"This is all just supposition. No one really knows," said Malcolm Macdonald, an internationally recognized space technology engineer and professor at the University of Strathclyde Glasgow in Scotland.

And it's important to point that out at the start. We don't know a lot about the technology behind the balloon, or, indeed, whether it is or is not a Chinese spy balloon as alleged by US military and security officials.  

What we know for sure, said Macdonald, is what the Americans have said publicly.

"The Americans have said the Chinese have done this, they've flown over Guan and Hawaii [in the Pacific], it's been over the USA and Canada for a few days now, it was spotted on Thursday by a member of the public and the military has had a look and said, 'Yes, that's what it is.'"

Macdonald said images of the balloon suggest it has solar panels that could power fans to steer it, but other than that, it would be nothing more than a standard helium balloon that may have drifted in the wind.

Things got trickier, however, when we spoke about the possible surveillance technology onboard, what the Chinese would hope to achieve, if indeed this is a Chinese craft, and why the US has not shot it down.

It's 'probably' a Chinese spy balloon

US officials say they have a high level of confidence that the balloon is Chinese, but are not saying why.

It was sighted over the US state of Montana, which borders Canada, having flown over the Aleutian Islands, most of which belong to the US state of Alaska. Some belong to Russia.

The AFP news agency quotes a senior Pentagon official as saying the balloon was now over "a number of sensitive sites" — and that was possibly in reference, wrote AFP, to nuclear missile silos in Montana.

"Clearly, they're trying […] to collect information," the official added, but added that the balloon's technology was not "revolutionary" and that its observations were unlikely to be better than what China was capable with its spy satellites.

China, meanwhile, said on Friday that the balloon was an unmanned civilian airship used mainly for meteorological research and said it had been blown off course.

Spy balloons offer an element of surprise

In low-Earth orbit, which is at an altitude of about 250-300 kilometers (155-186 miles) in the sky, "the Chinese can image down to a special resolution of probably about 15 centimeters per pixel," said Macdonald. "If you bring that down to 20-30 kilometers altitude, where the balloon is, you're potentially talking one centimeter per pixel."

But the imaging equipment may just be standard camera technology that anyone can buy online, said Macdonald. And they are unlikely to be using radar technology, which can see through clouds.

"The Chinese will have to have assumed that the Americans would shoot it down and that they would try to recover all of the electronics onboard and reverse engineer it," said Macdonald. "Therefore, they're not going to put their best capability onto it, because they wouldn't want to just hand it over to the Americans."

However, spy balloons do have at least one advantage over satellites in espionage.

"Satellites have a very predictable motion, so you know when they are going to be overhead," said Macdonald, "so, a balloon offers an element of surprise. You might get a chance to image something that [the others] didn't expect you to be able to see. But at the same time, it would be odd for the Americans not to spot a balloon entering their airspace."

Is the balloon intercepting radio signals?

A statement released by the US Department of Defense on Thursday said "we assess that this balloon has limited additive value from an intelligence collective collection perspective, but we are taking steps, nevertheless, to protect against foreign intelligence collection of sensitive information."  

That could, said Macdonald, hint at the balloon's surveilling radio signals intelligence.

"There will be radio frequency emissions coming off the missile silos that the balloon is flying over and they probably can't pick them up from low-Earth orbit as they are probably too weak," said Macdonald. "But being 20-30 kilometers above them, the balloon may be able to pick up some radio signals that they couldn't get from orbit."

If the Americans know it's there, said Macdonald, they could turn off all the computers off and bring stuff indoors. "But it is still an inconvenience." 

So, why not shoot it down?

The balloon has flown over vastly unpopulated areas, so the Americans could have shot it down.

They have said, though, that they are concerned about debris falling to the ground. And that is plausible, but concerns over public safety may not be the main reason.

Shooting it down would be "relatively easy — you burst the balloon and it comes down," said Macdonald, "but, ideally, you'd want to catch it before it hits the ground because if it hits the ground, [the electronics onboard] will shatter."

The Americans will want to avoid that happening, so that they can work out what was onboard. It may, then, be a case of their waiting for the best opportunity to shoot and catch the balloon.

And if they do shoot it down, it has enough weight for it to drop straight down.

"We've done flights with helium balloons. They drift in the wind to about 15-20 kilometers and then they go straight up, more or less. They then come straight down, with a very slight drift. So, as they come down, the wind catch them, but it is more or less straight down," said Macdonald.

That drift may be over an area of about 100 kilometers, "but that's relatively confined," said Macdonald, and one which pales compared to the 1,400 km that Australian authorities recently scoured to find a radioactive capsule.

The other option is still that the US just lets the balloon go. They have done that before. They won't want China to know they can see the balloon, they won't want to confirm what they have in their air defense system, said Macdonald.

So, the question remains as to why the Chinese would have done this. "Perhaps there's a sense that the botched COVID-exit has weakened President Xi Jinping and this may be him wanting to project some strength," suggested Macdonald. 

Edited by: Rob Mudge