Here’s how spy balloons work, what they can be used for

Such balloons usually operate at an altitude of 80,000-120,000 feet.

US officials said on Thursday that a Chinese “surveillance balloon” has been flying over the United States for several days. Using high-altitude balloons for espionage and other military operations is a practice that dates back to the middle of the last century. Here’s how they work and what they can be used for:

• During World War II, the Japanese military tried to hurl incendiary bombs over American territory using balloons designed to float in jet stream air currents. No military targets were damaged, but several civilians were killed when a balloon crashed in the Oregon woods.
• Just after World War II, the US military began exploring the use of high-altitude spy balloons, which led to a large-scale series of missions known as Project Genetics. According to government documents, the project flew photographic balloons over Soviet bloc territory in the 1950s.
• Such balloons typically operate at 80,000–120,000 ft (24,000–37,000 m), where commercial air traffic flies – airliners almost never fly above 40,000 ft. The highest performing fighters typically do not operate above 65,000 feet, although spy planes such as the U-2 have a service ceiling of 80,000 feet or more.
• According to a 2009 report by the Air Command and Staff College of the US Air Force, the advantages of balloons over satellites include the ability to scan wide areas of territory at close range and being able to spend more time over the target area.
• Unlike satellites, which require space launchers that cost millions of dollars, balloons can be launched inexpensively.
• According to a 2005 study for the Air Force’s Airpower Research Institute, the balloons are not propelled directly, but can be guided roughly to the target area by varying altitude to capture different air currents.
• The US military has tracked other spy balloons in recent years, including before the administration of President Joe Biden, according to a senior US defense official.

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