2,748 research outputs found
Charging strategies for electrostatic control of spacecraft formations
Formation control by means of electrostatic forces, generating attractive or repulsive actions by charging the satellites’ surfaces, has been recently proposed for high altitude orbits to precisely maintain the configuration without risk of plume impingement. This paper focuses on electrostatic control and switching strategies for charge distribution in spacecraft formations, taking into account the limits on the power requirements. Two nonlinear global control approaches are presented and applied to two and three satellites’ formations. Then, an optimized charge distribution process among the satellites is discussed and applied to the three spacecraft formation case. Numerical simulations are performed in order to evaluate the advantages and drawbacks of this configuration control technique
Pose and Shape Reconstruction of a Noncooperative Spacecraft Using Camera and Range Measurements
Recent interest in on-orbit proximity operations has pushed towards the development of autonomous GNC strategies. In this sense, optical navigation enables a wide variety of possibilities as it can provide information not only about the kinematic state but also about the shape of the observed object. Various mission architectures have been either tested in space or studied on Earth. The present study deals with on-orbit relative pose and shape estimation with the use of a monocular camera and a distance sensor. The goal is to develop a filter which estimates an observed satellite's relative position, velocity, attitude, and angular velocity, along with its shape, with the measurements obtained by a camera and a distance sensor mounted on board a chaser which is on a relative trajectory around the target. The filter's efficiency is proved with a simulation on a virtual target object. The results of the simulation, even though relevant to a simplified scenario, show that the estimation process is successful and can be considered a promising strategy for a correct and safe docking maneuver
Theoretical Estimates of Stellar e-Captures. I. The half-life of 7Be in Evolved Stars
The Li enrichment in the Universe still presents various puzzles to
astrophysics. One open issue is that of obtaining estimates for the rate of
e-captures on 7Be, for T and rho conditions different from solar. This is
important to model the Galactic nucleosynthesis of Li. In this framework, we
present a new theoretical method for calculating the e-capture rate in
conditions typical of evolved stars. We show how our approach compares with
state-of-the-art techniques for solar conditions, where various estimates are
available. Our computations include: i) "traditional" calculations of the
electronic density at the nucleus, to which the e-capture rate for 7Be is
proportional, for different theoretical approaches including the Thomas--Fermi,
Poisson--Boltzmann and Debye--Hueckel (DH) models of screening, ii) a new
computation, based on a formalism that goes beyond the previous ones, adopting
a mean-field "adiabatic" approximation to the scattering process. The results
obtained with our approach as well as with the traditional ones and their
differences are discussed in some detail, starting from solar conditions, where
our method and the DH model converge to the same solution. We then analyze the
applicability of the various models to a rather broad range of T and rho
values, embracing those typical of red giant stars. We find that, over a wide
region of the parameter space explored, the DH approximation does not stand,
and the more general method we suggest is preferable. We then briefly reanalyze
the 7Li abundances in RGB and AGB stars of the Galactic Disk using the new
Be-decay rate. We also underline that the different values of the electron
density at the nucleus we find should induce effects on electron screening (for
p-captures on Li itself, as well as for other nuclei) so that our new approach
might have wide astrophysical consequences.Comment: Astrophts. Journal Feb. 1, 201
MS, S and C Stars in the Infrared. Luminosities and Mass Loss Rates
In this note I present an outline of infrared (IR) photometric AGB
properties, based on two samples of Galactic Long Period Variables (C- and
S-type respectively). I show the various selection criteria used during the
choice of the sources and describe the motivations of observing them at near-
and mid-IR wavelengths. I discuss the problems encountered in estimating their
luminosity and distance and motivate the methods I choose for this purpose.
Properties of the luminosity functions and of the Hertzsprung-Russell (HR)
diagrams obtained from the analysis are discussed. Finally, the choices made
for estimating of the mass loss rates are described and preliminary results
concerning them are shown.Comment: 10 pages, 6 figures, contribution from the IX Torino Workshop, to be
published by AI
Virtual and rapid prototyping of an underactuated space end effector
A fast and reliable verification of an initial concept is an important need in the field of mechatronics. Usually, the steps for a successful design require multiple iterations involving a sequence of design phases-the initial one and several improvements-and the tests of the resulting prototypes, in a trial and error scheme. Now a day’s software and hardware tools allow for a faster approach, in which the iterations between design and prototyping are by far reduced, even to just one in favorable situation. This work presents the design, manufacturing and testing of a robotic end effector for space applications, realized through virtual prototyping, followed by rapid prototyping realization. The first process allows realizing a mathematical model of the robotic system that, once all the simulations confirm the effectiveness of the design, can be directly used for the rapid prototyping by means of 3D printing. The workflow and the results of the process are described in detail in this paper, showing the qualitative and quantitative evaluation of the performance of both the virtual end effector and the actual physical robotic hand
Dynamos and Chemical Mixing in Evolved Stars
In low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars,
anomalous mixing must transport material near the hydrogen-burning shell to the
convective envelope. Recently, it was suggested that buoyant magnetic flux
tubes could supply the necessary transport rate (Busso et al. 2007). The fields
are assumed to originate from a dynamo operating in the stellar interior. Here,
we show what is required of an dynamo in the envelope of an AGB
star to maintain these fields. Differential rotation and rotation drain via
turbulent dissipation and Poynting flux, so if shear can be resupplied by
convection, then large-scale toroidal field strengths of
\left\simeq3\times10^4 G can be sustained at the base of the
convection zone.Comment: 7 pages, 3 figures. To appear in AIP Proceedings of the IXth Torino
Workshop on AGB Nucleosynthesi
Low temperature mean opacities for the carbon-rich regime
Asymptotic Giant Branch (AGB) stars undergo a change in their chemical
composition during their evolution. This in turn leads to an alteration of the
radiative opacities, especially in the cool layers of the envelope and the
atmosphere, where molecules are the dominant opacity sources. A key parameter
in this respect is the number ratio of carbon to oxygen atoms (C/O). In terms
of low temperature mean opacities, a variation of this parameter usually cannot
be followed in stellar evolution models, because up to now tabulated values
were only available for scaled solar metal mixtures (with C/O ~ 0.5). We thus
present a set of newly generated tables containing Rosseland mean opacity
coefficients covering both the oxygen-rich (C/O
1) regime. We compare our values to existing tabular data and investigate the
relevant molecular opacity contributors.Comment: 8 pages, 5 figures. To appear in the AIP Proceedings of the IXth
Torino Workshop on AGB Nucleosynthesi
Vitrification of human immature oocytes before and after in vitro maturation: a review
The use of immature oocytes subjected to in vitro maturation (IVM) opens interesting perspectives for fertility preservation where ovarian reserves are damaged by pathologies or therapies, as in PCO/PCOS and cancer patients. Human oocyte cryopreservation may offer some advantages compared to embryo freezing, such as fertility preservation in women at risk of losing fertility due to oncological treatment or chronic disease, egg donation and postponing childbirth. It also eliminates religious and/or other ethical, legal, and moral concerns of embryo freezing. In addition, a successful oocyte cryopreservation program could eliminate the need for donor and recipient menstrual cycle synchronization. Recent advances in vitrification technology have markedly improved the oocyte survival rate after warming, with fertilization and implantation rates comparable with those of fresh oocytes. Healthy live births can be achieved from the combination of IVM and vitrification, even if vitrification of in vivo matured oocytes is still more effective. Recently, attention is given to highlight whether vitrification procedures are more successful when performed before or after IVM, on immature GV-stage oocytes, or on in vitro matured MII-stage oocytes. In this review, we emphasize that, even if there are no differences in survival rates between oocytes vitrified prior to or post-IVM, reduced maturation rates of immature oocytes vitrified prior to IVM can be, at least in part, explained by underlying ultrastructural and biomolecular alterations
- …