In a first-of-its-kind feat, the Army used a high-energy laser built by TRW Inc. to heat the shell, fired from a howitzer at White Sands Missile Range in New Mexico, and cause it to explode in flight. The test was successfully repeated a second time.

The shell, moving at about 1,000 mph, was tracked by radar and heat-sensing infrared sensors, then locked onto and zapped by the laser beam traveling at light speed.

The so-called Mobile Tactical High-Energy Laser is a short-range weapon being co-developed with Israel, which wants it to destroy Katyusha rockets fired at its border villages by Hezbollah guerillas in Lebanon.

The chemically powered weapon, which looks like a searchlight, is one of a handful of laser devices the Pentagon is working on under the umbrella of missile defense.

In earlier tests, the Army used the tactical laser to shoot down 25 Katyushas, both singly and in salvos. Artillery shells, however, generate far less heat than do rockets and are more difficult to track, officials said. Also, since rockets are pressurized, they are easier to detonate than are shells.

``This was, science-wise, a significant accomplishment,创 said William Congo, a spokesman for the Army Space and Missile Defense Command.

Before, the only defense against a lobbed shell was to bulk up on armor, move out of the way or dig in, said Dan Goure, vice president of the Lexington Institute, a nonprofit think tank in Arlington, Va. The ability to intercept a shell changes that.

``Now, in theory, this kind of capability allows you to deny that kind of attack,创 Goure said. The tactical laser could enter use in 2007. Since development began in 1996, the Army, the Israeli Ministry of Defense and TRW have spent $250 million on the project.

It is designed for use against shells, mortars, short-range ballistic missiles, cruise missiles and air-to-surface munitions. It could also target helicopters and small aircraft, including robotic drones.

Officials hope to shrink the weapon enough to allow it to be mounted on a truck, allowing it to be deployed where needed.

``It磗 movable, it磗 not mobile. What we are moving toward is a much smaller, mobile device,创 Congo said. An artists rendering of the actual deployed weapon shows it assembled from two tractor-trailers, the laser protruding on top.

The weapon would also have to be nimble enough to destroy multiple rounds as quickly as they are fired.

``Shooting down a single artillery shell is pretty cool, but artillery shells don磘 come in ones,创 said Christopher Hellman, a senior analyst at the Center for Defense Information in Washington.

Other related weapons the U.S. military is developing include the Airborne Laser, a $3.7 billion project to mount a laser aboard a Boeing 747. The flying laser is being built to intercept and destroy ballistic missiles shortly after launch.

A July report by the General Accounting Office, the investigative arm of Congress, found the Air Force has underestimated the complexity - as well as time and cost - of developing the Airborne Laser system. Even today, it remains ``very difficult创 to calculate the project's cost and schedule, according to the report. Also under development are space-based lasers, which would also target ballistic missiles, and ground-based systems that could take out orbiting satellites, crippling enemy communications.