3,164 research outputs found
Contamination of spacecraft by recontact of dumped liquids
Liquids partially freeze when dumped from spacecraft producing particles which are released into free space at various velocities. Recontact of these particles with the spacecraft is possible for specific particle sizes and velocities and, therefore, can become contamination for experiments within the spacecraft or released experiments as a result of waste and potable water dumped from Space Shuttle. An examination of dump characteristics was conducted on STS-29 using both on-board video records and ground based measurements. A preliminary analysis of data from this flight indicates particle velocities are in the range of 30 to 75 ft/sec and recontact is possible for limited particle sizes
Vertical Assessment of Lidar Bathymetry
Validation of Airborne Light Detection And Ranging (LiDAR) Bathymetry (ALB) is necessary for the production of nautical charts to fill coastal charting gaps and to quickly and accurately measure shallow dynamic seafloors. Previous assessments concentrate on the validation of ALB data within overlapping regions of Multibeam Echo-Sounder (MBES) data. This paper presents new coverage environments to the MBES overlap validation method, and proposes an alternative method for comparison between ALB data and ENC vector information. Analysis of the validation methods favour comparisons using overlapping MBES data; however, the overlap of ALB and ENC information provides an additional assessment method with substantial overlapping regions. The ENC comparison proves to be useful at locations where MBES data is absent and in the assessment of chart adequacy.La validación de la batimetrÃa mediante la detección y el alcance de la luz aerotransportada (LiDAR – Light Detection And Ranging) es necesaria para que la producción de cartas náuticas complete los vacÃos de la cartografÃa costera y mida con rapidez y precisión los fondos marinos dinámicos poco profundos. Las evaluaciones anteriores se concentran en la validación de los datos ALB en el seno de las regiones de solapamiento de los datos de la Ecosonda Multihaz (MBES). Este documento presenta nuevos entornos de cobertura para el método de validación mediante el solapamiento MBES, y propone un método alternativo para la comparación entre los datos del ALB y la información vectorial de las ENCs. El análisis de los métodos de validación favorece las comparaciones utilizando los datos de solapamiento MBES; sin embargo, el solapamiento de la información ALB y ENC proporciona un método de evaluación adicional con importantes regiones de solapamiento. La comparación de las ENCs resulta útil en los lugares en que no hay datos MBES y en la evaluación de la idoneidad de las cartas.La validation des données bathymétriques issues de levés LiDAR (Light Detection And Ranging, en français : détection et estimation de la distance par la lumière) aéroporté (ALB) est nécessaire pour la production de cartes marines afin de combler les trous dans la cartographie côtière et de mesurer rapidement et efficacement l’évolution des planchers océaniques peu profonds. Les évaluations précédentes étaient axées sur la validation de données ALB au sein de régions où des données issues de sondages acoustiques multifaisceaux (SMF) se chevauchaient. Le présent article présente les nouvelles conditions de couverture de la méthode de validation par recoupement des données SMF, et propose une méthode alternative de comparaison entre les données ALB et les informations vectorielles des ENC. L’analyse de ces méthodes de validation favorise les comparaisons utilisant le recoupement des données SMF ; toutefois, le recoupement de données ALB et ENC fournit une méthode additionnelle pour les régions comprenant d’importants chevauchements. La comparaison d’ENC s’avère utile à des endroits où il n’existe pas de données SMF et pour l’évaluation de l’exactitude de la carte
An experimental testbed for NEAT to demonstrate micro-pixel accuracy
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. In NEAT, one fundamental aspect is the capability to measure stellar
centroids at the precision of 5e-6 pixel. Current state-of-the-art methods for
centroid estimation have reached a precision of about 4e-5 pixel at Nyquist
sampling. Simulations showed that a precision of 2 micro-pixels can be reached,
if intra and inter pixel quantum efficiency variations are calibrated and
corrected for by a metrology system. The European part of the NEAT consortium
is designing and building a testbed in vacuum in order to achieve 5e-6 pixel
precision for the centroid estimation. The goal is to provide a proof of
concept for the precision requirement of the NEAT spacecraft. In this paper we
give the basic relations and trade-offs that come into play for the design of a
centroid testbed and its metrology system. We detail the different conditions
necessary to reach the targeted precision, present the characteristics of our
current design and describe the present status of the demonstration.Comment: SPIE proceeding
High intensity 5 eV O-atom exposure facility for material degradation studies
An atomic oxygen exposure facility was developed for studies of material degradation. The goal of these studies is to provide design criteria and information for the manufacture of long life (20 to 30 years) construction materials for use in low Earth orbit. The studies that are being undertaken will provide: (1) absolute reaction cross sections for the engineering design problems, (2) formulations of reaction mechanisms for use in the selection of suitable existing materials and the design of new more resistant ones, and (3) the calibration of flight hardware (mass spectrometers, etc.) in order to directly relate experiments performed in low Earth orbit to ground based investigations. The facility consists of a CW laser sustained discharge source of O-atoms, an atomic beam formation and diagnostics system, a spinning rotor viscometer, and provision for using the system for calibration of actual flight instruments
Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
Virtually all detailed gas-phase models of the chemistry of dense
interstellar clouds exclude polycyclic aromatic hydrocarbons (PAH's). This
omission is unfortunate because from the few studies that have been done on the
subject, it is known that the inclusion of PAH's can affect the gas-phase
chemistry strongly. We have added PAH's to our network to determine the role
they play in the chemistry of cold dense cores. In the models presented here,
we include radiative attachment to form PAH-, mutual neutralization between PAH
anions and small positively-charged ions, and photodetachment. We also test the
sensitivity of our results to changes in the size and abundance of the PAH's.
Our results confirm that the inclusion of PAH's changes many of the calculated
abundances of smaller species considerably. In TMC-1, the general agreement
with observations is significantly improved contrary to L134N. This may
indicate a difference in PAH properties between the two regions. With the
inclusion of PAH's in dense cloud chemistry, high-metal elemental abundances
give a satisfactory agreement with observations. As a result, we do not need to
decrease the observed elemental abundances of all metals and we do not need to
vary the elemental C/O ratio in order to produce large abundances of carbon
species in TMC-1 (CP).Comment: Accepted to ApJ. Astrophysical Journal (2008) accepte
High angular resolution imaging and infrared spectroscopy of CoRoT candidates
Studies of transiting extrasolar planets are of key importance for
understanding the nature of planets outside our solar system because their
masses, diameters, and bulk densities can be measured. An important part of
transit-search programmes is the removal of false-positives. The critical
question is how many of the candidates that passed all previous tests are false
positives. For our study we selected 25 CoRoT candidates that have already been
screened against false-positives using detailed analysis of the light curves
and seeing-limited imaging, which has transits that are between 0.7 and 0.05%
deep. We observed 20 candidates with the adaptive optics imager NaCo and 18
with the high-resolution infrared spectrograph CRIRES. We found previously
unknown stars within 2 arcsec of the targets in seven of the candidates. All of
these are too faint and too close to the targets to have been previously
detected with seeing-limited telescopes in the optical. Our study thus leads to
the surprising results that if we remove all candidates excluded by the
sophisticated analysis of the light-curve, as well as carrying out deep imaging
with seeing-limited telescopes, still 28-35% of the remaining candidates are
found to possess companions that are bright enough to be false-positives. Given
that the companion-candidates cluster around the targets and that the J-K
colours are consistent with physical companions, we conclude that the
companion-candidates are more likely to be physical companions rather than
unrelated field stars.Comment: 12 pages, 12 figures, A&A in pres
Sequence structure emission in The Red Rectangle Bands
We report high resolution (R~37,000) integral field spectroscopy of the
central region (r<14arcsec) of the Red Rectangle nebula surrounding HD44179.
The observations focus on the 5800A emission feature, the bluest of the
yellow/red emission bands in the Red Rectangle. We propose that the emission
feature, widely believed to be a molecular emission band, is not a molecular
rotation contour, but a vibrational contour caused by overlapping sequence
bands from a molecule with an extended chromophore. We model the feature as
arising in a Polycyclic Aromatic Hydrocarbon (PAH) with 45-100 carbon atoms.Comment: 13 pages, 9 figures, accepted for publication in ApJ. A version of
the paper with full resolution figures is available at:
http://www.aao.gov.au/local/www/rgs/Sequence-Structure
Molecular Emission Line Formation in Prestellar Cores
We investigate general aspects of molecular line formation under conditions
which are typical of prestellar cores. Focusing on simple linear molecules, we
study formation of their rotational lines by radiative transfer simulations. We
present a thermalization diagram to show the effects of collisions and
radiation on the level excitation. We construct a detailed scheme (contribution
chart) to illustrate the formation of emission line profiles. This chart can be
used as an efficient tool to identify which parts of the cloud contribute to a
specific line profile. We show how molecular line characteristics for uniform
model clouds depend on hydrogen density, molecular column density, and kinetic
temperature. The results are presented in a 2D plane to illustrate cooperative
effects of the physical factors. We also use a core model with a non-uniform
density distribution and chemical stratification to study the effects of cloud
contraction and rotation on spectral line maps. We discuss the main issues that
should be taken into account when dealing with interpretation and simulation of
observed molecular lines.Comment: Accepted for publication in Ap
Dynamics of Spreading of Small Droplets of Chainlike Molecules on Surfaces
Dynamics of spreading of small droplets on surfaces has been studied by the
molecular dynamics method. Simulations have been performed for mixtures of
solvent and dimer, and solvent and tetramer droplets. For solvent particles and
dimers, layering occurs leading to stepped droplet shapes. For tetramers such
shapes occur for relatively deep and strong surface potentials only. For wider
and more shallow potentials, more rapid spreading and rounded droplet shapes
occur. These results are in accordance with experimental data on small non -
volatile polymer droplets. PACS numbers: 68.10Gw, 05.70.Ln, 61.20.Ja, 68.45GdComment: to appear in Europhys. Letters (1994), Latex, 12 page
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