[Exsubmariner]

Yes, you are correct. The atmosphere is not 300 miles thick. I am aware of that. But, all you would need would be to have a weapon of sufficient power to generate ionization in the atmosphere over the continental US. That is a simple problem of power. Making a more powerful nuke was easier for the russians than guidance systems for their missles. That's how they compensated for having such inaccurate missles. All you would need to know would be the amount of ionization in terms of coulombs needed to produce an EMP of the desired strength and calculate the I squared R ratio backwards. I bet I could probably find that information on the web within about an hour of intense search. Then, you just build a bomb powerful enough to give off the desired level of gamma burst to produce that amount of ionization.

You could very easily overcome the "rocket Scientist" aspects by calculating the speed of your rocket and putting a timer on it to go off at the proper altitude. It doesn't need to really be that accurate for it to work.

You are right, however, that it would be much simpler to deploy a weapon to this effect just outside the atmosphere. It would have a much smaller footprint and cover an area say the size of New York State. Getting it right the first time would be crucial in this type of attack because whoever it was would certainly not ever get a second chance.