Japan's problem-plagued Hayabusa spacecraft has enough power to make it back to Earth, tests on two of its four ion engines have revealed.
If the craft does return as planned in 2010, researchers would finally find out whether it collected the first-ever samples from an asteroid during its two landings on the tiny space rock Itokawa.
Hayabusa was meant to collect the samples by firing pellets into the surface of the 535-metre-long rock and scooping up the resulting debris. But data from two landings in November 2005 suggest the pellets never fired because the craft's onboard computer sent conflicting signals to its collection instruments. Still, mission officials hoped to bring the spacecraft back to Earth in case some asteroid dust had slipped into its collection chamber by chance.
But hope for the spacecraft's return faded after a fuel thruster leak in late November sent it spinning out of control. When some contact with the craft was restored at the end of January, mission controllers realised all of the fuel in the 12 chemical thrusters had seeped away.
The chemical thrusters were already compensating for the loss of two of three stabilising reaction wheels a few months earlier, so after the leak the craft had to rely solely on its four ion engines, both to control its orientation and provide the thrust to make the journey back to Earth. The engines ionise xenon gas and then use electric fields to accelerate the ions, providing a steady - though weak - thrust.
In good shape
But mission managers were worried that the fuel leak and resulting spin had damaged the engines. Now they have tested two of them and found they are "in very good shape", says mission manager Jun'ichiro Kawaguchi.
A third engine will be tested in January 2007 when the spacecraft's orbit takes it closer to the Sun, making it less prone to damage from the cold of space. The fourth engine will be reserved as a spare and will therefore not be tested.
Normally, three of the engines are used together while the fourth is kept as a backup. But Kawaguchi says: "The return cruise is possible even with two engines." And he says there should be enough fuel for the journey.
In fact, the spacecraft's spin rate was recently lowered to one rotation every five minutes in order to conserve the xenon gas, which - in neutral atomic form - is being vented to control the spacecraft's orientation.
Kawaguchi says mission controllers are having no problems communicating with or operating the spacecraft. However, they are not in the clear as yet: "The spacecraft was seriously injured and operating it and making it resume cruise are still very hard things."
Mission planners hope to begin the return trip in March 2007, when the distance between Earth and the asteroid is once again at its minimum. After a three-year journey, the spacecraft would drop a capsule in the Australian outback in June 2010.
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