All of the emerging threats the nation faces are fleeting and elusive -- from transnational terrorists to ground-hugging cruise missiles to illegal migrants to drug smugglers to fast-burning ballistic weapons. The military has ways of defeating all these dangers if they can be detected and tracked, but effective surveillance turns out to be very difficult.

So the Pentagon has embarked on a vast effort to improve its terrestrial, airborne and orbital reconnaissance capabilities, spending hundreds of billions of dollars on efforts to find the enemy. Satellites that can see insurgents from 200 miles in space. Radar planes that can track ground vehicles in a sandstorm. Submarines that can eavesdrop on cell-phone networks. Sensor arrays that can identify each of the dozen telltale "signatures" generated by an SUV. There are more than a hundred different surveillance systems either already in operation or being actively researched.

But in the rush to bolster global vigilance, sometimes the simplest ideas get neglected. Like balloons. Excuse me, not balloons, but blimps -- inflatable, unmanned airships that can carry radars and other sensors aloft for a month or longer to provide continuous surveillance over vast areas of the Earth's surface.

Once you get above the jet stream, around 60,000 feet, winds and weather largely disappear -- as does most of the air -- making it relatively easy to stay in one place continuously. From that altitude, sensors have an unobstructed view of more than 300 miles in any direction.

Relying on blimps for surveillance may not sound like the wave of the future, but a High Altitude Airship can deliver much greater persistence and/or precision than airborne and orbital alternatives at much lower cost.

The potential savings really are staggering, because it costs more than $1,000 an hour to operate a pound's worth of reconnaissance payload from low-Earth orbit, and more than $100 an hour to operate a pound's worth of payload from a high-end reconnaissance drone such as a Predator.

The cost for operating a one-pound payload of surveillance gear one hour on a blimp is less than $10. That's what engineers refer to as "orders of magnitude" difference in the cost. And unlike in the case of the satellite, you can bring the payload down for maintenance or upgrades anytime you feel like it.

The Missile Defense Agency grasped the potential of blimps years ago, when it awarded a contract to develop a High Altitude Airship that could carry a 4,000-pound payload aloft for a month. The plan was to use 10 of the systems to construct a perimeter defense of the nation against missile attack.

A lightweight radar would be built into the structure of the airship capable of simultaneously looking up to track ballistic missiles and down to track cruise missiles, with power provided by solar panels on the skin of the airship. The airship would be radio controlled from the ground and have a cruising speed of about 30 miles per hour, with wireless links to the rest of the joint force.

The High Altitude Airship prototype is supposed to debut at the end of this decade. If it proves successful then a vast array of other applications will open up, from military reconnaissance and communications overseas to border security here at home.

But first the program must be kept on track through a change of administrations, and that means fighting off proponents of less cost-effective solutions who might try to use the airship money for other purposes.

(Loren B. Thompson is chief executive officer of the Lexington Institute, an Arlington, Va.-based think tank that supports democracy and the free market.)

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