1,027 research outputs found
The Juno mission Gravity Science experiment: a semi-analytic theory
La missione spaziale Juno, progettata dalla NASA, ha come obiettivo lo studio del pianeta Giove. La sonda di Juno è stata lanciata da Cape Canaveral il 5 Agosto del 2011 e raggiungerà Giove nel 2016. All'arrivo, verrà inserita in un'orbita polare molto eccentrica con periodo di 11 giorni. La durata della missione nominale è di 32 orbite, durante le quali raccoglierà dati e informazioni che comunicherà al Deep Space Network in California. L'esperimento di Gravity Science consiste nella determinazione del campo gravitazionale di Giove per mezzo di misurazioni Doppler della sonda. Tale esperimento verrà effettuato durante i passaggi al pericentro delle orbite 8-32 ed è fondamentale per rivelare la struttura interna di Giove, in quanto aiuterà a mappare la distribuzione di massa all'interno del pianeta, dando indicazioni precise sulla massa del nucleo. Affinché ciò sia possibile, l'incertezza con cui verrà determinato il campo gravitazionale non dovrà superare una soglia precedentemente stabilita; è essenziale, dunque, uno studio preliminare sull'accuratezza dell'esperimento, che possa predire il minimo errore che ci si deve aspettare.
L'obiettivo di questa tesi è sviluppare una teoria semi-analitica per lo studio dell'incertezza con cui può essere determinato il campo gravitazionale di Giove, che sia dipendente esclusivamente dalla geometria dell'orbita della sonda Juno
On the Juno Radio Science Experiment: models, algorithms and sensitivity analysis
Juno is a NASA mission launched in 2011 with the goal of studying Jupiter.
The probe will arrive to the planet in 2016 and will be placed for one year in
a polar high-eccentric orbit to study the composition of the planet, the
gravity and the magnetic field. The Italian Space Agency (ASI) provided the
radio science instrument KaT (Ka-Band Translator) used for the gravity
experiment, which has the goal of studying the Jupiter's deep structure by
mapping the planet's gravity: such instrument takes advantage of synergies with
a similar tool in development for BepiColombo, the ESA cornerstone mission to
Mercury. The Celestial Mechanics Group of the University of Pisa, being part of
the Juno Italian team, is developing an orbit determination and parameters
estimation software for processing the real data independently from NASA
software ODP. This paper has a twofold goal: first, to tell about the
development of this software highlighting the models used, second, to perform a
sensitivity analysis on the parameters of interest to the mission.Comment: Accepted for publication in MONTHLY NOTICES of the Royal Astronomical
Society 2014 October 31. Received 2014 July 28; in original form 2013 October
A Fast Alternating Minimization Algorithm for Total Variation Deblurring Without Boundary Artifacts
Recently, a fast alternating minimization algorithm for total variation image
deblurring (FTVd) has been presented by Wang, Yang, Yin, and Zhang [{\em SIAM
J. Imaging Sci.}, 1 (2008), pp. 248--272]. The method in a nutshell consists of
a discrete Fourier transform-based alternating minimization algorithm with
periodic boundary conditions and in which two fast Fourier transforms (FFTs)
are required per iteration. In this paper, we propose an alternating
minimization algorithm for the continuous version of the total variation image
deblurring problem. We establish convergence of the proposed continuous
alternating minimization algorithm. The continuous setting is very useful to
have a unifying representation of the algorithm, independently of the discrete
approximation of the deconvolution problem, in particular concerning the
strategies for dealing with boundary artifacts. Indeed, an accurate restoration
of blurred and noisy images requires a proper treatment of the boundary. A
discrete version of our continuous alternating minimization algorithm is
obtained following two different strategies: the imposition of appropriate
boundary conditions and the enlargement of the domain. The first one is
computationally useful in the case of a symmetric blur, while the second one
can be efficiently applied for a nonsymmetric blur. Numerical tests show that
our algorithm generates higher quality images in comparable running times with
respect to the Fast Total Variation deconvolution algorithm
Addressing some critical aspects of the BepiColombo MORE relativity experiment
The Mercury Orbiter radio Science Experiment (MORE) is one of the experiments
on-board the ESA/JAXA BepiColombo mission to Mercury, to be launched in October
2018. Thanks to full on-board and on-ground instrumentation performing very
precise tracking from the Earth, MORE will have the chance to determine with
very high accuracy the Mercury-centric orbit of the spacecraft and the
heliocentric orbit of Mercury. This will allow to undertake an accurate test of
relativistic theories of gravitation (relativity experiment), which consists in
improving the knowledge of some post-Newtonian and related parameters, whose
value is predicted by General Relativity. This paper focuses on two critical
aspects of the BepiColombo relativity experiment. First of all, we address the
delicate issue of determining the orbits of Mercury and the Earth-Moon
barycenter at the level of accuracy required by the purposes of the experiment
and we discuss a strategy to cure the rank deficiencies that appear in the
problem. Secondly, we introduce and discuss the role of the solar
Lense-Thirring effect in the Mercury orbit determination problem and in the
relativistic parameters estimation.Comment: 29 pages, 5 figures. Presented at the Seventh International Meeting
on Celestial Mechanics, San Martino al Cimino (Viterbo, Italy), 3-9 September
201
Mineralogical and Chemical Investigations of the Amguid Crater (Algeria): Is there Evidence on an Impact Origin?
Mineralogical and chemical investigations were carried out on intra-craterial bedrocks (Lower Devonian sandstone) and regolithic residual soil deposits present around the Amguid structure, to discuss the hypothesis of its formation through a relatively recent (about 0.1 Ma) impact event. Observations with an optical microscope on intra-craterial rocks do not unequivocally confirm the presence of impact correlated microscopic planar deformation features (PDFs) in quartz crystals. Field observations, and optical and instrumental analysis (Raman spectroscopy) on rocks and soils (including different granulometric fractions) do not provide any incontrovertible pieces of evidence of high energy impact effects or products of impact (e.g., high pressure—temperature phases, partially or totally melted materials, etc.) either in target rocks or in soils. A series of selected main and trace elements (Al, Fe, Mg, Ni, Co and Cu) were analysed on rocks and soils to evaluate the presence in these materials of extraterrestrial sources. Comparative chemical data on rocks and soils suggest that these last are significantly enriched in Fe-poor Mg-rich materials, and in Co, Ni and Cu, in the order. A large number of EDAX-SEM analyses on separated soil magnetic particles indicate an abnormally high presence of Al-free Mg-rich sub-spherical or drop-like silicate particles, showing very similar bulk chemistries compatible with forsterite olivine. Some particles were found associated with a Ni-rich iron metal phase, and this association suggests a specific extraterrestrial origin for them. Electron microscope analysis made on a large number of soil magnetic particles indicates that 98% of them are terrestrial phases (almandine garnet, tourmaline and Fe-oxides, in abundance order), whereas, only a few grains are of questionable origin. One of the Mg-rich silicate particles was found to be a forsterite (Mg = 0.86) Mn-rich (MnO: 0.23%) Cr-free olivine, almost surely of extraterrestrial sources. Electron microprobe analysis of three soil particles allowed identification of uncommon Cr-rich (Cr2O3 about 8%) spinels, poorly compatible with an origin from terrestrial sources, and in particular from local source rocks. We propose a specific extraterrestrial origin for sub-spherical olivine particles characterised by quite similar magnesian character. Excluding any derivation of these particles from interplanetary dust, two other possible extraterrestrial sources should be considered for them, i.e., either normal micrometeorite fluxes or strongly un-equilibrated, or the Vigarano type Carbonaceous (CV) chondrite meteorite material. In this case, further studies will confirm an impact origin for Amguid, as such magnesian olivine components found in soils might represent the only remnants of a vaporised projectile of ordinary non-equilibrated meteoritic composition
A test of gravitational theories including torsion with the BepiColombo radio science experiment
The Mercury Orbiter radio Science Experiment (MORE) is one of the experiments
on-board the ESA/JAXA BepiColombo mission to Mercury, to be launched in October
2018. Thanks to full on-board and on-ground instrumentation performing very
precise tracking from the Earth, MORE will have the chance to determine with
very high accuracy the Mercury-centric orbit of the spacecraft and the
heliocentric orbit of Mercury. This will allow to undertake an accurate test of
relativistic theories of gravitation (relativity experiment), which consists in
improving the knowledge of some post-Newtonian and related parameters, whose
value is predicted by General Relativity. This paper focuses on two critical
aspects of the BepiColombo relativity experiment. First of all, we address the
delicate issue of determining the orbits of Mercury and the Earth-Moon
barycenter at the level of accuracy required by the purposes of the experiment
and we discuss a strategy to cure the rank deficiencies that appear in the
problem. Secondly, we introduce and discuss the role of the solar
Lense-Thirring effect in the Mercury orbit determination problem and in the
relativistic parameters estimation
Stability of Magnetized Disks and Implications for Planet Formation
This paper considers gravitational perturbations in geometrically thin disks
with rotation curves dominated by a central object, but with substantial
contributions from magnetic pressure and tension. The treatment is general, but
the application is to the circumstellar disks that arise during the
gravitational collapse phase of star formation. We find the dispersion relation
for spiral density waves in these generalized disks and derive the stability
criterion for axisymmetric disturbances (the analog of the Toomre
parameter ) for any radial distribution of the mass-to-flux ratio
. The magnetic effects work in two opposing directions: on one hand,
magnetic tension and pressure stabilize the disk against gravitational collapse
and fragmentation; on the other hand, they also lower the rotation rate making
the disk more unstable. For disks around young stars the first effect generally
dominates, so that magnetic fields allow disks to be stable for higher surface
densities and larger total masses. These results indicate that magnetic fields
act to suppress the formation of giant planets through gravitational
instability. Finally, even if gravitational instability can form a secondary
body, it must lose an enormous amount of magnetic flux in order to become a
planet; this latter requirement represents an additional constraint for planet
formation via gravitational instability and places a lower limit on the
electrical resistivity.Comment: accepted in Ap
Thermal tests of a scaled down mock-up of CP5.2 packaging system: Post-test analysis
Abstract In this study, the thermal performances of an Italian CP5.2 packaging system aimed at the transportation of bituminised wastes (i.e. engulfing fire of 800 °C for 30 min according to the IAEA regulation) are presented. Due to the high risk of auto-ignition of the bituminised wastes, that are stowed in the drums, in turn, immersed in the cement matrix of the CP 5.2, it was decided to test firstly a small scale mock-up. The mock up, containing only one drum with bituminised waste, was designed and built at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa. The experimental test was carried out at Lab. Guerrini of the University of Pisa. Results demonstrated that after half an hour of fire exposure at 800 °C, the temperature in the bituminised waste package is below that of auto-ignition of the bitumen. The obtained results allowed in addition to set up the test procedure to adopt for fire test of a full scale CP5.2 system. Post test analysis, which was carried out by performing FEM analysis, is also presented and results compared to the experimental ones
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