>>25
I'll give it a last try.

>Ballistic Missiles are in fact capible of reaching orbit.

"Orbit" being of course a ~300 km low earth orbit.

>or press a button and fire somthing from a silo

You don't have a clue about ICBMs either, do you?
Flight control for an ICBM is laid out for a ballistic curve, not for interplanetary space. As soon as the booster rocket runs out the warhead is unpowered. No course corrections by a control center, no cameras, no other sensors of any kind. And designed to split up into a dozen MIRVs.
So one would need a new warhead, a new flight control system, and a vastly more powerful booster. There's your custom build.

>if we make a miscalculation on the mass of the asteroid

(M = mass of the spacecraft, m = mass of the asteroid, a = acceleration imparted on the asteroid, r = distance between spacecraft and asteroid, G = gravitational constant)

G*M*m/r^2 = m*a
=> a = G*M/r^2

The mass of the asteroid is irrelevant for the tractor.

A nuke on the other hand gives you a fixed impulse p. The imparted change in velocity would be:

v = p/m

A miscalculation of the asteroids mass is only a problem if you use a nuke.