2,543 research outputs found
Working group written presentation: Atomic oxygen
Earlier Shuttle flight experiments have shown NASA and SDIO spacecraft designed for operation in low-Earth orbit (LEO) must take into consideration the highly oxidative characteristics of the ambient flight environment. Materials most adversely affected by atomic oxygen interactions include organic films, advanced (carbon-based) composites, thermal control coatings, organic-based paints, optical coatings, and thermal control blankets commonly used in spacecraft applications. Earlier results of NASA flight experiments have shown prolonged exposure of sensitive spacecraft materials to the LEO environment will result in degraded systems performance or, more importantly, lead to requirements for excessive on-orbit maintenance, with both conditions contributing significantly to increased mission costs and reduced mission objectives. Flight data obtained from previous Space Shuttle missions and results of the Solar Max recovery mission are limited in terms of atomic oxygen exposure and accuracy of fluence estimates. The results of laboratory studies to investigate the long-term (15 to 30 yrs) effects of AO exposure on spacecraft surfaces are only recently available, and qualitative correlations of laboratory results with flight results have been obtained for only a limited number of materials. The working group recommended the most promising ground-based laboratories now under development be made operational as soon as possible to study the full-life effects of atomic oxygen exposure on spacecraft systems
Space shuttle mechanistic studies to characterize atomic oxygen interactions with surfaces
A materials interaction experiment has been approved to study atomic oxygen interaction mechanisms and develop coatings for Space Station elements requiring long-lived operation in the LEO environment. A brief summary of this experiment is presented and the required exposure conditions are reviewed
Material interactions with the Low Earth Orbital (LEO) environment: Accurate reaction rate measurements
To resolve uncertainties in estimated LEO atomic oxygen fluence and provide reaction product composition data for comparison to data obtained in ground-based simulation laboratories, a flight experiment has been proposed for the space shuttle which utilizes an ion-neutral mass spectrometer to obtain in-situ ambient density measurements and identify reaction products from modeled polymers exposed to the atomic oxygen environment. An overview of this experiment is presented and the methodology of calibrating the flight mass spectrometer in a neutral beam facility prior to its use on the space shuttle is established. The experiment, designated EOIM-3 (Evaluation of Oxygen Interactions with Materials, third series), will provide a reliable materials interaction data base for future spacecraft design and will furnish insight into the basic chemical mechanisms leading to atomic oxygen interactions with surfaces
Atomic oxygen
The effects of atomic oxygen (AO) on materials in aerospace environments are examined. Materials are categorized according to their susceptibility to AO. The degradation effects of AO are examined to determine which materials are most vulnerable. Classes of spacecraft materials are listed and correlated with a performance category. The spacecraft orbits sensitive to AO interactions are also listed. Correlations are presented of AO effects on materials and of spacecraft glow effects
Polarized Infrared Emission by Polycyclic Aromatic Hydrocarbons resulting from Anisotropic Illumination
We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons
(PAHs), when anisotropically illuminated by UV photons. PAH molecules are
modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As
first pointed out by Leger (1988), infrared emission features resulting from
in-plane and out-of-plane modes should have orthogonal polarization directions.
We show analytically how the degree of polarization depends on the viewing
geometry and the molecule's internal alignment between principal axis of
inertia and angular momentum, which gets worse after photon absorption. Longer
wavelength features, emitted after better internal alignment is recovered,
should be more strongly polarized. The degree of polarization for
uni-directional illumination (e.g., by a star) is larger than for diffuse
illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the
Cold Neutral Medium, the predicted polarization is probably too small to
distinguish from the contribution of linear dichroism by aligned foreground
dust. The level of polarization predicted for PAH emission from the Orion Bar
is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger
value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately
suprathermal rotation rates. Future observations of (or upper limits on) the
degree of polarization for the Orion Bar or for dust above edge-on galaxies
(e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs,
thus providing clues to their rotational dynamics.Comment: 9 pages, 4 figures, 1 table, submitted to Ap
Evaluation of bone excision on occipital area of simulated human skull
Surgical effects of bone and soft tissue tumours,
whether for biopsy or full excision have been
researched from as early as the 1970’s [1]. These
researches though have as main focus the biological
(histological) rather the mechanical aspects of the
effects [2]. With technological advances in biomedical
and biomechanical modelling, a plethora of researchers
have been exploring the possibilities of understanding
[3] or even predicting musculoskeletal behaviour under
different loading conditions [4]. This research is
seeking to bridge these two different facets by looking
into the mechanical effects bone tumour surgery might
have to the structural rigidity of a simulated human
skull
A New Family of Planets ? "Ocean Planets"
A new family of planets is considered which is between rochy terrestrial
planets and gaseous giant ones: "Ocean-Planets". We present the possible
formation, composition and internal models of these putative planets, including
that of their ocean, as well as their possible Exobiology interest. These
planets should be detectable by planet detection missions such as Eddington and
Kepler, and possibly COROT (lauch scheduled in 2006). They would be ideal
targets for spectroscopic missions such as Darwin/TPF.Comment: 15 pages, 3 figures submitted to Icarus notes (10 july 2003
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
Photodesorption of CO ice
At the high densities and low temperatures found in star forming regions, all
molecules other than H2 should stick on dust grains on timescales shorter than
the cloud lifetimes. Yet these clouds are detected in the millimeter lines of
gaseous CO. At these temperatures, thermal desorption is negligible and hence a
non-thermal desorption mechanism is necessary to maintain molecules in the gas
phase. Here, the first laboratory study of the photodesorption of pure CO ice
under ultra high vacuum is presented, which gives a desorption rate of 3E-3 CO
molecules per UV (7-10.5 eV) photon at 15 K. This rate is factors of 1E2-1E5
larger than previously estimated and is comparable to estimates of other
non-thermal desorption rates. The experiments constrains the mechanism to a
single photon desorption process of ice surface molecules. The measured
efficiency of this process shows that the role of CO photodesorption in
preventing total removal of molecules in the gas has been underestimated.Comment: 5 pages, 4 figures, accepted by ApJ
Molecular and Ionic shocks in the Supernova Remnant 3C391
New observations of the supernova remnant 3C391 are in the H2 2.12 micron and
[Fe II] 1.64 micron narrow-band filters at the Palomar 200-inch telescope, and
in the 5-15 micron CVF on ISOCAM. Shocked H2 emission was detected from the
region 3C391:BML, where broad millimeter CO and CS lines had previously been
detected. A new H2 clump was confirmed to have broad CO emission, demonstrating
that the near-infrared H2 images can trace previously undetected molecular
shocks. The [Fe II] emission has a significantly different distribution, being
brightest in the bright radio bar, at the interface between the supernova
remnant and the giant molecular cloud, and following filaments in the radio
shell. The near-infrared [Fe II] and the mid-infrared 12-18 micron filter
images are the first images to reveal the radiative shell of 3C391. The
mid-infrared spectrum is dominated by bright ionic lines and H2 S(2) through
S(7). There are no aromatic hydrocarbons associated with the shocks, nor is
their any mid-infrared continuum, suggesting that macromolecules and very small
grains are destroyed. Comparing 3C391 to the better-studied IC443, both
remnants have molecular- and ionic-dominated regions; for 3C391, the
ionic-dominated region is the interface into the giant molecular cloud, showing
that the main bodies of giant molecular clouds contain significant regions with
densities 100 to 1000/cm^3 and a small filling factor with higher-density. The
molecular shocked region resolves into 16 clumps of H2 emission, with some
fainter diffuse emission but with no associated near-infrared continuum
sources. One of the clumps is coincident with a previously-detected OH 1720 MHz
maser. These clumps are interpreted as a cluster of pre-stellar, dense
molecular cores that are presently being shocked by the supernova blast wave
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