They may be barely visible to the naked eye, but the specks of dust now being examined in a Japanese lab have prompted celebrations among astronomers world-wide. And small wonder, as these tiny grains could inspire one of the biggest re-thinks in our understanding of the solar system since the time of Copernicus.
They are the very first samples of material from an asteroid, one of the myriad chunks of cosmic debris that orbit the sun between Mars and Jupiter. In 2005, the Japanese-built probe Hayabusa ("Falcon") made a rendezvous with one such body, the half-kilometre wide potato-shaped Itokawa, at a distance of around 300 million kilometres from Earth. Its mission was to retrieve some samples from the surface of the asteroid and return them to Earth for laboratory analysis.
But just about everything that could go wrong did. The probe was supposed to hover close to the asteroid while a lander dropped to the surface, but a malfunction sent the lander spinning off into space. Attempts to fire a projectile into the asteroid to kick up plumes of dust for sampling also failed.
Faced with their plucky little probe coming home empty-handed, mission controllers decided to instruct Hayabusa to land on the asteroid itself. Communication problems meant they had no idea whether their ploy had succeeded in putting some asteroid dust in the probe's retrieval canister. They had no choice but to wait for five years while the probe limped back to Earth. Even when the sample canister finally crash-landed in Australia last June it was unclear whether the specks within it were anything more than contamination.
But last week, the Japanese space agency announced that tests carried out on the specks showed that most of them were unlike anything found on Earth. Their chemical composition was, however, similar to that of certain meteorites known to be debris broken off asteroids during collisions.
With the samples now validated, astronomers can use them to solve the mystery of how our solar system came to acquire a "cosmic junkyard" between Mars and Jupiter. And this, in turn, may confirm a striking new vision of sun's family of planets.
Ever since its discovery around 200 years ago, the asteroid belt has been something of an enigma. Originally it was thought to be the home of just one object, the 1,000-km wide minor planet Ceres, discovered on the first day of the 19th century. But within a few decades, many more had been found - leading to speculation that they were all chunks of a far larger planet that had been destroyed in the early days of the solar system.
Over the years, a more prosaic explanation came to be accepted: that the asteroids are just the "builder's rubble" left over from a planet that failed to form, its attempts to coalesce wrecked by the gravity of Jupiter. But now a far more dramatic story is starting to emerge, based on computer simulations of events that took place in the early solar system over four billion years ago.
This suggests that many asteroids were originally stationed around the frigid outskirts of the solar system, and were brought to their current location when the outer planets scooted around the solar system like pinballs.
Hints of this bizarre possibility have been circulating for decades, with studies of meteorites showing that asteroids come in all shapes and sizes, from vast balls of rock hundreds of kilometres wide to small, fragile "dirty snowballs" of ice and dust. Explaining how such radically different objects could all be formed in the same part of the solar system has long been a challenge.
Over the last few years, an international team led by Dr Hal Levison of the Southwest Research Institute, Colorado, has been piecing together some answers using supercomputers. Their simulations form the basis of the so-called Nice Model, named after the city in southern France where some of the team reside. And their results suggest that the placid "celestial clockwork" of today's solar system emerged from a period of appalling chaos and catastrophe.
According to the simulations, the original solar system that formed around 4,500 million years ago was considerably more compact than it is today. In particular, Jupiter and Saturn were much closer to one another - so close, in fact, that after 650 million years they became locked in a gravitational bond which affected the orbits of Uranus and Neptune. Both these giant planets ended up being catapulted away from the sun, crashing into a giant disc of debris on the outskirts of the solar system. Over the next 150 million years, chaos reigned in the solar system, with debris being hurled in all directions. Most of it ended up even further from the sun, becoming the source of objects like Pluto. But some of it was sent towards the sun - and became the asteroid belt.
What really impresses astronomers about the Nice Model is its ability to resolve some long-standing conundrums. For example, conventional theories struggle to explain how a giant planet like Neptune could have formed so far from the sun. The Nice model suggests it actually formed much closer in, and was ejected out to its current orbit by gravitational effects.
Then there's the mystery surrounding the formation of craters on the Moon, many of which seemed to have been formed around the same time: around 3,800 million years ago. According to the Nice model, this coincides with the time when the outer solar system went haywire, and huge chunks of debris were hurled deep into the solar system.
If the Nice Model is correct, those tiny specks now being examined by Japanese scientists may be pristine samples of material that once lay billions of kilometres from the sun, far beyond the orbit of present-day Neptune. By that reckoning, Hayabusa's mission was equivalent to a taxi journey to the airport to pick up a traveller from a distant and little-known land. No wonder scientists are keen to hear his story.
Robert Matthews is visiting reader in Science at Aston University, Birmingham, England
