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The Lunar Gateway will be in a "Near-Rectilinear Halo Orbit"



After months of discussion, the space agencies behind the Lunar Gateway have decided how the space station will orbit the Moon. NASA and ESA are developing the Lunar Gateway jointly, and the orbital path that it will follow around Moon is a key part of mission design. NASA and the ESA have decided on what is called a near-rectilinear halo orbit (NRHO.) That means Gateway will take over all the vital aspects of the mission, including how the spacecraft will be rendezvous and land at the station.

follow an excentric orbit around the moon. Sometimes it will be as close as 3,000 km to the lunar surface, and it will be 7,000 km away.

"Finding a lunar orbit for the gateway is not a trivial thing," said Markus Landgraf, Architecture Analyst working with ESA's Human and Robotic Exploration activities, in a press release.

The Lunar Gateway orbit will rotate together with the Moon. It is called a halo orbit, because as seen from Earth, it looks like a halo around the Moon.

The Gateway's NRHO is a seven-day cycle. Every seven days it will make its closest approach to Moon.

The NRHO makes use of the gravitationally balanced points that exist in the Solar System.

The NRHO makes use of the gravitationally balanced points that exist in the Solar System. Due to the interplay between the Earth and the Moon's gravity, the Lunar Gateway can sit in this halo orbit, almost like it is trapped by the gravity of the two bodies

The stability of these points in space is ideal for long- term missions like the Gateway. It's not perfect, because over time it will become unstable. But it will not take much energy to correct it.

"If you want to stay there for several years, the near rectilinear halo orbit is slightly unstable and objects in this orbit have a tendency of drifting away," said Landgraf.

The orbit also dictates some aspects of the Gateway's design. But since it's modular, there's a lot of flexibility.

The Gateway Configuration Concept. Image Credit: NASA

A key aspect of the choice of orbits is energy.

To escape Earth, spacecraft need a lot of energy. Once and the spacecraft arrives at the Moon, it needs to get rid of that energy in order to land safely. That means carrying enough fuel and thrusters to slow itself. Then, when it's time to return to Earth, it needs a big boost of energy again.

"In human spaceflight we do not fly a single, monolithic spacecraft," explains Florian Renk, Mission Analyst at ESOC's Flight Dynamics Division. "Instead we fly bits and pieces, placing parts together in space and soon on the surface of the moon. Some parts we leave behind, some bring back – the structures are forever evolving. "

The key point is the energy that needs to be scrubbed to land on the Moon. By docking on the moving Gateway, and the spacecraft can leave some parts of itself at Gateway and save some of that energy. It takes a lot less energy to launch from Gateway to the Moon, and back, than it does to launch from Earth to the Moon and back.

That means that a trip to Moon and back will not require a massive rocket like the Saturn V that took the Apollo astronauts to the Moon. Much smaller rockets like the Ariane can do the job. In a way, the Gateway will be like an energy bank that gives missions to Moon much more flexibility and efficiency.

 The massive Saturn V rocket launches the Apollo 11 mission to the Moon on July 16, 1969. Image: NASA
The massive Saturn in the rocket launches the Apollo 11 mission to the Moon on July 16, 1969. The Lunar Gateway will mean that missions to the Moon can be launched with much smaller rockets. NASA

The permanent Lunar Gateway in this orbit around the Moon will be a staging point for lunar exploration. Parts can be left behind, picked up and assembled. The Lunar Gateway will be built during the 2020s as not only a staging area for missions to the Moon, both crewed and robotic, but it will also be a scientific laboratory. It will also build on our understanding of space travel, and will be a step towards exploring Mars. We can stockpile supplies there for trips to Mars, or other possible destinations deep into the Solar System. Since it's removed from Earth's magnetic field, it's a good place to test technologies that can not be tested at ISS or near Earth.

"Our analysts and flight dynamics experts provide support for a full range of missions, including some of the most complex and exciting ones in the lunar Gateway," said Rolf Densing, ESA's Director of Operations. "

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