17,034 research outputs found
Effects of microgravity on rat muscle
It is well known that humans exposed to long term spaceflight experience undesirable progressive muscle weakness and increased fatigability. This problem has prompted the implementation of inflight exercise programs because most investigators believe that the major cause of diminished muscle performance is a combination of disuse and decreased workload. Inflight exercise has improved muscle health, but deficits have persisted, indicating that either the regimens utilized were suboptimal or there existed additional debilitating factors which were not remedied by exercise. Clarification of this question requires an improved understanding of the cellular and molecular basis of spaceflight-induced muscle deterioration. To this end, multiple investigations have been performed on the muscles from rats orbited 5 to 22 days in Cosmos biosatellites and Spacelab-3 (2,4,5,8,10 to 14,16,18,19,21 to 23,25,27,28). The eight Cosmos 1887 investigations examined the structural and biochemical changes in skeletal and cardiac muscles of rats exposed to microgravity for 12.5 days and returned to terrestrial gravity 2.3 days before tissues were collected. Even though interpretation of these results was complicated by the combination of inflight and postflight induced alterations, the consensus is that there is marked heterogeneity in both degree and type of responses from the whole muscle level down to the molecular level. Collectively, the muscle investigations of Cosmos 1887 clearly illustrate the wide diversity of muscle tissue responses to spaceflight. Judging from the summary report of this mission, heterogeneity of responses is not unique to muscle tissue. Elucidating the mechanism underlying this heterogeneity holds the key to explaining adaptation of the organism to prolonged spaceflight
A Ka-band (32 GHz) beacon link experiment (KABLE) with Mars Observer
A proposal for a Ka-Band (32 GHz) Link Experiment (KABLE) with the Mars Observer mission was submitted to NASA. The experiment will rely on the fourth harmonic of the spacecraft X-band transmitter to generate a 33.6 GHz signal. The experiment will rely also on the Deep Space Network (DSN) receiving station equipped to simultaneously receive X- and Ka-band signals. The experiment will accurately measure the spacecraft-to-Earth telecommunication link performance at Ka-band and X-band (8.4 GHz)
Redundant Arrays of IDE Drives
The next generation of high-energy physics experiments is expected to gather
prodigious amounts of data. New methods must be developed to handle this data
and make analysis at universities possible. We examine some techniques that use
recent developments in commodity hardware. We test redundant arrays of
integrated drive electronics (IDE) disk drives for use in offline high-energy
physics data analysis. IDE redundant array of inexpensive disks (RAID) prices
now equal the cost per terabyte of million-dollar tape robots! The arrays can
be scaled to sizes affordable to institutions without robots and used when fast
random access at low cost is important. We also explore three methods of moving
data between sites; internet transfers, hot pluggable IDE disks in FireWire
cases, and writable digital video disks (DVD-R).Comment: Submitted to IEEE Transactions On Nuclear Science, for the 2001 IEEE
Nuclear Science Symposium and Medical Imaging Conference, 8 pages, 1 figure,
uses IEEEtran.cls. Revised March 19, 2002 and published August 200
Second Epoch Global VLBI Observations of Compact Radio Sources in the M82 Starburst Galaxy
We have presented the results of a second epoch of global Very Long Baseline
Interferometry observations, taken on 23 February 2001 at a wavelength of 18
cm, of the central kiloparsec of the nearby starburst galaxy Messier 82. These
observations were aimed at studying the structural and flux evolution of some
of the compact radio sources in the central region that have been identified as
supernova remnants. The objects 41.95+575 and 43.31+592 have been studied,
expansion velocities of 2500 +/- 1200 km/s and 7350 +/- 2100 km/s respectively
have been derived. Flux densities of 31.1 +/- 0.3 mJy and 17.4 +/- 0.3 mJy have
been measured for the two objects. These results are consistent with
measurements and predictions from previous epochs.Comment: 5 pages, 3 figures. To be published on the accompanying CD of the
Proceedings of IAU Colloquium 192: Supernova
KASR: A Reliable and Practical Approach to Attack Surface Reduction of Commodity OS Kernels
Commodity OS kernels have broad attack surfaces due to the large code base
and the numerous features such as device drivers. For a real-world use case
(e.g., an Apache Server), many kernel services are unused and only a small
amount of kernel code is used. Within the used code, a certain part is invoked
only at runtime while the rest are executed at startup and/or shutdown phases
in the kernel's lifetime run. In this paper, we propose a reliable and
practical system, named KASR, which transparently reduces attack surfaces of
commodity OS kernels at runtime without requiring their source code. The KASR
system, residing in a trusted hypervisor, achieves the attack surface reduction
through a two-step approach: (1) reliably depriving unused code of executable
permissions, and (2) transparently segmenting used code and selectively
activating them. We implement a prototype of KASR on Xen-4.8.2 hypervisor and
evaluate its security effectiveness on Linux kernel-4.4.0-87-generic. Our
evaluation shows that KASR reduces the kernel attack surface by 64% and trims
off 40% of CVE vulnerabilities. Besides, KASR successfully detects and blocks
all 6 real-world kernel rootkits. We measure its performance overhead with
three benchmark tools (i.e., SPECINT, httperf and bonnie++). The experimental
results indicate that KASR imposes less than 1% performance overhead (compared
to an unmodified Xen hypervisor) on all the benchmarks.Comment: The work has been accepted at the 21st International Symposium on
Research in Attacks, Intrusions, and Defenses 201
Ka-band MMIC beam steered transmitter array
A 32-GHz six-element linear transmitter array utilizing monolithic microwave integrated circuit (MMIC) phase shifters and power amplifiers was designed and tested as part of the development of a spacecraft array feed for NASA deep-space communications applications. Measurements of the performance of individual phase shifters, power amplifiers, and microstrip radiators were carried out, and electronic beam steering of the linear array was demonstrated. The switched-line phase shifters were accurate to within 7 percent on average and the power amplifier 1-dB compressed output power varied over 0.3 dB. The array had a beamwidth of 7.5 deg and demonstrated acceptable beam steering over + or - 8 deg. From the results, it can be concluded that this MMIC phased array has adequate beam-scanning capability for use in the two-dimensional array. The areas that need to be improved are the efficiency of the MMIC power amplifier and the insertion loss of the MMIC phase shifter
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