[weikel]

Doesn't a nuke have side effects like fallout EMP etc an asteroid would not have( while increased dust in the atmosphere would be a side effect of an asteroid).

[RightWhale]

A nuke would emit lots of short wavelength radiation such as ultraviolet and x-rays. An asteroid would emit ordinary light and heat. The energy of a photon is supposedly inversely proportional to its wavelength, and the nuke would have a lot more high energy reactions. Although the total energy could be the same for a small nuke and a large asteroid, the distribution of energy in the spectrum would be different.