MIDAS: Micro-Imaging Dust Analysis System
MIDAS (Micro-Imaging Dust Analysis System) will study the dust environment around the comet. In particular, MIDAS will perform microtextural and statistical analysis of cometary dust particles. The instrument is based on the technique of atomic force microscopy. This technique, under the conditions expected to prevail, both at the comet and during the cruise phase, permits textural and other analysis of dust particles to be performed down to a spatial resolution of 4 nm. MIDAS also studied the dust environment of the two asteroids, Steins and Lutetia, that were flown by during the journey to comet 67P/Churyumov-Gerasimenko.
During the rendezvous with the comet MIDAS will provide the following information:
- Images of single particles with a spatial resolution of 4 nm
- Statistical evaluation of the particles according to size, volume, and shape
- Size distribution of particles ranging from about 4 nm to a few µm
- Shape, volume and topographic structure of individual particles
- Temporal variation of particle fluxes
- Spatial variation of particle fluxes
- Measurements on local elastic properties if studies show that they do not affect the tip lifetime
During the cruise phase to the comet MIDAS provided:
- Characterization of the dust environment in the vicinity of the asteroids Steins and Lutetia
- Characterization of dust particles detected in the vicinity of the comet
MIDAS will deliver global images, that is complete images of the entire scan field, and images of individual dust particles. The individual images are contained in the global images, since candidate particles for detailed imaging are identified using the global image. The selected particles are then re-scanned with a much higher resolution.
MIDAS is designed to analyse microdust particles collected in the interplanetary and cometary environment, irrespective of their electrical conductivity and shape, by means of atomic force microscopy. The sizes of the particles range from about 4 nm to a few µm. The dust collector includes a mechanism which controls the particle flux onto a wheel made of polished silicon. After analysis, another of the 64 facets of the wheel is exposed to the ambient dust flux. The MIDAS microscope consists of five functional parts: a one shot cover and a funnel to protect the aperture on the ground and during launch, the shutter to define the exposure time to the dust flux, the robotics system for manipulation of the dust particles, the scanner head, and the supporting electronics.
The heart of the atomic force microscope (AFM) is a very small tip which maps the surface of the particle. An AFM is capable, in principle, of imaging details down to atomic resolution. In the simplest case, the tip remains in permanent contact with the surface and follows its height variations with a control mechanism which keeps a constant force on the tip (contact mode). In a technically more complex mode, the tip scans the surface while its supporting cantilever vibrates at one of its natural resonance frequencies. There are two dynamic modes: (a) the tip does not come closer to the surface than a few tenths of a nanometre (non-contact mode) or (b) the tip hits the surface during its sinusoidal oscillation (tapping mode). In all three modes it is essential either to keep the force constant or to measure it accurately in order to derive an image of the surface.
The tip must move over the surface in a reproducible manner, which can be relatively easily achieved by piezo electric scanners in three independent directions. The combination of the tip, supporting cantilever, and piezo-electric actuators is called scanner head. Due to lifetime requirements, several tips will be employed.
The MIDAS instrument consists of one mechanical unit. The top part of the instrument unit houses the elements of the atomic force microscope and the system to collect and transport the dust samples to the head of the microscope. The dust intake system is connected to the instrument unit and protrudes through the outer spacecraft wall. The control electronics which must be near the sensor and the actuators are also accommodated in the top part of the instrument unit. The lower part of the instrument unit contains the remaining digital and analogue electronics and the interfaces to the spacecraft.
The dust intake system consists of a dust cover at the outside and a funnel. The cover is opened after launch by a pyro actuator. The path of the dust particles leads through a funnel through the spacecraft skin. The inner edge of the funnel points towards the entrance slit of the main instrument box, with some minimum clearance towards the box. The particles can enter the AFM via a slit. Beneath the slit there is a shutter which can be opened and closed in order to control the optimum exposure time of the facets on the dust collector; 64 facets, with an area of about 3.5 mm2 each, are located on the circumference of the collector wheel. This surface area is defined with a diaphragm located between the slit and the facet. After exposure, the facet rotates from the position behind the slit to the analysis position.
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Last Update: 02 November 2015