Spy balloons: How they work and why China is using one over the US
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 spying and other military missions is a practice that dates to the middle of the last century.
Here is what is known about how they operate and why China is using them now.
What are spy balloons?
During the second world was the Japanese military tried to loft incendiary bombs into US territory using balloons designed to float in jet stream air currents. No military targets were damaged, but several civilians were killed when one of the balloons crashed in an Oregon forest.
Just after the second world war, the US military started exploring the use of high-altitude spy balloons, which led to a large-scale series of missions called Project Genetrix. The project flew photographic balloons over Soviet bloc territory in the 1950s, according to government documents.
How do they work?
Such balloons typically operate at 80,000-120,000 feet (24,000-37,000m), well above where commercial air traffic flies - airliners almost never fly higher than 40,000 feet. The highest-performing fighter aircraft 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.
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.
Why is China doing this?
The US military has tracked other spy balloons in recent years, including before President Joe Biden's administration, according to a senior US defense official.
Analysts suggest China sent the balloon partly as a message. Its low altitude over sensitive targets has led some to believe Beijing wanted it to be spotted.
The images of a Chinese spy balloon drifting so easily over US territory may have been intended to embarrass the White House ahead of the security of state, Anthony Blinken, landing in Beijing to meet president Xi Jinping this weekend.
The timing is also auspicious coming after the US announced a string of new military deals with allies directly facing China amid growing tensions over Taiwan.
Alexander Neill, an analyst at Hawaii’s Pacific Forum think tank, said the balloon's intelligence gathering was likely limited, suggesting its real intention may be more political.
"China has its own constellation of spy and military satellites that are far more important and effective in terms of watching the US, so I think it is a fair assumption that the intelligence gain is not huge," Mr Neill said.