The trip to Beijing by U.S. Secretary of State Antony Blinken has been postponed, according to the U.S. State Department, due to concerns over a Chinese surveillance balloon that was observed flying in U.S. airspace.

The Pentagon expressed “very high confidence” that the balloon, operating at a high altitude, originated from China and was being used to gather information from sensitive locations.

China’s foreign affairs ministry confirmed on Friday that the balloon is theirs, but called it a “civilian airship used for research, mainly meteorological purposes,” that had gone off course by accident.

“Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course,” it said in a statement. “The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure. The Chinese side will continue communicating with the US side and properly handle this unexpected situation caused by force majeure.

U.S. defense officials believe it does not pose a “military or physical threat to people on the ground,” as NORAD and U.S. Northern Command said in a Thursday statement, and recommended against shooting it down.

Mao Ning, China’s foreign ministry spokesperson, previously told reporters that China has “no intention of violating the territory or airspace of any sovereign country” and urged people to remain calm and avoid speculation until all the facts are clear.

Still, some Republican lawmakers — including House Speaker Kevin McCarthy and Montana Rep. Ryan Zinke — are accusing China of provoking the U.S. and calling for the U.S. to address it.

The incident comes at a tense time for U.S.-China relations, with American national security concerns ranging from TikTok to Taiwan and just before Blinken was set to meet with Chinese President Xi Jinping in Beijing as part of an effort to diffuse those tensions.

The trip would have followed up on President Biden’s meeting with Xi in Bali in November. Blinken was due to depart Friday, and would have been the first Biden administration Cabinet secretary to visit China.

Here’s what else we know so far.

What is a surveillance balloon?

These large balloons carry cameras or other equipment and float above a given area, usually at an altitude of about 80,000 to 120,000 feet, The Guardian reports. In contrast, commercial planes generally fly at about 35,000 feet.

Pentagon press secretary Brig. Gen. Patrick Ryder has said the balloon is “currently traveling at an altitude well above commercial air traffic.”

The National Parks Service says the use of spy balloons within the U.S. dates back to the Civil War, when both sides used hot air balloons (usually tethered to the ground and reaching about 1,000 feet) to collect reconnaissance and direct artillery towards the other.

The practice became more widespread during World War I and then during the Cold War, when the U.S. launched hundreds of balloons to gather intelligence on China and the Soviet Union, Bloomberg reports.

Surveillance balloons have been overshadowed by satellites and unmanned drones in recent decades, though they still offer certain advantages (for instance, they’re cheaper to launch and can conduct long-duration surveillance missions). Politico reported last year that the Pentagon was expanding investment in high-altitude balloon projects.

What is it looking for?

U.S. officials said Thursday that the U.S. had been tracking the balloon since it entered U.S. airspace a few days ago. They didn’t specify its exact timeline or path other than to confirm it floated over Montana at one point (the white circle is so large that people could spot it from the ground).

What is the purpose of the surveillance?

Officials from the U.S. have confirmed that they have been monitoring the balloon since its entry into U.S. airspace a few days ago. While they did not provide specific details regarding its timeline or trajectory, they confirmed that it was seen floating over Montana at one point and was visible from the ground due to its large size

But, he said, the U.S. is using some of those fighter jets to “take a peek” at the balloon in its efforts to keep monitoring it.

What happened in Canada?

Canada also says it’s monitoring a high-altitude surveillance balloon, though it’s not clear if it’s the same one. U.S. officials said the balloon they are tracking flew over the Aleutian Islands and through Canada before it was spotted Wednesday over Billings, Montana.

“Canadians are safe and Canada is taking steps to ensure the security of its airspace, including the monitoring of a potential second incident,” the Canadian Department of National Defence said in a brief statement on Thursday.

It added that it is in frequent contact with the U.S. as the situation develops.

“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,” it said.

Have we seen this kind of thing before?

The Pentagon said similar balloon activity has been seen on occasion in recent years.

A defense official “wouldn’t characterize this as revolutionary,” Myre reports, adding that what seems to be different this time around is the altitude of the balloon and the length of time that it’s staying over the U.S.

The U.S. Naval Institute pointed to one not-so-recent memory in a Thursday evening tweet: During World War II, the crew of the USS New York spotted a sphere in the sky that they suspected was a Japanese balloon. After multiple successful attempts at shooting it down, they realized they were actually shooting at Venus.

What was Blinken planning to do in China?

Tensions were already high this week, when the U.S. announced it would expand its presence in Southeast Asia with access to more bases in the Philippines — a move China said would threaten security in the region.

The balloon situation creates more friction between the countries and has given China hawks in the U.S. more material to seize on, as NPR’s Beijing correspondent Emily Feng tells Morning Edition.

“The U.S. and China have been trying to stabilize their relationship, but this incident and ongoing bipartisan efforts to increase sanctions on China get in the way of that,” she explains.

CUT Blinken’s trip to China hasn’t been officially confirmed by either side, Feng adds, though he is expected to leave on Sunday.

Even if the meeting had gone as planned, nothing much was expected to come from it. U.S. officials and former diplomats tell Feng it might result in a joint statement, perhaps about combating climate change or against the use of nuclear weapons in Ukraine, but no major breakthroughs are likely.

Still, they had said the fact that Blinken was visiting China at all was a sign of progress.

“There are some meaty issues at the heart of their relationship over human rights, combating climate change, technological competition,” Feng says. “So that involves getting China on board, even if the two countries don’t see eye to eye.”