Calibration activity in support of GIADA, part of the ESA/Rosetta Spacecraft payload.

Comets formed about 4.6 billion years ago from the Pre-Solar Nebula and spent most of their existence in the outer and colder regions of the Solar System, beyond the Neptune’s orbit. These small celestial bodies, with masses ranging between 1014g and 1021g, could be considered the cosmo-chemical record of the primordial Solar Nebula. Therefore studying comets means investigating the physic-chemical properties of the material that formed the Solar System. With the aim of exploring the nature and the composition of these fascinating objects, several cometary space missions have been realized since the 1980’s. The first comet to be observed by a space probe, the NASA/ISEE 3 spacecraft, was comet 21P/Giacobini-Zimmer in 1985. One year after comet 1P/Halley was deeply studied by five different space probes: Vega 1 and Vega 2 (Soviet Union), Sakigake and Suisei (Japan), and GIOTTO (ESA). More recently two NASA missions, Deep Space 1 and Stardust, observed comet 19P/Borrelly and 81P/Wild 2, respectively, showing a considerable and unexpected mineral diversity in these objects. Despite the improvements provided by all these space missions, our knowledge about cometary nuclei, comae and activity remains rather primitive in many aspects. An incredible opportunity to make a critical step forward in cometary science and more in general in the knowledge of the Solar System origin is represented by the ESA space mission, Rosetta (Chapter 1). Thanks to the encounter with 67P/Churyumov-Gerasimenko (67P/CG), Rosetta will allow an extensive study of the mineral variety present in this Jupiter Family comet. Moreover this mission, for the first time, will undertake a comet long-term monitoring at close quarters along its orbit, allowing the study of the comet behaviour from the moment the cometary activity will start, when it will reach its maximum at perihelion and after it when it will slowly decrease. Among the instruments mounted on-board Rosetta, there is the Grain Impact Analyser and Dust Accumulator (GIADA) developed with the aim of studying the cometary dust environment, the evolution of the dust flow and the dynamics of each single grain as a function of time and position, i.e. at variable distances of the spacecraft from the nucleus and of the comet from the Sun. GIADA is designed as a single instrument composed of three different detection sub-systems: 1- Grain Detection System (GDS): detects each incoming grain, providing its optical equivalent size and measuring its speed, without affecting its dynamical properties. 2- Impact Sensor (IS): measures the momentum released from each grain impacting its sensitive surface. 3- Micro-Balances System (MBS): a network of five Quartz Crystal Microbalances, pointing toward different directions, that measure the cumulative dust deposition in time and monitor the dust flux ejected from the comet. Thanks to its features, described in Chapter 2, GIADA will contribute to the achievement of two of the five Main Goals assigned to the Rosetta space mission: - Define the physical properties and interrelation of volatiles and refractories in a cometary nucleus; - Study the development of cometary activity and processes in the surface layer of the nucleus and in the inner coma (dust-gas interaction). Rosetta was launched on the 2nd March 2004 and will reach, at a distance of 4 AU from the Sun, the comet 67P/CG on May 2014, after a long trek lasted 10 years around the Sun. In order to limit power and fuel consumption, and to minimise operating costs, Rosetta was put in hibernation on the 8th of January 2011. The seven years after launch and prior to hibernation are called Cruise Phase. During the Cruise Phase 13 Payload Checkouts (PCs) have been executed in order to verify the instruments functionalities and to perform the maintenance. GIADA detection sub-systems were switched on to monitor their health state and verify their performances. The results of the analysis of the data collected by GIADA during these tests, reported in Chapter 3, show that GIADA functionalities and performances have maintained their nominal behaviour; only the cover mechanism showed a non-nominal behaviour, which had a small impact on the microbalance sub-system: contamination deposited on the QCM because of the cover remained open caused a small decrease of their dynamical range. This will not affect the GIADA measurement capability during the Rosetta comet encounter and escort phase. Rosetta has awakened from hibernation on the 20th January 2014 and now its payloads are undergoing the post-hibernation commissioning, a phase devoted to the instruments re-activation-maintenance and checkout activities in preparation to the rendez-vous with comet 67P/CG. GIADA has successfully executed its first post-hibernation commissioning on the 27th March 2014. In preparation to comet encounter, the GIADA Team planned an extended calibration activity (Chapter 4) devoted to create a database of the sub-systems response to different kind of cometary dust analogue grains (composition, mineralogy, ice and carbon coatings) that will be used to support the analysis of the data that GIADA will collect during the comet phase. This activity was planned taking into account the new knowledge on cometary dust composition reached after the NASA/Stardust space mission. In order to well characterize the instrument responsivity with respect to realistic cometary analogue materials, some dust samples were prepared, taking into account the GIADA sub-systems sensitivities (GDS and IS), in four distinct size classes: 20 μm<O<50 μm, 50 μm<O<100 μm, 100 μm<O<250 μm and 250 μm<O<500 μm. Single grains of these selected materials were shot into the GIADA Proto Flight Model, i.e. the spare of the instrument on board Rosetta operating in a clean room in our laboratory, with velocities in the range 1-100 ms-1. In order to capture, manipulate and shoot these small grains, an innovative Electrostatic Micromanipulator has been developed. The use of this tool resulted to be very useful during the calibration activities. In addition, two new empirical methods were developed with the aim to improve the capability of reconstructing the IS impact position and the GDS crossing position of each single grain entering into GIADA. These are key issues to scale the signal detected by the two sub-systems using the sensitivity maps measured during the pre-launch calibrations. The method developed for the GDS measurements was used during the present calibration activity with cometary dust analogues that led to the assembly of the calibration curves for each different material. The results obtained after the work performed during the three years of PhD represent a significant step forward within the GIADA project that will surely critically contribute to make GIADA performances, data acquisition and reduction a success.