Swansong for Rosetta as it lands on the duck-shaped comet

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It’s been an epic journey, much more than 12 years in the making, but Rosetta is just about to go out in a blaze of glory. The final commands were uploaded mid-morning on 29th September, and now there is no going back. Rosetta is programmed to touchdown on comet 67P sometime late morning of 30th September. It is not a suicide crash, as some have described the manoeuvre, but should (we all hope) be a graceful, slow motion glide onto the surface.

So what has been achieved by this mission? What do we know now that we didn’t know before? For a start, a new discipline has been invented : cometary morphology, the study of a comet’s surface. The images sent back, first by the navigation cameras, and then by the higher resolution Osiris instrument, have been unbelievable. Take a black and white picture of a dried up river bed, or the layers of rock exposed in the walls of a narrow, steep-sided canyon on Earth, and compare them with some of the landscapes we have seen on 67P, and you could be looking at the same features. But we know that rivers have never carved their way across the surface of a comet, or that sediments have been laid down in shallow seas. So how do we interpret these landscapes if we do not understand the processes that have caused them?

Of course, perhaps the most spectacular achievement of the Rosetta mission was the landing of Philae on the surface of 67P. Although the lander bounced a couple of times before finally coming to rest on its side, in the dark, under a cliff, rather than in the middle of a flat, open, sunlit plain, the instruments on-board Philae managed to achieve almost all the planned science goals. Those that were not completed required specimens to be retrieved by the drill – which was not possible, given that Philae was not upright. The Philae teams were resigned over a year ago to the idea that their direct involvement in the Rosetta mission was over, and that Philae was consigned to eternal hibernation. But at the beginning of September, months of close and careful investigation of images of 67P’s surface paid off, and Philae’s resting place was discovered. The delight of the instrument teams was not merely an emotional response to locating the missing lander: finding Philae has enabled additional information to be recovered from data acquired by the lander’s instruments, as the physical context in which the data were gathered is now known. This will enable a more complete interpretation of structural and environmental data, which had been recorded without knowing the conditions under which they were collected.

Rosetta’s sojourn close to the cometary nucleus has not been without peril: sudden jets from the pits described as ‘goosebumps’ released bursts of gas and dust which, on several occasions, confused Rosetta. This was because the spacecraft navigated by the stars – and a cloud of dust grains reflecting sunlight bears a close resemblance to a starfield of many thousands of stars, resulting in sending Rosetta’s startracker astray. Each time this happened, Rosetta might have crashed into 67P, or sent itself flying off into the deep unknown – but each time, swift response by engineers back on Earth were able to rescue the craft from the brink of danger. During closest approach to the Sun, Rosetta wove a convoluted path around the comet, sometimes nearer, sometimes further away, depending on surface activity. This behaviour ensured that there was always at least one very happy instrument team, and one very irritated one, depending on whether the dust and gas released from the comet was regarded as of huge importance or a real nuisance.

Downloading and reducing results from Rosetta and Philae’s instruments is well under way; great efforts are in progress to interpret the data and understand their significance – but it will be some time yet before we manage to integrate all the findings into updated models of Solar System formation. Maybe we will have got it finished before the next cometary mission launches…..?

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