With the exception of far-less-common biological or radiological threats, explosive devices are the nightmare scenario for people charged with protecting people and places. In most cases, no one knows of the device until detonation, and it may take out first responders as well as private citizens.
This is because devices reside inside commonplace objects like books or backpacks that people see and don't notice. They may trigger via a timer, a switch activated by pressure, proximity or movement, or by remote command. Reliable detection of the devices is difficult because of their small size relative to their lethality. Science is getting better at this, but the more traditional methods also have their place.
Know Your Explosives
The earliest explosive was black powder or gunpowder, made of sulfur, charcoal, and potassium nitrate (also called saltpeter). Anyone reading this article is probably familiar with the characteristics of gunpowder. It's fairly easy to manufacture, but it's a low explosive, capable of propelling a bullet out of a gun barrel, but not destroying the gun itself (unless one overpacks the cartridge, which is more of a function of stupid than physics).
Nitroglycerin came along a bit later. It has a greater energy density than black powder, but its sensitivity to shock makes it difficult to work with, and it is rarely used today for anything except as a vasodilator medication.
Alfred Nobel invented dynamite by combining nitroglycerine with sodium carbonate and diatomaceous earth. The latter ingredient is available at any garden nursery, made of ancient decomposed sea shells. Dynamite was the first commercially viable explosive, and made Nobel wealthy enough to endow an annual prize for advancements in science and in peacemaking.
Dynamite is usually formed into sticks and wrapped in paper. Over time, the nitroglycerine in the dynamite will "weep" or "sweat," forming unstable crystals on the wrappers. Proper storage of dynamite dictates the packages be turned periodically to prevent this.
Some people think trinitrotoluene or TNT and dynamite are the same thing, but they aren't. TNT starts with toluene, a common solvent used in model airplane glue, among other things. The explosive is produced by repeatedly nitrating it with nitric and sulfuric acids. TNT is more stable than dynamite, and can be poured (it melts at 176 degrees Fahrenheit) into molds or combined with other explosives. Maybe that's why it's the most common explosive in conventional military munitions such as artillery and mortar shells.
TNT is the standard for measuring the destructive power of other compounds and weapons. The "Little Boy" atomic bomb dropped on Hiroshima in World War II was equivalent to about 15,000 tons (15 kilotons) of TNT. Modern nuclear weapons are measured in megatons, or million tons of TNT. To provide perspective, a kiloton of solid TNT would be a cube measuring about 10 yards on each side. Interesting bit of trivia: TNT is yellow, and skin that's in repeated contact with it turns the same color.
Terrorist bomb makers prefer plastic explosives. These are compounds containing one or more of the explosives already described, plus pentaerythritol tetranitrate (PETN) or RDX, plus wax or some other plasticizer. The resulting compound is a moldable, castable substance that can take almost any physical shape. PETN is a major ingredient of Semtex, while RDX is used in Torpex and C-4.
Umar Abdulmytallab stashed PETN in his underwear in December 2009 in his unsuccessful attempt to destroy Northwest Airlines Flight 253 between Amsterdam and Detroit. RDX was the primary explosive in the 2006 railroad bombings in Mumbai, India, and in the bombing of the Moscow Metro earlier this year.
Most, if not all, explosives developed since black powder have nitrate compounds as a base component. It's these nitrate molecules and ions that high-tech bomb "sniffers" look for. Plastic explosives tend to be denser than other common materials carried by travelers, and the explosives are sometimes surrounded by wires, batteries, or electronics that complete the bomb. These are detectable with X-rays and other visual scanning methods.
When these methods fail, or when the gadget locates a suspicious lump that might be a bomb, somebody has to take a look with his or her own eyes or a remotely operated device. Of course, you can always blow the thing up wherever it happens to be, but the people who own the venues usually prefer you not do that.