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All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

The dynamic tensile strength of ice and ice-silicate mixtures

The dynamic tensile strength of icy media is measured at strain rates on the order of 10,000/sec to aid in the understanding of impact and cratering phenomena. Compressed samples consisting of ice and ice-silicate mixtures with 5 and 30 wt % sand were impacted at temperatures between 230 and 250 K by projectile plexiglas plates imparting the required strain rates in less than 0.75 microsec. Taking the tensile stress corresponding to the transition from intact to spalled or fragmented samples as the dynamic tensile strength, strengths of 17, 20 and 22 MPa were obtained for the pure ice, 5 wt % sand, and 30 wt % sand specimens, respectively. The values lie considerably above those observed in static testing. A continuum fracturing model is used to obtain relations between tensile strength and stress rate as well as to derive stress and damage histories during tensile loading and the size distribution of icy fragments as a function of strain rate.

Lange, M. A.↗
NASA NTRS1995

Modeling of a possible conformational change associated with the catalytic mechanism in the hammerhead ribozyme

Here we describe a possible model of the cleavage mechanism in the hammerhead ribozyme. In this model, the 2' hydroxyl of C17 is moved into an appropriate orientation for an in-line attack on the G1.1 phosphate through a change in its sugar pucker from C3' endo to C2' endo. This conformational change in the active site is caused by a change in the uridine turn placing the N2 and N3 atoms of G5 of the conserved core in hydrogen bonding geometry with the N3 and N2 atoms on the conserved G16.2 residue. The observed conformational change in the uridine turn suggests an explanation for the conservation of G5. In the crystal structure of H.M. Pley et al., Nature 372, 68-74 (1994), G5 is situated 5.3A away from G16.2. However, the uridine turn is sufficiently flexible to allow this conformational change with relatively modest changes in the backbone torsion angles (average change of 14.2 degrees). Two magnesium ions were modeled into the active site with positions analogous to those described in the functionally similar Klenow fragment 3'-5' exonuclease (L.S. Beese and T.A. Steitz, EMBO J. 10, 25-33 (1991)), the Group I intron (T.A. Steitz and J.A. Steitz, P.N.A.S. U.S.A. 90, 6498-6502 (1993); R.F. Setlik et al., J. Biomol. Str. Dyn. 10, 945-972 (1993)) and other phosphotransferases. Comparison of this model with one in which the uridine turn conformation was not changed showed that although the changes in the C17 sugar pucker could be modeled, insufficient space existed for the magnesium ions in the active site.

Non-NASA Center↗
NASA NTRS1993

Modeling study on the cleavage step of the self-splicing reaction in group I introns

A three-dimensional model of the Tetrahymena thermophila group I intron is used to further explore the catalytic mechanism of the transphosphorylation reaction of the cleavage step. Based on the coordinates of the catalytic core model proposed by Michel and Westhof (Michel, F., Westhof, E. J. Mol. Biol. 216, 585-610 (1990)), we first converted their ligation step model into a model of the cleavage step by the substitution of several bases and the removal of helix P9. Next, an attempt to place a trigonal bipyramidal transition state model in the active site revealed that this modified model for the cleavage step could not accommodate the transition state due to insufficient space. A lowering of P1 helix relative to surrounding helices provided the additional space required. Simultaneously, it provided a better starting geometry to model the molecular contacts proposed by Pyle et al. (Pyle, A. M., Murphy, F. L., Cech, T. R. Nature 358, 123-128. (1992)), based on mutational studies involving the J8/7 segment. Two hydrated Mg2+ complexes were placed in the active site of the ribozyme model, using the crystal structure of the functionally similar Klenow fragment (Beese, L.S., Steitz, T.A. EMBO J. 10, 25-33 (1991)) as a guide. The presence of two metal ions in the active site of the intron differs from previous models, which incorporate one metal ion in the catalytic site to fulfill the postulated roles of Mg2+ in catalysis. The reaction profile is simulated based on a trigonal bipyramidal transition state, and the role of the hydrated Mg2+ complexes in catalysis is further explored using molecular orbital calculations.

NASA Discipline Exobiology↗
NASA NTRS1989

The fragmentation of 670A MeV neon-20 as a function of depth in water. I. Experiment

We present the final analysis of an experiment to study the interaction of a beam of 670A MeV neon ions incident on a water column set to different thicknesses. The atomic number Z (and, in some cases, the isotopic mass A) of primary beam particles and of the products of nuclear interactions emerging from the water column close to the central axis of the beam was obtained for nuclei between Be (Z = 4) and Ne (Z = 10) using a time-of-flight telescope to measure the velocity and a set of silicon detectors to measure the energy loss of each particle. The fluence of particles of a given charge was obtained and normalized to the incident beam intensity. Corrections were made for accidental coincidences between multiple particles triggering the TOF telescope and for interactions in the detector. The background due to beam particles interacting in beam line elements upstream of the detector was calculated. Sources of experimental artifacts and background in particle identification experiments designed to characterize heavy ion beams for radiobiological research are summarized, and some of the difficulties inherent in this work are discussed. Complete tables of absolutely normalized fluence spectra as a function of LET are included for reference purposes.

NASA Discipline Radiation Health↗
DOE OSTI2025

Global Simulations of Gravitational Instability in Protostellar Disks with Full Radiation Transport. I. Stochastic Fragmentation with Optical-depth-dependent Rate and Universal Fragment Mass

Fragmentation in a gravitationally unstable accretion disk can be an important pathway for forming stellar/planetary companions. To characterize quantitatively the condition for and outcome of fragmentation under realistic thermodynamics, we perform global 3D simulations of gravitationally unstable disks at various cooling rates and cooling types, including the first global simulations of gravitational instability that employ full radiation transport. We find that fragmentation is a stochastic process, with the fragment generation rate per disk area p frag showing an exponential dependence on the parameter β ≡ Ω K t cool , where Ω K is the Keplerian rotation frequency, and t cool is the average cooling timescale. Compared to a prescribed constant β, radiative cooling in the optically thin/thick regime makes p frag decrease slower/faster in β; the critical β corresponding to ∼1 fragment per orbit is ≈3, 5, and 2 for constant β, optically thin, and optically thick cooling, respectively. The distribution function of the initial fragment mass is remarkably insensitive to disk thermodynamics. Regardless of cooling rate and optical depth, the typical initial fragment mass is m frag ≈ 40M tot h 3 , with M tot being the total (star+disk) mass and h = H/R being the disk aspect ratio. Applying this result to typical Class 0/I protostellar disks, we find m frag ∼ 20M J , suggesting that fragmentation more likely forms brown dwarfs. Given the finite width of the m frag distribution, forming massive planets is also possible.

gravitational instability↗
NASA NTRS2013

Observations of Titan 3C-4 Transtage Fragmentation Debris

The fragmentation of a Titan 3C-4 Transtage (1968-081) on 21 February 1992 is one of only two known break-ups in or near geosynchronous orbit. The original rocket body and 24 pieces of debris are currently being tracked by the US Space Surveillance Network (SSN). The rocket body (SSN# 3432) and several of the original fragments (SSN# 25000, 25001, 30000, and 33511) were observed in survey mode during 2004-2010 using the 0.6-m Michigan Orbital DEbris Survey Telescope (MODEST) in Chile using a broad R filter. This paper will present a size distribution for all calibrated magnitude data acquired on MODEST. Size distribution plots will also be shown using historical models for small fragmentation debris (down to 10 cm) believed to be associated with the Titan break-up. In November 2010, visible broadband photometry (Johnson/Kron-Cousins BVRI) was acquired with the 0.9-m Small and Moderate Aperture Research Telescope System (SMARTS) at the Cerro Tololo Inter-American Observatory (CTIO) in Chile on several Titan fragments (SSN# 25001, 33509, 33510) and the parent rocket body. Color index data will be used to determine the fragment brightness distribution and how the data compares to spacecraft materials measured in the laboratory using similar photometric measurement techniques. In 2012, the SSN added 16 additional fragments to the catalogue. MODEST acquired magnitude data on ten Titan fragments in late 2012 and early 2013. The magnitude distribution of all the observed fragments are analyzed as a function of time. In order to better characterize the breakup fragments spectral measurements were acquired on the original rocket body and five Titan fragments using the 6.5-m Magellan telescopes at Las Campanas Observatory in Chile. The telescopic spectra are compared with laboratory acquired spectra of materials (e.g., Aluminum and various paints) and categorized based on known absorption features for spacecraft materials.

Cowardin, Heather↗
NASA NTRS2019

Experimentally-Derived Phase Function Approximations in Support of the Orbital Debris Program Office

The NASA Orbital Debris Program Office (ODPO) has used various optical assets to acquire photometric data of Earth-orbiting objects to define the orbital debris environment. To better characterize and model optical data acquired from ground-based telescopes, the Optical Measurements Center (OMC) at NASA Johnson Space Center emulates illumination conditions seen in space by using equipment and techniques that parallel telescopic observations and source-target-sensor orientations. One of the OMC goals is to improve the size calculation used for optical data by developing an optical-based Size Estimation Model. The current size estimation requires applying a Lambertian phase function, a set albedo value, and range to the observed magnitude. The first step to improving the sampled brightness of laboratory targets is to remove aspect-angle dependencies. Then, the volume of possible object viewing angles is sampled at 21 combinations of azimuth and elevation angles for each solar phase angle. Finally, the acquired images are input into an image processing program that generates approximations for the object’s Bidirectional Reflectance Distribution Function (BRDF) and phase function. The BRDF is a radiometric concept that identifies an object’s material composition by matching a BRDF approximated with photometric data collected by ground-based telescopes with a BRDF generated experimentally from a known object in the laboratory. This paper presents the initial BRDF and phase function approximations for various fragments/targets acquired in the OMC and how the findings will be incorporated into ODPO models. A Lambertian sphere is used as a baseline for initial size estimation calculations and phase function comparisons. Spacecraft materials and fragments from hypervelocity laboratory impact tests are also presented to compare against the current assumed Lambertian phase function used for size estimates. This paper presents the preliminary phase function analysis and plan forward to utilize a laboratory-based phase function to improve the current optical size estimates using BRDF measurements for a large volume of targets composed of various shapes, sizes, and materials.

Hostetler, J.↗
NASA NTRS1992

Hypervelocity impact effects on solar cells

One of the space hazards of concern is the problem of natural matter and space debris impacting spacecraft. In addition to mechanical damage, impact velocities greater than 5 km/sec can produce shock induced ionization effects with resultant surface charging and complex chemical interactions. The upper limit of the velocity distribution for these particles is on the order of 70 km/sec. The second source of particulate matter is due to the presence of man and the machinery needed to place satellites in orbit. This 'man made' component of the space debris consists of waste, rocket exhaust, and debris caused by satellite break-up. Most of the particles are small. However as the size increases, debris purposefully thrown overboard such as garbage and human waste, combined with paint chips, plastic, wire fragments, bolts, etc., become formidable hazards which completely dominate the distribution function for some orbits. These larger fragments can produce penetration and spalling of the thick metallic structures associated with spacecraft. The particles most often encountered are aluminum oxide, associated with fuel residue, and paint chips. These debris types can have a wide range of particle sizes. It has been stated that the design of spacecraft will have to take the debris evolution into account and provide additional suitable armor for key components in the near future. The purpose of this work was to subject samples from solar power arrays, one of the key components of any spacecraft, to a debris flux typical of what might be found in space, and measure the degradation of the power panels after impact.

Rose, M. Frank↗
DOE OSTI2025

Fragmentation dynamics through geometrical distortion in low-energy electron attachment to carbon disulfide

The dissociation dynamics of dissociative electron attachment to carbon disulfide is studied using the velocity map imaging technique. Ion yields of the S− and CS− fragments are measured as a function of incident electron energy between 2 and 12 eV. Energy and angle-differential yields of the S− fragments are analyzed to investigate the dynamics of the CS2− anion around the 3.8-eV peak. The resonances involved in this fragmentation are identified and characterized in calculations using the well-known R-matrix method. Based on our measurements and the geometry dependence of the calculated resonances, we propose that a geometrical distortion of the transient negative-ion state of CS2 is necessary for S-CS bond dissociation to occur.

Paul, Anirban↗
DOE OSTI2025

Low-energy electron driven reactions in 2-bromo-5-nitrothiazole

Thiazole derivatives are biologically relevant molecules, used also in pharmaceutical applications. Herein, we report results for electron attachment to 2-bromo-5-nitrothiazole (BNT) in the gas phase. Employing two crossed electron–molecule beam experiments, we determined the efficiency curves of various fragment anions as a function of the initial electron energy between about 0 and 10 eV as well as the emission angle and kinetic energy distributions of Br− and NO2− ions formed from a resonance near 4 eV. The experiments were supported by quantum chemical calculations, exploring possible dissociation pathways along with their reaction energies. We also compared the electron attachment characteristics of BNT with those of the native thiazole molecule by performing electron attachment experiments and calculations for this molecule as well. Compared to thiazole, which is primarily degraded only by electrons with kinetic energies between about 5 and 10 eV, BNT is susceptible to low-energy electrons near 0 eV with enhanced cross section for (dissociative) electron attachment. However, although BNT offers two localization sites with high electron affinity (Br and NO2 moieties), we do not find the corresponding anions as the dominant negatively charged species formed upon electron attachment. Instead, the reaction channels with an abstraction of Br and NO2 as neutral radicals prevail, accompanied by the opening of the thiazole ring due to the relatively weak C–S bond.

Chen, Jiakuan (ORCID:0009000729608641)↗
NASA NTRS1976

The electron Boltzmann equation in a plasma generated by fission fragments

A Boltzmann equation formulation is presented for the determination of the electron distribution function in a plasma generated by fission fragments. The formulation takes into consideration ambipolar diffusion, elastic and inelastic collisions, recombination and ionization, and allows for the fact that the primary electrons are not monoenergetic. Calculations for He in a tube coated with fissionable material show that, over a wide pressure and neutron flux range, the distribution function is non-Maxwellian, but the electrons are essentially thermal. Moreover, about a third of the energy of the primary electrons is transferred into the inelastic levels of He. This fraction of energy transfer is almost independent of pressure and neutron flux but increases sharply in the presence of a sustainer electric field.

Hassan, H. A.↗
NASA NTRS1975

Fast metastable fragments produced by dissociative excitation of carbonyl sulfide

Dissociative excitation of OCS by electron impact has been studied using the method of translational spectroscopy. Time-of-flight distributions and excitation functions of the fast metastable fragments have been measured. The results are compared with similar measurements on CO2 and show that a variety of metastable fragments including CO(a 3Pi), S(5S), O(5S) as well as long-lived high-lying atomic and molecular Rydberg fragments can contribute to dissociation.

Van Brunt, R. J.↗
DOE OSTI2025

Bridging the gap between molecules and materials in quantum chemistry with localized active spaces

The number of materials that “bridge the gap” between single molecules and extended solids, such as metal-organic frameworks and organic semiconductors, has been increasing. Consequently, there is a growing need for modeling approaches that effectively integrate the real-space molecular perspective employed by computational chemists and the reciprocal-space dispersive perspective employed by computational physicists. Here, we propose the localized active space (LAS) approach as a promising method to successfully bridge this gap. The LAS approach extends the active space concept from multiconfigurational methods such as complete active space self-consistent field theory to multiple molecular fragments via a product-form wave function ansatz. Here, we apply this method to solid state phenomena by treating each unit cell as a fragment with different sets of local quantum numbers (e.g., charge and excitation number). State interaction between these LAS states (LASSI) thus provides a comprehensive basis for the study of charge and energy transfer, meeting and surpassing the capabilities of single-reference fragmentation approaches such as constrained density functional theory (cDFT). Most centrally, we show how combining this LASSI approach with multiconfigurational pair-density functional theory (MC-PDFT) provides an elegant and efficient method to compute band structures that capture multiconfigurational character. We apply the LASSI band structure approach to the computation of band gaps in stretched hydrogen chain, polyacetylene, and bulk nickel oxide (NiO), finding good or excellent quantitative agreement with reference values in all cases. Additionally, we use the LAS basis in one-dimensional model systems to demonstrate its ability to treat difficult solid-state phenomena such as exciton transfer and excitation at p-n junctions.

method development↗
NASA NTRS1992

Effects of fragmentation parameter variations on estimates of galactic cosmic ray exposure: Dose sensitivity studies for aluminum shields

Initial studies of the sensitivities of estimates of particle fluence, absorbed dose, and dose equivalent to fragmentation parameter variations are undertaken by using the LaRC galactic cosmic ray transport code (HZETRN). The new results, presented as a function of aluminum shield thickness, include upper and lower bounds on dose/dose equivalent corresponding to the physically realistic extremes of the fragmentation process and the percentage of variation of the dose/dose equivalent as a function of fragmentation parameter variation.

Townsend, Lawrence W.↗
NASA NTRS1979

A statistical model for the initial stellar mass function

The nonlinear stages of the fragmentation of a collapsing molecular cloud are modeled by coagulation theory. Several distinct physical processes are discussed, including protostellar fragment coalescence, gas accretion, and binary formation. This work confirms and extends the earlier result of Nakano (1966) that an approximately self-similar limiting form of the mass spectrum develops after several mean initial collision times. An approximate solution to the velocity-averaged coagulation equation is given for an arbitrary power-law dependence of the coalescence rate on mass, with dimensional dependence proportional to m to the power lambda; i.e., the asymptotic mass spectrum varies as m to the -3 lambda/2 power at small masses and cuts off exponentially at large masses, the characteristic mass depending on the number of collision times elapsed. Simple physical arguments suggest that lambda may increase with increasing mass, but is restricted to the range from 2/3 to 4/3. A large fraction of collisions could result in binary formation.

Silk, J.↗
DOE OSTI2023

Enhanced Optical Contrast and Switching in Near‐Infrared Electrochromic Devices by Optimizing Conjugated Polymer Oligo(Ethylene Glycol) Sidechain Content and Gel Electrolyte Composition

Abstract A detailed investigation addressing the effects of functionalizing conjugated polymers with oligo(ethylene glycol) (EG n ) sidechains on the performance and polymer‐electrolyte compatibility of electrochromic devices (ECDs) is reported. The electrochemistry for a series of donor‐acceptor copolymers having near‐infrared (NIR)‐optical absorption, where the donor fragment is 3,4‐ethylenedioxythiophene (EDOT) or an EG n functionalized bithiophene (g2T) and the acceptor fragment is diketopyrrolopyrrole (DPP) functionalized with branched alkyl or EG n sidechains, is extensively probed. ECDs are next fabricated and it is found that EG n sidechain incorporation must be finely balanced to promote polymer‐electrolyte compatibility and provide efficient ion exchange. Proper electrolyte‐cation pairing and polymer structural tuning affords a 2x increase in optical contrast (from 12% to 24%) and >60x reduction in switching time (from 20 to 0.3 s). Atomic force microscopy (AFM)/grazing incidence wide‐angle X‐ray scattering (GIWAXS) characterization of the polymer film morphology/microstructure reveals that an over‐abundance of EG n sidechains generates large polymer crystallites, which can suppress ion exchange. Lastly, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) indicates sidechain/electrolyte identity does not influence the electrolyte penetration depth into the films, and EG n sidechain inclusion increases electrolyte cation uptake. The material structural design insight and guidelines regarding the polymer‐electrolyte ion insertion/expulsion dynamics reported here should be of significant utility for developing next‐generation mixed ionic‐electronic conducting materials.

Chemistry↗
NASA NTRS1972

Evacuation areas for transportation accidents involving propellant tank pressure bursts

Evacuation areas are defined for those transportation accidents where volatile chemical propellant tanks are exposed to fire in the wreckage and eventually explode with consequent risks from fragments in surrounding populated areas. An evacuation area with a minimum radius of 600 m (2000 ft) is recommended to limit the statistical probability of fatality to one in 100 such accidents. The result was made possible by the derivation of a distribution function of distances reached by fragments from bursting chemical car tanks. Data concerning fragments was obtained from reports or tank car pressure bursts between 1958 and 1971.

Siewert, R. D.↗