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When Eugene Cernan of Apollo 17 became the last human to walk on the Moon back in December 1972, no one would have guessed that 50 years later we still would not have returned.
A manned return to the Moon is finally about to happen, led by Nasa, and this time we will be staying. A key part of the plan is Lunar Gateway, a space station that will orbit the Moon acting as a link back to earth, and a launchpad for missions to Mars.
The International Space Station (ISS), which is coming to the end of its natural life, was a radical departure when it became operational 22 years ago – it was the first time humans had established a permanent presence in space.
The ISS facilitated experiments in microgravity not possible on the ground, and scientists learned a lot about the effects that long-term living in space produced on the human body. It's time for the next step.
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It can be seen by the naked eye on clear nights, orbits 400km overhead, and its usual crew of six can look out its viewing windows and see the reassuring sight of Earth passing below.
The Gateway project, however, puts a four-person crew in an elliptical orbit around the Moon and several days travel away from home. It will be the space station in deep space, and an important stepping stone for future explorations.
The Gateway is part of the international Artemis programme, led by Nasa, which aims to return astronauts to the lunar surface before the end of 2024, and also involves the European Space Agency (ESA), the Japanese Space Agency (JAXA) and the Canadian Space Agency (CSA). Despite the challenges of Covid-19, the programme is on schedule.
This lunar space station will use solar panels to generate electrical energy, with a power and propulsion capability capable of manoeuvring the space station if needed.
There's a refuelling module, a habitation module designed and built by ESA, a reusable lunar lander, a logistics module holding food supplies and equipment, an air lock that permits the crew to get outside for space walks, a life-support module which recycles oxygen and makes drinking water and an Orion spacecraft which carries the crew to and from Earth.
Base camp
The Gateway has been described by some as an interplanetary way station, where resources, crew and cargo are gathered to support human exploration and settlement of the Moon, firstly, and then Mars. Others like to think of it as a kind of base camp where pioneering astronaut-explorers are supported with everything they need for success.
There are similarities between the Gateway lunar space station and the ISS low earth orbit space station, but differences too.
Sara Pastor, ESA's international habitation module project manager for Lunar Gateway, also worked on the ISS Columbus module. She is responsible for designing, developing and testing the pressurised living quarters for the Gateway crew, who must make do with about 700 square feet of living space.
“The crew will feel a little more crowded than compared to the ISS,” says Pastor. “The astronauts tested out the design in virtual reality to see if they can operate in it. It is designed to absolutely limit the need for maintenance using low weight materials. There will be equipment for exercising like ISS, but it will have less mass and less volume.”
The priorities are different for this moon shot compared to the Apollo missions in the 1960s and 1970s. Back then Nasa wished to demonstrate that humans could travel to the Moon, exist on its surface and return safely to Earth. Now it is about achieving sustainable living in space, and laying the groundwork for manned missions to Mars.
"On Apollo everything was brought up on that one mission," says Sean Fuller, Nasa's international partner manager for the Gateway programme. "It limited the duration of stay on the lunar surface to just a few days, whereas with Gateway we can build the pieces together with multiple flights there, and then spend a longer duration on the lunar surface to do more science, more research, and then bring the crew back to Earth.
Low-earth orbit
The Gateway will be composed of a four-person crew, compared to the six crew that typically inhabit the ISS at any one time. The Gateway crew will be onboard for a shorter time, between 30 and 90 days, while the ISS crew members can be onboard for a year or more. The ISS is permanently inhabited too, while Gateway is mostly not.
The Gateway will reside beyond low-earth orbit and the earth’s magnetosphere, which means its crew are less protected from potentially harmful solar and cosmic radiation than the crew of the ISS. Scientists want to know how such radiation will affect the Gateway crew and what can be done about it – a key consideration for Mars missions.
The orbit that the Gateway takes around the Moon is elliptical, taking it to within about 1,500km of the north pole of the Moon, and 70,000km of the south pole. This orbit is easily maintained as it is where the gravitational forces of the Moon, the Earth and Mars cancel out, so precious fuel does not have to be burned to keep it on track.
The Gateway can just sit in this stable elliptical orbit without having to use much propellant to remain in the correct position. There are big costs in terms of carrying fuel propellant on a rocket from Earth, so the less that is required the better it is. It takes 6½ days for the Gateway to complete one elliptical orbit around the Moon, whereas the ISS takes just 90 minutes to do a complete orbit of the earth.
The Gateway project can be seen as a natural evolution of the international cooperation on ISS. The planning for Gateway began in 2010 when agencies started to ponder what comes after the ISS in the context of no nation being able to afford to go it alone.
The first couple of elements were built by Nasa, but the life support system was built by ESA, with JAXA life support systems included in it. The ESA also built a refuelling module which means that Gateway will be capable of remaining in orbit for up to 15 years.
For its part CSA provided a robotic arm which is similar to the one it supplied for the ISS and Space Shuttle missions. This arm permits crew to conduct activities outside, such as inspections.
Nasa plans to build the Gateway by deploying a new heavily-lifting rocket, the Space Launch System, and Orion deep-space capsule. The Gateway crew will, once in place, be able to control and operate robotic rovers on the lunar surface with virtually no latency time, as would be the case if controllers were on Earth. There the crew will collect and analyse data, and this activity will continue even when the space station is unmanned.
The crew themselves will be the subject of data-gathering as doctors and scientists on earth monitor them closely to see how their bodies and minds are bearing up in this deep-space environment. These findings are crucial to long duration manned Mars missions.
Once the crew arrives at Gateway they will begin preparing to travel down on to the Moon, the first humans to do so in half a century.
"If you think about exploring here and explain it like the western US, the first thing out were the scouts, and after that we built the outposts to operate from," Fuller says. "That's what we are really doing here."