A small hopping robot meant to explore the asteroid Itokawa was lost in space after being released from Japan's Hayabusa spacecraft on Saturday. But mission officials say the main probe will still try to land and collect samples of the space rock at least once before beginning its return flight to Earth in December 2005.
The 10-centimetre-tall robot, called Minerva (Micro/Nano Experimental Robot Vehicle for Asteroid), was designed to hop around the 600-metre-long rock, snapping close-up images with three cameras and taking temperature measurements of the surface.
It was originally to be released on 4 November while Hayabusa was approaching the asteroid in a test of its "autopilot" landing system. But this "dress rehearsal" was cut short because mission controllers could not accurately guide the spacecraft using its fuel thrusters - a contingency solution devised following the failure of two of the craft's three stabilising reaction wheels.
Now a similar problem on a rehearsal descent on Saturday has prevented the robot from ever reaching its target.
Breakdown in communication
In the early phases of the spacecraft's descent, mission officials had used height readings from an onboard laser altimeter and the craft's speed to estimate when Hayabusa would be at Minerva's release point, 70 metres above the asteroid.
But the slope of the asteroid's surface had apparently caused the altimeter to misjudge those first estimates of the craft's altitude. So mission officials were shocked when later readings revealed the spacecraft was actually much closer to the asteroid than anticipated.
"As soon as we realised we were below 100 metres, we decided Minerva should be separated," project manager Jun'ichiro Kawaguchi told New Scientist.
Unfortunately, that point came when the main data link with the spacecraft was being switched from an antenna in Japan to one in Australia. During this 40-minute antenna change, information about the spacecraft's vertical motion was unavailable to ground controllers.
So when they sent the release command, they did not realise the spacecraft had just fired its thrusters to maintain a minimum height above the asteroid - an adjustment it performed every 10 minutes or so during the descent.
The command took about 16 minutes to travel from the Earth to the spacecraft, and when it arrived, the thruster firing had raised Hayabusa's altitude to about 200 metres. So Minerva was released while its mother ship was moving away from the asteroid at about 15 centimetres per second - faster than the space rock's 13 cm per second escape velocity - and the robot simply drifted into space.
"This is [our] responsibility," says Kawaguchi, who says officials simply did not account for all possible mission scenarios during the antenna changeover. "Our readiness was not so complete."
Nonetheless, he says the mother ship's practice descent was a success. It approached the asteroid to within 55 m and its laser range-finder - which had never before been tested - successfully helped the craft maintain its orientation with respect to the asteroid's surface.
Still, he admits that mission controllers do not fully understand how to deal with the spacecraft's motion after the periodic thruster firings. So rather than attempt two sample collection landings as planned on 19 and 25 November, officials may decide to do another rehearsal descent on 19 November.
"We may end up cancelling the second sampling," says Kawaguchi. During the sample-collecting rendezvous, the spacecraft will attempt to briefly touch down on an expanse of fine dust in the middle of the asteroid called MUSES-Sea, fire a pellet into the surface and scoop up the resulting debris.
After gathering its sample, it will leave Itokawa in December and then drop the first-ever asteroid samples back to Earth, over Australia, in July 2007.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
Have your say
Only subscribers may leave comments on this article. Please log in.
Only personal subscribers may leave comments on this article