Using high-altitude balloons for spying and other military missions is a practice that dates to the middle of the last century.
No military targets were damaged, but several civilians were killed when one of the balloons crashed in an Oregon forest.
* Just after World War 2, the U.S. military started exploring the use of high-altitude spy balloons, which led to a large-scale series of missions called Project Genetrix.
* 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 U.S. military has tracked other spy balloons in recent years, including before President Joe Biden's administration, according to a senior U.S. defense official.