No other agency notices the Comet?

Our government did nothing to stop Al-Qaeda on 9/11, so yes, this is absolutely within probability. No agency did anything even though the threat was staring them in the face. So what happens in this movie lines up.

To quote billy Bob Thorton in Armegeddon: "It's a big ass sky!"

Yeah, someone did.

Did you watch the movie, it's exactly about that: someone noticing a comet that comes toward earth ...

OF COURSE someone is the first to notice ...

Actually, most comets are discovered by amateur astronomers. As someone else said, it's a big sky, and professionals are usually looking at particular areas of interest. It's easier to find a comet with a lower power telescope that accompasses a wide angle view and compare images day to day. Not something professional astronomers do. New comets appear randomly and the fact that there are so many more amateurs with lower powered scopes leads to the discoveries.

Actually the least scientifically believable part of the story is that such a large comet was seen for the first time by a professional when it was only 6 months out rather than an amateur when it was further away. Unless they explained they were looking for near earth objects and I missed that.

I did like that they depicted a double tail on the comet!

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They did, these guys were just the first.