The MIDAS experiment for the Rosetta Mission

This paper describes the scientific objectives and the basic concept of the instrument MIDAS, which has been selected for the preliminary payload of the Rosetta Orbiter. The instrument is considered essential for this mission as, for the first time, it has the capability of three dimensional imaging of interplanetary and pristine cometary particles in the nanometer to micrometer range. According to cometary dust models and experience gained from Giotto and VEGA, it appears that there is an abundance of particles in this size range, which also covers the building blocks of pristine interplanetary dust particles. Some models assume that they consist of silicate core particles (0.1 - 0.2 #mu#m diameter) and #approx# 0.01 #mu#m refractory organic mantles. The instrument will collect and image particles by atomic force microscopy and provide -morphological and statistical information on the dust population, including texture, shape, size and flux. NMAS is based on an atomic force microscope (AFM), a type of scanning microprobe capable of three-dimensional imaging of small structures. The instrument features a dust collector wheel with #approx# 200 facets on its perimeter, each providing hundreds of independent scan areas of up to 50 by 50 #mu#m. The dust collector points to the comet and collects particles drifting outward from the cometary surface. Although the AFM is a proven laboratory technology, MIDAS will be its first application in space. An extensive design verification programme has been initiated.