Physical risks of landing on a cometary nucleus

Comets originate from the outer solar system where the density and temperature of the planetary nebula were rather low. They form a distinct class of small solar system bodies and their physical properties differ considerably from those of any other known objects. The present knowledge of the cometary nucleus is cursory and driven more by imagination than by observational facts. Landing on a cometary nucleus, as planned during the ROSETTA mission, is of high scientific interest and will substantially improve our knowledge of the nature and origin of comets. However, successful landing and operating the instruments is an extraordinary challenge to engineers and scientists. Even a close approach to an inactive nucleus may jeopardize the orbiter. A wide range of uncommon and risky environmental conditions such as outgassing, a fragile surface, deposition of dust grains, extreme temperature variations, and pronounced topography have to be considered. These are described in physical terms based on model calculations ranging from simple one-dimensional heat transfer to gas-dynamic acceleration of dust particles. Environmental constraints relevant for landing and operating instruments on a cometary nucleus are presented such as sublimation and recondensation of water on the cold parts of the lander and varying illumination conditions over the extent of the mission. Aspects of risk reduction are discussed. The paper also summarizes the present knowledge about physical and structural properties of cometary nuclei relevant for the development of landing strategies.