Modelización tridimensional de una figura por métodos fotogramétricos y escaneado de precisión. Verificación y comparación de resultados.

El objetivo principal de este proyecto es la obtención del modelo tridimensional de la escultura del patrón San Isidoro de la E.T.S.I. Topografía, Geodesia y Cartografía fabricada en yeso con unas dimensiones de 225 mm de largo, 182 mm de ancho y 318 mm de alto. Se realizará mediante dos métodos diferentes: por medios fotogramétricos utilizando correlación,y por métodos de topografía industrial. En la toma de imágenes para la restitución fotogramétrica se utilizó una cámara no métrica, cuya calibración tuvo que ser realizada comparando dos sistemas de coordenadas, el sistema imagen determinado por técnicas fotogramétricas, y el sistema objeto o terreno determinado por técnicas de topografía clásica. De esta forma se obtienen los datos internos de la cámara. La tolerancia establecida a priori para fotogrametría terrestre es de 1 mm, verificando y contrastando los resultados obtenidos con equipos de trazabilidad superior con los que se procede a la extracción del modelo digital por métodos de topografía industrial, mediante escaneado de precisión. El modelo tridimensional obtenido podría ser útil para múltiples tareas, como disponer de documentación geométrica digital de la figura, su estudio para una posible restauración,etc.

Analysis of tape tether survival in LEO against orbital debris

The low earth orbit (LEO) environment contains a large number of artificial debris, of which a significant portion is due to dead satellites and fragments of satellites resulted from explosions and in-orbit collisions. Deorbiting defunct satellites at the end of their life can be achieved by a successful operation of an Electrodynamic Tether (EDT) system. The effectiveness of an EDT greatly depends on the survivability of the tether, which can become debris itself if cut by debris particles; a tether can be completely cut by debris having some minimal diameter. The objective of this paper is to develop an accurate model using power laws for debris-size ranges, in both ORDEM2000 and MASTER2009 debris flux models, to calculate tape tether survivability. The analytical model, which depends on tape dimensions (width, thickness) and orbital parameters (inclinations, altitudes) is then verified with fully numerical results to compare for different orbit inclinations, altitudes and tape width for both ORDEM2000 and MASTER2009 flux data.

Drifting plasma collection by a positive biased tether wire in LEO-like plasma conditions: current measurement and plasma diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earth's magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of- ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites.

An universal system to de-orbit satellites at end of life

A 3-year Project financed by the European Commission is aimed at developing a universal system to de-orbit satellites at their end of life, as a fundamental contribution to limit the increase of debris in the Space environment. The operational system involves a conductive tapetether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. The Project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. Starting at the second year, mission selection is carried out while developing numerical codes to implement control laws on tether dynamics in/off the orbital plane; performing numerical simulations and plasma chamber measurements on tether-plasma interaction; and completing design of subsystems: electronejecting plasma contactor, power module, interface elements, deployment mechanism, and tether-tape/end-mass. This will be followed by subsystems manufacturing and by currentcollection, free-fall, and hypervelocity impact tests.

Electrodynamic tether for scientific mission in low Jovian orbit

An electrodynamic bare tether is shown to allow carrying out scientific observations very close to Jupiter, for exploration of its surface and subsurface, and ionospheric and atmospheric in-situ measurements. Starting at a circular equatorial orbit of radius about 1.3/1.4 times the Jovian radius, continuous propellantless Lorentz drag on a thin-tape tether in the 1-5 km length range would make a spacecraft many times as heavy as the tape slowly spiral in, over a period of many months, while generating power at a load plugged in the tether circuit for powering instruments in science data acquisition and transmission. Lying under the Jovian radiation belts, the tape would avoid the most severe problem facing tethers in Jupiter, which are capable of producing both power and propulsion but, operating slowly, could otherwise accumulate too high a radiation dose . The tether would be made to spin in its orbit to keep taut; how to balance the Lorentz torque is discussed. Constraints on heating and bowing are also discussed, comparing conditions for prograde versus retrograde orbits. The system adapts well to the moderate changes in plasma density and motional electric field through the limited radial range in their steep gradients near Jupiter.

A proposed bare-tether experiment on board a sounding rocket

A mission on board a sounding rocket to carry out two bare-tether experiments is proposed: a test of orbital-motion-limited (OML) collection and the proof-of-flight of a technique to determine the (neutral) density vertical profile in the critical E-layer. Since full bias from the motional field will be small (~ 20V), corresponding to a tape 1 km long and V rocket <8 km/s, a power source with a range of supply voltages of few kV would be used. First, the negative terminal of the supply would be connected to the tape, and the positive terminal to a round, conductive boom of length 10 - 20 m; electrons collected by the boom cross the supply into the tape, where they leak out at the rate of ion impact plus secondary emission. Determination of the density profile from measurements of auroral emissions observed from the rocket, as secondaries racing down the magnetic field reach an E-layer footprint, are discussed. Next the positive terminal of the voltage supply is connected to the tape, and the negative terminal to a Hollow Cathode (HC); electrons now collected by the tape cross the supply, and are ejected at the HC. The opposite connections, with current collection operated by tape and boom, and operating on electrons and ions, and through partial switching in the supply, allow testing OML collection in almost all respects it depends on.

Floating Bare Tether as Upper Atmosphere Probe

Use of a (bare) conductive tape electrically floating in LEO as an effective e-beam source that produces artificial auroras, and is free of problems that have marred standard beams, is considered. Ambient ions impacting the tape with KeV energies over most of its length liberate secondary electrons, which race down the magnetic field and excite neutrals in the E-layer, resulting in auroral emissions. The tether would operate at night-time with both a power supply and a plasma contactor off; power and contactor would be on at daytime for reboost. The optimal tape thickness yielding a minimum mass for an autonomous system is determined; the alternative use of an electric thruster for day reboost, depending on mission duration, is discussed. Measurements of emission brightness from the spacecraft could allow determination of the (neutral) density vertical profile in the critical E-layer; the flux and energy in the beam, varying along the tether, allow imaging line-of-sight integrated emissions that mix effects with altitude-dependent neutral density and lead to a brightness peak in the beam footprint at the E-layer. Difficulties in tomographic inversion, to determine the density profile, result from beam broadening, due to elastic collisions, which flattens the peak, and to the highly nonlinear functional dependency of line-of-sight brightness. Some dynamical issues are discussed.

Conductive-tether design for de-orbiting from given altitude and inclination

A bare tether with thin-tape cross section is both i) the most effective electrodinamic tether for given length and mass, and ii) capable of effective design for an arbitrary mission through its three disparate dimensions. It handily beats the fully insulated tether that exchanges current at both ends, a result resting in advantages of 2D current collection as against 3D collection; it has much greater perimeter than the round bare tether and much lower fatal debris-impact rate, leading to greatly faster de-orbiting and greatly higher probability of survival; and it only allows multi-line tethers reaching a few hundred lines to stand competitive. In selecting the disparate values of length L, width w, and thickness h for a de-orbit mission, performance involves three criteria: a) tether-tospacecraft mass ratio must be small; b) probability of survival against the debris environment must be high; and c) de-orbiting must be fast to reduce manoeuvres for avoiding catastrophic collisions with big active/passive satellites around. Beyond determining tether mass through the product Lwh, main dimension parameters affecting performance are L/h2li characterizing ohmic effects, and w determining electron collection. An algorithm for optimal selection of tape dimensions is elaborated.

Survivability analysis of tape-tether against multiple impact with tiny debris

We show that for a tether at 800 km altitude, which is 5 km long, 2 cm wide and 0.05 mm thick, the risk of substantial damage during a 3 month period due to multiple impacts with debris or micrometeoroids is low, of about 1.4%. By substantial damage we mean that if the tape is divided in 2 cm2 cm squares, then in some square the damaged area by bombardment with debris or micrometeoroids exceeds 11% of the area of the square. Furthermore, we show that the danger posed by the micrometeoroids is negligible compared to the risk posed by the debris.

La physique de l'encensoir

Depuis sept siècles, un mécanisme ingénieux fait osciller un encensoir géant dans la cathédrale de Saint-Jacques-de-Compostelle. Ce dispositif témoigne des connaissances empiriques des mécaniciens du Moyen Age.

Física del botafumeiro

Hace siete siglos se instaló un mecanismo destinado al culto en la catedral de Santiago de Compostela, que introduce efectos dinámicos refinados. Pudo haber originado las primeras experiencias sobre caos determinista.