102,499 research outputs found
Displaced geostationary orbits using hybrid low-thrust propulsion
No full textIn this paper, displaced geostationary orbits using hybrid low-thrust propulsion, a complementary combination of Solar Electric Propulsion (SEP) and solar sailing, are investigated to increase the capacity of the geostationary ring that is starting to become congested. The SEP propellant consumption is minimized in order to maximize the mission lifetime by deriving semi-analytical formulae for the optimal steering laws for the SEP and solar sail accelerations. By considering the spacecraft mass budget, the performance is also expressed in terms of payload mass capacity. The analyses are performed both for the use of pure SEP and hybrid low-thrust propulsion to allow for a comparison. It is found that hybrid low-thrust control outperforms the pure SEP case both in terms of payload mass capacity and mission lifetime for all displacements considered. Hybrid low-thrust propulsion enables payloads of 255 to 487 kg to be maintained in a 35 km displaced orbit for 10 to 15 years. Adding the influence of the J2 and J22 terms of the Earth’s gravity field has a small effect on this lifetime, which becomes almost negligible for small values of the sail lightness number. Finally, two SEP transfers that allow for an improvement in the performance of hybrid low-thrust control are optimized for the propellant consumption by solving the accompanying optimal control problem using a direct pseudospectral method. The first type of transfer enables a transit between orbits displaced above and below the equatorial plane, while the second type of transfer enables customized service for which a spacecraft is transferred to a Keplerian parking orbit when geostationary coverage is not needed. While the latter requires a modest propellant budget, the first type of transfer comes at the cost of an almost negligible SEP propellant consumption
CONTAMINAZIONE DELLE ACQUE SOTTERRANEE DA ARSENICO GEOGENICO: UNA PANORAMICA DEI MECCANISMI DI MOBILIZZAZIONE E DELLE TECNICHE DI INDIVIDUAZIONE DELLE SORGENTI
No full textWe aim at: (i) providing a general overview on the contamination of geogenic arsenic resulting from natural rock-water interactions in aquifer systems and (ii) suggesting a general approach to identifying possible source mechanisms. The presence of non-negligible concentrations of arsenic in groundwater is observed in many shallow and deep aquifers all over the world and affects more than 140 millions of people. Identifying the sources of geogenic arsenic remains one the most important contemporary challenges to minimize the portion of population exposed to arsenic hazard and to define possible mitigation strategies to contamination taking place in the system. Here, we present an overview of the geochemical mechanisms which are typically considered to be the driving source of geogenic arsenic in groundwater. These include: oxidation of arseno-pyrite and pyrite; reductive dissolution of iron hydroxides; adsorption and desorption; and dissolution of arseno-sulfides. For each mechanism, we provide a summary of the reaction network and of the boundary and environmental conditions that might trigger the release of arsenic in the natural water bodies. The description of the mechanisms is accomplished through a review of the methods and techniques that are commonly used to track back the geogenic source of arsenic and to define the conceptual model descriptive of arsenic mobilization: statistical analysis of field data; laboratory experiments; and numerical modeling. Strengths and limitations of these techniques are pinpointed and commented. The general picture presented in this work highlights the complexity and the difficulties that are typically faced during the investigations aimed at identifying the source of geogenic arsenic in natural subsurface water bodies. The critical discussion of source mechanisms and source identification techniques is finalized to suggest a possible structured approach, transferable to a variety of case studies, to define one or more conceptual models descriptive of the arsenic release in a case study of interest. This approach combines the diverse source identification techniques reviewed in this work and organizes these according to a set of sequential steps aiming at maximizing the benefit of their strengths and compensating possible limitations specific to a given technique
Automatic goal allocation for a planetary rover with DSmT
No full textIn this chapter, we propose an approach for assigning aninterest level to the goals of a planetary rover. Assigning an interest level to goals, allows the rover to autonomously transform and reallocate the goals. The interest level is defined by data-fusing payload and navigation information. The fusion yields an 'interest map',that quantifies the level of interest of each area around the rover. In this way the planner can choose the most interesting scientific objectives to be analysed, with limited human intervention, and reallocates its goals autonomously. The Dezert-Smarandache Theory of Plausible and Paradoxical Reasoning was used for information fusion: this theory allows dealing with vague and conflicting data. In particular, it allows us to directly model the behaviour of the scientists that have to evaluate the relevance of a particular set of goals. This chaptershows an application of the proposed approach to the generation of a reliable interest map
Conjugate gradient heat bath for ill-conditioned actions.
No full textWe present a method for performing sampling from a Boltzmann distribution of an ill-conditioned quadratic action. This method is based on heat-bath thermalization along a set of conjugate directions, generated via a conjugate-gradient procedure. The resulting scheme outperforms local updates for matrices with very high condition number, since it avoids the slowing down of modes with lower eigenvalue, and has some advantages over the global heat-bath approach, compared to which it is more stable and allows for more freedom in devising case-specific optimizations
Mission analysis and systems design of a near-term and far-term pole-sitter mission
No full textThis paper provides a detailed mission analysis and systems design of a near-term and far-term polesitter mission. The pole-sitter concept was previously introduced as a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high-latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the north or south pole and, as such, can provide real-time, continuous and hemispherical coverage of the polar regions. Being on a non-Keplerian orbit, a continuous thrust is required to maintain the pole-sitter position. For this, two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar electric propulsion (SEP) and a far-term pole-sitter mission where the SEP thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. In order to maximize the spacecraft mass at the start of the operations phase of the mission, the transfer from Earth to the pole-sitter orbit is designed and optimized assuming either a Soyuz or an Ariane 5 launch. The maximized mass upon injection into the pole-sitter orbit is subsequently used in a detailed mass budget analysis that will allow for a trade-off between mission lifetime and payload mass capacity. Also, candidate payloads for a range of applications are investigated. Finally, transfers between north and south pole-sitter orbits are considered to overcome the limitations in observations due to the tilt of the Earth’s rotational axis that causes the poles to be alternately situated in darkness. It will be shown that in some cases these transfers allow for propellant savings, enabling a further extension of the pole-sitter mission
recensione a G. Lazzi, P. Ventrone, Simonetta Vespucci. La nascita della Venere fiorentina
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Recensione a G. Marchesi, Quei laboriosi valligiani. Economia e società nella montagna bresciana tra tardo Settecento e gli anni postunitari
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Design of optimal Earth pole-sitter transfers using low thrust propulsion
No full textRecent studies have shown the feasibility of an Earth pole-sitter mission using low-thrust propulsion. This mission concept involves a spacecraft following the Earth's polar axis to have a continuous, hemispherical view of one of the Earth's poles. Such a view will enhance future Earth observation and telecommunications for high latitude and polar regions. To assess the accessibility of the pole-sitter orbit, this paper investigates optimum Earth pole-sitter transfers employing low-thrust propulsion. A launch from low Earth orbit (LEO) by a Soyuz Fregat upper stage is assumed after which a solar-electric-propulsion thruster transfers the spacecraft to the pole-sitter orbit. The objective is to minimise the mass in LEO for a given spacecraft mass to be inserted into the pole-sitter orbit. The results are compared with a ballistic transfer that exploits the manifolds winding off the pole-sitter orbit. It is shown that, with respect to the ballistic case, low-thrust propulsion can achieve significant mass savings in excess of 200 kg for a pole-sitter spacecraft of 1000 kg upon insertion. To finally obtain a full low-thrust transfer from LEO up to the pole-sitter orbit, the Fregat launch is replaced by a low-thrust, minimum time spiral through an orbital averaging technique, which provides further mass savings, but at the cost of an increased time of flight
La circolazione libraria a Varese e dintorni nella prima età moderna. Avvio di una indagine su scala locale
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