Editor's Note: Steve Chapman is on vacation. The following column was originally published in February 2008.

"Death tugs at my ear and says, 'Live, I am coming.'" Were Supreme Court Justice Oliver Wendell Holmes Jr. alive today, he might ascribe that line not to death but to nuclear terrorism.

Ever since Sept. 11, 2001, Americans have had to live with the knowledge that the next time the terrorists strike, it could be not with airplanes capable of killing thousands but atomic bombs capable of killing hundreds of thousands.

The prospect has created a sense of profound vulnerability. It has shaped our view of government policies aimed at combating terrorism (filtered through Jack Bauer). It helped mobilize support for the Iraq war.

Why are we worried? Bomb designs can be found on the Internet. Fissile material may be smuggled out of Russia. Iran, a longtime sponsor of terrorist groups, is trying to acquire nuclear weapons. A layperson may figure it's only a matter of time before the unimaginable comes to pass. Harvard's Graham Allison, in his book "Nuclear Terrorism," concludes, "On the current course, nuclear terrorism is inevitable."

But remember: After Sept. 11, 2001, we all thought more attacks were a certainty. Yet al-Qaida and its ideological kin have proved unable to mount a second strike.

Given their inability to do something simple—say, shoot up a shopping mall or set off a truck bomb—it's reasonable to ask whether they have a chance at something much more ambitious. Far from being plausible, argued Ohio State University professor John Mueller in a presentation at the University of Chicago, "the likelihood that a terrorist group will come up with an atomic bomb seems to be vanishingly small."

The events required to make that happen comprise a multitude of Herculean tasks. First, a terrorist group has to get a bomb or fissile material, perhaps from Russia's inventory of decommissioned warheads. If that were easy, one would have already gone missing.

Besides, those devices are probably no longer a danger, since weapons that are not scrupulously maintained (as those have not been) quickly become what one expert calls "radioactive scrap metal." If terrorists were able to steal a Pakistani bomb, they would still have to defeat the arming codes and other safeguards designed to prevent unauthorized use.

As for Iran, no nuclear state has ever given a bomb to an ally—for reasons even the Iranians can grasp.

Stealing some 100 pounds of bomb fuel would require help from rogue individuals inside some government who are prepared to jeopardize their own lives. The terrorists, notes Mueller, would then have to spirit it "hundreds of miles out of the country over unfamiliar terrain, and probably while being pursued by security forces."

Then comes the task of building a bomb. It's not something you can gin up with spare parts and power tools in your garage. It requires millions of dollars, a safe haven and advanced equipment—plus people with specialized skills, lots of time and a willingness to die for the cause. And if al-Qaida could make a prototype, another obstacle would emerge: There is no guarantee it would work, and there is no way to test it.

Assuming the jihadists vault over those Himalayas, they would have to deliver the weapon onto American soil. Sure, drug smugglers bring in contraband all the time - but seeking their help would confront the plotters with possible exposure or extortion. This, like every other step in the entire process, means expanding the circle of people who know what's going on, multiplying the chance someone will blab, back out or screw up.

Mueller recalls that after the Irish Republican Army failed in an attempt to blow up British Prime Minister Margaret Thatcher, it said, "We only have to be lucky once. You will have to be lucky always." Al-Qaida, he says, faces a very different challenge: For it to carry out a nuclear attack, everything has to go right. For us to escape, only one thing has to go wrong.

That has heartening implications. If al-Qaida embarks on the project, it has only a minuscule chance of seeing it bear fruit. Given the formidable odds, it probably won't bother.

None of this means we should stop trying to minimize the risk by securing nuclear stockpiles, monitoring terrorist communications and improving port screening. But it offers good reason to think that in this war, it appears, the worst eventuality is one that will never happen.

Steve Chapman blogs daily at newsblogs.chicagotribune.com/steve_chapman.