How difficult is it to softly land on an alien planet?

So far, the list of members of the "Lunar Soft Landing Club" remains at four: the United States, the Soviet Union/Russia, China, and India. Is a soft landing really that difficult? Indeed, it is.

So far, humans have tried to soft-land on the Earth, the Moon, Mars, asteroids, and comets. Only the United States has successfully soft-landed on Mars, and Europe's lander broke. Only Japan has successfully landed on an asteroid, and twice in a row. Europe once soft-landed the Philae probe on the Churyumov-Gerasimenko Comet, but the landing was not ideal, so it is hard to say whether it was a success.

The moon is closest to the earth, and people have more comprehensive observations and surveys of the front side of the moon. Therefore, landing on the front side of the moon seems to be the easiest.

No more than 70%

The world's first probe to soft-land on the moon was Luna 5, launched by the Soviet Union in 1965. It crashed due to the failure of its reverse thrust rocket. Luna 7 and 8 also attempted soft landings but failed. Luna 9, launched in 1966, finally achieved the first soft landing on the moon. Less than four months after Luna 9 landed, the US lander followed, and Prospector 1 successfully landed on the lunar surface. At the end of the year, the Soviet Union's Luna 13 achieved another successful soft landing.

From the late 1960s to the early 1970s, the United States launched a large number of lunar landers to pave the way for the Apollo manned moon landing. After Prospector 1, Prospector 2 to 7 were launched. Among them, Prospector 2 and 4 failed to land, while the others succeeded. The work of these landers opened the prelude to the Apollo moon landing. Apollo 11, 12, 14, 15, 16, and 17 successively achieved successful moon landings, but Apollo 13 did not enter the moon orbit.

Apollo 11's lunar module

During this period, the Soviets were not idle either. Luna 16 and 17 landed successfully in 1970, and Luna 20 landed successfully in 1972. Luna 18 lost contact after soft landing in 1971. Soviet scientists judged that it might have fallen into a steep mountain and broken down. In 1973, Luna 21 successfully landed softly and released the lunar rover. Luna 23 landed on the moon in 1974. It was a sampling return model, but it fell into a pit and lost its balance during landing. Although it sent back some data, the sampling work failed. Luna 24 was launched in 1976 and successfully brought back lunar rock samples.

Thus, the first phase of human soft landing on the moon came to an end. Among the 25 soft landing probes that successfully entered the lunar orbit, 7 failed to land. The Soviet Union attempted soft landings 12 times and failed 5 times. The United States attempted soft landings 13 times and failed 2 times. If the manned Apollo spacecraft is excluded, the United States attempted to land the "Explorer" unmanned probe 7 times and failed 2 times. In general, the combined success rate of unmanned probe soft landings of the United States and the Soviet Union is not high, only 63.5%.

Since the 1990s, the international space community has resumed its exploration of the moon. However, in the past 30 years, the United States, Europe, Japan, and India have all launched lunar orbiters, and only China, India, and Israel have attempted soft landings on the moon. After the United States proposed returning to the moon, the lunar commercial cargo program, and the Lunar X Prize, although many organizations have proposed various concepts of lunar soft landers, none of them have been actually launched.

Where is the difficulty

Why is it so difficult to softly land on the moon? This is because it takes several thrilling stages to get from the earth to the surface of the moon. The first is lunar capture, which requires accurate calculation of the intersection position of the lander's orbit and the moon's orbit, so that the probe is attracted to the moon and enters the orbit around the moon. If you miss it, you have to wait until next month or even several months. When capturing the moon, you also need to accurately control the size and direction of the lander's speed. If you rush too close, you will crash directly into the moon and there will be no chance of a soft landing. "Genesis" finally completed this task this time, which should be said to be a great achievement.

Then comes the landing. This work needs to be discussed in several stages. The first is the landing site selection, which is done before the lander is launched. Theoretically, if you land on the front side of the moon, you can find a lot of reference materials on the lunar surface topography. Israel and the United States have a good relationship, and it is entirely possible to find high-precision maps to accurately select a wide and flat location. In contrast, there are no sufficiently accurate maps of the back side of the moon where Chang'e 4 landed. Chang'e 4 relied on its own probe and artificial intelligence algorithm to explore and descend, and finally landed safely.

Chang'e 4 lunar probe

When the lander enters the lunar orbit, it will observe the lunar surface, find the intended landing site, and then enter the landing procedure. The content and risks of this process have been explained in the previous article.

Any problem with the above procedures will lead to a failed soft landing. Moreover, launching a rocket from the Earth also has risks. In history, several interstellar probes had problems during launch and failed to even enter the initial Earth orbit. Others had problems in Earth orbit, such as the famous Apollo 13.