How do you drive a rover on Mars?
The latest rover to land on Mars, Curiosity, represents a huge leap forward in technology for exploring the planet. At 875 kilograms and ten feet long, it is about twice as long and five times as heavy as NASA’s twin Mars Exploration Rovers, Spirit and Opportunity, which launched in 2003.
Landing, powering and controlling such a complex and sophisticated machine from hundreds of millions of miles away represents a big challenge (especially if you want to avoid this happening).
The first challenge is the landing – we’re used to seeing footage of rovers ‘crashlanding’ into the surface of Mars, and bouncing around in a protective tetrahedron of air bags, before unfolding themselves. This is how Opportunity and Spirit were both landed back in 2004.
But Curiosity, being much larger and heavier, needed to land with much more precision. The spacecraft has been designed to steer itself during descent through the atmosphere, with the rover attached to its underbelly. Slowing itself down with parachutes and retro rockets, it lowered the upright rover on a tether onto the surface of Mars, touching down at about 6.30 am on Monday morning, UK time.
Even with meticulous planning, it made for nervous viewing at NASA:
NASA’s Jet Propulsion Laboratory in Pasadena has engineered Curiosity to be able to roll over obstacles up to 65 cm high and to travel about 200 metres per day. But, as Caltech’s John Grotzinger explained during a 2010 Shell Lecture at the Society, sometimes things can still go wrong. Speaking about the Opportunity rover, he said;
‘We woke up one morning and realised we were bogged down in the sand. So the engineers used a simulation with a rover on earth and did a series of experiments. The rover drivers predicted that if they did this, this and that, after 200 wheel revolutions the rover would just lift up in the sand and drive away. This took about six months to work out, we waited nervously, then we started driving it. After about 220 revolutions the rover popped out, and off we went.
They put a speed limit on us after that.’
Opportunity was intended to last for ninety Martian days, which it spent exploring a small crater it had landed in.
‘When we landed, bouncing along in the tetrahedron, we rolled into a small crater, and spent the next sixty days looking at the rocks there.’
‘We were worried that after ninety days, something bad would happen. So we raced over to a larger crater…hoping there would be more to look at.’
‘We reached the crater, spent almost a year there, came back out..everyone was worried the rover was going to die. So we drove as fast as we could, – at one point more than 200 metres in one day – the engineers were trying to see how fast we could go.’
It was at that point the rover got stuck in sand dunes, but after escaping, it just kept on going.
‘The power source comes from solar power. Originally, we thought dust would settle and our power would slowly dissipate until the onboard computer couldn’t keep warm anymore and you’d have massive system failure.’
‘Once a year on Mars it gets really dusty. Dust storms encircle the entire planet. But it turns out that after the dusty season there’s a windy season that generates a lot of dust devils. They go flying by the rover, and they don’t tip it over but they clear away the dust. So the rover emerges from the dust storms and we have as much power as we did at the start of the mission.’
After eight and a half years, Opportunity is still exploring the Martian surface, carrying out experiments and sending results back to Earth.
So will Curiosity manage to outlive its own life expectancy by such a huge amount?
Probably not. Being nervous about the issue of dust, NASA had already designed the next rover to have a nuclear power source – a much more reliable form of power. This is expected to last for two Earth years, producing electricity from the heat of the radioactive decay of plutonium-238. During that time, Curiosity will carry out sampling and analysis of rocks by drilling holes into them, even vaporising them, to determine their composition.
The results will be transmitted back to Earth using radio relays via Mars orbiters, taking about fifteen minutes to reach us.
‘With every rover we send, the technology gets more sophisticated’ said Grotzinger back in 2010. The next stage will be a rover which is capable of collecting samples and bringing them back to Earth. That could happen in the next decade, if proposals from NASA and ESA go ahead.
‘That’s an even bigger challenge. To land hardware on the surface of mars that must then take something from the surface, escape its gravitational pull and bring it back to earth. It’s a natural precursor to human exploration.’
It will be a long time before that step is taken – but you can’t help wondering if the first human on Mars might bump into the Opportunity rover, still crater hopping its way across the planet.