If you were planning a road trip, chances are you would give your car a once-over before you set off. Top up the essential fluids, check the lights and kick the tyres.
But what if you were heading to Mars?
First, make sure the heater works, because it’s going to be chilly. Temperatures on Mars can drop as low as minus 107 degrees Centigrade. That is beyond the point at which rubber transitions to a glass-like material. Kick those tyres and they might shatter. Small wonder then that Martian rovers do not get around on rubber.
And if the low temperatures did not rule out such material, the weight of a thick rubber tyre, plus weaving and rim, would. Mars rovers have to be light for a variety of reasons – but mainly because of the difficulties involved in getting the craft to land safely on another planet without any human involvement. The margins for error are tiny.
The Martian surface of loose dust, soil and broken rock – called regolith – covering solid rock is unforgiving. If the rover is too heavy at the moment it touches down it might crash rather than land and the mission would be over before it began.
If the rover is too heavy at the moment it touches down it might crash rather than land and the mission would be over before it began
Rock and roll
That is why thin is in when it comes to rover wheels. Typically, each one is machined from a single block of aluminium, pared back to the bare minimum. On Curiosity, which landed on Mars in August 2012, each wheel is about 0.75mm thick. They could have been thicker but that would have added weight. Possibly too much. NASA estimated that making the wheels one millimetre thicker would add about 10kg of mass. Instruments might have to be sacrificed to offset such an increase. Scientists who make rovers are always seeking the best trade-off between weight and utility.
But the current slimline wheels meant NASA had to confront other problems, says Emily Lakdawalla from the Planetary Society. Some of which only became clear as the rovers were leaving tracks in the rich red dust.
The Martian surface, says Ms Lakdawalla, has proved to be far more punishing than anyone expected. NASA has characterised some of the surface conditions that Curiosity is traversing as like "sharks' teeth embedded in concrete".
That is more of a problem than it might appear because of the way the suspension system on Curiosity, and other rovers, works. If one wheel hits a rock or a pothole, the suspension distributes the load around the others. This has meant a wheel sometimes gets driven much harder into a surface feature than expected.
Getting a grip
That has left punctures on the thin skin of the wheels, creating holes that only get bigger as the craft moves around. The holes slow down Curiosity so it has to burn more power to travel about, which saps energy that might otherwise be used to gather data or send it back to Earth.
NASA is now working on the latest rover-wheel designs for the future; the next surface mission is due to launch in 2020 and clearly there will need to be some change. “They haven't settled on a new design yet; they're still testing,” says Ms Lakdawalla. “I doubt the materials will be different but the design will be.”
“It's not just about the holes,” she adds, “it's also about flotation on sand, which continues to be a maddening problem.” Curiosity, and all other rovers, have flanges or “grousers” on their wheels that are supposed to help them grip the surface in the low Martian gravity – about 40 per cent of that on Earth.
The red planet's surface has characteristics, apart from low gravity, that make getting around challenging
They need those because, according to Dr Terence Richards from the University of Cranfield, who has studied how different wheel types perform on Martian soils, the red planet's surface has characteristics, apart from low gravity, that make getting around challenging.
To begin with, he says, Mars is very dry. “That means the surface material tends to get less compact so it can carry less weight. Sinkage is a big issue. If your sinkage goes up, your rolling resistance goes up and then your power demands increase.”
Deformable wheels can help with this as they give a broader contact patch and help spread out weight. Get it wrong and your rover – and your mission – might be going nowhere.
“The one thing you do not want is to get stuck,” says Dr Richards.
The innovations NASA is putting into its rovers are not only seeing use on Mars. In 2015, it built a demonstration electric car, called the Modular Robotic Vehicle, which used some of the same technology. Its wheels were independently steerable and were controlled by a joystick – making it very easy to park. It used little power, could zip along at 70kph (44mph) and was controlled by computer. There is currently no sign of it becoming a production vehicle for use in everyday life on Earth, though it does show that wherever they are used, rovers are blazing a trail.