One of the biggest threats to a robotic vehicle exploring Mars isn't necessarily a large rock or a steep slope, but rather fine, loose sand. Regular wheels can slip, gradually sink, and dig themselves a hole from which the vehicle can't extricate itself.
Researchers at the University of Würzburg in Germany propose tackling the problem with wheels that don't just roll. The new design mimics the motion of a medical cart (Scincus scincus), a desert lizard known in English Sandfish Skink Due to its ability to move through sand as if it were swimming in it.
The prototype was developed by a team led by Prof. Marco Schmidt, an expert in embedded systems and Earth observation sensors. The project is part of the VaMEx initiative, which stands for Valles Marineris Explorer, led by the German Aerospace Center (DLR).
The fool who is not afraid of dance
The sandpiper lives in the deserts of North Africa, Sinai, and Israel. Its body is adapted for burrowing and moving in loose sand. After diving below the surface, it tucks its legs into its body and moves its body and tail in an undulating motion, similar to swimming.
Previous studies using X-rays to image the sandworm have shown that it moves through the sand using a wave that propagates along its body. This movement allows it to reach speeds of about 1.5 body lengths per second in dense granular material.
The Würzburg researchers did not attempt to build a vehicle that crawls entirely like a lizard. Instead, they translated the principle of motion into the shape and movement of the wheels.
According to Amenosis Lopez, a researcher on Schmidt's team, the wheels mimic the way a slob exerts force on sand. In this way, they create not only a longitudinal force that propels the vehicle forward, but also a lateral force. The result is a motion reminiscent of swimming, leaving behind wavy, or sinusoidal, tracks in the sand.
Why does a regular wheel get hard in sand?
When driving on solid ground, a round wheel is able to efficiently transfer engine power to forward motion. In loose sand, the situation is more complex: the sand moves and folds under the wheel, some of the energy is wasted on moving the grains, and the wheel may spin without moving the vehicle forward.
As the wheel slips, it may sink deeper. As it sinks, the resistance to movement increases and more energy is required. This creates a loop in which slippage and sinking reinforce each other.
The problem is not merely theoretical. The vehicleMars NASA's Spirit became stuck in soft sandy soil in 2009. The mission team tried for months to rescue it, but was ultimately forced to use it as a stationary research station. After failing to adjust the solar panels to an angle that would provide it with enough energy in the winter, contact with it was lost.
The new wheels also sank initially
The prototype tests were conducted on sandy surfaces and in an open area, in collaboration with the German Research Center for Artificial Intelligence (DFKI) in Bremen and the University of Bremen.
According to the researchers, the vehicle was able to move stably on sand. However, the initial tests also revealed obvious weaknesses in the design: the first Sandfish wheels were heavier and narrower than similar pneumatic wheels. As a result, greater pressure was exerted on the ground and the vehicle sank into the sand.
The team changed the design, widening the wheels and reducing their weight. The wider wheel distributed the vehicle's weight over a larger area, reducing pressure on the sand and reducing subsidence. The reduction in weight and slip also improved the vehicle's stability and controllability.
The researchers estimate that further changes to the tire's surface texture could improve performance on mixed terrain, including sand, rocks, and gravel. However, the university's announcement does not include full numerical data, a detailed comparison of tire types, or a reference to a peer-reviewed scientific paper. So it's still too early to say how effective the method will be in real Martian conditions.
Not just wheels: the vehicle will need to know it is sinking
The next phase of the project doesn't just focus on the mechanical structure. The team plans to develop a control system that will detect slippage and subsidence and adjust wheel movement to ground properties in real time.
Such a system could use data from motors, sensors and cameras to assess whether the wheels are moving at their rotational speed, or if they are slipping in place. The control software could change the speed of the wheels, their direction of movement and perhaps the vehicle's trajectory before it sinks too deep.
The need for autonomy is especially important on Mars, because the distance from Earth does not allow for real-time vehicle control. Communication delays can reach many minutes in each direction, so a vehicle that gets stuck in sand or approaches a dangerous slope must recognize the problem and make decisions on its own.
Part of a swarm of robots to explore Valles Marineris
The VaMEx initiative isn't focused on a single vehicle. It aims to explore how a swarm of vehicles, crawling robots, and unmanned aerial vehicles could autonomously explore Valles Marineris, the vast canyon system on Mars.
Valles Marineris is about 4,000 kilometers long and more than seven kilometers deep in some places. The terrain includes slopes, canyons, caves, channels, and areas covered in fine sand. Due to the distance from Earth and the complexity of the terrain, the robots will have to navigate, share information, and independently decide which paths are safe.
The Sandfish wheels are currently a prototype tested on Earth, not a system approved for flight to Mars. However, the development demonstrates how mimicking an animal that evolved over millions of years in a sandy environment could offer engineers a new way to tackle one of the most stubborn problems in planetary exploration.
Questions and Answers
What is the medical term?
The medical scum (Scincus scincus) is a species of the family of skinks that lives in sandy areas in North Africa, Sinai, and Israel. In English, it is called the Sandfish Skink due to its ability to move under the sand in a movement reminiscent of swimming.
Does the Mars rover really swim under the sand?
No. The vehicle remains above the sand. The term “swimming” refers to the special movement of the wheels and the longitudinal and lateral forces they exert on the sand.
How are the wheels different from regular wheels?
They don't just rely on rolling forward. Their shape and movement create a wavy pattern, which helps push the vehicle through the sand and leaves sinusoidal tracks.
Has the vehicle already been tested on Mars?
No. So far, experiments have been conducted on sand and open areas on Earth. The gravity, atmosphere, temperature, and soil conditions on Mars are different, so further testing and development will be required.
Are the new wheels already better than any regular wheel?
The university reported stable movement and improvement after widening the wheels and reducing their weight. However, the announcement did not include full comparative data or a peer-reviewed article, so there is no basis for a blanket statement yet.
What is the VaMEx initiative?
VaMEx – Valles Marineris Explorer – is an initiative of the German Aerospace Center to develop technologies that will allow a swarm of various robots to autonomously explore the Valles Marineris region of Mars.
More on the subject on the science website
- Proposal: Test samples from Mars in a quarantine facility on the Moon before bringing them to Earth
- An emergency plan to get Opportunity out of the mud
- Baseball-sized rover successfully operates autonomously on the moon
- NASA has lost contact with the Mars Rover Opportunity
- Material from Earth may have reached Venus' clouds – but this is still not proof of life
