Geostationary secular dynamics revisited: application to high area-to-mass ratio objects

The long-term dynamics of the geostationary Earth orbits (GEO) is revisited through the application of canonical perturbation theory. We consider a Hamiltonian model accounting for all major perturbations: geopotential at order and degree two, lunisolar perturbations with a realistic model for the Sun and Moon orbits, and solar radiation pressure. The long-term dynamics of the GEO region has been studied both numerically and analytically, in view of the relevance of such studies to the issue of space debris or to the disposal of GEO satellites. Past studies focused on the orbital evolution of objects around a nominal solution, hereafter called the forced equilibrium solution, which shows a particularly strong dependence on the area-to-mass ratio. Here, we i) give theoretical estimates for the long-term behavior of such orbits, and ii) we examine the nature of the forced equilibrium itself. In the lowest approximation, the forced equilibrium implies motion with a constant non-zero average `forced eccentricity', as well as a constant non-zero average inclination, otherwise known in satellite dynamics as the inclination of the invariant `Laplace plane'. Using a higher order normal form, we demonstrate that this equilibrium actually represents not a point in phase space, but a trajectory taking place on a lower-dimensional torus. We give analytical expressions for this special trajectory, and we compare our results to those found by numerical orbit propagation. We finally discuss the use of proper elements, i.e., approximate integrals of motion for the GEO orbits.Comment: Accepted for publication in CMD

From order to chaos in Earth satellite orbits

We consider Earth satellite orbits in the range of semi-major axes where the perturbing effects of Earth's oblateness and lunisolar gravity are of comparable order. This range covers the medium-Earth orbits (MEO) of the Global Navigation Satellite Systems and the geosynchronous orbits (GEO) of the communication satellites. We recall a secular and quadrupolar model, based on the Milankovitch vector formulation of perturbation theory, which governs the long-term orbital evolution subject to the predominant gravitational interactions. We study the global dynamics of this two-and-a-half degrees-of-freedom Hamiltonian system by means of the fast Lyapunov indicator (FLI), used in a statistical sense. Specifically, we characterize the degree of chaoticity of the action space using angle-averaged normalized FLI maps, thereby overcoming the angle dependencies of the conventional stability maps. Emphasis is placed upon the phase-space structures near secular resonances, which are of first importance to the space debris community. We confirm and quantify the transition from order to chaos in MEO, stemming from the critical inclinations, and find that highly inclined GEO orbits are particularly unstable. Despite their reputed normality, Earth satellite orbits can possess an extraordinarily rich spectrum of dynamical behaviors, and, from a mathematical perspective, have all the complications that make them very interesting candidates for testing the modern tools of chaos theory.Comment: 30 pages, 9 figures. Accepted for publication in the Astronomical Journa

Cytoskeletal-based mechanisms differently regulate <i>in vivo</i> and <i>in vitro</i> proplatelet formation

Platelets are produced by bone marrow megakaryocytes through cytoplasmic protrusions, named native proplatelets (nPPT), into blood vessels. Proplatelets also refer to protrusions observed in megakaryocyte culture (cPPT) that are morphologically different. Contrary to cPPT, the mechanisms of nPPT formation are poorly understood. We show here in living mice that nPPT elongation is in equilibrium between protrusive and retraction forces mediated by myosin-IIA. We also found, using WT and β1-tubulin-deficient mice, that microtubule behavior differs between cPPT and nPPT, being absolutely required in vitro, while less critical in vivo. Remarkably, microtubule depolymerization in myosin-deficient mice did not affect nPPT elongation. We then calculated that blood Stokes'forces may be sufficient to promote nPPT extension, independently of myosin and microtubules. Together, we propose a new mechanism for nPPT extension that might explain contradictions between severely affected cPPT production and moderate platelet count defects in some patients and animal models

Optimized low-thrust mission to the Atira asteroids

Atira asteroids are recently-discovered celestial bodies characterized by orbits lying completely inside the Earth’s. The study of these objects is difficult due to the limitations of ground-based observations: objects can only be detected when the Sun is not in the field of view of the telescope. However, many asteroids are expected to exist in the inner region of the Solar System, many of which could pose a significant threat to our planet. In this paper, a mission to improve knowledge of the known Atira asteroids in terms of ephemerides and composition and to observe inner-Earth asteroids is presented. The mission is realized using electric propulsion, which in recent years has proven to be a viable option for interplanetary flight. The trajectory is optimized in such a way as to visit the maximum possible number of asteroids of the Atira group with the minimum propellant consumption; the mission ends with a transfer to an orbit with perigee equal to Venus’s orbit radius, to maximize the observations of asteroids in the inner part of the Solar System

A simple ionospheric correction method for radar-based space surveillance systems, with performance assessment on GRAVES data

Ionospheric effects degrade the quality of radar data, which are critical for the precision of the satellite ephemeris produced by space surveillance systems; this degradation is especially noticeable for radars such as GRAVES that operate in the very high frequency range. This article presents a simple and effective method to correct for ionospheric effects, with an evaluation on data obtained with GRAVES, the French space surveillance radar. This method relies on GPS data, and our evaluation relies on GRAVES and DORIS data. We found that the gain in terms of evaluated radial velocity can be as high as 1.76$\kappa$, where $\kappa$ is the typical root mean square of the noise on radial velocity measurements for GRAVES (excluding ionospheric effects): the error decreases from 2.60$\kappa$ to 0.83$\kappa$ for daytime satellite overhead passes. Our conclusion is that, while this method is very simple to implement, it has proven to be a good correction for ionospheric effects in practice.Comment: Accepted for publication in Advances in Space Research. 6 pages, 4 figure

Soil depth detection by seeds and diurnally fluctuating temperatures: different dynamics in 10 annual plants

International audienceBackground and Aims Seeds buried in the soil detect burial depth through light and diurnally fluctuating temperatures (DFT) and in this way limit losses due to germination too deep in the soil. DFTs and germination also increase in vegetation gaps. However, dry open environments with high DFTs can also increase seedling mortality, creating conflicting selection pressures for reaction to DFTs. Since this questions the general function of DFT detection, we therefore tested if interspecific differences in DFT detection are related to mortality in different soil depths. Methods We buried seeds of ten annual plants including species pairs of increasing and decreasing germination in response to DFTs. Seeds were buried in 5, 10 and 25cm soil depth and exhumed after two different burial times. Seed viability was tested using germination in growth chambers and tetrazolium. We also measured DFTs at these depths using temperature data loggers. Results DFT detection was not related to differences in mortality at three burial depths. Three species showed a clear pattern of depth dependent mortality, however inconsistently related to DFT detection. Conclusions Depth detection mechanisms are more species-specific than expected. Hence, interspecific differences in seed mortalities are difficult to predict by DFT detection alone and alternative soil depth sensing mechanisms should be explored in future

Importance of environmental stiffness for megakaryocyte differentiation and proplatelet formation.

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Broader expression of the mouse platelet factor 4-cre transgene beyond the megakaryocyte lineage

Background : Transgenic mice expressing cre recombinase under the control of the platelet factor4 (Pf4) promoter, in the context of a 100-kb bacterial artificial chromosome, have become a valuable tool with which to study genetic modifications in the platelet lineage. However, the specificity of cre expression has recently been questioned, and the time of its onset during megakaryopoiesis remains unknown. Objectives/MethodsTo characterize the expression of this transgene, we used double-fluorescent cre reporter mice. ResultsIn the bone marrow, Pf4-cre-mediated recombination had occurred in all CD42-positive megakaryocytes as early as stageI of maturation, and in rare CD42-negative cells. In circulating blood, all platelets had recombined, along with only a minor fraction of CD45-positive cells. However, we found that all tissues contained recombined cells of monocyte/macrophage origin. When recombined, these cells might potentially modify the function of the tissues under particular conditions, especially inflammatory conditions, which further increase recombination in immune cells. Unexpectedly, a subset of epithelial cells from the distal colon showed signs of recombination resulting from endogenous Pf4-cre expression. This is probably the basis of the unexplained colon tumors developed by Apc(flox/flox);Pf4-cre mice, generated in a separate study on the role of Apc in platelet formation. ConclusionAltogether, our results indicate early recombination with full penetrance in megakaryopoiesis, and confirm the value of Pf4-cre mice for the genetic engineering of megakaryocytes and platelets. However, care must be taken when investigating the role of platelets in processes outside hemostasis, especially when immune cells might be involved