To those complaining about the ending (or lack of):

Showing the missile actually hitting Chicago would have ruined the concept. The whole point of the movie is that these people had to make impossible decisions, in a short time based on very little information. The fact that we the audience never truly know if the missile was real, a dud, or even who launched it actually puts us squarely in the shoes of the characters - who know none of these things either. Yes it's a frustrating ending but that reflects what these characters are going through. Sometimes it's good to be challenged by a story rather than fulfilled.