The next chapter in cosmic exploration is starting to take shape: NASA engineers have proposed a space station that—if Congress approves its funding—would begin orbiting the moon in about a decade. A primary goal is to develop the infrastructure and experience to one day land humans on Mars.

The Deep Space Gateway (DSG) project would likely be a collaboration among the U.S., Russia and other international partners. It would sit in a lunar orbit about 240,000 miles from Earth—1,000 times farther than the International Space Station (ISS). This would put it outside Earth's protective magnetic field, letting scientists measure the effects of deep-space radiation on humans and instruments. The station could also be a relay point for expeditions to the moon's surface. Plans for lunar landers—bearing humans or robots, or both—are still under discussion. NASA officials say astronauts and construction materials could be ferried to lunar orbit in four Orion rocket launches sometime after 2019.

Credit: Katie Peek

But the proposal has its critics. After the Columbia space shuttle disaster in 2003, NASA vowed to launch humans separately from cargo—a principle the DSG plan appears to violate. Some space policy experts warn that lunar operations are expensive and could be more of a distraction from Mars than a step toward it. Others question whether the month-long stays planned for the new station would teach NASA enough about how the human body responds to deep space, given that the journey to Mars requires at least six months. One lunar engineer has expressed concern about intermittent and unpredictable solar storms. Such events might subject astronauts to dangerous radiation levels in the absence of a shield, such as a thick layer of water, which would need to be built into the space station's design.

Despite these reservations, many experts agree the moon could be a crucial training and proving ground—not just for astronauts but also for the Earth-based operations and equipment to support human spaceflight. “In my mind, it is clear you have to do the lunar exploration first,” says David Kring, a scientist at the Lunar and Planetary Institute in Houston, who has worked extensively on moon mission planning. Humans have not landed on the moon since the 1970s. The current generation of spaceflight engineers needs to learn to work on an extraterrestrial surface, Kring says, “and the best place to do that is three days away.”