An army of inflatable, spherical robots might one day roll around on the Martian surface. Engineers who have designed the lightweight probes say they could economically explore vast regions of other planets.
“Our inflatable rovers are lightweight, travel great distances, use very low energy and will be fairly cheap,” says Fredrik Bruhn of Ǻngström Aerospace in Uppsala, Sweden, who initiated the idea that has now been developed by a team of engineers. “One battery charge will let such a rover travel around 100 kilometres.”
While researchers have proposed rolling spherical rovers before, no-one has previously suggested making them inflatable. With funding from the Swedish National Space Board, Bruhn’s team has designed an inflatable version with a diameter of just 30 centimetres when it gets pumped full of xenon gas from an internal cartridge on landing.
Crucially, when deflated and stowed, the inflatable occupies just half the volume a wheeled rover with equivalent sensing instrumentation would require, says Bruhn.
The technology would allow the descendents of NASA’s Spirit and Opportunity Mars rovers to carry mini-rovers of their own – dispatching them to scout out new sites of scientific interest.
‘Groundbot’ in tests
Bruhn is confident the idea will work because a larger (though not inflatable) version of the spherical robot technology, made by Rotundus of Stockholm, is already up and running. That company’s spherical ‘Groundbot’, is currently being tested for use in security and surveillance applications by the defence division of Saab.
“It can happily patrol large areas like harbours, travelling on snow, sand or gravel – it doesn’t matter what the surface is,” Bruhn told New Scientist. “As a sphere, only a single point ever touches the ground, so the friction is close to zero. It’s the most energy-efficient geometry for roving on unpredictable surfaces.”
The inflatable version of the robot would comprise an inflatable shell made of polyaryletheretherketone, an ultra-strong plastic commonly used in space flight applications that’s capable of withstanding high temperatures. Inside it, a hollow metal axle stretches from one side of the sphere to the other, supporting all of the rover’s electronics on a pendulum that hangs off it.
The pendulum is crucial because it is the sphere’s drive mechanism. When a motor forces the pendulum’s hanging mass to move forwards, the sphere rotates to meet the change in the centre of gravity. To steer, the pendulum can be moved sideways too.
The ball gets its energy from thin-film solar panels on some of the hexagonal panels making up its soccer-ball-like surface. To keep them clear of dust, an ultrasonic cleaner on the inside vibrates the shell, shaking it off.
Atmospheric sensors, cameras and grippers are stashed inside the hollow axles – the pendulum can allow the ball to lean sideways to sample the ground. In addition, the surface of the ball is coated with electrodes that are used to sense ground electrical properties like conductance and resistivity.
The inflatable rover then reports back to its orbiter or master rover by radio on any potentially interesting terrain it finds.
Bruhn’s design team, which includes engineers from Uppsala University and NASA’s Jet Propulsion Laboratory in Pasadena, California, US, envisages two landing scenarios. In one, a lander or large rover can land as normal and then inflate and deploy the spherical scouts when they are needed.
“They would make a very good complement to a wheel-based robot, travelling at up to 30 kilometres per hour on sand and loose, gravelly solid surfaces – and giving scientists many more chances to see where the best science can be done,” says Bruhn.
In another, less likely scenario, an orbiter arriving at Mars could inflate a flock of rovers and dispatch them into the atmosphere, with a heat shield to protect them from atmospheric heating.
The inflatable was originally dreamt up as part of the European Space Agency’s BepiColombo mission to Mercury in 2013, which was to have included a lander that would have explored the planet’s cooler polar regions. But the lander was cancelled.
Neither is there a berth for the inflatable on ESA’s ExoMars rover, also launching in 2013. Bruhn’s team is waiting to find out if ESA has another slot: “ESA is a slow animal,” he says.
Journal reference: Acta Astronautica (in press)