Exposing the Weakest Link: As Airline Passenger Security Tightens, Bombers Target Cargo Holds

The latest terrorism scare, involving a pair of explosive packages bound for Chicago from Yemen, has shed light on a new target for bombers—aircraft traveling to the U.S. whose cargo holds either have not been inspected, or if they have, by x-rays and bomb-sniffing dogs  that are not sensitive enough to root out certain types of explosives. Would-be aircraft bombers have proved successful at smuggling pentaerythritol tetranitrate (PETN) into aircraft cabins concealed in shoes and underwear, but fortunately have been unable to detonate this high-powered, military-grade explosive in flight. The latest plot placed one PETN bomb and detonator in a cargo plane and another in the cargo hold of a passenger plane, where they were less likely to be detected.

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The October 29 incident indicates that more attention must be paid to screening shipped packages. Yet, even if all cargo were screened, explosives, including PETN are difficult to detect because they have a very low vapor pressure, which means very little of the explosive material gets into the air around the bomb where it can be detected. "Normally, whenever there is a solid material sitting on a surface, there is a certain concentration of that substance lingering in the air above it in a gas phase," says physicist Kurt Becker, Polytechnic Institute of New York University's associate provost for research and technology initiatives. "All explosives are notorious for having a very low vapor pressure at room temperature." Cheese, by comparison, has a very high vapor pressure at room temperature and is easy to detect through its aroma.

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