We know.
The only time you detonate a nuke in orbit is to disable a wide array of communications with your emp, which would cover a much broader area (it is actually what MI for the US is most afraid of, instead of terrorist detonating a nuke within a city since at a certain height even a tactical nuke could disable a huge portion of the eastern seaboard, or if you placed one high enough in the atmosphere the entirity of mainland usa).
To do damage to an enemy who needed no tech to keep coming after you however, a direct strike would be neccessary.