On December 22, 2016, astronaut Piers Sellers knew his time was running out.

In a hospital bed in Houston, Texas, the former Kent schoolboy's life was ebbing away.

A little over a year earlier he had been diagnosed with stage four pancreatic cancer, missed initially by medics, and told he had 12 to 18 months to live.

As he spent his final day on an Earth he had seen from space, he was joined at his bedside by one of his dearest friends.

Compton Tucker - or Tuckeroo as Piers nicknamed him - had known him since he first arrived in the US in 1982. The two had worked together for many years at Nasa's Goodard Space Flight Centre in Maryland. They were good friends and, after Piers had hung up his spacesuit, been neighbours.

"He rang me," explains the senior scientist from Nasa's Earth Science division, "and said 'I don't think I'm going to be here much longer'. So I spent 18 hours with him by his bedside in the hospital the day before he died. It gave Mandy, his ex-wife, a break.

"He went through all the things he wanted me to do after he was gone. Which of his friends would get this or that. He left one of them some money, and I said 'well what should I tell this person' and he said 'tell her she has to buy a more reliable used car this time'. Even to the end he was still very funny.

"When an astronaut passes on, they plant a tree for them in a memorial grove at the Johnson Space Centre in Houston so, in one of my last conversations with him, I said what type of tree do you want planted for you?

"He looked me in the eye and said 'I don't want a tree. I want a shrubbery, one that looks nice, and not too expensive' - from the Monty Python movie the Holy Grail.

"There are an endless cycle of stories like this. He always had a great sense of humour."

It was typical of a man who had grasped the nettle of life with a determined dedication to fulfil not only his own scientific ambitions but to highlight the existential threat of climate change.

Someone who had been inspired by watching the Apollo Moon missions while attending Cranbrook School and devoted his life to reaching for the stars.

He fought tooth and nail not only to get to join the ranks of Nasa but to join its astronaut programme - despite challenges which would deter the majority.

And when he retired after blasting into space on three separate occasions, he worked harder than ever to not only attain a senior position in Nasa but to raise awareness of the challenges facing the planet which even saw him appear in a film alongside Hollywood actor Leonardo DiCaprio.

"He was really unusual and very funny," explains Compton Tucker. "He had wit, he had intelligence, he was sincere, he treated everyone well, but he also had a high quotient for monkey business.

"He loved pulling pranks and things like that. He gave everyone nicknames but never in a demeaning way - always humorous.

"He was an extremely kind person."

Born in 1955, and growing up in Crowborough, near Tunbridge Wells, just over the East Sussex border, he was sent to board at Cranbrook as his father was in the Army.

While at the school he joined the Royal Air Force Cadets - learning how to fly gliders.

Leaving in 1973, he attended the University of Edinburgh and obtained a degree in ecological science, before gaining a doctorate in biometeorology at the University of Leeds.

Determined to work for Nasa, he left the UK in 1982 and worked for the space agency as a research meteorologist.

Within two years he started to apply to be accepted on the astronaut programme. But it was no simple route to the stars.

"He was deeply motivated by science, but he also wanted to be an astronaut," says Nasa scientist Compton Tucker.

"He became an astronaut in 1996 but had applied earlier but was rejected because of a torn retina or some retinal damage. One of the astronauts told him he would never be an astronaut.

"But Piers persisted, he went to several eye specialists and was ultimately accepted."

It was also boosted considerably by his becoming a US citizen in 1991 - a move which lifted the additional barrier of his UK passport.

In 2002 his childhood dream was finally realised. Selected to be part of a space shuttle mission to help the on-going assembly of the International Space Station, he flew into orbit in the October of that year.

While on board the 10-day mission, he took part in three space walks.

Allowed to take a dozen personal items with him into space he contacted his old Kent school asking them for something he could take on his journey.

The governors immediately thought of the school's foundation charter, signed by Elizabeth I, but the document kept in a glass case in the school library was too fragile to risk. Instead, they had a copy made, which he took with him.

His timing was fortunate. Just three months after he safely touched back on ground, the space shuttle Columbia disintegrated on its return to Earth, killing all its crew.

He knew the risk was what elevated astronauts in people's estimations.

His friend recalled: "I asked him once 'what it's like to be an astronaut' and he said with a smile 'well, astronauts are the rock stars of science because we do dangerous things and we could be blown up at any minute'."

Undeterred, he would go into space on two more occasions - the next only the second after the Columbia disaster where the mission, primarily, was to check on new safety and repair techniques in a bid to ensure no repeat of the tragedy.

At the time, he said of what he witnessed from on high: “The atmosphere that we rely on for life is a very, very thin, finite volume of gas, so it’s easily affected by things we do. The oceans, too. There’s not much of that. It’s just a thin layer of water here and there on the planet, so the bit of the biosphere that we enjoy is a very thin shell around this large planet.”

Retiring as an astronaut in 2011 - the same year in which he was made an OBE for services to science in the New Year's Honours List - he continued his fascination with the Earth's climate - rising to become director of of the Earth sciences division at Nasa's Goddard Space Flight Centre in Maryland.

"Any project he took on," says Compton Tucker, "he did the work and would work long hours where necessary, because he was in charge and he never led from the rear, he always led from the front.

"He was a good scientist and he had fun with what he did."

In 2015, however, he started feeling unwell as he approached his 60th birthday.

Explains his friend, colleague: "Because he lived next door to me, I would take him to numerous doctor appointments or pick him up, and physicians just missed the diagnosis. It was inexcusable.

"He really wasn't feeling well, but he couldn't find out what the illness was.

"He said later 'I should have had the intelligence to go for the worse first' instead of him going in and people saying perhaps it was his gall bladder or something like that.

"He really had no clue, but he wasn't feeling well for all of 2015."

But by the October of that year, the cause of his declining health was revealed. He had stage four pancreatic cancer - which meant the cancer was spreading around his body.

His doctors gave him 12-18 months to live.

He handled the diagnoses in typically stoic fashion.

"When he knew of his diagnosis, he said 'look, I don't want any sympathy in public, I don't want any prayer or any fuss made over me. I want to be treated exactly as people would treat me if I was in perfect health'." explains Compton Tucker.

"So if people got soppy, it was the role of his friends to tell people 'look, this is inappropriate, you're distressing Piers, don't do it'.

"He was trying to finish a book. He'd written a few chapters of it. But I have no idea what happened to it.

"He worked diligently in the office at least through October 2016 and into early November."

In an email to colleagues telling them of his condition, he quipped it had "drastically simplified my retirement planning" and "got me out of jury duty".

He went public with his diagnosis in January 2016 in an article he penned for the New York Times.

In it, he used the platform not to dwell on his own condition, but to continue to raise awareness of the changes happening to our world and which he had devoted himself to monitoring.

"I’ve no complaints," he wrote in the article, "I’m very grateful for the experiences I’ve had on this planet.

"As an astronaut I space-walked 220 miles above the Earth. Floating alongside the International Space Station, I watched hurricanes cartwheel across oceans, the Amazon snake its way to the sea through a brilliant green carpet of forest, and gigantic night-time thunderstorms flash and flare for hundreds of miles along the Equator. From this God’s-eye-view, I saw how fragile and infinitely precious the Earth is."

As part of trying to reach as wide an audience as possible, he appeared, in the last few months of his life, on Before The Flood, a film directed by Leonardo DiCaprio, in which he was interviewed by the star, designed to highlight the crisis unfolding before us.

"Let’s find a way out of it," the former astronaut said, "and there are ways out of it.

"If we stopped burning fossil fuel right now, the planet would still keep warming for a little while before cooling off again.

"The odds are I won’t be around for very long. That’s really motivated me to think about what’s important to do, and what can I contribute in the time I have left.

“I have faith in people. I really do have faith in people."

Added Compton Tucker: "He was a pragmatist. He was not overly optimistic (about climate change) but he felt that our species have responded to emergencies that are immediate and this is immediate. But a lot of people are too stupid to realise it, although he wouldn't say that.

"He thought what Nasa and the European Space Agency were doing with Earth observation, would convince people that something was happening and that people would understand the geophysics of what was happening - and realise the consequences are very serious.

"At the same time, he realised that everyone on the planet is in this together. He would always tell me, when he was in orbit, and looking down at the Earth, 'I didn't see any national boundaries, countries just blend into the next'.

"He thought it would be obvious something needed to be done. He didn't know if that was five or 10 years hence, but he knew it would happen sometime within the next 10 to 20 years."

But it would be too late for him to see any notable progress.

By late December 2016, his old friend was receiving that final phone call to come to his bedside to say his final farewell.

Piers Sellers died on December 23 that year. He was 61.

Concludes Compton: "Piers often said that seeing the Earth from above gave him a different perspective.

"He spoke about how, when he was on a space walk, he would lean forward so he couldn't see the edge of his space suit and that it was like being in an aquarium.

"Of how, when you're flying along at 7km a second, looking down and seeing the clouds, the oceans, you see all the cities at night, at daytime you see all the land features. He said every time that happened it was as inspiring as it was the first time."