Did Yutu crash due to speeding? How is that possible?

"Yutu was damaged by racing?"

There are rumors online that the Yutu operator was too excited and drove the Yutu rover too fast, resulting in a malfunction.

Rumor Analysis

This is a misunderstanding of what the experts said.

In fact, due to the limitations of Yutu's performance and operating specifications, the so-called "damage caused by speeding" is impossible.

Not long ago, a piece of news circulated on the Internet, saying that Yutu was damaged due to drag racing. In fact, this was a misunderstanding, which turned Professor Zhang Yuhua's popular science introduction into a joke.

Picture from Weibo

Let me first state the conclusion: In fact, this statement is a misunderstanding and exaggeration, because on the one hand the lunar rover really cannot fly, and on the other hand the operator would never be so casual.

Let’s talk about it in detail next.

Speed ​​racing? Is it possible to speed racing at this speed?

Yutu can actually be considered a high-tech "remote control car", but it is not like the RC cars that we see RC enthusiasts playing in the community square, running all the way at a speed of tens of kilometers per hour. Every time Yutu moves, it needs to carefully observe the surrounding environment and report to the commander on Earth, and then it can move a small step, continue to observe the surroundings, and determine the next move plan, and so on. There are fewer moves and more stops.

Image from the National Space Administration

Why is there such a difference? The remote control car in the community runs fast because it takes advantage of the short distance. If it is placed on the moon 380,000 kilometers away, and the owner and the remote control are left on the earth, it will not be able to run fast.

Because the transmission of electromagnetic wave signals takes time, any scene the owner sees is outdated information from 1.26 seconds ago, and his instructions will be sent back to the car and executed 1.26 seconds later. At this time, the owner will involuntarily slow down to give himself and his car enough reaction time.

What's more, Yutu is not an ordinary "remote control car", but a "treasure" with a heavy mission. Its 136-kilogram body is equipped with a large number of valuable scientific equipment. It is itself a mobile scientific research station. It has to stop and explore after every step, which makes it even more difficult to "speed up".

From the landing on the moon on December 15, 2013 to the report of the failure on January 25, 2014, Yutu slept for 20 days in total. In the remaining 21 days of stopping and starting, its route was as follows:

Yutu's trajectory (Zhang Jinhai, Yang Wei, Hu Sen, etc.)

In this picture, A and B are two impact craters, the red five-point star is the landing point of Chang'e 3, the blue dot is the location where Yutu drove past, and the white numbers mark "the number of days and points", for example, 107 means the seventh point of the first day. The cyan CD and LS mark the locations analyzed by scientific instruments, which have little to do with this article, so you can ignore them.

Although there is a scale on the map, it is still difficult for us to feel its actual size due to the lack of familiar reference objects. So let's find another picture of the earth and superimpose this track on it in equal proportion:

Yutu's driving trajectory and crossroads (drawn by the author)

It can be seen that in 21 days, Yutu has only traveled a single intersection in the city. Its total driving distance is only 114.8 meters, and it moves an average of 23 centimeters per hour. In terms of average speed, even a baby that has just learned to crawl is faster than this, let alone a car.

Moreover, Yutu-2 is designed to travel at a speed of only 200 m/h, which is only slightly faster than 5 cm/s, while some large tortoises can travel at speeds of about 2,000 m/h (the tortoise won this time). So, in terms of speed alone, it is not "racing", and Yutu is on an exploration mission, not a race on the moon.

Where did the term "speed racing" come from?

Let’s take a look at Professor Zhang Yuhua’s original words.

After giving a popular science lecture titled "Unveiling the Story Behind Chang'e 5" at the Shanghai Popular Science Forum on December 19, 2020, she talked about Yutu with the host and said:

…(Yutu) continued to work for a while, and when it was almost the second lunar day, we suddenly discovered that there was a problem. After analysis and verification, we found two aspects.

One aspect is that we don't imagine the temperature on the lunar surface to be that high. In fact, let's say we have a section of cable and we want to tie the cable tie around this half circle. However, on Earth, temperatures of more than 20 or 30 degrees Celsius are no problem, but on the moon, during the day, the temperature may be 110 or 120 degrees. The cable will become softer and longer, and its insulation may be weaker.

Then the second thing is that when we were reviewing the situation, we analyzed it with the North Command and Director Li Jian. Our operators were too brave at the time. They just kept moving forward and would not fall, so they thought there was no problem. So we hit it - it got hooked on a rock and pulled the cable out. After pulling it out, after moving a few more times, the cable skin might have been broken on the 0.7 mm thick wall, which caused a short circuit, and the electricity could not be supplied.

…

Let me explain to you why it is so slow. There is no one on board. It moves forward using an obstacle avoidance camera and a navigation camera. It is on the back of the moon, and the obstacle avoidance camera takes a picture, which has to be transmitted to the ground via the Queqiao relay satellite. After it reaches the ground, we have to make a judgment and give instructions. It takes seven hours for it to move one step, and it can only move 7 meters at most in one step. If it moves longer than that, it will not be allowed to move, so it moves very slowly.

We have actually set up automatic obstacle avoidance driving, but we don't think we are going there to run. The main purpose is to test these control technologies, and also to see what else we can pay attention to. For example, some of our scientists have published many papers, and they say that you should not compete with others in how far you can run. We need to take our time and there will be many scientific discoveries.

From Professor Zhang Yuhua's words, we can see that "vigorous" refers to the rapid progress (driving forward without reversing), not the speed of the car. Moreover, the cable sags due to heat and hooks on the stone. If the cable is hooked, as long as you keep moving forward, the cable will be pulled out, and you don't need to drive very fast. This situation should be considered an accident. After all, many friends who drive by themselves accidentally bottom out may not be able to discover it in time, let alone when they are at the distance of the earth and the moon.

The case was solved, but the "killer" on the moon

Maybe more than one

The cause of the Yutu failure introduced by Professor Zhang Yuhua is a speculation based on the ambient temperature transmitted by Yutu after repeated tests and verification on the ground. In fact, the lunar environment is complex and there are many potential natural "killers".

For example, after Yutu reported a fault, the initial guess (which cannot be completely ruled out now) was that lunar dust carrying static electricity drifted into the mechanical structure or damaged the control circuit. Because lunar dust has sharp edges and is everywhere, its destructive power is very large, and lunar exploration equipment cannot be taken lightly.

Chang'e-3 waited one day after landing on the moon to wait for the lunar dust to settle before opening the hatch of the lunar telescope and the cover of the extreme ultraviolet camera. In addition, the gravity on the lunar surface is only one-sixth of that on Earth. Low gravity leads to reduced friction, which may cause the wheels to slip, stir up lunar dust or cause mechanical failure.

The temperature difference between day and night and even between the yin and yang sides is also a potential threat. After Yutu landed on the moon and completed the two-device mutual photography, it did not start working immediately, but took a nap for four days. The reason is that the sun-facing side of the vehicle can reach more than 110℃, while the shady side is only -10℃, so the lunar rover must be extremely cautious. Due to the extreme temperature on the moon, Chang'e 3's terrain camera is designed to work only during one lunar day, and it did stop working when the second lunar day arrived.

Due to the lack of atmospheric and magnetic field protection, micrometeorites and solar particles can reach the lunar surface directly. The probability of micrometeorites hitting Yutu is indeed very small, but 2013-2014 was the peak period of crazy solar output, and it is not impossible for solar particles to penetrate protective equipment and damage circuit devices.

In early January 2014, the huge sunspot AR1944 was photographed by the author.

In summary, aerospace engineering projects are handled with great care and caution, strictly in accordance with the sixteen-character principle summarized and guided by Premier Zhou Enlai at that time: "serious, thoughtful, meticulous, reliable, and foolproof". When problems are found, repeated tests will be conducted to draw inferences from them and make revisions. There are various risks and accidents in lunar and even space exploration, but China's aerospace scientific equipment is stronger than the previous generation, and the operators are more stable than the previous generation. Please give full trust. Accidents are possible, but drag racing is of course impossible.

Looking in the mirror of rumors

Sometimes, experts’ explanations may be misrepresented or “joke-ized.” Although such jokes can make people laugh, they can cause us to ignore details and even go against the facts.

Planning and production

Author: Qu Jiong Communication Technology Engineer

Review丨Liu Yong, Researcher, National Space Science Center, Chinese Academy of Sciences

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