The Strength of Cometary Surface Material: Relevance of Deep Impact Results for Philae Landing on a Comet

We discuss the Deep Impact estimates of strength of the surface material of comet Tempel 1. It appears doubtful that the tensile strength is as low as originally published. The method applied neglects the weakening by the shock wave, the acceleration by gas and the ongoing discussion whether gravity or strength craters are consistent; with Hie observations. The various definitions of strength (tensile, shear, compressive, dynamic strength) and its size-dependency are discussed. Even for extremely low cohesion, a lower limit of approximate to 1 kPa for the quasi-static compressive strength at, din-scales is derived; far more likely is a compressive strength of the order of approximate to 10 kPa for very soft comet surfaces. The Rosetta lander, Philae, will touchdown on comet Churyumov-Gerasimenko in November 2014. A soft comet surface with a compressive strength >= 4 kPa indeed facilitates landing and leads to a penetration of the order of only 20 cm according to our model calculations.