Curiosity Wheel Destruction Does Not Endanger Mars Rover Mission

Created by NASA engineers, the Curiosity rover continues to move along a given route on the surface of Mars, studying the characteristics of the atmosphere and lithosphere of the neighboring planet. Now the total distance covered by the device is already 16 kilometers. Overall, the rover is in good condition, although the problem with its wheels continues to deteriorate. On aluminum "tires" there are new cracks, cracks and holes.

Checking the condition of the wheels of the device is carried out regularly since the first dents and scratches on them were noticed. After some time, it turned out that the wheel shell collapses faster than the experts had planned. This year, NASA scientists, after examining the condition of the rover wheels, concluded that the wheel part had exhausted its resources by 60%. But the rover is in excellent condition, and can continue to fulfill its mission.

"All six wheels are more than in acceptable condition so that the rover continues to move in any of the planned directions to continue the mission," said Curioisty program manager Jim Erickson. “For us, this was not a surprise, but now it is already clear that the left middle wheel is approaching the maximum point of wear.” The agency has already stated that wheel wear will not change the scientific plans regarding the study of mineralogy of Mount Sharp.
')
Now the rover is on the sandy hills of Murray, studying the geology of the region. In the future, Curiosity should visit the hematite-rich region of the Vera Rubin mountain range. There is a lot of clay, sulphates, and scientists would like more information about this place.

The rover gradually climbed higher, exploring the younger layers of the foot of Mount Sharp. Scientists are counting on the fact that thanks to the data collected in this area, it will be possible to obtain information on the climate change of Mars billions of years ago. Rocks bear the traces of these changes, and from them one can judge how the planet has changed in general. Curiosity has already proved that earlier in some areas of Mars conditions were suitable for life, at least microbial. Higher than now, the temperature made possible the existence of liquid water on the surface of Mars. According to modern scientists, water is the most important criterion for the existence of life.

Another 6 km is left to the region rich in hematite. Curiosity program participants believe that this distance will be overcome without difficulty - the resource of the rover is sufficient. In addition, over the past four years, scientists have learned how to plan the promotion of the device, noting potentially dangerous places with sharp pieces of sharp rock in order to avoid damage to the wheels of the device.

The outer diameter of each wheel is 50 centimeters. "Tires" are made of aluminum. They are not too thick, so most pieces of rock are not able to pierce a layer of aluminum. Damage to the wheels of the rover are divided into several main types. These are holes, scratches, dents and breaks in the metal.


The condition of the wheels of the rover 708 sol. The photos were taken with the MAHLI manipulator with the camera. Source: NASA / JPL / MSSS / Emily Lakdawalla

The left central wheel is the hardest hit. The photo below shows the development of damage from salt 546 to salt 708

What causes the wheels to be damaged?

After scientists learned that the wheels of the rover are badly damaged in the process of moving the vehicle, they began to look for the cause. She was not identified immediately. When developing a rover, scientists envisioned possible holes and damage, but not in such numbers.



Moreover, the size of the damage was more than estimated. “When we saw the photos of the wheels, we realized that one of the holes is much larger than we expected. This did not happen in any of the passed tests. We did not know what was the cause. We did not know whether this would continue, ”says one of the rover operators Matt Heverly. “It was the moment when we realized that the method of movement needed to be radically changed. But how? This could not be said before the new tests. "

Tests were carried out for a year - both on Earth and on Mars, in the field. It turned out that one of the reasons that causes damage to the aluminum tires of the rover is material fatigue. Wheels constantly pass through uneven terrain. Thin aluminum undergoes deformations and eventually bursts. A good illustration of what happened is a breakdown of a regular paper clip, which is constantly bent in different directions in the same place. The wheels were designed in such a way that in case of a collision with a stone, it would be deformed and returned to its normal state. But, as you know, the planned service life of the rover is several dozen times less than the current life of the Curiosity. Tests to which the wheels were subjected showed that they should last for an estimated service life. But after this period, the material of the wheels began to gradually "get tired."

As for the holes, they leave the sharp edges of the rock found in many regions of Mars.



It is clear that the presence of such debris on Mars is not a surprise for the team of the rover. The designers were well aware that there are many such stones on the surface of the planet, but they did not expect that some of them would be stronger than the calculated parameters. In the tests on Earth, the apparatus rolled on fragments of different size and sharpness, but there were no special problems. Metal on wheels is designed for frequent visits to stones of different sizes, so the fact of hitting a sharp piece of rock does not mean a hole. The reason is different.

As it turned out, the reason for the mechanical wheel balancing system during the trip. A wheel when hitting a stone can withstand about 1/6 of the mass of the apparatus. In some cases, for example, when moving on a steep slope, the main load goes to the left or right row of wheels. When hitting a sharp stone, the load increases even more, and the design features of Curiosity lead to the maximum load on the middle wheel. As a result, it simply does not support almost the full weight of Curiosity and sometimes the metal breaks through with a stone.

True, not every sharp stone, on which the rover fell on the whole mass, will necessarily pierce the wheel. The calculation was based on the fact that when hitting a heavy machine simply knock a stone, so that the sharp end will not threaten the wheel. But on the path of the rover, sometimes there are very hard, fixed sharp stones. It is a strong rock that has been subject to weathering for millions or even tens of millions of years. They represent a particular danger. One of the engineers on the team demonstrated the problem by hitting a copy of Curiosity on Earth at the tip of the spear. The edge pierced the aluminum bus rover without any problems. Scientists, considering the surface of Mars before the rover’s departure there, did not find any particularly sharp stones, the tip of which was directed exactly upwards. Unfortunately, it turned out that they are relatively common. In November 2014, the rover drove through the region, where there are many fragments of this type.

What is the duration of the wheels designed, and what happens after?

According to experts from NASA, in order for the wheels to completely collapse, it’s enough to drive only 8 km through the breed, the type of which is described above. What exactly is meant by the "destruction of the wheel"?



As we see, in the mass of their damage are concentrated in the center. This is what can happen on Mars - a wheel can simply be cut in half. If stiffening ribs are broken, the wheel can fail. Fortunately, so far the situation is not so complicated. The wheels work, although some of them are badly damaged.

The good news is that now Curiosity has left the area with rock fragments. As already mentioned, he rides on the slopes of sand hills.

NASA experts have calculated how much a rover can travel on surfaces of various types until the wheels are destroyed:

• Sharp and hard stones: 8 km;
• A large number of ordinary stones: 13-14 km;
• Sandstone: 30–40 kilometers or more;
• Plain solid surface or sand with a small amount of stones: probably not defined tens or even hundreds of kilometers.

How can the wheel life be extended?

Of course, Curiosity has already exceeded its scientific program, but if it is in good condition, why should it be abandoned? Therefore, scientists think how to extend the life of the wheels. Here are some of the methods proposed by NASA.

Manage the rover more carefully . Try to check every point that is in front, look around, avoid sharp stones and especially problem terraces with such stones.

Plan a route to places with a flat surface or sand . One of the best ways is to pave the route through places covered with soft material with a minimum of stones. Over the past few months, scientists are trying to plan the path of the rover, referring to the photos of extremely high resolution. Such photos are obtained using the CRISM tool from the MRO ( Mars Reconnaissance Orbiter ) apparatus. Experts compare images from orbit with photos of the surface, which are obtained by the rover itself.

Modification of the software rover, in order to "teach" the machine to determine the load on the wheels . This part has not yet been implemented, although it has already begun to be implemented. The point is that when a rover moves, all data on the various parameters of its elements are analyzed by specialized software. In this case, it will be possible to avoid situations with strong pressure of sharp stones on the rover wheels.

It is not for the first time that NASA is going to change software on a remote device. In this case, the rover is much more advanced, on board is a relatively modern computer with the same modern software. So the update should not be a problem. On the other hand, the mission members are concerned that the same software is rather complicated. “The more complex the software, the more likely it is that something will go wrong. You will get surprises both during the tests and when performing various operations, ”said one of the mission representatives. However, scientists hope that everything will go well.

As for the heir to Curiosity, the rover of the Mars 2020 , then NASA has already announced that they will take into account the mistakes of the previous mission. In this case, most likely, the wheels will not significantly modify. Instead, scientists will simply shorten the possible path that the rover needs to drive. If, in the case of Curiosity, he first sat down and then began to look for interesting targets, then Mars 2020 is planning to plant in the vicinity of the targets of interest to scientists. The fact is that several orbital probes are currently working in orbit, capable of receiving high-resolution photographs of the surface of Mars. And it makes it easier to choose a goal to learn. The new rover is intended for astrobiological studies of the ancient environment on Mars, the surface of the planet, geological processes and history, including the assessment of the past habitability of the planet and the search for evidence of life within the available geological materials.