Why is the U.S. falling behind on its promises to destroy old nuclear weapons? Here’s one reason given to government auditors (pdf):

United States Government Accountability Office

That’s right, the U.S. isn’t dismantling its old nuclear weapons, because we might need them to destroy an asteroid hurtling toward earth. To clarify some of the bureaucratic language above, NNSA is the “National Nuclear Security Administration”; CSAs are “canned subassemblies” that contain highly enriched uranium for use in nuclear weapons. And senior-level government evaluation means that somewhere in government, there is contingency planning going on around what to do in the event of an asteroid heading toward earth.

The Wall Street Journal, which first reported this item, notes that scientists aren’t necessarily concerned about a massive, planetary-extinction level asteroid more than kilometer in diameter, like the on scientists believe led to the extinction of dinosaurs—at least for the next hundred years or so.

But since an asteroid just 20 meters in diameter crashed in Chelyabinsk, Russia, last year, injuring over a thousand people, scientists are increasingly concerned about the threat presented by smaller asteroids and are working on ways to track and combat them. Some scientists think that if such an asteroid were heading for earth, a nuclear blast in space could be the best way to break avert the threat.