I guess the probability of impact won't reach the >50% level until a few months before impact due to variations in earth's orbit and by that point the ability to do anything about it is limited.
I don't see how this could possibly be correct. Earth's orbit is very precisely known.
-the size of the object
-the orbit of the object
-the velocity of the object
-effect of outgassing and solar wind on the object (as mentioned in the top comment)
Rather than uncertainties in the earth's orbit.