Driving on Mars is not easy: the wheels are very different from those on Earth

Produced by: Science Popularization China

Author: Zhang Junwei (National Space Science Center, Chinese Academy of Sciences)

Producer: China Science Expo

The Tianwen-1 Mars probe was launched on July 23, 2020, by a Long March 5 Y4 carrier rocket from the Wenchang Space Launch Center and successfully entered the planned orbit. As of September 15, 2022, the Tianwen-1 Mars probe has been in orbit for more than 780 days, and the Zhurong Mars rover has traveled a total of 1,921 meters, completed the established scientific exploration mission, and obtained 1,480GB of original scientific exploration data.

The Zhurong rover has a clever design on its tires: the designer left two Chinese characters "中" on the rear tires of the Zhurong rover. As the rover moves forward, it will leave a "中" mark on the ground of Mars. Currently, Zhurong has left nearly 4,000 "中" characters. Jia Yang, deputy chief designer of the Tianwen-1 Mars rover, said that this design is not only commemorative, but also has a technical role. Scientists can judge whether the rover is at risk of slipping based on the spacing between the characters.

Wheels are the walking part of the Zhurong. Although tires have long been the most common equipment in life on Earth, since the environment on Mars is completely different from that on Earth, designing suitable tires for the Mars rover is an important issue.

Tires have long been part of our lives, but tires adapted to the Martian environment need to be redesigned

(Photo source: veer photo gallery)

Why is the tire design of the Mars rover important?

The tire design of the Mars rover is extremely important, which is related to many factors.

The first is the extreme climate of Mars. Due to its distance from the sun and the lack of an atmosphere, the climate of Mars is extremely extreme. The average temperature on Mars is -63°C, with a maximum temperature of 30°C and a minimum temperature of -140°C. In comparison, the highest temperature in Beijing in July 2023 may exceed the highest temperature on the surface of Mars.

Figure 2 Temperature comparison between Earth and Mars

(Image credit: NASA)

This huge temperature difference poses a severe test to tire rubber. Usually, rubber's elasticity decreases at low temperatures, which causes the rubber to harden, become brittle, and be easily damaged. At the same time, due to the hardened surface, rubber tires are prone to slipping at low temperatures. Ordinary car tires can only withstand temperatures as low as -20°C. For lower temperatures, the rubber will harden, not to mention the extreme cold of more than 100 degrees below zero. Therefore, it is not advisable to use rubber tires on the Mars rover. The Mars rover needs materials that are more resistant to low temperatures to ensure the stability of the rover's operation.

The second issue is the special terrain of Mars. Mars has a variety of terrains like Earth, with mountains, plains and canyons. Mars is basically a desert planet with sand dunes and gravel all over the surface. Faced with countless possible sharp objects, and in order to cope with the tire slippage that may be encountered in the desert and steep slopes, the design of the Mars rover tires should be strong, non-slip, and not easy to deform.

The second issue is the lifespan of the rover. For Mars exploration, the Zhurong rover is designed to last 90 Martian days (about 92.5 Earth days), but in the actual design, sufficient "design redundancy" must be considered, especially as the most important part of the propulsion device, the wheels of the rover must be durable enough.

In short, the tires of the Mars rover need to be resistant to the environment, radiation, strong and non-slip, and not easy to deform. These are the biggest challenges for tire designers. At the same time, tires are also one of the most important parts of the Mars rover. Once a problem occurs, the entire system will be affected. In order to ensure the completion of scientific missions, the design of the Mars rover tires is crucial.

What scientific explorations are involved in the design of the Mars rover’s tires?

NASA, which has already landed several rovers on Mars, has ample experience in tire design, but this experience also has lessons to learn. On July 7, 2019, the Curiosity rover, which has been in service for five years, took a selfie of its wheels using the "handheld lens imager" on its robotic arm. The images sent back show that the huge wheels have been scarred by the erosion of Martian rocks. Not only have the rocks created a large number of potholes on the six wheels, but the aluminum alloy material on the tire surface has also been severely damaged, with holes and punctures occupying more than half of the tire.

In this photo taken on July 7, 2019, the Curiosity rover shows some damage to its wheels.

(Image credit: NASA/JPL-Caltech/MSSS)

Therefore, scientists around the world are racking their brains to avoid a similar problem as Curiosity. Relevant scientific research teams have conducted more research on the tire design and movement design of deep space probes in extreme climates.

First, NASA replaced the wheels of Curiosity's brother, the Mars 2020 Rover, with new ones. The newly designed wheels are still made of aluminum alloy, and have 48 track patterns added. They are also larger in diameter and thicker in tires. At the same time, the tire width has been appropriately reduced to reduce the ground contact area.

Secondly, NASA's Glenn Laboratory has developed a new metal mesh tire made of a new memory alloy called "nickel-titanium" to ensure the durability and high reliability of future Mars rovers. This tire launched by NASA is woven from some rolled steel wires and looks like a flexible mesh. The biggest advantage of this structure is that it can change with the terrain, making it convenient to walk on various rugged roads. At the same time, it is also elastic enough to quickly restore its original shape after rolling over rocks on the road, allowing it to walk on relatively flat roads.

Figure 4: Test mesh tire under manufacture.

(Image credit: NASA)

At the same time, some special and innovative tire movement methods have gradually been proposed. The ground on Mars is full of gravel, and the soil on some parts of the planet is soft, which requires the Mars rover to have the ability to cross obstacles and strong traction.

In order to meet the above needs, China's "Zhurong" Mars rover is equipped with advanced active suspension, which has multiple movement modes such as creeping, lifting wheels, and body lifting. Simply put, "Zhurong" has six wheels in the front, middle and rear, and its suspension system corresponds to three modes: front wheel lifting, middle wheel retracting and rear wheel lifting. When a wheel is stuck or stuck in the earth, you only need to start the suspension system and actively lift the corresponding wheel to avoid obstacles or get out of trouble. The design team said that this design draws inspiration from the insect "inchworm", which does not crawl when moving, but fixes its front legs on the surface of the object, then lifts the middle of the body, and then drags the hind legs forward. The Mars rover equipped with active suspension no longer has to worry about the wheels sinking, and will not lose the ability to move even if a single wheel fails.

Another lunar rover being designed by NASA, the Volatiles Investigating Polar Exploration Rover (VIPER), will also move its wheels in a special, caterpillar-like coordinated way to help the rover get out of trouble. Currently, the prototype of the rover has completed ground testing at NASA's Glenn Research Center in Cleveland.

Scientists are even considering abandoning the tire design to complete planetary science exploration. According to the British Daily Mail, NASA's future Mars rover is designed to be shaped like a bag filled with gelatinous substances, which crawls like a worm and can crawl freely on Mars. This patent was approved by the US Patent and Trademark Office and is a major breakthrough in design over existing rover. At the same time, NASA is also developing a "polymer battery robot" that also uses the expansion and contraction of membrane cells to propel the rover on the surface of the planet.

For future deep space exploration, in the face of more and more planetary surface exploration and sampling missions, the mobility of probes, including the design and research of tires, will always be an unavoidable topic. I believe that with the further development of future technology, more suitable and durable materials and design solutions will emerge to support the completion of deep space planetary exploration missions.

References:

【1】CNET. NASA's Mars Curiosity rover still taking a beating from red planet rocks[EB/OL]. [2023-07-11]. CNET.

【2】NASA Chinese. Basic facts about Mars [EB/OL]. [2023-07-11]. NASA Chinese.

【3】Li Xiuquan Studio of Rubber Technology. Overview of the cold resistance principle and general solutions of rubber products [EB/OL]. [2023-07-11]. Sohu.

【4】Sheng Yang. NASA's future Mars rover may crawl like a worm in a gelatinous state [EB/OL]. [2023.07.18]. China.cn.

【5】Xinran Laboratory. What do you think of the Curiosity Mars rover's wheels being scrapped after only 20 kilometers? [EB/OL]. [2023.07.18]. Zhihu.