IT’S AMAZING what nature throws at us – literally. Each year, thousands of tonnes of rubble from space rains down on the Earth’s atmosphere, the largest chunks streaking across the sky in startling fireballs. Those big enough to plonk meteorites on the Earth’s surface could reveal amazing secrets about distant parts of the solar system. Unchanged since the birth of the solar system, these space rocks could tell us about the chilly outposts where they were born.
Except that meteorites, sadly, don’t carry birth certificates. “We have no idea where any of them come from,” says Phil Bland, a planetary scientist at Imperial College London. “Trying to understand the early solar system from meteorites is like trying to understand the geology of Britain from a trailer load of random rocks dumped in your backyard.”
Bland hopes to change all that. He and his colleagues are building a network of robotic cameras in the Australian desert to spot fireballs as they plummet through the night sky and record their trajectories. From a fireball’s path, they will work out where the meteorite has landed and send out a search party to find it. They will also be able to calculate the rock’s orbit through space, allowing them to trace its origin.
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Tens of thousands of meteorites already lie in museum vaults around the world. They were probably chipped off several hundred different parent bodies, mostly asteroids that mill around in the main belt between Mars and Jupiter. However, all theories of their origins are just educated guesswork, since we can’t travel the solar system gathering samples. “We have a tentative feel, a little sniff of many things that meteorites are trying to tell us about the solar system,” says Bland. “But because we don’t know where they came from, there’s a fundamental gap in our knowledge. There is a hell of a lot we really haven’t hammered down yet.” For instance, how did the chemistry of the cloud of gas surrounding the young sun change with distance from the sun? Do any meteorites come from comets? (See “Top five meteor mysteries”)
Bland hopes his fireball network will help answer these questions. The idea has been tried before – an observatory in what is now the Czech Republic started a programme to capture meteors on film in 1951. This expanded to become the European Fireball Network, which started up in 1963 and still operates today. Another two networks have come and gone: one in the US mid-west from 1964 to 1973 and another in Canada from 1971 to 1985.
While these networks spotted hundreds of fireballs and allowed scientists to calculate the meteor orbits, only four of the meteorites that hit the ground were ever found. The problem was that it was very difficult to locate them. “Recovering the rocks was a nightmare,” says Bland. “There was so much vegetation. If you’re looking for a small dark rock in rural Canada, for instance, searching a few square metres is a problem, never mind a few square kilometres.”
Bland decided to build a network somewhere where it would be easy to find meteorites. He plumped for the Nullarbor desert of Western Australia, which is a limestone plateau covering about 250,000 square kilometres, nearly twice the area of England. Collectors have found hundreds of meteorites there, their charred black surfaces standing out like a sore thumb against barren red soil.
Along with Martin Towner of the Open University in Milton Keynes, UK, and engineers from the Czech Republic who built the first automatic camera for the European Fireball Network, Bland has designed a new station to record fireballs at Nullarbor. They shipped the prototype to Australia last year,and installed it at an accessible test site in the outback, where it began observations last October.
Each night, weather sensors test the air for rainfall and a CCD camera counts the stars to decide whether the sky is clear. If so, it opens the shutter on a wide-angle camera, which watches the sky all night and records the tracks of any bright meteors on a single film. A light meter clocks the exact time of the meteor fall, and an acoustic sensor records the sonic boom of any nearby meteorites plunging to the ground. At the end of the night, the station tucks the film away in a dark canister and replaces it with a fresh one.
Equipped with a magazine of 32 films, the station can operate on its own for four-and-a-half weeks or longer, depending on the number of clear nights. The station has a two-way satellite internet connection, and if the system senses that something is going wrong, it sends Bland an email.
At the end of October, the station completed its first year of operation. “It worked like a dream,” says Bland. “After years of work, to finally have something out there taking pictures was just fantastic.” The station spotted some 70 fireballs, and a dozen of these were spectacular enough to have dropped meteorites big enough to find.
However, the lone station cannot work out by itself where a meteorite landed. Bland is seeking funding for the second stage of the project in which two more stations will be built. The three will be about 150 kilometres apart, and they will be able to work out by triangulation where a meteorite has landed to the nearest couple of square kilometres. All the stations will be automated so that they recognise the light from a fireball and send an email or a text message alert. “I love the idea of being out for dinner or a beer, and hearing this little bleep – my camera’s seen a fireball!” says Bland.
Then it’s time to send out the search party. To prevent moisture rusting the meteorite, the aim is to find it within a few days or weeks. A team of up to eight people with four-wheel-drive vehicles is on standby, ready to scan the target area. That would take about three to six days.
“Trying to understand the solar system from meteorites is like trying to understand Earth’s geology from a trailer load of random rocks”
Observations so far suggest that the team will harvest about five meteorites with known orbits each year. It might even be possible to trace some meteorites back to individual parent bodies, which would be a bit like getting a spacecraft sample-return mission for free.
Such missions don’t come cheap, as the Japanese Institute of Space and Astronautical Science knows. In May last year JAXA, the Japanese space agency, launched a spacecraft called Hayabusa (formerly Muses-C), which is now en route to an asteroid called Itokawa, where it will arrive in June 2005. It will map the 0.7-kilometre rock then skip across its surface to collect samples, just 1 gram in total, at three sites and seal them into a capsule.
Hayabusa will then return the samples to Earth in June 2007. The capsule will parachute down in Australia, delivering our first asteroid sample whose origins are certain. It is a high-risk, amazingly complicated mission to test new technologies, and cost $100 million. If the Desert Fireball Network can match a meteorite to its parent asteroid, it will have effectively returned a sample for peanuts.
“Until we get going, we don’t really know if that’s possible or not. There’s a lot more work to do,” says Bland. He just looks forward to the time the network allows his team to bag its first, freshly fallen meteorite.
“That’ll be the real moment, when the camera sees a meteorite, we can work out its orbit and location, and we can drive out and find it,” he says. “Somewhere, in the middle of nowhere, we’ll be drinking a lot of champagne.”
Top five meteor mysteries
Knowing exactly where meteorites come from could help resolve a host of questions:
WHAT WERE THE BUILDING BLOCKS OF THE SOLAR SYSTEM?
Finding meteorites that originated from different parts of the solar system would provide the first samples showing how the composition of the debris surrounding the young sun varied with distance from it.
ARE METEORITES JUST A CHIP OFF THE OLD BLOCK?
It’s not clear whether meteorites that land on Earth have exactly the same composition as the asteroid they came from. Scientists assume that the most common meteorites come from the stony asteroids that dominate the main asteroid belt between Mars and Jupiter. Certainly, the meteorites’ composition matches the composition inferred from the spectra of these asteroids. However, no one has tested that directly. Meteorites may not look like their parent asteroids because something happens to change the rocks as they zoom around in space.
DO ANY COME FROM COMETS?
Carbonaceous chondrites, a rare subset of stony meteorites, are rich in water, sulphur and organic compounds. Scientists think that meteorites like these delivered volatile materials to the newly formed Earth and helped establish our hospitable atmosphere. Without them, life on Earth may never have arisen.
These rocks have never been exposed to temperatures above 50 °C, suggesting they originate far out in the solar system. Some scientists suspect they come from icy, distant comets in the Kuiper belt, a disc of icy chunks about 30 to 100 times further away from the sun than the Earth, or the Oort cloud, some 1000 times further still (see Graphic).
DO ASTEROIDS LEAVE RUBBLY TRAILS?
Comets leave behind a trail of dusty debris that burns up in the Earth’s atmosphere if the Earth ploughs through the trail during its orbit. This causes annual meteor showers like the Leonids, which peak around 17 November. Theory suggests that rocky asteroids cannot leave long-lasting rubble trails. But there’s one intriguing hint that they do. Of the few known meteorites for which an orbit has been calculated, two seem to have followed the same route.
One landed in Czechoslovakia on 7 April 1959, the other fell onto a mountain in Germany on 6 April 2002. That might mean asteroids leave a trail of debris that lasts for decades, an idea that the Desert Fireball Network will test.
HOW BIG A THREAT DO THEY POSE?
Astronomers would love to know exactly how much space rubble falls onto the Earth each year. Earlier observations by fireball networks accurately measured the numbers of meteors of different brightness streaking through the atmosphere. By calibrating the brightness of tens or hundreds of fireballs with the mass of the meteorites they dump on the ground, the Desert Fireball Network could turn fireball data, old and new, into better estimates of the numbers of space rocks that hit the Earth, including large and menacing ones.
