So now we know a 30-foot rock can sneak up on us and not be seen until the last second. No surprise. But one question I’ve never seen answered is, how big is the largest rock (moving at, say, 30 mi/sec) which could sneak up on us and not be seen until it is too late? To avoid detection until the very last minute, a large rock would probably have to come in from the direction of the Sun and so be lost in the glare. Anyhoo... 300ft?... 3000ft?... One mile? The dinosaur in me is dying to know.
(Thanks for the ping!)
In answer, any size (or dark material) that isn’t likely to be detected by optical telescopes; radioastronomy to detect near-Earth objects doesn’t (yet) cover the entire sky, which is a huge mistake IMHO.
The impactor that produced the 3/4 mile wide Meteor Crater was less than 80 feet in diameter.
The Tunguska object detonated before striking the ground (there’s been a recent renewal of interest in looking for metallic chunks embedded near ground zero) and knocked down many thousands of trees for miles in all directions, and was probably 100 feet in diameter (mostly ices).