Scientists at the the US space agency, the National Aeronautics and Space Administration, are now trying to get one of the Mars rovers out of sand, where it has been stuck for a while.

This is rather like some great video game, because the rover is stuck in sand on Mars and people are sitting on planet earth, wiggling joysticks; well, actually not quite – it is much more complex than that.

Two Mars explorers named Spirit and Oppurtunity were launched some time ago. They have turned out to be spectacu- larly successful. Their orignial mission design life has now lasted more than 22 times longer than planned.

This is amazing.

Further, the original plans were for the two rovers to roam around reasonably close to where they landed. They did all that was originally planned, and were still going strong, so the mission planners decided to be courageous and send the two rovers on long-ranging missions much further from the landing sites. All this has been most succesful.

Then the right front wheel of Spirit broke. After some head scratching, the scientists decided to keep going but to let Spirit drive backwards so that the craft could drag the inoperable wheel.

That worked fine until the rover drove onto a thin crust of sand, which then broke, allowing the rover to sink into some bright-toned slippery soft sand. After a few attempts to move the rover, it sank deeper.

Compare this with driving your car on the beach and your wheels sink into the sand. Usually, the driver tries to get out, only to sink a deeper. Then you get out and find some sticks or stones to push under the wheels to pro- vide traction, and then try again.

But on Mars there is nobody to put some stones under the wheels. The exercise of trying to get the rover out of the sand then becomes a major mathematical and physical problem, and not just a video game.

What they have to do, for example, is to calculate the sun angle so that, when the scientists switch on the electric motors of the rover, the sun is providing maximum electricity to the solar panels, which are the only energy source for the rover.

They also calculate the temperature of the sand to determine a time of day when the sand will give most traction.

They will also devise routines for rocking forwards and backwards, or whatever they eventually figure out to do. Most important of all, they will think very carefully before making any decisions, because they do not want to make the situation worse.

The scientists decided that the first command to be sent was an order to rotate Spirit’s five working wheels for- wards by six revolutions. Data only returns a day later because of the distance from the earth to Mars, so the whole thing is a slow exercise.

Data shows that Spirit is straddling the edge of an 8-m-wide crater that was filled in long ago with sulphate-bearing sand, produced in a hot water environment, so it is, scientifically, an interesting piece of ground. The material churned up by the wheels of Spirit have the highest sulphur content of any material measured on Mars.

The crater is filled with different colours of sand, which is all capped with a crust like a crème brûlée. It was this crust that the rover’s wheels broke through. The first straight-ahead plan devised was to get the rover’s centre of mass past a rock underneath the craft, so one can appreciate how much mathe- matical thought has gone into the ‘video game’.

Spirit and Oppurtunity have explored Mars for five years, which is amazing. Let us hope that some more productive life can be squeezed out of this project.