1,687 research outputs found
Analytical and numerical study of the ground-track resonances of Dawn orbiting Vesta
The aim of Dawn mission is the acquisition of data from orbits around two
bodies, (4)Vesta and (1)Ceres, the two most massive asteroids. Due to the low
thrust propulsion, Dawn will slowly cross and transit through ground-track
resonances, where the perturbations on Dawn orbit may be significant. In this
context, to safety go the Dawn mission from the approach orbit to the lowest
science orbit, it is essential to know the properties of the crossed
resonances. This paper analytically investigates the properties of the major
ground-track resonances (1:1, 1:2, 2:3 and 3:2) appearing for Vesta orbiters:
location of the equilibria, aperture of the resonances and period at the stable
equilibria. We develop a general method using an averaged Hamiltonian
formulation with a spherical harmonic approximation of the gravity field. If
the values of the gravity field coefficient change, our method stays correct
and applicable. We also discuss the effect of one uncertainty on the C20 and
C22 coefficients on the properties of the 1:1 resonance. These results are
checked by numerical tests. We determine that the increase of the eccentricity
appearing in the 2:3 resonance is due to the C22 and S22 coefficients.
Our method can be easily adapted to missions similar to Dawn because,
contrarily to the numerical results, the analytical formalism stays the same
and is valid for a wide range of physical parameters of the asteroid (namely
the shape and the mass) as well as for different spacecraft orbits.
Finally we numerically study the probability of the capture in resonance 1:1.
Our paper reproduces, explains and supplements the results of Tricarico and
Sykes (2010).Comment: 34 pages, 9 figures, 10 Table
Symplectic integration of space debris motion considering several Earth's shadowing models
In this work, we present a symplectic integration scheme to numerically
compute space debris motion. Such an integrator is particularly suitable to
obtain reliable trajectories of objects lying on high orbits, especially
geostationary ones. Indeed, it has already been demonstrated that such objects
could stay there for hundreds of years. Our model takes into account the
Earth's gravitational potential, luni-solar and planetary gravitational
perturbations and direct solar radiation pressure. Based on the analysis of the
energy conservation and on a comparison with a high order non-symplectic
integrator, we show that our algorithm allows us to use large time steps and
keep accurate results. We also propose an innovative method to model Earth's
shadow crossings by means of a smooth shadow function. In the particular
framework of symplectic integration, such a function needs to be included
analytically in the equations of motion in order to prevent numerical drifts of
the energy. For the sake of completeness, both cylindrical shadows and penumbra
transitions models are considered. We show that both models are not equivalent
and that big discrepancies actually appear between associated orbits,
especially for high area-to-mass ratios
NIMASTEP: a software to modelize, study and analyze the dynamics of various small objects orbiting specific bodies
NIMASTEP is a dedicated numerical software developed by us, which allows one
to integrate the osculating motion (using cartesian coordinates) in a Newtonian
approach of an object considered as a point-mass orbiting a homogeneous central
body that rotates with a constant rate around its axis of smallest inertia. The
code can be applied to objects such as particles, artificial or natural
satellites or space debris. The central body can be either any terrestrial
planet of the solar system, any dwarf-planet, or even an asteroid. In addition,
very many perturbations can be taken into account, such as the combined
third-body attraction of the Sun, the Moon, or the planets, the direct solar
radiation pressure (with the central body shadow), the non-homogeneous
gravitational field caused by the non-sphericity of the central body, and even
some thrust forces. The simulations were performed using different integration
algorithms. Two additional tools were integrated in the software package; the
indicator of chaos MEGNO and the frequency analysis NAFF. NIMASTEP is designed
in a flexible modular style and allows one to (de)select very many options
without compromising the performance. It also allows one to easily add other
possibilities of use. The code has been validated through several tests such as
comparisons with numerical integrations made with other softwares or with
semi-analytical and analytical studies. The various possibilities of NIMASTEP
are described and explained and some tests of astrophysical interest are
presented. At present, the code is proprietary but it will be released for use
by the community in the near future. Information for contacting its authors and
(in the near future) for obtaining the software are available on the web site
http://www.fundp.ac.be/en/research/projects/page_view/10278201/Comment: Astronomy & Astrophysics - Received: 25 November 2011 / Accepted: 27
February 2012 -- 14 pages, 4 figure
Design x Science cards
A deck of cards to support collaborative ideation in multidisciplinary Design and Science projects
The language of intervention: A review of concepts and terminology in wetland ecosystem repair
As programmes and projects aimed at addressing wetland degradation gain momentum in South Africa, it is critical that related ideas are communicated among and between researchers, practitioners, management agencies, land-owners and the general public in a common language. This paper explores the meaning of ‘restoration’ and ‘rehabilitation’; terms that we suggest are key to understanding and advancing South Africa’s efforts to address wetland degradation. In its essence, the paper is a critical review of wetland ecosystem repair concepts and terminology from local and international literature. The major products of the paper are proposed definitions of the terms ‘restoration’ and ‘rehabilitation’ in a South African wetland science and management context. Although the terms are often used interchangeably, we argue that their absolute distinction will allow scientists and practitioners to better understand what it is that ecosystem repair interventions aim to achieve. We suggest that the terms be distinguished on the basis of what could be considered their respective ecological starting points, where ‘restoration’ applies to part of a system or a system in its entirety that has been completely and permanently, but not irreparably altered, and essentially removed from the landscape, and ‘rehabilitation’ applies to part of a system or a system in its entirety that has not been removed from the landscape through complete and permanent alteration, but is in a degraded state. Thus, ‘wetland restoration’ is defined as the process of reinstating natural ecological driving forces within part or the whole of a completely and permanently altered wetland to recover former or desired ecosystem structure, function, biotic composition and ecosystem services, while ‘wetland rehabilitation’ is defined as the process of reinstating natural ecological driving forces within part or the whole of a degraded wetland to recover former or desired ecosystem structure, function, biotic composition and ecosystem services
Aspergillus strain typing in the genomics era
Multiple reasons may justify a need for strain typing purposes, but the
most common reason is to delineate the epidemiological relationships between
isolates. The availability of whole genome sequences has greatly influenced
our ability to develop highly targeted and efficient strain typing methods fur
these purposes. Some strain typing methods may serve dual goals: not only can
they be used to discriminate between multiple isolates of a certain species,
they can also aid in the recognition, identification, description and
validation process of a fungal species
Structural plasticity of the social brain: Differential change after socio-affective and cognitive mental training
Although neuroscientific research has revealed experience-dependent brain changes across the life span in sensory, motor, and cognitive domains, plasticity relating to social capacities remains largely unknown. To investigate whether the targeted mental training of different cognitive and social skills can induce specific changes in brain morphology, we collected longitudinal magnetic resonance imaging (MRI) data throughout a 9-month mental training intervention from a large sample of adults between 20 and 55 years of age. By means of various daily mental exercises and weekly instructed group sessions, training protocols specifically addressed three functional domains: (i) mindfulness-based attention and interoception, (ii) socio-affective skills (compassion, dealing with difficult emotions, and prosocial motivation), and (iii) socio-cognitive skills (cognitive perspective-taking on self and others and metacognition). MRI-based cortical thickness analyses, contrasting the different training modules against each other, indicated spatially diverging changes in cortical morphology. Training of present-moment focused attention mostly led to increases in cortical thickness in prefrontal regions, socio-affective training induced plasticity in frontoinsular regions, and socio-cognitive training included change in inferior frontal and lateral temporal cortices. Module-specific structural brain changes correlated with training-induced behavioral improvements in the same individuals in domain-specific measures of attention, compassion, and cognitive perspective-taking, respectively, and overlapped with task-relevant functional networks. Our longitudinal findings indicate structural plasticity in well-known socio-affective and socio-cognitive brain networks in healthy adults based on targeted short daily mental practices. These findings could promote the development of evidence-based mental training interventions in clinical, educational, and corporate settings aimed at cultivating social intelligence, prosocial motivation, and cooperation
Scalable, high power line focus diode laser for crystallizing of silicon thin films
We present the design and performance of a diode laser module producing a high intensity line focus at 808 nm for material processing. The design is based on a linear array of 7 laser bars and beam forming optics featuring a micro-optic homogenizer. The module delivers a total output power of 900 W at 140 A and peak intensity created in the focus area of 10.3 kW/cm2. Two systems with line length of 5 cm and 10 cm at a large working distance of 110 mm have been realized. The chosen concept allows scaling in length by joining multiple modules which is of interest for material processing in industrial applications. Application results from laser crystallization of amorphous silicon seed layers used in the fabrication of photovoltaic cells for solar panels are given
Mechanism of environmentally driven conformational changes that modulate H-NS DNA-bridging activity
Adaptive Tracking of Angular Velocity for a Planar Rigid Body With Unknown Models for Inertia and Input Nonlinearity
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57795/1/AdaptiveTrackingTACTTCST1D.pd
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