I would say it's physical presence in a nation's territory. Given altitude is unlimited, territory has to end somewhere and that seems to be space, from the co

I would say it's physical presence in a nation's territory. Given altitude is unlimited, territory has to end somewhere and that seems to be space, from the cold war period. Also there are agreements not to weaponise space which reduces threat levels. Also when this became the norm sattelites were far less capable which may have help this become acceptable, though not confident on that. Also consider nations grabbing comms is considered reasonable. They protest occasionally but largely this is considered fair game as you are again not physically present, where as spy caught in a building type event is a big deal. There needs to be some boundaries. Physical presence in 'territory' seems to be it.