Mars rovers: the Red Planet welcomes careful drivers
Image credit: Nasa
Heat, rocks, dust (lots of it), scanty maps and nowhere to refuel. Driving on Mars is the ultimate extreme challenge.
In March 2010, with the Martian winter descending on it, the rover Spirit had one last chance to survive. It needed to reposition itself so that its solar panels could capture enough of the meagre sunlight to keep the batteries charged. But the rover was stuck in a deep patch of Martian dust, struggling to break free. “We turned the wheels and it got deeper and deeper in the sand,” explains Olivier Toupet, research technologist at Nasa’s Jet Propulsion Laboratory. “Eventually it wiggled itself free, but not in time to get the aspect it needed to recharge.”
Spirit’s companion Opportunity is still mobile but faces the continual danger of the fine, sandy dust that cakes the Red Planet’s otherwise rocky surface. It’s a permanent headache for scientists and a nightmare for rover drivers.
“Dust holds no scientific interest,” says ESA ExoMars project scientist Jorge Vago. “It has been moved around by the wind and sterilised by ultraviolet light.” There’s no life in the Martian topsoil; instead it’s a suffocating barrier to the drill system that is the European rover’s prime tool in its search for exobiology.
It is difficult to prepare for Martian dust, too. Toupet has driven three American rovers and says that, despite repeated efforts to simulate Mars using a combination of fine sand and chlorine-rich chemicals used for cleaning swimming pools, the team at JPL has “never quite managed to recreate Martian dust on Earth”.
What’s more, every day they drive on Mars, the rovers go somewhere no robot has ever been before. Toupet recounts an incident with the solar-powered rover Opportunity. “We had this little episode about a year ago where we entered a little valley. We always consult with soil-property scientists to make sure it’s safe. They said ‘no problem’ going through the ripples in the dust ahead, and then the rover seemed to get stuck,” he sighs. The rover backed out and survived to fight another sol, while the rover drivers marked a new no-go zone on their route across Mars.
As well as stopping rovers in their tracks, the dust can smother solar panels, robbing the electronics and motors of vital energy. And dust is far from the only danger. Extremes of temperature eat into the lifespan of your electronics. And the rocks offer little respite from the dust: sharp rocks push out of the surface with the ability to tear holes in the fine metallic wheels.
So, there is only one way to drive on Mars: carefully. Progress is always slow. On a really good day, a Mars rover driver can cover a distance of perhaps 100 metres.
Four rovers have made the journey to Mars and they are all American: Sojourner, Spirit, Opportunity, and Curiosity. Look at their wheel tracks in the soil and you see the robot equivalent of Aldrin and Armstrong’s boot prints on the Moon, proof of our presence and our pioneering efforts to explore this hostile world.
Soon to join those tracks will be those of ExoMars, the ESA-Roscosmos rover that is the first to directly search for signs of life, and Nasa’s Mars 2020 rover, which will collect samples for future analysis back on Earth. Both launch in July 2020.
The current king of Mars rovers is Curiosity, a car-sized nuclear-powered rolling laboratory, which descended to the surface from a rocket-powered sky crane. Yet even Curiosity is beginning to suffer.
“The skin on the wheels is very light, as thin as a sheet of paper,” says Toupet. “After a year of driving we started seeing holes and cracks in the wheels, some big enough to cause a problem.” Curiosity cannot turn and drive at the same time, so it stops to rotate its wheels left or right, and then drives forwards or backwards. If it turns while over a sharp rock, the wheel can be pierced. Nasa’s solution? “We were instructed to drive around the pointy rocks,” Toupet recalls.
The ExoMars team think they have the pointy rock problem covered. Its wheels are made from what Airbus Defence and Space engineer Paul Meacham describes as “bendable steel that’s coiled into the wheel, allowing it to compress”. ExoMars will also aim to land in an ancient river delta or basin where the environment should be more benign than the sharp outcrops of Gale Crater, where Curiosity roams. Nasa’s Mars 2020 rover gets around the punctured wheel problem by making them narrower and thicker.
Once you have landed on Mars, you have to decide where to drive. The reason why we take rovers in the first place is that “our present landing capability is not precise enough for us to target a region of interest with pinpoint accuracy” explains ESA’s Vago. Since there are no high-resolution maps of Mars, the rover has to have enough autonomy to be able to avoid obstacles it detects in its path. JPL’s Toupet and his colleagues use similar technology to that used in driverless cars on Earth, albeit operating at a far slower rate.
The rover scans the Martian terrain ahead and creates a 3D image from which the route planners at JPL set its commands accordingly. The rover then moves forwards, and after one metre of travel it takes another measurement to work out if it has gone off course or not. The ExoMars rover will have more autonomy, using its NavCam instruments and software to drive towards the target the scientists have selected. “It’s able to recognise the slopes that are too steep, the rocks that are too big: they are classified as ‘forbidden zones’,” says Airbus’s Meacham.
With each generation of Mars rover our ability to drive on the Red Planet improves. But still, the difficulties engineers and scientists face persist at the most basic level. Toupet provides an example: “You never know exactly where the rover is, so you may think a wheel is clearing a rock, when in fact it’s next to the rock.” Next to a rock isn’t necessarily better than on top of it, as the rover may be about to dig itself into the sandy dust.
“One thing we’ll do is constantly monitor wheel slip versus expected travel distance,” says Vago. “A slip ratio higher than 80 per cent indicates a risk that the rover may get stuck.” What do you do then? “Drive backwards and seek another path.”
En route to a promising outcrop or ancient riverbed that the scientists would like to sample or photograph, the rovers will repeatedly check, measure, and re-check where they are. The ExoMars rover has a mirror on board to look underneath the chassis and to help it avoid getting stuck. It will also use wide-angle cameras on its mast and fixed cameras on the base to take images every 10 seconds, and compare them to what was seen before. Basically the rover feels its way forward, making its own map as it goes along.
The entire process is painstakingly slow. Curiosity, with its larger wheels, is capable of 4.2cm per second, while the 300kg ExoMars will aim for just one or two centimetres per second. Mars is roughly half the diameter of Earth, and while an average family car could easily circle our planet ten times with nothing but routine maintenance, a Mars rover is never designed to travel far. The ExoMars team at Airbus DS are briefed to build a machine that will cover 4km in its lifetime.
What else could possibly go wrong? In the film ‘The Martian’ Matt Damon’s rover vehicle topples over, and indeed that is a fear of real-life Mars rover designers. “If it attempted an obstacle that was too large for it, that would be the end of the mission. There’s no self-righting mechanism on the ExoMars rover,” says Meacham.
In National Geographic’s Mars series, the crew breaks a rover’s suspension by driving too fast. That won’t happen in real life, as even 10km/h is what Vago describes as “breakneck speed”.
To deal with problem of power-sapping dust build-up on the solar cells, the ESA-Roscosmos team is making the photovoltaic array on ExoMars larger.
Nasa has learned a few tips over the years. Taking care to park its veteran Opportunity rover tilted towards the Sun to maximise recharge, experience has demonstrated that a few times a year small whirlwinds on Mars will come along and blow the dust away.
An alternative answer to the power problem is is to bring it with you, as Nasa does with Curiosity and Mars 2020. They are equipped with a system that generates electricity from the heat of plutonium’s decay. Not everyone agrees with the ethics of this approach, but it does increase the chances of success.
The dust storms that can engulf all of Mars often play havoc with Hollywood’s human colonists, but in reality the atmospheric pressure is so low that winds of 100km/h would feel like a gentle breeze, and wouldn’t harm any of a rover’s components. Mission planners are also alert to the fact that dust storms appear mostly in winter, and Earth’s rovers land in spring, so the nominal mission can be completed before the bad weather comes along.
Even in summertime, though, Mars is exceptionally cold, and that eats into your rover’s lifespan. “For me the biggest engineering challenge is the temperature: 0 to +10°C by day but as low as -120°C at night, every night,” explains Professor Andrew Coates, whose Mullard Space Science Laboratory in the UK is behind the PanCam and NavCam instruments on ExoMars. Those low overnight temperatures inform every decision about the materials used in the rover, with each component being tested far beyond normal space standards to be sure it can survive.
Overall, the life of a Mars rover is a cold slog at glacial speeds across unknown terrain far from help. But those that have made it have been remarkably successful: despite its dusty demise Spirit lasted 2,210 sols and drove 7.7km; Opportunity is still going strong after 4,600 sols and 44km covered. Curiosity has driven 15km over the course of 1,600 sols. They have explored just a tiny fraction of the planet, yet far exceeded expectations already, and they have proved that with meticulous planning, careful operation and ingenious design, it is possible to drive successfully on Mars.