The strength of comets

A highlight of the Rosetta mission to comet 67P was the deployment of the Philae lander onto the comet’s surface. Although this aspect of the mission did not go entirely to plan (Philae bounced twice on touchdown, ending up in a non-ideal position), it has nevertheless yielded some unique insights into the surface properties of a comet. And there is still more to be learned. As Laurence O’Rourke and colleagues describe here, even the seemingly random aspects of the touchdown have something to tell us about the comet. In a feat of cosmic detective work, they have reconstructed the trajectory of the bouncing lander and identified in imagery the previously unknown site of the second touchdown. It appears that Philae fortuitously sliced into a boulder, exposing its pristine icy interior for scrutiny; from the geometry of the impression left by the lander, the authors are able to calculate the intrinsic strength of that boulder. The very low compressive strength inferred will be an important constraint in the planning of future cometary landers, and might even offer some clues into the formation processes of comets themselves.