By Michael O’Hanlon, Special to CNN

Michael O’Hanlon is senior fellow at Brookings and author of The Wounded Giant: America’s Armed Forces in an Age of Austerity. The views expressed are his own.

Military strikes against the uranium enrichment facilities at Natanz and Qom, together with other possible targets related to Iran’s nuclear infrastructure, could last for a single day and single sortie – or they could last for several days or even weeks. The latter possibility of course implies American participation too, and probably requires the use of air bases in one or more Gulf states as well, given the likely U.S. interest in using stealthy planes that at present don’t fly from aircraft carriers (though B-2 bombers could fly from Diego Garcia, for example).

So what is the likely effectiveness, and what are the likely risks, of each possible approach? I’d argue that there is there is significant unpredictability about how well an air campaign by Israel in particular would work – not least in terms of how much of the existing Iranian nuclear infrastructure it would destroy, and how long it might take Iran to recover (and that’s even leaving aside the huge issue of how Iran might retaliate).

First, though, it’s worth reviewing some basic information on previous, somewhat analogous, air campaigns, as well as the nature and locations of Iran’s nuclear facilities.

In Operation Desert Fox, in 1998, the United States and Britain used 650 aircraft sorties and 415 cruise missile strikes, against 97 targets (11 WMD facilities, 18 command and control, 8 Republican Guard barracks, six airfields, 19 key regime sites such as Special Republican Guard barracks). They were reasonably effective. But of course we had been monitoring Iraq closely for nearly a decade by this point, on the ground and in the air, and Saddam Hussein had already abandoned his nuclear aspirations, we now know.

Or consider the Kosovo campaign of the spring of 1999. In the first month of the campaign, daily strikes averaged 5 against air defenses, 2.5 against lines of communication, and 4 against command and control (for the rest of the campaign, the respective figures were 3, 5, and nearly 7). Total sorties per day averaged 250 in the first week, and close to 500 in April. This campaign had a much different purpose, but underscored the difficulty of achieving major goals with a limited number of air strikes. Initial forecasts of a quick Milosevic capitulation, in Washington and elsewhere, were badly off.

All this suggests that a single-day air campaign against a large nuclear infrastructure would be quite unusual in the annals of modern air warfare. To be sure, the United States sometimes uses more assets than perhaps required simply because it can. But typical attacks last several days and involve hundreds of sorties. Often, those attacks that were originally expected to take just a few days take weeks, as well. Nuclear facilities, meanwhile, are typically quite large, meaning that a planeload or two of ordnance will quite often be far short of what’s required.

Recent improvements in precision strike technology and GPS guidance make it quite possible that we could successfully drop numerous munitions into the same hole, digging deeper and deeper, even in bad weather and even if dust and other debris would make laser guidance challenging. In addition, it appears likely that we have tested this methodology fairly carefully, and it appears likely the Israelis could carry it out too. Even in a single-sortie attack, successive planes reaching a target could perhaps therefore execute such a mission.

However, air defenses could be quite problematic for non-stealthy planes and perhaps even for stealthy planes, since the locations of attack can be anticipated extremely well. This challenge would place a greater premium on a larger attack force, to allow attacks against radars and command centers and SAM launchers, and argues against the single-sortie mission. Recent precedents suggest that dozens of bombs could be needed to deal with an air defense system, especially since Iran’s enrichment facilities aren’t near its borders.

In addition, bomb damage assessment could be challenging. We may be able to dig deep with bombs, but we may not know if we were successful right away. Protracted examination of the site after initial strikes might be needed to confirm if any activity continued at those locations, meaning that the need for follow-on strikes might not be immediately apparent. This is especially true at the second uranium enrichment site near Qom. Again, this would be a downside of the single-sortie option.

All that said, we need to remain cognizant of the risks of even the prolonged attack scenario as well – going well beyond the well-known risks of Iranian retaliation, even greater internal Iranian consensus on the desirability of going nuclear after the attack, and the likely weakening of international sanctions after the attack as well. There could be a third enrichment site that we fail to attack simply because we don’t know it currently exists. The Iranians may bring in a group of civilians to “inspect” the site of the first attack, making follow-on waves difficult with the use of such human shields. (This would be risky but much less dangerous than how the regime used its own young soldiers in the Iran-Iraq war.) Air defenses could be repaired and/or relocated as well, while any attack involving hundreds of sorties or more would very likely involve the loss of one or more airplanes due to Iranian defensive action or simple aircraft malfunction.

We frequently talk pseudo-scientifically about the likelihood of a one to two year delay in Iran’s program after such a strike. I believe the range is more like 6 months to 3 or 4 years, given how hard it would be to predict the damage to Qom, as well as other uncertainties such as the possible existence of other enrichment sites.

All this means that there’s greater uncertainty in the outcome of an Israeli or U.S. air campaign against Iran’s nuclear facilities than frequently appreciated, whether it is carried out quickly or over a period of days or even weeks.