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Crossover: 720 seconds of Chang´e-3 moon landing

12-14-2013 20:29 BJT Special Report: Chang’e-3 Lunar Probe |

Now let’s turn to Wang Mangmang again for more about the Chang’e 3 mission, which is about to land on the moon. So Mangmang, now we know that the rover is going to land in the area called the "Bay of Rainbow", tell us, why did scientists choose that particular place, and how does it look like?

Answer: Well, actually according to the original plan, scientists circled 5 potential landing spots for Chang’E. Among them, Sinus Iridum, or Bay of Rainbow, is the most level and the most suitable. And during the previous mission, Chang’E-2 well surveyed this area, clearing the way for its successor.

Landing on the Moon

Welcome to the moon. We are now in the Rainbow Bay Area, the landing site of the Chang’e 3 rover. When you look at the moon, you may notice a big black shadow. This is called the lunar mare. I’m standing in the bay of the mare, a colossal crater wall with a diameter of 260 kilometers. Chang’e 2 had conducted a careful survey of this area to prepare for the arrival of Chang’e 3.

Now, let’s take a look at Chang’e 3. It will begin descending 15km away from the perilune. Of course it will not drop vertically, but will arc in a graceful, curving course towards the moon, at some 400 km. Throughout the process, the spacecraft needs to aim at the target, lower speed, and avoid obstacles to finally make a soft landing.

The change of speed depends on the engine. The main engine of Chang’e 3 rover is 7500 newton, which is the most powerful engine ever used on Chinese spacecrafts. Not only that, it is also a variable thrust engine. The main engine of the rover is located at its bottom, controlling landing speed by changing the thrust. When the engine gives a stronger push, Chang’e 3 will slow down. Under the combined influence of the push from the engine and the moon’s gravity, the rover will gradually descend. When the rover speed reached zero, the thrust and gravity will equalise, pushing in opposite directions. This will enable the Chang’e to hover in the air.
Now that we understand the role of variable thrust engine, we can simulate the landing process of Chang’e 3.

First is the main braking stage, which goes from 15km away from the perilune to 3km away from the moon’s surface. In this stage, the descending speed of the rover will decrease from 2km per second to 70 meters per second. And after the rapid adjustment stage, the rover will be at 2.4 km from the moon’s surface.

Then the rover enters the near-moon stage, in which it will descend from 2.4km to 100 meters above the landing site. The task in this stage is to avoid big rocks and pits. The moon’s surface is very dangerous, with mountains, ditches and pits. In order for the rover to make a safe landing, our scientists have designed a sophisticated "brain" and sharp "eyes" for it. The lander can detect rocks and pits as large as one meter in diameter through optical imaging sensor, thereby finding a safe landing site.

Now comes the key stage. At 100 meters above the moon’s surface, the rover needs to pause for a while, just like a hovering helicopter. The rover will stop and survey the situation beneath, using its "eyes" -- a three dimensional imaging sensor, to scan the landing venue and transmit the information to the navigation guidance controller, which will decide on a safe landing site. The time available for this "critical judgement" is very little -- less than 30 seconds. Most importantly, the whole thinking process is done by Chang’e 3 on its own.

Now the rover can make the next move after seeing what’s beneath clearly. When it descends from 100 meters to 30 meters, it will move to avoid small rocks and pits, and find the safest landing area. As it gets closer to the moon’s surface, it moves slower, at 2 meters per second. The speed is controlled by the main engine of the rover, which can sense the distance to the ground and adjust its speed accordingly, to avoid a hard landing. According to estimates by scientists, when it gets 30 meters above ground, the engine will stir up moon dirt. That’s when it enters the final stage -- the slow descending stage. When it’s only three meters above the ground, the engine will be switched off, enabling the rover to free fall and make a soft landing through its own gravity. Why does the engine need to be switched off? Because even if the rover avoids the bumps and hollows, the moon surface is not completely even. If the engine is on, the thrust might make the rover fall over. So it’s safer to enable a free fall.

Of course, in order to make for a stable free fall, the rover needs to have good legs as well. The four "legs" are made of special materials with strong scalability. So its standing posture won’t be affected even if it lands on slopes or thin obstacles. Let’s take a closer look. At the bottom of each leg is a sucker-like object. We call it the rover’s "feet". What are they there for? According to previous scientific study, there is a four meter thick soil on the moon’s surface. Without the big "feet", the rover may fall into the soil!

Now Chang’e 3 rover has made a stable landing on the moon’s surface. This is the first Chinese spacecraft ever to land on an extraterrestial body. But it won’t start to work right away. It needs to rest for several hours. After a journey lasting over ten days, the rover is very exhausted. All its parts and components need to take a break. It can only start working after getting used to the environment on the moon. Okay, lets take a break.

The lander rover Yutu, and photos taken between the rover and the lander

Hello and welcome to Hongwan on the moon, the new home for Chang’e 3. Now a brief introduction to the Lunar rover Yutu. Yutu is now out of the lander, onto the moon, and moving freely. On the front side, we see China’s national flag. First let’s take a closer look at its appearance. I’m now standing beside it. I’m barely 5’5’’ on my heels, so you can see that Yutu is about 4’9’’. Then let’s compare its width to my outstretched arms. A grown up person has the same height as the width of his outstretched arms. But Yutu is different. This is because Yutu has lots of solar panels which will provide the energy it needs.

Now we move on to its weight. Yutu weighs about 140 kilograms. It’s rather slim. Remember this figure, because the weight of the moon rover plays a key role in its functions. I’ve asked our space scientists, can we have Yutu move faster? Can Yutu bring more equipment with it? The answer is, the more Yutu weighs, the more it can do. This is just the prototype of China’s moon mission; let the scientists take their time.

Now I would like to introduce some of Yutu’s major parts. Yutu needs to have eyes if it wants to move around. We have one pair of eyes, while Yutu has three pairs. Let’s find them. First, this pair of eyes on top of Yutu, the dark eyes are called navigation camera. They can see clearly ten meters around. Under the dark eyes, we can see the pair of white eyes, called panoramic camera. They can see clearly 30 meters around. Data taken on these two cameras are sent back to Earth, so scientists can study it and help make judgement for the vehicle. Then they can control the vehicle and tell it where to go and how to get there. Yutu also has a pair of eyes for itself. Let’s look down. We can see China’s national flag, very clearly, and then the two red buttons here, and then down here, we can see a pair of black eyes, called Obstacle avoidance camera. They send data to Yutu’s brain so the vehicle can avoid obstacles and find its way. So we need these three pairs of eyes working together. With remote control from Earth and its own judgement, the lunar rover vehicle can thus see clearly and move easily.

Thousand-Mile Eye is not enough. The lunar vehicle also needs Wind-Accompanying Ear. Where is it? Let’s have a look. Yes, this big round disk that can swing. Its function is to find Earth, no matter how the relative position changes between Earth and Moon. In this way, Yutu and earth can be closely connected.

We are moving on to its legs, these six wheels. They are called Elastic mesh wheels. We will understand why they are called Elastic mesh wheels when we see the details. The design reduces the weight, and also helps the vehicle move on the soft moon soil. Their elasticity also allows the wheels to absorb shocks. People who often drive, know that a car has different driving system: front drive, back drive or four-wheel drive. Yutu has six drives, and each wheel can run in its own way, even when the vehicle is moving on the moon’s soil. What is it good for? Let’s have a look. We see that the wheels are spinning but the rover is not moving forward. Each of its six wheels is moving in a different way. It’s OK if you didn’t see clearly. Let’s have a look again.

Well, now let’s look at a small but vital part. Pay attention to the suspension system. It has a main swinging arm and a supportive swinging arm. The design guarantees the lunar rover vehicle moves smoothly on the moon soil, even on a bumpy road. Actually, some institutes and hobbyists have designed their own lunar vehicles, some of which have four wheels. The question is, why six wheels? Let’s do an experiment. Let’s assume that a four wheel lunar vehicle is landed on the moon...let’s see what will happen. Now we see this lunar rover vehicle. It has a similar structure, but with four wheels. Come to the moon! Immediately it encounters a large rock, and then it loses its balance and finally falls down. Not like Earth, you can’t find a rescue team on the moon. So if the vehicle falls down, we need to say goodbye forever. Now you can see how important it is to have six wheels with swinging arms.

Now you have a general idea about Yutu. Let’s get more information from its first task. Yutu’s first task happens on the first moon day, called two vehicles taking photos of each other. That’s the rover vehicle taking photos of the lander, and vice-versa. Pay attention to this photo, a photo of the rover taken from the lander. We see the Chinese national flag on the rover, the first of its kind taken on the moon surface.

 

Editor:James |Source: CCTV.com

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