EUROPE鈥檚 Smart-1 spacecraft, which is en route to the moon, has an engine problem that could leave the vehicle stranded in space. Engineers at the European Space Agency are hard at work on software they hope will rescue the probe, and plan to transmit it to the spacecraft next week.
Smart-1 is powered by an ion engine, which produces thrust in one direction by accelerating xenon ions in an electric field in the other direction. Although this creates only about the same force as the weight of a postcard, the engine works continuously, gradually increasing the size of the spacecraft鈥檚 elliptical orbit until it is captured by the moon鈥檚 gravitational field, a process that takes 15 months (see Graphic). Although this is not the most direct route, the engine is extremely fuel-efficient, and ESA is testing it out for a mission to Mercury.
Soon after the spacecraft was launched, the engine started switching off repeatedly. The spacecraft鈥檚 circuitry is sensitive to strikes from high-energy protons from the sun, which generate rogue voltage spikes. Engineers routinely build capacitors into circuits to mop up any voltages induced this way. But after launch, the team found to its dismay that the mopping up feature had been omitted from some key circuits. Each time a high-energy proton hits a particular optical sensor, it generates a spike that causes the on-board computer to switch off the engine, called a 鈥渇lameout鈥�.
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The problem has been exacerbated by the high radiation levels the spacecraft experienced while passing through the Earth鈥檚 radiation belts, and by the increased solar activity in recent months. 鈥淚t鈥檚 probably the worst radiation environment we have ever experienced on a mission,鈥� says Anders Elfving, the engineer who led the vehicle鈥檚 design team. The engine has flamed out 18 times since the mission began in September.
A flameout is usually just a minor headache, as ground control can switch the engine back on next time it contacts the craft. But Smart-1鈥檚 unusual trajectory makes the problem more serious. The spacecraft鈥檚 elliptical orbits must be controlled so the craft reaches its furthest point from the Earth at exactly the same time as it makes its closest approach to the moon. This 鈥渞esonance鈥� gives the vehicle the extra tug it needs to complete its journey.
This is a delicate operation. 鈥淭here are three or four crucial points in the mission when if the engine flames out, the spacecraft will not achieve resonance,鈥� says Luca Stagnaro, the mission鈥檚 software specialist. 鈥淚f we miss two or three, we will not have enough fuel to make it to the moon.鈥�
The team has come up with a fix, however. The spacecraft is controlled by the most powerful computer ever to fly on a European spacecraft, and late last year Stagnaro began writing a program that would allow the computer to spot flameouts and switch the engine back on at once.
Mike McKay who manages the spacecraft鈥檚 ground operations from the European Space Operations Centre in Darmstadt, Germany, is confident this will do the trick: 鈥淚t鈥檚 taken a bit of detective work, but we鈥檝e characterised the problem and we know how to fix it.鈥� But much depends on Stagnaro鈥檚 solution. The team first attempted a software fix in December, but the program overloaded the computer. Now Stagnaro has developed a streamlined version, to be uploaded to the spacecraft next week.