The US military recently shot down a Chinese surveillance balloon off the coast of South Carolina, and the incident has raised questions about the use of spy balloons in the 21st century.

In an age of satellites and high-altitude spy planes, why would anyone use a balloon for surveillance? What capabilities do these balloons have? And what does the incident tell us about the nature of international airspace and the limits of a nation’s sovereignty?

To answer these questions, we spoke with aerospace engineer Iain Boyd of the University of Colorado Boulder. Boyd explained that spy balloons are gas-filled balloons that fly at an altitude of around 60,000 feet, equipped with sophisticated cameras and imaging technology. These balloons can provide a closer view than satellites, and they also have a degree of persistence, allowing them to capture images continuously.

Boyd said that the balloon shot down by the US military was likely collecting images across different regions of the electromagnetic spectrum, including visible and infrared. He also noted that balloons are easier to shoot down and are not completely controllable, which is why they are not usually used for spying these days.

The incident has also raised questions about the limits of a nation’s airspace. Boyd explained that the internationally accepted boundary is the Kármán Line at 62 miles (100 kilometers) altitude, and the Chinese balloon was well below that, so it was definitely in US airspace.

Finally, Boyd said that the incident is likely a political message from China, rather than a real military threat. He said that China has often engaged in provocative behavior, and this could be a form of experimenting to see how far they can push things.

The US military’s decision to shoot down the Chinese surveillance balloon off the coast of South Carolina has raised important questions about the use of spy balloons, the capabilities of these balloons, and the limits of a nation’s sovereignty. Through our conversation with aerospace engineer Iain Boyd, we have gained a better understanding of this incident and its implications for international relations.