North Korea detonated something on Monday and it seems to have been atomic. The reason we begin this way is that the precise yield of the device is still uncertain, but current estimates are in the range of half a kiloton (save for the Russians, who have said it was over a kiloton and was certainly a nuclear weapon). That is about one-fortieth of the Nagasaki blast — which was primitive — and is substantially smaller than first tests by other nuclear powers. Unless the yield-estimate consensus is revised upward dramatically in the next day or so, this is all a bit odd.

There are three possible explanations for the apparently small yield: the North Koreans deliberately detonated a very small device, they tested a larger device but it failed to execute properly, or the explosion was not caused by an atomic device.

The first explanation could be true. Possibly the North Koreans wanted to show that they had the technology but did not want to appear too threatening, so they minimized the size. Or they could be demonstrating the ability to use lower-yield nuclear mines or artillery shells that would protect North Korea by blocking strategic passes into the country, and would possibly threaten Seoul but would not pose a significant threat elsewhere. Also, the water table is high in the area of the blast; maybe they were being careful not to break into the aquifer.

These are all good reasons, but the counterargument is that if you are going to go nuclear, go nuclear. North Korea does not have a pressing need — or history — of being subtle, so a small blast doesn’t fit in with its plan. This brings us to the second explanation, that the fissioning process failed at some point, causing a partial detonation. We are not nuclear physicists, so we can’t really say whether this is plausible; but some physicists have said that it is theoretically possible.

What if the North Koreans didn’t go nuclear, but detonated a large chemical explosive in an underground chamber? It would take a lot of explosive to yield that result, but it is not impossible. A chemical explosion would have a different seismic signature than a nuclear one, and therefore geologists should have already discounted this theory; but the analysis is going to take up to two days, according to the White House. It is certainly not beyond the North Koreans to fake a nuclear explosion, and there have been some big explosions in North Korea that have been mistaken, for a short period of time, for something nuclear. But there is no evidence, beyond our speculation, for this theory.

We really don’t think the North Koreans exploded a baby weapon deliberately. Following the failures in their missile launches, they needed to demonstrate a robust capability. We have to believe that they wanted an unambiguous explosion. We would suspect that, had it been a chemical blast, that fact would be already known and widely disseminated. That leaves us with the idea of a partially successful detonation. If we had to pick now, that’s what we’d go for.

The reason this matters is credibility. What the North Koreans were buying with this blast was international credibility as a significant nuclear power not to be trifled with. They did not need a repeat of the missile fiasco, when just about everything went wrong with multiple launches. The failure to detonate according to plan cuts into their credibility. So knowing what the plan was really does matter. It tells us something about how close they are to actually having a deliverable weapon. If this test was in some sense a failure, it creates more time until a weapon is ready, potentially creates a crisis of confidence in North Korea and opens the door for other powers to pressure Pyongyang. If it was according to plan, the time required to produce a weapon decreases, meaning the North Koreans will be as cocky as only they can be and they will be doing the pressuring.