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Padova, Italy

Fortunato L.,National Institute of Nuclear Physics, Italy | Sartori L.,CISAS G. Colombo
Communications in Theoretical Physics | Year: 2010

We analyze in detail the quantum phase transitions that arise in models based on the u(2) algebraic description for bosonic systems with two types of scalar bosons. First we discuss the quantum phase transition that occurs in hamiltonians that admix the two dynamical symmetry chains u(2) ⊃ u(1) and u(2) ⊃ so(2) by diagonalizing the problem exactly in the u(1) basis. Then we apply the coherent state formalism to determine the energy functional. Finally we show that a quantum phase transition of a different nature, but displaying similar characteristics, may arise also within a single chain just by including higher order terms in the hamiltonian. © Chinese Physical Society and IOP Publishing Ltd. Source

Chiaradia M.,University of Padua | Rodeghiero G.,University of Padua | Di Donato G.,University of Padua | Barilaro L.,University of Padua | And 7 more authors.
European Space Agency, (Special Publication) ESA SP | Year: 2011

This paper reports an overview of SCRAT experiment which is one of the three experiments that successfully flew on board of BEXUS 10 balloon the 9th of October 2010 from ESRANGE Space Center in Sweden. SCRAT is an acronym which stands for Spherical Compact Rechargeable Air Thruster, the experiment in fact aimed to develop and test a small cold-gas-actuator, CGA, using the atmospheric air as a propellant, in the extreme stratospheric environment offered by the BEXUS platform. The experiment has been developed in the REXUS/BEXUS Programme (RX/BX Programme) framework; the RX/BX Programme is the result of the collaboration among ESA, the Swedish Space Corporation (SSC), the Swedish National Space Board (SNSB) and DLR, and it provides the opportunity of fly an experiment of a students team on board of a sounding rocket (REXUS) or a stratospheric balloon (BEXUS). The paper traces all the steps of the RX/BX experience from the concept and the design of SCRAT until the launch campaign and the analysis of retrieved scientific results. Source

Segato E.,CISAS G. Colombo | Da Deppo V.,CNR Institute for Photonics and Nanotechnologies | Cremonese G.,National institute for astrophysics
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

Optical instruments for space missions work in hostile environment, it's thus necessary to accurately study the effects of ambient parameters variations on the equipment performance. In particular, optical instruments are very sensitive to ambient conditions, especially temperature. This variable can cause dilatation and misalignment of the optical elements, and can also lead to rise of dangerous stresses in the optics. Optical elements displacements and surface deformations degrade the quality of the sampled images. In this work a method for simulating and studying the effects of the thermal deformations, particularly the impact on the expected optical performance, is presented. Optical elements and their mountings are modelled and processed by a thermo-mechanical Finite Element Model (FEM) analysis, reproducing expected operative conditions. The FEM output is elaborated into a MATLAB optimisation code; a non-linear least square algorithm is used to determine the equation of the best fitting nth degree polynomial, or the spherical surface of the deformed lenses and mirrors; model accuracy is 10-8 m. The obtained mathematical surface representations are then directly imported into ZEMAX raytracing software for sequential raytrace analysis. The results are spot diagrams, chief ray coordinates on the detector, MTF curves and Diffraction Encircled Energy variations due to simulated thermal loads. This analysis helps to design and compare different optical housing systems for finding a feasible mounting solution. The described method has been applied successfully to the optics and mountings of a stereo-camera for the BepiColombo mission. Different types of lenses and prisms constraints have been designed and analysed. The results show the preferable use of kinematic constraints, instead of using glue, to correctly maintain the instrument focus in orbit around Mercury considering an operative temperature range between -20°C and +30°C. © 2010 SPIE. Source

Segato E.,CISAS G. Colombo | Da Deppo V.,CNR Institute for Photonics and Nanotechnologies | Cremonese G.,National institute for astrophysics | Cherubini G.,SELEX Galileo
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

In this paper the results of the thermo-elastic analysis performed on the Stereo Imaging Channel of the imaging system SIMBIO-SYS for the BepiColombo ESA mission to Mercury is presented. The aim of the work is to determine the expected stereo reconstruction accuracy of the surface of the planet Mercury, i.e. the target of BepiColombo mission, due to the effects of the optics misalignments and deformations induced by temperature changes during the mission lifetime. The camera optics and their mountings are modeled and processed by a thermo-mechanical Finite Element Model (FEM) program, which reproduces the expected optics and structure thermo-elastic variations in the instrument foreseen operative temperature range, i.e. between -20°C and 30°C. The FEM outputs are elaborated using a MATLAB optimization routine: a non-linear least square algorithm is adopted to determine the surface equation (plane, spherical, nth polynomial) which best fits the deformed optical surfaces. The obtained surfaces are then directly imported into ZEMAX raytracing code for sequential raytrace analysis. Variations of the optical center position, boresight direction, focal length and distortion are then computed together with the corresponding image shift on the detector. The overall analysis shows the preferable use of kinematic constraints, instead of glue classical solution, for optical element mountings, this minimize the uncertainty on the Mercury Digital Terrain Model (DTM) reconstructed via a stereo-vision algorithm based on the triangulation from two optical channels. © 2010 Copyright SPIE - The International Society for Optical Engineering. Source

Tadini P.,Polytechnic of Milan | Tancredi U.,Parthenope University of Naples | Grassi M.,University of Naples Federico II | Anselmo L.,CNR Institute of Information Science and Technologies Alessandro Faedo | And 10 more authors.
Acta Astronautica | Year: 2014

During the last 40 years, the mass of the artificial objects in orbit increased quite steadily at the rate of about 145 metric tons annually, leading to about 7000 metric tons. Most of the cross-sectional area and mass (97% in low Earth orbit) is concentrated in about 4500 intact abandoned objects plus a further 1000 operational spacecraft. Analyses have shown that the most effective mitigation strategy should focus on the disposal of objects with larger cross-sectional area and mass from densely populated orbits. Recent NASA results have shown that the worldwide adoption of mitigation measures in conjunction with active yearly removal of approximately 0.2-0.5% of the abandoned objects would stabilize the debris population. Targets would have typical masses between 500 and 1000 kg in the case of spacecraft, and of more than 1000 kg for rocket upper stages. In the case of Cosmos-3M second stages, more than one object is located nearly in the same orbital plane. This provides the opportunity of multi-removal missions, more suitable for yearly removal rate and cost reduction needs. This paper deals with the feasibility study of a mission for the active removal of large abandoned objects in low Earth orbit. In particular, a mission is studied in which the removal of two Cosmos-3M second stages, that are numerous in low Earth orbit, is considered. The removal system relies on a Chaser spacecraft which performs rendezvous maneuvers with the two targets. The first Cosmos-3M stage is captured and an autonomous de-orbiting kit, carried by the Chaser, is attached to it. The de-orbiting kit includes a Hybrid Propulsion Module, which is remotely ignited to perform stage disposal and controlled reentry after Chaser separation. Then, the second Cosmos-3M stage is captured and, in this case, the primary propulsion system of the Chaser is used for the disposal of the mated configuration. Critical mission aspects and related technologies are investigated at a preliminary level. In particular, an innovative electro-adhesive system for target capture, a mechanical system for the hard docking with the target and a hybrid propulsion system suitable for rendezvous, de-orbiting and controlled reentry operations are analyzed. This is performed on the basis of a preliminary mission profile, in which suitable rendezvous and disposal strategies have been considered and investigated by numerical analysis. A preliminary system mass budget is also performed, showing that the Chaser overall mass is about 1350 kg, including a primary propulsion system of about 300 kg and a de-orbiting kit with a mass of about 200 kg. This system is suitable to be launched with VEGA, actually the cheapest European space launcher. © 2014 IAA. Source

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