Space exploration: Flight of the Falcon

A Japanese vessel is due to become the first to land on an asteroid - and what it finds may save us if Earth is threatened. Peter Bond reports
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Planetary scientists around the world are watching enthralled as a dramatic saga unfolds in the depths of space. More than 180 million miles from Earth, a small, relatively inexpensive Japanese spacecraft named Hayabusa, or "Falcon", is attempting to make history by capturing samples of a tiny asteroid.

In a $170m (£98m) mission, Hayabusa is designed to become the first craft to touch down on an asteroid, seize some fragments and bring them back to Earth. However, such milestones are rarely achieved without a few nerve-racking moments. On 4 November, an unexpected "anomalous signal" brought the dress rehearsal to a sudden halt. A second dry-run on Saturday was rated a success - although a small lander, Minerva, failed to reach its target, raising questions about whether the Falcon has bitten off more than it can chew.

So what is Hayabusa doing up there? Samples of cosmic material are not too hard to find. Since time immemorial, people have been picking up meteorites that have fallen from the sky. Most are unremarkable chunks of stone or metal, although some of the rarer examples contain organic compounds, water and tiny diamonds. In recent years, a few dozen specimens that originated on the Moon and Mars have also been recognised.

Unfortunately, although the ages and compositions of these meteorites can be measured, there is no way to determine their precise source regions on the Moon and Mars. They all seem to be leftovers, billions of years old, from the formation of the planets. Most of them are thought to be fragments of larger, rocky bodies - asteroids - that have been chipped away during innumerable collisions. But there is no solid proof that this is the case.

"At present, we can only infer that meteorites came from asteroids," says Monica Grady, a meteorite expert at The Open University. "There is no proven asteroid match for the most common type of meteorite." The importance of "ground truth" was demonstrated by the rock-filled boxes brought back from the Moon by the Apollo astronauts more than three decades ago. Each sample was meticulously photographed and recorded by the Moon walkers before it was dropped into the sample bags. This made it fairly straightforward for scientists to piece together the history of each landing site over the past 4 billion years.

However, rocks on the Moon and Mars have been altered by huge impacts, volcanism and melting. In contrast, small asteroids may contain pristine material that has changed little since the birth of the Solar System. A soil sample from an asteroid can provide clues about the raw materials from which the planets formed and the conditions that prevailed in the cloud of gas and dust that surrounded the Sun at that time.

The importance of the Hayabusa mission lies in its potential to examine a particular asteroid from close range and then bring back pieces for analysis in terrestrial laboratories. Hayabusa's target, known as Itokawa, was chosen because is thought to be one of the ubiquitous S-type (stony) objects that populate the inner Solar System.

This celestial fossil also seems to be a fairly typical representative of the small, rocky objects that regularly cross Earth's orbit. Indeed, calculations suggest that it may collide with our planet within the next million years. By learning more about the density and internal structure of this potential weapon of mass destruction, we may be providing ourselves with a form of insurance against future devastation.

Hayabusa's measurements of Itokawa's density have already indicated that the asteroid may be nothing more than a loose pile of rubble, held together by gravity. "One of the things we need to find out is its coherence," says Monica Grady. "We don't know the overall hardness of asteroids or how easy it would be to deflect them if they were on a collision course with Earth."

Hayabusa has already overcome many obstacles on its expedition. Launched on 9 May 2003, the spacecraft used its fuel-efficient ion engine to spiral out slowly towards its target. Hit by a solar flare that slightly degraded the efficiency of its solar panels, Hayabusa's arrival at the asteroid was subsequently delayed from mid-summer until September 2005. It has also lost two of the three reaction wheels that are used to maintain its attitude and orientation.

Despite these setbacks, the spacecraft has successfully spent the past two months hovering over Itokawa, taking pictures and measurements of the 1,800ft-long "deformed potato". One particular surprise has been the plethora of large boulders, with the occasional smooth, dusty plain that offers a safe landing zone.

This month, the spacecraft was scheduled to make three slow descents towards the boulder-free surface. The first of these "dress rehearsals" was aborted at an altitude of about 2,300ft on 4 November. Then, on 12 November, Hayabusa tested its laser range-finder and jettisoned a tiny lander, Minerva. Weighing less than a bag of sugar, Minerva carries three small digital cameras that are designed to image the surface as it hops from place to place. Unfortunately, Itokawa's gravity was too weak to draw Minerva on to its surface and the little lander now seems to be stranded in orbit around the asteroid.

The historic sampling of the surface will take place on Saturday and the following Friday, 25 November, when two attempts will be made to collect fragments of material from the asteroid. Since Itokawa's surface gravity is more than 100,000 times lower than the Earth's, any attempt to grab a sample would be unlikely to succeed without using some form of anchor. Japanese scientists have overcome this problem by designing Hayabusa to make a fleeting low-speed touchdown that will last no more than a second.

During this brief contact, Hayabusa will fire a tiny pellet of tantulum - a rare, hard metal - at the pristine surface. Travelling at a speed of 1,000 ft per second, this hi-tech musket ball will scatter a cloud of dust and rock fragments. Pieces stirred up by the impact will be captured by a horn-shaped device and brought into the spacecraft's sample chamber.

Hayabusa is expected to stay at the asteroid until early December, when its ion engine will fire once more to send the spacecraft on a leisurely homeward journey. The final obstacle of its "mission impossible" will be faced in June 2007, when the capsule containing the precious samples is scheduled to slam into the Earth's atmosphere and parachute to the ground in the Australian outback, ready for scientists to unravel some of the secrets of one of the Solar System's oldest inhabitants.

Peter Bond's latest book, Distant Worlds, will be published by Copernicus next spring