Humanity must head to the stars to survive, says Stephen Hawking
The top physicist repeated his warning that the human race is doomed unless it can find a new home beyond the Earth.
Speaking at the Starmus science and arts festival in Trondheim, Norway, he argued that interstellar travel - impossible with today's technology - should be a definite long-term aim "in the next 200 to 500 years".
The professor imagined nuclear fusion-powered ships propelled by light, Star Trek-style matter-antimatter reactors, or "some completely new form of energy".
A small first step was already being taken by Russian billionaire Yuri Milner's Breakthrough Start Shot project, said Prof Hawking.
This envisages sending a fleet of tiny "nanocraft" carrying light sails on a four light-year journey to Alpha Centauri, the nearest star system to Earth.
The camera-equipped miniature probes, would be sent on their way by tens of gigawatts of focused power from an array of lasers, reaching their destination in about 20 years.
Travelling to the stars is not a fanciful idea but a necessity, according to the professor, whose address was broadcast via a video link to the festival.
Adopting the role of "cosmic sloths" enjoying the universe from the comfort of Earth was not an option, he maintained.
Prof Hawking said: "The Earth is under threat from so many areas that it is difficult for me to be positive. The threats are too big and too numerous.
"Our physical resources are being drained, at an alarming rate. We have given our planet the disastrous gift of climate change. Rising temperatures, reduction of the polar ice caps, deforestation, and decimation of animal species. We can be an ignorant, unthinking lot.
"When we have reached similar crises in our history, there has usually been somewhere else to colonise. Columbus did it in 1492 when he discovered the New World. But now there is no new world. No Utopia around the corner.
"We are running out of space and the only places to go to are other worlds. It is time to explore other solar systems. Spreading out may be the only thing that saves us from ourselves. I am convinced that humans need to leave Earth."
There are an estimated 1,000 stars within 30 light years of Earth, he pointed out.
"Even if only 1% of these have Earth-sized planets, we have 10 candidate New Worlds," said the professor.
Pondering on the likelihood of intelligent alien life, Professor Hawking said there were three reasons why we had not yet heard from ET.
"First, it may be that the probability of primitive life appearing on a suitable planet is very low.
"Second, even if the probability of primitive life is reasonably high, the probability of that life developing intelligence like ours may be very low.
"And third, it could be that alien life develops into a fully functioning civilisation, but it then destroys itself with war, disease and weapons of mass destruction.
"I believe this is the trajectory we are on. Personally, I prefer the second reason: that primitive life is relatively common, but that intelligent life is very rare. Some would say it has yet to occur on Earth."
Professor Hawking said he believed the human race was "standing at the threshold of a new era".
He added: "Human colonisation on other planets is no longer science fiction. It can be science fact.
"The human race has existed as a separate species for about two million years. Civilisation began about ten thousand years ago, and the rate of development has been steadily increasing.
"If humanity is to continue for another million years, our future lies in boldly going where no-one else has gone before."