The atmospheric scientist, who gained his PhD at Leeds University, was forced to abandon his country of birth to follow his dream of getting into orbit.
He is riding on the Atlantis space shuttle with the Stars and Stripes on his shoulder - not the Union Flag.
The UK Government believes human spaceflight is a waste of time and money, and will not support any manned programme, even through the European Space Agency of which it is a member.
Birthday surprise
The Blair administration briefly toyed with the idea of sending a Brit into orbit when it came to power in 1997. It was drawn to the potential publicity spin-offs but eventually ran shy of putting funds behind the project.
UK money is directed instead at space science and the satellite industry.
This policy, endorsed by successive governments, has left Dr Sellers, and Michael Foale before him, with little choice but to get hold of a US passport.
Sellers, who was born in Crowborough in Sussex, was inspired to become an astronaut by Yuri Gagarin. The Russian's pioneering journey into orbit occurred a day after Seller's sixth birthday.
Flashing light
"My dad told me all about it," he said in a recent Nasa interview. "And, I thought it was just a continuation of my birthday. This was, you know, somehow connected. It wasn't the case, as it turned out.
Piers J Sellers
Born 11 April, 1955, in Crowborough, Sussex
Married with two children
Educated at Cranbrook School in Kent
BSc in ecological science from the Edinburgh University
PhD in biometeorology from Leeds University
Russian speaker
"But, he explained to me how this guy had got into this rocket, was shot up, and was going around and around the world really fast. And, he held an orange up, and he had his finger going round this orange, in and out of the light, because he had a light shining on it from one side.
"So, he showed how Yuri Gagarin would be seeing the sunlight for a little while, travelling all the way over the Earth, and then flashing back into the dark and coming back - night, day, night, day.
"I thought this was fascinating to me. I couldn't imagine anything better."
Wonderful opportunity
Shortly after finishing his doctorate research in biometeorology Sellers went to the US space agency's (Nasa) Goddard Space Flight Center in Maryland to work on climate computer models.
That was 1982 and within two years he was filling out forms to become an astronaut. But he was unlikely to ever get a place among the elite so long as he was a British citizen.
That was rectified in 1991 when he became naturalised; acceptance on the astronaut training programme followed five years later.
Now he has his chance - and a wonderful opportunity to make his mark.
'Hard-working Brit'
Dr Sellers is due to attempt three spacewalks during the STS-112 mission, and hopes to beat the record for a single spacewalk of eight hours and 56 minutes set by Americans James Voss and Susan Helms in 2000.
Piers Sellers' wife Mandy said that her husband had reached this point through "sheer determination and hard work".
She said: "When I met him as a student he had the model of Apollo on the wall, and I used to think: 'Yeah, that won't ever happen' - and I think he thought the same.
"Piers is not interested in becoming famous; he is just a hard-working Brit who had to become an American to go into space."