urn:esa:psa:context:instrument:ro.midas 1.0 1.11.0.0 Product_Context urn:nasa:pds:context:instrument:midas.ro 2021-02-24 1.0 Changed inst LIDs from u:n:p:c:i:instID.scID to u:n:p:c:i:scID.instID Changed LIDs from urn:nasa:pds: to urn:esa:psa: And per "Guide toPDS4 Context Products" v1.7, changed all lidvid_reference to lid_reference urn:esa:psa:context:instrument_host:spacecraft.ro instrument_to_instrument_host Riedler, W. et al., MIDAS - The Micro-Imaging Dust Analysis System for the Rosetta Mission, Space Sci. Rev., 128(1-4), 869-904, Feb. 2007. reference.RIEDLERETAL2007 MICRO-IMAGING DUST ANALYSIS SYSTEM Microscope not applicable not applicable This catalog file contains excerpts from Riedler et al, 2006. Refer to this paper for an in depth description of the instrument. Scientific Objectives ===================== The main objective of the instrument MIDAS (Micro-Imaging Dust Analysis System) on board of the Rosetta Orbiter is the analysis of particles collected in the coma of Comet 67P/Churyumov-Gerasimenko by means of atomic force microscopy. The instrument will provide 3D images and statistical parameters of the particles in the nm-um range. The main anticipated results include - 3D images of single particles, - images of the textural complexity of particle aggregates, - identification of crystalline material, - identification of sub-features on clean surfaces which provides insight into the growth conditions (e.g. twinning defects) and/or storage environment (e.g. dissolution marks), - statistical evaluation of the particles according to size, volume and shape, - variation of particle fluxes between individual exposures of the collector unit on time scales of hours. Measurement Principle ===================== An Atomic Force Microscope forms the heart of MIDAS. A sharp tip scanning the surface of a sample is used to derive information from the tip-sample interaction, which depends on the nature of the tip and is based on mechanical force and - in connection with suitable sensors - magnetic force. The surface features can be resolved with almost atomic resolution. MIDAS adopts dynamic AFM as the main working mode and employs piezo-resistive cantilevers that can detect their own deflection electrically. In this mode, the cantilever is excited at its natural mechanical resonance frequency (~100 kHz) at close distance to the sample. The amplitude of the cantilever vibration is of the order 100 nm. Close to the surface, the resonance frequency of the cantilever changes due to the tip-surface interaction forces. The changes of resonance frequency and amplitude are used to derive the topography of the sample in a closed loop system. The basic elements of an AFM are a piezoelectric scanner for cantilever movement, a tip mounted near the edge of the cantilever, a detection system for cantilever deflection, and a feedback system to control the vertical tip position. In addition, a coarse approach system is needed to bring the tip to within the working distance of the piezoelectric scanner. Instrument Overview =================== The MIDAS instrument is a single mechanical unit. The top part houses the elements of the AFM and the system to collect and transport the dust samples from the exposure position to the head of the microscope. A funnel-shaped dust intake system, which protrudes through the outer spacecraft wall, is attached to the upper part of the box. An external cover protects the internal elements against contamination during ground operations and launch. A shutter at the outer end of the funnel controls the exposure time of one facet at a time on the dust collector wheel inside the box. This wheel can be rotated and translated to transport the dust samples to one of 16 sensor elements (cantilevers) in the AFM after exposure. The array of cantilevers is fixed to the Z microscope stage, which is attached to the high-resolution XY positioning stage. All stages are driven by piezoelectric systems, and the displacements can be measured by a strain gauge and capacitive sensors. In addition, the voltages applied to the piezoelectric actuators provide the same displacement information through the respective calibration curves. To begin the imaging process, the XYZ-stage is brought into contact with the sample on the dust collector wheel by the approach mechanism. As soon as the selected cantilever senses contact with the sample (indicated by a deflection of the cantilever in contact mode, or a change of amplitude in dynamic mode), the scanning operation can start. After the measurement, the XYZ-stage has to be retracted, and another approach procedure has to be performed before the measurement on another area. The 16 available needles have slightly different characteristics and shapes, and four out of them are additionally sensitive to magnetic forces. As the measurements by the AFM are sensitive to microvibrations induced by other active mechanisms aboard Rosetta, the AFM element is fixed to the base plate of the box by four studs of highly flexible silicone material with a high damping capability. During ground operations and launch, the AFM platform was locked in its zero position by a clamping device, which is unlatched after launch by a mechanism using paraffin actuators. The lower part of the instrument serves as the electronics box, which could be decoupled from the upper section to help the assembly process. Most of the printed circuit boards are located in the lower section, but a few sensitive amplifiers reside in the AFM area close to the sensors. The electronics box has a connector panel for redundant electrical interfaces to data, power and checkout. Some additional connectors, e.g. for pyrotechnic actuators, are mounted on the AFM box. Operational Modes and On-Board Software ======================================= MIDAS has to acquire and analyse samples of cometary dust at regular intervals throughout all phases of the mission. At first, the selected facet of the dust collector wheel has to be exposed to the ambient dust flux until the required surface coverage by dust is achieved. The best coverage value remains to be determined experimentally and will also depend on the type of analysis planned (imaging of individual particles or statistics). Current estimates indicate a surface coverage of 0.1% should be sufficient for most purposes. The required exposure times range from fractions of an hour during high cometary activity at close distances to the nucleus (few cometary radii) to several days during moderate activity and/or larger distances. Apart from the facet selection and shutter operations at the beginning and end, MIDAS may be left unpowered. Alternatively, if it remains in standby mode, it is able to listen to broadcast messages that may be used to adjust the exposure time autonomously. If MIDAS is powered during exposure, it may also simultaneously process previously obtained images. After exposure, the shutter is closed and the target wheel can be turned to another position for the exposure of another facet, or a facet can be placed underneath the scanner head. AFM images of the exposed target areas are obtained by scanning. At the beginning of the scan mode, an area on the collector wheel is selected by rotating the chosen facet under the scanner head. In each scan, only a small part of the exposed surface is imaged by the microscope. The selection of scan areas within a facet is achieved by fine control of the angle and lateral position of the wheel. A single exposure period will be followed by several scanning operations on the previously exposed facet at varying resolutions. It is also possible to scan or rescan facets that have been exposed much earlier. The time needed to scan a single area on a facet depends on the operational settings and is of the order of several hours. Processing and transmitting the acquired data is typically performed after completion of a scan (image acquisition). Typically, processing time is short compared to the image acquisition time. The baseline MIDAS operations involve alternating between exposure, scanning and processing, but data collection simultaneously with the processing and transmission of collected images is also possible. Some images will be reduced to simple statistical parameters for transmission (for example, the number and sizes of dust grains), while others will be studied at full resolution. The standard modes for exposure, scanning, and image processing as well as the associated positioning of the target surfaces are complemented by modes for checkout and in-flight calibration. The MIDAS software is structured into a low-level software kernel and a high-level main program. Every time the instrument is switched on, the kernel program is loaded into the Random Access Memory (RAM) and executed. After a successful check of the Electrically Erasable Programmable Read-Only Memory (EEPROM), the main program is started and the instrument enters standby mode. The software kernel handles the interface between the hardware and the main program, basic telecommand processing, generation of basic housekeeping telemetry, software maintenance and uploads, timing and low- level control of the hardware. Main program tasks include high-level instrument control, scientific and extended housekeeping data generation, and image processing. Control of the instrument relies on sequences of low-level commands combined into larger packets. Recognised packets are processed with high or normal priority, depending on the telecommand type. Commands with high priority are executed immediately. They include time-critical commands, such as program abort, error reporting, and all commands processed by the kernel such as telecommand acknowledgements. Commands with normal priority are stored in a command buffer and executed sequentially. The low priority buffer is used for housekeeping and science data transfer where the packets are grouped into larger blocks. Two different subtypes of housekeeping data packets and six subtypes of science data packets have been defined. Science data packets are generated during or after acquisition of an image and processing of the image data. In general, data from one image will extend over many packets, depending on the resolution. Among the routinely used modes, only the scan mode produces raw science data that are stored temporarily in instrument memory. The size of an image can be set to multiples of 32 pixels in each direction, with a maximum of 512 pixels. A typical single image of 256x256 pixels requires 128 Kbyte of memory. As 1 Mbyte is available for raw data, eight typical images can be held. Onboard image data processing is foreseen for two main reasons: the reduction of the data volume, and the identification and characterisation of dust grains to determine the coordinates of following scans (zoom-in). Key Capabilities and Characteristics ==================================== Maximum scan field 94 x 94 um Minimum scan field 0.97 x 0.97 um Image resolution 256 x 256 pixels (typ.), 512 x 512 pixels (max.) Lateral resolution variable 3.8 nm ... 400 nm Height resolution 0.16 nm Number of AFM tips 16 Number of targets 61 Mass 8.2 kg Power consumption 8.3 to 13.4 W (peak power <17 W) Telemetry rate 100 bit s-1