All wrong. As everybody should know by now it has been the former residents of Atlantis who disabled them in order to remain the only superior nation on the planet's face. That was shortly before a inner political force succeeded and they started space travel and submerged their old city.

In a way, there are. It's theorized to be the ancient city of Sodom which was nuked by an "alien".

Time machine failures, I like to imagine.

In fairness, the odds of an asteroid hitting so close to a major city is so small, that an intentional alien attack should not be entirely ruled out.

A berserker in far solar orbit could conceivably stunt human development every couple thousand years with an asteroid, while a more effective extermination ship is being dispatched.