If we ever achieve some form of reasonable, in-system spaceflight, then we are really going to have to start worrying about every crazy person with a spaceship bumping a smallish space rock into an impact orbit with earth. Or even just unfortunate accidents. But it wouldn't take much of an asteroid to burn down through the atmosphere and create a Tunguska-level impact.
The most recent Expanse novel is an interesting take on how apocalyptic such an event could be http://amzn.to/2aD2AFl
The most recent Expanse novel is an interesting take on how apocalyptic such an event could be http://amzn.to/2aD2AFl