Every nuke is an EMP. The higher you detonate a nuke above ground, the less physical damage is done, but the EMP spreads further the higher up it is. Any super power with dreams of taking over the US, wont nuke our cities. They will keep the cities intact. Instead they will wipe out our power grid with high altitude nuclear detonations, which will soften us up for a ground invasion.
Nukes in space doesn't make much sense to me, unless its only purpose is to cut down our response time against a nuke targeted at the US, but im told that these nukes in space are just to take out satellites. ICBM's already go into space. If you want to take out satellites, then just launch a nuke and detonate it in orbit.