US, China trade spying charges: Why are balloons usually sent into air, can they be used for surveillance?

The United States and China are currently involved in an extraordinary standoff over allegations of espionage through high-altitude balloons. Last week, the US downed three unidentified 'objects' flying over its airspace and that of Canada. The downed objects have not yet been recovered, so it is not clear whether these were balloons. But the first incident, on February 4, involved a Chinese balloon that Beijing claimed was meant for research purposes and was accidentally blown off course by wind. The US has also claimed that Chinese balloons entered its airspace on at least four earlier occasions, remaining undetected at that…