3,762 research outputs found
Onboard processor technology review
The general need and requirements for the onboard embedded processors necessary to control and manipulate data in spacecraft systems are discussed. The current known requirements are reviewed from a user perspective, based on current practices in the spacecraft development process. The current capabilities of available processor technologies are then discussed, and these are projected to the generation of spacecraft computers currently under identified, funded development. An appraisal is provided for the current national developmental effort
On the Early Evolution of Forming Jovian Planets I: Initial Conditions, Systematics and Qualitative Comparisons to Theory
(abridged) We analyze the formation and migration of a proto-Jovian companion
in a circumstellar disk in 2d, during the period in which the companion makes
its transition from `Type I' to `Type II' migration, using a PPM code. Spiral
waves are generated by the gravitational torque of the planet on the disk.
Their effects are to cause the planet to migrate inward and the disk to form a
deep (low surface density) gap. Until a transition to slower Type II migration,
the migration rate of the planet is of order 1 AU/10 yr, and varies by less
than a factor of two with a factor twenty change in planet mass, but depends
near linearly on the disk mass. Although the disk is stable to self gravitating
perturbations (Toomre everywhere), migration is faster by a factor of two
or more when self gravity is suppressed. Migration is equally sensitive to the
disk's mass distribution within 1--2 Hill radii of the planet, as demonstrated
by our simulations' sensitivity to the planet's assumed gravitational softening
parameter. Rapid migration can continue after gap formation. Gaps are typically
several AU in width and display the \mplan proportionality predicted by
theory. Beginning from an initially unperturbed 0.05\msun disk, planets of mass
\mj can open a gap deep and wide enough to complete the
transition to slower \ttwo migration. Lower mass objects continue to migrate
rapidly, eventually impacting the inner boundary of our grid. This transition
mass is much larger than that predicted as the `Shiva mass' discussed in Ward
and Hahn (2000), making the survival of forming planets even more precarious
than they would predict.Comment: 39 pages incl 13 figures. High resolution color figures at
http://www.maths.ed.ac.uk/~andy/publications.htm
Formation and long-term evolution of 3D vortices in protoplanetary discs
In the context of planet formation, anticyclonic vortices have recently
received lots of attention for the role they can play in planetesimals
formation. Radial migration of intermediate size solids toward the central star
may prevent their growth to larger solid grains. On the other hand, vortices
can trap the dust and accelerate this growth, counteracting fast radial
transport. Multiple effects have been shown to affect this scenario, such as
vortex migration or decay. The aim of this paper is to study the formation of
vortices by the Rossby wave instability and their long term evolution in a full
three dimensional protoplanetary disc. We use a robust numerical scheme
combined with adaptive mesh refinement in cylindrical coordinates, allowing to
affordably compute long term 3D evolutions. We consider a full disc stratified
both radially and vertically that is prone to formation of vortices by the
Rossby wave instability. We show that the 3D Rossby vortices grow and survive
over hundreds of years without migration. The localized overdensity which
initiated the instability and vortex formation survives the growth of the
Rossby wave instability for very long times. When the vortices are no longer
sustained by the Rossby wave instability, their shape changes toward more
elliptical vortices. This allows them to survive shear-driven destruction, but
they may be prone to elliptical instability and slow decay. When the conditions
for growing Rossby wave-related instabilities are maintained in the disc,
large-scale vortices can survive over very long timescales and may be able to
concentrate solids.Comment: Accepted for publication in A&
Device and method for frictionally testing materials for ignitability
Test apparatus for determining ignition characteristics of various metal in oxidizer environments simulating operating conditions for materials is invented. The test apparatus has a chamber through which the oxidizing agent flows, and means for mounting a stationary test sample therein, a powered, rotating shaft in the chamber rigidly mounts a second test sample. The shaft is axially movable to bring the samples into frictional engagement and heated to the ignition point. Instrumentation connected to the apparatus provides for observation of temperatures, pressures, loads on and speeds of the rotating shaft, and torques whereby components of stressed oxygen systems can be selected which will avoid accidental fires under working conditions
Data Mining Atmospheric/Oceanic Parameters in the Design of a Long-Range Nephelometric Forecast Tool
The Department of Defense calls for long-range forecasts to aid in the planning of operations. The goal of this research was to explore the feasibility of predicting, one month in advance, the total monthly cloud cover over the country of Afghanistan. In an attempt to reach this goal, the following objectives were achieved: (1) climatological synoptic study of Afghanistan; (2) survey of Real Time Nephanalysis, outgoing longwave radiation (OLR), and surface observational data; (3) examination of teleconnection indices and sea surface temperatures; (4) standard statistical analysis for prediction; and (5) classification tree analysis (CART), In addition, due to current world events, CART analysis was also applied over the country of Iraq (see Appendix C). Data were examined using standard statistical regression techniques, including linear and multiple linear regression, and then CART analysis was used for exploring possible concealed predictive structures. Standard statistics showed a strong negative correlation between monthly average OLR and surface observational total cloud cover from the fall through spring months. However, linear regression revealed very weak relationships between the predictor and predictand variables. As well, CART results contained misclassification rates that exceeded established thresholds for operational use. Further studies using CART for atmospheric science applications should be pursued
Planet formation models: the interplay with the planetesimal disc
According to the sequential accretion model, giant planet formation is based
first on the formation of a solid core which, when massive enough, can
gravitationally bind gas from the nebula to form the envelope. In order to
trigger the accretion of gas, the core has to grow up to several Earth masses
before the gas component of the protoplanetary disc dissipates. We compute the
formation of planets, considering the oligarchic regime for the growth of the
solid core. Embryos growing in the disc stir their neighbour planetesimals,
exciting their relative velocities, which makes accretion more difficult. We
compute the excitation state of planetesimals, as a result of stirring by
forming planets, and gas-solid interactions. We find that the formation of
giant planets is favoured by the accretion of small planetesimals, as their
random velocities are more easily damped by the gas drag of the nebula.
Moreover, the capture radius of a protoplanet with a (tiny) envelope is also
larger for small planetesimals. However, planets migrate as a result of
disc-planet angular momentum exchange, with important consequences for their
survival: due to the slow growth of a protoplanet in the oligarchic regime,
rapid inward type I migration has important implications on intermediate mass
planets that have not started yet their runaway accretion phase of gas. Most of
these planets are lost in the central star. Surviving planets have either
masses below 10 ME or above several Jupiter masses. To form giant planets
before the dissipation of the disc, small planetesimals (~ 0.1 km) have to be
the major contributors of the solid accretion process. However, the combination
of oligarchic growth and fast inward migration leads to the absence of
intermediate mass planets. Other processes must therefore be at work in order
to explain the population of extrasolar planets presently known.Comment: Accepted for publication in Astronomy and Astrophysic
VLA, PHOENIX, and BATSE observations of an X1 flare
We present observations of an X1 flare (18 Jul. 1991) detected simultaneously with the Very Large Array (VLA), the PHOENIX Digital Radio Spectrometer and the Burst and Transient Source Experiment (BATSE) aboard the Gamma Ray Observatory (GRO). The VLA was used to produce snapshot maps of the impulsive acceleration in the higher corona several minutes before the onset of the hard x ray burst detected by BATSE. Comparisons with high spectral and temporal observations by PHOENIX reveal a variety of radio bursts at 20 cm, such as type 3 bursts, intermediate drift bursts, and quasi-periodic pulsations during different stages of the X1 flare. From the drift rates of these radio bursts we derive information on local density scale heights, the speed of radio exciters, and the local magnetic field. Radio emission at 90 cm shows a type 4 burst moving outward with a constant velocity of 240 km/s. The described X1 flare is unique in the sense that it appeared at the east limb (N06/E88), providing the most accurate information on the vertical structure of different flare tracers visible in radio wavelengths
Neutral and Ionized Hydrides in Star-forming Regions -- Observations with Herschel/HIFI
The cosmic abundance of hydrides depends critically on high-energy UV, X-ray,
and particle irradiation. Here we study hydrides in star-forming regions where
irradiation by the young stellar object can be substantial, and density and
temperature can be much enhanced over interstellar values. Lines of OH, CH, NH,
SH and their ions OH+, CH+, NH+, SH+, H2O+, and H3O+ were observed in
star-forming regions by the HIFI spectrometer onboard the Herschel Space
Observatory. Molecular column densities are derived from observed ground-state
lines, models, or rotational diagrams. We report here on two prototypical
high-mass regions, AFGL 2591 and W3 IRS5, and compare them to chemical
calculations making assumptions on the high-energy irradiation. A model
assuming no ionizing protostellar emission is compared with (i) a model
assuming strong protostellar X-ray emission and (ii) a two-dimensional (2D)
model including emission in the far UV (FUV, 6 -- 13.6 eV) irradiating the
outflow walls that separate the outflowing gas and infalling envelope material.
We confirm that the effect of FUV in two dimensional models with enlarged
irradiated surfaces is clearly noticeable. A molecule that is very sensitive to
FUV irradiation is CH+, enhanced in abundance by more than 5 orders of
magnitude. The HIFI observations of CH+ lines agree with the two-dimensional
FUV model by Bruderer et al. which computes abundances, non-LTE excitation and
line radiative transfer.{Ref 20} It is concluded that CH+ is a good FUV tracer
in star-forming regions. The effect of potential X-ray irradiation is not
excluded, but cannot be demonstrated by the present data.Comment: 8 pages, 4 figures, Journal of Physical Chemistry in pres
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