Designing tires for wear and tear 400 million miles from a spare
I’m not completely up on the art of selfies, but I’m confident that most of them are not taken to check for potentially crippling damage to one’s mobility. If you’re stuck on another planet with nobody to check up on you, that becomes necessary. On the 411th day of its mission, the Curiosity Mars rover revealed the early signs of such problems, with some puncture damage spotted in the front and middle tires. While nobody on Curiosity’s command team was happy to see this kind of damage, nobody was surprised either. They’d seen it all before back home.
Training wheels
Before Curiosity went to Mars, a full-sized prototype, called Scarecrow, was built and driven around the deserts of California and Arizona. The chassis and wheels were a match for the final product, the payload of this rover was dead weight, intended as a proxy for the scientific instruments that would be sent on Curiosity itself. Scarecrow wasn’t used for sampling the sands of California, but was instead used to test the mobility and durability of the design and materials, including the six, aluminum tires.
The metal tires were designed to balance weight and strength. Each was carved out of a single block of aluminum to a mere .75 millimeters thickness (with tread chevrons being around ten times thicker.) With a diameter of around 20 inches across, the proportions of these tires are obviously quite a bit different from the radials on your car, but the weight of the nearly 2000 pound rover needed to be minimized in order to get Curiosity safely to the surface of Mars. The thin wheels have actually been quite durable, and while the test versions on Scarecrow were torn a bit on Earth, Mars’ weaker gravity plus fewer intentional encounters with rough rock have helped delay these cracks and punctures.
Progress in reverse
After four years of journeying to Mount Sharp, Curiosity’s wheels aren’t doing great, but the rover isn’t expected to get stuck any time soon either. The current damage levels seem to be the result of continued flexing back and forth of the aluminum as the wheels roll over bedrock, coupled with weight occasionally being unevenly distributed to some wheels more than others. This has lead to the front and middle row of tires showing more wear and tear, but also offered an opportunity. As engineers discovered when working with Scarecrow, the relatively unscathed rear tires are available to take on the some of the heavy lifting as Curiosity continues forward. It just means that it’s time to start driving the rover backwards for a while. The middle tires will continue to take damage, but it should allow the mission to continue for years to come.
Source: The Achilles Wheel Of The Mars Curiosity Rover by Ethan Siegal, Forbes
