When she dreams of space – as she still does from time to time – Helen Sharman sees how blue the Earth looks from 200 miles up.

“You can’t imagine how deep the colour is,” she says. “And the detail: you can see continents, but also the wake of a ship. And, at night, the lights of cities shine up to you. There was a window where I slept, and waking up to the world right outside ... wonderful.”

Next month, it will be 25 years since Sharman became the first Briton to journey into space. Her eight-day mission, in which she joined a Soviet Union crew aboard the Russian station Mir, transformed the then 27-year-old into a national hero. Successive prime ministers welcomed her to Downing Street, while streets and school buildings were named after her. In her home city of Sheffield, a star was put in the pavement. She spent the 90s travelling the country talking about her experiences.

And then she disappeared.

For 15 years, she led an intensely private life. For 10 of those years, she didn’t work. There was a rumour that she had grown to despise being asked how to go to the toilet in zero gravity. No one could find out if it was true, because all interview requests were refused.

Helen Sharman. Photograph: Glenn Copus/Evening Standard/Eyevine

Today, at 52, Sharman is operations manager of Imperial College London’s chemistry department. We meet in her office. Her self-imposed anonymity has gradually come to an end now the intensity of her fame has receded, yet most students here are unaware that among the staff is one of only 59 women to go to space. “I’m treated with the same lack of respect as everyone else,” she smiles.

Why did she withdraw from the limelight? “I wanted my privacy back,” she says. “I’m a scientist, but I found myself in interviews being asked where I bought my clothes. Irrelevant. And I always felt I had to be photo-ready. Fame was the downside of space.”

Nonetheless, she was surprised in 2013 to find the UK Space Agency apparently writing her out of history. In statements, it described Major Tim Peake – who travelled to the International Space Station last December – as the UK’s first official astronaut. “I asked them: ‘What happened to me?’” she says. “I asked what ‘official’ even meant, and reminded them my mission was part of the Soviet Union space programme. The British government didn’t fund it but it was still official.”

Was she angry? There’s a long pause.

“These days, to get headlines, you have to be first or best or some superlative, and I suspect someone thought that title would get Tim more attention,” she replies. “And, actually, he deserved that because what he’s doing is incredible.” Before Peake blasted off, she met him. She gave him a book – Road to the Stars by Yuri Gagarin – and one piece of advice: look out the window as much as you can.

Sharman’s own journey into space started with a radio advert. After completing a PhD at Birkbeck University in London, she was working as a research chemist when, in 1989, she heard a commercial for Project Juno. The programme – partially designed to boost London-Moscow relations – would send a Briton to Mir. The strapline was simple: “Astronaut wanted, no experience necessary.”

“It wasn’t so much going to space as the training that appealed,” she says. “Living in Russia, learning the language, doing advanced mechanics. It was a way out [of] the rat race.” Some 13,000 people applied. Physical exams, psychological tests, aptitude analysis and a standard job application (“inevitably, the hardest bit”) sifted them down. Two were eventually picked to train at Moscow’s Star City. Then, after 18 months there, Dr Sharman was picked for the mission.

“I can only surmise why me,” she shrugs. “I was physically fit, good in a team and not too excitable, which was important. You can’t have people losing it in space. I think it was just my normality.”

Photograph: Alamy

This was a more rudimentary space age. The crew travelled in a Soyuz craft designed in the 60s. Once there, contact with Earth was limited to a few hours a day. Most disconcertingly, perhaps, the morning alarm was the same as the emergency siren.

“You’d wake up unsure if it was time to get up or if you were leaking oxygen,” says Sharman. “It got us out of our sleeping bags pretty quick.”

Power failures were common. The station’s solar panels were damaged. As a result, its batteries would overload, plunging the craft into darkness. “You couldn’t see your hand in front of your face,” she recalls. “We would sit there, just waiting.”

Her time was mainly spent running medical and agricultural experiments. When her eight days were up, she wished she could stay longer.

She wasn’t the only astronaut to feel that way. Many suffered with the mundanity of life back on Earth. Neil Armstrong became a recluse; Buzz Aldrin turned to drink. Apollo 12 astronaut Alan Bean quit science to spend his days painting the stars.

Was her own withdrawal inspired by a similar sense of anticlimax? “I never felt empty afterwards,” she shakes her head. “There’s this suggestion nothing can match going to space – and I’m not saying it isn’t fantastic – but there are equally enjoyable things on Earth. For me, standing on a hill in the Lake District and looking out at fields is just as special.”

All the same, she would like more Brits to follow in her footsteps. Of late, she has started speaking on the need for increased UK space funding. “It’s about understanding the universe and how, one day, humans can survive off this planet,” she says. “Britain should be helping with that research. It’s expensive, but it’s important. And it’s good value. Space missions capture the imagination. It turns people on to science.”

Sharman lights the Olympic flame at the Sheffield World Student Games in 1991. Photograph: Eye Ubiquitous/Rex/Shutterstock

She is fully in support of manned missions to Mars. And she herself would love to be on the maiden voyage.

“It won’t happen now, but if I was younger I’d do it in an instant,” she says. “I’d still love to go back up. It’s a wonderful thing.”