Surveillance balloons track prevailing winds
The US has accused China of dropping a surveillance balloon over sensitive US territory, but what evidence is there that it came from China?
Chinese authorities now say it is theirs but say it is for scientific research and has gone off the rails.
Where was the ball seen?
According to US authorities, the balloon was monitored as it passed over the Aleutian Islands of Alaska and Canadian territory before emerging over the town of Billings, Montana on Wednesday, where it was been spotted in the sky by locals.
Map showing Montana, Billings and Malmstrom Air Force Base
The Pentagon says it tracked the balloon over US airspace with manned aircraft and the surveillance device flew over sensitive locations.
Do we know where the ball comes from?
Not sure. We don’t have flight tracking data like we would have for airliners, for example, so we have to look to other sources of information.
One technique used to estimate the trajectory of particles or objects at high altitudes is to use a model based on wind speed and direction.
Map showing the balloon’s possible route from China to the United States
The United States National Oceanic and Atmospheric Department (NOAA) has developed such a model (known as HYSPLIT) based on winds at altitudes above 14,000 m (46,000 ft).
“Its main application,” says BBC Weather’s Simon King, “is to calculate the transport and spread of things like pollutants and hazardous substances through the atmosphere.
“The model also works in reverse – what is called the return trajectory – where we can see where anything that is carried in the air is coming from, such as pollutants, ash or other materials.
“In the case of the balloon over the United States, this return path can show where the air carrying the balloon is coming from and, by analyzing the direction and speed of the wind, where it will fly in the future. “
American meteorologist Dan Satterfield used this model to calculate a possible route that the surveillance balloon took and took shared their findings online.
From the position sighted in Montana on Feb. 1, he estimated a possible return route for the ball based on wind data from central China.
It should be emphasized that this is not the actual trajectory of the surveillance balloon, but an analysis based on the model developed by the US government agency NOAA.
Graphic of a high-altitude balloon showing a helium-filled balloon, solar panels, and an instrument bay that can hold cameras, radar, and communications equipment. They can fly at altitudes of 80,000 to 120,000 feet, higher than fighter jets and airliners
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