A navigation system for the moon

Four months after its launch, NASA's Capstone satellite reached the moon this week and entered its intended orbit. The satellite, which is the size of an orange crate and weighs 25 kilograms, is also a scout for the planned "Gateway" space station, which is supposed to orbit the moon in a few years' time.

As part of the "Artemis" program, NASA is planning to put people on the moon again from the middle of the decade. The "Gateway" space station is intended to serve as an intermediate station for those traveling to the moon. A lander docked there can take them to any destination on the lunar surface.

In the case of a direct flight from Earth without a stopover in lunar orbit, the flexibility would not be as great. Later, the "Gateway" station, in which NASA, the European and Canadian space organizations are involved, may also be used as a starting point for manned flights to Mars.

The Gateway space station is designed to orbit the moon in an unusual, highly elliptical orbit. At the greatest distance to the moon, "Gateway" will be 70,000 kilometers from the south pole of the earth's satellite. On the other side of the orbit, the lunar space station approaches the North Pole of the moon to within 3000 kilometers. That would be the point at which a lander would undock and fly down to the lunar surface.

Never before has a satellite been placed in such an extreme orbit around a celestial body. But is such an orbit even stable over long periods of time? NASA researchers have explored this with computer simulations. The result: The runway planned for "Gateway" has a high level of stability.

But you don't want to rely on calculations alone in this sky-dynamic new territory. After all, this is a so-called three-body problem, because in this orbit, in addition to the gravitational pull of the moon on the "gateway", the influence of the gravitational force emanating from the earth cannot be neglected. Therefore, in the coming months, "Capstone" will be tested under real conditions to determine whether the path planned for "Gateway" is actually as stable as the computer simulations suggest.

In order to be able to navigate with high accuracy when approaching a landing site on the moon or during excursions with a rover on the earth satellite, one would ideally need something like a lunar GPS system. To do this, one would have to bring a fleet of satellites into lunar orbits – analogous to the terrestrial GPS navigation system.

The space organization Esa has identified this aspect as another possible contribution from Europe to NASA's extensive lunar program. Under the project name "Moonlight", she is currently exploring how European space companies could benefit from the moon boom.

However, NASA is already testing its own navigation system as part of the “Capstone” mission. It is intended to determine the position of the small lunar satellite relative to the "Lunar Reconnaissance Orbiter" (LRO), which has been orbiting the moon since June 2009.

"Moonlight" is not only about navigation and orientation for space witnesses, lunar robots and astronauts. At least as important is the communication that must take place between the various actors on the moon and between them and the earth.

During the Apollo missions, the astronauts in their capsule were always cut off from communication with the ground station in Houston when they were just behind the moon as seen from Earth. This communication gap could be closed with a network of "Moonlight" satellites. In addition, a communication link via satellite could be set up from any point on the moon's surface to any other point.

This of course includes contact with the "Lunar Gateway" and the ground stations on earth. Radio contact with an unmanned research station on the back of the moon would also be conceivable. For example, a radio telescope could be operated there, which would be shielded from interfering terrestrial waves.

Experts assume that a data connection between the earth and the moon with a capacity of several hundred megabits per second is required for the numerous planned moon activities.

Esa's "Moonlight" plans are not yet concrete. There are a number of ideas on the table as to how the network of navigation and communications satellites could look in detail. A first prototype could start to the moon in 2025 at the earliest.

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