I always see videos of people randomly filming and a rock breaches the Earths atmosphere. I presume if there was a large one we would see it a lot earlier. Is there a size specifically that astronomers look for and anything smaller than that they ignore?
In: Planetary Science
it depends. the smallest asteroid we have found in space is 6ft, but it is much easier to find asteroids outside of our orbit than between us and the sun. The way we look for asteroids is by their reflection, so its both size and surface. If an asteroid is a small icy rock, it might be brighter than a larger matt black rock. We can find larger asteroids if we know where to look by how they eclipse distant stars, but this is less reliable.
As for destruction, we only really need to worry about 100ft large asteroids, and we are pretty sure we have found all of them outside of our orbit, and mapped them for the next 100 years to ensure they wont collide with earth, and we would need 10 years of warning to prevent one from doing so. Anything smaller than 25ft will burn up completely before hitting the ground and isn’t really a concern.
As for space missions, we mostly just ignore asteroids. space is so big and asteroids are so small, even in the most dense region of space, you can just fire a probe through and expect to not hit anything without having to specifically check. Hitting an asteroid is the trick, not missing one.
There are plans to put in a sun orbiting satellite to check the zone we currently cant, but not enough people thought there even might be an asteroid of concern in that region to actually fund it. https://en.wikipedia.org/wiki/Sentinel_Space_Telescope
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