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Unless and until terrorists acquire nuclear weapons, blowing up airplanes will be their most effective strategy for creating mass casualties. Timothy McVeigh needed about two tonnes of explosives to kill 168 people. By leveraging the deadly power of gravity, a terrorist who attacks a commercial airliner can kill the same number with a charge one thousandth the size.

In response to the threat, governments have taken measures to prevent suicide terrorists from sneaking on board airplanes with bombs and knives. But that's just one way to bring down a plane. As usual, the West is building a Maginot defence against yesterday's terror attacks, but ignoring tomorrow's.

In May, a Saudi security patrol found a tube from an SA-7 shoulder-mounted missile-launcher -- a Russian version of the heat-seeking U.S.-supplied Stingers the Afghan mujahedeen used to shoot down more than 200 Soviet aircraft in the 1980s -- near the U.S. Air Force's Prince Sultan base. A May 22 FBI intelligence bulletin concluded "the discovery is likely related to al-Qaeda targeting efforts against U.S.-led forces on the Arabian peninsula." Nine days later, The Washington Times reported that Stinger-armed Islamic terrorists may already be on U.S. soil.

I have no special insight as to whether these reports are accurate. But it would surprise me if they weren't. There are thousands of Stinger-type missiles in worldwide circulation, and the black-market price is reportedly in the mid-five figures.

Although the number of deaths from a Stinger strike would be low by the standards set by the World Trade Center disaster, the psychological effect would be devastating. After Sept. 11, our leaders convinced us they could keep us safe by beefing up airport security. And we believed them. But such self-deception would not survive a missile attack: All the bomb-sniffing dogs and racial profiling in the world won't stop a Stinger. Since the tiny missiles can hit targets up to 10,000 feet up in the air (13,000 feet in the case of the Russian version), an al-Qaeda Stinger team wouldn't even have to come near an airport. An abandoned industrial park a few miles away would do fine.

You might think we're powerless against such a threat. But, thanks to the much-reviled American military-industrial complex, we're not. Northrop Grumman is now in limited production of a plane-mounted anti-missile system that, in live-fire tests, has blocked a variety of infrared-guided missile types with virtually 100% reliability.

Competing systems typically rely on flare decoys, which distract heat-guided missiles but also pose a fire hazard to nearby buildings. But the new system operates on an entirely different principle. It works like this: When a Stinger is launched, the heat signature of its plume is detected by a plane-mounted sensor. An infrared emitter housed in a small turret mounted on the hull then automatically directs a stream of radiation at the missile, blinding its guidance system and sending it off course. The whole package weighs about 260 kg and takes up about as much space as a few large suitcases.

Northrop Grumman has already sold its system -- code-named Nemesis -- to the U.S. military, which means that in the next war, U.S. planes may be able to safely fly missions at whatever altitude they choose. By installing the Nemesis on passenger airliners, we could safeguard domestic skies against Stingers too. But according to Northrop Grumman, not a single commercial airline has committed to the system. Nor is there any interest in Washington. Congress' Electronic Warfare Working Group has promoted the Nemesis in recent years, but only for combat applications.

Are economic factors holding the technology back? I doubt it. Sources at Northrop Grumman told me that installing Nemesis on a single plane would cost about US$5-million. But according to Philip J. Klass, who has written about the Nemesis system for Aviation Week & Space Technology, that figure would apply only to a one-off job. Outfitting a substantial fleet with the Nemesis system, he estimates, would cost only about $2-million to $3-million per plane -- or about 2% of the cost of a Boeing 747-400. The airlines already spend similar sums outfitting their cabins with personal Nintendo, phone and movie systems.

My guess is that no one will make a serious push to put Nemesis on commercial airliners until a plane does get shot down. (Some say that one already has been: TWA Flight 800, which crashed off Long Island in 1996. But that's a minority view.) That fits in with the traditional pattern: On the terrorism file, the West's game over the past two decades has been strictly reactive. In fact, the polite-but-uninterested response I got when I interviewed Electronic Warfare Working Group staff earlier this week was roughly what I would have heard if I'd called the FAA on Sept. 10 asking about cockpit security.

Remember that no one thought much about protecting U.S. military installations overseas until a truck loaded with explosives blew up a building full of sleeping Marines in 1983. Ditto the security situation at U.S. embassies until the 1998 bombings. Sept. 11 was supposed to mark the beginning of a "war" against -- terrorism. Yet the old peace-time pattern remains: We give the terrorists one free hit before we start defending ourselves in any serious way.