Philae: one year on comet 67P

On November 12, it was a year since Philae made its historic landing on the Comet 67P/Churyumov-Gerasimenko. Rosetta arrived at the Comet 67P on August 6 and Philae was delivered to the surface on November 12.

Rosetta did an initial survey before selecting the Agilkia region as the landing site. However, the Agilkia region did not turn out to be a perfect site as it had a very hard surface. The hard surface caused the Philae to bounce on landing.

A year after Philae landed at the Agilkia region, scientists have found why Philae bounced. Ice screws, harpoons and a small thruster were to be used to secure Philae after it landed on the comet surface. The ice screws were designed keeping in mind a relatively soft material, but the Agilkia region turned out to be very hard and so the screws failed to penetrate the surface, notes a European Space Agency release.

The harpoons, capable of working in both softer and harder material, were supposed to fire on contact and lock Philae to the surface. But that did not happen.

The problem was either with the four ‘bridge wires’ taking current to ignite the explosive that triggers the harpoons, or the explosive itself. The thruster on top of the lander was meant to push it down to counteract the recoil from the harpoon. But attempts to arm the thruster the night before had failed.

Besides helping in locking Philae to the surface on making contact, the harpoons were to serve another important function — they contain sensors that can measure the temperature below the surface. So the agency might attempt to retry firing the harpoons if contact with Philae is restored.

Despite the unplanned bouncing, Philae completed 80 per cent of its planned first science sequence before falling into hibernation when the primary battery got exhausted, the release claims. The secondary batteries could not be charged as there was not enough sunlight in Philae’s final location at Abydos. In the absence of fully powered secondary batteries, science measurements will not continue. Plan was to reactivate Philae once the batteries get charged as the comet moved nearer to the Sun, heading towards closest approach in August. But only eight intermittent contacts were made between June 13 and July 9.

Only intermittent contacts were made as the increasing sunlight led to increased activity on the comet. Rosetta was forced to retreat to several hundred kilometres for safety, well out of range with Philae. “Over the past few weeks, with the comet’s activity now subsiding, Rosetta has started to approach again. This week it reached 200 km, the limit for making good contact with Philae, and on November 12 it dips to within 170 km,” the release notes.

The comet is now heading out beyond the orbit of Mars and the temperatures are dipping. Beyond January-end Philae cannot work as the temperature will dip below – 51 degree C. The plan is to end the mission with a ‘controlled impact’ of Rosetta on the surface in September 2016.

Published in The Hindu on November 16, 2015

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