Why is landing on the moon so difficult?

Why is landing on the moon so difficult?

Luna-25 at the best of times. On Sunday, the Russian lunar lander crashed into the lunar surface.A.P.’s photo

How often do moon landings go wrong?

The moon is a cemetery for fallen cosmic dreams. In recent history alone, one mission after another has failed. In 2019, lunar landers from Israel (Beresheet) and India (Vikram) were killed. This year, Japan (Hakuto-R) and this weekend also Russia (Luna-25) have been added. The probe did not enter the correct orbit due to a fault in the engines. In all, about half of all attempts to perform a so-called “soft landing” on the moon have failed since the beginning of the space age.

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George van Hall writes for De Volkskrant about astronomy, physics and space travel. He has published books on everything from the universe to the smallest building blocks of reality.

In the 1960s and 1970s, the United States and the Soviet Union landed on the moon the entire time. Why is it still so difficult half a century later?

Oddly enough, we ask ourselves this question about the moon landing, but not about—take it—climbing Mount Everest. After all, success in the past is no guarantee of the future. And that during the flight to the Moon, even with an unmanned probe, is at least a feat of heroism.

“You have to decelerate from over 5,000 kilometers per hour at 100 kilometers above the surface to zero speed, exactly on the surface of the moon,” says independent space consultant Eric Lann. And the hardest part: You can’t practice this entire sequence on the floor. So landing on the moon is by definition the first and only attempt.

For example, when a space probe returning to Earth encounters Earth’s atmosphere and thus slows down dramatically, the one on its way to the Moon has to come to a complete stop using thrusters. Thus, India’s previous attempt to land the Vikram lander failed. The lander’s rockets gave it more thrust than expected. The onboard software didn’t know how to handle this. “You can hardly fully test this on the ground,” says Lan.

And one more thing: the Moon has only one sixth of the Earth’s gravity. How does fuel flow in a car, for example? You can simulate it at most, but it is not possible to try it. With parabolic flight, you can simulate low gravity for twenty seconds, but then you can’t fully simulate such a descent.

Things can also go wrong during the descent itself. Although the surface of the Moon is well mapped using satellites, there is no GPS for the Moon, for example, to determine its exact location. “Neil Armstrong had to move the Apollo 11 landing site at the last moment by another five hundred meters because otherwise he would have hit a rock,” says Lan.

What do China and, in the past, the United States and the Soviet Union do better than the rest?

Lan says this is primarily a matter of time and money. “China has invested a lot, both in terms of money and people,” he says. Then you can do additional simulations, additional tests, and then the chance of success increases.

However, every mission remains a gamble, especially the first time around. Each lunar lander is unique: it was developed by other countries, with different programs, different components, and so on. In the 1960s and 1970s, the United States and the Soviet Union were able to build on previous attempts and create a successful series of missions. Now that knowledge is gone. The devices they used at that time are no longer available. So, by definition, you get a whole new car that you have to test again,” says Lan.

So India will take advantage of the failed landing in 2019 this week, during its second attempt at a soft landing on the Moon. “Now they know how to compensate for the slightly more aggressive braking rockets,” says Lan. “So I have a good feeling, but it remains very exciting whether they will actually succeed.”

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