The NASA robot continues to rove the unforgiving slopes of Mount Sharp, but dramatic signs of damage are appearing on its aluminum wheels.

In 2013, earlier than expected signs of damage to Curiosity’s wheels were causing concern. Four years on and, unsurprisingly, the damage has gotten worse. The visible signs of damage have now gone beyond superficial scratches, holes and splits — on Curiosity’s middle-left wheel (pictured above), there are two breaks in the raised zigzag tread, known as “grousers.” Although this was to be expected, it’s not great news.

The damage, which mission managers think occurred some time after the last wheel check on Jan. 27, “is the first sign that the left middle wheel is nearing a wheel-wear milestone,” said Curiosity Project Manager Jim Erickson, at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., in a statement.

After the 2013 realization that Curiosity’s aluminum wheels were accumulating wear and tear faster than hoped, tests on Earth were carried out to understand when the wheels would start to fail. To limit the damage, new driving strategies were developed, including using observations from orbiting spacecraft to help rover drivers chart smoother routes.

It was determined that once a wheel suffers three grouser breaks, the wheel would have reached 60 percent of its useful life. Evidently, the middle left wheel is almost there. According to NASA, Curiosity is still on course for fulfilling its science goals regardless of the current levels of wheel damage.

“This is an expected part of the life cycle of the wheels and at this point does not change our current science plans or diminish our chances of studying key transitions in mineralogy higher on Mount Sharp,” added Ashwin Vasavada, Curiosity’s Project Scientist also at JPL.

While this may be the case, it’s a bit of a downer if you were hoping to see Curiosity continue to explore Mars many years beyond its primary mission objectives. Previous rover missions, after all, have set the bar very high — NASA’s Mars Exploration Rover Opportunity continues to explore Meridiani Planum over 13 years since landing in January 2004! But Curiosity is a very different mission; it’s bigger, more complex and exploring a harsher terrain, all presenting very different engineering challenges.

Currently, the six-wheeled rover is studying dunes at the Murray formation and will continue to drive up Mount Sharp to its next science destination — the hematite-containing “Vera Rubin Ridge.” After that, it will explore a “clay-containing geological unit above that ridge, and a sulfate-containing unit above the clay unit,” writes NASA.

Since landing on Mars in August 2012, the rover has accomplished an incredible array of science, adding amazing depth to our understanding of the Red Planet’s habitable potential. To do this, it has driven 9.9 miles (16 kilometers) — and she’s not done yet, not by a long shot.