Hitching a ride on a comet

The Rosetta spacecraft and the Philae robotic landing craft above comet 67P/Churyumov-Gerasimenko.

Scientists are hoping a high-risk operation to place a lander on a flying block of ice will give them answers to key questions about life, writesRobin McKie

Within a few days, scientists will manoeuvre Europe's £1 billion ($2.1 billion) Rosetta spacecraft directly above a massive ball of ice, dust and organic chemicals called Comet 67P/Churyumov-Gerasimenko. Then they will transmit a signal to the probe instructing it to release a tiny lander craft, called Philae, and send it gliding down to the comet.

The little lander will take seven hours to make its uncontrolled 20km descent. Then it will touch down on the boulder-strewn comet and -- if all goes well and it avoids the boulders -- it will begin a series of experiments designed to help answer a host of key scientific questions. How did the solar system form? Did comets bring water to Earth and form our oceans? Did they seed our world with organic chemicals that were the building blocks of life?

Philae, which will be the first probe to land on a comet if all goes well on November 12, is part of the most complex mission undertaken by the European Space Agency (Esa). It took 10 years to plan and build Rosetta, and a further 10 years to send it on a highly complex route into deep space for its rendezvous with Comet 67P. Now it is set to carry out its riskiest, most intricate manoeuvre using a probe that has no rocket landing thrusters or guidance system. Philae will simply be pointed at its target and released into space. A target area of 1sq km has been pinpointed on the comet's surface and Philae could land anywhere inside it, say mission controllers.

"You plan for 20 years and then, in the end, you realise the probe you have worked on may touch down on a nice flat area and you are fine or it could move 10m to one side and hit a boulder and everything is lost," said Stephan Ulamec, manager of the Philae landing mission. "Twenty years of my career -- and those of my colleagues -- will be boiled down to those few seconds. So, yes, it will certainly get emotional in the control room as we wait for a signal to say Philae has landed safely."

Rosetta, with Philae on board, was launched from Kourou in French Guiana on March 2, 2004, and placed on a trajectory that involved three Earth fly-bys and one of Mars on its route to Comet 67P. It was not until Rosetta reached its journey's last stretch this year that astronomers got their first idea of their target's appearance, its cameras revealing an object 4km in diameter that is made up of a large body with a neck of rock attached to a smaller head-like chunk.

"It looks like a big rubber duck," said Rosetta Flight director Andrea Accomazzo.

For the past three months, Rosetta has been sweeping round the comet, mapping its surface and analysing the gases and dust that are beginning to pour from its surface as it warms up as it nears the sun. "They have also revealed a world littered with boulders, cliffs, crags and areas of erosion," said Accomazzo. As a result, it has taken astronomers weeks of careful study to pick a landing area that seems relatively smooth and which has a sunny enough aspect to ensure Philae's solar cells remain well charged.

"On all previous space missions that have involved landings on other worlds -- on the moon or Mars, for example -- there have been precursor missions that have pinpointed good spots for touchdown," said Paolo Ferri, head of operations for Esa. "This is the first space mission where we have gone to a totally unknown place and have then had to work out where to land after we arrived."

Rosetta staff and engineers remain confident of success but acknowledge there is a more than 20 per cent chance the lander will strike a boulder or drop on to a sloping slab of rock and flip over, leaving it unable to transmit signals to Rosetta and Earth.

"Essentially we are attempting a highly complex landing on an object we barely understand," said Mark McCaughrean, a senior science adviser at Esa. "Comet 67P weighs 10 billion tonnes and is the size of Mont Blanc. But if you were to drop it into an ocean it would float. It has the density of pinewood."

The comet is also extraordinarily dark. "It only reflects 4 per cent of the light it receives from the sun back into space," said McCaughrean. "It's as black as charcoal."

Scientists are confident ice does lie below this charcoal coating -- but are unsure of its exact depth. And this also poses problems. If Philae drops into a thick black layer of powder, its instruments and solar panels may become coated with dirt, limiting the craft's operations.

Philae is also fitted with ice screws and harpoons that will be automatically deployed when it touches down -- because Comet 67P's gravity field is too weak to prevent the lander from rebounding into space. While the ice screws need a hard surface to drill into, the harpoons are designed to cope with both hard and soft material. Nothing like it has been attempted before.

The scientists insist that Rosetta is already a resounding success, however. Even if Philae were to fail in some way next week, the mission is still destined to complete more than 80 per cent of its objectives. "We have already done the really hard part in getting to a comet and have started studying it close up, something we will be doing for another year," said mission scientist Matt Taylor. "A successful landing by Philae would be the icing on the cake, albeit a very dark, dusty, dirty cake."

If Philae survives the hurdles that lie ahead, the lander will begin experiments aimed at unravelling the composition of comets, objects that are thought to be made up of primordial material left over from the birth of the solar system 4.6 billion years ago. A drill will carry samples from under the surface into the lander where different devices will test them. Ptolemy, designed by Professor Ian Wright of the Open University, consists of a gas chromatograph mass spectrometer that will analyse the ratios of the different isotopes of carbon, nitrogen, oxygen and other elements found on Comet 67P. In this way it should be possible to determine the ratio of atoms of hydrogen to atoms of deuterium (an isotope of hydrogen) in the comet's ice.

"If that is very similar to the ratio we find in water on Earth, that will be another piece of evidence to suggest comets provided Earth with its oceans," says Wright.

Other instruments on Philae will include an alpha proton x-ray spectrometer that will study the chemical composition of sub-surface samples by irradiating them with x-rays and alpha particles while the Rosetta lander's magnetometer and plasma monitor will study the comet's magnetic field.

At the same time instruments on Rosetta will study the plasma and dust being thrown off the comet as it travels close to the sun.

Given that Rosetta's instruments are scheduled to operate for more than a year as the comet swings past the sun, the information they will provide will transform our knowledge about comets.

However, this work -- particularly that of Philae -- is under considerable time constraints, Rosetta mission manager Fred Jansen pointed out. "If there are problems, for example with the lander's battery power levels, on November 12 we will halt its release, though that will mean a delay of two or three weeks before we can manoeuvre Rosetta back to the right point above the landing zone."

And that in turn will have serious implications for the mission. Philae is not expected to survive on the surface of comet 67P beyond March. "By then, it will have got so hot on the comet, as it gets closer and closer to the sun, that Philae will no longer be able to operate," said Jansen. "So any delay means time will be lost for science. The clock is ticking all the time for this mission."

Comet 67P is beginning to warm up gently as it heads for its closest approach to the sun. When Rosetta arrived three months ago, instruments showed about a third of a litre of water a second was evaporating from its surface. Now that rate has reached more than five litres a second, an increase that indicates the celestial fireworks that lie ahead.

As Comet 67P gets closer to the sun, the rate at which ice below its surface sublimates will increase dramatically and will flow away from the comet, dragging dust and organics to produce great filaments of material, which will form a glowing tail that will arc behind it. "The comet is only losing a few litres of water a second at present," said McCaughrean. "That will increase to tonnes per second and by this time next year, after it has completed its journey past the sun, Comet 67P may well have lost about a metre of material from its entire surface."

Rosetta will witness this whole operation, with backing from Philae. Among the signs that will be closely sought by researchers will be indications of the presence of amino acids that form the building blocks of life on Earth and which are thought likely to be found on comets. Other studies should reveal clues that will indicate one way or the other that water on Earth has cometary origins.

"There is no doubt we are going to revolutionise our knowledge about comets," said mission expert Gerard Schwem. "However, there is a limit to what these little instrument packages on Rosetta and Philae can do.

"The real goal must be to bring comet material back to Earth so we can put it through the very best instruments in our laboratories. For me, that is still the ultimate goal. Really, we have only just started this business of studying comets."