EMPs are set off in the atmosphere not space. It must re-enter to be effective.
EMP blasts can occur at 40 - 400 kilometers / 24 - 240 miles altitude.
A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi) and 2,000 kilometers (1,200 mi).
So you can detonate an EMP weapon in LEO at 160 - 400 km. That is most assuredly outside the atmosphere and in orbit.
You can park a satellite at a higher altitude, then bring it down to a lower altitude for detonation when you want to attack. The satellite orbit won’t degrade at the higher altitude where it is parked.
This is FAR easier than building a re-entry vehicle.
The NORKs have had significant EMP technology transferred to them, BTW.
"In July, 1962, the US carried out the Starfish Prime test, exploding a 1.44 megaton bomb 400 kilometres (250 mi) above the mid-Pacific Ocean. This demonstrated that the effects of a high-altitude nuclear explosion were much larger than had been previously calculated. Starfish Prime made those effects known to the public by causing electrical damage in Hawaii, about 1,445 kilometres (898 mi) away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms and damaging a microwave link.
Starfish Prime was the first success in the series of United States high-altitude nuclear tests in 1962 known as Operation Fishbowl. Subsequent tests gathered more data on the high-altitude EMP phenomenon.
The Bluegill Triple Prime and Kingfish high-altitude nuclear tests of October and November 1962 in Operation Fishbowl provided data that was clear enough to enable physicists to accurately identify the physical mechanisms behind the electromagnetic pulses.
The EMP damage of the Starfish Prime test was quickly repaired due, in part, to the fact that the EMP pulse over Hawaii was relatively weak compared to what could be produced over the northern U.S. with a more intense pulse, and in part due to the relative ruggedness (compared to today) of Hawaii's electrical and electronic infrastructure in 1962.
The relatively small magnitude of the Starfish Prime EMP in Hawaii (about 5.6 kilovolts/metre) and the relatively small amount of damage (for example, only 1 to 3 percent of streetlights extinguished)[11] led some scientists to believe, in the early days of EMP research, that the problem might not be significant. Newer calculations[10] showed that if the Starfish Prime warhead had been detonated over the northern continental United States, the magnitude of the EMP would have been much larger (22 to 30 kV/m) because of the greater strength of the Earth's magnetic field over the United States, as well as its different orientation at high latitudes. These calculations, combined with the accelerating reliance on EMP-sensitive microelectronics, heightened awareness that EMP could be a significant problem.[citation needed]