Trial balloon.
Really.
If this one gets shot down, I bet the wreckage would show a big "nothing".
But if this is accepted, then the next one will be the one with viruses, or an EMP. Imagine three balloons -- Los Angeles, Chicago, NYC/Washington -- all timed to hit us with EMPs simultaneously. We'd never recover. And, no, I don't think our subs would launch nukes against China -- because, gosh darn it, we couldn't really be sure it was China, could we? And we sure wouldn't want to nuke an innocent country, right? So we would sit back and take it.
People have said that anything they surveil with this balloon could be known from satellites or other intelligence.
I suspect the thought is that this has some other purpose.
What if this balloon is carrying an EMP? There’s strategic locations in the heartland that would cause an apocalypse in the US if detonated. Shoot this POS down!