In preparation for the European Space Agency’s ExoMars mission to the Red Planet in 2018, the agency’s engineers are playing in a large sandpit in Noordwijk in the Netherlands. But they aren’t using plastic buckets and spades.
The 90-square-metre playpen, known as the Planetary Utilisation Testbed is filled with sand, soil, gravel and rocks designed to recreate the look and feel of the Mars surface.
The ESA engineers are using the area to test the sense of direction of a prototype of the six-wheeled ExoMars rover. The final version will have to travel across the surface without the aid of a map, as well as to drill 2 metres beneath the Martian surface in search of life. Ensuring the rover can look after itself is vital to the mission’s success.
A ring of eight Vicon infrared cameras – designed to capture human motion for game- or moviemaking – record the rover’s every move in three dimensions, tracking six reflective markers attached to its chassis.
These records of the rover’s motion are compared to what its onboard software “thinks” has happened; revealing in detail how well the rover manages when exploring unaided, as it will have to do on Mars. The several minutes it takes for signals to reach the planet from Earth rule out it being driven remotely.
“We have to rely on the intelligence and autonomy of the systems we have onboard,” says Pantelis Poulakis, a robotics engineer working on the rover.
As it rolls over the ground, the rover attempts to work out its changing position by combining information from its stereo panoramic cameras, gyroscope, accelerometer, sun sensor and other instruments.
Like NASA’s Spirit and Opportunity rovers, ExoMars will use stereoscopic views to create a 3D representation of its surroundings, allowing route-planning algorithms to plot a course around obstacles. This process is repeated every 10 metres, requiring ExoMars to stop for 30 seconds while its computers churn away to add a new section to its internal map and plan its next steps.
Spirit and Opportunity do the same thing and must stop for nearly two minutes to plan each move, but ExoMars will carry “substantially more” computing power, says Anders Elfving, ESA’s rover manager on the ExoMars project. NASA’s rover can manage 40 metres in a day, but Exomars will be capable of 100 metres, he says.
The team has also been working on a system that checks the rover’s progress as measured from its 3D models against its distance-measuring odometer, which works like that in a car. Any mismatch suggests the rover is spinning its wheels without moving, perhaps bogged down in a sand trap.
NASA’s rovers were not launched with that capability and Opportunity was almost stuck forever after spending a whole day spinning its wheels in loose ground, says Elfving.
NASA has patched its rovers’ software to improve their ability to avoid such problems, but ESA thinks its rover can be even safer as it travels on Mars.
More on these topics: