Looking for fresh ways to build a missile defense system, the Pentagon issued a broad public appeal earlier this year inviting anyone with "new and innovative concepts" to write in with them.

Although the United States already has spent more than $120 billion over the past half-century trying to come up with a workable weapon for defending the country against ballistic missile attack, none exists. With the Bush administration again making missile defense a high priority and investing nearly $8 billion a year in various efforts, Pentagon officials wanted to make sure they weren't overlooking any promising approaches.

In response to the appeal, all kinds of ideas have poured in from all kinds of contributors -- academicians, small businesses, major defense contractors, scientists, hobbyists. Some have been more fanciful than others.

Take, for instance, the one from a small California company that proposed developing a stealthy airplane, armed with lasers and carrying a contingent of 50 Special Forces troops, that would land and zap enemy missiles being readied for launch.

Or the one from a Chinese citizen who submitted a vague plan for an interceptor that would sneak up on an enemy missile from behind in a kind of tail chase.

Or a suggestion for X-ray lasers that would orbit in space.

Sorting through the 194 proposals received since February, Gary Payton, director of the Advanced Concepts Office at the Missile Defense Agency, has instructed his staff to focus on technical feasibility and cost-effectiveness.

"I tell my folks, for the initial round of peer review, if the ideas violate no more than two laws of physics, we'll keep them," Payton said. "The one on X-ray lasers violated several laws of both physics and economics."

Payton's office isn't the only one canvassing the public for bright ideas about missile defense. Two other appeals, known formally as "broad area announcements," went out over the past year -- one searching for ways of building interceptors that would knock down missiles shortly after takeoff in their "boost phase," the other for improving the "producibility and manufacturing" of all missile defense elements.

"What we wanted to do was understand completely what was out there, what potentially could be out there, that would be applicable to this kind of problem," said Terry Little, who oversees development of kinetic boost-phase systems.

One response his office received proposed placing a huge interceptor on an unmanned airship that would patrol at an altitude of 80,000 feet. Another company simply offered its services, saying in effect: "We have lots of smart people, give us all your money, and we'll produce the products for whenever you need them."

Not all the "white papers," as they are called, have exceeded the bounds of practicality. Some of the more promising ideas, in fact, have ended up incorporated in the Pentagon's budget plans for next year, officials said. Among the proposals being seriously pursued is one for an array of solar cells that would power a high-flying airship for detecting incoming missiles. Another promotes the further miniaturization of interceptor "kill vehicles" for homing in on and obliterating enemy warheads.

A number of submissions from defense contractors helped persuade Little that a boost-phase system actually stands a chance of working. He had questioned whether it would be possible to build an interceptor that is fast enough to catch an enemy missile after launch.

"I didn't know where we were with that kind of technology," Little said. "My comfort level came when I saw that boosters here today, things we've actually built, could achieve this kind of velocity."

A longtime defense acquisition specialist who has managed several of the Air Force's most successful munitions and missile programs, Little still isn't ready to declare himself totally sold on the feasibility of boost-phase systems. "I would say today I'm at a 60 percent confidence level," he said.

Pentagon officials are further along in developing a system of land-based interceptors intended to strike enemy missiles once they reach space, in their "midcourse phase." But the boost-phase approach has strong support among some missile defense advocates, and the Pentagon plans to issue contracts in the spring for conceptual designs for a boost-phase system.

Little said such a system could conceivably be fielded by 2008. His office intends to focus first on building a system of mobile, land-based interceptors, then expand to sea-launched ones.