> On the other hand, what's the chance of something like this happening with a much bigger asteroid.
Bigger asteroids are easier to see.
Bigger asteroids are easier to see.
another metric that affects the ... impact ... of such an event is also the speed of the asteroid. Unlike size, I suspect higher speeds would make it harder to spot (and once spotted there would be less time to take action)
Though warning times will be shorter the higher the relative speed is.
If we are only surveying a portion of the sky at a time and a faster asteroid spends less time traversing that portion, the likelihood of detection is lower.
Assuming you are not. What kind of "streak" you are thinking about? Are you thinking about comets with their tails? Or motion blur?
Because if motion blur I would expect an asteroid on a collision course to have none. (at least in the short timeframe before the collision) Because "Constant bearing, decreasing range" is how a collision looks like from a first person perspective.
(Of course, if it's going to hit you, the faster it's going, the more straight at you its path is at the same distance. But for the same amount of "not straight at you", faster leaves a bigger streak.)