惑星科学：着陸機フィラエがバウンドしたために得られた、彗星に関する新知見

The second touchdown site of the Philae lander as it bounced on the surface of comet 67P/Churyumov–Gerasimenko is identified in a study published in Nature. An analysis of the impressions made by Philae on the icy boulders at this site — named ‘skull-top ridge’ — provides insights into the strength of the ice freshly exposed by the landing.

On 12 November 2014, the Philae lander descended towards 67P/Churyumov–Gerasimenko. The lander bounced twice on touchdown before coming to rest under an overhang in the Abydos region of the comet. Although the first and third landing points were identified previously, the location of the second site remained unknown.

On the basis of a new landing trajectory analysis, Laurence O’Rourke and colleagues set out to identify the second touchdown point of the Philae lander. Using a comparative analysis of pre- and post-landing imagery from the Rosetta spacecraft — which was in orbit around 67P/Churyumov–Gerasimenko — the authors observed changes in the features of two adjoining boulders on the surface of a ridge, which could only be explained by Philae’s presence. They determined that the lander spent nearly two minutes at this site, making four distinct surface contacts. In doing so, it exposed water ice in the interior of the boulders. At the third contact point, an approximately 0.25-metre impression made by Philae was observed in the ice. From this, the authors calculated that the strength of the ice in the boulder was very low (less than 12 pascals, softer than freshly fallen light snow). The findings provide insights into the mechanical processes that will be needed to collect ice samples in future comet missions, they conclude.