Lets assume the asteroid is composed mainly of nickel with a density of around 8900 kg/m^3. Therefore the mass of an extinction event asteroid with a 10 km diameter would be around 524 billion kilograms. At an impact velocity of 40 km/sec it would release 4.19E+20 Joules of energy, equivalent to 100 billion tons of TNT. We don't want this hitting the earth. A one-megaton nuke would be 50% efficient at best, but probably only 25% of its energy would be used (guessing here), giving the asteroid a delta-v of 63 m/s. This assumes it does not disintegrate. To miss the earth by 1 lunar distance would require at least 71 days warning. This also assumes the asteroid's trajectory is not made worse by the earth's gravity. Personally I think we're screwed. But that may just be my mood. Lots of assumptions here.
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Nuclear Deflection of asteroids