This isn't really correct; near-Earth asteroids big enough to cause global catastrophe are much brighter than smaller asteroids like the one talked about in the article, so they're much easier to detect. We've already mapped the vast majority (over 90%) of >1km asteroids and most of the ones in the >140m range. If any of these were found to have a high probability of hitting Earth, we could send a redirection mission years ahead of time.

There are also (ground-based) systems like ATLAS which are monitoring the sky 24/7 and would likely provide a few days to weeks of advance warning for small to medium asteroids. That's too late to redirect the asteroid, but enough time to evacuate an area if necessary.

[–] Rhaedas@fedia.io 4 points 22 hours ago

Large NEOs are very unlikely at this point for the reasons you've given. It's advanced warning on anything else that's needed. Redirection is another topic, but we have no experience and only theory on the best ways to do it, and wouldn't know which technique to try before a close examination (which means time to find it and get to it just to figure that out). As for advanced warning for evacuation, that's probably the most likely scenario, but it's also morbidly funny to mention in a thread discussing near misses found days before its passing, and often we see articles about the rock after the fact. (Although maybe it was known about before by experts, just didn't hit the press in time)