Scientific Objectives

Dust particles emitted from cometary nuclei form a major source of information for the understanding of primitive matter, because they are probably the least modified material since the formation of the Solar System 4.5 Gyr ago.

Cometary material was analysed in situ for the first time in 1986. The mass of the particles collected in the coma has been estimated between 10^-15g and 10^-17g, which corresponds to a radius between 0.01 and 0.5 µm. The prime scientific objective of this investigation is to image the micro-topography, and micro-textural units of cometary dust particles, thereby allowing to characterise the nature of these particles (for example, building blocks), but also the definition of sub-features on clean surfaces provides insight into either the growth conditions (for example, twinning defects) and/or storage environment (for example, dissolution marks, brecciation).

In principle surface images give the opportunity to identify objects according their specific shape or texture. Furthermore endogenous and/or exogenous processes often form and change the appearance of a surface. For example on radar images of a planetary landscape one can identify volcanoes and deep trenches that are results of sub-crustal magmatic and tectonic activities.

Additionally if loose debris would be detected, the presence of erosion by some exogenous media like liquids streams and wind could be stated. A similar scenario applies for the microcosm, for example the shape and outer appearance of complex but nevertheless small (micrometre-sized) particles is heavily influenced by the interior. The habitus of crystalline phases is driven by growth conditions. A crystalline surface may show traces of interaction with liquids, gases and radiation. High-resolution images of cometary material therefore provide information about the material and the environment.

Following the mapping of single particles with a resolution in the few nanometre-range many statistical parameters can be used to describe the cometary environment. This comprises the statistical evaluation of the collected particles according to size, volume and shape, but also temporal and spatial variations of the particle flux can be deduced.

In summary, MIDAS will meet the scientific objectives when the following information can be obtained during the rendezvous with the comet:

• 3-D images of single particles with a resolution better than 10 nm
• Search for "very small particles" (10 nm)
• Search for evidence of euhedral crystals
• Possible detection of ferro-magnetic minerals
• Size distribution of particles
• Variation of particle fluxes on time scales between hours and days

Payload

An outside view on the MIDAS instrument as it is mounted on the Rosetta spacecraft. The funnel penetrates the spacecraft hull and if opened dust particles can enter the instrument.

The dust grains in the size range from 4 µm down to 4 nm will be imaged in 3-D by means of atomic force microscopy (AFM). AFM makes use of tiny physical forces (van der Waals, interatomic, magnetic etc.) that act on a sensor in closest distance to a surface.

The sensor is a 600 µm long cantilever arm with an extremely sharp 7 µm long tip mounted underneath. The sensor is controlled by a piezoelectric mechanical system which scans above the surface and senses its topography.

The dust enters the instrument via a funnel penetrating the spacecraft's hull and hits the collector surface. Sixty-four of these targets, coated silicon facets, are mounted on the perimeter of the dust collector wheel. Via rotation the facet exposed to the dust stream is presented to the microscope which approaches the surface automatically and starts the scanning (imaging) process.

Usually an overview image will be used to identify collected particles and the automatic zoom-in function takes a detailed high resolution image (nanometre-scale). A defined scanning strategy collects data covering all grain size classes in the working range of the microscope.

Looking inside MIDAS, with the main components labelled.

Performance Specifications

Selected instrument performance specifications are summarized in the table below. The table compares specifications as stated in the original instrument proposal with their realization in the MIDAS flight model.

It becomes immediately obvious that the flight instrument matches the predictions from the original proposal. On top of that the flight model has enhanced capabilities due to additional working modes and data channels. The only variance is the length of time in which images will be acquired or further processed. Due to less processing capabilities as originally planned, the scan duration for one image exceeds the primarily envisaged 600 seconds. However, neither the scientific value of the images nor the capability to obtain a sufficient database is limited by this design change. The overall scientific performance has been increased when compared to the initial proposal due to implementation of more data channels and advanced working modes.

Instrument commissioning, 5-9 April 2004

After a near perfect orbit insertion of the Rosetta spacecraft by Ariane 5 rocket on 2 March 2004 the spacecraft went onto an interplanetary course. Subsequently spacecraft and scientific payload have been tested extensively.

In the test sequence of the MIDAS instrument, which ran over five contact passes from the ground station to the spacecraft, the following results were obtained:

• Complete electronics checkout
• Cover opening by a pyrolitic device
• Unlocking of all clamp mechanisms
• Movement of linear stage and approach mechanism out of launch position
• Verification of all motors and mechanisms
• Verification of all 16 sensors by resonance search
• Verification of the scanner by imaging calibration surfaces
• Initial characterization of mechanical noise environment

Collaborating Institutions

Principle Investigator

• W. Riedler, K. Torkar
• IWF Graz, Austria

Hardware contribution by:

• IWF Graz, Austria
• ESA/ESTEC, Science Payload and Advanced Concepts Office, Netherlands
• Universität Kassel, Germany
• Technische Univeristät Wien, Austria

Scientific advice by:

• Institut für Geologische Wissenschaften, Universität Halle, Germany
• Department of Physics, University of Sheffield, Great Britain
• Institut für Geochemie, Universität Wien, Austria
• Sterrewacht Leiden, Netherlands
• Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany
• University of Tromso, Auroral Observatory, Tromso, Norway
• Université Paris 6, Aéronomie CNRS, Service d'Aéronomie, Verrières, France
• Astrophysique du Solide, C.S.N.S.M., Orsay, France

Web Links

• ESA Science and Technology Pages
• Space Research Institute, Graz, Austria