NASA's Curiosity rover has been operating on Mars since August 2012 — its aluminium wheels are now riddled with holes and cracks, and every metre it travels is planned in centimetres by a team on Earth working overnight shifts on Mars time

When Curiosity rover landed in Gale Crater in August 2012, each wheel carried only twelve small holes, and those were deliberate: drilled at the factory as markers to help track how far the rover had rolled. Everything else you see now was done by Mars.

Curiosity has been on the surface since that landing. It moves in a stop-and-go rhythm, one planned drive at a time, stopping often to take pictures and do science. It has become one of the clearest physical records of what long, hard surface work does to hardware.

Wheels built thin, ground down fast

The design helps explain the damage. Each of the six wheels is carved from a single block of aluminium, about 50 centimetres across. The skin that carries the rover’s weight is only 0.75 millimetres thick, roughly half the thickness of a US dime, and made that thin because it was the thinnest the machining could reliably produce. Nineteen raised ribs called grousers run around each rim to give the wheel grip.

Engineers began seeing dents and holes pile up faster than expected in 2013, about fourteen months into the mission. The cause was terrain the ground tests had never reproduced. Matt Heverly, an engineer on the mission, later described what the rover had rolled across: “We had exposed bedrock that had been sharpened by millions of years of wind erosion, creating what geologists call ‘ventifacts,’ which are wind-sharpened rocks.”  The thin wheel skin took the full force. 

Heverly was blunt about why the testing had missed it. He said “the problem with the holes in the wheels was that we did not test in a relevant way,” and, while some damage had been expected, “the rate of damage accumulation was much faster than we expected.” The first two grousers broke in early 2017, after only about 16 kilometres of driving. By 2021 the count had reached four broken grousers.

Every drive plotted in centimetres, on Mars time

None of this can be fixed as it happens. A radio signal takes minutes to travel between the two planets, so there is no way to steer around a hazard the moment it appears. As a JPL mission release put it, “Because of the radio signal delay between Earth and Mars, they can’t simply move the rover forward with a joystick.” Planners on Earth write out each day’s drive in advance, and the rover carries out the instructions the next Martian day.

The planning is painstaking. A single drive can take four to five hours to plan, with several people writing and checking hundreds of lines of code. Distance is measured by the wheels themselves: one full turn with no slipping covers about 157 centimetres. Every drive is worked out at that scale, one wheel turn at a time.

The wheel damage also shaped how much the rover could drive itself. Jennifer Trosper, later the Perseverance project manager, said “Curiosity couldn’t AutoNav because of the wheel-wear issue” early on, meaning the onboard autonomy that could otherwise have steered around hazards was largely off the table.

Adapting the route to save the metal

The team chose to change how Curiosity drives rather than accept a shorter mission. JPL engineers wrote a software update that slows the wheels over rough ground to ease the stress that cracks the skin. Testing in JPL’s Mars Yard gave them a way to judge how much life a wheel has left: one with three broken grousers has used up roughly 60 percent of its useful life. Route planning now weighs terrain against wear, steering the rover away from the sharpest bedrock where possible.

The wheels were designed to last 10 to 20 kilometres, and the rover has long since passed the low end of that range. In 2017, project manager Jim Erickson said “All six wheels have more than enough working lifespan remaining to get the vehicle to all destinations planned for the mission,” and project scientist Ashwin Vasavada called the wear “an expected part of the lifecycle of the wheels” that did not, at that point, change the science plans.

We’d say the holes read less like a fault than like a tally. Each puncture and torn tread marks ground that a machine the size of a small car has crossed, on another planet, one counted wheel turn at a time.