asteroids - How can they tell no asteriods will hit earth in the next hundreds years?

A couple of points based on some basic orbital mechanics

They don't need to get a "good" view, like the clear, crisp photos of Pluto to see one coming. They only need to get a picture over time to calculate trajectory. The unclear snapshots work just fine to calculate if it'll hit us or miss us.

Also, an object as far as Mars at it's closest pass to earth, a bit over 1/2 AU, would still take a few months to reach earth if it's in solar orbit. Mostly we don't need to track anything as far out as Pluto, they can look much closer to the Earth and still have sufficient warming time. The hard part, is tracking things that approach from the Sun side, cause those are harder to see. That's why the Chelyabinsk meteor wasn't spotted. it was also on the small side, smaller than NASA is currently looking for.

The good news is that, we don't get struck by things that size very often. The Solar system is pretty enormous and pretty empty and pretty big strikes like that one are rare, like, maybe once a century.

Also, virtually all of the injuries from the Chelyabinsk meteor were from people who didn't know what to do. If you see a big fireball in the sky, it's human nature to watch it, but use some common sense. A space rock of that size will make a shock-wave that travels at roughly the speed of sound and the shock-wave can break windows, even knock over trees and buildings if it's big enough. You don't want to be standing in-front of a window when the shock wave its. Lay down next to a couch or under a table in case your building gets shaken and cover your ears. If everyone had done that, there would have been very few injuries. You only need to wait maybe 2 minutes or so to be on the safe side.

If you're in a car, stop, cause the shock-wave could knock down trees or debris in-front of you and stay in the car, cause that's safer than being outside. All told, the damage to buildings was tiny compared to natural disasters like Earthquakes, floods or volcanoes which happen to us several times a year. It's good that NASA is watching for this kind of thing, but it's also a pretty rare event.