Japan's latest space mission seems to have succeeded in its second attempt to collect pieces of a small asteroid. If so, this will be the first time a sample has been collected for return to Earth from any object in the Solar System apart from the Moon. However, engine trouble casts doubts on whether the craft can return home safely.

On 25 November, the Japan Aerospace Exploration Agency (JAXA) said data sent from Hayabusa show that all stages of the sampling process went well. Agency engineers said it was almost certain that Hayabusa's sampler had touched down on the Itokawa asteroid as planned, and shot two metal pellets into the rock to throw up fragments of the surface. “I think we collected a sample,” said project manager Jun'ichiro Kawaguchi.

Hayabusa was launched in May 2003, and arrived this September at Itokawa — a potato-shaped, 540-metre-long asteroid located about 300 million kilometres from Earth. Hayabusa is expected to leave Itokawa by early December for its return journey. Whether a sample was definitely collected will not be known until the craft reaches Earth in the summer of 2007.

To reach this stage, the members of the Hayabusa team have endured a rough time. Two of the craft's three reaction wheels, which stabilize the probe and help navigation, stopped working; the first failed in July and the second in October. Chemical engines on board were used instead. But their lower accuracy made landing more difficult. In the first landing attempt, on 20 November, Hayabusa seemed to park on the asteroid's hot surface for more than 30 minutes, and failed to collect a sample.

Hayabusa snaps its own shadow as it descends. Credit: JAXA

Time and fuel were running short, so the 25 November try was almost the last chance to collect a sample. “It was learning in real time,” says Donald Yeomans, US project scientist for Hayabusa and senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. With each attempt, he says, “they learned more and more about how the spacecraft behaves”.

Still, concerns remain. Hayabusa lost its balance soon after departing from the asteroid. Engineers are investigating the cause, but one possibility is that the craft's long stay on Itokawa's hot surface during the first landing attempt damaged one or more of its 12 chemical engines. JAXA said that if the trouble could not be fixed, it would be difficult for Hayabusa to return to Earth.

But astronomers have praised Hayabusa's achievements so far as showing the way for future asteroid missions — especially those involving the operation of ion-propulsion engines and delivery of high-resolution images. Analysing the sample, assuming it makes it back to Earth, would also help to answer questions about how the Solar System was created.

The mission is renewing Japan's confidence in space activities. JAXA has recently tried a string of high-risk missions, but has seen many failures over the past few years. “Hayabusa's success has become a tailwind for Japan's space development,” Hajime Inoue, JAXA's executive director, said at a press conference. “It proves that the way we have been doing things wasn't wrong.”