But you can also gather different kinds of information in other parts of the electromagnetic spectrum. Another fairly well-known one is infrared. If it’s nighttime, a camera operating in the visible part of the spectrum is not going to show you anything. It’s all going to be dark. But an infrared camera can pick up things from heat in the dark.

Most of these balloons literally go where the wind blows. There can be a little bit of navigation, but there are certainly not people aboard them. They are at the mercy of whatever the weather is. They sometimes have guiding apparatus on them that change a balloon’s altitude to catch winds going in particular directions. But it’s pretty much wherever the wind’s blowing, that’s where you’re going.

There are machine learning-types of approaches that would seek to optimize your path, so that if you’re trying to get from A to B, you can get closer. But if the prevailing winds are just going completely in the opposite direction to where you want to go, there’s really no way to get there with a balloon.

There is an internationally accepted boundary called the Kármán Line at 62 miles (100 kilometers) altitude. This balloon is well below that, so it is absolutely, definitely in U.S. airspace.

The Pentagon has had programs over the last few decades studying balloons, different aspects of what can be done with balloons that couldn’t be in the past. Maybe they’re bigger, maybe they can go higher in the atmosphere so they’re more difficult to shoot down or disable. Maybe they could be more persistent. But I’m not aware of any countries actively using spy balloons these days. There have been unconfirmed reports of potential spy balloons in Asia that have been attributed to China.

The U.S. flew many balloons over the Soviet Union in the 1940s and 1950s, and those were eventually replaced by the high-altitude spy airplanes, the U-2s, and they were subsequently replaced by satellites.

I’m sure a number of countries around the world have periodically gone back to reevaluate: Are there other things we could do now with balloons that we couldn’t do before? Do they close some gaps we have from satellites and airplanes?

China has complained for many years about the U.S. spying on China through satellites, through ships. And China is also well known for engaging in somewhat provocative behavior, like in the South China Sea, sailing close to other nations’ boundaries and saber-rattling. I think it falls into that category.

The balloon doesn’t pose any real threat to the U.S. I think sometimes China is just experimenting to see how far they can push things. This isn’t really very advanced technology. It’s not serving any real military purpose. I think it’s much more likely some kind of political message.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. Like this article? Subscribe to our weekly newsletter.

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Iain Boyd receives funding from the U.S. Department of Defense, the U.S. Department of Energy, NASA, and Lockheed-Martin.