Defense Secretary Robert Gates has ordered deployment of the Terminal High Altitude Area Defense missile system to Hawaii, according to American Forces Press Service.

The system, called THAAD, is designed to destroy short- to medium-range ballistic missiles at high altitudes, even outside the atmosphere.
The THAAD system has been successfully tested at Kauai’s Pacific Missile Range.
Since 2005, the system — using mobile launchers at Barking Sands on Kauai's west coast — has been a perfect 5 for 5 in knocking out an incoming missile.
Gates also has ordered use of the sea-based SBX radar.
THAAD is one of two ground-based mobile missile interceptor systems available to the U.S. military, according to the Department of Defense’s Missile Defense Agency. The other is the Patriot Advanced Capability 3.
Both systems work in tandem with radar and involve missiles that directly hit incoming missiles.
A third system — the Airborne Laser, or ABL — involves a laser aboard a modified Boeing 747 that destroys missiles in flight by heating them until they fail structurally.
The Patriot, which gained notoriety by knocking down Iraqi SCUDs during the Gulf War, is designed primarily as a hit-to-kill interceptor of short-range ballistic missiles, cruise missiles and aircraft.
For more on the THAAD system, go to:
http://www.mda.mil/mdalink/html/terminal.html