Professor Stephen Hawking told listeners of BBC Today programme that he thought humans would almost definitely colonise Mars, but he warned against encouraging alien encounters.

As Stephen Hawking’s 70th birthday approaches, the scientist who was told he wouldn’t live past 40 answered some of the public’s questions focusing on humanity's destiny to inhabit outer space.

A bit of a futurist, Hawking's latest book, George's Secret Key To The Universe is an explanation of the world aimed at children, a lesson for the next generation.

It’s not the first time Hawking has advised people to “reach out to the stars.” The physicist has long been outspoken on the subject of humanity’s intergalactic future.

He was the first quadriplegic to experience zero gravity, after a sub orbital space flight in 2009. At the time, he justified the £100,000 needed (and donated by Richard Branson) by saying:

"Many people have asked me why I am taking this flight. I am doing it for many reasons.

"First of all, I believe that life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden nuclear war, a genetically engineered virus, or other dangers.

"I think the human race has no future if it doesn't go into space. I therefore want to encourage public interest in space."

This anxiety was picked up again in Hawking’s answer on the Today programme. He was asked "Do you think the human race will survive all potential disasters and eventually colonise the stars?"

His response reflects worries of biological and particularly nuclear warfare:

"It is possible that the human race could become extinct, but it is not inevitable.

"I think it is almost certain that a disaster such as nuclear war or global warming will befall the Earth within a thousand years."

This is part of the reason why the Cambridge University cosmologist and theoretical physicist insists on colonisation.

"It is essential that we colonise space. I believe that we will eventually establish self-sustaining colonies on Mars, and other bodies in the solar system, although probably not within the next 100 years."

Ever-ambitious, Hawking limits human expansion to beyond the solar system, spiralling out into other galaxies:

"I am optimistic that progress within science and technology will eventually allow humans to spread beyond the solar system and out into the far-reaches of the universe,"

"The discovery of intelligent life elsewhere in the universe would be the greatest scientific discovery ever. But it would be very risky to attempt to communicate with an alien civilisation.

Fleshing out the possibilities of alien interaction, Hawking went on: "If aliens decided to visit us, then the outcome might be similar to when Europeans arrived in the Americas. That did not turn out well for the Native Americans."

Hawking’s birthday is on Sunday 8 January, and Cambridge University Centre of Theorectical Cosmology is holding a public symposium for the professor who was given only 2 years to live when he was diagnosed with a form of motor neurone disease in 1963.

Since then, Hawking has not only lectured as a distinguished academic but has transformed the perception of the universe, published best selling books and led a busy family life: he has three children and three grandchildren.