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When they confiscated your nail file at the airport, did you feel more secure? Perhaps, if it happened in the month following September 11. At least somebody was doing something, you thought, even if you couldn’t figure out how a nail file could be used to a hijack an airplane.

Your instincts were right. The confiscation accomplished nothing. After September 11, no plane could be hijacked with a nail file, a pocket knife, a gun, or even a bomb. A terrorist could kill passengers, or blow up the airplane, but he couldn’t hijack it. The passengers and crew wouldn’t let him, and he knows it. He might as well do his killing at the mall.

So does the bothersome airport security really make any sense? Why protect an airplane more carefully than a public library? Are the delays just an annoyance that accomplishes nothing?

Not all of them; some security measures truly are worthwhile. But my choices for the most important may surprise you. They are:

*Checking passengers’ shoes
*Requiring all checked luggage match passengers on the plane
*”Random” checks of passengers at the gate

Why did I chose these as worthwhile? I begin by asking what kind of attack is al Qaeda still capable of executing that could have the impact of September 11. My answer: a dozen planes destroyed over the United States in one hour, from explosives carried onboard or hidden in checked luggage. The deaths and the horror would rival the World Trade Center disaster.

It is important to recognize that there is no good way to detect carefully prepared explosives. Neutron activation, which detects the nitrogen in explosives, has received the most attention. But this technique generates too many false alarms–typically several per full flight–from leather and other nitrogenous materials. What do you do with luggage that sets off a bomb detector? Open it? Where? Blow it up? There is no good solution, as long as there are abundant false alarms.

Better explosive detectors are under development. Electric nuclear quadrupole resonance–a method that detects the chemical environment of the nitrogen nucleus–offers real hope with few false alarms, but it is not yet ready to put into airports. The best bet today is the ion mobility time-of-flight spectrometer. These are the “sniffers” that are in wide use at airports to analyze swabs. They cost less than $50,000 and have a false alarm rate under one in a thousand. But they would miss a carefully wrapped explosive, unless the outside of the package (or the person carrying it) was contaminated.

On a recent trip to France, I was stopped after an x-ray inspector noted something suspicious in my carry-on luggage (probably the bag full of chargers for my video camera, digital still camera, cell phone, iPod, and computer). How could he check all these? He didn’t–instead, he asked me to take off my shoes, and put them in a sniffer. Smart, I thought! If I really were really a terrorist, there might be residue from explosives on my shoes.

Remember Richard Reid, the al Qaeda terrorist who tried to light a fuse on his shoe and failed when attacked by other passengers? The intelligence experts have concluded that Reid himself didn’t (couldn’t have?) designed that shoe. Was Reid on an official al Qaeda terrorist mission? I’m guessing the answer is no. Reid became frustrated at the lack of communications and orders (al Qaeda has been badly broken) and he decided to go ahead and blow up a plane himself. That was very, very stupid. Al Qaeda is not interested in blowing up one plane; they want to blow up a dozen. They knew they could smuggle explosive-laden shoes on board, and (I am guessing) they had a dozen of these shoes all set for simultaneous attack. Reid, in his impatience, blew the secret of the scheme. I’ll bet the other eleven shoes are still out there. But now shoes are checked, and as long as this is done, the larger plan will be impossible.

Maybe you have had your luggage run through a large, expensive-looking machine near the airline check-in counter. That is actually a high resolution x-ray device, and they are looking for bombs, not raw explosives, attached to electronics or timers or altimeters. The machines are powerful, precise, and expensive. As a test I left a role of film in my luggage, and the developed negatives came back partially exposed by x-rays. This device is the best we have for detecting bombs on checked luggage, but its value is not yet clear.

Some people think it useless to require checked luggage match the passenger list. (And it makes it difficult to switch flights, if yours is delayed.) After all, we know the terrorists are willing to commit suicide! But that criticism misses the point.

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