How Do Investigators Measure Bomb Strength?

By checking the windows.

Moscow's Domodedovo international airport. Click image to expand.
Police officers patrol outside Moscow’s Domodedovo international airport soon after the explosion.

A bomb blast killed at least 23 people in the baggage claim area of Moscow’s Domodedovo airport on Monday afternoon. * Within 100 minutes of the explosion, authorities had determined that the bomb’s strength was equivalent to seven kilograms of TNT. How did they figure that out so fast?

By checking the walls, probably. Forensic investigators can estimate a bomb’s strength by examining its effects on nearby walls, windows, cars, and human victims. For example, a seven-kilogram TNT bomb detonated in a relatively open space might crack a brick wall 45 feet away. The explosion would damage plaster walls and asbestos shingles at around 115 feet. Single-strength window glass would shatter at a distance of 210 feet. Human eardrums could be damaged within 40 feet, and there’s a chance that anyone within 12 feet would die. These estimates, however, are extremely rough. In an enclosed area like an airport, the pressure waves bounce off of the ceiling and walls, increasing the bomb’s destructive force. And injuries will vary depending on how a person is oriented in relation to the blast.

Although early reports suggested that the Moscow bomb was composed of seven kg of TNT, it’s likely that Russian news agency Interfax, which provided this piece of information, was referring to relative effectiveness—that’s the strength of the bomb expressed in units of TNT—rather than the bomb’s actual chemical composition. (Unless investigators got a look at the bomb in a security video.) Military scientists have been using TNT as a measuring stick for explosiveness since at least World War II. In fact, after the first successful nuclear weapon test in New Mexico in 1945, physicist Enrico Fermi estimated that the blast strength was equivalent to 10,000 tons of TNT. (He arrived at this figure by dropp ing sheets of paper from a height of six feet and measuring their landing positions.) The actual number was about twice Fermi’s guess. Although media and military types continue to speak in terms of pounds, kilos, or tons of TNTs, scientists now prefer the power index, in which picric acid is the reference explosive.

Russian investigators should know by now what the explosive was made of, if not the precise amount involved. Most bomb units carry ion mobility detectors. The handheld units, which look like cordless drills, suck in a sample of air and look for tiny bits of unexploded chemicals. If they find even a single nanogram of explosive, a red light indicates its presence. The technology is the same as that used in airport security screening, but more portable. The sooner the machine gets to the scene, the better. Explosives aren’t particularly volatile, so they don’t hang around in the air for long.

It’s entirely possible that the Moscow airport bombers used TNT, since Russian attackers have used the material in the past. The chemical isn’t the terrorist’s bomb of choice internationally, however. It’s very hard to make on a sub-industrial scale, and in most countries it’s not easy to get a hold of. In the United States, it’s limited to mostly military uses. Mining companies and road crews have resorted to ammonium nitrates, which heave rocks rather than obliterating them the way TNT does.

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Explainer thanks John Goodpaster of Indiana University-Purdue University Indianapolis, and Peter Harrington and Glen Jackson of Ohio University.

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Correction, Jan. 24, 2011: This article originally stated that the explosion at Domodedovo airport occurred on Monday night. It was Monday afternoon. (Return to the corrected sentence.)