Balloons are a great way for China to antagonise other countries

The advantage of floating an intelligence-gathering balloon from one US state to the next would have been negligible. But tens of millions of Americans who pay no attention at all to international affairs have now received a clear message China regards them as enemies - and is even willing to violate their borders in its machinations against them.

Talk about own goals.

Taiwan just got a bit safer. US voters are now more likely to want to support it. So we just got a bit safer, too.

The Chinese military sent the balloon - and, we now hear, others before it - to collect information using sensors that would certainly have included radio receivers.

Armed forces use radio transmissions for communication, controlling weapons and, with radars, detecting things. So all sufficiently capable countries listen to and analyse the signals of potential enemies (and friends, if they're not unusually chummy) to see what they're up to.

This is a valuable but inherently low-profile activity, so no government talks about it very much.

The eavesdropping equipment can be in satellites which, being in space, are not regarded as entering another country's territory as they pass over. Alternatively, ships and aircraft can hang around near the edge of a target country, and ground stations are always trying to pick up signals from afar.

What governments talk about even less is unlawfully sending intelligence-gathering aircraft into other countries to get better radio reception than satellites can achieve from orbit. Intruding aircraft may also get good photos and, unlike satellites, don't pass overhead at precisely predictable times dictated by Sir Isaac Newton.

How many countries do that is unclear, though the US is certainly one that has - for example, to keep an eye on Iran.

The preferred aircraft these days are drones, since the target country may detect the intrusion and start shooting.

So China is just doing what some others do, though, unusually, it is using balloons instead of pilotless aeroplanes.

But the idea is not to get caught - or, more precisely, not to run a risk that's out of proportion with the value of the information you expect to pick up.

The minimal value to China of this stickybeak ballooning is obvious from the US having not bothered to stop it until now. It began in 2018, though working out what China was trying to do took a while, officials tell the Washington Post.

Last week's balloon wandered close to US military bases and, in principle, could have collected certain radio signals that, being absorbed in the atmosphere, could not be picked up by satellites.

But bases on US territory would use landlines for most communication. Signals from radios and radars in moveable equipment such as aircraft may be interesting, but they can be analysed when they turn up closer to China.

Also, sensitive transmissions can be switched off when the balloon approaches, which is just what the Americans did. Shutting things down is inconvenient, however, especially since a balloon, unlike a satellite, can loiter for hours, changing altitude to catch winds with different directions.

The latest intelligence balloon turned up in US airspace near Alaska on January 28, and the Biden administration obviously intended to let it just proceed on its merry way. But the NBC television network broke the story.

Only the resulting public outrage forced what should have happened to the balloon near Alaska: a super-hot F-22 fighter shot it down.

That action had to wait until the balloon got to the Atlantic Ocean on Sunday. The delay supposedly avoided endangering life but the real purpose was no doubt to improve the chance some pieces would survive hitting the surface. My bet is the equipment will turn out to be uninteresting.

Far more important is the damage this really stupid, low-value intelligence program has caused to China's reputation - not just by inevitably getting caught but by getting caught with a really attention-grabbing activity above a series of US states.

"It's right here, over Missouri," they were saying in St Louis, outraged.

"It's right here, over Tennessee," they were saying in Memphis the next day.

But it's not just Americans who will be annoyed. Another Chinese balloon turned up over Colombia last week, maybe because it was trying to get to Florida. So now Colombians know their territory has been violated, too.

In fact, such balloons have flown over five continents, and US officials this week briefed 40 countries on the matter, the Washington Post reports. We can guess the continents were North and South America, Europe, Asia and Africa.

What about Australia? If we haven't been visited already, the Chinese military balloonists have no doubt intended to get to us eventually, assuming they can find suitable wind patterns.

Detecting a balloon might not be a problem for us. Unless it came from the south, we'd soon notice it with our magnificent Jindalee over-the-horizon radars, which can see for thousands of kilometres.

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Then what should we do?

The answer is obvious. It would be an intruding foreign military aircraft. We should shoot it down.

Our fighters can't fly as high as the balloons' limit, which is at least 24,000 metres. But a fighter pilot could accelerate to high speed then zoom up briefly to a higher altitude and release a missile, which would climb the rest of the way to hit the nosy gas bag.