Japan's Hayabusa is on its way home after its rendezvous with Itokawa. Credit: Akihiro Ikeshita / Japanese Aerospace Exploration Agency

Yesterday in the south central Australian village of Woomera, Japanese scientists, with US and Australian colleagues, celebrated news from their Hayabusa spacecraft — the first round-trip space mission to an asteroid. Successful manoeuvres that day had put the spacecraft on course to parachute its recovery capsule into the nearby desert on 13 June. Anticipation will continue to build until the capsule's lid is opened, offering, they hope, the first peek at asteroid dust.

Scientists anticipate that such samples will provide greater knowledge of these 'little planets', shedding light on the beginnings of the Solar System, the origins of life on Earth (see 'Asteroid ice hints at rocky start to life on Earth') and the connection between asteroids and the tens of thousands of meteorites that have been found on Earth. "It could establish a bridge for the first time," says Michael Zolensky, a NASA scientist who will analyse some of the Hayabusa samples.

The original aim of Hayabusa was as a technical mission to test various engineering technologies, such as ion-thrust engines and autonomous navigation systems, needed for landing on an asteroid and returning to Earth. The spacecraft, launched in May 2003, landed on the 535-metre-long Itokawa asteroid twice in November 2005.

The mission is lucky to get this far, says Hajime Yano, a Hayabusa project scientist with the Japan Aerospace Exploration Agency (JAXA). After the spacecraft landed on Itokawa a second and final time, a massive fuel leak resulted in loss of communication for several months. Hayabusa's engines also experienced a range of problems, and its return was uncertain until 8 June, when its faltering thrusters completed the last necessary trajectory correction. "It's like a critical patient who can't walk well," says Yano. "Up until yesterday, anything could have happened."

Mission accomplished

Although three years behind schedule, the mission has achieved its major goals, and has already yielded scientific data about Itokawa's dimensions and composition, which will provide a benchmark for future asteroid studies.

The remaining mission goals are retrieving the sample-return capsule from the desert and analysing any samples within it. The capsule is a disk-shaped aluminium container about 40 centimetres in diameter, and is scheduled to separate from the craft on 13 June before entering Earth's atmosphere. "From their perspective, it's all gravy," says Scott Sandford, another NASA scientist working on the project. "Everything we're getting now is a bonus, but to me it's the most interesting part."

From their perspective, it's all gravy. ,

The incoming capsule has a radio beacon to signal its location, and infrared-detection devices on helicopters will also be used to find it. Even if these fail, monitoring stations will track the burning wreckage of Hayabusa through the atmosphere as a guide to the trajectory of the nearby capsule.

JAXA scientists will retrieve the capsule on 14 June, and by the end of the week, they hope to have it back in their laboratory in Sagamihara, just south of Tokyo. There they will first take X-ray images to estimate the amount of dust picked up — if any.

High hopes

Zolensky says they were originally hoping to have "spoonfuls, several grams of sample", made up of "chips like peanut-size grains". But sample collection didn't go as planned. Pellets designed to hit Itokawa to dislodge loose fragments seem not to have fired. "It's very likely we won't be able to see anything with the naked eye," says Makoto Yoshikawa, also a Hayabusa project scientist at JAXA.

Still, the scientists have high hopes. A hard impact on Hayabusa's first landing attempt on the asteroid is likely to have kicked up dust, says Yoshikawa. Hayabusa sat on the surface for 30 minutes after that. Zolensky agrees: "Just the fact that it landed should cover it in dust. The astronauts on the Moon got covered with dust just walking around. The mission team might not get grams, but they'll have something. With today's technologies, even microbe-sized particles should be enough."

On the basis of the X-ray results, the researchers will decide when and how to open the lid and distribute the contents between analysis and storage. Any dust that is found will be analysed over the next six months as scientists look at oxygen isotopes, helium content, water traces, and other clues that might provide insight into Itokawa's history. "We want to look at its elemental composition and structure," says Yoshikawa. "When this asteroid was born, what kind of matter was around? This will tell us about Earth and the Solar System."