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A high-altitude balloon floats over Billings, Mont., on Feb. 1.Chris Jorgensen/The Associated Press

On Thursday, the Pentagon said it was tracking a suspected Chinese surveillance balloon that has been spotted over U.S. airspace just days ahead of a planned trip to Beijing by Secretary of State Antony Blinken.

The balloon was spotted in Montana, home to one of the U.S.’s three nuclear missile silo fields at Malmstrom Air Force Base. The Pentagon, however, said it decided not to shoot it down over concerns of hurting people on the ground. Canada also said it summoned China’s ambassador due to information that the high-altitude balloon may have spent time in Canadian airspace.

The balloon’s appearance has heightened already tense relations between the U.S. and China, with Mr. Blinken postponing his Beijing trip because of the incident.

Here’s what we know about the high-altitude balloon so far.

Did the balloon fly into Canadian airspace? How is Ottawa responding?

The Canadian government said it met with China’s ambassador, Cong Peiwu, on Thursday after the balloon was spotted over U.S. airspace and drifted across Western Canada.

Steffan Watkins, an Ottawa-based research consultant, told The Globe and Mail that it appears Canada dispatched a CP-140 surveillance plan from Comox, B.C., to monitor the balloon January 31 while it was flying over the Canadian Rockies.

He said flight tracking data also suggests Canada also intercepted the balloon with CF-18s on January 31, and the balloon appears to have drifted from Alaska into Northern B.C. and then across the Canada-U.S. border over Montana.

The Department of National Defence has said in a statement that the balloon’s movements are being tracked by the North American Aerospace Defence Command.

“Canadians are safe and Canada is taking steps to ensure the security of its airspace, including the monitoring of a potential second incident,” National Defence said. “NORAD, the Canadian Armed Forces, the Department of National Defence, and other partners have been assessing the situation and working in close co-ordination.”

National Defence also said Canada’s intelligence agencies are “working with American partners and continue to take all necessary measures to safeguard Canada’s sensitive information from foreign intelligence threats.”

What’s the response from the United States?

On Thursday, U.S. Air Force Brigadier-General Patrick Ryder told reporters that the government was tracking a high-altitude surveillance balloon over the United States and said it was “traveling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground.”

He said similar balloon activity has been seen in the past several years, and added that the U.S. took steps to ensure it did not collect sensitive information.

Military officials said the U.S. prepared fighter jets, including F-22s, to shoot down the balloon if ordered. The Pentagon ultimately recommended against it and President Joe Biden accepted that recommendation because of the safety risk from debris, U.S. officials said on Thursday.

On Friday, Mr. Blinken postponed his diplomatic trip to China due to the balloon incident. The development came just before Mr. Blinken had been due to depart Washington for Beijing, marking a blow for strained U.S.-China relations.

What’s the response from the Chinese government?

In a statement Friday, China’s Foreign Ministry acknowledged the balloon was Chinese but said it was a “civilian airship used for research, mainly meteorological purposes.”

The balloon was blown off course, the statement said, adding “the Chinese side regrets the unintended entry of the airship into US airspace.”

It said it would continue to communicate with the United States to “properly handle” the unexpected situation. A Chinese government spokesperson said earlier that “China has no intention of violating the land territory and airspace of any sovereign country.”

On Friday, the U.S. Department of Defence rejected China’s claim that the balloon was for weather monitoring purposes and confirmed that it was a spy ship.

“The fact is, we know that it’s a surveillance balloon,” Brig.-Gen. Pat Ryder told a briefing at the Pentagon. “The balloon has violated U.S. airspace and international law, which is unacceptable.”

What, exactly, is a spy balloon?

The lightweight device is filled with gas, usually helium, and attached to a piece of spy equipment such as a radar or solar-powered camera.

The practice of using spy balloons can be traced back to dynastic China or the French Revolutionary wars, making them one of the oldest forms of surveillance technology.

During the Second World War, Japanese military tried to loft incendiary bombs into U.S. territory using balloons designed to float in jet stream air currents. The balloons were also widely used for reconnaissance missions by the U.S. and the Soviet Union during the Cold War. And more recently, in 2021, the European Union’s border agency also used a surveillance balloon to detect migrants boats.

How big is the Chinese balloon and how high is it flying?

The high-altitude balloon is believed to be the size of three buses, according to a CNN report.

The balloon is travelling at 60,000 feet, or more than 18,000 metres, above the ground, far higher than commercial airline flights, Brig.-Gen. Ryder said. Surveillance balloons typically operate at 24,000 to 37,000 metres, well above where commercial air traffic flies.

As of midday Friday, it was moving east and had arrived over the central U.S.

With reports from Steven Chase, Robert Fife, James Griffiths, Reuters and The Associated Press.