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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.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Comments on the Rotational State and Non-Gravitational Forces of Comet 46/WIRTANEN

We apply our experience of modeling the rotational state and non-gravitational forces of comet 1 P/Halley and other comets to comet 46P/Wirtanen. While the paucity of physical data on 46P/Wirtanen makes this process somewhat speculative, this comet's place as target for the important Rosetta mission gives significance to such a study. Our arguments are based on the summary of observational data provided by Jorda and Rickman (1995) and a comparative study of the behavior of other periodic comets. We find 46P/Wirtanen to have a level of surface activity relative to its mass that is dynamically more akin to that found in comet 1 P/Halley than in a typical periodic comet. We show through an illustrative numerical example that this apparent fact should likely lead to an excited spin state for this comet and that significant changes in the spin period could occur in a single pass through perihelion. We argue that the available observations are not sufficient to substantiate the claim of Jorda and Rickman (1995) that the nucleus is undergoing retrograde rotation and it is possible that the rotation is either prograde as well as retrograde. The substantial requirements that must be placed on any future observing program necessary to determine the precise rotational state are outlined. We advocate an extended (approx. two month) southern hemisphere observing campaign to determine the nuclear rotational state in 1996 if possible before activity turns on.

Samarasinha, Nalin H.↗
NASA NTRS1991

Geomagnetic activity associated with earth passage of interplanetary shock disturbances and coronal mass ejections

Coronal mass ejection events (CMEs) are important occasional sources of plasma and magnetic field in the solar wind at 1 AU, accounting for approximately 10 percent of all solar wind measurements in the ecliptic plane during the last solar activity maximum. Using a recently appreciated capability for distinguishing CMEs in solar wind data in the form of counterstreaming solar wind electron events, this paper explores the overall effectiveness of shock wave disturbances and CMFs in general in stimulating geomagnetic activity. The study is confined to the interval from mid-August 1978 through mid-October 1982, spanning the last solar activity maximum, when ISEE 3 was in orbit about the L1 Lagrange point 220 Re upstream from earth. It is found that all but one of the 37 largest geomagnetic storms in that era were associated with earth passage of CMEs and/or shock disturbances, with the large majority of these storms (27 out of 37) being associated with interplanetary events where earth encountered both a shock and the CME driving the shock (shock/CME events). Although CMEs and/or shock disturbances were increasingly the cause of geomagnetic activity as the level of geomagnetic activity increased, many smaller geomagnetic disturbances were unrelated to these events.

Gosling, J. T.↗
NASA NTRS2018

Statistical Analysis of Torus and Kink Instabilities in Solar Eruptions

A recent laboratory experiment of ideal magnetohydrodynamic instabilities revealed four distinct eruption regimes readily distinguished by the torus instability (TI) and helical kink instability (KI) parameters. To establish its observational counterpart, we collected 38 solar flares (stronger than GOES-class M5 in general) that took place within 45° of disk center during 2011–2017, 26 of which are associated with a halo or partial halo coronal mass ejection (CME; i.e., ejective events), while the others are CME-less (i.e., confined events). This is a complete sample of solar events satisfying our selection criteria detailed in the paper. For each event, we calculate a decay index n of the potential strapping field above the magnetic flux rope (MFR) in and around the flaring magnetic polarity inversion line (a TI parameter) and the unsigned twist number T(sub w) of the nonlinear force-free field lines forming the same MFR (a KI parameter). We then construct an n–T(sub w) diagram to investigate how the eruptiveness depends on these parameters. We find that (1) T(sub w) appears to play little role in discriminating between confined and ejective events; (2) the events with n ≳ 0.8 are all ejective, and all confined events have n ≲ 0.8. However, n ≳ 0.8 is not a necessary condition for eruption because some events with n ≲ 0.8 also erupted. In addition, we investigate the MFR’s geometrical parameters, apex height, and distance between footpoints, as a possible factors for the eruptiveness. We briefly discuss the difference of the present result for solar eruptions with that of the laboratory result in terms of the role played by magnetic reconnection.

Solar activity↗
NASA NTRS1991

Probing the core of Cepheus A - Millimeter and submillimeter observations

Moderate and high angular resolution (40-7 arcsec) maps are presented of the core of the Cepheus A star-forming region using CS J = 3-2 and J = 7-6 emission, which traces the dense gas component of the cloud core, and using far-infrared (450 and 800 microns) continuum emission tracing the warm dust component. Three regimes in the core are traced by these observations: (1) a small (about 0.14 pc), nearly circular central core of high density (1-10 x 10 to the 6th/cu cm) and temperature (30-100 K) containing at least 25 percent of the mass and which contains the active early-type star formation; (2) an extended (0.5 x 0.25 pc), NE-SW oriented core of mass 200-300 solar masses, temperature 30-40 K, and average density nH2 of about 10 to the 5th/cu cm and which, together with the central core, contains 60-80 percent of the total core mass; and (3) an extended core envelope of dimensions 0.5 x 0.85 pc oriented primarily north-south, and a lower density. The velocity structure of the core suggests that it is being disrupted by the high-velocity winds driving the molecular outflow and is not due to a rotating circumstellar disk.

Moriarty-Schieven, G. H.↗
NASA NTRS1974

Trace geochemistry of lunar material

The lunar samples from the Apollo 16 and 17 flights which were analyzed include soil, igneous rock, anorthositic gabbro, orange soil, subfloor basalt, and norite breccia. Up to 57 elements including majors, minors, rare earths and other trace elements were determined in the lunar samples. The analytical techniques used were spark source mass spectrometry and neutron activation analysis. The latter was done either instrumentally or with group radiochemical separations. The differences in abundances of the elements in lunar soils at the various sites are discussed. With regard to the major elements only Si is about the same at all the sites. A detailed analysis which was performed on a sample of the Allende meteorite is summarized.

Morrison, G. H.↗
NASA NTRS1984

Photovoltaic concentrator pointing dynamics and plasma interaction study

The objectives of this experiment are to use the Space Technology Experiments Platform (STEP) system to demonstrate the viability of concentrator photovoltaic arrays by: (1) configuring a deployable mast on the STEP pallet with concentrator mass models and some active photovoltaic modules; (2) measuring the array pointing dynamics under normal rotation as well as disturbance conditions; (3) performing an array plasma interaction experiment to determine the steady-state plasma losses under various voltage conditions; and (4) providing active distributed control of the support truss to determine the improvement in dynamic response. Experiment approach and test control and instrumentation are described.

Stern, T. G.↗
NASA NTRS1984

Oxygen diffusion in alpha-Al2O3

Oxygen self diffusion coefficients were determined in single crystal alpha-Al2O3 using the gas exchange technique. The samples were semi-infinite slabs cut from five different boules with varying background impurities. The diffusion direction was parallel to the c-axis. The tracer profiles were determined by two techniques, single spectrum proton activation and secondary ion mass spectrometry. The SIMS proved to be a more useful tool. The determined diffusion coefficients, which were insensitive to impurity levels and oxygen partial pressure, could be described by D = .00151 exp (-572kJ/RT) sq m/s. The insensitivities are discussed in terms of point defect clustering. Two independent models are consistent with the findings, the first considers the clusters as immobile point defect traps which buffer changes in the defect chemistry. The second considers clusters to be mobile and oxygen diffusion to be intrinsic behavior, the mechanism for oxygen transport involving neutral clusters of Schottky quintuplets.

Cawley, J. D.↗
NASA NTRS1986

Exposure history of shergottites

The cosmogenic nuclides Cl-36, Al-26, Be-10, and Mn-53 were analyzed in Shergotty, ALHA 77005, and EETA 79001 shergottites by means of accelerator mass spectrometry and neutron activation. The cosmogenic radionuclide data were combined with noble gas data and cosmic ray track data to obtain the exposure ages, terrestrial ages, preatmospheric radii, and ablation depths for the three shergottites. The results indicate that none of the three meteorites was irradiated measurably by cosmic rays on its parent body, and that all three objects were ejected from greater than a 3-m depth in their parent bodies. The EETA 79001 meteorite was ejected in an event distinct from that of ALHA 77005 and Shergotty. All three shergottites show a very small amount of ablation, suggesting low velocities on entry into the earth's atmosphere.

Nishizumi, K.↗
NASA NTRS1989

Optimal terminal maneuver for a cooperative impulsive rendezvous

An optimal terminal maneuver is presently defined for the cooperative impulsive rendezvous of two spacecraft, in which each vehicle is capable of furnishing all or a part of the velocity change required for the rendezvous. In this maneuver, the final masses of the two vehicles are maximized in a fashion that is equivalent to minimum total propellant consumption. If neither propellant mass fraction constraint is active, one vehicle will supply all of the required velocity change.

Prussing, John E.↗
NASA NTRS1988

A review of the R Aquarii system

Results of IR, radio, optical, UV, and X-ray observations of the D-type symbiotic R Aquarii system, obtained since early observations by Lampland in the 1920s, are described. These data are discussed in context with models which were proposed to explain the appearance of the brilliant jet. It is proposed that R Aqr might constitute a prototype of directed mass loss or jet activity in symbiotic systems.

Michalitsianos, A. G.↗
NASA NTRS1992

Proposed Ka-band array-fed near field Cassegrain system for NASA's Polar Probe Mission

A Ka-band array-fed near-field Cassegrain antenna envisioned for NASA's Solar Probe Mission is the basis of the present spacecraft telecommunications system, which is designed for operation at X-band up/downlink and Ka-band downlink. The hexagonal array employed, which is located at the Cassegrain point of the 0.75-m reflector, encompasses 37 active elements; each of these contains 1/4-W HEMT amplifiers. Relative to a TWTA-based system, this active array feed reduces mass, increases dc power efficiency, enhanced reliability, and reduces volume requirements.

Kellogg, K.↗
NASA NTRS1992

Demonstration of active vibration control on a stirling-cycle cryocooler testbed

SatCon Technology Corporation has demonstrated excellent vibration reduction performance using active control on the JPL Stirling-cycle cryocooler testbed. The authors address the use of classical narrowband feedback control to meet the cryocooler vibration specifications using one cryocooler in a self-cancellation configuration. Similar vibration reduction performance was obtained using a cryocooler back-to-back configuration by actively controlling a reaction mass actuator that was used to mimic the second cooler.

Johnson, Bruce G.↗
NASA NTRS1994

A deep atmospheric eclipse of AL Velorum

I have obtained new International Ultraviolet Explorer (IUE) observations in 1992 to search for an accretion column in and to analyze an atmospheric eclipse of AL Vel, a low luminosity zeta Aurigae binary. Rich shell spectra recorded at three phases of atmospheric eclipse give column densities and excitation temperatures for the chromospheric gas. The chromospheric density falls off exponentially with a scale height of 3 solar radius, an order of magnitude greater than expected for support by thermal motions. Excitation temperatures at 2 - 5 scale heights into the chromosphere are approximately 7000 K, with Doppler widths near 17 km/s. Absorption features that could be formed in an accretion column were detected at phases 0.70 and 0.75 in a series of spectra spanning the range 0.63-0.80 in orbital phase. They imply a very slow acceleration of the wind, consistent with results for other zeta Aur binaries, but the wind speed implied is inconsistent with profiles of wind lines. The terminal velocity of this wind, greater than or approximately = 360 km/s, is higher than that in any of the other zeta Aur binaries. Wind and chromospheric features in 1992 were much more prominent than in 1988, which implies that both the wind and chromosphere were much more massive: we seem to have witnessed in 1992 a burst of activity with a mass-loss rate greater than 10(exp -8) solar mass/yr.

Eaton, Joel A.↗
NASA NTRS1997

X-Ray Flare Characteristics in the B2e Star Lambda Eridani (ROSAT)

We document the results of a simultaneous wavelength monitoring on the B2e star (lambda) Eri. This campaign was carried out from ground stations and with the ROSAT, ASCA, IUE, and Voyager 2 space platforms during a week in February-March 1995; a smaller follow-up was conducted in September 1995. During the first of these intervals (lambda) Eri exhibited extraordinary wind and disk-ejection activity. The ROSAT/HRI X-ray light curves showed no large flares such as the one the ROSAT/PSCA observed in 1991. However, possible low level fluctuations in the February-March ROSAT data occurred at the same time as unusual activity in H(alpha) He I (lambda)6678, He II (lambda)1640, and the C IV doublet. For example, the hydrogen and helium lines exhibited an emission in the blue half of their profiles, probably lasting several hours. The C IV lines showed a strong high-velocity Discrete Absorption Component (DAC) accompanied by unusually strong absorption at lower velocities. The helium line activity suggests that a mass ejection occurred at the base of the wind while the strong C III (Voyager) and C IV (IUE) lines implies that shock interactions occurred in the wind flow. It is not clear that the X-ray elevations are directly related to the strong C IV absorptions because the former changed on a much more rapid timescale than absorptions in the C IV lines. Within hours of the mild X-ray flux variations found by ROSAT on February 28, the Voyager UVS observed a "ringing" that decayed over three 3-hr. cycles. The amplitude of these fluctuations was strong (50%) at (lambda)(lambda)950-1100, decreased rapidly with wavelength, and faded to nondetection longward of (lambda)1300. Various considerations indicate that these continuum variations were not due to an instrumental pathology in the UVS. Rather, they appear to be due to a time-dependent flux deficit in the (lambda)(lambda)950-1250 region. We outline a scenario in which a dense plasma structure over the star's surface is heated and cooled quasi-periodically to produce such flux changes. Observations of new examples of this phenomenon are badly needed. Amateur astronomers can make a significant contribution to its understanding by searching for ringing in light curves of Be stars during their outburst phases. Finally we draw attention to an increase in the emission of the H(alpha) line that occurred at about the time the FUV ringing started. This increased emission hints that approximately 50,000K plasma near the star's surface can influence the circumstellar disc at approximately 12R. by its increased Lyman continuum flux.

Smith, Myron A.↗
NASA NTRS1999

The TRMM Precipitation Radar: Opportunities and Challenges

Although studies on the feasibility of spaceborne weather radar date back to the 1960's, it was only with the launch of the Tropical Rainfall Measuring Mission (TRMM) Satellite in November 1997 that the first weather radar was placed into low earth orbit. The long delay between the initial concept and implementation was caused not only by the demanding requirements of active sensors such as mass, power, and reliability, but because of scientific and technological challenges. For example, the demand for adequate spatial resolution arises from the need to resolve the horizontal structure of convective storm cells and to avoid surface contamination of the rain return at off-nadir angles. To achieve a horizontal resolution on the order of 4 km from low earth orbit with a modest antenna size of 2 m requires the use of a much higher frequency (Ku-band) than those typically used for ground-based weather radars (S- and C-band). Higher frequencies are subject to higher attenuation. As Hitschfeld and Bordan (1954) showed in their classic paper, attenuation correction with a single-wavelength radar is inherently unstable at high attenuations unless the drop size distribution and the radar constant are known precisely. Since these conditions are seldom met, much work over the last decade has been devoted to formulating and testing alternative methods of attenuation correction. The operational method used in the TRMM radar processing is discussed in section 3 of the paper.

Meneghini, R.↗