A Chinese spy balloon was detected over the US, the Department of Defense said.
Spy balloons date back centuries, but still have some advantages today.
Experts say spy balloons can do things satellites can't, and China may have wanted to get caught.
The US Department of Defense raised the alarm Thursday about a intelligence-gathering balloon it said it spotted over Montana.
The balloon, the Pentagon said, was "most certainly" sent by China to spy on the US. After first declining to say, China on Friday admitted the balloon was its own, though claimed it was just for monitoring the weather.
The unusual technology prompted a slew of questions, and some experts to explain why China might want to deploy a huge balloon for espionage.
What is the Chinese spy balloon?
A spy balloon is a balloon with any kind of surveillance equipment is attached. This one was white with a series of panels hanging underneath, according to photos of the balloon taken by local media in Montana.
US officials declined to give specifics, but made clear the balloon was large, and visible from far away. Several media outlets gave an estimate that it was the size of three buses.
In a Pentagon briefing Thursday, a senior US defense official said spy balloons have been seen several times over the US in recent years.
The latest one, however, appeared to be hanging around specific locations for longer than usual.
Using balloons for spying dates back hundreds of years, even as planes and later satellites gave higher-tech options to modern intelligence agencies.
They were first recorded in use in the late 1700s, and lastest well into the age of aviation, with the US deploying spy balloons over the Soviet Union in the 1950s. The latest news brought them to the fore again.
Why would China send a spy balloon over the US?
The US alleged that the balloon was "clearly" there for surveillance, and noted that it was seen not far from sensitive sites including the base for some of the US nuclear arsenal.
China, after initially not admitting the balloon was Chinese, later admitted it was, and said it was a weather-monitoring device that had been blown badly off course.
Insider asked Marina Miron, a defense researcher at Kings College London, what China might have to gain if the balloon was indeed for spying.
She told Insider there was likely "symbolic value" to China being able to brazenly fly a balloon over the US, even when it also has access to monitoring technology from satellites beyond the reach of the US.
She said Beijing was likely "reminding the US that China can just conduct surveillance with impunity."
Miron said that China may have wanted to get caught for this reason. "My take is that China almost wanted to have the balloon spotted," she said.
Miron speculated that China might have been reacting to events closer to home, wanting to send a signal after the nearby US upset China by striking a deal with the Phillippines to set up a new base there.
Miron added that China may also have been influenced by the Russia's invasion of Ukraine, perhaps concluding that the US may not be able to "contain China at the same time" as supporting Ukraine.
Why use a spy balloon rather than a satellite?
Spy balloons still offer advantages today over other surveillance methods like satellites, experts say.
Peter Layton, a fellow at the Griffith Asia Institute in Australia and former Royal Australian Air Force officer, told CNN that they have cost and speed advantages: "Balloon payloads can now weigh less and so the balloons can be smaller, cheaper and easier to launch."
Miron told Insider that the balloons can collect information that satellites can't, as different types of equipment can be attached to them.
And they can be hard to detect, said Blake Herzinger, an expert in Indo-Pacific defense policy at the American Enterprise Institute who spoke to CNN.
They can also stay and hover in one spot to collect data, he added, unlike a satellite.
Is the balloon dangerous?
US officials said the balloon appeared to pose little threat to the US, both physically and in terms of what intelligence it might gather.
Pentagon Press Secretary Air Force Brig. Gen. Pat Ryder said the balloon was much higher than commercial air traffic, which tends to operate between 30,000 feet and 40,000 feet.
This means there is little risk of a plan crashing into it.
Ryder said the US "acted immediately" to stop the balloon gathering any sensitive information, but gave no specifics about how.
Why not shoot it down?
The US said it considered shooting the balloon down, but decided not to.
The senior US defense official said on Thursday that shooting it down could hurt civilian, and that it didn't pose enough of a threat to justify that.
"Currently, we assess that this balloon has limited additive value from an intelligence collective collection perspective," the official said. "But we are taking steps, nevertheless, to protect against foreign intelligence collection of sensitive information."
Canada also said that it was monitoring the balloon, and mentioned an unspecific "second incident" it was monitoring.
Its authorities also concluded that "Canadians are safe" as were its state secrets.
Herzinger also told CNN that it couldn't be ruled out that the balloon ending up over the US was an error resulting from the balloon being blown off course.
"There's at least some possibility that this was a mistake and the balloon ended up somewhere Beijing didn't expect" — the justification that China eventaully gave.
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