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.
It comes as the US military on Friday said it was tracking a suspected Chinese spy balloon, described as being the size of three buses, that has been flying over northwestern America in recent days.
Image: A high-altitude balloon floats over Billings in Montana
A senior defence official said the US has “very high confidence” it is 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.
Beijing admitted that the 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.
US Secretary of State Antony Blinken is postponing a high-profile visit to China which had been due to begin on Sunday.
What are spy balloons?
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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.
Image: Professor Michael Clarke
“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.
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0:19
Mao Ning, a Chinese foreign ministry spokesperson, says that those involved should be ‘cool-headed’
Military and defence leaders said they considered shooting the balloon out of the sky but decided against it 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.”
Mr Biden was diagnosed on Friday, with tests revealing the cancer had spread to his bones.
The former president posted an image of himself and his wife Jill on X on Monday and wrote: “Cancer touches us all. Like so many of you, Jill and I have learned that we are strongest in the broken places. Thank you for lifting us up with love and support.”
Image: The King and Joe Biden at Windsor Castle in July 2023. Pic: Reuters
The King’s letter comes after British Prime Minister Sir Keir Starmer said he was “very sorry to hear President Biden has prostate cancer”.
“All the very best to Joe, his wife Jill and their family, and wishing the President swift and successful treatment,” he added.
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Meanwhile US President Donald Trump said he was “saddened” by the news, adding: “We extend our warmest and best wishes to Jill and the family, and we wish Joe a fast and successful recovery.”
Former US president Barack Obama said: “Michelle and I are thinking of the entire Biden family.
“Nobody has done more to find breakthrough treatments for cancer in all its forms than Joe, and I am certain he will fight this challenge with his trademark resolve and grace. We pray for a fast and full recovery.”
Image: Barack Obama (right) with Joe Biden at a campaign event in 2022. File pic: Reuters
After a poor debate performance against Mr Trump and amid escalating concerns around his age and fitness to serve, Mr Biden withdrew from the 2024 presidential election race and endorsed his vice president Kamala Harris.
Ms Harris wrote on X after his diagnosis: “We are keeping him, Dr. Biden, and their entire family in our hearts and prayers during this time.
“Joe is a fighter – and I know he will face this challenge with the same strength, resilience, and optimism that have always defined his life and leadership. We are hopeful for a full and speedy recovery.”
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1:16
Mr Biden’s diagnosis: What we know
Former US president Bill Clinton wrote on social media: “My friend Joe Biden’s always been a fighter. Hillary and I are rooting for him and are keeping him, Jill, and the entire family in our thoughts.”
Hillary Clinton, who unsuccessfully ran for president in 2016, said she was “thinking of the Bidens as they take on cancer, a disease they’ve done so much to try to spare other families from”.
Speaker of the US House of Representatives Mike Johnson said it was “sad news” and his family “will be joining the countless others who are praying” for Mr Biden.
Former House speaker Nancy Pelosi described Mr Biden as a “great American patriot” and said she was “praying for him to have strength and a swift recovery”.
Mr Trump’s secretary of state, Marco Rubio, wrote on social media that he and his wife were “united in prayer for the Biden Family during this difficult time”.
US President Donald Trump said he was “saddened” by the news, adding: “We extend our warmest and best wishes to Jill and the family, and we wish Joe a fast and successful recovery.”
Former US president Barack Obama said: “Michelle and I are thinking of the entire Biden family.
“Nobody has done more to find breakthrough treatments for cancer in all its forms than Joe, and I am certain he will fight this challenge with his trademark resolve and grace. We pray for a fast and full recovery.”
Image: Barack Obama (right) with Joe Biden at a campaign event in 2022. File pic: Reuters
British Prime Minister Sir Keir Starmer said he was “very sorry to hear President Biden has prostate cancer”.
“All the very best to Joe, his wife Jill and their family, and wishing the President swift and successful treatment,” he added.
After a poor debate performance against Mr Trump and amid escalating concerns for his health, Mr Biden withdrew from the 2024 election and endorsed his vice president Kamala Harris.
Ms Harris wrote on X: “We are keeping him, Dr. Biden, and their entire family in our hearts and prayers during this time.
“Joe is a fighter – and I know he will face this challenge with the same strength, resilience, and optimism that have always defined his life and leadership. We are hopeful for a full and speedy recovery.”
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1:16
Mr Biden’s diagnosis: What we know
Former US president Bill Clinton wrote on social media: “My friend Joe Biden’s always been a fighter. Hillary and I are rooting for him and are keeping him, Jill, and the entire family in our thoughts.”
Hillary Clinton, who unsuccessfully ran for president in 2016, said she was “thinking of the Bidens as they take on cancer, a disease they’ve done so much to try to spare other families from”.
Speaker of the US House Of Representatives Mike Johnson said it was “sad news” and his family “will be joining the countless others who are praying” for Mr Biden.
Former House speaker Nancy Pelosi described Mr Biden as a “great American patriot” and said she was “praying for him to have strength and a swift recovery”.
Mr Trump’s secretary of state, Marco Rubio, wrote on social media he and his wife were “united in prayer for the Biden Family during this difficult time”.
Following President Trump’s Middle East trip – which the White House is touting as an unbridled success – Sky News’ Martha Kelner sits down with Barbara Leaf, who was US ambassador to the United Arab Emirates during Trump’s first term and assistant secretary of state for Near Eastern affairs in the Biden administration.
She was also in the team that formed the first formal US presence in Syria after more than a decade.