Inspired by YR4 2024 to write a story about a catastrophic asteroid impact. Would anyone care to consult my very new/rough idea?

I have some questions about how long it would take to measure an asteroid’s impact, size, composition, etc. For example - would it be possible to discover an asteroid that will potentially be dangerous in 6 years, only to discover 3 years later that it absolutely will be catastrophic? I also would like to ask for insight about what kind of defensive tactics (like DART) would be used in the event of an asteroid and how they can succeed/fail Or anyone that would like to poke holes in what I have so far. It’s very new and mostly just an idea. ChatGPT is helping me organize my layout and then I will work on a proper outline. Thanks!