'Scuse me while I kiss the sky

The final countdown has begun for an ambitious space mission to kiss an asteroid called Eros on St Valentine's day. If the romance works out, it could lay the foundations for a new love affair with precious rocks that could be more meaningful than a visit to Tiffany. The rendezvous with Eros, a city-sized chunk of rock, may one day lead to mining on an epic scale, millions of miles from Earth.

The final countdown has begun for an ambitious space mission to kiss an asteroid called Eros on St Valentine's day. If the romance works out, it could lay the foundations for a new love affair with precious rocks that could be more meaningful than a visit to Tiffany. The rendezvous with Eros, a city-sized chunk of rock, may one day lead to mining on an epic scale, millions of miles from Earth.

Mission controllers at the American National Aeronautics and Space Administration (Nasa) have begun a 24-hour watch on Eros, some 150 million miles away in deep space. Nasa is on the look-out for a speck of light about 26,000 million miles farther beyond the floating piece of geology, namely the Near Earth Asteroid Rendezvous (Near) spacecraft. Over the next few weeks, until 14 February, Near will move closer and closer to Eros.

As Eros moves against the fixed background pattern of stars, its trajectory will be analysed, and Near will fire its thrusters to edge closer. If all goes to plan, Near will become the first craft to go into orbit around an asteroid. What Near will see beneath it will be an ancient, pock-marked object riven with valleys that holds under its bleak, airless surface the secrets of the solar system's past and possibly the fate of our species.

Older than the oldest rocks on Earth, older than those on the Moon, Eros has remained unchanged in composition for billions of years, since the time the solar system was born, long before Earth came into existence. Eros is one of thousands of known asteroids that orbit the Sun, mainly in a giant ring between the orbits of Mars and Jupiter. Perhaps some of them are the debris of a much larger body shattered by impacts when the solar system was young; perhaps some of them have always been small bodies. Though scattered across vast tracts of space, together they would not make up the mass of the Moon.

The first asteroid, Ceres, was discovered on 1 January 1801; others were soon found. Measuring about 35km by 14km (22 by nine miles), the potato-shaped Eros is larger than the other three asteroids that spacecraft have flitted past. It circles the Sun every 643 days and spins every five hours. It is a tiny world of bright sunlight, deep shadows and minimal change.

Near will use its magnetometer to look for a magnetic field around the asteroid, which would tell scientists something about the asteroid's iron content. An X-ray/gamma ray spectrometer would analyse tiny amounts of high-energy radiation, looking for spectral signatures of silicon, magnesium, iron, uranium, thorium and potassium. A near infra-red spectrometer will map out the surface composition.

Near's path to Eros has been a rocky one, so to speak. The craft was launched flawlessly in February 1996, and in June 1997 it flew past the large, dark asteroid Mathilde. So angular was that tiny world, that, when approaching from its dark side, Near's camera was able to see only a series of rocky ridges with deep shadows in between. To some astronomers it looked like a prehistoric flying dinosaur. One researcher stuck a picture of it above his desk, labelled: "Pterodactyl".

When the time came for Near to fire its rocket in December 1998 to guide itself towards Eros, disaster almost struck. The rocket shut down after only two seconds, and mission controllers had to scramble to rescue the spacecraft. When they regained control, it was clear it was too late to orbit Eros. Emergency observations of it were made as it went sailing by in the interplanetary dark. Another rocket, fired in January 1999, put it back on course for the rendezvous, but this time it would be a year away on the other side of the Sun. So, for the second time, Near is approaching Eros.

The craft will tell us more about asteroids in just a few weeks than we have ever learnt. What we know already should fascinate us, for asteroids may play a role in the economic and technological development of Earth. The fact is, a smallish, 1km asteroid contains about eight billion tonnes of metal, worth over $50,000,000bn. Its iron and copper could supply the Earth for a decade. Its nickel would satisfy us for one millennium, its cobalt for several. Such mineral riches, yet you could walk round it in 15 minutes.

Near will be circling the 2,900 cubic km of Eros, in which there is more aluminium, gold, silver, zinc and other base and precious metals than has ever been mined or indeed ever could be from the outer layers of Earth's crust. Forget conventional mining and all its environmental problems. One Eros in orbit around the Earth, or preferably the Moon, is all we would need for almost all time. But perhaps Eros is not the right asteroid to mine. Better would be the more numerous 100-metre ones. Standing on one of those would be a hazardous occupation, as too vigorous a step would send you careering off into space. Lift a piece of rock and drop it, and it would take five minutes to reach the ground.

Mining the metal would not be complicated. Scoop the surface (it will be loose) and crush the fragments in a centrifuge, extracting metal fragments with a magnet. Take what is left and heat it in a solar furnace - huge mirrors made of foil will do - and all sorts of useful material will be vaporised that can be extracted with the kind of condenser seen in any oil refinery. Even oxygen, a rocket fuel, can be obtained.

It is difficult to say how much Eros is worth. One low estimate is that its mineral wealth would sell on the open market for $200,000bn. But the resources are way out in space. One way to transport the metals back would be to mine them on Eros and send the refined ore to Earth. It takes about 2,000 calories to boil a gram of iron, so the equivalent of 20,000-200,000 megatonnes of TNT would be needed to start liberating substantial quantities of iron from the asteroid. The energy could be obtained from the Sun. If you wanted to mine one section of Eros at a time, a huge solar energy collector - a sheet only a few kilometres in size - could collect enough energy to power a smelting plant on Eros.

Those are all "guesstimate" figures. But they show how mining one small asteroid such as Eros would revolutionise the availability of many raw materials on Earth. There is another reason to take a keen interest in Eros. One dynamical analysis of its orbit suggests that it is unstable on a time-scale of 100 million years or so; after that it has a 5 per cent chance of colliding with Earth. That is almost certainly an alarmist figure, but sooner or later something like Eros will threaten us, so we had better find out more about it. Hopefully, unlike Bruce Willis, we will not blow it up but nudge it into a safe orbit and mine it. At the moment, such gold-mines in space are tantalisingly out of reach. No one knows how much a robot mission to mine an asteroid would cost, but it is a safe bet that it would be the best return on an investment since Leonardo da Vinci bought a sketch-pad.

David Whitehouse is science editor of BBC News Online