What is a suspected Chinese spy balloon doing above the US?
News that the Pentagon is monitoring a suspected Chinese surveillance balloon in the skies over the continental United States raises a series of questions – not least among them, what exactly it might be doing.
US officials have said the flight path of the balloon, first spotted over Montana on Thursday, could potentially take it over a "number of sensitive sites" and say they are taking steps to "protect against foreign intelligence collection."
But what's less clear is why Chinese spies would want to use a balloon, rather than a satellite to gather information.
This is not the first time a Chinese balloon has been spotted over the US, but this seems to be acting differently to previous ones, a US defense official said.
"It is appearing to hang out for a longer period of time, this time around, [and is] more persistent than in previous instances. That would be one distinguishing factor," the official said.
Don't spies use satellites now?
Using balloons as spy platforms goes back to the early days of the Cold War. Since then the US has used hundreds of them to monitor its adversaries, said Peter Layton, a fellow at the Griffith Asia Institute in Australia and former Royal Australian Air Force officer.
But with the advent of modern satellite technology enabling the gathering of overflight intelligence data from space, the use of surveillance balloons had been going out of fashion.
Or at least until now.
Recent advances in the miniaturization of electronics mean the floating intelligence platforms may be making a comeback in the modern spying toolkit.
"Balloon payloads can now weigh less and so the balloons can be smaller, cheaper and easier to launch" than satellites, Layton said.
Blake Herzinger, an expert in Indo-Pacific defense policy at the American Enterprise Institute, said despite their slow speeds, balloons aren't always easy to spot.
"They're very low signature and low-to-zero emission, so hard to pick up with traditional situational awareness or surveillance technology," Herzinger said.
And balloons can do some things that satellites can't.
"Space-based systems are just as good but they are more predictable in their orbital dynamics," Layton said.
"An advantage of balloons is that they can be steered using onboard computers to take advantage of winds and they can go up and down to a limited degree. This means they can loiter to a limited extent.
"A satellite can't loiter and so many are needed to criss-cross an area of interest to maintain surveillance," he said.
What might it be spying on?
According to Layton, the suspected Chinese balloon is likely collecting information on US communication systems and radars.
"Some of these systems use extremely high frequencies that are short range, can be absorbed by the atmosphere and being line-of-sight are very directional. It's possible a balloon might be a better collection platform for such specific technical collection than a satellite," he said.
Retired US Air Force Col. Cedric Leighton, a CNN military analyst, echoed those thoughts.
"They could be scooping up signals intelligence, in other words, they're looking at our cell phone traffic, our radio traffic," Leighton told CNN's Erin Burnett.
Intelligence data collected by the balloon could be relayed in real time via a satellite link back to China, Layton said.
Analysts also noted that Montana and nearby states are home to US intercontinental ballistic missile silos and strategic bomber bases.
US officials say they have taken actions to ensure the balloon cannot collect any sensitive data. They decided against shooting it down because of the risk to lives and property by falling debris.
And if the US could bring down the balloon within its territory without destroying it then the balloon might reveal some secrets of its own, Layton added.
But maybe there are no secrets or spying involved. This could be just an accident, with the balloon blown off course or Chinese operators losing control of it somehow.
"There's at least some possibility that this was a mistake and the balloon ended up somewhere Beijing didn't expect," Herzinger said.
For its part, China says it's looking into things.
"We are aware of reports [of the balloon] and are trying to understand the circumstances and verify the details of the situation," a Foreign Ministry spokesperson said Friday. "I'd like to stress that before it becomes clear what happened, any deliberate speculation or hyping up would not help handling of the matter."