I can't resolve a problem I thought of in special relativity, was hoping somebody here could help. This is going to sound somewhat strange, but say I'm floating in space, no accelerations involved. Suddenly a spacecraft flies past me, inside are a bunch of incompetent physicists trying to make a nuclear bomb and test it in their spacecraft (leading to their certain demise). From their perspective, the nuclear fission reaction fails, say, because they didn't make the nuclear fuel dense enough for the reaction to happen properly (it wasn't dense enough to get the reaction going) so the experiment fails and they survive.
However from my perspective, they are travelling near to the speed of light, and their length is shortened in their direction of motion. This means that from my perspective, the nuclear fuel is now much more dense (its been squashed lengthways). This happens to just be enough to get the nuclear fuel to critical density and the reaction doesn't fail from my perspective, the bomb explodes and the ship gets destroyed.
...I feel I'm missing something painfully obvious, thanks in advance if you can explain this.