(Reuters) -U.S. officials said on Thursday that a Chinese "surveillance balloon" has been flying over the United States for several days.
Using&nbsp;high-altitude&nbsp;balloons&nbsp;for&nbsp;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 what they can be used for:
* During World War 2, the Japanese military tried to loft incendiary bombs into U.S. territory using&nbsp;balloons&nbsp;designed to float in jet stream air currents. No military targets were damaged, but several civilians were killed when one of the&nbsp;balloons&nbsp;crashed in an Oregon forest.
* Just after World War 2, the U.S. military started exploring the use of&nbsp;high-altitude&nbsp;spy&nbsp;balloons, which led to a large-scale series of missions called Project Genetrix. The project flew photographic&nbsp;balloons&nbsp;over Soviet bloc territory in the 1950s, according to government documents.
* Such&nbsp;balloons&nbsp;typically operate at 80,000-120,000 feet (24,000-37,000m), well above where commercial air traffic flies - airliners almost never fly&nbsp;higher than 40,000 feet. The&nbsp;highest-performing fighter aircraft typically do not operate above 65,000 feet, although&nbsp;spy&nbsp;planes such as the U-2 have a service ceiling of 80,000 feet or more.
* The advantages of&nbsp;balloons&nbsp;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 U.S. Air Force's Air Command and Staff College.
* Unlike satellites, which require space launchers that cost hundreds of millions of dollars,&nbsp;balloons&nbsp;can be launched cheaply.
* The&nbsp;balloons&nbsp;are not directly steered, but can be roughly guided to a target area by changing&nbsp;altitudes to catch different wind currents, according to a 2005 study for the Air Force's Airpower Research Institute.
* The U.S. military has tracked other&nbsp;spy&nbsp;balloons&nbsp;in recent years, including before President Joe Biden's administration, according to a senior U.S. defense official.
(Reporting by Gerry Doyle; Editing by Don Durfee and Stephen Coates)

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