At a briefing on Monday, a US defence official said the size and make-up of the object informed the decision not to shoot it down while it was over land. Remnants of the object - which the US believes is a spy balloon but China says is a weather monitoring device blown astray - have been collected from a roughly 1,500m (4,920 ft) by 1,500m sized area, but it is thought debris is spread over a far larger site. Adm Mike Mullen, former chair of the US Joint Chiefs of Staff, rejected China's suggestion it might have blown off course, saying it was manoeuvrable because "it has propellers on it"