Stretch a Slinky toy too far, and eventually the metal coil will be warped so much it won't be able to return to its original spring shape. That's a problem also faced by the metal spring tyres designed to roll across our Moon, and other planets our rovers are exploring. But NASA has created an alternative, made from titanium, that can tackle any terrain and always return to its original tyre shape.
Because puncturing an inflatable tyre while driving around the surface of the Moon would have been a disaster, the Lunar Rover, delivered by Apollo 15, instead featured airless tyres made of hollow metal springs. They absorb bumps like a rubber air-filled tyre does, but over time, those metal springs get warped and deformed until they're misshapen and don't roll as efficiently.
What's happening is that over time, the bonds between the atomic structures that make up the materials in a metal spring tyre are stretched to the point where they can't return to their original arrangement, which is a problem when repair technicians are millions of kilometres away from a vehicle.
The solution to the problem is a new type of metal spring tyre made from a nickel titanium alloy, whose atomic bonds are instead re-arranged as the tyre deforms and stretches when rolling over uneven terrain. Known as "shape memory alloys", the metals are assembled in what looks like a tyre made from chain mail, whose structure can be compressed right to the hub and still spring back to its original shape afterwards.
The advantage to using a tyre like this here on Earth is immediately obvious, especially if you've ever had to change a flat tyre on the side of the road in the cold of night. But having a tyre that can last for years with minimal maintenance is even more important when sending rovers to the other planets in our Solar System. The tyres used for the Curiosity Rover deteriorated faster than expected, and when you're spending hundreds of millions of dollars to send an autonomous explorer to another world, the last thing you want to derail your experiment is a damaged tyre.