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Ten years after leaving Earth, one of humanity鈥檚 most ambitious space missions is ready for its climax 鈥� a nail-biting drop onto the surface of a comet

MATT TAYLOR will soon experience the most agonising 28 minutes and 20 seconds of his life. That鈥檚 how long it takes a signal to travel the 500 million kilometres from the surface of comet 67P Churyumov-Gerasimenko to the European Space Agency鈥檚 mission control in Darmstadt, Germany. So that鈥檚 how long Taylor must wait to find out if his team has made history by landing a spacecraft on a comet for the very first time.

The European Space Agency (ESA) craft, Rosetta, has been orbiting comet 67P since August. But the most daring part of the mission begins on 12 November. At 0835 GMT, Rosetta will cast adrift a washing-machine-sized lander called Philae. For the next 7 hours or so, Philae will be pulled down by the comet鈥檚 puny gravity, dropping 23 kilometres. Touchdown is scheduled for around 1530 GMT, when the nail-biting wait to hear a signal from Philae will begin.

Can mission controllers pull off the toughest landing ever? (Image: ESA/Rosetta/Philae/CIVA)

Plenty could go wrong. Philae could overshoot its target. It could bounce off the comet. Or crash land. It could even be blasted to pieces by jets of gas erupting from the comet. 鈥淭his is the most challenging mission ESA has ever attempted,鈥� says Taylor. By 1600 GMT, we will know if Philae has earned a place in the space hall of fame and started its pioneering scientific investigations, or become an extremely expensive piece of space junk.

There is a great deal we want to find out about 67P. 鈥淢any of us are now convinced that life was possible on Earth only because water and certain organics were brought by the comets,鈥� says Philae鈥檚 lead scientist Jean-Pierre Bibring of the Space Astrophysics Institute in Orsay, France.

Comets are often called dirty snowballs. They appeared early in the solar system鈥檚 history and formed anywhere water can freeze. Back then, dust and rocks merged into ever larger proto-worlds that went on to merge and make planets. Comets were also part of this process and brought their supply of water to the forming planets.

The comets that remain today are the astronomical leftovers 鈥� the getaways. They were slingshotted to the farthest reaches of the solar system by the gravity of Jupiter and Saturn. Now they wander in darkness, bound to the sun by only the weakest of gravitational threads. Occasionally, one falls back towards the sun and we catch a window back in time to when they filled the solar system.

Ground-based observations of about 150 comets have revealed that most have an abundance of organic compounds. There is little doubt that they helped to supply our planet with the molecular building blocks of life. 鈥淎 fundamental question to answer is what are all the complex molecules that are on the comet?鈥� says Mike A鈥橦earn at the University of Maryland in College Park.

Identifying these molecules from Earth is difficult because it relies on the comet throwing them into space to form its tail. The molecules seldom survive intact and so astronomers see only fragments, usually just pairs and triplets of carbon atoms. The unbroken molecules on a comet鈥檚 surface can sometimes be seen using infrared equipment, but rarely do comets become bright enough at these wavelengths to produce adequate signals.

A better way is to visit a comet. In 2005, A鈥橦earn headed a NASA mission called Deep Impact that fired a copper cannonball into comet Tempel 1 and observed the cloud of dust and gas the collision threw up. The trouble was that the impact also broke up many of the organic molecules in which they were interested.

From a distance, Rosetta has already caught a passing whiff of rotten eggs, cat urine and bitter almonds.

Having tried a slap, it鈥檚 time for a tickle. Philae will put its mechanical hands into the undamaged carbon compounds and analyse them with its many instruments. 鈥淧hilae should sort this all out,鈥� says A鈥橦earn.

But first it has to get there. Usually, space flight is a comparative piece of cake. With no air pressure or friction to disturb spacecraft, they follow precise orbits that can be predicted using Newton鈥檚 law of gravity. Around a comet, things are different. First, comets boast hardly any gravity. Rosetta had to get within 30 kilometres to experience 67P鈥檚 gravitational pull. Before then thrusters had to be fired now and again to manoeuvre the spacecraft around its target.

But that鈥檚 not the worst of it. Heat from the sun boils away the ice on the comet to create an atmosphere, known as the coma, that streams away to become the characteristic tails. This is not a gradual or a uniform process. The activity comes from specific sites on the comet, and is highly variable in the amount of gas that is driven away.

Peril at every turn

Rosetta鈥檚 giant solar panels, which measure 70 square metres, act like sails and mean the spacecraft is constantly buffeted, unpredictably changing its course and its orientation. The ground crew have had to become used to the navigation-camera images showing empty space when they should be showing the comet. 鈥淲ith Rosetta, you鈥檙e never where you think you are,鈥� says ESA鈥檚 Nicolas Altobelli, who works on the mission.

Although mission controllers are now adept at correcting these meanderings, there is no way to predict them in advance. And as the comet warms up as it gets closer to the sun, the activity increases and the problem gets worse.

The upshot is that the team cannot know exactly where Rosetta will be when the lander is released. Neither can they know exactly how Philae will drop to the surface. Taking all the uncertainty into account, the lander could touchdown anywhere within half a kilometre of its designated target spot 鈥� and that鈥檚 a big problem.

At the beginning of the mission, the design team had assumed that the comet was a potato-shaped object with large, smooth areas on its surface. They pegged their chances of a successful landing at 70 to 75 per cent. Now, all bets are off.

鈥淎t the beginning, mission planners assumed the comet was potato shaped鈥�

The reason is the comet鈥檚 dramatic and unexpected shape. When new images from Rosetta arrived at Bibring鈥檚 laboratory on 14 July, they showed that 67P looks more like a rubber duck than a potato. Even now, the smaller lobe is still referred to as the head and the larger lobe as the body.

Worse, they showed that 67P was rotating every 12 hours around an axis that passed at an angle through the duck鈥檚 neck rather than from head to toe. 鈥淚t was juggling around in all different directions,鈥� says Bibring, 鈥淲hen we saw that, we feared this is just not going to happen. There will be nowhere that we can land safely. We were excited, but desperate.鈥�

But you only live once, so the flight-dynamics team began to look for ways to make it happen. To everyone鈥檚 surprise, landings were possible in a number of locations, albeit at the highest touchdown speed that the lander was designed to survive. Spirits lifted.

Then more detailed pictures arrived. 鈥淥ur fears returned when we saw a large variety of surface features we hadn鈥檛 envisioned. Instead of being flat, the surface looks terribly odd,鈥� says Bibring.

Of five candidate sites, the most promising was labelled B. It was the only one that looked more or less flat across the whole landing area. But there was more bad news just around the corner. Higher-resolution images showed that the landing field was strewn with boulders 2 to 10 metres in size. Tens of thousands of smaller boulders might be there too. Each rock could upturn or wreck Philae if the lander happened to come down on top of one.

So mission controllers finally plumped for landing site J on their shortlist. J is located on the top of the duck鈥檚 head. It鈥檚 not the flattest area but there are fewer slopes steeper than 30 degrees, the maximum Philae is designed to cope with. 鈥淚t is not necessarily the best site for science, but it offers the best chance of success,鈥� says Bibring.

If Philae makes it, its science investigations will start as soon as possible (see diagram). These will answer one of the most obvious questions about 67P: is it two objects stuck together or one gradually eroding into two?

Bibring is keeping an open mind but points out that a comet鈥檚 activity is driven by its shape. Sunlight entering a crack in the surface, for example, drives off volatile ice underneath and widens the crack. This enhances the activity and sets up a feedback loop. Where you once had a crack, now you have a crevasse. This chimes with what might have happened at 67P. Most of the activity observed so far is coming from the neck, meaning that it is inexorably becoming narrower and weaker. 鈥淚f we are lucky, it will fall to pieces the day after we land,鈥� he says, only half-jokingly.FIG-mg29940601.jpg

Rosetta鈥檚 instruments have already started sizing up 67P. They show that the comet is 4 kilometres across at its widest point and isn鈥檛 very dense at all 鈥� in fact, its density is much lower than water ice. This means that 67P must be more than 60 per cent empty space. Perhaps that means there are large caverns inside 鈥� cathedral-sized spaces that could burst open as the comet becomes more active. 鈥淲e just don鈥檛 know yet,鈥� says Martin P盲tzold at the University of Cologne in Germany. It could be that the dust and gas is instead in a fluffy, loosely bound aggregate. 鈥淲hatever it is, it will help tell us how comets are formed. We really don鈥檛 know that yet either,鈥� he says.

If Philae lands successfully, its instruments will give us answers. While the first science sequence is taking place, the confirmation signal of landing will arrive at Earth. This will contain information on Philae鈥檚 position, orientation and condition. As the lander team analyse this, Philae will begin drilling into the comet to begin its chemistry experiments.

These will continue for as long as Philae鈥檚 initial battery power lasts. Once that runs out 鈥� anytime between 40 and 50 hours after landing 鈥� it will switch to rechargeable batteries powered from solar panels. All being well, Philae will then enter the long-term science phase in which the experiments on the surface will continue with increasing refinement.

So long as the rechargeable batteries hold out, and there is no untoward comet activity that damages the lander, the mission鈥檚 natural end will come in March 2015. By then, the comet will be closer to the sun, and the temperature will begin to affect the lander鈥檚 electronics.

If Philae survives that far, it will be a major triumph. Its measurements will keep cometary scientists busy for years. For Bibring, it is the organic chemistry and its role in the origin of life on Earth 鈥� and potentially elsewhere too 鈥� that holds the greatest promise. 鈥淭he question 鈥榓re we alone in the universe?鈥� is directly connected to what we are investigating with this mission,鈥� he says.

鈥淭he question 鈥榓re we alone in the universe鈥� is directly connected to this mission鈥�

After all, exoplanet hunters have catalogued orbiting more than 1100 stars. They may not look much like our solar system, but does that mean they are all barren? Not necessarily. Excitingly, researchers have spotted comets in 11 other solar systems. So understanding the complex organic chemistry that takes place in comets and young solar systems may be the clue we need to unlock how life gains its foothold and turns a young planet into a potential habitat.

So perhaps it is no wonder that when Taylor stood in front of hundreds of planetary scientists to give a status report in September, he elicited a round of spontaneous applause with his first sentence: 鈥淭his is the sexiest mission that鈥檚 ever been flown in space.鈥�

But make no mistake; sexy comes with risks. The chances of failure are high. While the main orbiter mission will continue regardless of whether or not Philae makes it, the world will be watching on 12 November.

鈥淚f we fail because of something that we could not predict, then ok,鈥� says Bibring. 鈥淲orse would be if we have underestimated something we should not have underestimated.鈥� That鈥檚 why he and every other Rosetta team member is working overtime to think through every eventuality and prepare as best as possible. 鈥淚 look this old because it鈥檚 such hard work,鈥� quips 41-year-old Taylor.

Rosetta itself was first discussed 25 years ago, and Bibring was there. 鈥淵ou put a lot of yourself into a mission over that length of time,鈥� he says. 鈥淎s someone once said: failure is not an option. So we won鈥檛 fail. This is it. What else can I say?鈥�

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This article appeared in print under the headline 鈥淛ust one shot at this parking spot鈥�