British police have arrested 24 people allegedly linked to a plot to blow up multiple flights across the Atlantic from the U.K. to the U.S. Although details of the plot remained sketchy, officials revealed that the plan included the construction of bombs onboard using liquid explosives.

Terrorists and other malefactors have known about liquid explosives for a long time. The hijackers of 9/11 knew that the jet fuel in the airplanes would act as an incendiary, and even gasoline could deliver a devastating explosion if packaged properly. The British shoe bomber, Richard Reid, employed a compound made by mixing liquids available at the local pharmacy, though he failed to detonate it. And a plot from the mid-1990s to attack multiple U.S.-bound airplanes from Asia succeeded in trial runs in sneaking nitroglycerin onto planes in the saline solution bottles typically carried by contact lens wearers, killing one passenger on a Philippines Airlines flight. "This has always been the case," notes chemist Nick Turro of Columbia University. "It was known that you could do this."

Chemists in the Department of Homeland Security (DHS) declined to comment on the composition of the explosive, but government officials hinted that it would have been based on peroxides, commonly available as hydrogen peroxide--a household cleaner or bleach. Of course, the laundry list of chemicals that could conceivably be used--in either liquid or solid form--is essentially endless, given the proper expertise. "There are a lot of liquid explosives," says chemist James Tour of Rice University, who works on retarding chemical weapons attacks. "There are books on these things." But terrorists often employ the same or similar compounds. Favorites include triacetone triperoxide (TATP) and nitroglycerin, the components of which are individually harmless, but dangerous when combined.

Neither of these is easy to work with, however. TATP is an atypical explosive, relying as it does on the formation of four gas molecules for every solid TATP molecule when triggered--a powerful "entropy burst," as researchers have called it. But TATP is relatively unstable--simply hitting the powder can set off the reaction--and it requires some time to produce the powerful compound. Terrorists would have to blend the constituent chemical brew and then wait for the more explosive powder to precipitate over the course of the next day, according to Tour, making it unlikely to be made on board a flight, although the brew itself is also dangerous. Nitroglycerin--the explosive ingredient in dynamite (and a common heart medication in small quantities)--would be even more difficult to handle if made on board; early experimenters suffered a slew of explosive mishaps due to its volatile nature. And many other explosives immediately fume or otherwise react as the chemicals are blended, as well as being highly sensitive to jostling. "Peroxides are very dangerous and shock sensitive," notes one university chemist who asked not to be identified.

But it would take only small quantities of various liquid explosives or fuels to generate the force necessary to punch a hole in an aircraft or more seriously disable it. "If it's packaged just right, a few ounces is all you need," Tour says. "The heel of a shoe would do it," notes Neal Langerman, former chair of chemical safety for the American Chemical Society. "These are highly energetic compounds." Furthermore, some chemicals can be mixed to create a toxic gas capable of killing people in an enclosed space such as an airplane.

Based on information that the suspects planned to conceal the explosives in question as beverages to detonate with an electronic device, government officials have immediately banned all gels, liquids and lotions from U.S. flights as a precaution. And because detonation of such bombs could be accomplished with any battery--be it from a cell phone, watch or laptop computer--these common electrical devices have also been curtailed. "While we're in the process of assessing [countermeasures]," noted DHS Secretary Michael Chertoff in a press conference, "and, you know, honestly, some of these are pretty difficult--we want to, frankly, take the most protective stance."

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