Search NASA⌕ Search

SEARCH · Search NASA

Results for “NUCLEUS”

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

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

At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Scattering and stopping of hadrons in nuclear matter

It was observed, in the 180 litre xenon bubble chamber, that when hadrons with kinetic energy higher than the pion production threshold fall on a layer of nuclear matter - on an atomic nucleus in other words - in many cases they can pass through it without causing particles production but they are deflected through some deflection angles; if the energy is lower than a few GeV and the nuclear matter layer is thick enough, the hadrons can be stopped in it. The amount of the deflection at a given incident hadron energy varies with the way the hadron strikes the atomic nucleus; the probability of the occurrence of stopping depends on the incident hadron identity and energy, and on the way the hadron passed through the nucleus, as well.

Strugalski, Z.↗
NASA NTRS1985

Measurements of hadron mean free path for the particle-producing collisions in nuclear matter

It is not obvious a priority that the cross-section for a process in hadron collisions with free nucleons is the same as that for the process in hadron collisions with nucleons inside a target nucleus. The question arises: what is the cross-section for a process in a hadron collision with nucleon on inside the atomic nucleus. The answer to it must be found in experiments. The mean free path for particle-producing collisions of pions in nuclear matter is determined experimentally using pion-xenon nucleus collisions at 3.5 GeV/c momentum. Relation between the mean free path in question lambda sub in nucleons fm squared and the cross-section in units of fm squared/nucleon for collisions of the hadron with free nucleon is: lambda sub i = k/cross section sub i, where k = 3.00 plus or minus 0.26.

Strugalski, Z.↗
NASA NTRS1984

A radar study of Comet IRAS-Araki-Alcock 1983d

Radar echoes from Comet IRAS-Araki-Alcock at wavelengths 3.54 and 12.9 cm indicate that the comet's nucleus is very rough on a scale larger than the radar wavelengths; however, the low polarization ratio (25 percent at 3.54 cm) indicates that the scattering is not dominated by multiple reflections, internal reflections, or large abundances of sharp edges, cracks, and pits. The shape of the nucleus probably departs greatly from a sphere with average radii near 3-4 km. The nucleus does not appear to look significantly different from a number of Apollo and Amor asteroids except that: (1) there is a suggestion that minor structure moves rapidly across the spectra, and (2) the debris not gravitationally bound to the comet was detected, and contributes 25 percent of the total radar cross section at the 12.9-cm wavelength. Other considerations suggest that the pole was at least 45 deg away from the line of sight on two days of observation, and that the rotation period is approximately 1-2 days.

Goldstein, R. M.↗
NASA NTRS1984

An improved thermal model for cometary nuclei

A thermal model for cometary nuclei is presented and used in comparisons of predictions with data on Halley's comet. The model considers the surface boundary conditions and heat diffusion equations, the sublimation rate of ice, the energy balance, and the dust production rate. Projections were made for the Halley nucleus over 18 latitudes from -85 to +85 deg, from 185 d before to 110 d after perihelion, and in 20 layers of its surface in 1.3 cm increments. Sublimation is predicted to increase by a factor of 2.2 every orbit, thereby allowing 320 returns for a nucleus with a coma, 710 returns with a bare ice nucleus. The gas production during the Giotto intercept at 0.9 AU will be 4.4 x 10 to the 29th molecules/sec, close to an estimate made during the 1910 return.

Weissman, P. R.↗
NASA NTRS1985

Surface photometry of comet P/Encke

A scheme to clean cometary digital images from offending background-star trails, and this technique has been applied to a pair of deep Kitt Peak 4-m plates of comet P/Encke, taken in October 1980. Simultaneous and subsequent digital spectra have been obtained at Lick Observatory. The non-polluted coma images show a strong asymmetric sunward-oriented fan/jet, and an extended and rounder (mostly gaseous) main coma, out to approximately 100,000 km radius. The stellar-trail point-spread function has a narrow width (sigma approximately 0.6 arcsec), so that spatial resolution better than approximately 300 km is achieved at the comet. The photometric gradient near the nucleus is very steep, strongly suggesting an icy-grain component which evaporates quickly (at radii equal to or less than 500 km) in the sunlight. Further from the nucleus, the profile becomes shallower, bluer, and more gas dominated. The effect of solar radiation pressure on C2, CN, and other molecules is probably responsible for the rounding of the outer, fainter isophotes. The source of the molecules is likely to be larger than the nucleus itself, and a substantial fraction may originate in the jet. The technique described here may also be applicable in surface photometry of galaxies, in cases where the heavy image pollution by foreground stars is present.

Djorgovski, S.↗
NASA NTRS1984

X-ray inverse Compton emission from the radio halo of M87

A significant fraction of known galaxies contain an active galactic nucleus (AGN) at their cores, the site of violent activity and non-stellar radiation seen across the entire electromagnetic spectrum. This activity is thought to be due to the accretion of gas onto a massive black hole. A fraction of AGNs also eject collimated beams of energetic material, usually seen by virtue of its synchrotron emission in the radio band. Efforts to study these jets from AGNs in the X-ray band with the Einstein Observatory has led to several detections, most notably the jets in the nearby radio galaxies Centaurus A and Virgo A = M87. In their study of M87, Schreier, Gorenstein and Feigelson (1982) noted that, in addition to the synchrotron jet 10"-20" from the nucleus, X-rays appear to be generated in the diffuse radio halo 2'-5' from the nucleus. This finding may be particularly important as it may constitute the first known case of X-ray inverse Compton emission from AGN ejecta, allowing for the first time direct determination of the magnetic field strengths.

Feigelson, E. D.↗
NASA NTRS1987

Temperatures and minimum thickness of the inactive surface layer of comet Halley

The effects of a nonvolatile mantle on the thermal state of a comet nucleus are investigated. The original computer model was modified so that temperatures can be computed through a thin dust mantle to the center of a 5 km spherical nucleus in the orbit of P/Halley. No attempt is made to simulate the formation of the mantle. Results are obtained for various specified values of initial mantle thickness and thermal conductivity to determine their effects on temperature profiles through the mantle. The minimum thickness of mantle that can withstand ejection by sublimating gasses is also calculated as a function of mantle thermal conductivity. Calculations were performed for ten or more orbits until temperatures in the mantle reached a near steady state. Results indicate that mantles as thin as 4 cm and 14 cm, for thermal conductivities of 600 and 6000 ergs/cm/s K, respectively, will remain intact. Surface temperatures as high as 511 K at perihelion and 400 K at the position of spacecraft encounter were computed at 0 degree latitude for an upright, rotating nucleus. Ice interface temperatures were raised by different amounts during each orbit, depending on mantle thickness and thermal conductivity, until steady state was reached. These results imply that relatively small nonvolatile masses emplaced randomly in comet nuclei could produce an irregular, permanently mantled surface and could also account for the apparently random location of active areas.

Fanale, F. P.↗
NASA NTRS1987

The relation between star formation and active nuclei

Three questions relevant to the relation between an active nucleus and surrounding star formation are discussed. The infrared stellar CO absorption bands can be used to identify galaxies with large populations of young, massive stars and thus can identify strong starburst unambiguously, such as in NGC 6240, and can help identify composite active/starburst systems such as Arp 220. An active nucleus is probably not required for LINER spectral characteristics; dusty starburst galaxies, particularly if they are nearly edge-on, can produce LINER spectra through the shock heating of their interstellar media by supernovae combined with the obscuration of their nuclei in the optical. The Galactic Center would be an ideal laboratory for studying the interaction of starbursts and active nuclei, if both could be demonstrated to occur there. Failure to detect a cusp in the stellar distribution raises questions about the presence of an active nucleus, which should be answered by additional observations in the near future.

Rieke, G. H.↗
NASA NTRS1986

On the CO and N2 abundance in Comet Halley

The mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is evaluated. At 1000 km from the nucleus number density n(CO)/ n(H2O) is less than or = 0.07. The production rate of CO as a parent molecule directly from the nucleus is thus less than 7% of the H2O production rate. However, CO is also produced from an extended source in the inner coma (r is less than 20,000 km) and at 20,000 km from the nucleus, for the total equivalent CO production rate 0.05 is less than or = Q(CO)/Q(H2O) is less than or = 0.15. For N2 an upper limit Q(N2)/Q(H2O) is less than or = 0.1 is derived. No parent molecule for the CO is identified in agreement with the NMS measurements. It is proposed that CO or a very short-lived parent is released in the coma from cometary dust grains, such as the CHON particles.

Eberhardt, P.↗
NASA NTRS1986

Brightness curve and porosity of cometary nuclei

The brightness curve of a comet is analyzed in terms of porosity and nature of volatiles in its nucleus. Mobility of pores, densification, phase changes, differential thermal expansion and heat flow into and out of the nucleus are discussed leading to a method for estimating the effective heat conductivity and porosity of the nucleus. The method is applied to comets P/Giacobini-Zinner, P/Halley, P/Kopff, P/Encke, P/Tempel 2 and P/d'Arrest.

Smoluchowski, R.↗
NASA NTRS1986

Photometric and spectrophotometric activity of P/Halley during 1984-85

Imaging and spectroscopic observations of P/Halley were performed during 1984 and 1985 using a CCD camera and an Image Dissector Scanner Spectrograph, and the results are reported. P/Halley activity appears to begin around 6.23 AU and varies as r exp -5.3 for heliocentric distances between 5.8 and 4.2 AU. The observed radial brightness distribution falls off much faster than the inverse of the projected distance from the nucleus, indicating that the particles are disappearing or evaporating within about 20,000 km of the nucleus. Weak CN emission was detected in spectra obtained during February 1985, corresponding to a column density of 3 x 10 to the 8th molecules/sq cm. The spectroscopic results indicate that the excess of the observed emission over that from the bare nucleus is dominated by scattered solar continuum from grains, rather than by molecular coma emissions.

Barker, E. S.↗
NASA NTRS1986

Thermal-infrared and visual imaging of comet Giacobini-Zinner

Thermal-infrared and visual images obtained simultaneously of comet Giacobini-Zinner on UT 1985 August 4.4 are presented. The 10.8-micron map, which spans several arc minutes with 8-arcsec resolution, is the first ground-based infrared image of a comet. Considerable spatial structure is evident in the map, including a curved dust tail consisting primarily of dust grains larger than 100 microns. The width of this large-grain tail implies that these grains depart the nucleus with velocities less than 3 m/s. The distribution of grain albedo is determined for the central 1 arcmin. It is speculated that an observed increase in the albedo by a factor of 2 with distance from the nucleus and prolonged radius vector results from the confinement to the orbital plane of large fluffy grains previously considered for GZ by others; the lower albedo of this dust could result from multiple internal scattering increasing the probability of absorption of incident light. It is estimated that dust was departing the nucleus at the rate of about 8 x 10 to the 5th g/s, comparable to that derived for many other comets.

Telesco, C. M.↗
NASA NTRS1987

Navigation of the Mariner Tempel 2 mission - Orbit phase and penetrator deployment

The navigation of the Mariner Mark II spacecraft about the comet Tempel 2 in the Comet Rendezvous Asteroid Flyby (CRAF) mission is described. This mission is the first of a series using the Mariner Mark II spacecraft planned for the 1990's and thereafter. The primary purpose of the mission is to orbit the nucleus of the periodic comet Tempel 2 and study it at close range. A secondary purpose is to fly by and observe one or two asteroids enroute to the comet. During the orbital phase of the mission, a penetrator will be deployed from the spacecraft to impact the surface of the comet and measure the composition and other properties of the nucleus. Navigation accuracies are given in this paper for spacecraft and penetrator trajectory prediction and control. The orbit determination performance in support of science observations while in orbit about the nucleus is also described.

Miller, James K.↗
NASA NTRS1987

Comet West: A view from the HELIOS zodiacal light photometers

Comet West passed through perihelion on February 25, 1976. The comet crossed the HELIOS A and B spacecraft zodiacal light photometer fields of view as the spacecraft orbited the Sun, allowing them to record the brightness, polarization, and color of the comet and its surrounding interplanetary medium. Data from the U, B, and V photometers across the tail shows a distinct bluing followed by a slight reddening corresponding to the ion and dust tails, respectively, entering the field of view. The non-Earth perspective of the HELIOS photometers allows a comparison of the tail with Earth observations at the same time. Precise location of the nucleus and tail allow the photometer data to be searched for evidence of the comet bow shock and orbital dust. A brightness bump present in the data before the comet reaches some photometer positions, can be shown to approximately form a parabolic shape Sunward and ahead of the orbital motion of the Comet West nucleus. If this is the comet bow shock or bow compression, then it corresponds to a density enhancement of the ambient medium by 1.5 to 2 times in the vicinity of the comet. The distance of the brightness increase from the nucleus by comparison with Comet Halley implies a neutral gas production rate of approximately 3 times that of Halley.

Benensohn, R. M.↗
NASA NTRS1987

The loss and depth of CO2 ice in comet nuclei

The chemical differentiation of a homogeneous, initially unmantled cometary nucleus whose composition encompasses water ice, unclathrated CO2 ice, and silicate dust, is presently simulated by an analytical model to test whether the selective sublimation of any free CO2 ice in the new comet will produce a surface layer of water ice and dust covering the undifferentiated core. The model encompasses the effects of nucleus rotation, arbitrary rotatiuon axis orientation, latitude, nucleus heat conduction, and CO2 gas outflow restriction by the water ice and dust layer.

Fanale, Fraser P.↗
NASA NTRS1988

A 2 kiloparsec accretion disk (?) inside a 17 kiloparsec H-alpha bar in the cooling flow cluster 2A 0335+096

Observations of the central region of the galaxy cluster associated with 2A 0335+096, obtained in H-alpha, R, I, and b45 (4506 A with FWHM 355 A) using an 800 x 800-pixel CCD detector on the 4-m telescope at KPNO on December 27 and 30, 1986, are reported. The data are presented graphically and analyzed. Features discussed include a 17-kpc-long bar of H-alpha emission surrounded by filaments, a smaller elongated H-alpha structure (possibly an accretion disk) centered on the nucleus of the central galaxy, a secondary galaxy with a red H-alpha-bright nucleus located within the 17-kpc bar about 6 arcsec from the central-galaxy nucleus, and evidence for excess blue emission (indicating star formation at about 8 solar mass/yr in the cooling flow). From the presence of dust in the emission-line gas it is inferred that theoretical models based on preferential formation of low-mass stars may not be applicable to this object.

Romanishin, W.↗
NASA NTRS1988

Outgassing asymmetry of periodic comet Encke. I - Apparitions 1924-1984

A concept of a collimated flow of ejecta from a discrete source (vent) on the surface of a rotating cometary nucleus is applied to P/Encke, based on positional observations of its sunward fanlike coma at 13 apparitions between 1924 and 1984. The major results include a finding that the observations are consistent with an invariable position of the rotation pole during the 60 yr (yielding an obliquity of 70 deg), although this is not to be interpreted as an indication that the nucleus is not precessing. Two vents are identified on the nucleus surface: one on the northern hemisphere at a latitude of +55 deg, of an estimated sublimitation area of 0.4 sq km, which is active along the inbound leg of the comet's orbit until a few days preperihelion and again beginning several weeks postperihelion; the other on the southern hemisphere at -75 deg, of an estimated area of 0.6 sq km, whose activity prevails in the intervening time. There appears to be a significant difference between the two regions in terms of dust content in their emissions.

Sekanina, Z.↗
NASA NTRS1988

Chemical abundance of comets

Observations of NH2, (OI) and molecular ion spectra in comets represent virtually all of the volatile fraction of a comet nucleus. Their study leads to the N2, NH3, H2O, CO2, CO content of the nucleus, and thus to important constraints on models of comet formation and chemical processing in the primitive solar nebula. The observations of Comet Halley provide the opportunity for the first comprehensive determination of the abundances in a comet nucleus. The carbon isotope abundance ratio 12 C/13 C = 65 plus or minus 8 has been determined for Comet Halley from resolved rotational line structure in the CN B-X (0,0) band. The ratio is approximately 30 pct lower than the solar system value, 89, indicating either an enhancement of 13CN or a depletion of 12CN in the comet. Scenarios consistent with the observed carbon isotope ratio are: (1) formation of the comet at the periphery of the solar nebula in a fractionation-enriched 13CN region, or hidden from 12CN enrichment sources, and (2) capture of an interestellar comet. Long-slit charge coupled device (CCD) spectra obtained at the time of the spacecraft encounter of Comet Halley have also been analyzed. Scale lengths, production rates and column densities of CH, CN, C2 and NH2 were determined.

Wyckoff, Susan↗