2,864 research outputs found
Controlled Ecological Life Support Systems (CELSS) physiochemical waste management systems evaluation
Parametric data for six waste management subsystems considered for use on the Space Station are compared, i.e.: (1) dry incineration; (2) wet oxidation; (3) supercritical water oxidation; (4) vapor compression distillation; (5) thermoelectric integrated membrane evaporation system; and (6) vapor phase catalytic ammonia removal. The parameters selected for comparison are on-orbit weight and volume, resupply and return to Earth logistics, power consumption, and heat rejection. Trades studies are performed on subsystem parameters derived from the most recent literature. The Boeing Engineering Trade Study (BETS), an environmental control and life support system (ECLSS) trade study computer program developed by Boeing Aerospace Company, is used to properly size the subsystems under study. The six waste treatment subsystems modeled in this program are sized to process the wastes for a 90-day Space Station mission with an 8-person crew, and an emergency supply period of 28 days. The resulting subsystem parameters are compared not only on an individual subsystem level but also as part of an integrated ECLSS
Exclusion of Tiny Interstellar Dust Grains from the Heliosphere
The distribution of interstellar dust grains (ISDG) observed in the Solar
System depends on the nature of the interstellar medium-solar wind interaction.
The charge of the grains couples them to the interstellar magnetic field (ISMF)
resulting in some fraction of grains being excluded from the heliosphere while
grains on the larger end of the size distribution, with gyroradii comparable to
the size of the heliosphere, penetrate the termination shock. This results in a
skewing the size distribution detected in the Solar System.
We present new calculations of grain trajectories and the resultant grain
density distribution for small ISDGs propagating through the heliosphere. We
make use of detailed heliosphere model results, using three-dimensional (3-D)
magnetohydrodynamic/kinetic models designed to match data on the shape of the
termination shock and the relative deflection of interstellar neutral H and He
flowing into the heliosphere. We find that the necessary inclination of the
ISMF relative to the inflow direction results in an asymmetry in the
distribution of the larger grains (0.1 micron) that penetrate the heliopause.
Smaller grains (0.01 micron) are completely excluded from the Solar System at
the heliopause.Comment: 5 pages, 5 figures, accepted for publication in the Solar Wind 12
conference proceeding
The Origin of Soft X-rays in DQ Herculis
DQ Herculis (Nova Herculis 1934) is a deeply eclipsing cataclysmic variable
containing a magnetic white dwarf primary. The accretion disk is thought to
block our line of sight to the white dwarf at all orbital phases due to its
extreme inclination angle. Nevertheless, soft X-rays were detected from DQ Her
with ROSAT PSPC. To probe the origin of these soft X-rays, we have performed
Chandra ACIS observations. We confirm that DQ Her is an X-ray source. The bulk
of the X-rays are from a point-like source and exhibit a shallow partial
eclipse. We interpret this as due to scattering of the unseen central X-ray
source, probably in an accretion disk wind. At the same time, we observe what
appear to be weak extended X-ray features around DQ Her, which we interpret as
an X-ray emitting knot in the nova shell.Comment: 18 pages including 4 figures, accepted for publication in
Astrphyisical Journa
Active current sheets and hot flow anomalies in Mercury's bow shock
Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities
interacting with collisionless bow shocks. They are typically formed when the
normal component of motional (convective) electric field points toward the
embedded current sheet on at least one of its sides. The core region of an HFA
contains hot and highly deflected ion flows and rather low and turbulent
magnetic field. In this paper, we report first observations of HFA-like events
at Mercury identified over a course of two planetary years. Using data from the
orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) mission, we identify a representative ensemble of active
current sheets magnetically connected to Mercury's bow shock. We show that some
of these events exhibit unambiguous magnetic and particle signatures of HFAs
similar to those observed earlier at other planets, and present their key
physical characteristics. Our analysis suggests that Mercury's bow shock does
not only mediate the flow of supersonic solar wind plasma but also provides
conditions for local particle acceleration and heating as predicted by previous
numerical simulations. Together with earlier observations of HFA activity at
Earth, Venus and Saturn, our results confirm that hot flow anomalies are a
common property of planetary bow shocks, and show that the characteristic size
of these events is of the order of one planetary radius.Comment: 39 pages, 15 figures, 2 table
Dust in the Local Interstellar Wind
The gas-to-dust mass ratios found for interstellar dust within the Solar
System, versus values determined astronomically for the cloud around the Solar
System, suggest that large and small interstellar grains have separate
histories, and that large interstellar grains preferentially detected by
spacecraft are not formed exclusively by mass exchange with nearby interstellar
gas. Observations by the Ulysses and Galileo satellites of the mass spectrum
and flux rate of interstellar dust within the heliosphere are combined with
information about the density, composition, and relative flow speed and
direction of interstellar gas in the cloud surrounding the solar system to
derive an in situ value for the gas-to-dust mass ratio, . Hubble observations of the cloud surrounding the solar system
yield a gas-to-dust mass ratio of Rg/d=551+61-251 when B-star reference
abundances are assumed. The exclusion of small dust grains from the heliosheath
and heliosphere regions are modeled, increasing the discrepancy between
interstellar and in situ observations. The shock destruction of interstellar
grains is considered, and comparisons are made with interplanetary and presolar
dust grains.Comment: 87 pages, 9 figures, 6 tables, accepted for publication in
Astrophysical Journal. Uses AASTe
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