The Chinese balloon that the Pentagon said was flying over sensitive US ballistic missile sites might be guided by advanced artificial intelligence technology.
William Kim, a specialist in surveillance balloons at the Marathon Initiative think tank in Washington, said they are a valuable means of observation and difficult to shoot down.
The US military reported the presence of a Chinese spy balloon over the state of Montana on Thursday, prompting Secretary of State Antony Blinken to postpone a visit to China. Another "Chinese surveillance balloon" was passing over Latin America, the Pentagon said late on Friday without giving details.
China has said the balloon seen over Montana was a meteorological research balloon with limited self-steering capabilities that went off course due to winds.
Mr Kim said balloon looked like a normal weather balloon but had distinct characteristics.
It has a quite large, visible "payload" — the electronics for guidance and collecting information, powered by large solar panels. And it appears to have advanced steering technologies that the US military has not yet put in the air, he told AFP.
Artificial intelligence has made it possible for a balloon, just by reading the changes in the air around it, to adjust its altitude to guide it where it wants to go, Mr Kim said.
"Before you either had to have a tether … or you just send it up and it just goes wherever the wind takes it," he said.
"What's happened very recently with advances in AI is that you can have a balloon that doesn't need its own motion system. Merely by adjusting the altitude it can control its direction."
That could also involve radio communications from its home base, he said.
But "if the point of it is to monitor [intercontinental ballistic missile] silos, which is one of the theories … you wouldn't necessarily need to tell it to adjust its location," he added.
Mr Kim said that as satellites become more vulnerable to being attacked from the Earth and space, balloons have distinct advantages.
Firstly, they do not easily show up on radars.
"These are materials that don't reflect, they're not metal. So even though these balloons expand to quite large, detecting the balloon itself is going to be a problem," he said.
And the payload, if small enough, can be overlooked.
Balloons also have the advantage of holding relatively stationary positions over a surveillance target, compared to constantly orbiting satellites used by spy agencies to take photographs.
"These things can stay overhead, they can stay over one spot months at a time, compared to the low-Earth-orbit satellites," Mr Kim said.
He said there was a "real possibility" that a Chinese balloon may have been intended to collect data from outside US boundaries or from much higher, but malfunctioned.
"These balloons don't always work perfectly," he said.
The balloons usually operate at altitudes of 20,000-30,000 metres, and this one was detected at around 14,000 metres, he said.
"That's definitely a little low. If you wanted it to be harder to spot, if you want it to be harder to shoot down, then it would make sense to operate at higher altitudes."
The Pentagon said it did not intend to shoot down the balloon as it moved east over the US mainland, citing the risk from falling debris, but Mr Kim said doing so would not have been easy as it sounds.
"These balloons use helium. It's not the Hindenburg — you can't just shoot it and then and then it goes up in flames."
"If you do punch holes in it, it's just kind of going to leak out very slowly."
Mr Kim recalled that in 1998 the Canadian air force sent up F-18 fighter jets to try to shoot down a rogue weather balloon.
"They fired a thousand 20-millimetre cannon rounds into it. And it still took six days before it finally came down. These are not things that explode or pop when you shoot at them."
Other experts suggested that, rather than surveillance, the purpose of the balloon was to gauge Washington's response.
"This is a way to test how does the other side respond, not in a military sense. But politically, what do you do about it? Do you keep it quiet?" said Dean Cheng, senior adviser to the China programme at the US Institute of Peace.
"If there have been in fact many and this is not the first time, then it raises an interesting question. What happened to the previous ones? Did we shoot them down?” he said.
Mike Rounds, a Republican member of the US Senate Armed Services Committee, told Fox News it would be good to recover the balloon to see "if it was designed to actually collect data or if it was designed to test our response capabilities".
Andrew Antonio, co-founder of high-altitude balloon start-up Urban Sky, said the wind currents that high-altitude balloons depend on for steering on long-distance trips were least favourable in the winter, suggesting China's intentions might not have been be to target any specific location in the US.
"Specifically targeting a certain military base with that balloon from a launch in China, in January or February, in the Northern Hemisphere, is very difficult to do, if not impossible," Mr Antonio said, speculating that the balloon's venture into US airspace could have been the result of a failed experiment, or some failure in its self-termination system.
With reporting from AFP and AP