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Where to land on a comet?


Philae will aim for one of five sites on Comet 67P/Churyumov-Gerasimenko when it lands in November


Spare a thought for scientists on the European Space Agency (ESA) Rosetta mission, who will spend this weekend dissecting the ins and outs of five patches of land on a comet 440 million kilometres away.

They will be selecting a landing spot for the washing-machine-sized robotic probe Philae. On 11 November they plan to land the 100-kilogramme probe on 67P/Churyumov-Gerasimenko, in what will be the first ever attempt to soft land on a comet.

Picking the site will be a trade-off between scientific interest and operational safety — based largely on data from Rosetta’s instruments that are just weeks old, taken since the spacecraft caught up with its target on 6 August. High-resolution visible range, ultraviolet and temperature maps of the rubber-duck shaped comet, and data on the pressure and density of gas around the nucleus, already make the comet the most studied in history, says Rosetta project scientist at ESA, Matt Taylor.

At a meeting this weekend at the French National Centre for Space Studies (CNES) in Toulouse, scientists will debate the pros and cons of five shortlisted sites, three on the duck’s head and two on its body (see picture), selected last month. None are perfect, says Taylor. “There isn’t a great spot there. We’re going to have to pick the best of what we can achieve.”

Uncertainties in Rosetta’s navigation near the comet, as well as the distance from which it must release Philae, mean that the smallest spot the scientists can specify to land in is an ellipse with an area of 1 square kilometre. To maximize the chances of a safe landing, engineers will want the site to be as flat and smooth as possible, as well as somewhere on the comet that is feasible to reach, given safe orbits for Rosetta.

To thrive once there, Philae will need the spot to have just the right amount of daylight to power the lander’s solar panels and recharge its batteries. This means at least 6 hours per 12.4-hour comet rotation, but constant sunlight will cause the craft to overheat, says Mark McCaughrean, senior science adviser at the ESA directorate of science and robotic exploration.

Scientific interest is also balanced in the landing-site trade-off. Philae is armed with an on-board chemistry laboratory and instruments that allow it, in conjunction with Rosetta, to use radio waves to map the interior of the comet. Scientists will want to land Philae where they can use these instruments to learn the most.

Site A, for example, is scientifically exciting as it gives a view of both the head and body section, between which most of the comet’s gas is being produced.  Site J has advantages for radio mapping the nucleus.  Site B seems to be a safer choice in terms of landing. The wide crater has already been nicknamed “the heliport” because of its flatness, says Stephan Ulamec, Philae lander manager at the German Aerospace Center (DLR).

After examining the case for each, the lander team and Rosetta scientists will announce their favourite, and a backup, in Paris on Monday 15 September at 11 a.m. local time.

Rosetta is currently in orbit just under 30 kilometres from the comet’s surface, caught within its gravitational field. In the coming weeks, depending on the amount of gas and dust the comet releases, Rosetta will try to creep into closer orbits, taking pictures from as low as 10 kilometres, to best prepare for the landing in November.


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