For years, President Obama has been saying the U.S. must send humans to Mars. Permanently.

There was the 2010 speech when he said, "By the mid-2030s, I believe we can send humans to orbit Mars and return them safely to Earth. And a landing on Mars will follow. And I expect to be around to see it."

There was that time in 2012 when he said the Curiosity rover was inspiring kids to tell "their moms and dads they want to be part of a Mars mission — maybe even the first person to walk on Mars." And there were those other times he told kids visiting the White House that they might go to Mars someday.

And, of course, the 2015 State of the Union address when he called for the U.S. to push out into the solar system "not just to visit, but to stay."

Today, the president said it again.

"We have set a clear goal vital to the next chapter of America's story in space: sending humans to Mars by the 2030s and returning them safely to Earth, with the ultimate ambition to one day remain there for an extended time," the president wrote in an op-ed published by CNN.

"Getting to Mars will require continued cooperation between government and private innovators, and we're already well on our way," he wrote. "Within the next two years, private companies will for the first time send astronauts to the International Space Station."

The president continued:

"The next step is to reach beyond the bounds of Earth's orbit. I'm excited to announce that we are working with our commercial partners to build new habitats that can sustain and transport astronauts on long-duration missions in deep space. These missions will teach us how humans can live far from Earth — something we'll need for the long journey to Mars."

Several commercial spaceflight companies have also announced plans to aim for Mars in the coming decades.

Last week, for example, Boeing's CEO announced, "I'm convinced the first person to step foot on Mars will arrive there riding a Boeing rocket." Blue Origin, owned by Amazon founder Jeff Bezos, is testing rockets to bring humans into orbit "and beyond." And the defense and space contractor Orbital ATK is building and testing rocket boosters for a future human mission to Mars.

A Dutch venture called Mars One hopes to colonize Mars by 2025, and has been taking applications from would-be travelers for years, although it acknowledges that it will rely on "major aerospace companies" for transportation.

In September, SpaceX CEO Elon Musk announced a yet-to-be-built rocket system that he said would put people on Mars by 2025, and would eventually transport enough people to the red planet to start a permanent colony.

Musk predicted any Mars mission would require "a huge public-private partnership."

A 2014 report by the National Research Council found federal funding for human spaceflight programs has not been robust enough to achieve the president's goals for Mars. "Pronouncements by multiple presidents of bold new ventures by Americans to the Moon, to Mars, and to an asteroid in its native orbit, have not been matched by the same commitment that accompanied President Kennedy's now fabled 1961 speech," the report's overview reads.

But Obama appears undeterred.

"Someday I hope to hoist my own grandchildren onto my shoulders," the president writes. "We'll still look to the stars in wonder, as humans have since the beginning of time. But instead of eagerly awaiting the return of our intrepid explorers, we'll know that because of the choices we make now, they've gone to space not just to visit, but to stay — and in doing so, to make our lives better here on Earth."

Later this week, Obama will host the White House Frontiers Conference in Pittsburgh, where the White House says the five "frontiers of innovation" up for discussion are personal, local, national, global and interplanetary.

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