Primer: Why Liquid Explosives? Liquid explosives can make it easily by airport screeners. But such chemicals are notoriously unstable and can be set off by an accidental bump.
NPR logo Primer: Why Liquid Explosives?

Primer: Why Liquid Explosives?

The alleged plot to blow up airliners over the Atlantic involved the use of liquid explosives, according to officials in the United Kingdom and the United States. Authorities suggest that the plotters planned to carry explosive ingredients aboard the aircraft in hand luggage -- perhaps in cosmetics containers or beverage bottles -- and detonate them. Here, a primer on liquid explosives.

How do liquid explosives work?

This type of explosive is simply a chemical compound that in liquid form is relatively unstable. A physical, chemical, or electrical jolt causes it to explode, usually producing a fair amount of heat, light and a destructive shock wave (a very rapid change in air or water pressure). Like solid explosives, liquid explosives vary in power, depending on their recipe and how they are packed into a container.

Why would terrorists want to use liquid explosives?

Security screeners have gotten much better at detecting solid explosives, while liquid explosives have proven much harder to detect. In addition, liquid explosives can be made from fairly common ingredients, including substances typically found in your garage or under your sink. Finally, a terrorist could assemble an explosive from components carried aboard a plane in separate containers or by different people -- making it even harder for screeners to spot. One person could carry ingredient A on to the plane, another ingredient B, and then they could be combined.

Do liquid explosives have disadvantages?

Like many homemade explosives, they can be very volatile. Some can be set off by a stray spark, a change in temperature, exposure to sunlight, or even an accidental bump. For instance, the liquid nitroglycerin -- made famous by Alfred Nobel, the inventor of dynamite and father of the Nobel Prize -- is notoriously unstable. It can be set off by an accidental jolt. In fact, Nobel's brother died when a dynamite factory exploded. That's why the invention of more stable explosives, like sticks of TNT, was important. TNT typically needs to be ignited with a powerful blasting cap; it can even be melted safely.

Terrorists have tried several times in the past to blow up airliners with liquid explosives...

The U.S. government has focused on the threat of liquid explosives since at least 1995. That's when they learned about a plot in the Philippines to plant bombs on 11 airliners bound for the United States. The plotters apparently wanted to use liquid nitroglycerin. They planned to smuggle it aboard in bottles made for contact lens solution. But the plan came apart when there was a fire in the apartment where the bombs were being made. (A test run for that plot, a plane bound for Tokyo, resulted in the detonation of a liquid explosive in-air. One person was killed, and pilots were able to land the plane.)

Experts believe that terrorists have since moved on to use the type of explosives previously described, where the components are carried separately and then mixed at the site, to reduce their instability.

How hard is it to screen for liquid explosives?

They're a real threat to airlines. Currently security agencies have no test for liquid explosives. Technology is in the works, and several devices have been tested in airports. One kind uses microwaves to distinguish safe, water-based liquids (like coffee or soda) from solvents and other dangerous chemicals used in explosives. But this device can’t see through metal containers. Another kind of device sends laser light through clear glass or plastic. The light bounces back with a scatter signature that can be compared to a database of worrisome liquids. But this technology can’t see through opaque containers.

Why aren't some of these devices already installed in airports?

Experts have known for years that screening liquids is a weak spot in airport security, but other needs have taken priority. For example, one government report said that research and development was recently delayed when the Transportation Security Administration had to transfer funds away from R&D to other expenses, like personnel costs for screeners.

With reporting by NPR's Nell Boyce and Jon Hamilton.