Updated

Teams led by BAE Systems and Northrop Grumman (search) were chosen Wednesday to build and test prototypes for anti-missile systems to defend U.S. commercial planes against shoulder-fired rockets.

The Homeland Security Department (search) said the companies will each get $45 million over the next 18 months. It's the second phase of the agency's effort to determine whether affordable, effective anti-missile systems can be deployed on commercial planes.

Last January, BAE, Northrop and United Airlines were chosen from among 24 companies that sought $2 million contracts to develop plans for using anti-missile technology and to analyze the economic, manufacturing and maintenance issues for placing such systems on civilian aircraft.

All three came up with plans that use lasers to redirect heat-seeking rockets away from aircraft engines. United's plan was not considered as advanced, so it was dropped.

"We have been impressed with the progress made by these teams over the past six months," Homeland Security Secretary Tom Ridge (search) said. "Taking aggressive action to understand how technology can address this potential threat is a high priority for us, and today we take the next significant step in developing systems that could protect commercial aircraft from enemies that would do us harm."

Military aircraft and planes that carry heads of state, such as Air Force One, already deploy anti-missile systems. The systems, though, are not reliable enough for commercial aviation, require a great deal of maintenance and can go off spontaneously while a plane is on the ground, creating a safety hazard.

The two companies will have to overcome those problems in a cost-effective way and obtain approval for the devices from the Federal Aviation Administration, said Parney Albright, assistant secretary for Homeland Security.

"There's a lot of challenges ahead," Albright said.

Israel announced in July that it had successfully tested a system for commercial planes that uses computerized radar to launch flares that divert the missiles.

Albright, though, said that Israel's airline, El Al, only has about 30 planes that fly out of a single airport.

That, he said, allows Israel to deploy a round-the-clock "on-call pit crew at Ben-Gurion Airport" to maintain the systems — something not feasible in the United States, which has about 6,800 commercial airplanes flying out of hundreds of airports.

Outfitting every commercial airliner with anti-missile systems would cost billions of dollars. Airlines are reluctant to shoulder such a cost, which the Air Transport Association estimates could be between $50 billion and $100 billion over 20 years. The group represents major airlines.

Concerns about terrorists using lightweight rocket launchers to take down commercial airliners were raised in November 2002 when terrorists fired two SA-7 missiles that narrowly missed an Israeli passenger jet after it took off from Mombasa, Kenya. U.S. officials concluded Al Qaeda probably was behind the attack, which coincided with a bomb blast at a nearby hotel.

Hundreds, and perhaps thousands, of Soviet-style SA-7s — heat-seeking rockets that can hit low-flying aircraft within a range of three miles — are said to be available on the weapons market worldwide.

California Sen. Barbara Boxer is among congressional Democrats who have said the Bush administration is moving too slowly to develop an anti-missile system that would address an existing threat.