Dang.
So we will not make it the 9.27 years we have left to stop global climate change because we will be wiped out by an asteroid? Damn!
Two things. We are finding new asteroids with large orbits which we have not seen in hundreds of years. Some have shown up this year that we did not even know existed. And secondly, there is always the combination. An asteroid can hit another object and part of that combination can form new orbits and they could hit earth. We seem to be fine as long as everything keeps its orbit and hits nothing else. The reality is that space things are hitting other space things all the time. And you cannot mathematically calculate the orbit of the pieces. You just have to wait and see the outcome.
Any nuclear explosion would have to be planned to occur far enough away from earth to ensure that Earth would not get too strong of an electromagnetic pulse (EMP) that would know out electrical grids.