I’m well placed to write the character of the daughter of a scientist,” says Lucy Hawking. Lucy may not dress up in tutus or spacesuits, as her heroine Annie does in her new science fiction book for children, George’s Secret Key to the Universe, out this week. But she shares Annie’s taste for the theatrical and she knows what it’s like having a scientist for a father. Stephen Hawking, co-author of the new book, is probably the most famous scientist in the world.

Hawking is Lucasian Professor of Mathematics and Theoretical Physics at the University of Cambridge, and his work on black holes regions in the universe where gravity is so strong that no light can escape has led to comparisons with Einstein. His book, A Brief History of Time, has sold 12 million copies worldwide.

The new book by father and daughter unites her love of storytelling with his passion for communicating scientific ideas in simple terms. It is a passion that has survived the cruel blow fate has dealt him, imprisoning his brilliant mind in an increasingly frail body. Now 65, he was diagnosed with motor neurone disease more than 40 years ago and given only one or two years to live. Lucy, born in 1970, can only remember him in a wheelchair. He communicates through a speaking machine; clenching a muscle in his cheek to select words from a screen.

Lucy has a personal reason for wanting to write about science in a way that a child can understand: she wants to explain to William, her nine-year-old son, what his grandfather has been doing. William is autistic and cannot yet read, but Lucy hopes he will soon be able to enjoy the book, either by reading it himself or by having it read to him.

Although Lucy could have become a scientist she won top grades in maths and three sciences in O-levels at the Perse School in Cambridge she opted to follow her mother into modern languages, specialising in Russian, before becoming a journalist and writer.

Did her father try to persuade her into science? “After my O-level results, he did ask if I wanted to become a scientist,” she says. “When I said that was not the direction I wanted to take, he said: ‘Fine, it’s your life’. The only pressure on me was to be good at whatever I did.”

George’s Secret Key is aimed at anyone over the age of eight. It’s the story of a boy who follows his pet pig through a hole in the fence to the house next door. There he meets Annie, Eric, her scientist father, and Cosmos, his super intelligent computer. Cosmos can take the initiated through a portal to the furthest reaches of the universe. And yes, a black hole plays a part.

The chapter on black holes is cunningly disguised as notes written by Eric to explain his discoveries to the children. Every word was written by Stephen Hawking. Yet the book should not be seen as a simplified version of A Brief History of Time, says Lucy. “This is a story,” she stresses.

It’s a rollicking yarn, illustrated in a style that recalls Antoine de Saint Exupery’s Little Prince. George’s parents lentil-eating, Guardian-reading, concerned about the planet yet opposed to the technology that might save it make a fine contrast with kind, scatty Eric and Susan, his practical wife. There is also a satisfyingly creepy villain, a jealous science teacher called G Reeper.

Is Eric really Stephen Hawking? Eric has a talent for explaining, enthusiasm, wit and affability, says Lucy. But there are also elements of her father’s character curiosity and determination in George.

Although the story contains a lot of science unobtrusively woven in, it is also interrupted by fact boxes as well as photographs and models of the solar system. Incorporating all the science in the narrative would have made it unmanageable, say the authors.

“All the people I know who’ve read it have read it in different ways,” says Christophe Galfard, a former PhD student of Stephen Hawking’s who has acted as scientific consultant on the book. “Some skip the science bits and go back to them at the end. Some start with the science and then read the story. Some read both as they go along.

“If a parent is reading this to a younger child and the child asks a question about the science, the parent will find the answer in the box,” he says.

Christophe, who has turned from research to writing and talking about science, shares his mentor’s views about making science fun. “I don’t think all scientific knowledge should be spread through school,” he says. “It should be accessible and entertaining.”

So: a gripping novel, parents’ helper, teaching aid and reference book, all in one. The book’s reception at the recent Edinburgh Books Festival suggest the formula is a winner. Deals are already signed to translate the book into 30 languages. And there are two books in the series still to come.

Lucy has already published two books for adults, but writing this, her first book for children, has been exhausting and has occupied her for 18 months.

How much of the science did she know before she started the book? “Not much,” she says.

“When I set out, I felt ‘gosh, I wish I’d listened when I was young’. It was mind-expanding.”

She has started on the sequel and reckons she has another two years to go before completing the trilogy. And then? A return to adult fiction?

“Take a flight into space go and colonise another planet,” suggests Lucy.

For there is one thing on which father, daughter and disciple agree: finding another planet to live on is the only way for humanity to survive

George’s Secret Key to the Universe by Lucy and Stephen Hawking is published by Doubleday, pound;12.99