Spy Balloons: Are They Really a Threat?

US-China relations remained strained after a suspected Chinese spy balloon was shot down by the US military in early February. China has denied the balloon was being used for espionage, asserting it was merely a weather-monitoring airship that had blown off course. But what do we really know about spy balloons? And how much of a threat are they? The BBC's Security Correspondent Gordon Corera looks at the evidence.