The Mars Curiosity rover, which soft-landed on that planet on Aug. 6, 2012 after being lowered by a dramatic “sky crane” system, was expected to have a viable lifetime of one to two years on the planet’s surface. Likely sources of failure included the rover's various onboard mechanisms, its batteries, and its 110-W, nuclear-powered radioisotope thermoelectric generator (RTG). Amazingly, the rover is still functioning fairly well after five years, exploring farther and farther from the landing spot, and deliberately being directed through more-challenging terrain.
But those extra travels are spurring another problem that was originally not a concern. The treads on its wheels are wearing out from the extra distance traveled, and even more so due to climbing over sharp-edged rocks and other obstacles. Since tire replacement is certainly not an option, two researchers at the Jet Propulsion Laboratory (JPL), which manages the mission for NASA, instead developed an algorithm that should extend tread life. After 18 months of testing on Earth, the new wheel-drive software was uploaded in March and approved for use by project management in June.
The tread-wear problem is primarily due to uneven ground. On level ground, all of the rover's six wheels (three on each side) obviously turn at the same speed. On uneven terrain, however, any incline causes one or more of the wheels to start slipping relative to other wheels.
Further, when an individual wheel is traversing pointed, embedded rocks, the wheels in front pull the trailing wheels into rocks and the rear wheels push the leading wheels into rocks. Thus, the climbing wheel can end up experiencing higher forces, leading to cracks and punctures. While the tread sections on the wheels are designed for climbing rocks, the spaces between them are more at risk.
An Earth-based duplicate of the Mars Curiosity rover drives over a sensor as part of the evaluation of a new wheel-control algorithm designed to reduce wear on the rover’s tread sections and so extend tire life. (Credit: NASA/JPL-Caltech)
Each aluminum wheel is about 20 inches (50 cm) in diameter and about 16 inches (40 cm) wide. A January photo showed no major damage, but a subsequent March 19 photo revealed that the middle left wheel had two broken tread sections. When three of these tread sections—called grousers—are broken, NASA feels that the wheel has reached about 60% of its useful life.
The improved traction-control algorithm adjusts each wheel's speed based on real-time data, measuring changes to the suspension system to determine the contact points of each wheel. It then calculates the correct speed to avoid slippage, improving the rover's traction.
In Earth-based tests, the wheels were driven over a 6-in. (15-cm) force-torque sensor on flat terrain (see figure). Using the new algorithm, the leading wheels experienced a 20% load reduction, while middle wheels experienced an 11% load reduction.
The algorithm also addresses the problem of “wheelies,” where a climbing wheel keeps rising and eventually lifts high enough off the surface of a rock to spin freely. The resultant loss of contact of that wheel increases the forces on the wheels that are still in contact with terrain, thus increasing their wear. When the new algorithm detects a wheelie’s occurrence, it adjusts the speeds of the other wheels until the rising wheel comes back into contact with the ground.
Everything about the Mars Curiosity mission has been astonishing, especially its radical “sky crane” approach to the landing, and is detailed in the 2016 book The Right Kind of Crazy: A True Story of Teamwork, Leadership, and High-Stakes Innovation. Written by Adam Steltzner (who led the Entry, Descent, and Landing team) and William Patrick, the book covers the overall mission, but focuses on how the “it can’t possibly work” sky-crane landing idea was conceived, “sold” internally, simulated, implemented, and successfully executed after an eight-month journey.