The U.S. Defense Department is considering equipping some Navy ships with a limited capability to shoot down ballistic missiles near the end of their flight, according to Missile Defense Agency (MDA) officials.

A sea-based, terminal missile defense capability could help the military respond quickly to a missile attack in the early phases of a deployment when ground-based missile defense systems might still be incomplete, MDA officials said.

The Pentagon conducted the first successful intercept demonstration of this capability May 24, according to an MDA news release. The military previously has used the Aegis Ballistic Missile Defense System to destroy missiles in their midcourse phase of flight during testing, and has declared six of the seven tests successes.

Intercepting missiles in their terminal phase — the final minutes or seconds before they strike — gives the military a much tighter timeline for the shoot-down than intercepting missiles in the midcourse phase of their flight. One advantage, though, is that it makes an intercept somewhat easier than a midcourse intercept because decoys deployed by the incoming missile to fool the interceptor are not likely to remain near the target by the terminal phase , the MDA official said.

The Aegis ship uses the same firing system for both the midcourse intercepts and the terminal phase shoot-down but employs a different interceptor rocket for the two tasks. The firing system was developed by Lockheed Martin Maritime Systems and Sensors of Moorestown, N.J.

The ship used in the May 24 test fired a modified Standard Missile (SM)-2 Block 4 rocket for the terminal intercept, and used an SM-3 for the midcourse shots. Raytheon Missile Systems of Tucson, Ariz., builds both interceptors.

The two rockets use the same first and second stage, according to Sara Hammond, a Raytheon spokeswoman. The SM-3 features a third stage motor that takes the rocket out of the atmosphere and into space for the midcourse intercept, which is not necessary for terminal shots, Hammond said.

The two rockets also use different kill vehicles. The SM-3 destroys missiles purely by kinetic impact, Hammond said. The SM-2 Block 4 is capable of destroying incoming missiles in the terminal phase through either direct impact or by exploding close to its target in case it is unable to make a direct hit, she said.

In the May 24 demonstration, the SM-2 Block 4 missile used both of those capabilities to destroy the incoming target, according to the MDA news release. The interceptor was fired by the USS Lake Erie, an Aegis cruiser based at Pearl Harbor, at a short-range ballistic missile target launched from the Pacific Missile Range Facility in Kauai, Hawaii.

Landon Hutchens, a spokesman for the Naval Sea Systems Command, said that the program executive office for integrated warfare systems could not respond by press time to questions about the demonstration.

Rear Adm. Barry McCullough, director of surface warfare on the staff of the chief of naval operations, said in the MDA news release that the demonstration is “an important step” along the way towards fielding an operational sea-based terminal phase interceptor capability.

The Pentagon has a “significant number” of SM-2 Block 4 missiles in its inventory, and senior officials are reviewing the possibility of modifying them for the missile defense mission, Air Force Lt. Gen. Henry “Trey” Obering, MDA director, said in the news release.

An MDA official said the Pentagon is considering modifying 100 existing SM-2 Block 4 interceptors over the course of a year that could be deployed on Aegis ships that have been equipped for the ballistic missile defense mission. The Pentagon currently has three Aegis ships equipped for this mission, and plans to have a total of 18 ready by 2009.

These missiles would not be as capable as the missiles that were envisioned under the Navy Area program, which the Pentagon terminated in 2001 due to cost growth. However, they could provide a limited defensive capability while the Pentagon develops a more robust capability, the MDA official said.

The only other existing option for destroying missiles in their terminal phase is the U.S. Army’s Patriot interceptor system. The Pentagon also is reviewing the possibility of applying the Patriot Advanced Capability-3 rockets, which are built by Lockheed Martin Missiles and Fire Control of Dallas, for sea-based defense, McCullough said.

Baker Spring, a missile defense expert at the Heritage Foundation, a think tank here, said he hoped the May 24 intercept proved to Pentagon officials that the Navy can play a role in terminal phase missile defense intercepts.

Spring said he was pleased to see the successful demonstration, but expressed disappointment that it took so long for the Pentagon to conduct this type of demonstration with existing air defense rockets.

Philip Coyle, the Pentagon’s former director of operational test and evaluation, said ships are likely to offer a significant capability for the military for terminal missile defense, but said the military is likely a long way from fielding an operational system.

While the Pentagon has not released details of the test, an initial demonstration is likely not nearly as challenging as an operational scenario, and the military likely needs years before it is ready to conduct such a test, said Coyle, who currently serves as a senior adviser to the Center for Defense Information, a think tank here.