[davidu]

There are wide swaths of the US airspace that do not have strong radar coverage, certainly not at 80,000 feet.

Most of the ocean is unmonitored, too. Remember MH370? We lost it in the air and we couldn't find it in the ocean. :-)

[bigmattystyles]

I mean, it doesn’t mean we couldn’t, just that it was not worth revealing our ability to do so over one airliner crash where no one was likely to survive. They didn’t directly act on intelligence in enigma for the same reason either if memory serves. Tin foil hats are itchy…

[JackFr]

Umm, NORAD is able to track Santa - this balloon is much easier.

[AlbertCory]

If your argument is "MH370", it's a weak one. No one has radar coverage in the southern Indian Ocean because no one needs it.

There was an article about that very topic on HN just recently. It was not even remotely over US airspace, and the pilot had evidently threaded the needle through the radar coverage of several nations.