Humans are overdue for a return trip to the moon, Stephen Hawking has just opined.

Speaking this week at the Starmus Festival, a science-slash-musical gathering, the astrophysicist offered two parts doom cut with one part scientific optimism. He argued that we should prepare for a cosmic exodus to take place in the next 200 to 500 years.

"We are running out of space, and the only place we can go to are other worlds. It is time to explore other solar systems," he said via video link to the audience gathered in Trondheim, Norway. "Spreading out may be the only thing that saves us from ourselves. I am convinced that humans need to leave Earth."

Hawking's plan to boogie off this planet is ambitious: Countries should collaborate to construct a moon colony within 30 years. We can reach Mars "in the next 15 years," he said, with a base to follow a few decades later.

The head of the European Space Agency said last year that a "moon village" would take 20 years to plan and construct. NASA's long-term plans include sending humans to Mars by the 2030s.

Astronauts last walked on the moon in 1972, the same year that Elton John's Rocket Man debuted on vinyl. The final lunar visitor, Eugene Cernan, died in January. Cernan remained a lifelong advocate for space travel, testifying before Congress in 2011 that American space exploration was on "a path of decay" after the Obama administration shuttered NASA's Constellation moon program.

Hawking's gloom goes beyond decay into eschatology. In November, he said we had about 1,000 years left before escaping to the stars. In May, he chopped that timetable to the next hundred years. During his speech Tuesday, titled "The future of humanity," the 75-year-old black hole expert said that "Earth is under threat from so many areas that it is difficult for me to be positive."

There are extraterrestrial apocalypses, such as asteroid impacts "guaranteed by the laws of physics and probability." On Earth, Hawking cited melting polar ice caps, loss of animal life and dwindling physical resources, among other ill portents.

"The Earth is becoming too small for us," he said. Global warming is a threat, too, a view he knows is not shared by President Donald Trump, "who may just have taken the most serious and wrong decision on climate change this world has seen. I am arguing for the future of humanity and a long-term strategy to achieve this."

So let us set our sights on other worlds. At a neighbourly 4.37 light-years away, the planet Proxima B in the Alpha Centauri system is a promising target, Hawking said - except that with current technology, interstellar travel is "utterly impractical."

He outlined some of the theoretical technology behind Breakthrough Starshot, a mission he supports along with Russian tycoon Yuri Milner. The goal is to send tiny probes all 25 million miles to Proxima B and have them beam back information.

In theory, a powerful light source, blasting tens of kilowatts of power into space, could propel nanocraft like sailboats caught in a mighty wind. The probes would fly by Mars in an hour, Pluto in days and Alpha Centauri in 20 years, Hawking said. (He does not envision such a system being useful for human interstellar travel, though, in part because light-propelled craft have no brakes to pump.)

"The human race has existed as a separate species for about 2 million years. Civilisation began about 10,000 years ago, and the rate of development has been steadily increasing," Hawking said. "If humanity is to continue for another million years, our future lies in boldly going where no one else has gone before."

The Washington Post.