After travelling 8.5 kilometres on Mars, NASA’s Curiosity rover is now facing some of the most dangerous terrain it has ever encountered.

The car-sized rover is currently crossing a stretch of hard, rocky ground of the sort that previously dented and punctured its aluminium wheels. Winds at Gale Crater, Curiosity’s landing site, have whittled and sharpened rocks into piercing points unlike that seen by NASA’s three earlier Mars rovers. Curiosity needs to travel about 200 metres of this sharp ‘caprock’ before it can descend into a sandy, more wheel-friendly depression dubbed Hidden Valley.

“This is awful stuff,” says John Grotzinger, the mission’s chief scientist and a geologist at the California Institute of Technology (Caltech) in Pasadena. He spoke on 16 July in a public lecture associated with a week-long Mars conference on the Caltech campus.

Grotzinger and his team of scientists and engineers have spent much of the past few months figuring out a way to get Curiosity closer to its ultimate target — a 5-kilometre-high mountain known as Mount Sharp — without destroying its wheels along the way. The problem became apparent last December, when Curiosity sent back close-up images of its wheels that revealed more wear and tear than engineers were expecting. Over the next few months, the wheels rapidly deteriorated. One ripped across nearly half of its width in a giant gash. “When you have a metal wheel and you can see the planet through it, that’s not a good thing,” says Grotzinger.

Each of the rover’s six wheels is machined from a single piece of aluminium, measures 40 centimetres across and weighs just 3 kilograms. That size saved weight at launch, but means that the aluminium skin — just three-quarters of a millimetre thick — is prone to tearing, says rover driver Chris Roumeliotis, of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena.

The damage was particularly bad on the rover’s pairs of front and middle wheels. To figure out why, mission engineers hauled out a mockup of Curiosity and rolled it over piles of sharp rocks in the ‘Mars yard’ test site at the JPL.

In one particularly gruesome test, the wheels went over a sharp metal point nicknamed the Impaler. “Hearing the aluminium crack and puncture like that just gives me chills,” says Roumeliotis.

Soon the team figured out that Curiosity could minimize damage by driving backwards over sharp rocks, which lessened the load on the wheels just as pivoting from pushing to pulling luggage changes the stress. The rover has been scuttling along mostly in reverse ever since.

But the team cannot avoid the fact that sharp rocks must be crossed. Using images from orbiting spacecraft, Grotzinger and his colleagues have mapped out ten types of terrain, colour-coded from green (kind to wheels) to an extreme red (full of pointy rocks). At times, they have opted to take the long way between two locations to cross over the least-damaging terrain possible.

But there was no avoiding the fact that Curiosity had to cross a swath of red at a place called Zabriskie Plateau. Next week, rover planners will send it slowly rolling over the last stretch of dangerous caprock and descending into Hidden Valley below. “We will go in and out of the valleys, trying to work at the interface between the wheel-damaging caprock and where we would like to be,” says Grotzinger.

That could take a while. Curiosity still has 3.5 kilometres to travel just to make it to the base of Mount Sharp.