Stephen Hawking died at age 76 on Wednesday, his family announced in a statement. The famed British physicist, whose theories on black hole radiation and cosmology transformed him into a household name, died peacefully in his Cambridge home. Toward the end of his life, Hawking strongly advocated for a future where humanity left the Earth and made new settlements on other planets.

“We are running out of space, and the only place we can go to are other worlds,” Hawking said to an audience gathered around his video livestream at Starmus science and music festival in Trondheim, Norway, in June 2017. “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.”

In the livestream, he set out a clear timeline for how quickly he would have liked to see development progress. Within 30 years, he wanted to see humans build an international moon colony. A few decades after that, he wanted a Mars colony to take shape. Within the next 200 to 500 years, humanity needs to prepare to venture forth to outer worlds.

See also: “Stephen Hawking’s 5 Predictions About the Future”

This was not a one-time whim for Hawking, who had warned about the future of humanity for the past few years. Hawking was signatory to a letter opposing autonomous weapons in July 2015, and later said in January 2016 that “most of the threats we face come from the progress we’ve made in science and technology,” calling for space colonization as a way to ensure these advancements don’t threaten the species. In April 2016, he was announced as a board member for Breakthrough Starshot, which aims to use gram-scale nanocrafts to leave Earth and arrive at Alpha Centauri within around 20 years. Hawking said at a Starshot press conference that he was supporting the venture because “Earth is a wonderful place, but it might not last forever.”

“The human race has existed as a separate species for about 2 million years,” Hawking said in the Trondheim festival livestream. “Civilization began about 10,000 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.”

Artist's impression of a Martian city. SpaceX

Hawking was not alone on this mission. SpaceX, the space-faring company headed by Elon Musk, plans a manned mission to Mars that could take place as soon as 2024, establishing a base on the red planet to power a future interplanetary rocket network. Buzz Aldrin, the second person to walk on the moon, has held a campaign dubbed “Get Your Ass to Mars” to encourage humanity to explore further. NASA has produced a concept vehicle and the Chinese government has simulated a colony, both of which are intended to fuel interest in Mars exploration.

Hawking’s legacy may be felt in these ambitions to set up homes on distant worlds. In a statement to the press, Hawking’s children — Lucy, Robert, and Tim — said he was “a great scientist and an extraordinary man whose work and legacy will live on for many years.”

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