GIADA: Grain Impact Analyser and Dust Accumulator
GIADA (Grain Impact Analyser and Dust Accumulator) will measure the number, mass, momentum and velocity distribution of dust grains in the near-comet environment. Giada will analyse both grains that travel directly from the nucleus to the spacecraft and those that arrive from other directions having had their ejection momentum altered by solar radiation pressure.
The primary scientific objectives of GIADA (Grain Impact Analyser and Dust Accumulator) are:
- Dust flux measurement for "direct" and "reflected" grains
Two populations of cometary grains exist: "direct" (coming directly from the nucleus) and "reflected" grains (coming from the Sun direction, under the action of the solar radiation pressure). The two populations undergo very dissimilar dynamic evolution in the coma and have different times of ejection from the nucleus. In the case of Rosetta, "direct" and "reflected" grains can be collected simultaneously. The relative amount will depend on the probe position along its orbit. GIADA will be able to monitor grain fluxes coming from different directions and will allow, for the first time, discrimination between the two dust populations. This task is fundamental to the determination of the original dust size distribution. In turn, this information is required to define the dust mass loss rate.
- Analysis of the dust velocity distribution
The dust ejection velocity depends both on the grain size and on time. Moreover, grains with a given size have a wide dust velocity distribution. GIADA will allow the measurement of scalar velocity and momentum for grains coming from the nucleus direction so as to give mass and impact velocity of each analysed "direct" grain. From this information it will be possible to derive grain mass and ejection velocity from the nucleus surface. For the first time we will obtain:
|- ||the size dependence of the dust ejection velocity|
|- ||the relation between most probable dust velocity and dust mass|
|- ||the velocity distribution for each dust mass|
|- ||the link between velocity dispersion and dust mass|
- Study of dust evolution in the coma
Once ejected from the nucleus, grains may change their physical properties due to several processes, including, for example, fragmentation. These modifications may alter the grain size distribution. The size distribution of grains collected by GIADA in the nucleus direction should not be affected by the dust velocity dispersion. Thus, changes in the dust distribution at different nucleus distances can be linked directly to actual variations in the dust size distribution and correlation can be found with dust fragmentation and/or with emission from active areas on the nucleus.
- Correlation of dust changes with nucleus evolution and emission anisotropy
The dust environment characteristics depend on the comet-Sun distance and on the time evolution of the nucleus. The continuous monitoring by GIADA of dust flux and dynamic properties will offer the best opportunity to characterise the time evolution of the dust environment as a function of heliocentric distance. Nucleus imaging will allow us to link observed changes to the nucleus evolution and to its spin state.
- Determination of dust to gas ratio
One of the crucial parameters characterising the comet nucleus is the dust to gas ratio. Dust flux monitoring by GIADA is needed to estimate the dust to gas ratio. This will be possible in combination with results of other experiments.
- Other objectives
The data provided by GIADA about dust fluxes and grain dynamic properties are very important for the correct interpretation of images of the coma and nucleus and mass spectrometer data.
GIADA will help in the selection of the surface science package landing site. The characterisation of dust emitting areas, and possibly of the dust population of different active areas, will be necessary for the site selection process to achieve a proper balance between safety and scientific interest.
GIADA will play an important role for the health and the safety of various experiments and the spacecraft itself, as it will be able to provide information about dust flux in several directions. Optical surfaces of experiments and other devices pointing to the nucleus will be polluted by the dust flux. GIADA data will allow the prediction of deposition rates and informed decision making for mission planning and operations. Data from GIADA will be the only resource to predict and allow control of the performance degradation of critical devices such as passive radiators and solar panels.
The instrument comprises three modules: GIADA 1 measures momentum, scalar velocity and mass of single grains entering the instrument by the Grain Detection System (GDS) and the Impact Sensor (IS), placed in cascade. The GIADA 2 module contains the main electronics (ME); it controls the acquisition of data from the sensors and the operation of the other subsystems. It also provides the power supply for the whole experiment. The GIADA 3 module measures the cumulative dust flux and fluence from different directions by means of five microbalances. One microbalance points towards the nucleus, while the other four cover the widest possible solid angle.
In the GDS, four laser diodes with their fore-optics are used to form a thin (3 mm) light curtain (100 cm2). For each grain passing through it, the scattered/reflected light is detected by two series of four detectors (photodiodes) placed at 90 deg with respect to the sources. In front of each photodiode a Winston cone is placed to achieve a uniform sensitivity in the detection area.
The IS is a thin (0.5 mm) aluminium square diaphragm (sensitive area 100 cm²) equipped with five piezoelectric sensors, placed below the corners and its centre. When a grain impacts the sensing plate, flexural waves are generated on the plate, and are detected by the piezoelectric crystals. The maximum displacement of these systems is directly proportional to the impulse imparted, and the displacement of the crystal produces a proportional potential. Through calibration, a known impulse may be equated with a specific charge produced on the electrodes of the PZT crystals. The detected signal is proportional to the momentum of the incident grain through the factor (1+e), where e is the coefficient of restitution.
When a grain enters GIADA 1, the GDS gives a first estimate of the grain speed and starts a time counter that is stopped when the IS detects the grain impact and the momentum is measured. In this way, for each entering grain, speed, time-of-flight, momentum and, therefore, mass are measured.
The microbalances in GIADA 3 each consist of two quartz crystals oscillating at a frequency of about 15 MHz, one acting as a sensor, the other as a reference. The measured physical quantity is the beat frequency between the two crystals. The resonance frequency of the sensor quartz oscillator, exposed to the dust environment, changes due to the variation of its mass as a result of material accretion, while the reference crystal is not exposed to the dust flux. Thus, the output signal is proportional to the mass deposited on the sensor and dust flux and fluence are measured in time. The use of a reference crystal ensures extremely small dependence on temperature and power supply fluctuations and, thus, high sensitivity.
||COSIMA: Cometary Secondary Ion Mass Analyser
||MIDAS: Micro-Imaging Dust Analysis System
Last Update: 25 May 2010