Two aerostats are due to arrive in the Washington area soon for a test that might last three years. (REUTERS)

A pair of big, blimplike craft, moored to the ground and flying as high as 10,000 feet, are to be added to a high-tech shield designed to protect Washington from air attack, at least for a while.

The bulbous, helium-filled aerostats — each is 243 feet long, more than three-quarters the length of a football field — are to be stitched into existing defenses as part of a test of new technology ordered by the Defense Department.

The coming addition to the umbrella over Washington is known as Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System, or JLENS. Raytheon is the prime contractor.

“We’re trying to determine how the surveillance radar information from the JLENS platforms can be integrated with existing systems in the national capital region,” said Michael Kucharek, a spokesman for the North American Aerospace Defense Command. NORAD is responsible for defending air space over the United States and Canada.

The most significant air attack in the area took place on Sept. 11, 2001, when al-Qaeda militants hijacked a Boeing 757 and crashed it into the Pentagon.

To expand the time available to detect and defend against attacks from commercial aircraft, major changes were made under Operation Noble Eagle, a program of combat air patrols begun after the Sept. 11 attacks. Airspace restrictions were extended. Sentinel radars for low-altitude coverage and short-range Stinger/Avenger missile batteries were deployed.

Washington’s skies are now guarded by a system that includes Federal Aviation Administration radars and Department of Homeland Security helicopters; in addition, fixed-wing aircraft are on alert at Reagan National Airport to intercept slow, low-flying aircraft.

The JLENS craft are expected to arrive by Sept. 30, according to Kucharek. A “capabilities demonstration,” as the test is called, is expected to last as long as three years. Its location is being withheld, pending notification of lawmakers and others.

JLENS craft work in pairs, each tethered to mobile moorings. One of the aerostats carries a long-range surveillance radar that can reach out to 340 miles. The other carries a radar used for targeting. A pair costs roughly $450 million.

Operating as high as 10,000 feet for up to 30 days at a time, JLENS is meant to give the military more time than ground-based radar does to detect and react to threats, including cruise missiles and manned and unmanned aircraft.

The system is also designed to defend against tactical ballistic missiles, large-caliber rockets and surface vehicles that could be used for attacks, including boats, cars and trucks.

A successful test in the Washington area could give a boost to the JLENS program, which has been scaled back sharply along with the Pentagon’s other lighter-than-air vehicle efforts.

Blimplike craft offer several advantages compared with fixed-wing aircraft, including lower cost, larger payload capacity and extended time aloft. However, their funding is scheduled to fall sharply as Pentagon spending shrinks to help pare federal deficits.