"The system that we test today is a developmental system that's being flown for the first time and we look forward to understanding the results so we continue to mature the system and stay ahead of the threat."
The U.S. system has successfully hit its target in only nine of 17 tests since 1999. The most recent test was in 2014.
North Korea has dramatically ramped up the pace of its missile tests over the past year, with a goal of developing an ICBM that can strike the U.S. mainland.
The continental United States is around 9,000 km (5,500 miles) from North Korea. ICBMs have a minimum range of about 5,500 km (3,400 miles), but some are designed to travel 10,000 km (6,200 miles) or farther.
The Missile Defense Agency said the test will involve launching a simulated ICBM from a test site on Kwajalein Atoll in the Marshall Islands toward the United States.
U.S. forces, using data from satellites and radar, will fire a ground-based interceptor from Vandenberg Air Force Base, California.
A successful intercept would be much like hitting a bullet with a bullet, experts say, cautioning that the ICBM would be traveling faster than any missile in previous GMD tests.
Riki Ellison, the founder of the Missile Defense Advocacy Alliance, described the test as "vital."
"We are replicating our ability to defend the United States of America from North Korea, today," Ellison said.
Still, there were no guarantees that Tuesday's test would be successful and any failure of the GMD could deepen concerns about a program that according to one estimate has already cost more than $40 billion to develop.
In the fiscal year 2018 budget proposal sent to Congress last week, the Pentagon requested $7.9 billion for the Missile Defense Agency, including about $1.5 billion for the GMD program.
A 2016 assessment released by the Pentagon's weapons testing office in January said that U.S. ground-based interceptors meant to knock out any incoming ICBM still had low reliability, giving the system itself a limited capability of shielding the United States.
"There are already significant questions about the capability of this system and how much protection it actually provides and I think if the test fails, you are going to hear even louder concerns and criticisms," said Kingston Reif at the Arms Control Association. (Editing by Steve Orlofsky)