The human race must start leaving Earth within 30 years to avoid being wiped out by over-population and climate change, Professor Stephen Hawking has warned.
Making an impassioned speech at the Starmus science festival in Trondheim, Norway, the astrophysicist said it was crucial to establish colonies on Mars and the moon, and take a Noah's Ark of plants, animals, fungi and insects, to start creating a new world.
Hawking said it was only a matter of time before the Earth as we know it is destroyed by an asteroid strike, soaring temperatures or over-population.
He said that becoming a "cosmic sloth" was not an option because "the threats are too big and too numerous".
"I am convinced that humans need to leave Earth. The Earth is becoming too small for us, our physical resources are being drained at an alarming rate.
"We have given our planet the disastrous gift of climate change, rising temperatures, the reducing of polar ice caps , deforestation and decimation of animal species.
"When we have reached similar crisis 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."
Hawking told the audience that the Earth would eventually be hit by a devastating asteroid strike.
"This is not science fiction it is guaranteed by the laws of physics and probability," he said. "To stay risks being annihilated.
"Spreading out into space will completely change the future of humanity. It may also determine whether we have any future at all.
"Wherever we go we will need to build a civilisation, we will need to take the practical means of establishing a whole new ecosystem that will survive in an environment that we know very little about and we will need to consider transporting several thousands of people, animals, plants, fungi, bacteria and insects."
Hawking said the moon and Mars were the best sites to begin the first colonies, stating that a lunar base could be established within 30 years and a Martian outpost within 50. But he also suggested leaving the solar system and venturing to our nearest star system, Alpha Centauri, where scientists believe there exists a habitable planet known as Proxima B.
The professor said nuclear fusion-powered ships propelled by light, Star Trek-style matter-antimatter reactors, or "some completely new form of energy" could allow humans to travel light years.
A small first step was already being taken by Russian billionaire Yuri Milner's Breakthrough Start Shot project, said Hawking.
This envisages sending a fleet of tiny "nanocraft" carrying light sails on a four light-year journey to Alpha Centauri.
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.
"If we succeed we will send a probe to Alpha Centauri within the lifetime of some of you alive today," he said.
"It is clear we are entering a new space age. We are standing at the threshold of a new era. Human colonisation and other planets is no longer science fiction, it can be science fact.
"The human race has existed a separate species for about two million years. Civilisation came about 10,000 years ago and the rate of development has been steadily increasing, If humanity is to continue for another million years it relies on boldly going where no one has gone before. I hope for the best. I have to. We have no other option."
Ahead of Asteroid Day, next Friday, Professor Alan Fitzsimmons from Queen University's Astrophysics Research Centre also warned that an asteroid collision with Earth could easily destroy a major city.
Asteroid Day commemorates the Tunguska strike in Siberia in 1908 which devastated 800 square miles.
Fitzsimmons said: "It is important to know that scientists and engineers have made great strides in detecting Near-Earth Asteroids and understanding the threat posed by them.
"Over 1800 potentially hazardous objects have been discovered so far, but there are many more waiting to be found.
"Astronomers find Near-Earth Asteroids every day and most are harmless. But it is still possible the next Tunguska would take us by surprise, and although we are much better at finding larger asteroids, that does us no good if we are not prepared to do something about them."