PHYSICAL RISKS OF LANDING ON A COMETARY NUCLEUS

Comets Originate from the outer solar system where the density and tem perature of the planetary nebula were rather low. They form a distinct class of: small solar system bodies and their physical properties dif fer considerably from those of any other known present knowledge of th e cometary nucleus is cursory and driven more by imagination than obse rvational 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, su ccessful landing and operating the instruments is an extraordinary cha llenge to engineers and Even a close approach to an inactive nucleus m ay jeopardize the orbiter. A wide range of uncommon and risky environm ental 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 beat transfer to gas-dynamic acceleration of dust pal-tides. Environmental constrain ts relevant for landing and operating instruments on a cometary nucleu s 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 structu ral properties of cometary nuclei relevant for the development of land ing strategies. (C) 1997 Published by Elsevier Science Ltd.