This summer many of Europe's space scientists are planning one of the most ambitious space missions ever devised. As they go on holiday a stream of faxes and electronic messages follow them. At last, almost 10 years to the day after it was proposed to the European Space Agency, the Rosetta Project is taking shape. Over the years the design of the space probe has altered and its target has changed. But the Rosetta mission, named after the stone in the British Museum that was the key to understanding the ancient world, today reads like science fiction. So imagine this report by David Whitehouse appearing in The Daily Telegraph as the mission ends in the year 2013...
Riding bareback on a comet
THE SPACE probe encountered Comet Wirtanen as it rounded the orbit of Jupiter nearly
five times further from the Sun than the Earth. The dark comet, quiet and inert, was easy
to track as it moved against the background stars. The probe, Rosetta, adjusted its orbit to
match that of its companion-to-be. It began the first of many close approaches to the six-mile
wide rocky nucleus of the comet, gently teased by its tiny gravity.
On the comet, life is boring most of the time. When far from the Sun, little happens to disturb
the tranquillity of its frozen landscape. If you could stand on a solid patch of rock, you could
leap 20 miles into space and take over a week to come down again, seeing the frozen iceberg
rotate a few times beneath you.
Stride to the equator and you could throw snowballs against the nucleus's spin and watch them float almost motionless before you. You could reach out, take one and throw it away for ever. It is only when the comet approaches the Sun that things happen.
Just beyond the orbit of Mars, Wirtanen stirs; there are movements on the surface, tiny puffs
of gas and dust leak into space, at first intermittently but with growing frequency as the frozen ices are warmed by the ever-nearing Sun.
Rosetta's instruments begin mapping the dust and gas geysers; keeping the probe away from
the geysers is tricky as the force from them perturbs the comet's spin. There was a danger
from new geysers, even though scientists thought they could spot where they would originate.
Rosetta watches the months pass.
Now the space around the probe becomes brilliant. Streamers - some straight, some curved -
reach into space, trailing into spirals following the comet's spin. All are stopped and blown
backwards by unseen forces from the Sun; the comet's tail is forming.
Rosetta measures particle density, chemical composition and energy structure. Its cameras
allow earthbound scientists to chart Wirtanen's surface, map the glaciers of ice, the geysers
of gas and dust, searching for a spot on the surface that is relatively quiet.
Rosetta has been stationed to keep a relatively safe 25 miles from the restless comet. During
its last close pass, it mapped two promising regions. As they are brought into view by the
comet's spin, Rosetta fires a small probe at great speed towards one of the quiescent regions.
Tiny buffer jets correct its trajectory. The penetrator lands one foot deep into the fluffy surface. Clouds of particles are thrown up, turning into a myriad flashing points of light as they strike the sunlight.
Throughout the impact, the penetrator has been in constant touch with the mother ship, its
accelerometer revealing how abruptly it was brought to rest and telling scientists how firm
the surface is. It is a prelude to the most important part of the mission.
With luck, Rosetta will be the centrepiece of the European Space Agency's science programme in the first decade of the next century and one of the most exciting space missions ever - which means that its scientific payload and parameters are being decided now.
At the onboard computer's timed command, cable cutters sever the electrical umbilicals that connect the surface probe with the mother ship. Explosive bolts fire and springs push the spacecraft apart.
THE MOTHER ship remains in orbit around the restless bulk of Wirtanen that now occupies a quarter of the sky. The surface probe turns, its radio telemetry establishes contact with the mother craft and it fires its thrusters to fall cometwards. Unable to bring a piece of a comet to the laboratory, the laboratory is going to the comet.
The craft's neural net computer controls the descent, avoiding potentially damaging streams of gas and dust. Thrusters fire to bring it down on a point on the comet's surface identified by the net's map matrix. Stabilising arms unfold and, to prevent bouncing, a "hold-down" thruster fires on contact.
There is no time to be lost; the batteries will last only a day or so and there is always the possibility of a surface eruption beneath the probe. Seismic monitors are deployed, environmental measurements are made and a picture is taken.
Relayed back to Earth via the mother ship and received by Nasa's Deep Space Tracking Network, the first-ever photograph of a cometary landscape takes an hour to reach the expectant scientists' monitor screens.
At the tip of a probe is an X-ray spectrometer. Cosmic rays that have traversed the galaxy and have hit the comet cause its atoms to give off gamma rays. By analysing them, the scientists can tell what the comet is made of. A small claw scoops a tiny sample and pulls it into a chamber in which it is heated in a gas chromatograph to reveal its composition.
Comets such as Wirtanen are important. They come from far beyond the most distant planet in the solar system's cold, dark and lonely outer reaches. It is thought that there are trillions of comets, frozen snowballs of dust and ice, left over from the formation of the solar system. They form a cloud, called the Oort cloud, perhaps one third of the distance to the nearest star.
Astronomers estimate that the Oort cloud contains about 25 earth masses of comets but spread out over a space so vast that the average distance between each comet is about that between the Earth and the distant planet Uranus. Not much happens in the Oort cloud.
Sometimes, though, the tiny tug of gravity from a distant star, a passing molecular cloud or passage through a spiral arm causes one of these comets to fall Sunward towards a very distant, and very dim, point of light.
Most comets pass through the inner solar system only once before receding into deep space for tens of thousands of years. But some, purely by chance, pass too close to Jupiter, whose gravity warps their orbit and confines them to the inner solar system.
HERE, each passage past the Sun loses them dust and ice until they fizzle away, becoming a meteor stream. Observe a comet before this happens and you can read secrets written before the planets were formed. Wirtanen is now approaching perihelion, its closest approach to the Sun. It is now far removed from its tranquil self near the orbit of Jupiter.
Hillocks of frozen dust and ice crumble and are splayed into space; the surface probe has been dead for weeks. But before the density of dust can rise to levels that pose a threat, the Rosetta mother ship has moved, slowing its orbit slightly and falling behind the comet.
Thus it begins its 30,000-mile foray into the comet's coma, about which few measurements have been made. The flotilla of space probes that flew past Halley's Comet nearly 30 years before paid scant attention to its coma.
By now, Rosetta is showing its age, having been in space for 10 years, the last two in a very hazardous environment. As it passes perihelion, the comet's tail now moves ahead of it as it begins the long climb out of the inner solar system back towards the distant planets.
Standing on the surface of the comet, you notice the skies are clearing. The dust and gas geysers that sprayed into the sky are beginning to subside. Parts of the sky still remain bright with streamers and lanes of shimmering lights. The dark, reddish brown landscape around you still seethes and erupts, casting material into space, though less so than of late.
The solar wind at its back, the tail points the way outward. Then something strange and unusual catches your eye. Almost hidden by the solar glare is a tiny but brilliant point
of light. It moves against the background of the few stars you can see. Rosetta is coming.
Before Rosetta dies, it will crash on to the nucleus. Hundreds of millions of miles away, the Hubble space telescope will detect a new jet emerge from the nucleus, forming a train of gas and dust perhaps hundreds of millions of miles long.
Rosetta and the comet are one, for ever.
Dr David Whitehouse is the BBC's Science Correspondent.
