10,081 research outputs found
An altitude-dependent spacecraft charging model
A model for the altitude dependence of the hot plasma parameters responsible for the electrostatic charging of spacecraft was developed. Based upon plasma orbit theory, the directed velocity is a function of the ambient magnetic field flux density. A consequence of this approach is that while the thermal velocity distributions (assumed to be Maxwellian) of the plasma particles are independent of the magnetic field strength (and hence altitude), the particle densities increase with magnetic field strength. Thus, according to this model, while the equilibrium voltage is independent of altitude, the charging current density increases with decreasing altitude. However, the probability of such spacecraft charging decreases with decreasing altitude
The Biochemical and Genetic Odyssey to the Function of a Nicastrin-Like Protein
gamma-Secretase is a high-molecular-weight protein complex required for the proteolytic processing of various transmembrane proteins including the Alzheimer's disease-associated amyloid precursor protein and the signaling receptor Notch. One of the gamma-secretase complex components is the type I transmembrane protein nicastrin. Here we review the odyssey to a cyclopic fish, which at the end allowed the functional analysis of nicalin, a novel member of the nicastrin protein family. This 60-kDa protein is part of a previously unknown membrane protein complex unrelated to gamma-secretase and binds to Nomo (Nodal modulator, previously known as pM5), a novel 130-kDa transmembrane protein. Both proteins are highly conserved in metazoans and show almost identical tissue distribution in humans. Functional studies in zebrafish embryos and cultured human cells revealed that nicalin and Nomo collaborate to antagonize the Nodal/TGF beta signaling pathway. Thus, nicastrin and nicalin are both associated with protein complexes involved in cell fate decisions during early embryonic development. Copyright (C) 2004 S. Karger AG, Base
Cellular functions of gamma-secretase-related proteins
Amyloid-beta pepticle (A beta) is generated by gamma-secretase, a membrane protein complex with an unusual aspartyl protease activity consisting of the four components presenilin, nicastrin, APH-1 and PEN-2. Presenilin is considered the catalytic subunit of this complex since it represents the prototype of the new family of intramembrane-cleaving GxGD-type aspartyl proteases. Recently, five novel members of this family and a nicastrin-like protein were identified. Whereas one of the GxGD-type proteins was shown to be identical with signal pepticle peptidase (SPP), the function of the others, now called SPP-like proteins (SPPLs), is not known. We therefore analyzed SPPL2b and SPPL3 and demonstrated that they localize to different subcellular compartments suggesting nonredundant functions. This was supported by different phenotypes obtained in knockdown studies in zebrafish embryos. In addition, these phenotypes could be phenocopied by ectopic expression of putative active site mutants, providing strong evidence for a proteolytic function of SPPL2b and SPPL3. We also identified and characterized the nicastrin-like protein nicalin which, together with the 130-kDa protein NOMO (Nodal modulator), forms a membrane protein complex different from gamma-secretase. We found that during zebrafish embryogenesis this complex is involved in the patterning of the axial mesendoderm, a process controlled by the Nodal signaling pathway. Copyright (c) 2006 S. Karger AG, Basel
Ionization of Infalling Gas
H-alpha emission from neutral halo clouds probes the radiation and
hydrodynamic conditions in the halo. Armed with such measurements, we can
explore how radiation escapes from the Galactic plane and how infalling gas can
survive a trip through the halo. The Wisconsin H-Alpha Mapper (WHAM) is one of
the most sensitive instruments for detecting and mapping optical emission from
the ISM. Here, we present recent results exploring the ionization of two
infallling high-velocity complexes. First, we report on our progress mapping
H-alpha emission covering the full extent of Complex A. Intensities are faint
(<100 mR; EM <0.2 pc cm^-6 but correlate on the sky and in velocity with 21-cm
emission. Second, we explore the ionized component of some Anti-Center Complex
clouds studied by Peek et al. (2007) that show dynamic shaping from interaction
with the Galactic halo.Comment: 4 pages, 2 figures; to appear in proceedings of "The Role of
Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?" held in
Espinho, Portugal during 2008 Augus
Environmental effects on spacecraft materials
The effects on the natural space environments on materials are presented, which may be used for SDI applications. The current state-of-the-art knowledge of those effects was studied, and a literature search, a questionnaire mailing, and some visits to NASA and Air Force research facilities were performed. Phase 2 will be a study of what materials may be used for SDI applications and to what natural space environments they may be vulnerable. Deficiencies in knowledge of the effects of the natural space environments on these materials are to be identified and recommendations are to be made to eliminate these knowledge deficiencies
Strengthening of foamed composite materials
We investigate the shear elastic modulus of soft polymer foams loaded with
hard spherical particles and we show that, for constant bubble size and gas
volume fraction, strengthening is strongly dependent on the size of those
inclusions. Through an accurate control of the ratio that compares
the particle size to the thickness of the struts in the foam structure, we
evidence a transition in the mechanical behavior at . For
, every particle loading leads to a strengthening effect whose
magnitude depends only on the particle volume fraction. On the contrary, for
, the strengthening effect weakens abruptly as a function of
and a softening effect is even observed for .
This transition in the mechanical behavior is reminiscent of the so-called
"particle exclusion transition" that has been recently reported within the
framework of drainage of foamy granular suspensions [Haffner B, Khidas Y,
Pitois O. The drainage of foamy granular suspensions. J Colloid Interface Sci
2015. In Press.]. It involves the evolution for the geometrical configuration
of the particles with respect to the foam network, and it appears to control
the mechanics of such foamy systems
Flow and Jamming of Granular Suspensions in Foams
The drainage of particulate foams is studied under conditions where the
particles are not trapped individually by constrictions of the interstitial
pore space. The drainage velocity decreases continuously as the particle volume
fraction increases. The suspensions jam - and therefore drainage
stops - for values which reveal a strong effect of the particle
size. In accounting for the particular geometry of the foam, we show that
accounts for unusual confinement effects when the particles pack
into the foam network. We model quantitatively the overall behavior of the
suspension - from flow to jamming - by taking into account explicitly the
divergence of its effective viscosity at . Beyond the scope of
drainage, the reported jamming transition is expected to have a deep
significance for all aspects related to particulate foams, from aging to
mechanical properties
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