Search NASA⌕ Search

SEARCH · Search NASA

Results for “LET”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Deoxyribonucleoprotein structure and radiation injury - Cellular radiosensitivity is determined by LET-infinity-dependent DNA damage in hydrated deoxyribonucleoproteins and the extent of its repair

Until recently, OH radicals formed in bulk nuclear water were believed to be the major causes of DNA damage that results in cell death, especially for sparsely ionizing radiations. That hypothesis has now been challenged, if not refuted. Lethal genomic DNA damage is determined mainly by energy deposition in deoxyribonucleoproteins, and their hydration shells, and charge (energy) transfer processes within those structures.

Lett, J. T.↗
NASA NTRS1992

Comparative study of spermatogonial survival after X-ray exposure, high LET (HZE) irradiation or spaceflight

Spermatogonial cell loss has been observed in rats flown on Space Lab 3, Cosmos 1887, Cosmos 2044 and in mice following irradiation with X-ray or with HZE particle beams. Spermatogonial loss is determined by cell counting in maturation stage-6 seminferous tubules. With the exception of iron, laboratory irradiation experiments (with mice) revealed a similar pattern of spermatogonial loss proportional to the radiation dose at levels less than 0.1 Gy. Helium and argon irradiation resulted in a 5-percent loss of spermatogonia after only 0.01 Gy exposure. Significant spermatogonial loss (45 percent) occurred at this radiation level with iron particle beams. The loss of spermatogonia during each spaceflight was less than 10 percent when compared to control (nonflight) animals.

Sapp, W. J.↗
NASA NTRS1993

Let your fingers do the walking: The projects most invaluable tool

The barrage of information pertaining to the software being developed for a project can be overwhelming. Current status information, as well as the statistics and history of software releases, should be 'at the fingertips' of project management and key technical personnel. This paper discusses the development, configuration, capabilities, and operation of a relational database, the System Engineering Database (SEDB) which was designed to assist management in monitoring of the tasks performed by the Network Control Center (NCC) Project. This database has proven to be an invaluable project tool and is utilized daily to support all project personnel.

Zirk, Deborah A.↗
NASA NTRS1994

A model of solar energtic particles for use in calculating LET spectra developed from ONR-604 data

A model of Solar Energetic Particles (SEP) has been developed and is applied to solar flares during the 1990/1991 Combined Release and Radiation Effects Satellite (CRRES) mission using data measured by the University of Chicago instrument, ONR-604. The model includes the time-dependent behavior, heavy-ion content, energy spectrum and influence, and can accurately represent the observed SEP events in the energy range between 40 to 500 MeV/nucleon. Results are presented for the March and June, 1991 flare periods.

Chen, J.↗
NASA NTRS1995

Determination Of LETs Of SRAMs By Use Of A Laser

Report describes experimental study of use of microelectronic advanced laser scanner (MEALS) to cause single-event upsets (SEUs) in integrated logic circuits. Basic concepts of SEU testing by use of MEALS described in "Laser Scanner Tests for Single-Event Upsets" (NPO-18216), "Single-Event-Upset Laser Scanner With Optical Bias" (NPO-18217), and "More About Laser Scanner Tests for Single-Event Upsets" (NPO-18494). Study part of continuing effort to study SEU effects of ionizing radiation on such circuits and to use MEALS as relatively inexpensive SEU-prescreening laboratory apparatus serving as alternative to heavy-ion acclerator.

Kim, Quiesup↗
NASA NTRS2001

Large Engine Technology (LET) Task XXXVII Low-Bypass Ratio Mixed Turbofan Engine Subsonic Jet Noise Reduction Program Test Report

NASA Glenn Research Center supported a three year effort to develop the technology for reducing jet noise from low-bypass ratio engines. This effort concentrated on both analytical and experimental approaches using various mixer designs. CFD and MGB predictions are compared with LDV and noise data, respectively. While former predictions matched well with data, experiment shows a need for improving the latter predictions. Data also show that mixing noise can be sensitive to engine hardware upstream of the mixing exit plane.

Hauser, Joseph R.↗
NASA NTRS2002

Letting Our Cells Do the Fighting: Flight-Induced Changes in the Immune Response

The organisms that make us ill, such as bacteria, viruses, and fungi, are like attacking armies. We now know a great deal more about this unseen world of microscopic invaders. Fortunately for us, the human immune system is ever vigilant against them. Microorganisms such as bacteria, viruses, and fungi occupy almost every corner of the Earth, and even parts of the human body. Some organisms are beneficial to us, helping to produce milk, cheese or yogurt. Others are potentially harmful, yet we don#t always develop illnesses from them; they are kept in check by the sentinels of our immune system. Our immune system is routinely challenged by these organisms every day. When the immune response is diminished, our ability to fight off these "bugs" is lowered. And that's when we become ill. Space flight presents a challenge to the immune system. Scientists believe that the stressful conditions of space flight - launch into orbit, adapting to microgravity, heavy workloads, and isolation from family and friends, to name but a few - reduce the astronauts' immunity. This immune suppression makes them more susceptible to common illnesses from bacteria and to re-infections from latent viruses in the body. In addition, risk of spreading illness in the confined environment of the Space Shuttle is high. Understanding changes in immune function will help scientists develop ways to keep astronauts healthy in space. This knowledge can also benefit earthbound populations. This experiment will give scientists insight into the immune system by comparing how certain cells of astronauts' innate immune system - the first line of defense against invaders - function after flight compared to before flight.

Pierson, Duane↗
NASA NTRS2002

Letting Thoughts Take Wing

Scientists are conducting research into electroencephalograms (EEGs) of brainwave activity, and electromyography (EMG) of muscle activity, in order to develop systems which can control an aircraft with only a pilot's thoughts. This article describes some EEG and EMG signals, and how they might be analyzed and interpreted to operate an aircraft. The development of a system to detect and interpret fine muscle movements is also profiled in the article.

Jorgensen, Chuck↗
NASA NTRS2001

Clustered DNA damages induced by high and low LET radiation, including heavy ions

Clustered DNA damages--here defined as two or more lesions (strand breaks, oxidized purines, oxidized pyrimidines or abasic sites) within a few helical turns--have been postulated as difficult to repair accurately, and thus highly significant biological lesions. Further, attempted repair of clusters may produce double strand breaks (DSBs). However, until recently, there was no way to measure ionizing radiation-induced clustered damages, except DSB. We recently described an approach for measuring classes of clustered damages (oxidized purine clusters, oxidized pyrimidine clusters, abasic clusters, along with DSB). We showed that ionizing radiation (gamma rays and Fe ions, 1 GeV/amu) does induce such clusters in genomic DNA in solution and in human cells. These studies also showed that each damage cluster results from one radiation hit (and its track), thus indicating that they can be induced by very low doses of radiation, i.e. two independent hits are not required for cluster induction. Further, among all complex damages, double strand breaks comprise--at most-- ~20%, with the other clustered damages being at least 80%.

NASA Discipline Radiation Health↗
NASA NTRS1996

Experimental models for cellular radiation targets: LET, RBE and radioprotectors

The topics of this presentation are: a brief review of the early research, the ideas it stimulated and the ways they are used in current efforts to explain cellular radiosensitivity; an analysis of the strengths and weaknesses of two experimental models used in vitro for simulating the induction of double strand breaks (DSB) and single strand breaks (SSB) in situ. Note that when alkali is used to denature cellular DNA for the determination of strand breaks, both overt SSB and the SSB that result from DSB in the denaturation process are recorded as total strand breaks (TSB). All information is taken from published literature.

NASA Discipline Number 93-10↗
NASA NTRS1992

Radiobiological studies with the nematode Caenorhabditis elegans. Genetic and developmental effects of high LET radiation

The biological effects of heavy charged particle (HZE) radiation are of particular interest to travellers and planners for long-duration space flights where exposure levels represent a potential health hazard. The unique feature of HZE radiation is the structured pattern of its energy deposition in targets. There are many consequences of this feature to biological endpoints when compared with effects of ionizing photons. Dose vs response and dose-rate kinetics may be modified, DNA and cellular repair systems may be altered in their abilities to cope with damage, and the qualitative features of damage may be unique for different ions. The nematode Caenorhabditis elegans is being used to address these and related questions associated with exposure to radiation. HZE-induced mutation, chromosome aberration, cell inactivation and altered organogenesis are discussed along with plans for radiobiological experiments in space.

short duration↗
NASA NTRS1986

Introduction: "Let there be (some) life"

The author introduces a meeting session about the origins of life by briefly reviewing theories of chemical and molecular evolution. The discussion focuses on structures of early life, sequences of chemical synthesis, and evolution of biomolecules.

Non-NASA Center↗
NASA NTRS1994

A model of solar energetic particles for use in calculating LET spectra developed from ONR-604 data

A model of solar energetic particles (SEP) has been developed and is applied to solar flares during the 1990/1991 CRRES mission using data measured by the University of Chicago instrument, ONR-604. The model includes the time-dependent behavior, heavy-ion content, energy spectrum and fluence, and can accurately represent the observed SEP events in the energy range between 40 to 500 MeV/nucleon. Results are presented for the March and June, 1991 flare periods.

unmanned↗
NASA NTRS2004

Let's Orbit Mars: A Proposal to Explore Mars Now

Mars is an exciting target for the human exploration; the next destination toward the ultimate human colonization of the solar system. But the price of proposed missions to Mars is a daunting barrier. Expensive missions make it a slow and difficult process to achieve the political consensus to make a commitment to exploration. In today's deficit-conscious era (and what era is not?), it is as difficult-- perhaps impossible-- task to justify to a skeptical and cost-conscious public the need to invest in exploration. It seems far too easy to postpone exploration into a future that never seems to arrive. It would be terrific to explore Mars in small steps, where each step makes progress toward human exploration and settlement, and each step also is not only exciting to the public, but also justifiable on its own scientific merits.

Landis, Geoffrey A.↗
NASA NTRS2006

Large Engine Technology (LET) Short Haul Civil Tiltrotor Contingency Power Materials Knowledge and Lifing Methodologies

This report documents the results of an experimental program conducted on two advanced metallic alloy systems (Rene' 142 directionally solidified alloy (DS) and Rene' N6 single crystal alloy) and the characterization of two distinct internal state variable inelastic constitutive models. The long term objective of the study was to develop a computational life prediction methodology that can integrate the obtained material data. A specialized test matrix for characterizing advanced unified viscoplastic models was specified and conducted. This matrix included strain controlled tensile tests with intermittent relaxtion test with 2 hr hold times, constant stress creep tests, stepped creep tests, mixed creep and plasticity tests, cyclic temperature creep tests and tests in which temperature overloads were present to simulate actual operation conditions for validation of the models. The selected internal state variable models where shown to be capable of representing the material behavior exhibited by the experimental results; however the program ended prior to final validation of the models.

Spring, Samuel D.↗
NASA NTRS1993

Hands-On Activities: Let's Get Real

This paper describes ways to make to hands-on activities more closely aligned with the science education guidelines of the National Science Standards and the California Science Framework.

N/A↗
NASA NTRS2007

High-LET Patterns of DSBs in DNA Loops, the HPRT Gene and Phosphorylation Foci

We present new results obtained with our model based on the track structure and chromatin geometry that predicts the DSB spatial and genomic distributions in a cell nucleus with the full genome represented. The model generates stochastic patterns of DSBs in the physical space of the nucleus filled with the realistic configuration of human chromosomes. The model was re-used to find the distribution of DSBs in a physical volume corresponding to a visible phosphorylation focus believed to be associated with a DSB. The data shows whether there must more than one DSB per foci due to finite size of the visible focus, even if a single DSB is radiochemically responsible for the phosphorylation of DNA in its vicinity. The same model can predict patterns of closely located DSBs in a given gene, or in a DNA loop, one of the large-scale chromatin structures. We demonstrated for the example of the HPRT gene, how different sorts of radiation lead to proximity effect in DSB locations, which is important for modeling gene deletions. The spectrum of intron deletions and total gene deletions was simulated for the HPRT gene. The same proximity effect of DSBs in a loop can hinder DSB restitutions, as parts of the loop between DSBs is deleted with a higher likelihood. The distributions of DSBs and deletions of DNA in a loop are presented.

Ponomarev, Artem L.↗
NASA NTRS2007

Summary of LET Requirements/Testing

The performance of the Michigan State SEETF during its inaugural runs is evaluated. Beam profiles and other diagnostics are presented, and prospects for future development and testing are evaluated.

Ladbury, Ray↗