Spacecraft on course to score a first with asteroid samples

Japanese probe poised for collect-and-return mission.

Japan's most ambitious space mission entered a final, critical phase this week as the Hayabusa spacecraft parked itself 20 kilometres from the asteroid Itokawa. If all goes to plan, the mission will be the first to return samples of an asteroid to Earth.

The US$100-million Hayabusa mission is primarily a test of technology, and will carry out “a lot of firsts for the Japanese”, says Andrew Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Cheng is one of several Americans on the project's science team.

A Japanese-built ion propulsion system delivered the craft to the 600-metre-long asteroid, and Cheng considers this a success in itself. A strong solar flare in 2003 degraded the spacecraft's solar panels, reducing the power output of the ion drive engines and causing a three-month delay in reaching the asteroid. One of three reaction wheels that help Hayabusa to hold a steady position has also failed, although project officials say the effects on the observing programme will be relatively minor.

More hurdles lie ahead. After mapping the asteroid's surface with cameras, and X-ray and infrared spectrometers, Hayabusa will move in for a closer survey. In November, it will make brief contact with Itokawa, firing tantalum ‘bullets’ into the asteroid, then capturing the material that flies up in a horn-like collecting tube. Scientists hope to gather a gram or more of material from two or three locations.

On target: the Japanese craft Hayabusa (top) will collect material from the asteroid Itokawa (above) and return it to Earth.

Just before firing the first bullet, the craft will drop a tiny ‘hopper’ called Minerva on to the asteroid.This cube, equipped with cameras and thermometers, will flip around the surface like a tiddlywink in the low gravity. Finally, in December, Hayabusa will head home, its samples tucked inside a hermetically sealed capsule that will land near Woomera, Australia, in 2007.

Cheng praises the mission as “very innovative”, and says the scientific pay-off should be high if all the technology works as hoped. No spacecraft has ever visited such a small asteroid. Ground observations suggest that Itokawa, which was only discovered in 1998, is an S-class asteroid similar in make-up to a class of stony meteorites known as chondrites.

The samples from Hayabusa will be compared with telescope observations and meteorites on Earth. So far, scientists have matched only one meteorite to an asteroid source — Vesta — and even that link is debatable, says Michael Zolensky of NASA's Johnson Space Center in Houston, Texas, who will help to analyse the samples that come back. Having a piece of a well-studied object such as Itokawa will give researchers greater confidence in making assumptions about asteroids based solely on spectral analysis from Earth.

Once the samples arrive they will stay in Japan for a year, where a team of Japanese, US and Australian researchers will have first crack at them. The samples will then be shared with other international researchers, and some 10% will be turned over to NASA permanently.

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Reichhardt, T. Spacecraft on course to score a first with asteroid samples. Nature 437, 306 (2005). https://doi.org/10.1038/437306b

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