You've seen or heard the story about Abdullah Al Muhajir, the former Chicago street gang member turned terrorist who allegedly had plans to detonate a radiological bomb in the USA, probably in Washington. He is now in the custody of the U.S. military as an (unlawful) enemy combatant.

In many ways, "dirty bombs" fit the classic "4th Generation Warfare" (4GW) template: an act of war against non-military targets, whose main effect is not physical but psychological.

This particular incident is revealing on several levels, but let's start with the basics:

• "Dirty Bombs" are conventional bombs, with additional material included: chemicals, radioactive matter, or (least likely) biological agents.
  
• They have been used recently in Israel, and there was a radiological incident in Chechnya in 1995.
  
• Their effects are more psychological and economic than physical. Which doesn't make them useless by any means. If you understand what they're for, they're actually extremely effective.
  
• The USA is unsure what stage this plot was at, but it doesn't appear to have included shipment of radioactive materials from abroad... which means Al-Qaeda probably intended to find and use local US sources.
  
• That's comforting on one level. It means Al-Qaeda's documented efforts in this field remain problematic for them. Recall those heroic Afghani physicists we talked about a month or so ago.
  
• That said, US sources do exist and security in those places may not be what it should be. Winds of Change explains, with examples.
  
• It's obvious that Pakistan played a role in this investigation on some level, and that the sources involved are sources the USA badly wants to conceal. Hence the military detention.
  
• It's not about trial formats - he's a US citizen, and the government has said it won't place Muhajir before a military tribunal (even though legal precedent says it could). He can be tried later for sedition or treason, however, and in the meantime the USA may detain him 'for the duration of the conflict.'
  
• While the USA claims information from Zubaydeh helped with this operation, that could well be a two-edged bluff. As long as Al-Qaeda isn't certain of that, however, it doesn't matter.

-- Dirty Bombs --

A "dirty bomb" is an explosive device with radioactive materials, chemicals, or biological agents packed in the device. Palestinian terrorists have already used chemical weapons like rat poison and cyanide with their suicide bombs. Mundane devices like nails don't count as "dirty bombs," though their physical effects may be more severe.

Fortunately, experiments in police laboratories have shown that most easily purchased chemical compounds dissipate in the heat of explosions, without causing any damage. The cyanide bomb at the Netanya Passover Massacre, which was based on spreading a gas, failed because of a technical mishap.

"Radiological bombs" just use radioactive materials around an explosive core, instead of chemicals. USA today has a quick interactive demo explaining the concept. The Center for Foreign Relations has a more in-depth page, meanwhile, that answers a whole range questions about radiological bombs. If you have other questions like "what does it take to make one" and such, this page is a great source.

-- Radiological Bombs & 4GW --

A radiological bomb has already been used in a 4GW scenario. Chechen rebels left an unexploded dirty bomb in Moscow in 1995, then alerted the media. The bomb was defused, and it turned out the radioactivity level was low (by Russian standards, at least). The stunt didn't really achieve very much, except perhaps to increase russian ruthlessness.

So radiological bombs aren't great weapons. Well, it depends what you want to accomplish, and whom you're targeting. In terms of destructive ability, no, they're nothing special. As a harassment and psychological tool that isn't likely to induce a full nuclear response, however, they're unparalleled.

They're especially good against advanced economies whose citizens have low risk thresholds - a description that certainly fits the USA and Europe.

The LA Times reports that at a Senate hearing in March, Steven E. Koonin, provost of Caltech, testified that under U.S. government guidelines the dispersion of a fraction of a gram of a certain isotope over a square-mile area would make the area uninhabitable.

Yet, exposing the population to that quantity of the material would only add 4 more cases of cancer per 100,000 people - vs. the 20,000 cases likely to result from all other causes.

All in all, the direct economic damage per bomb could easily run into the billions. Koonin said he could imagine an attack that exposed people in 100 square blocks of a business district to 3x the acceptable level of radioactivity. This probably wouldn't kill anyone, but the area could be sealed off for months of decontamination. Hundreds of thousands of people would visit hospitals for screening. It could be cheaper to raze dozens of buildings rather than decontaminate them. The fear factor would also have huge indirect economic effects on stock exchanges etc.

Which is, of course, the whole idea. Create additional economic and political pressure against resistance, and divert official attention from fighting terrorism to fixing the economy.

A series of these would be a potent equalizer for a terrorist network determined to eat away at our capabilities.

It's all classic 4th Generation Warfare. Acts of war committed by a non-national force, using unconventional delivery and/or weapons, and relying on attrition and paralysis to weaken and eventually defeat a larger enemy.