In an era of orbital satellites so advanced that they are able to make out objects half the size of cars from space, a spy balloon might seem like a bit of a relic.

They were a prominent tool for reconnaissance during the Cold War and were even used in a more basic form for intelligence gathering in the Napoleonic Wars more than 200 years ago.

But security experts say the balloons are just the “tip of a revolution” in the development and use of new high-altitude surveillance craft, with the UK even investing millions in a project to develop spy balloons last year.

On Saturday, the US shot down a suspected Chinese spy balloon that had been flying over its airspace.

A senior defence official had previously said the US has “very high confidence” it was a Chinese high-altitude balloon and was flying over sensitive sites to collect information, while China has not immediately denied the balloon belonged to them.

The Pentagon acknowledged reports of a second balloon that was spotted flying over Latin America, saying: “We now assess it is another Chinese surveillance balloon”.

The sightings prompted US Secretary of State Antony Blinken to postpone a high-profile visit to China which had been due to begin on Sunday, while the Pentagon accused Beijing of spying on sensitive military sites.

Beijing admitted that the initial balloon had come from China, but insisted it was a “civilian airship” that had strayed into American airspace and that it was for meteorological and other scientific research.

What are spy balloons?

The devices are lightweight balloons, filled with gas, usually helium, and attached to a piece of spying equipment such as a long-range camera.

They can be launched from the ground and are sent up into the air where they can reach heights of between 60,000ft (18,000m) and 150,000ft (45,000m), above the flight paths of commercial aircraft in an area known as “near space”.

Once in the air, they travel using a mixture of air currents and pressurised air pockets, which can act as a form of steering.

Why are they still useful in the satellite era?

According to defence and security analyst Professor Michael Clarke, the biggest advantage of spy balloons over satellites are that they can study an area over a longer period of time.

“The advantage is they can stay in one place for a long time,” he told Sky News.

“Because of the way the Earth rotates, unless a satellite is over the Equator, you need three to five satellites going all the time to track the same spot.

“These balloons are also relatively cheap, and much easier to launch than a satellite.”

Will balloons continue to be used in future for spying?

Very much so, according to Professor Clarke.

Despite the wide use of satellite technology, countries including the UK are also focusing on the development and use of spycraft to operate in the upper atmosphere.

In August, it was announced the Ministry of Defence had agreed a £100m deal with US defence company Sierra Nevada to provide high-altitude unmanned balloons to be used for surveillance and reconnaissance.

Professor Clarke said: “(These balloons) are the very tip of the revolution for passive upper atmosphere aircraft.”

He said other defence firms, such as BAE, were working on ultralight solar-powered drones which are able to operate in the upper atmosphere and stay in place for up to 20 months.

Why have China used them now?

According to Professor Clarke, the use of these balloons, if indeed they were launched by China, will likely have been a message to the US following its decision to open new military bases in the Philippines.

“I think it’s a challenge,” he said.

“They (China) are signalling that if the US is going to come closer to them then they will be more aggressive with their surveillance.

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“It is also caused a political issue in the US now, because it will be seen as a sign of weakness not to shoot it down.

“This causes some embarrassment, but the US doesn’t need to respond.”

The balloon was spotted over Billings, Montana, on Wednesday – close to one of the US’s three nuclear missile silo fields at Malmstrom Air Force Base.

Military and defence leaders initially decided against shooting down the balloon due to the safety risk from falling debris.

Professor Clarke added: “I think the debris issue is a bit of an excuse. It was over one of the least densely populated areas of the US and if they needed to they could have asked everyone to stay inside.

“I don’t think they wanted to make it a bigger issue, because China are daring them to shoot it down and make it an international issue.”