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Such balloons typically operate at 24,000m to 37,000m, well above where commercial air traffic flies – airliners almost never fly higher than 40,000ft (12,000m). The highest-performing fighter aircraft typically do not operate above 65,000ft, although spy planes such as the U-2 have a service ceiling of 80,000ft or more.

The advantages of balloons over satellites include the ability to scan wide swathes of territory from closer in, and to be able to spend more time over a target area, according to a 2009 report to the US Air Force’s Air Command and Staff College.

Unlike satellites, which require space launchers that cost hundreds of millions of dollars, balloons can be launched cheaply.

The balloons are not directly steered, but can be roughly guided to a target area by changing altitudes to catch different wind currents, according to a 2005 study for the Air Force’s Airpower Research Institute.

The US military has tracked other spy balloons in recent years, including before President Joe Biden’s administration, according to a senior US defence official.