Professor Stephen Hawking has been forced to miss a symposium to mark his 70th birthday because of ill-health.
The world's most famous scientist was given only a few years to live after being diagnosed with motor neurone disease in 1963 at the age of 22.
The University of Cambridge, where the event was being held, says he followed the symposium on cosmology via webcast after being discharged from hospital on Friday.
The BBC reports a recording of Professor Hawking speaking was played to those attending the meeting on Sunday. Through his voice synthesizer, he told the symposium people must keep on exploring space.
"Most recent advances in cosmology have been achieved from space, but we must also continue to go into space for the future of humanity. I don't think we will survive another thousand years without escaping beyond our fragile planet."
Professor Hawking recently experienced first-hand what space travel feels like by taking a zero-gravity flight in a specially modified plane.
The BBC reports only 5% of people with the form of MND that he has - a condition called amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease - survive for more than a decade after diagnosis. Most die within a few years of diagnosis.
The former Lucasian professor of mathematics and theoretical physics at Cambridge, a post he held for 30 years, is widely regarded as one of the most brilliant theoretical physicists since Einstein.
Professor Hawking has 12 honorary degrees, a CBE, and was awarded the US Presidential Medal of Freedom in 2009.
''I'm sure my disability has a bearing on why I'm well known,'' he said.
''People are fascinated by the contrast between my very limited physical powers, and the vast nature of the universe I deal with.
''I'm the archetype of a disabled genius, or should I say a physically challenged genius, to be politically correct. At least I'm obviously physically challenged. Whether I'm a genius is more open to doubt.''
Many experts say Stephen Hawking has defied medical science by reaching the age of 70 with such a debilitating condition.
