The Pentagon has been tracking a suspected Chinese surveillance balloon which flew over sensitive areas in Montana in recent days, apparently the latest of several such incursions. President Biden reportedly proposed that the intruder be shot down, but the Pentagon opposed this action, citing the risk of civilian casualties.

In fact, bringing down this type of balloon may be extremely difficult, as it is likely to be highly resistant to the available weapons. It may look fragile, but the sheer size and construction of a stratospheric balloon makes it all but invulnerable.

There is also the problem of getting to it. Such balloons ride high in the stratosphere, far above the airliners. They do this to take advantage of the change in wind direction with altitude to steer themselves to where they are needed like sailing ships, then circling above a point of interest. The U.S. military has its own stratospheric spy balloon program with project like COLD STAR – short for COvert Long Dwell STratospheric ARchitecture, designed to lurk undetected in enemy airspace.

Such balloons typically fly at 80,000 feet or more – NASA’s version cruises at 120,000 feet.. The U.S. Air Force’s F-15 Eagle and F-22 Raptor both have a stated operating altitude of around 65,000 feet. While they might be able to get close enough to fire a missile, the balloon may be too high for them to shoot.

When people think of military balloons they probably think of the WW1 German Zeppelin raids, and the balloon-busting biplane pilots who brought them crashing down in flames. The giant German airships were filled with highly flammable hydrogen gas, and could be ignited with a few bursts of incendiary bullets, creating an the same effect as the Hindenburg Disaster. However, in this case the balloon is filled with non-flammable helium rather than hydrogen.

You might still think that simply puncturing the balloon envelope would be enough. It might not pop like a toy balloon, but letting the gas out should be enough to bring the balloon down.

The problem though is one of scale. Stratospheric balloons are colossal. NASA’s standard balloons are 40 million cubic feet, a volume equivalent to more than 195 Goodyear GT blimps: you could fit en entire football stadium inside one. The balloon envelope is made of plastic material no thicker than sandwich wrap, and the pressure difference between the inside and outside is small. Attempting to let the air out by punching a few holes is like expecting to ventilate an entire warehouse with fresh air by opening one small window.

We know that large balloons are hard to shoot down from previous experience. In 1998 a rogue Canadian weather balloon drifted towards Russian airspace. Fighter jets from Canada, Norway and Sweden attempted to bring it down without success. Two Canadian air force CF-18 fighters hit the balloon with more than 1,000 rounds of 20mm cannon fire off the coast of Newfoundland, riddling it with holes. This was not enough to let a significant amount of gas out, and the balloon continued drifting.

A volley of 2.75” rockets was equally ineffective, as the high-explosive rockets simply flew though the balloon without detonating. This may be the Air Force’s real concern with intercepting the Chinese balloon: any missile fired at it may be a much greater hazard to civilians below than the balloon itself, which is likely to descend slowly if at all. (The Canadian balloon drifted into Russian territory and is believed to have come down in the Arctic Sea).

Interceptor aircraft may in principle be able to get a missile lock and hit the balloon’s small gondola, the suspended capsule containing its cameras, control systems and radio communications. Destroying this would put the balloon out of action and prevent it from carrying out espionage. However the balloon would continue to drift over U.S. territory and claiming a ‘kill’ would be difficult. Worse, a failed attempt to bring it down would be a public relations disaster.

This is not the first time balloons have been used for spying. Back in the 1950s, before they had satellites, the CIA’s Genetrix program sent ‘weather balloons’ to drift at random over the Soviet Union taking pictures. The project was not a success, but caused considerable alarm in Russia, which even developed a ‘balloon destroyer’ version of its M-17 Mystic high-altitude aircraft.

The modern balloon spies are far more effective. New control algorithms and an understanding of stratospheric winds means they are steerable and can go anywhere at will, with solar panels providing indefinite flight duration. Unlike satellites, they can remain over a site of interest for a prolonged period — but being inside foreign airspace rather than in the international territory of space means they can be downed.

Dealing with such spies is a new challenge for the 21st century. The fact that the U.S. has not attempted to bring down previous intruders suggests it may be a tough one. A new and surprising chapter in the history of aerial warfare is about to begin.

UPDATE 9:30am 3rd Feb: China is now claiming that the balloon is "a civilian airship used for research, mainly meteorological, purposes" and that it has "eviated far from its planned course." Which is exactly the same excuse that the U.S. gave for the 'weather balloons' in its spying program in the 1950s.