Why is it so difficult to land on the moon?

Missions to the Moon continue. The American company Intuitive Machines will attempt to land its Nova-C lander on the Earth’s natural satellite on Thursday, February 22. The machine, which carries six scientific instruments for NASA, must land at the South Pole. Success would be an important step for the United States: it would be the first American apparatus on the Moon since the end of the Apollo program in 1972. Especially since intuitive machines are involved in the ambitious Artemis program, launched by the American space agency. , which aims to return humans to the Moon in a sustainable manner.

In recent months, Japanese, Russian and American missions have failed to land on the moon. The Japanese space agency, JAXA, experienced a semi-success (or semi-failure) in January: it managed to land its slim moon lander, entering a very exclusive club of countries to have achieved such a feat (along with the United States, the Soviet Union, China and India). ). Except Slim is upside down, his battery is almost empty and he’s asleep. “Moon landing is a very difficult challenge”The leader of the Japanese Institute of Space and Astronautical Science responded. “We are fully aware of the huge challenges ahead”, declared Steve Altemus, CEO of Intuitive Machines. Franceinfo details why landing on the moon is so complicated.

Because the moon has no atmosphere

“Earth is big. To escape, you need a lot of speed: 11.2 km/s (or 40,320 km/h). You get to the moon very fast”, explains planetologist Silvestre Maurice, professor at the University of Toulouse. To land softly, you need to achieve zero or nearly zero speed. Apart from that there is no atmosphere on the moon. So it is not possible to deploy a parachute at slow speeds like on Earth.

landing on the moon, “The only way to slow down is to counter-thrust, i.e. turn on the engine to create what we call ‘specific impulse'”Sylvestre Maurice, who worked on missions targeting the Moon and Mars, explains. “This thrust must be perfectly calibrated to control descent and control speed”, he insists. Which is very difficult because everything is automated and requires extreme finesse.

Because there is no geolocation system

Although probes have made it possible to map the Moon precisely, it’s hard to get your bearings right once you get there. When a device gets there, it cannot rely on a satellite geolocation system like GPS or Galileo, like Earth, to know very precisely where it is located. A must for every device “Try to get data, take images and correlate it with knowledge that it might have to find its way”Jean Bluvac, head of research and manned flight programs at the National Center for Space Studies (Cnes), explains.

The absence of a geolocation system around the Moon is not inevitable. “This may change in the future as humanity begins to build the infrastructure for a permanent human settlement on the Moon, but for now, a network of lunar GPS and radar stations is still far from becoming a reality”, the American company Astrobotic wrote in a lengthy press release in December 2023 before the failure of its Peregrine mission. In the meantime, we must do without and deal with uncertainties.

And they are important. On Earth, the accuracy with the European Galileo system is of the order of one meter. By placing a slim module on the lunar surface, the Japanese initially set themselves a precision margin of about a hundred meters. According to experts interviewed by Franceinfo, they eventually reached their landing point with a margin of error of 55 meters, better than expected and already significant.

However, the accuracy of the arrival location is crucial because the Moon is not completely flat and smooth. It has relief, craters, rocks, slopes, which can complicate the arrival of the device on its surface. without being extreme, “The terrain is not very smooth”Comments Silvestre Maurice.

During landing, it is also necessary to accurately determine the altitude. That’s why the devices carry radar that constantly provides the best information possible. Is the surface 15 or 18 meters, then 7 or 10 meters, etc.? ? An inaccuracy of 3 meters, for example, could be fatal for the device, Sylvestre Maurice notes, noting that the ship is only allowed one attempt.

Because piloting is done at (very great) distances

On the scale of the universe, the Milky Way, or even the Solar System, the Moon is next to nothing. However, it is about 384,000 km from Earth.

“The moon is not very far away. But piloting in real time is not possible: there is a delay of little more than a second.”

Silvestre Maurice, Astronomer

at franceinfo

A second of delay? This period of time seems short but it turns out to be immense during the fast pace and moon landing. So the final stage of landing is automated. “These phases usually start about a hundred kilometers above the surface of the moon, which we call low lunar orbit.”, explains Jean Bluvac. During this phase, “The systems will use the radar signal to know the relative altitude and give instructions so that the speed can be adapted at the very last moment to touch down”, summarizes Jean Bluwack. Added to all this is the fact that the Moon introduces local anomalies in the gravity field in places, he notes. “Additional Complications”, breathing head of Cnes. In total, for the moment, about one in two lunar missions ends in a crash.