Some time in the next two weeks, European scientists hope to launch a spaceship the size of a delivery van and lob it across more than 4-billion miles of space to rendezvous 10 years from now with a dark lump of rock and ice the size of a city block.
The spacecraft — called Rosetta after the stone that provided the key to the mystery of Ancient Egypt — will inch alongside the object at a speed of 37,8kms a second, and go into orbit around it. It will be humankind’s closest encounter with a comet, one of the bits of primeval breezeblock from which the solar system was made.
Then, after studying the surface of Comet Wirtanen, it will disgorge a second craft roughly the size of a washing machine. This will amble across the remaining space at walking speed, harpoon the comet as if it were a whale, and then reel itself in, to touch down and begin probing its properties. Then orbiter, lander and comet will slowly accelerate towards a close encounter with the sun.
Comets have been implicated in episodes of mass destruction. One is believed to have crashed into the Earth at about 37 miles a second, 65-million years ago, bringing the era of the dinosaurs to a close. Comets may have delivered all the water in the Earth’s oceans, and may even have provided the chemical building blocks from which life was assembled.
”This is vital for working out how unique mankind is in the universe,” said Sue Horne, who heads space science within Britain’s particle physics and astronomy research council.
Insight into origins
”If we can demonstrate that the water on Earth may have arisen from comets arriving on the surface of the primitive planet,” said Ian Wright of the Open University, ”then what this enables us to do is to look now at comets today and see what other kinds of things can have been brought along with them. It will give us some kind of insight into our own origins. ”It also has the potential to come down and wipe life out. These are fascinating things to try and understand.”
The Rosetta mission is one of the boldest ever devised. The spacecraft was put together by 39 subcontractors from 14 nations, and carries instruments from six teams in five European countries. Its launch is an occasion for nailbiting: the Ariane 5 rocket due to put it into space failed and self-destroyed as it tried to launch another payload last month. But if Rosetta is to meet Wirtanen in 2012, it must take off before January 31 from Kourou in French Guiana. So Europe’s space chiefs have to decide whether to go now with the same rocket, or delay for months and search for another comet.
Once in space, the challenges multiply. To collect power so far from the sun, the spaceship must unfurl solar panels that stretch for 32 metres — half as long again as a tennis court. To get up speed from its launch velocity of around 20 miles a second, it must first loop round Mars to gather extra oomph with a technique called ”gravity slingshot” and then do the same around Earth, twice, before winding up to an orbit that will reach across the asteroid belt and coincide with Wirtanen.
During this journey, Rosetta will have to go into long hibernations to save power, ”waking up” to look more closely at Mars and two little rocky asteroids that it will encounter on its travels. It will have to survive temperatures as low as minus 170C, and near the end endure solar radiation that could bring its surface temperature to levels hotter than boiling water.
When it makes contact with the comet in 2012, it will be so far away that messages from Earth will take 45 minutes to arrive. Instruments packed into a tiny box will examine the detail of the cometary surface (”we don’t know whether it is as dense as concrete or candy floss” one engineer has said) and select a place for touchdown. The long timescale creates other puzzles. Some of the scientists who began working on Rosetta a decade ago will have retired or died nine years from now, and researchers back on Earth will be using a computer technology nine years more advanced than anything aboard the spaceship.
But the prize will be a profound insight into the origins of the planets and the life that clings to one of them. There could be billions of comets occupying the outer rim of the solar system: now and then one is dislodged and sent hurtling towards the sun. This mysterious lump of something — water, dust, chemical compounds — begins its journey as little more than a dirty snowball that might be only half a mile in diameter. But as it nears the sun, it begins to release a coma of hot gas and dust that may grow to 600 000 miles across.
Both the US and the European space agencies have ambitious comet missions. Giotto flew off to meet comet Halley more than a decade ago, and astronomers were astonished to find that the nucleus was black, rather than white. A spacecraft called Stardust is poised next year to fly through the tail of Comet Wild-2 and bring a handful of comet dust back to Earth. – Guardian Unlimited Â