General Discussion

It would have been visible in the sky to the naked eye for a few days before impact, growing bigger and bigger. But our modern NASA would've seen it ages off. It was chilling around dipping into our orbit regularly for millions of years before gravity finally got it just so. We would've been tracking it like we do other near-Earth objects now (and a six mile asteroid is super visible to our technology that close).

The good news is, we know where most of the dangerous asteroids around the Earth are and none of them are predicted to smack us any time soon. Last I saw, it was something like 90% estimated found, tracked, and orbits calculated.

Our real problem is the Oort cloud. There could be asteroids or comets with high eccentricity that only come around every few hundred or thousand years. By the time we saw something really damaging coming, not much to be done for it.

But we are on it as much as one can be: https://science.nasa.gov/planetary-defense

All that said. There's always a chance some random ass asteroid that could go off like a nuke might pop in out of nowhere. Very, very small chance, but not non-zero.