LONDON (AFP) ― When Stephen Hawking was diagnosed with motor neurone disease aged just 21, he was given only a few years to live. But the British scientist marks his 70th birthday on Sunday, as questioning as ever.

Despite spending most of his life crippled in a wheelchair and able to speak only through a computer, the theoretical physicist’s quest for the secrets of the universe has made him arguably the most famous scientist in the world.

“I’m sure my disability has a bearing on why I’m well known,” Hawking once said. “People are fascinated by the contrast between my very limited physical powers, and the vast nature of the universe I deal with.”

Much of his work has centered on bringing together relativity (the nature of space and time) and quantum theory (how the smallest particles in the universe behave) to explain the creation of the universe and how it is governed.

In 1974, aged just 32, he became one of the youngest fellows of Britain’s prestigious Royal Society. Five years later he became Lucasian Professor of Mathematics at Cambridge University, a post once held by Isaac Newton.

His fame moved beyond academia in 1988 with the publication of his book “A Brief History of Time,” which explained the nature of the universe to non-scientists, and sold millions of copies worldwide.

Hawking’s stardom was later cemented in cameos in “Star Trek” and “The Simpsons,” where he tells the rotund Homer Simpson that he likes his theory of a “doughnut-shaped universe,” and may have to steal it.

Martin Rees, Britain’s Astronomer Royal and a former president of the Royal Society, first met Hawking when they were both research students “and it was thought he might not live long enough to finish his Ph.D. degree.”

Rees said his survival made him a “medical marvel.” but stressed that it was his work that would prove his lasting legacy.

“His fame should not overshadow his scientific contributions because even though most scientists are not as famous as he is, he has undoubtedly done more than anyone else since Einstein to improve our knowledge of gravity,” he said.

Hawking was just 21 when he was diagnosed with amyotrophic lateral sclerosis, a former of motor neurone disease that attacks the nerves controlling voluntary movement.

Brian Dickie, research director of the MND Association, says most sufferers live for less than five years ― “the fact that Stephen Hawking has lived with the disease for close to 50 years makes him exceptional.”

Hawking admitted he felt “somewhat of a tragic character” after diagnosis, but he soon returned to work, securing a fellowship at Cambridge, and married Jane Wilde, with whom he had three children.

Professor Kip Thorne, the acclaimed U.S. theoretical physicist who will speak at a special symposium at Cambridge for Hawking’s birthday on Sunday, said his illness had in fact been instrumental to his work.

“When Stephen lost the use of his hands and could no longer manipulate equations on paper, he compensated by training himself to manipulate complex shapes and topologies in his mind at great speed,” he said.

“That ability has enabled him to see the solutions to deep physics problems that nobody else could solve, and that he probably would not have been able to solve, himself, without his newfound skill.”

In addition to Sunday’s symposium on ‘the state of the universe,’ Hawking’s birthday will be marked by a new exhibition celebrating his achievements which opens at London’s Science Museum on Jan. 20.

Hawking retired as Lucasian Professor of Maths when he reached 67, but his fascination with the world remains.

He is watching the progress of the Large Hadron Collider closely, having bet $100 in 2009 that it will not find an elusive particle seen as the holy grail of cosmic science.

He has also long had the ambition of going into space, which he has argued is mankind’s only hope as it uses up the Earth’s resources. However, he has cautioned that any contact with aliens would likely end badly for humans.

Other mysteries closer to home puzzle him, too.

In an interview with the New Scientist magazine this week, Hawking ― who divorced his second wife in 2006 ― was asked what he thought about most during the day. He replied: “Women. They are a complete mystery.”