The controversy over the Chinese spy balloon shot down by the U.S. off the coast of South Carolina on Saturday is raising questions about why Beijing would use a seemingly old technology to gather intelligence when more sophisticated instruments exist.

According to Timothy Heath, a senior international defense researcher at the Rand Corporation, one reason is that the balloons are hard to detect, giving them an advantage over satellites, which tend to be predictable in their movements.

There are a few reasons surveillance balloons can be hard to detect.

“Newer technologies allow little motors to be attached so you can make adjustments to the position of the balloon and then they can just stay in a place for quite a while,” Heath said. 

“Once you’ve identified a satellite in space, you can track its orbit and you can accurately predict when it will pass and when it will not be watching,” Heath said.  

This could be partly why China chose to use a balloon: Its movements are more erratic and thus more difficult to track.

The Biden administration has said several Chinese spy balloons flew over the United States during the Trump administration. Officials said the information was not even known by officials until after Trump left office.

“I will tell you that we did not detect those threats,” said Gen. Glen VanHerck, commander of North American Aerospace Defense Command and U.S. Northern Command. “And that’s a domain awareness gap that we have to figure out.”

The use of surveillance balloons, Heath noted, dates back to the Civil War. 

“[Historically] the U.S. has used balloons to carry surveillance equipment, intelligence collection equipment,” he told the Hill. “The appeal of balloons is that, one, they’re relatively cheap; two, they can be hard to detect because there’s not a lot of metal on them, so radars will not easily find them; and then, three, you can position them in a place and just keep them there.”

Because balloons are a more inexpensive form of surveillance, it isn’t a huge loss if another country strikes one down, like the U.S. did with this stray balloon.

Additionally, the balloons are often constructed out of softer materials that make them more difficult to track on radar than metal-rich satellites. If a craft is not showing up on radar, it’s going to take longer for officials to spot. 

China described the balloon as a high-altitude “weather” balloon that had drifted off course due to its limited steering capacity. The balloon was soaring above Alaska at roughly 60,000 to 65,000 feet before it drifted into Canada and then back into U.S. airspace, where it spent a week, slowly drifting across the country.  

Heath said the use of balloons is a surveillance method that has been used by a number of countries.

“Yes, it’s probably one of many methods the Chinese use to collect intelligence, just like the U.S. government uses many methods to collect intelligence on China,” Heath said. “It’s not especially noteworthy to me that China uses different intelligence methods; the U.S. does the same.”