CASSE — The ROSETTA Lander Comet Acoustic Surface Sounding Experiment — status of some aspects, the technical realisation and laboratory simulations

Abstract

In 2003 the ROSETTA space mission to Comet 46P/Wirtanen will be launched by the European Space Agency (ESA). On board of the spacecraft there will be a Lander platform, which opens for the first time the possibility to carry out “in situ” measurements on a comet surface. The ROSETTA Lander experiment CASSE aims to investigate the outermost surface of the comet by means of transmitting, receiving and even passively monitoring acoustic waves in a frequency range from a few hundred to several kilohertz. In particular, by transmitting and recording compression and shear waves, the mechanical properties of the cometary surface will be investigated. To guarantee sufficient ground contact in any foreseeable surface topography, and in any composition of the material to be encountered, e.g. dust, sand, ice and mixtures of these materials, the acoustic transmitters (actuators), stacked piezoceramics, and triaxial commercial piezoelectric accelerometers as receivers, will be integrated in each of the three feet of the ROSETTA Lander. The Lander feet thus act as antennas for the transmitters and receivers. By switching different actuators and accelerometers, an analysis of the cometary surface material will become possible by direct foot to foot transmission of the acoustic signals. Further, an in-depth sounding will allow the detection of layered structures or embedded local inhomogeneities. The ground contact of the Lander will be strengthened by a harpoon providing a fixation force up to at least 5 N per foot.

This paper describes the first design of the Lander feet incorporating piezoceramic composite stacks as sound transmitters and receivers, and the experiments to test their efficiency. In the first part of this paper, the results of acoustic wave propagation experiments in ice, sand and olivine dust, performed in the laboratory are reported. These experiments verify that a signal strength of at least 20 dB above noise can be expected for acoustic wave propagation in cometary analogue materials. The second part of the paper deals with the outdoor experiments, performed with sensors integrated into the Lander feet prototypes and placed in a sand filled construction container. The experiments serve to estimate the minimal range of detectability of the sounding signals.

Introduction

After the successful space missions to comet 1P/Halley in 1986, experiments with “artificial comets” were made in the Space Simulator of the Institute of Space Simulation of DLR Cologne. This “Cometary Simulation” (KOSI, an acronym of the German expression “Kometensimulation”) programme, financially supported by the Deutsche Forschungsgemeinschaft, was performed from 1987 to 1993 (Kochan et al., 1999, Sears et al., 1999). The physical processes near and in the surface of the artificial comets were studied under simulated space conditions. The input parameters for the simulation were derived from results gathered by the Giotto mission to comet 1P/Halley and from Fred Whipple’s model of a “dirty snowball” (Whipple, 1950, Whipple, 1951), expressing the hypothesis that cometary nuclei consist mainly of a mixture of minerals and water ice. Comet 1P/Halley, taken as a prototype for all comets, consists of fluffy ice, comparable to dark velvet, with a density of around 0.3 g/cm³ and an albedo of 4% (Keller et al., 1987), although these numbers have been debated (Sagdeev et al., 1988, Colangeli et al., 1989, Peale, 1989, Rickman, 1989).

For the expected nature of the comet surface, it is very important to note that comet 46P/Wirtanen is of rather small size. Astronomical observations and model calculations indicate a radius of about 550 m (Jorda and Rickman, 1995, Cartwright et al., 1997, Benkhoff, 1999, Möhlmann, 1999). This might be a hint that 46P/Wirtanen, as a member of the Jupiter-family of short period comets, is in itself a building-block of a former larger body, remaining from earlier splits, or that it consists of only a few of such smaller building-blocks. This strengthens the interest in studying especially this comet and its internal structure. Such an experiment may provide deeper insight in the earlier growth processes in the preplanetary disk. The investigation of the nucleus’ structure will help to understand the processes of formation and origin of cometary nuclei (accretion from smaller bodies, disruption of parent bodies, homogeneous growth, etc.). Knowledge of the mechanical properties of the surface layers (in the dm- to m-range) is a key in understanding the evolution and future development of comet nuclei with respect to, e.g. outgassing, crustal formation, surface evolution, layering, increasing thickness of surface layers or quenching of outgassing, and especially the phenomenon of variable cometary activity, related to the volatile (water) budget of the surface layers. These data about the “pristine” composition will help to understand the origin and evolution of cometary nuclei. This is the main scientific objective of the ROSETTA mission and the Lander experiments. Furthermore, a detailed knowledge of the upper surface layers of the nucleus of P/46 Wirtanen will be a necessary prerequisite to derive the pristine composition and structure of this nucleus. The gases propagating from the sites, where they are released, are monitored by the Lander as well as by the orbiter. This gas emission is strongly influenced by the physical status of the upper surface layers.

Large cometary nuclei with a sufficiently strong gravitation are assumed to have a gravitationally-caused formation of at least local mantles or surface covers (Kührt and Keller, 1994, Möhlmann, 1994, Möhlmann, 1996a, Möhlmann, 1996b), formed out of the sublimation residua, the refractory material. Due to the small mass of comet 46P/Wirtanen, it is expected that its surface is made up mainly of a bare ice-crust.

In some parts of the cometary surface, e.g. in polar latitudes not irradiated by the sun, or even in valleys, regolith material may be accumulated, e.g. by avalanches (Grün et al., 1993). The surface and the subsurface region are expected to consist of layers, made up of hardened ice-mineral mixtures, with a cohesion ranging in hardness from a few kPa to some MPa, caused by sintering processes within the granular material and the recondensation of the different sublimated ice constituents. Our knowledge of the physical properties of cometary matter and the related structure of cometary nuclei is limited, and the nature of the interface between the feet of the Lander and the cometary surface is largely unknown. This requires the design of Lander feet which on one hand, ensures sufficient acoustic coupling to the cometary ground and on the other hand, it guarantees a safe landing without the danger that the Lander crashes on or into the ground.

To cope with the unknown mechanical conditions on and in the cometary surface, acoustic wave propagation experiments were made in various materials, to simulate different cometary conditions. Material analogues to regolithic dust and sand on one hand, and hardened ice dust mixtures on the other, were studied in the laboratory and in the outdoors. The technical design of the CASSE space experiment (Möhlmann, 1995) to be performed on comet Wirtanen requires extensive laboratory simulations and this paper reports some of these experiments.