Mars rover wheels are metal, not rubber. NASA's are aluminium: Curiosity's are machined from solid blocks, 50.8 cm across, with a skin 0.75 mm thick between raised treads called grousers, and they carry two kinds of holes. The neat rectangular ones are deliberate: they spell "JPL" in Morse code, printing a pattern into the ground that the rover's cameras use to measure wheel slip. The ragged ones are damage, torn by sharp rocks since 2013. Mars Rally Championship, a free browser rally game, runs its rover on rubber rally tires instead.
What Mars rover wheels are made of
Every wheel NASA has driven on Mars is metal. Curiosity's and Perseverance's are each machined from a single block of flight-grade aluminium and joined to their hubs by curved titanium spokes, for what NASA calls springy support.
Those spokes are the nearest thing to a spring in the running gear: the rocker-bogie suspension carrying all six wheels is a linkage of pivots with none.
Curiosity's skin is 0.75 mm of aluminium, about half the thickness of a US dime and about as thin as it could be machined. The grousers carry the load and give the wheel its structure; the wide, thin skin between them only has to float an 899 kg rover on loose sand.
Why not rubber tires?
Rubber stiffens and cracks in deep cold, degrades under strong ultraviolet, and an air-filled tire is a pressure vessel nobody can patch. A metal wheel cannot go flat; it can only wear through. For future vehicles, NASA's Glenn Research Center has been developing airless tires built from nickel-titanium shape-memory-alloy springs.
Why do Mars rover wheels have holes?
Curiosity's wheels have holes on purpose. Each of the six carries short and long rectangular slots through the skin that spell J, P and L in Morse code — .--- .--. .-.. — for NASA's Jet Propulsion Laboratory. Every turn of a wheel, about 1.6 m of travel, stamps the pattern into the ground.
The letters are a signature; the pattern is a tool. Curiosity measures its real progress with visual odometry, tracking rocks, shadows and its own track patterns from image to image; any shortfall against what the wheel rotations predict is slip. Smooth sand offers almost nothing to track, so the rover prints its own high-contrast mark; it never reads the Morse.
The idea began as a side effect. Spirit's and Opportunity's wheels had been bolted to their landing platform, and the bolt holes printed marks that proved critical for Opportunity's visual odometry on the relatively featureless plains of Meridiani Planum. For Curiosity, JPL cut the holes on purpose: several small ones instead of one big one, laid out as Morse.
The other holes in Curiosity's wheels are damage.
How sharp rocks tore Curiosity's wheels
Curiosity's wheel damage comes from rocks fixed in the ground, and from the rover's own wheels forcing each other onto them. Holes and tears began accumulating faster than expected in late 2013, on wind-scoured ground in Gale Crater studded with angular rocks that could not roll aside or sink in.
Weight alone was not the problem. The Planetary Society's analysis cited wheels photographed perched on sharp rocks without harm, and rear wheels with no punctures, though the load is shared evenly by all six. The likely culprit was the wheels behind, each with its own motor, shoving the middle and front wheels onto the points.
Speed was not a factor either. At Curiosity's top speed of about 4 cm/s, the whole rover carries under a joule of kinetic energy; this was a slow squeeze, thin aluminium pressed onto a stone tooth until it gave.
JPL changed how Curiosity drives. Planners used orbital images to favour smoother ground and sand, which does the wheels no damage: a wheel on loose Martian regolith sinks and spreads its load instead of balancing on a point. On 18 February 2014 Curiosity made its first long reverse drive, 100.3 m; the rocker-bogie is asymmetric, and driven backwards its wheels are less prone to damage.
Curiosity's grousers, and the 2017 breaks
The grousers are the zigzag ribs on the tread, and structurally they matter more than the skin. JPL's wheel-wear reports count 19 per wheel, about 7.5 mm tall, with chevron points to resist sideways slip; NASA's 2020 Perseverance announcement counts Curiosity's chevrons as 24. They carry much of the rover's weight and most of its traction, which is why a wheel keeps working with much of its skin gone.
The first two broke on the left middle wheel, seen in images taken on 19 March 2017. Ground tests set the rule of thumb: once three grousers on a wheel have broken, it has used about 60% of its useful life. A JPL assessment counted four broken by 2 August 2021, three of them on the left middle wheel.
JPL has a contingency for the worst case: snag a failing wheel's inner section on a rock and tear it away, leaving a "rimmed wheel", the outer third. Tests on Scarecrow, its mobility test rover on Earth, showed a rimmed wheel still grips well enough to drive.
In July 2026, on sol 4963, Curiosity photographed its wheels again: cracked, holed, and still turning after more than 37 km.
The 2017 fix was software: traction control
In 2017 JPL gave Curiosity a traction-control algorithm that sets each wheel's speed to suit the ground under it, so that no wheel pushes or pulls another into a rock.
The rocker-bogie became a sensor: the software reads the angles of its pivots to work out where each wheel is touching. The algorithm went up in March 2017 after 18 months of testing and was cleared for routine driving that June. JPL put the load reduction at about 20% on the leading wheels and about 11% on the middle ones.
What Perseverance changed about its wheels
Perseverance's wheels are narrower than Curiosity's, slightly larger at 52.6 cm across against 50.8 cm, and have thicker skin. In place of chevrons they carry 48 gently curved treads, and JPL's Mars Yard tests of the redesigned wheels showed they resist sharp rocks better and grip at least as well in sand. The aluminium and the titanium spokes stayed.
By December 2025 Perseverance had driven almost 40 km, and JPL had certified the actuators that turn its wheels for at least another 60 km.
Sojourner, Spirit and Opportunity: smaller wheels, different failures
Sojourner, the first rover to drive on Mars, rolled on 13 cm aluminium wheels with stainless-steel treads and cleats, and its wheels were never the story.
Spirit and Opportunity had 25 cm aluminium wheels with cleats and built-in flexures to absorb shocks, and their wheel trouble came from motors. Spirit's right-front drive motor quit on 13 March 2006; its drivers spent three years driving mostly backwards, dragging the dead wheel, which in 2007 scraped up soil about 90% silica, pointing to hot springs or steam vents in Gusev Crater's past. In 2009 Spirit broke through a crust into soft soil at a site called Troy, and never drove out.
Opportunity lost the steering on its right-front wheel in April 2005, and in June 2017 its left-front steering stalled with the wheel turned outward more than 30°. It still covered more than 45 km before a planet-wide dust storm silenced it in 2018.
Mars rover wheels compared, from Sojourner to Perseverance
Wheel diameter roughly doubled from Sojourner to the twins and again to Curiosity, and the trouble moved from motors to the metal itself.
| Rover (landed) | Wheel diameter | Construction | Tread | What happened to the wheels |
|---|---|---|---|---|
| Sojourner (1997) | 13 cm | Aluminium, stainless-steel treads | Cleats | Nothing that mattered |
| Spirit (2004) | 25 cm | Aluminium, built-in flexures | Cleats | Right-front drive motor failed 2006; stuck in soft soil 2009 |
| Opportunity (2004) | 25 cm | Aluminium, built-in flexures | Cleats | Front steering failures 2005 and 2017; drove over 45 km |
| Curiosity (2012) | 50.8 cm, 40 cm wide | Machined aluminium, 0.75 mm skin, titanium spokes | 19 zigzag grousers, about 7.5 mm tall | Punctures and tears from 2013; grousers breaking since 2017 |
| Perseverance (2021) | 52.6 cm, narrower | Machined aluminium, thicker skin, titanium spokes | 48 gently curved treads | Wheel actuators certified in Dec 2025 for at least 60 km more |
| Mars Rally Championship rover | 0.9 m | Rubber gravel-rally tires | Knobby lugs | Per-tire health; impacts do nearly all the damage |
What Mars Rally Championship does with its tires
Mars Rally Championship puts its fictional 3,000 kg, four-wheeled rover on rubber: gravel-rally tires on 0.45 m-radius wheels, with a friction coefficient of 0.70 on regolith. Real Mars argues against rubber; the game keeps it because a rally game lives on its tire model, here four tires each computing their own forces 480 times a second.
Rocks do the damage here too, by the opposite mechanism: at rally speed a rock is an impact, not a slow squeeze. Each tire has its own 0–100 health. A direct hit at about 80 km/h of closing speed takes 18 HP from the tire on the nearest corner, mesas hit 2.5 times harder, a hard landing spreads a smaller hit across all four, and sliding costs under 5 HP a minute even fully sideways.
A worn tire's grip falls in a straight line to 40% of new at 20% health; below that, in the blowout regime, it collapses toward 20% and the tire visibly squashes. Stop within 8 m of the start and, after 3 seconds, the repair zone heals tires at 10 HP a second; how the game simulates vehicle damage has the whole model.
What the game does not simulate is most of this page: thin aluminium skins, grousers, Morse-code odometry, six wheels pacing each other over rock. The rover does leave tracks, a rut under every wheel with the tread pattern pressed in, but nothing reads them. Its traction control is a car's, not Curiosity's: it caps each tire's drive at 90% of that tire's grip, so full throttle still leaves cornering grip. Same name, different problem.
Where to try this
- Free roam, with no timer: clip a few rocks at speed until the steering pulls to one side, then limp back to the repair zone at the start.
- Stage 1, "The Opener" is a 1,164 m timed line with its rocks fixed in place, all at least 15 m off the route.
- Mars Rally Championship is free at /play with no download and no account, on a laptop, a phone in landscape or a school Chromebook.