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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Conventional and Bimodal Nuclear Thermal Rocket (NTR) Artificial Gravity Mars Transfer Vehicle Concepts

A variety of countermeasures have been developed to address the debilitating physiological effects of zero-gravity (0-g) experienced by cosmonauts and astronauts during their approximately 0.5 to 1.2 year long stays in low Earth orbit (LEO). Longer interplanetary flights, combined with possible prolonged stays in Mars orbit, could subject crewmembers to up to approximately 2.5 years of weightlessness. In view of known and recently diagnosed problems associated with 0-g, an artificial gravity (AG) spacecraft offers many advantages and may indeed be an enabling technology for human flights to Mars. A number of important human factors must be taken into account in selecting the rotation radius, rotation rate, and orientation of the habitation module or modules. These factors include the gravity gradient effect, radial and tangential Coriolis forces, along with cross-coupled acceleration effects. Artificial gravity Mars transfer vehicle (MTV) concepts are presented that utilize both conventional NTR, as well as, enhanced bimodal nuclear thermal rocket (BNTR) propulsion. The NTR is a proven technology that generates high thrust and has a specific impulse (Isp) capability of approximately 900 s-twice that of today's best chemical rockets. The AG/MTV concepts using conventional Nuclear Thermal Propulsion (NTP) carry twin cylindrical International Space Station (ISS)- type habitation modules with their long axes oriented either perpendicular or parallel to the longitudinal spin axis of the MTV and utilize photovoltaic arrays (PVAs) for spacecraft power. The twin habitat modules are connected to a central operations hub located at the front of the MTV via two pressurized tunnels that provide the rotation radius for the habitat modules. For the BNTR AG/MTV option, each engine has its own closed secondary helium(He)-xenon (Xe) gas loop and Brayton Rotating Unit (BRU) that can generate 10s of kilowatts (kWe) of spacecraft electrical power during the mission coast phase eliminating the need for large PVAs. A single inflatable TransHab-type habitation module is also used with multiple vertical floors oriented radial to the MTV spin axis. The BNTR MTV's geometry-long and linear-is naturally compatible with AG operation. By rotating the vehicle about its center-of-mass (CM) and perpendicular to its flight vector at approximately 3.0 to 5.2 rpm, a centrifugal force and AG environment corresponding to approximately 0.38 to 1.0 g can be established to help maintain crew fitness out to Mars and back. Vehicles using NTP/ Bimodal Nuclear Thermal Propulsion (BNTP) can more readily accommodate the heavier payload mass and increased RCS propellant loading associated with AG operation, and can travel faster to and from Mars thereby reducing the crew's exposure to galactic cosmic radiation and solar flares. Mission scenario descriptions, key vehicle features and operational characteristics for each propulsion option are presented using the lift capability and payload volumes estimated for the Space Launch System (SLS)-1B and followon Heavy Lift Vehicle (HLV).

Nuclear Propulsion↗
NASA NTRS1979

Fan-shaped coma, orientation of rotation axis, and surface structure of a cometary nucleus. I - Test of a model on four comets

Pronounced anisotropy in the outgassing from comets, especially from short-period ones, appears to be a factor responsible for the frequent occurrence of a fan-shaped coma, extending in the general direction of the sun. It is suggested that the pattern of deviations from the sunward direction contains information on the orientation of the spin axis and on the time lag in the sublimation process, which in turn provides insight into the nature of the nuclear surface. A simple model of a spherical rotating nucleus is formulated and a trial-and-error technique is devised to determine the axis-orientation constants and a lag angle, a measure of the time lag in units of the rotation period. The method is applied to the comets Encke, Tempel 2, Borrelly, and Schwassmann-Wachmann 3.

Sekanina, Z.↗
NASA NTRS1995

Propagation of nuclear burning fronts on accreting neutron stars: X-ray bursts and sub-hertz noise

We identify a new regime of time dependent helium burning for high accretion rate neutron stars and suggest that this burning is the origin of the low-level luminosity variations (on timescales of 10-10(exp 4) s, designated the 'very low-frequency noise'(VLFN) by van der Klis and collaborators) always detected in the brightest accreting X-ray sources. Only two nuclear burning regimes were previously recognized. At accretion rates in excess of the Eddington limit (dot-M approximately greater than (1-3) x 10(exp -8) solar mass/yr), the accreted matter fuses steadily. At very low dot-M, the star's entire surface is rapidly (approximately less than 10 s) burned by a fast propagating convective burning front at regular intervals, giving quasi-periodic Type I X-ray bursts. We show that for the observationally interesting range of 5 x 10(exp -10) solar mass/yr approximately less than dot-M approximately less than 10(exp -8) solar mass/yr, parts of the stellar surface burn slowly. At these accretion rates, a local thermonuclear instability starts a fire which propagates horizontally at v approximately 300 cm/s. The fire propagates around the flammable surface in roughly the same time it takes to accrete enough fuel for the next instability (approximately 10(exp 3)-10(exp 4), so that only a few fires are burning at once, giving rise to large luminosity flares. Nuclear burning is always time dependent for sub-Eddington local accretion rates: a local patch undergoes a recurrent cycle, accumulation fuel for hours until it becomes thermally unstable or is 'ignited' by a nearby burning region. The global pattern of burning and the resulting luminosity are thus very dependent on how fast nuclear fires spread around the star. The nuclear burning luminosity is not uniform over the stellar surface and so may provide a handle on measuring, or constraining, the spin periods of these neutron stars.

Bildsten, Lars↗
NASA NTRS1998

Secondary Fragmentation of Comet Shoemaker-Levy 9 and the Ramifications for the Progenitor's Breakup in July 1992

Comprehensive analysis of discrete events of secondary fragmentation leads to a conceptually new understanding of the process of disintegration of comet Shoemaker-Levy 9. We submit that the jovian tidal forces inflicted extensive cracks throughout the interior of the original nucleus but did not split it apart. The initial disruption was apparently accomplished by stresses exerted on the cracked object by its fast rotation during the early post-perijove period of time. We argue that this disruption was in fact a rapid sequence of episodes during July 1992 that gave birth to the 12 on-train, or primary, fragments: A, C, D, E, G, H, K, L, Q (later Q(sub 1)), R, S, and W. The discrete events of secondary fragmentation, which gave birth to the off-train fragments, are understood in this scenario as stochastic manifestations of the continuing process of progressive disintegration. Of the 13 off-train fragments considered, nine were secondary--B, F, G(sub 2), M, N, P (later P(sub 2) or P(sub 2a)), Q(sub 2), U, and V--and four tertiary (J, P(sub 1), P(sub 2b), and T). The separation parameters of 11 off-train fragments were determined. The vectorial distribution of separation velocities of these fragments shows a strong concentration toward a great circle, unquestionably an effect of the approximately conserved angular momentum of the progenitor comet since the time of its initial disruption. Also apparent is their clumping (except for P(sub 1)) to a segment along the great circle, implying that the fragments were consistently released from one side of their parents, thus explaining for the first time why the off-train fragments preferentially appeared on one side of the nuclear train. In order to obtain a consistent solution, our model requires that the points of separation be on the antisolar side of the parent fragments, where thermal stresses are likely to enhance the effect of rotation. The episodes of secondary fragmentation are found to have nine months after the close encounter with Jupiter in early July 1992, and the separation velocities ranged between 0.36 and 1.7 m/s. The spin-axis position is determined to have been nearly in the jovicentric orbit plane, which rules out the Asphaug-Benz-Solem strengthless aggregate model as a plausible breakup hypothesis. Since the separation velocities are rotational in nature, they cannot substantially exceed the critical limit for centrifugal breakup and offer an estimate for the original nuclear dimensions. The comet's nucleus is found to have been approximately 10 km in diameter and spinning rapidly. With the exception of P(sub 1) and apparently also P(sub 2) and F, no nongravitational deceleration was detected in the motions of the off-train fragments. Serious doubts are cast on continuing appreciable activity of any of these fragments. Indeed, when it was necessary to introduce a deceleration into the equations of motion, the effect appears to have been due to the action of solar radiation pressure on the centroid of centimeter-sized particulates in the disintegrating condensations.

Sekanina, Z.↗
NASA NTRS1994

Rapidly rotating neutron stars in general relativity: Realistic equations of state

We construct equilibrium sequences of rotating neutron stars in general relativity. We compare results for 14 nuclear matter equations of state. We determine a number of important physical parameters for such stars, including the maximum mass and maximum spin rate. The stability of the configurations to quasi-radial perturbations is assessed. We employ a numerical scheme particularly well suited to handle rapid rotation and large departures from spherical symmetry. We provide an extensive tabulation of models for future reference. Two classes of evolutionary sequences of fixed baryon rest mass and entropy are explored: normal sequences, which behave very much like Newtonian sequences, and supramassive sequences, which exist for neutron stars solely because of general relativistic effects. Adiabatic dissipation of energy and angular momentum causes a star to evolve in quasi-stationary fashion along an evolutionary sequence. Supramassive sequences have masses exceeding the maximum mass of a nonrotating neutron star. A supramassive star evolves toward eventual catastrophic collapse to a black hole. Prior to collapse, the star actually spins up as it loses angular momentum, an effect that may provide an observable precursor to gravitational collapse to a black hole.

Cook, Gregory B.↗
NASA NTRS2014

Simultaneous NuSTAR and XMM-Newton 0.5-80 KeV Spectroscopy of the Narrow-Line Seyfert 1 Galaxy SWIFT J2127.4+5654

We present a broad-band spectral analysis of the joint XMM-Newton and Nuclear Spectroscopic Telescope Array observational campaign of the narrow-line Seyfert 1 SWIFT J2127.4+5654, consisting of 300 kiloseconds performed during three XMM-Newton orbits. We detect a relativistic broadened iron K-alpha line originating from the innermost regions of the accretion disc surrounding the central black hole, from which we infer an intermediate spin of a = 0.58 (sup +0.11) (sub −0.17). The intrinsic spectrum is steep (gamma = 2.08 plus or minus 0.01) as commonly found in narrow-line Seyfert 1 galaxies, while the cutoff energy (E (sub c) = 108 (sup +11) (sub −10) kiloelectronvolts) falls within the range observed in broad-line Seyfert 1 galaxies. We measure a low-frequency lag that increases steadily with energy, while at high frequencies, there is a clear lag following the shape of the broad Fe K emission line. Interestingly, the observed Fe K lag in SWIFT J2127.4+5654 is not as broad as in other sources that have maximally spinning black holes. The lag amplitude suggests a continuum-to-reprocessor distance of about 10-20 radius of gyration. These timing results independently support an intermediate black hole spin and a compact corona.

Nuclear Spectroscopic Telescope Array↗
NASA NTRS1983

Deuteron NMR (Nuclear Magnetic Resonance) in relation to the glass transition in polymers

H-2NMR is introduced as a tool for investigating slow molecular motion in the glass transition region of amorphous polymers. In particular, we compare H-2 spin alignment echo spectra of chain deuterated polystyrene with model calculations for restricted rotational Brownian motion. Molecular motion in the polyztyrene-toluene system has been investigated by analyzing H-2NMR of partially deuterated polystyrene and toluene, respectively. The diluent mobility in the mixed glass has been decomposed into solid and liquid components where the respective average correlation times differ by more than 5 decades.

Roessler, E.↗
NASA NTRS1983

Theory of nuclear magnetic relaxation

A theory of nuclear magnetic interaction is based on the study of the stochastic rotation operator. The theory is applied explicitly to relaxation by anisotropic chemical shift and to spin-rotational interactions. It is applicable also to dipole-dipole and quadrupole interactions.

Mcconnell, J.↗
NASA NTRS1988

A cosmic and solar X-ray and gamma-ray instrument for a scout launch

An overview is presented for a set of simple and robust X-ray and gamma ray instruments which have both cosmic and solar objectives. The primary solar scientific objective is the study of the beaming of energetic electrons and ions in solar flares. The instrument will measure spectra and polarization of flare emissions up to 10 MeV. At X-ray energies both the directly emitted flux and the reflected albedo flux will be measured with a complement of six X-ray sensors. Each of these detectors will have a different high Z filter selected to optimize both the energy resolution and high rate capabilities in the energy band 10 to 300 keV. At energies greater than 100 keV seven 7.6 x 7.6 cm NaI and a set of 30 concentric plastic scattering detectors will record the spectra and polarization of electron bremsstrahlung and nuclear gamma rays. All of the components of the instrument are in existence and have passed flight tests for earlier space missions. The instrument will use a spinning solar oriented Scout spacecraft. The NaI detectors will act as a self-modulating gamma ray detector for cosmic sources in a broad angular band which lies at 90 degrees to the Sun-Earth vector and hence will scan the entire sky in 6 months.

Forrest, D. J.↗
NASA NTRS1991

Characterization of the rotation of cometary nuclei

The primary methods used to determine the spin state of cometary nuclei and the pitfalls and successes experienced in their use are reviewed. Attention is given to the theory of rigid body rotation from the standpoint of remote (astronomical) observers, with emphasis on what is known of the effects of nutation of lightcurves, the influence of torques induced by jet activity, the effects of internal energy and mass dissipation, and nuclear splitting. The available knowledge on rotation for eight comets, including Comet P/Halley, is reviewed. The following questions are addressed: achievement of a consensus on the rotational state of P/Halley, the possibility of accurately determining the amplitude of the transverse nongravitational force associated with rotation, and whether the orientations of fanlike comas are a valid indicator of the orientation of the spin vector. The prerequisites for answering these questions are discussed.

Belton, Michael J. S.↗
NASA NTRS1984

The study of the physics of cometary nuclei

The observations of comet P/Holmes 1892III, exhibiting two 8 to 10 magnitude bursts, were carefully analyzed. The phenomena are consistent with the grazing encounter of a small satellite with the nucleus. The grazing encounter produced, besides the first great burst, an active area on the nucleus, which was rotating retrograde with a period of 16.3 hr and inclination nearly 180 deg. After the final encounter, the spin period was essentially unchanged, but two areas became active, separated some 164 deg in longitude on the nucleus. After the first burst the total magnitude fell less than two magnitudes, while the nuclear region remained diffuse or complex, rarely if ever showing a stellar appearance. The fading was much more rapid after the second burst (barely naked eye at maximum) while the nucleus frequently stellar after the first day. It seems reasonable to conclude that the grazing encounter distributed a volume of large chunks in the neighborhood of the nucleus, maintaining activity for weeks.

Whipple, F. L.↗
NASA NTRS2003

Journey to the Center of a Neutron Star

A neutron star is not a place most would want to visit. This dense remnant of a collapsed star has a magnetic field billions of times stronger than Earth's, enough to shuffle your body's molecules long before you even land. The featureless surface is no fun either. Crushing gravity ensures that the star is a near perfect sphere, compressing all matter so that a sand-grain-sized scoop of neutron star material would weigh as much as a battleship on Earth. At least black holes offer the promise of funky singularity, time warps, and the Odyssean temptation to venture beyond a point of no return. What s a journey to a neutron star good for, one might ask? Well, for starters, it offers the possibility of confirming a theorized state of matter called quark-gluon plasma, which likely existed for a moment after the Big Bang and now might only exist in the superdense interiors of neutron stars. Beneath the neutron star crust, a kilometer-thick plate of crystalline matter, lies the great unknown. The popular theory is that the neutron star interior is made up of a neutron superfluid - a fluid without friction. With the help of two NASA satellites - the Rossi X-Ray Timing Explorer and the Chandra X-Ray Observatory - scientists are journeying to the center of a neutron star. Matter might be so compressed there that it breaks down into quarks, the building blocks of protons and neutrons, and gluons, the carrier of the strong nuclear force. To dig inside a neutron star, no simple drill bit will do. Scientists gain insight into the interior through events called glitches, a sudden change in the neutron star s precise spin rate. 'Glitches are one of the few ways we have to study the neutron star interior,' says Frank Marshall of NASA s Goddard Space Flight Center, who has used the Rossi Explorer to follow the escapades of the glitchiest of all neutron stars, dubbed the Big Glitcher and known scientifically as PSR J0537-6910.

Wanjek, Christopher↗
NASA NTRS2006

Isotopic Dependence of GCR Fluence behind Shielding

In this paper we consider the effects of the isotopic composition of the primary galactic cosmic rays (GCR), nuclear fragmentation cross-sections, and isotopic-grid on the solution to transport models used for shielding studies. Satellite measurements are used to describe the isotopic composition of the GCR. For the nuclear interaction data-base and transport solution, we use the quantum multiple-scattering theory of nuclear fragmentation (QMSFRG) and high-charge and energy (HZETRN) transport code, respectively. The QMSFRG model is shown to accurately describe existing fragmentation data including proper description of the odd-even effects as function of the iso-spin dependence on the projectile nucleus. The principle finding of this study is that large errors (+/-100%) will occur in the mass-fluence spectra when comparing transport models that use a complete isotopic-grid (approx.170 ions) to ones that use a reduced isotopic-grid, for example the 59 ion-grid used in the HZETRN code in the past, however less significant errors (<+/-20%) occur in the elemental-fluence spectra. Because a complete isotopic-grid is readily handled on small computer workstations and is needed for several applications studying GCR propagation and scattering, it is recommended that they be used for future GCR studies.

Cucinotta, Francis A.↗
NASA NTRS1972

The calculation of neutron capture gamma-ray yields for space shielding applications

The application of nuclear models to the calculation of neutron capture and inelastic scattering gamma yields is discussed. The gamma ray cascade model describes the cascade process in terms of parameters which either: (1) embody statistical assumptions regarding electric and magnetic multipole transition strengths, level densities, and spin and parity distributions or (2) are fixed by experiment such as measured energies, spin and parity values, and transition probabilities for low lying states.

Yost, K. J.↗
NASA NTRS2005

UltraSail - Ultra-Lightweight Solar Sail Concept

UltraSail is a next-generation high-risk, high-payoff sail system for the launch, deployment, stabilization and control of very large (sq km class) solar sails enabling high payload mass fractions for high (Delta)V. Ultrasail is an innovative, non-traditional approach to propulsion technology achieved by combining propulsion and control systems developed for formation-flying micro-satellites with an innovative solar sail architecture to achieve controllable sail areas approaching 1 sq km, sail subsystem area densities approaching 1 g/sq m, and thrust levels many times those of ion thrusters used for comparable deep space missions. Ultrasail can achieve outer planetary rendezvous, a deep space capability now reserved for high-mass nuclear and chemical systems. One of the primary innovations is the near-elimination of sail supporting structures by attaching each blade tip to a formation-flying micro-satellite which deploys the sail, and then articulates the sail to provide attitude control, including spin stabilization and precession of the spin axis. These tip micro-satellites are controlled by 3-axis micro-thruster propulsion and an on-board metrology system. It is shown that an optimum spin rate exists which maximizes payload mass.

Burton, Rodney L.↗
NASA NTRS2014

NuSTAR Discovery Of A Young, Energetic Pulsar Associated with the Luminous Gamma-Ray Source HESS J1640-465

We report the discovery of a 206 ms pulsar associated with the TeV gamme-ray source HESS J1640−465 using the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory. PSR J1640−4631 lies within the shelltype supernova remnant (SNR) G338.3−0.0, and coincides with an X-ray point source and putative pulsar wind nebula (PWN) previously identified in XMM-Newton and Chandra images. It is spinning down rapidly with period derivative P = 9.758(44) × 10(exp −13), yielding a spin-down luminosity E = 4.4 × 10(exp 36) erg s(exp −1), characteristic age tau(sub c) if and only if P/2 P = 3350 yr, and surface dipole magnetic field strength B(sub s) = 1.4×10(exp 13) G. For the measured distance of 12 kpc to G338.3−0.0, the 0.2-10 TeV luminosity of HESS J1640−465 is 6% of the pulsar's present E. The Fermi source 1FHL J1640.5−4634 is marginally coincident with PSR J1640−4631, but we find no gamma-ray pulsations in a search using five years of Fermi Large Area Telescope (LAT) data. The pulsar energetics support an evolutionary PWN model for the broadband spectrum of HESS J1640−465, provided that the pulsar's braking index is n approximately equal to 2, and that its initial spin period was P(sub 0) approximately 15 ms.

pulsars: individual↗
NASA NTRS2014

Conventional and Bimodal Nuclear Thermal Rocket (NTR) Artificial Gravity Mars Transfer Vehicle Concepts

A variety of countermeasures have been developed to address the debilitating physiological effects of "zero-gravity" (0-g) experienced by cosmonauts and astronauts during their approximately 0.5-1.2 year long stays in LEO (Low Earth Orbit). Longer interplanetary flights, combined with possible prolonged stays in Mars orbit, could subject crewmembers to up to approximately 2.5 years of weightlessness. In view of known and recently diagnosed problems associated with 0-g, an artificial gravity spacecraft offers many advantages and may indeed be an enabling technology for human flights to Mars. A number of important human factors must be taken into account in selecting the rotation radius, rotation rate, and orientation of the habitation module or modules. These factors include the gravity gradient effect, radial and tangential Coriolis forces, along with cross-coupled acceleration effects. Artificial gravity (AG) Mars transfer vehicle (MTV) concepts are presented that utilize both conventional NTR, as well as, enhanced "bimodal" nuclear thermal rocket (BNTR) propulsion. The NTR is a proven technology that generates high thrust and has a specific impulse (I (sub sp)) capability of approximately 900 s - twice that of today's best chemical rockets. The AG/MTV concepts using conventional NTP carry twin cylindrical "ISS-type" habitation modules with their long axes oriented either perpendicular or parallel to the longitudinal spin axis of the MTV and utilize photovoltaic arrays (PVAs) for spacecraft power. The twin habitat modules are connected to a central operations hub located at the front of the MTV via two pressurized tunnels that provide the rotation radius for the habitat modules. For the BNTR AG/MTV option, each engine has its own "closed" secondary helium-xenon gas loop and Brayton rotating unit that can generate tens of kilowatts (kW (sub e)) of spacecraft electrical power during the mission coast phase eliminating the need for large PVAs. A single inflatable "TransHab-type" habitation module is also used with multiple vertical floors oriented radial to the MTV spin axis. The BNTR MTV's geometry - long and linear - is naturally compatible with AG operation. By rotating the vehicle about its center-of-mass and perpendicular to its flight vector at approximately 3.0 - 5.2 rpm, a centrifugal force and AG environment corresponding to approximately 0.38 - 1.0 g can be established to help maintain crew fitness out to Mars and back. Vehicles using NTP/BNTP can more readily accommodate the heavier payload mass and increased RCS propellant loading associated with AG operation, and can travel faster to and from Mars thereby reducing the crew's exposure to galactic cosmic radiation and solar flares. Mission scenario descriptions, key vehicle features and operational characteristics for each propulsion options are presented using the lift capability and payload volumes estimated for the SLS-1A and HLV.

Nuclear Propulsion↗
NASA NTRS1994

Merging neutron stars. 1. Initial results for coalescence of noncorotating systems

We present three-dimensional Newtonian simulations of the coalescence of two neutron stars, using a smoothed particle hydrodynamics (SPH) code. We begin the simulations with the two stars in a hard, circular binary, and have them spiral together as angular momentum is lost through gravitational radiation at the rate predicted by modeling the system as two point masses. We model the neutron stars as hard polytropes (gamma = 2.4) of equal mass, and investigate the effect of the initial spin of the two stars on the coalescence. The process of coalescence, from initial contact to the formation of an axially symmetric object, takes only a few orbital periods. Some of the material from the two neutron stars is shed, forming a thick disk around the central, coalesced object. The mass of this disk is dependent on the initial neutron star spins; higher spin rates result in greater mass loss and thus more massive disks. For spin rates that are most likely to be applicable to real systems, the central coalesced object has a mass of 2.4 solar mass, which is tantalizingly close to the maximum mass allowed by any neutron star equation of state for an object that is supported in part by rotation. Using a realistic nuclear equation of state, we estimate the temperature of the material after the coalescence. We find that the central object is at a temperature of approximately 10 MeV, while the disk is heated by shocks to a temperature of 2 to 4 MeV.

Davies, M. B.↗