Comet would explain the "light night", as might a rocky meteor.

I can't buy the "spontanious nuke" theory, and I'd also discount the "lost motor home" one to- you can't cross space with a fission reactor, and if you have fusion availble to you why not use it on your scouts? And if you crack a fusion reactor... big deal, you let the cold air in and the plasma dies down. Yes, everyone in a few thousand yards is kinda hosed but you don't get a 20 Kt bang. :P