How much of a threat are Chinese spy balloons?

A surveillance balloon drifting on the high-altitude breeze is not cutting-edge spyware. But the Chinese device discovered earlier this week over the north-western US state of Montana has proven to be effective as a diplomatic irritant between the world’s largest economies.

Secretary of state Antony Blinken cancelled a trip to China in response to the incursion, even as Beijing claimed it was a “civilian airship used for research” that had been blown off course.

The Pentagon announced earlier this week it was tracking the balloon over Montana, where the US has nuclear intercontinental ballistic missile silos. At the time, it added that President Joe Biden had decided against shooting it down because of the risk to civilians. On Saturday, the US shot the balloon down off the east coast.

Earlier, the Pentagon said its decision not to shoot the balloon down reflected the fact it provided China with little information that it could not obtain from satellites. Canada said it was tracking the balloon in conjunction with the US, but was also monitoring a “second potential incident”.

What is a spy balloon?

First deployed during the French revolutionary wars, balloons operated by spies with binoculars were used during the American civil war to detect enemy troop movements.

They were used extensively during the cold war, as the US and Soviet Union sought ways to monitor each other’s military.

Lacking propulsion, they are subject to the vagaries of wind currents. But modern versions of the low-cost devices can house cameras, radars and radio devices.

Why would one be used?

The vast improvement in satellite technology and proliferation of spy satellites in recent decades, plus the increased use of unmanned drones, has rendered surveillance balloons technologically largely obsolete. But they are still being used, particularly for non-military observation.

The Pentagon said the balloon did not provide China with capabilities that went beyond its spy satellites, but…