Au contraire. It's reasonable to assume that future advances in science will enable humans to redirect an asteroid. If we can do so at sufficient distance, the difference between an asteroid hitting or missing the Earth, can be as little as 1 millimeter.
Well Mr Fairsheet it's my understanding that the US collectively spends less on detecting NEO's and potential impact problems than it does studying what the best tooth whitener is.
Point being here is by the time a private astronomer (Hobbyist most likely) spots one and manages to convince the powers that be we actually have a problem it's going to be close and in spite of Hollywood I'm not sure we as a world have the technology or the equipment to get a deterrent into the air/space and actually hit an asteroid with, I'd suppose a large nuke at enough distance to actually deflect enough to miss earth.
I'd like to see the world governments take the threat a lot more seriously though as if we were to suffer an impact, even a moderate one it could ruin our days here on earth as a species.
To make it even uglier let's suppose it's a rock say 1.5 to 2 miles in diameter and we actually do manage to scramble around and hit it with a large nuke what would the odds be of it breaking up into dozens of smaller asteroids, small being a relative term like 100 yards in diameter, and the now dozens of pieces of soon to be meteors continue on in roughly the same path and some still strike earth.
Just playing with the numbers a two cubic mile rock would work out to roughly 10,903,552,000 cubic yards and estimating 2000 pounds per cu yd we end up with roughly 11,252,313,096 metric tons of weight and mass potentially impacting the earth.
I have no idea how to estimate the mega joules of energy released per pound on a meteoric impact, however I do know beyond any shadow of a doubt I don't want to be anywhere near even a small one.