Perhaps the "Launch Codes" also include the "Abort Codes"
I shold also explain... the launch codes have NOTHING to do with actually releasing your weapons. All the PALs do is give you PERMISSION and orders. Not to make light of the situation, but its like Mom saying you can go get a cookie out of the jar. Whereas here, the cookie is an ICBM with 8 MIRVs. The men in the silos were the ones turning the keys. In the game, if the aliens got the launch codes, they'd have to hack into the secure comm networks and transmit a false order. By having Also, to be a bit more realistic, if an officer that had first hand knowledge of the PALs went missing, the old codes would be invalidated and new PALs would be created. I'm oversimplifying this of course, so...
Oh, one other thing, there are no abort code. What I mean is, once the birds fly.... no force on earth is going to stop them. Either they're going to get shot down by an ABM or they're going to their target. That being said, up until the point of launch, with the right orders, we can always stop.