China says that a balloon spotted over United States airspace was being used for weather research and was blown off course, despite Pentagon suspicion it was for "surveillance".
The US had previously said in a defence briefing to reporters that it was confident the balloon belonged to China and that they were tracking its movements.
But what else do we know about the balloon that has caused such alarm?
How big is the balloon?
Briefing US reporters on Thursday, a US defence official did not specify the size of the balloon, but said it was large enough that commercial pilots could see it and that shooting it down would create a debris field large enough to put people on the ground at risk.
Eyewitnesses in the north-western state of Montana have also taken photos and video of what appears to be the balloon flying high above the state.
US network ABC News additionally reported that a senior US defence official had described the balloon as the size of three buses.
What is the balloon carrying?
The senior US defence official said the balloon featured a technology bay.
But they said they "wouldn't characterise" the surveillance technology on board as "revolutionary".
Still, Malcom Davis, a senior analyst at the Australian Strategic Policy Initiative, told ABC News the instrumentation package underneath the balloon would be "really valuable in terms of an intelligence find".
"If you look at the picture of the balloon which you've got up, it's got big solar panels there on either side of an instrumentation package, so it's gathering intelligence in that regard," he said.
So, what's it looking for?
According to Dr Davis, Montana is the site of a number of highly sensitive US military facilities.
Some of the United States' nuclear arsenal is located in Montana, with one of the nation's three nuclear-missile silo fields located at the state's Malmstrom Air Force Base.
A US defence official told reporters on Thursday they were concerned the balloon was trying to collect intelligence on the silo fields.
"… clearly the intent of this balloon is for surveillance," the official told reporters. "And so the current flight path does carry it over a number of sensitive sites."
But Dr Davis also said it was likely the balloon was intended to intercept radio transmissions, analyse them and send them back to China.
"Well, it's a fairly archaic way of gathering intelligence, but I think what it's probably doing is undertaking what's known as signals intelligence," Dr Davis said.
"Because if you were taking photos of military sites, you would do that using spy satellites, and the Chinese have a large number of spy satellites.
"So, if you're doing sig-int (signals intelligence), it seems to me you would want something that would linger over a territory and suck up radio information and radio transmissions from a large area. And so a balloon would actually make sense in that regard."
Is the balloon's presence designed to send a message?
Dr Davis said the balloon might have also been sent as a "provocative" political message.
"It's provocative in the sense that the Chinese are flying essentially military hardware across the US — at very high altitude so it's not a danger to commercial air traffic — but nevertheless, it is a Chinese military asset that is gathering intelligence, so it is quite provocative," he said.
"And it does beg the question as to why are they seeking to gather intelligence in this manner, and potentially signals intelligence or electronic emissions, at this time."
What has China said about the balloon?
In a statement late on Friday, China said that the balloon is for civilian meteorological and other scientific purposes and that it regrets that the airship strayed into US airspace.
"The airship is from China and is civilian in nature, used for meteorological and other scientific research," China's defence ministry said.
"Due to the influence of westerly winds and its limited control capability, the airship deviated from its intended course."
"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure," the statement continued, citing a legal term used to refer to events beyond one's control.
"China will continue to maintain communication with the US side to properly handle this accident."
Earlier, China's foreign ministry had said it was looking into the balloon's movements, and called for calm.
"China is a responsible country and has always strictly abided by international laws, and China has no intention to violate the territory and airspace of any sovereign countries," foreign ministry spokesperson Mao Ning said.
She said politicians and the public should withhold judgement "before we have a clear understanding of the facts".
How long have spy balloons been around for?
Using high-altitude balloons for spying and other military missions is a practice that dates to the middle of last century.
During World War II, the Japanese military tried to loft incendiary bombs into US territory using balloons designed to float in jet-stream air currents.
No military targets were damaged, but several civilians were killed when one of the balloons crashed in an Oregon forest.
Just after World War II, the US military started exploring the use of high-altitude spy balloons, which led to a large-scale series of missions called Project Genetrix.
The project flew photographic balloons over Soviet bloc territory in the 1950s, according to government documents.
The US military has tracked "a handful" of other spy balloons over the country in recent years, including before current President Joe Biden took office, according to the senior US defence official.
But the official said this balloon was different.
"It is appearing to hang out for a long period of time this time around, more persistent than in previous instances," he said.
"So that would be one distinguishing factor."
On Friday, Canada's defence ministry said it had also detected a "high-altitude surveillance balloon" and it was monitoring a "potential second incident".
How are they operated?
Spy balloons are not directly steered, but they can be roughly guided to a target area by changing their altitude so they catch different wind currents, according to a 2005 study for the US Air Force's Airpower Research Institute.
The balloon identified by US defence officials is said to be travelling at an altitude "well above commercial air traffic", which is typical of spy balloons.
Spy balloons typically operate at an altitude of 24 kilometres to 37km, which is well above where commercial planes fly. Airliners almost never fly higher than 40,000 feet (12.19km).
The highest-performing fighter aircraft typically do not operate above 65,000 feet (19.8km), although spy planes such as the U-2 have a service ceiling of 80,000 feet (24.38km) or more.
Are they more effective than satellites?
Unlike satellites, which require space launchers that cost hundreds of millions of dollars, balloons can be launched cheaply.
The advantages of balloons over satellites include their ability to scan wide swathes of territory from closer in, and they can also spend more time over a target area, according to a 2009 report to the US Air Force's Air Command and Staff College.
But the US is confident that the balloon spotted over the country is unlikely to extract any intelligence beyond what China could collect through satellites in low orbit.
ABC/wires
