Hawking, the author of the international bestseller "A Brief History of Time," was diagnosed with motor neuron disease in 1963 and told he had barely two years to live. He has since been hailed as one of the most brilliant theoretical physicists since Einstein.

In the speech played out at a symposium in his honor at Cambridge University, he said his excitement and enthusiasm for his subject drove him on, and urged others to seek out the same inspiration.

"Remember to look up at the stars, not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist. Be curious," Hawking said in the speech he had been due to give in person.

Hawking's plans to speak Sunday at Cambridge, where as a PhD student he first became fascinated with cosmology and the state of the universe, were scrapped after his doctor advised him he was too ill to attend the event, officials said.

Hawking had recently been in hospital and was discharged on January 6, Cambridge's Vice-Chancellor Leszek Borysiewicz said.

"Unfortunately ... his recovery has not been fast enough for him to be with us today," Borysiewicz told a disappointed audience of scientists, students and celebrities at the event.

Almost completely paralyzed by a form of motor neuron disease called amyotrophic lateral sclerosis (ALS), which attacks the nerves that control muscles and gradually stops them

functioning, Hawking is wheelchair-bound and uses a computerized voice synthesizer to speak.

DEPRESSED

When as a bright and enthusiastic 21-year-old he was diagnosed with the disease, doctors told him he would probably not make it beyond the age of 23.

"At first I became depressed," Hawking said. "There didn't seem to be any point working on my PhD because I didn't know if I would live long enough to finish it."

Yet in the almost half a century since, Hawking has broken new frontiers research into theories of time, space, relativity and black holes. He is often hailed as a modern-day Einstein and his work has shed light on the origin of the cosmos, the nature of time, and the ultimate fate of the universe.

Currently the director of research at the Department of Applied Mathematics and Theoretical Physics at Cambridge, Hawking also founded the university's Center for Theoretical Cosmology and only recently retired from a post known as the Lucasian Professorship of Mathematics, a title once held by Isaac Newton.

Looking back on his life and work in the speech entitled "A Brief History of Mine," Hawking said it had been a "glorious time" to be alive and be researching theoretical physics.

"Our picture of the universe has changed a great deal in the last 40 years and I'm happy if I have made a small contribution," he said.

'DON'T GIVE UP'

The 70-year-old urged fellow researchers and cosmology enthusiasts to encourage public interest in space and to keep going there to witness what he described as the "uninterrupted views of our vast and beautiful universe."

"We must also continue to go into space for the future of humanity," he said. "I don't think we will survive another thousand years without escaping beyond our fragile planet."

Hailing Hawking's achievements, Cambridge University's Borysiewicz said the physicist had "changed our perception of the universe in so many ways."

Britain's Astronomer Royal Martin Rees, who also spoke at the symposium, said Hawking had defied all medical and scientific odds.

"It's wonderful that we are celebrating Stephen's 70th birthday. It's a chance to thank him for the many insights he's given us about the universe, and ... for the inspiration he's offered to millions by achieving so much against all the odds," he said.

Despite having a mind that appears to work on a far higher level than most other human beings', Hawking has always made an effort to bring science to the masses.

He has featured on the hit U.S. cartoon show The Simpsons several times, and in Star Trek as a hologram of himself. His voice, famous across the world, also featured in Pink Floyd's 1994 album Division Bell.

Hawking's health has deteriorated over the years and he now uses twitches in the muscles in his cheek to choose letters or words on his voice computer to allow him to communicate. This means his speech has slowed dramatically, to a current rate of around one word per minute.

Hawking still appeared to be undaunted by his disability.

"However difficult life may seem, there is always something you can do and succeed at," he said. "It matters that you don't just give up."

(Editing by Alessandra Rizzo)

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