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At least 451 records · Page 25

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NASA NTRS1976

An equipotential model for auroral arcs

Shaped charge barium release data and high-speed auroral image data show the likely existence of anomalously large (about 1 V/m referred to the 100-km level) electric fields at distances the order of one earth radius above the earth. A model of the electrostatic field above an auroral arc is proposed in which the magnetic field lines are not all equipotentials. Use is made of recent theoretical work on oblique current-driven electrostatic shocks to demonstrate the plausibility of nonequipotential field lines. The model is addressed to the structure of inverted-V electron spectra, current continuity between magnetosphere and ionosphere, and the results of recent shaped charge barium experiments. The model is used to identify specific gaps in our understanding of auroral phenomena.

Swift, D. W.↗

NASA NTRS1976

The inconsistency between proton charge exchange and the observed ring current decay

The equatorial pitch-angle distributions of ring-current ions observed during a storm recovery phase at L values between 3 and 4 are compared with the pitch-angle distributions predicted by proton charge exchange with neutral hydrogen. Large disagreements are found, and three alternative explanations are explored. (1) A strong proton source acts to mask the effects of charge exchange. It is believed that the required strong continual source with a unique pitch-angle and energy dependence is unrealistic at these low L values. (2) Presently accepted neutral hydrogen density models have densities well over an order of magnitude too large for a storm recovery phase. No evidence is known to support the required large errors in the densities. (3) The ring current at particle energies not exceeding 50 keV was dominated by some ion species other than protons during the storm recovery phase. Such ions must have much longer lifetimes for charge exchange with hydrogen than do protons. This alternative is strongly favored, with He(+) being an attractive candidate.

Lyons, L. R.↗

NASA NTRS1972

Ultraviolet absorption spectra of mercuric halides.

The gas phase transitions of the mercuric halides were observed in the UV region by operating at temperatures above 400 K and at vapor pressures on the order of 0.5 mm. Spectral features exhibited by the chloride, bromide, and iodide of mercury correlate energetically with bands previously designated as intermolecular charge transfer transitions. The solution spectra of mercuric iodide and deep color of the crystals (if not due to some solid state interactions) indicate that this molecule may also have longer wavelength transitions.

Templet, P.↗

NASA NTRS1974

Observation of a cosmic gamma-ray burst on Apollo 16. II - X-ray time profile and source location

A burst of X-rays was detected during the trans-earth coast phase of Apollo 16 on Apr. 27, 1972 at 10:68 UT, simultaneously with the observation of a transient event by a gamma-ray spectrometer aboard the same spacecraft. The two instruments provide a broad energy range of more than three orders of magnitude for describing the spectral distribution of this event. The conclusion that the incident flux was X-rays and not charged particles is based on the fact that the particle flux detectors in the Apollo gamma ray spectrometer and on the Vela 6A, which also observed the event, did not respond. The time variation of the total count rate in the X-ray range before and after corrections for detector geometry and the analysis for source direction is presented.

Trombka, J. I.↗

NASA NTRS1979

An investigation and analysis of the density and thermal balance of the Martian ionosphere

The major photochemistry consisted of solar EUV and photoelectrons comprising 70 percent and 30 percent respectively, of the initial source of CO2(+) and O(+). The energetic O2(+) provided a substantial source of energy to the ambient ions, distributing of the order of 1.6 x 10 to the -7 power W/sq m at an average of 160 km. This input can be compared to that from the ambient electrons of 1.3 x 10 to the -7 power W/sq m with average deposition at 145 km and from the calculated thermal conduction of 1 x 10 to the -9 power W/sq m at 270 km and 1 x 10 to the -8 power at 230 km for assumed dip angles of 2 deg and 12 deg respectively, for a 10nT magnetic field. At altitudes above 250 km upward, vertical fluxes of the order 6 x 10 to the 10th power/sq m/sec for the thermal ions were calculated. The net ionization of O(+) and CO2(+) by charge exchange with incoming solar wind protons varied from 5 x 10 to the 8th power to 5 x 10 to the 12th power /sq m/sec for assumed field strengths of 50nT to 2nT on the dayside of the planet.

Rohrbaugh, R. P.↗

NASA NTRS1978

Chemical releases

A Chemical Release Module to be carried into orbit by the shuttle is described. The module would release chemicals from orbiting satellites in order understand processes within the Earth's magnetosphere, atmosphere and ionosphere. A large number of potential experiments are identified, ranging from introducing traceable material into the solar wind in order to study its transport to and within the magnetosphere, to injecting material into the outer magnetosphere to simulate the precipitation of trapped charged particles, to the release of material at lower altitudes to create waves both mechanical and electromagnetic.

Heppner, J. P.↗

NASA NTRS1980

Trigger, an active release experiment that stimulated auroral particle precipitation and wave emissions

The experimental design by which a cesium vapor cloud was suddenly released in order to stimulate auroral particle precipitation is described along with the general results obtained. A drastic increase of the field-aligned charged-particle flux was observed with subsequent particle bursts. It is suggested that low-energy acceleration was due to parallel electric fields created by an instability which was driven by field-aligned currents resulting from the plasma injection. Pitch angle scattering in the deep magnetosphere may account for particle precipitation continuing for 130 sec after release.

Holmgren, G.↗

NASA NTRS1986

Characterization of hydrogen embrittlement in nickel base superalloy single crystals

In order to study the role of CMSX2 single crystal microstructure on the combined stress-hydrogen environment effects, hydrogen was introduced by cathodic charging. Concentration measurements were carried out to investigate the dependence of hydrogen solubility and trapping on microstructure. Mechanical properties were measured at room temperature on smooth tensile specimens as a function of heat treatment, crystal orientation and H charging conditions. SEM and TEM allow to study H induced cracks initiation and propagation. A large amount of hydrogen can be dissolved and trapped in CMSX2 single crystals when exposed to a high hydrogen fugacity environment. The strong H trapping evidenced in voids explains the predominant role of these defects as crack initiation sites. The strong detrimental effect of hydrogen on the material tenacity is discussed.

Chene, J.↗

NASA NTRS1990

Solvent-tuned intramolecular charge-recombination rates in a conjugated donor-acceptor molecule

The nonradiative charge-recombination rates from the charge-transfer state of a new conjugated donor-acceptor molecule (p-cyano-p-prime-methylthiodiphenylacetylene) can be tuned over almost an order of magnitude by varying the polarity of the solvent. These measurements of intramolecular recombination show a turnover of rates as a function of emission energy, consistent with the 'normal' and 'inverted' behavior of Marcus theory. Steady-state spectra and time-resolved measurements make it possible to quantitatively compare thermal and optical electron-transfer rates as a function of driving force and demonstrate their correspondence.

Khundkar, Lutfur R.↗

NASA NTRS2003

Synchrotron Characterization of Hydrogen and Ferric Iron in Martian Meteorites

The hydrogen budget of the Martian interior is distributed among several phases: melts, hydrous minerals, and nominally anhydrous minerals like olivine, pyroxene, and garnet. All these phases are vulnerable to loss of hydrogen during shock, excavation and transport via the mechanism of dehydrogenation, in which the charge on the H protons is left behind as polarons on Fe atoms. Thus, both H and F(3x) must be analyzed in order to reconstruct hydrogen and oxygen fugacities on Mars. To date, SIMS data have elucidated D/H and H contents of hydrous phases in SNC meteorites, but anhydrous martian minerals have not been systematically examined for trace hydrogen. Ferric iron has been quantified using XANES in many marital phases, but integrated studies of both Fe(3x) and H on the same spots are really needed to address the H budget. Here, we measure and profile H and Fe(3x) abundances in and across individual grains of glass and silicates in Martian meteorites. We use the new technology of synchrotron microFI'lR spectroscopy to measure the hydrogen contents of hydrous and nominally anhydrous minerals in martian meteorites on 30-100 microns thick, doubly polished thin sections on spots down to 3 x 3 microns. Synchrotron microXANES was used to analyze Fe(3x) on the same scale, and complementary SIMS D/H data will be collected where possible, though at a slightly larger scale. Development of this combination of techniques is critical because future sample return missions will generate only microscopic samples for study. Results have been used to quantitatively assess the distribution of hydrogen and ferric iron among phases in the martian interior, which will better constrain the geodynamic processes of the interior, as well as the overall hydrogen and water budgets on Mars.

Dyar, Melinda D.↗

NASA NTRS2020

Capabilities Development: From International Space Station and the Moon to Mars

The President of the United States, in signing Space Policy Directive-1, directed the NASA Administrator “to lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities. Beginning with missions beyond low-Earth orbit (LEO), the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations.” NASA is charged to land American astronauts on the lunar South Pole in 2024 and to continue a campaign of sustainable lunar surface exploration in order to develop necessary technologies and capabilities to enable initial human missions to Mars. NASA’s lunar surface exploration plans are part of a continuum of activities utilizing platforms in low Earth orbit (LEO), cislunar space, and the lunar surface to demonstrate advanced technologies, advance operations concepts, and develop countermeasures to lessen the impacts of the space environment and long duration exposure on the crew working in space. NASA is using a capability-driven approach to identify critical gaps to be addressed as part of a focused program to reduce risk for future deep space exploration missions building to eventual human missions to the surface of Mars. Teams of discipline experts from across NASA identify capability gaps between the current state of the art and the needs of proposed exploration missions and develop integrated strategies and roadmaps for filling those gaps. These inputs include assessment of platform needs for demonstration and testing of new capabilities. Generally, the International Space Station (ISS) and Gateway are needed for demonstration of capabilities for Mars transit, while Lunar surface activities focus on development of capabilities and operational protocols for Mars surface. This paper discusses the activities required to advance critical exploration capabilities, focusing on selection of demonstration and test location based upon the unique environments and characteristics of the ISS, Gateway, and potential lunar surface assets. The optimal strategy will be a combination of ISS/LEO, Gateway, and lunar surface testing; however, not all capabilities require a deep space exploration missions.

Boggs, Kathleen Gallagher↗

NASA NTRSDate not supplied

Ultrahigh Areal Capacity Holey Graphene Air Cathodes for Li-O2 and Li-CO2 Batteries

Advanced lithium (Li) batteries using gaseous cathode reactants such as oxygen (O2) and carbon dioxide (CO2)are attractive energy storage platforms because the gases are obtained externally and thus not accounted for in the total battery weight when fully charged.The discharge products at the cathode, typically Li2O2 for Li-O2 batteries and Li2CO3 for Li-CO2 batteries, are insoluble in the electrolyte.Therefore, in order for such batteries to function properly, an “air cathode”,which is a conductive scaffold within the battery cell,is required as a physical location for cathode electrochemical reactions to occur. Prior research has identified many carbon nanomaterials such as carbon nanotubes and graphene as viable choices for air cathode scaffold, while various metallic and metal-free catalytic systems integrated onto carbon-based air cathodes have been developed to improve the sluggish discharge and charge reactions. For future practical applications, the air cathode must exhibit a usable capacity per unit electrode area, or areal capacity, a critical parameter that has been largely overlooked so far in this field. In order to achieve high areal capacity, the air cathode must exhibit a sufficient amount of accessible void volume per unit electrode area while maintaining the conductive scaffold integrity during the entire electrochemical process. Here we present an ultrathick,holey graphene-based air cathode platform fabricated from a facile dry compression process that exhibits remarkable areal capacity values. Holey graphene is a carbon nanomaterial derived from graphene, but with nanometer sized holes through the nanosheet thickness.The presence of these holes enhances mass transport through electrode thickness and also enables the unique dry-press fabrication process that is not achievable using other carbon scaffold materials.The dry-pressed holey graphene air cathode platform is not only compatible with catalyst incorporation to improve battery reaction kinetics, but also allows for novel engineering of electrode architectures that is not achievable using conventional electrode fabrication approaches.The applications of such highly versatile, ultrahigh areal capacity air cathode platforms to both Li-O2 and Li-CO2 battery chemistries will be discussed.

Li-O2 batteries, Li-CO2 batteries, holey graphene,↗

NASA NTRS1999

On Europa's Magnetospheric Interaction: A MHD Simulation of the E4 Flyby

The global three-dimensional interaction of Europa with the Jovian magnetosphere is modeled by using a complete set of ideal magnetohydrodynamic (MHD) equations. The model accounts for exospheric mass loading, ion-neutral charge exchange, recombination, and a possible intrinsic dipole magnetic field of Europa. The single-fluid MHD equations are solved by using a modem, finite volume, higher-order, Godunov-type method on an adoptively refined unstructured grid, which allows detailed modeling of the region near Europa while still resolving both the upstream region and the satellite's wake. The magnetic field and plasma density measured during Galileo's EGA flyby of December 19, 1996, are reproduced reasonably well in the simulation. We find the agreement between the data and our model particularly convincing if we assume that the plasma velocity during the EGA flyby deviated from the nominal corotation direction by approximately 20 deg. Evidence from the Galileo energetic particle detector also supports this assumption. In this case, we can fit the data using a dipole with orientation close to that of an induced dipole arising from the interaction of a hypothetical conducting subsurface layer on Europa with the periodically changing magnetic field of Jupiter. However, the magnitude of the dipole in our model is somewhat smaller (70%) than that suggested by Khurana et al. The total mass loading and ion-neutral charge exchange rates are consistent with the estimates of Europa's atmosphere and ionosphere.

Kabin, K.↗

NASA NTRS1974

The interstellar wake of the solar wind

The present work examines theoretically the cooling of the subsonic solar wind by the interstellar hydrogen gas entering the solar system. It is assumed that the interstellar hydrogen gas is distributed uniformly in the space where the solar wind is subsonic, and the solar-wind wake is represented by an idealized column flow of plasma in which the flow velocity is constant and the temperature varies along, not across, the wake. The results are therefore to be taken only in their order of magnitude. In the hot region near the shock sphere the electrons are cooled mainly through electron ionization while the protons cool through charge exchange. In the cooler regions far away from the shock, the protons are still cooled by charge exchange and the electron is cooled by collisions with protons. As the temperature of the solar wind decreases, the magnetic field becomes relatively more important. When the spin axis of the sun is parallel to the direction of the incident interstellar gas flow, the lines of force are in spiral form and the tension causes the plasma to be concentrated along the center of the wake.

Yu, G.↗

NASA NTRS1981

On rotational forces in the solar wind

Solar rotational forces affecting the flow of minor ions in the solar wind are considered as corotating with the sun. Cold, noninteracting charged particles in the magnetic and gravitational fields of the sun rotate with the angular velocity of the sun, and calculations of lowest bulk order velocities show that differences in particle velocities decrease with increasing distance from the sun. A centrifugal potential in the corotating frame implies that ion motion is independent of protons, with velocities determined by the potential, which monotonically decreases without limit. The potential dominates the initial kinetic energy of the particles, and the equality of velocities within the potential is not due to interactions between particles as claimed by Mackenzie et al. (1979).

Hollweg, J. V.↗

NASA NTRS1988

Comparative study of cross-field and field-aligned electron beams in active experiments

Beam-plasma interactions associated with the cross-field and field-aligned injection of electron beams from spacecraft were investigated using a two-dimensional (three velocity component) electrostatic particle simulations. It is shown that the beam properties and plasma response can be characterized well by the ratio between the stagnation time and the plasma response time, which depends on the ratio of the ambient plasma density to the beam density, the beam width, the beam energy, and the spacecraft length. It was found that the beams injected across the field lines tend to lose their coherence after about one or two gyrations due to space-charge oscillations induced by the beam, irrespective of the spacecraft charging. These oscillations scatter the beam electrons into a hollow cylinder of a radius equal to a beam electron gyroradius and thickness of the order of two beam Debye lengths. Parallel injected beams are subjected to similar oscillations, which cause the beam to expand to fill a solid cylinder of a comparable thickness.

Winglee, R. M.↗

NASA NTRS1991

Observational evidence of aging processes in comets

Emphasis was on searching for systematic differences among two groups of comets: periodic comets which spend most of their time in the vicinity of the inner Solar System and the new comets which are believed to be passing through the inner Solar System for the first time. Such differences are expected, but have never been observed, in part because there has never been a systematic observational program aimed at addressing this question. Understanding possible physical and compositional differences between these two groups will lead to a better understanding of the cometary formation conditions in the early Solar System. The employed method studies the activity in the comets as a function of distance by obtaining charge coupled device (CCD) observations of the comets at frequent intervals on both the pre- and post-perihelion legs of their orbits in order to ascertain the distances at the onset and turn-off of activity through comparison with sublimation models.

Meech, Karen J.↗

NASA NTRS1992

Low-Isp derated ion thruster operation

The performance and lifetime expectations of 30 cm xenon ion thruster technology at low values of specific impulse were evaluated, with emphasis on 1000-2500 s operation. Power levels of up to 2.0 kW, appropriate for auxiliary and orbit maneuvering propulsion, were processed at thrust-to-power ratios up to 57 mN/kW. These tests were conducted using a derated 30 cm ion thruster with high-perveance design two-grid ion optics with xenon propellent. Lifetime projections were made based on a simple analysis of critical component erosion rates, and it was found that a strong correlation exists with the ratio of the specific impulse-to-input power. Under all operating conditions for which the projected thruster lifetime is less than 10,000 hrs, the life-limiting component of this technology is erosion of the accelerator grid due to charge-exchange ions. The use of alternative grid materials such as carbon is estimated to increase useful thruster lifetimes by as much as an order of magnitude and may enable long-life high thrust-density, sub-2500 s Isp operation. The performance and life of the derated thruster appears similar to that of the Russian SPT-100 thruster in the 1.0-2.0 kW, 1600-2000 s operational envelope.

Patterson, Michael J.↗