2,232 research outputs found
Orbital debris research at NASA Johnson Space Center, 1986-1988
Research on orbital debris has intensified in recent years as the number of debris objects in orbit has grown. The population of small debris has now reached the level that orbital debris has become an important design factor for the Space Station. The most active center of research in this field has been the NASA Lyndon B. Johnson Space Center. Work is being done on the measurement of orbital debris, development of models of the debris population, and development of improved shielding against hypervelocity impacts. Significant advances have been made in these areas. The purpose of this document is to summarize these results and provide references for further study
Orbital debris environment for spacecraft designed to operate in low Earth orbit
The orbital debris environment model is intended to be used by the spacecraft community for the design and operation of spacecraft in low Earth orbit. This environment, when combined with material-dependent impact tests and spacecraft failure analysis, is intended to be used to evaluate spacecraft vulnerability, reliability, and shielding requirements. The environment represents a compromise between existing data to measure the environment, modeling of this data to predict the future environment, the uncertainty in both measurements and modeling, and the need to describe the environment so that various options concerning spacecraft design and operations can be easily evaluated
FMRI Clustering and False Positive Rates
Recently, Eklund et al. (2016) analyzed clustering methods in standard FMRI
packages: AFNI (which we maintain), FSL, and SPM [1]. They claimed: 1) false
positive rates (FPRs) in traditional approaches are greatly inflated,
questioning the validity of "countless published fMRI studies"; 2)
nonparametric methods produce valid, but slightly conservative, FPRs; 3) a
common flawed assumption is that the spatial autocorrelation function (ACF) of
FMRI noise is Gaussian-shaped; and 4) a 15-year-old bug in AFNI's 3dClustSim
significantly contributed to producing "particularly high" FPRs compared to
other software. We repeated simulations from [1] (Beijing-Zang data [2], see
[3]), and comment on each point briefly.Comment: 3 pages, 1 figure. A Letter accepted in PNA
Fully-Coupled Simulation of Cosmic Reionization. I: Numerical Methods and Tests
We describe an extension of the Enzo code to enable fully-coupled radiation
hydrodynamical simulation of inhomogeneous reionization in large cosmological volumes with thousands to millions of point sources. We
solve all dynamical, radiative transfer, thermal, and ionization processes
self-consistently on the same mesh, as opposed to a postprocessing approach
which coarse-grains the radiative transfer. We do, however, employ a simple
subgrid model for star formation which we calibrate to observations. Radiation
transport is done in the grey flux-limited diffusion (FLD) approximation, which
is solved by implicit time integration split off from the gas energy and
ionization equations, which are solved separately. This results in a faster and
more robust scheme for cosmological applications compared to the earlier
method. The FLD equation is solved using the hypre optimally scalable geometric
multigrid solver from LLNL. By treating the ionizing radiation as a grid field
as opposed to rays, our method is scalable with respect to the number of
ionizing sources, limited only by the parallel scaling properties of the
radiation solver. We test the speed and accuracy of our approach on a number of
standard verification and validation tests. We show by direct comparison with
Enzo's adaptive ray tracing method Moray that the well-known inability of FLD
to cast a shadow behind opaque clouds has a minor effect on the evolution of
ionized volume and mass fractions in a reionization simulation validation test.
We illustrate an application of our method to the problem of inhomogeneous
reionization in a 80 Mpc comoving box resolved with Eulerian grid
cells and dark matter particles.Comment: 32 pages, 23 figures. ApJ Supp accepted. New title and substantial
revisions re. v
Thickness dependence of spin-orbit torques generated by WTe2
We study current-induced torques in WTe2/permalloy bilayers as a function of
WTe2 thickness. We measure the torques using both second-harmonic Hall and
spin-torque ferromagnetic resonance measurements for samples with WTe2
thicknesses that span from 16 nm down to a single monolayer. We confirm the
existence of an out-of-plane antidamping torque, and show directly that the
sign of this torque component is reversed across a monolayer step in the WTe2.
The magnitude of the out-of-plane antidamping torque depends only weakly on
WTe2 thickness, such that even a single-monolayer WTe2 device provides a strong
torque that is comparable to much thicker samples. In contrast, the
out-of-plane field-like torque has a significant dependence on the WTe2
thickness. We demonstrate that this field-like component originates
predominantly from the Oersted field, thereby correcting a previous inference
drawn by our group based on a more limited set of samples.Comment: 8 pages, 8 figure
In-situ measurements of the optical absorption of dioxythiophene-based conjugated polymers
Conjugated polymers can be reversibly doped by electrochemical means. This
doping introduces new sub-bandgap optical absorption bands in the polymer while
decreasing the bandgap absorption. To study this behavior, we have prepared an
electrochemical cell allowing measurements of the optical properties of the
polymer. The cell consists of a thin polymer film deposited on gold-coated
Mylar behind which is another polymer that serves as a counterelectrode. An
infrared transparent window protects the upper polymer from ambient air. By
adding a gel electrolyte and making electrical connections to the
polymer-on-gold films, one may study electrochromism in a wide spectral range.
As the cell voltage (the potential difference between the two electrodes)
changes, the doping level of the conjugated polymer films is changed
reversibly. Our experiments address electrochromism in
poly(3,4-ethylene-dioxy-thiophene) (PEDOT) and
poly(3,4-dimethyl-propylene-dioxy-thiophene) (PProDOT-Me). This closed
electrochemical cell allows the study of the doping induced sub-bandgap
features (polaronic and bipolaronic modes) in these easily oxidized and highly
redox switchable polymers. We also study the changes in cell spectra as a
function of polymer thickness and investigate strategies to obtain cleaner
spectra, minimizing the contributions of water and gel electrolyte features
Oxazole and thiazole analogs of sulindac for cancer prevention
Aim: Experimental and epidemiological studies and clinical trials suggest that nonsteroidal anti-inflammatory drugs possess antitumor potential. Sulindac, a widely used nonsteroidal anti-inflammatory drug, can prevent adenomatous colorectal polyps and colon cancer, especially in patients with familial adenomatous polyposis. Sulindac sulfide amide (SSA) is an amide-linked sulindac sulfide analog that showed in vivo antitumor activity in a human colon tumor xenograft model. Results/methodology: A new analog series with heterocyclic rings such as oxazole or thiazole at the C-2 position of sulindac was prepared and screened against prostate, colon and breast cancer cell lines to probe the effect of these novel substitutions on the activity of sulindac analogs.Conclusion: In general, replacement of the amide function of SSA analogs had a negative impact on the cell lines tested. A small number of hits incorporating rigid oxazole or thiazole groups in the sulindac scaffold in place of the amide linkage show comparable activity to our lead agent SSA.</p
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