NASA's Curiosity rover has been driving across Mars for over four-and-a-half years without an overhaul and it's starting to show. According to the space agency, the six aluminum wheels that support the unmanned explorer are starting to show significant signs of wear, with two new small breaks in the metal treads of the left-side middle wheel. However, it's expected the wheels will go the distance to allow the rover to complete its planned mission.

The wheels on the Curiosity are the largest ever to roll on the sands of the Red Planet. Built by Tapemation in California, the aluminum wheels are 20 in (50 cm) in diameter and 16 in (40 cm) across, with 19 zigzag treads, called grousers, projecting out about a quarter inch to provide traction. Inside the wheels, there are curving titanium spokes for support.

So far, Curiosity has traversed almost 10 mi (16 km) and the damage to the wheels and treads was noticed during a routine check on March 19 – the first check since January when the breaks hadn't appeared. Previous dents and holes in the soft metal wheels were seen in 2013 and NASA became concerned because the wear was occurring faster than had been anticipated. Tests have indicated that when three treads on a wheel have broken, it has reached 60 percent of its service life.

NASA says that Curiosity is exploring the sand dunes of the Murray formation on the slopes of Mount Sharp as it climbs higher in search of younger geological formations to find signs of climate change billions of years ago. The rover has traveled 9.9 mi (16 km) since landing in August 2012 and needs to cover another 3.7 mi (6 km) to reach its final destination. To help it achieve this, mission control is choosing routes that avoid as many sharp rocks as possible.

"All six wheels have more than enough working lifespan remaining to get the vehicle to all destinations planned for the mission," says Curiosity Project Manager Jim Erickson at NASA's Jet Propulsion Laboratory, Pasadena, California. "While not unexpected, this damage is the first sign that the left middle wheel is nearing a wheel-wear milestone."

View gallery - 3 images