About this "The Simpsons" episode:

The Simpsons - Bart's Comet is the 14th episode of season 6 and number 117 overall in the series.
In this episode, during detention, Bart accidentally discovers a comet. It is then discovered that the comet is heading straight for Springfield, and a rocket is launched to destroy it. However, it misses and blows up the only bridge out of town, dooming everyone in Springfield. The Simpsons and the Flanders share the bomb shelter that Ned had built, to hopefully protect themselves from the comet. Soon, everyone in town joins them in the shelter, but the door cannot be closed unless somebody leaves. Homer decides that the only useless person there is Ned, and forces him to go. Homer then feels guilty about his decision and leaves, with the rest of the town following him. However the danger is averted when the comet enters the atmosphere and burns up in the thick layer of pollution.