The $2 billion Sea-Based X-Band Radar recently left Pearl Harbor to take part in an upcoming test of the nation’s ground-based ballistic missile defense system — a key element in the protection of Hawaii and the mainland from North Korean threats, officials said.

In the test, an interceptor missile fired from Vandenberg Air Force Base in California will try to hit a target missile launched from Kwajalein Atoll over the Pacific, said Riki Ellison, chairman of the Missile Defense Advocacy Alliance, a group that supports a strong missile defense.

Twenty-six interceptors are in silos at Fort Greely, Alaska, and four others are at Vandenberg, providing the backbone of the nation’s ballistic missile defense system.

The U.S. Government Accountability Office, the investigative arm of Congress, said in an April report that the U.S. Missile Defense Agency has spent about $90 billion since 2002 and plans to spend about $8 billion more per year through 2017 to continue developing a system of land-, sea- and space-based sensors and missiles to counter foreign threats.

The Missile Defense Agency said the ground-based interceptors have hit their targets in eight of 15 tests, with the most recent successful test in 2008.

"This is a pretty important test. If you look at the stats, it’s been quite a while since we’ve had a successful intercept test," Ellison said.

The interceptor missile to be tested carries what’s known as the CE-1 "exo-atmospheric kill vehicle," which is designed to slam into and destroy an enemy ballistic missile.

George Lewis, a missile defense expert, said on his website, mostlymissiledefense.com, that the CE-1 began deployment in 2004 and that the Missile Defense Agency began development of a new interceptor, the CE-2, in 2005.

The CE-1 kill vehicles successfully hit targets in flight tests in 2006, 2007 and 2008, Lewis said, while the CE-2 version failed in two intercept tests in 2010.

"A good portion of our missiles are this older generation that we haven’t tested (in an intercept) in five years or so, so it needs to be tested," Ellison said.

The Missile Defense Agency says it doesn’t release a test date until a "Notice to Airmen and Mariners" has been issued, and that no such notice had been issued as of Tuesday.

Controversy surrounds ground-based missile defense not only due to its reliability in technologically complex tests likened to trying to hit a bullet with a bullet, but also because of the additional challenges that likely would exist with decoy warheads and other countermeasures in incoming missiles.

Despite those challenges, Defense Secretary Chuck Hagel announced March 15 that to "stay ahead of the challenges posed by Iran and North Korea’s development of longer-range ballistic missile capabilities," the United States was adding 14 more ground-based interceptors in Alaska.

Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff, said at the same time that "we have confidence in" the CE-1 missile and that fixes to the CE-2 variant — still to be tested in intercepts — will prove it to be successful, as well.

"But we retain our confidence in the CE-1 missile, which is in silos up in Alaska right now," Winnefeld said. "So the American people should have faith in that missile and that we can defend ourselves against a potential North Korean threat as it exists today."

However, Bruce MacDonald, a former assistant director for national security at the White House who was part of a ballistic missile defense panel discussion this month, said in his presentation slides that the United States can’t count on its defensive systems working even reasonably well. Even so, they are a deterrent for foreign powers, better than no defense at all and shouldn’t be ignored, MacDonald said.

The GAO revealed in its April report that $2 billion has been spent on the one-of-a-kind Sea-Based X-Band Radar, which is used to track ballistic missiles, but the radar was downgraded from operational status to limited test status because of funding limitations.

The change allows the Missile Defense Agency to save $670 million in operations and maintenance costs for fiscal years 2013 through 2018, the GAO said.

Missile Defense Agency officials told the GAO that the floating radar, used primarily with the ground-based system, was developed "to assist in countering a threat that has not yet manifested and therefore, from an operational standpoint, the radar is not currently needed," the GAO said.