Science & technology | The fate of Philae

Where the shadows lie

Europe’s comet-lander worked, but not as well as had been hoped

The black cliffs of 67P/Churyumov-Gerasimenko

AS MIXED successes go, it was a spectacular one. On November 12th the European Space Agency (ESA) announced, with a mixture of relief and triumph, that Philae, a robotic probe, had landed on its target, a 4km-wide comet called 67P/Churyumov-Gerasimenko. But, as the minutes and hours passed, it became clear that things had not gone entirely to plan. Philae was indeed down, but it was down in the wrong place, and suffering from a serious shortage of sunshine to boot.

Landing on a comet is tricky, even by the standards of rocket science. Because comet 67P is so small, its gravity is feeble. Anything lifting off from its surface at a speed greater than about one metre a second will zoom away into space. It was vital, then, that Philae make a gentle landing, and have some means of staying put once it was down. That did not happen, thanks to what could only be called hard luck.

First, a small rocket intended to fire on touchdown to push the craft downwards in order to stop it rebounding, failed before Philae separated from its mother ship, Rosetta. ESA’s mission controllers decided to go ahead anyway, relying on a pair of harpoons designed to fire into the comet’s surface and anchor Philae in place. In the event, those did not work either.

Moreover, the comet appears to be made of stiffer stuff than expected. When Philae hit the surface, its flexible legs absorbed some of the impact energy—but not enough to prevent a bounce. Even a small jolt was enough to send it rebounding hundreds of metres back into space. As it was bouncing, the comet rotated beneath it. When it did come to rest, two hours later (and after a second, smaller bounce), it was far from its planned landing site.

Worse, it ended up lying at an awkward angle, in the shadow of a cliff (see photograph). Its solar panels were illuminated for just an hour and a half out of every 12-hour cometary day. With insufficient sunlight to recharge the craft’s batteries, mission controllers had to husband carefully what juice they had.

For two days, eight of its ten instruments collected data. On November 14th, with the batteries running low, ESA decided to try deploying Philae’s soil penetrator, but it seems not to have accomplished much. Then, with the collected data safely transmitted to Earth and the batteries deep in the red, the controllers moved the craft slightly to try to improve the amount of sunlight it received. On November 15th, contact was lost.

But not, perhaps, for ever. Rosetta continues to orbit the comet, gathering data of its own. And Philae seems undamaged by its ordeal. ESA hopes that, as 67P falls towards the sun and Philae’s solar panels are consequently able to harvest more and more photons, the probe might one day bring itself back to life.

This article appeared in the Science & technology section of the print edition under the headline "Where the shadows lie"

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