I'm not sure an explosion at 500 miles would affect even an unshielded satellite in geosynchronous orbit, which is over 22,000 miles out. This is where most of our comsats are located.

I'm also not sure which satellites "routinely" pass through the Van Allen belts, although satellites and probes on their way to geosynchronous and beyond must pass through them at least once on the way out.

In any case, many of our defense systems are set up quite specifically to detect rocket launches into space, and it is unlikely a nation could pull something like this off without pointing a finger straight at themselves.