China’s Fleet of Balloons Prove Hard to Detect as They Survey the World

The objective of the outreach, officials said, is to refute Beijing’s statements that the airship was a civilian research craft and to pressure China’s government to discontinue the surveillance efforts. A closed-door briefing on the balloon for all U.S. senators is scheduled for Thursday.

“I would not be surprised if the PRC starts to re-evaluate its dirigible collection program,” Pentagon spokesman Brig. Gen.

Pat Ryder,

the Pentagon spokesman, said at a media briefing, using the acronym for the People’s Republic of China.

China has ramped up its overall surveillance efforts in recent years in what current and former officials say poses a challenge to the U.S. and its allies. The Pentagon said that China had more than 260 satellites used for surveillance, reconnaissance and intelligence-gathering as of the end of 2021, nearly double the fleet size in 2018 and second only to the U.S. Last year, China said it had largely completed its satellite-based earth observation system.

Satellites provide Beijing with a trove of visual information from high-resolution cameras or radar signals, as well as allowing it to track objects such as warships through their electronic signals.

Balloons can loiter for long periods over a site, helping to supplement satellite images. Because balloons are slow moving and follow less predictable paths, they are harder for militaries to identify and more difficult to block with countermeasures, unlike satellites which can be closely tracked. Satellites usually orbit the Earth from several hundred miles above the ground, while the recent balloon was tracked at an altitude of around 60,000 feet, or 11.4 miles.

Balloons attributed to the Chinese surveillance program have been spotted in Japan, Taiwan, the Philippines and elsewhere. Following the attention on the balloon shot down by the U.S., Beijing confirmed this week that it was the source of a balloon detected flying over Latin America.

Pentagon officials said the latest incursion was notable for the amount of time the balloon lingered over the continental U.S. Detected by the military on Jan. 28 over the Aleutian Islands, the balloon traversed Alaska, parts of Canada and then re-entered the U.S. in Idaho before passing over a swath of the U.S. until it was shot down off the South Carolina coast on Saturday.

The U.S. scrambled jet fighters in February last year after a high-altitude balloon flew near the Hawaiian island of Kauai, the site of a missile test range and other military facilities, but opted not to shoot the object down. At the time, Hawaii’s top military official said the balloon was unmanned and didn’t have any observable markings. He didn’t say where the balloon was believed to have come from or its suspected purpose.

Florida has been the site of suspected past intrusions by China, according to Florida Rep.

Mike Waltz

(R., Fla.), who said he had been briefed about them by the secretary of defense’s office.

China has sent balloons over the continental U.S. on at least four previous occasions during the Trump and Biden administrations that weren’t detected until they were no longer in U.S. airspace, according to the Biden administration.

On Monday, Gen.

Glen VanHerck,

commander of the North American Aerospace Defense Command, acknowledged that the U.S. radar network didn’t identify those intrusions due to a “domain awareness gap.” Rather, U.S. intelligence agencies informed the command after the fact that the flights had occurred by drawing on their classified collection capabilities, he said.

Those previous flights were much shorter in duration, according to administration officials. The ability of the Chinese to exploit the limitations of the U.S. network of ground-based military and civilian radars during its previous flights may have emboldened Beijing to undertake its latest mission, former officials said.

In Washington, reports of the previous flights have led lawmakers to ask why the Biden administration didn’t shoot down the balloon when it first appeared near the Aleutian Islands on Jan. 28 and then flew over Alaska.

Gen. VanHerck said he was initially precluded from ordering it to be downed because the balloon didn’t demonstrate “hostile intent.” The situation changed on Jan. 31 when the balloon appeared over northern Idaho before drifting over Montana, and the White House and the Pentagon began to focus on military options.

Flight-tracking sites show that U.S. Air Force planes, including an RC-135 reconnaissance aircraft, closely monitored the balloon as it passed over the continental U.S. The U.S. military doesn’t have the authority to collect intelligence within the U.S. but Gen. VanHerck said “specific authorities were granted to collect intelligence against the balloon specifically, and we utilized specific capabilities to do that.”

According to Western officials, China’s military surveillance and intelligence-gathering operations are controlled by the People’s Liberation Army Strategic Support Force, a division of the military created by leader

Xi Jinping

as part of an organizational shake-up of the PLA in 2015 that was intended to modernize the armed forces. The information office of China’s cabinet, known as the State Council, didn’t respond to a request for comment.

“China’s reconnaissance capability is about the same as ours now. They’ve advanced a lot over the last 20 years,” said

Carl Schuster,

a former U.S. military analyst who now teaches at Hawaii Pacific University.

Beyond collecting visual data, balloons could have other uses, military analysts said. Scooping up details on atmospheric conditions around military facilities, for example, can help calculate the flight paths of hypersonic missiles that might be used to attack the facilities, Mr. Schuster said. Accurate data is essential because such missiles travel at more than five times the speed of sound and can easily miss their intended target with even a small error in the flight path.

Balloons could also be used by China to help intercept communications such as phone calls or electronic signals because they are closer to the ground than satellites, the military analysts said.

NATO Secretary-General

Jens Stoltenberg,

who appeared at a joint press conference with Mr. Blinken Wednesday, said that China’s intelligence activities are on the rise across Europe as Beijing invests more on new military capabilities, including different types of surveillance and intelligence platforms.

“They use satellites, they cyber, and as we’ve seen in the United States, also balloons, so we just have to be vigilant, we need to be aware of the constant risk of Chinese intelligence and then step up what we do to protect ourselves,” Mr. Stoltenberg said.

—Nancy A. Youssef contributed to this article.

Write to Alastair Gale at alastair.gale@wsj.com and Michael R. Gordon at michael.gordon@wsj.com