comment: >
  
  Probably airborne sensors. This thing is the size of three buses w/ a sizable observation bay. Deep ground penetrating radar from space has limits due to high orbit and the transition from space to atmosphere. Something that big probably can emit strong enough radar burst to get good look at deep underground military installations . Also just testing our response time and where our fighters would take off to intercept said balloon for North American defense.
context: Someone please make a Chinese version of "99 Luftballons" >
  
  This person got it started. >
  
  Can somebody inform me what a balloon can do that a spy satellite can’t?