As a spy balloon drifts across the US, what are they and what can they do?

The balloon floating over Montana. Credit: AP

A suspected Chinese spy balloon slowly crossing the United States has sparked a flurry of military, diplomatic and political activity in Washington - with many simply asking why not use a satellite?

The balloon was spotted on Thursday and filmed by a Montana resident in a clip that soon went viral.

The Pentagon quickly claimed it was a Chinese spycraft and the news has put a lot of hot air into the lungs of Republicans claiming President Joe Biden was weak on national defence.

What's happening with the balloon in the US?

Over Thursday evening and Friday morning, reports of a balloon entering Canadian and then American airspace began circulating.

The US military quickly said it had "very high confidence" the it was a Chinese spy balloon.

Watch the footage of the balloon captured drifting over Montana

China hasn't denied the balloon belongs to them, which to some commentators amounts to the tacit acknowledgement that it is theirs.

The Pentagon said the current flight path would carry the balloon over a number of sensitive sites, but did not give details.

Malmstrom Air Force Base in Montana is home to some of the US's nuclear missiles.The Pentagon said the spy balloon would provide "limited" value in terms of gathering intelligence that could not be obtained by satellites.

President Joe Biden was briefed on the balloon and he ultimately decided not to shoot it down due to the risk the debris would pose to people on the ground.

The balloon has raised questions over whether US Secretary of State Antony Blinken's imminent trip to China, which is aimed at cooling relations between the two superpowers, will go ahead.

Spy balloons have flown over the United States several times in recent years, but this balloon appeared to be lingering longer than in previous instances.

How does a spy balloon operate?

Such balloons typically operate at 80,000-120,000 feet (24,000-37,000m), well above where commercial air traffic flies.

The highest-performing fighter aircraft typically do not operate above 65,000 feet, although spy planes such as the U-2 have a service ceiling of 80,000 feet or more.

The balloon has overshadowed Antony Blinken's visit to China. Credit: AP

This means destroying the balloon is not too difficult, but actually intercepting it to possibly bring it back down to the ground and study it is surprisingly hard.

Unlike a drone, spy balloons aren't usually controlled with pinpoint precision.

Usually, they are given signals to nudge it in a particular direction to make use of different wind currents.

The US government said the balloon posed no threat to any aircraft in the region.

They can gather accurate information on most sites from the sky, although this can be done with far more stealth by a satellite.

Spy balloons can stay stationary or move very slowly over an area giving the operators longer to observe a particular target than a satellite which flies over targets at incredibly high speeds.

Why not use a satellite?

Craig Singleton, a China expert of the Foundation for Defense of Democracies, said that such balloons had been widely used by the United States and the Soviet Union during the Cold War and are a low-cost intelligence-gathering method.

Since the explosion of satellites in the past few decades the use of spy balloons has fallen out of favour.

But in recent years new technologies, particularly advancements in lasers, have put satellites under threat.

This is problematic as satellites can cost hundreds of millions to get into space, and if they can be disabled with relative ease they can quickly become unaffordable.

Balloons were used - and shot down - extensively during the Second World War. Credit: AP

Balloons on the other hand are extremely cheap to launch and although they can be destroyed relatively easy, as can be seen with the balloon this week, it doesn't come without problems.

If they aren't destroyed they are also far easier to return home and repurpose than a satellite.

How long have they been in use?

Modern hot air balloons were invented by the French in the 1780s and nations quickly began wondering if they could have a military use.

They were used in the French revolutionary wars and the American Civil War to great effect.

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Pilots would try and spy information on enemy troop movements using their vantage points and binoculars.

They were deployed extensively in the First World War to help provide accurate information on the enemies' position for artillery strikes.

But as technology developed and guns became more accurate with longer-range observation balloons became easy targets.

Although, sometimes extensive amounts of ammo could be used without hitting the target.

The US Naval Institute on Friday shared a story on Twitter of the USS New York in 1945.

They spotted what they thought was a Japanese spy balloon and the captain ordered it shot down.

After it was fired on several times but no one could hit their mark navigators eventually realised they were shooting at Venus.

Balloons were sent higher in the air during the Cold War to get out of range of land-based guns and they moved into a spying role rather than detailed ground information for combat purposes.

But eventually developments in reconnaissance aircraft, drones and satellites rendered them mostly obsolete.