After months of discussion, the space agencies behind the Lunar Gate have decided how the space station should orbit the moon. NASA and ESA are developing the Lunar Gateway together and the orbit that will follow it around the Moon is an integral part of mission design. This affects all important aspects of the mission, including the way the spacecraft gathers and lands on the station.
NASA and ESA have decided what is known as a near-linear Halo Orbit (NRHO) follows an eccentric orbit around the Moon. Sometimes it is up to 3,000 km to the lunar surface and sometimes up to 7,000 km.
"Finding a lunar orbit for the gateway is not a trivial task," said Markus Landgraf, architectural analyst working with ESA's Human and Robotic Exploration activities, in a press release.
The orbit of the moon gate rotates with the moon. It's called a halo orbit because it looks like a halo around the moon from Earth's point of view.
The gateway's NRHO is a seven-day cycle. Every seven days, it approaches the moon the next. This means that every seven days there will be a window to start from the station to the lunar surface and a window to return to the station.
The NRHO uses gravitationally balanced points in the solar system. Due to the interplay between the Earth and the gravity of the Moon, the Moon's Gate can sit in this orbit as if it were caught by the gravity of the two bodies.
The stability of these space points is ideal for term missions such as the Gateway. It is not perfect because it becomes unstable over time. But it will not take much energy to correct it.
"If you want to stay there for several years, the straight-line halo orbit is slightly unstable and objects in orbit tend to wander away," says Landgraf.
The orbit also determines some aspects of the design of the gateway. But because it's modular, there's a lot of flexibility.
A key aspect in the choice of orbits is the energy.
To escape Earth, spaceships need a lot of energy. Once a spaceship arrives on the moon, it has to get rid of that energy to land safely. That means carrying enough fuel and engines to slow down. Then, when it's time to return to Earth, it takes a big boost of energy again. The Lunar Gateway will change that.
"We do not fly a single monolithic spaceship in human spaceflight," explains Florian Renk, Mission Analyst in the ESOC Flight Dynamics Division. "Instead, we fly parts and put them together in space and soon on the surface of the moon. We leave some parts behind, others we bring back – the structures develop forever.
The key point is the energy that needs to be scrubbed to land on the moon. By docking with the moving gateway, a spaceship can leave some of its own at the gateway and save some of that energy. Starting from the Gate to the Moon and back requires much less energy than the start from Earth to the Moon and back.
This means that for a trip to the Moon and back, no massive rocket like the Saturn V is needed, with which the Apollo astronauts were brought to the moon. Much smaller missiles like the Ariane can do the job. In a sense, the gateway will be like an energy bank, giving much more flexibility and efficiency to missions to the moon.
The permanent Moon Gate in this orbit around the Moon will be a starting point for exploring the Moon. Parts can be left behind, picked up and assembled. After taking off from the moon, only a modest maneuver is required to slow down a spaceship that is on its way to the gateway.
The Lunar Gateway will be built not only as a launcher for missions to the Moon in the 2020s, with both crew and crew robots, but it will also be a science lab. It will also build on our understanding of space travel and be a step towards exploration of Mars. We can store supplies there for travel to Mars or other possible destinations deeper into the solar system. Since it has been removed from the Earth's magnetic field, it is a good place to test technologies that can not be tested on the ISS or near Earth.
"Our flight dynamics analysts and experts support a whole range of missions, including some of the most complex and exciting ones, including the Moongate," said Rolf Densing, ESA Director of Operations. "We can not wait to realize this ambitious international company."