Begun in 1998 and completed in 2011, the International Space Station is celebrated as a remarkable feat of engineering as well as a far-flung home away from home for the astronauts who have lived and worked aboard the orbiting outpost.

But with the ISS slated for retirement in 2028, NASA is now making bold plans for the next phase of human spaceflight. The plans call for astronauts to return to the moon and then venture deeper into space. As President Trump said in December, these will be the first steps toward “an eventual mission to Mars and perhaps someday to many worlds beyond.”

A new space station is key to NASA’s plans. Unlike the ISS, the proposed Deep Space Gateway (DSG) will orbit the moon rather than Earth. Its crew will live and work a whopping quarter of a million miles from home — a thousand times more distant than the ISS. While NASA has no immediate plan to put astronauts on the lunar surface — something we haven’t done since the Apollo missions came to an end in 1972 — putting up the DSG will mark the first time in 45 years that humans will have ventured beyond low-Earth orbit.

Lockheed Martin

It’s a substantial undertaking, with a powerful new rocket and a new crew capsule among the hardware requirements. The ultimate cost would almost certainly exceed that of the estimated $125 billion it took to build and operate the ISS. Even if Congress approves the necessary funds, building the DSG will require NASA to team up with international and private sector partners. NASA and Roscosmos, the Russian space agency, issued an informal joint statement on cooperation last September; the Japanese and Canadian space agencies have also expressed interest.

But nothing is certain, and space stations and human spaceflight in general have long had powerful critics. One of the most outspoken is Dr. Steven Weinberg, the Nobel-winning theoretical physicist. He's called the ISS an “orbital turkey” and said that “human beings don’t serve any useful function in space.”

A challenging environment

What would life be like on the Gateway? Lonely, for starters. From the ISS, Earth looms large. At night, bright blotches of light visible through the station’s windows mark the world’s great cities. But for astronauts on the DSG, our planet will appear barely wider than a thumb held at arm’s length. Help, if it’s needed, would be weeks away. In contrast, the ISS can be evacuated in a matter of hours.

And the deep-space environment is inherently risky; radiation, including high-energy cosmic rays, would pose an ever-present danger to astronauts.

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“It will be a challenging environment, for sure,” says Dr. Chris Impey, a University of Arizona astronomer and the author of "Beyond: Our Future in Space." “It would be a lot more cramped [than the ISS]… I think the psychological stresses of being on the Gateway...would start to pile up.”

Even getting to the DSG will be a challenge. It will require a new heavy-launch vehicle, known as the Space Launch System. Now under development, the SLS will be the most powerful rocket ever built, producing more thrust than the Saturn V rocket used for the Apollo missions. The SLS, in conjunction with another planned craft, the Orion crew capsule, will lift astronauts first into orbit around Earth and then on to lunar orbit.

NASA's Space Launch System, seen in this artist's rendering, is billed as the world's most powerful rocket. NASA

There are sound arguments for making the moon, or its immediate environment, the next logical destination for human spaceflight. “It is a natural progression,” says Impey. “The idea of eventually moving beyond the Earth depends on it being routine to live and work in space, to stay healthy, to feed yourself, and re-supply your station” — an effort that began in earnest with the ISS and that would continue with the proposed Gateway.

And, Impey says, in the DSG's low-gravity environment, “you’re kind of liberated to [pursue] solar system exploration.”

Stepping stone to Mars

After lunar orbit, the next obvious destination for DSG crews would be the lunar surface, last visited by Apollo 17 astronauts Eugene Cernan and Harrison Schmitt in December 1972. And then — eventually — Mars.

The DSG “would serve as a gateway to deep space,” Cheryl Warner, a spokesperson with NASA’s Human Exploration and Operations Mission Directorate, told NBC News MACH in an email, adding that the region of space near the moon — the technical name is “cislunar space” — offers “a true deep-space environment where we can gain experience for human missions that push farther into the solar system, including Mars.”

Warner notes that the station’s exact design has yet to be pinned down, though it will include a propulsion system and a crew habitat.

NASA is already far along on some of the key components. A non-crewed test flight of the SLS, dubbed Exploration Mission-1, is planned for December 2019. The aim is to send an Orion capsule around the moon, and to deploy a half-dozen satellites. A second mission, planned for 2022, would be the first with a crew. After that, NASA aims to conduct flights at the rate of about one per year.

NASA astronauts practice an emergency egress from the Orion capsule in the Gulf of Mexico in 2017. Mark Mulligan / Houston Chronicle via AP

By the end of the 2020s, NASA hopes to conduct a year-long crewed mission to “validate the readiness of the system to travel beyond the Earth-moon system to Mars and other destinations,” the agency said in a statement last spring.

Aside from sending humans farther from home than we’ve been in 50 years, the Gateway will facilitate cutting-edge science. Last fall, NASA put out a call for proposals for DSG-based experiments; these will be the subject of a workshop to be held in Denver next month. NASA expects proposals for “planetary science, astrophysics, Earth observations, heliophysics [solar physics], fundamental space biology and human health and performance.”

The imperative to explore

Many spaceflight experts believe that no matter how much science could be done aboard the DSG, the project can’t be justified on scientific grounds alone.

“Yes, you can get some science done on the side — but science is way down the list, and it is not the motivating factor,” says Dr. Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology. He says a massive undertaking like the Deep Space Gateway makes sense only as a stepping stone to the red planet.

“If we’re ever to go to Mars, we have to learn how to operate far from the Earth," he says. "We need that operational experience. And I think that is the motivation for the Deep Space Gateway — to gain operational experience away from the comfort zone of low-Earth orbit.”

Of course, the ultimate reason for the push for the Deep Space Gateway — and after it, Mars — comes from the conviction that humans must, eventually, leave our home planet. Binzel quotes rocket pioneer Konstantin Tsiolkovsky, who once said, “Earth is the cradle of humanity, but one cannot live in a cradle forever.”

Robotic space missions — which by now have visited all of the solar system’s planets — have been incredibly successful, and these have been far cheaper than any mission involving a human crew. And yet, Binzel says, the idea of humans in space sparks the imagination in a way that no robotic mission can: “There’s something impactful about a fellow human being in space, which is hard to quantify. I think it’s all about human curiosity, the human imperative to be explorers.”

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