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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Streaming Clumps Ejection Model and the Heterogeneous Inner Coma of Comet Wild 2

The conventional concept of cometary comae is that they are dominated by fine particulates released individually by sublimation of surface volatiles and subsequent entrainment in the near-surface gas. It has long been recognized that such particulates could be relatively large, with early estimates that objects perhaps up to one meter in size may be levitated from the surface of the typical cometary nucleus. However, the general uniformity and small average particulate size of observed comae and the relatively smooth, monotonic increases and decreases in particle density during the Giotto flythrough of comet Halley s coma in 1986 reinforced the view that the bulk of the particles are released at the surface, are fine-sized and inert. Jets have been interpreted as geometrically constrained release of these particulates. With major heterogeneities observed during the recent flythrough of the inner coma of comet Wild 2, these views deserve reconsideration.

Clark, B. C.↗
NASA NTRS1982

Geological implications of impacts of large asteroids and comets on the earth

The present conference discusses such topics as large object fluxes in near-earth space and the probabilities of terrestrial impacts, the geological record of impacts, dynamics modeling for large body impacts on continents and oceans, physical, chemical, and biological models of large impacts' atmospheric effects, dispersed impact ejecta and their signatures, general considerations concerning mass biological extinctions, the Cretaceous/Tertiary boundary event, geochemical signatures in the stratigraphic record, and other phanerozoic events. Attention is given to terrestrial impact rates for long- and short-period comets, estimates of crater size for large body impact, a first-order estimate of shock heating and vaporization in oceanic impacts, atmospheric effects in the first few minutes after an impact, a feasibility test for biogeographic extinction, and the planktonic and dinosaur extinctions.

Silver, L. T.↗
NASA NTRS1986

International Halley Watch: Discipline specialists for near-nucleus studies

The purpose of the Near-Nucleus Studies Net is to study the processes taking place in the near-nucleus environment as they relate to the nature of nucleus. This is accomplisghed by measuring the spatial and temporal distribution of dust, gases and ions in the coma on high resolution images taken from many observatories around the world. By modeling the motions of discrete dust features in Comet Halley, it is often possible to determine the locations of the emission sources on the surface and learn about the nucleus structure. In addition to the general goals shared by all IHW nets, the scientific goals of the net has been to determine (1)the gross surface structure of the nucleus, (2)the nucleus spin vector, (3)the distribution and evolution of jet sources and (4)the interrelationships between the gas, dust and ion components of the coma. An additional Comet Giacobini-Zinner watch was carried out by the NNSN in support of the NASA International Cometary Explorer flyby.

Larson, S.↗
NASA NTRS1987

Noiseless coding for the magnetometer

Future unmanned space missions will continue to seek a full understanding of magnetic fields throughout the solar system. Severely constrained data rates during certain portions of these missions could limit the possible science return. This publication investigates the application of universal noiseless coding techniques to more efficiently represent magnetometer data without any loss in data integrity. Performance results indicated that compression factors of 2:1 to 6:1 can be expected. Feasibility for general deep space application was demonstrated by implementing a microprocessor breadboard coder/decoder using the Intel 8086 processor. The Comet Rendezvous Asteroid Flyby mission will incorporate these techniques in a buffer feedback, rate-controlled configuration. The characteristics of this system are discussed.

Rice, Robert F.↗
NASA NTRS1992

Properties of mass-loading shocks. II - Magnetohydrodynamics

The one-dimensional magnetohydrodynamics of mass-loading shocks is examined. These shocks, which are distinct from MHD shocks of classical nonreacting fluid dynamics and of combustion theory and which are characterized by the addition of mass within the shock transition, are to be found at comets and, depending upon circumstances, at nonmagnetized and weakly magnetized planets such as Venus and Mars. A completely general mass-loading form of the Hugoniot equation is derived, and some of the most important differences between mass-loading and nonreacting classical MHD shocks are identified. Two new types of MHD shocks are described which have no classical MHD analogues.

Zank, G. P.↗
NASA NTRS1994

The End of Comet Shoemaker-Levy 9

The pieces of the former Comet Shoemaker-Levy 9 struck Jupiter during the week of July 16-22 1994. We divide the general impact phenomena into four phases: (1) Atmospheric entry should have been accompanied by an entry flash, probably only detectable by the Galileo spacecraft. (2) Aerodynamic forces cause the impactor to explode. We show that the apparent altitude of the explosions is consistent with the analytical model proposed by Zahnle and Chyba et al, with numerical models calculated by Mac Low and Zahnle and Yabe et al., and with dynamically-based estimates of fragment masses Suggested by Scotti and Melosh and Asphaug and Benz. (3) The explosion produces a fireball that propagates most quickly up the wake, as predicted by Takata et al. and Crawford et al. The fireball's luminosity fades over a timescale of 30-60 seconds. (4) Each explosion produced a plume that reached thousands of kilometers above the cloud-tops. The plumes appear to have been dusty and SO illuminated when they reached sunlight. The plumes rose and fell over a period of ten or twenty minutes, governed by the simple laws of ballistics. When plume material (dust) fell back on the atmosphere it was reheated and glowed brightly in the thermal infrared for a period of order ten to twenty minutes; a large fraction of the impact energy was spent thus. Geometry introduces additional factors that complicate interpretation of the observed light curves.

Zahnle, K. J.↗
NASA NTRS2018

Constraining the Ratio of Micrometeoroids from Short- and Long-Period Comets at 1 AU from LADEE Observations of the Lunar Dust Cloud

We interpret recent observations of the secondary dust ejecta cloud around the Moon from the Lunar Dust Experiment (LDEX) on board the NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft with help from dynamical models of meteoroids. Results suggest that in order to match the spatial structure of observed ejecta profiles, the flux of meteoroids on the Moon must be primarily provided by short-period comets with an excess ratio of at least 1.3:1 compared to long-period comets. This ratio increases significantly if the dependence of the ejecta yield on impactor velocity is stronger than generally believed. The model accounts for the orbital geometry of LADEE and shows no indication of a large asymmetry in the meteoroid flux impacting from the Helion and Anti-Helion directions.

Janches, Diego↗
NASA NTRS1988

The volatile composition of comets

Comets may be our best probes of the physical and chemical conditions in the outer regions of the solar nebula during that crucial period when the planets formed. The volatile composition of cometary nuclei can be used to decide whether comets are the product of a condensation sequence similar to that invoked to explain the compositions of the planets and asteroids, or if comets are simply agglomerations of interstellar grains which have been insignificantly modified by the events that shaped the other bodies in the solar system. Although cometary nuclei are not generally accessible to observation, observations of cometary comae can illuminate at least some of the mysteries of the nuclei provided one has a detailed knowledge of the excitation conditions in the coma and also has access to basic atomic and molecular data on the many species present in comets. Examined here is the status of our knowledge of the volatile composition of cometary nuclei and how these data are obtained.

Weaver, H. A.↗
NASA NTRS2011

Stratospheric Collection of Dust from Comet 73P/Schwassmann-Wachmann 3

Interplanetary dust particles (IDPs) collected in the stratosphere are unique materials that are compositionally distinct from meteorites. Astronomical observations and dynamical models indicate that both asteroids and short-period comets are significant sources of IDPs. IDPs having fragile, porous structures, unequilibrated, anhydrous mineralogy, and high atmospheric entry velocities are thought to derive from comets, whereas asteroidal IDPs are identified by their compact structure, hydrated mineralogy and low atmospheric entry velocities. Uncertainty remains in the classification of asteroidal and cometary IDPs owing to our limited sampling of comets and the asteroid belt and the complex dynamical histories of most IDPs in space. Most IDPs spend thousands of years in space prior to being accreted by the Earth. During this time, dust particles undergo orbital evolution, including gradual reduction in their perihelion and eccentricity as a result of Poynting-Robertson drag. Planetary encounters may also significantly change their orbital parameters. Consequently, it is generally not possible to identify the specific parent body of a given IDP. However, it has been proposed that it is possible to identify dust from comets that have formed Earth-crossing dust trails. In this case, the dust particles have been in space for such a short period of time (a few decades or less) that their orbits have not significantly changed. Furthermore, these fresh IDPs could be identified in the laboratory from their short space-exposure histories (low solar noble gas abundance and lack of solar flare tracks). NASA flew several dedicated IDP collection missions attempting to collect dust from comet 26P/Grigg-Skjellerup, the best candidate identified. Remarkably, many particles from those collectors exhibit unusual properties, including low abundances of solar noble gases and high abundances of presolar grains. These observations are consistent with the dust particles originating from comet Grigg-Skjellerup (hereafter G-S). This study considers the prospects for collection of dust from comet 73P/Schwassmann-Wachmann 3 (hereafter SW3). SW3 is a small (2 km diameter) Jupiter family comet whose perihelion is close to and just inside the Earth's orbit. The orbit of SW3 is suitable for producing a low-velocity Earth-crossing dust stream and is the likely parent of the Tau Herculid meteor stream. This study complements a previously published model of the SW3 meteor stream that predicted a very low level of activity for grains 100 micron -- 100 mm in size.

Messenger, Scott R.↗
NASA NTRS1985

The Pioneer Venus extended mission

A history of the Pioneer Venus Orbiter is given, as well as a presentation of the ongoing and future missions (through reentry in 1992). General characteristics of Venus including atmospheric and ionospheric data are reviewed. Development of the Pioneer program and studies including comet detection, gamma ray bursts, composition and structure of the atmosphere, ionosphere, clouds, and solar wind are discussed. Planned observations of Halley's Comet in 1986 and of past observations of the Comet Enke are examined in detail. Maps and diagrams illustrate the studies and observations of this extended mission. Specification of the spacecraft as well as details of its orbits are given.

Craig, R. A.↗
NASA NTRS1992

0.7- to 23-micron photometric observations of P/Halley 2986 III and six recent bright comets

Infrared photometry over 0.7-23 micron range is presented for P/Halley 1986 III and six other recent bright comets. It is concluded that comets can be classified by their thermal infrared energy distributions. IR Type I comets have low continuum superheat and the 10-micron silicate emission is muted or absent. IR Type II comets have large continuum superheat and strong silicate emission features. Simultaneous measurements of P/Halley and Bradfield 1980 XV are generally consistent with the steady-state model for nuclear ablation. P/Halley's dust coma had an average albedo of 0.20 at a scattering angle of 130 deg. The correlation between superheat and 20-micron silicate excess, the flattened forward-scattering peak of the albedo curve, and the relatively low backscattering albedo are all consistent with laboratory and theoretical results for nonspherical and fluffy grains. The results appear to be especially consistent with core-mantle grain models such as those proposed by Greenberg and Hage (1989).

Gehrz, R. D.↗
NASA NTRS2003

Impact Tsunami Calculations: Hydrodynamical Simulations vs. Linear Theory

Tsunamis generated by the impacts of asteroids and comets into the Earth oceans are widely recognized as a potential catastrophic hazard to the Earth s population. Our general conclusion is that linear theory is a reasonably accurate guide to behavior of tsunamis generated by impactors of moderate size, where the initial transient impact cavity is of moderate depth compared to the ocean depth. This is particularly the case for long wavelength waves that propagate fastest and would reach coastlines first. Such tsunamis would be generated in the open ocean by impactors of 300 meters in diameter, which might be expected to strike the Earth once every few thousand years, on the average. Larger impactors produce cavities deep enough to reach the ocean floor; even here, linear theory is applicable if the starting point is chosen at a later phase in the calculation when the impact crater has slumped back to produce a cavity of moderate depth and slope.

Korycansky, E.↗
NASA NTRS2016

What is Eating Ozone? Thermal Reactions between SO2 And O3: Implications for Icy Environments

Laboratory studies are presented, showing for the first time that thermally driven reactions in solid H2O+SO2+O3 mixtures can occur below 150 K, with the main sulfur-containing product being bisulfate (HSO4(-)). Using a technique not previously applied to the low-temperature kinetics of either interstellar or solar system ice analogs, we estimate an activation energy of 32 kJ per mol for HSO4(-) formation. These results show that at the temperatures of the Jovian satellites, SO2 and O3 will efficiently react making detection of these molecules in the same vicinity unlikely. Our results also explain why O3 has not been detected on Callisto and why the SO2 concentration on Callisto appears to be highest on that world's leading hemisphere. Furthermore, our results predict that the SO2 concentration on Ganymede will be lowest in the trailing hemisphere, where the concentration of O3 is the highest. Our work suggests that thermal reactions in ices play a much more important role in surface and sub-surface chemistry than generally appreciated, possibly explaining the low abundance of sulfur-containing molecules and the lack of ozone observed in comets and interstellar ices.

Loeffler, Mark J.↗
NASA NTRS1994

The fragmentation of dust in the innermost comae of comets: Possible evidence from ground-based images

Dust particles when released from the nucleus of a comet are entrained in the expanding gas flow created by the vaporization of ices (mainly water ice). Traditional approaches to dusty-gas dynamics in the inner comae of comets consider there to be an initial distribution of dust particle sizes which do not fragment or evaporate. The standard Finson-Probstein model (and subsequent variations) yields a one-to-one-to-one correspondence between the size of a dust particle, its terminal velocity owing to gas drag, and its radiation pressure acceleration which creates the notable cometary dust tail. The comparison of a newly developed dust coma model shows that the typical elongated shapes of isophotes in the dust comae of comets on the scale of greater than 10(exp 4) km from the nucleus requires that the one-to-one-to-one relationship between particle size, terminal velocity and radiation pressure acceleration cannot in general be correct. There must be a broad range of particles including those having a small velocity but large radiation pressure acceleration in order to explain the elongated shape. A straightforward way to create such a distribution is if particle fragmentation, or some combination of fragmentation with vaporization, routinely occurs within and/or just outside of the dusty-gas dynamic acceleration region (i.e., up to several hundred km). In this way initially large particles, which are accelerated to fairly slow velocities by gas-drag, fragment to form small particles which still move slowly but are subject to a relatively large radiation pressure acceleration. Fragmentation has already been suggested as one possible interpretation for the flattened gradient in the spatial profiles of dust extracted from Giotto images of Comet Halley. Grain vaporization has been suggested as a possible spatially extended source of coma gases. The general elongated isophote shapes seen in ground-based images for many years represents another possible signature of fragmentation.

Combi, Michael R.↗
NASA NTRS1995

Meteor stream activity. 2: Meteor outbursts

In the past two centuries, alert amateur and professional meteor astronomers have documented 35 outbursts of 17 individual meteor streams well enough to allow the construction of a homogeneous set of activity curves. These curves add to similar profiles of the annual streams in a previous paper (Paper 1). This paper attempts to define the type and range of phenomena that classify as meteor outbursts from which the following is concluded: Outbursts are associated with the return of the comet to perihelion (near-comet type outbursts), but occur also when the parent comet is far from perihelion and far from the Earth (far-comet type). All outbursts of a given type only, depending on encounter geometry. The activity curves, expressed in terms of Zenith Hourly Rates (ZHR), have a shape that is generally well described by: ZHR = ZHR(sub max) 10(sup(-B (the absolute value of lambda (sub dot in a circle) - lambda (sup max) (sub dot in a circle))). The steepness of the slopes varies from an exponent of B = 7 to B = 220 per degree of solar longitude, with a typical value of B = 30. In addition, most near-comet type outbursts have a broader component underlying the main peak with B approximately 1 - 7.The duration Delta t is approximately 1/B of the main peak is almost independent of location near the comet, while the background component varies considerably in duration and relative intensity from one return to another. The two components in the activity curve are due to two distinct structures in the dust distribution near the parent comet, where the main component can be due to a sheet of dust that emanates from the IRAS dust trail. This brings the total number of distinct structures in meteor streams to four, including the two structures from the annual stream activity in Paper 1.

Jenniskens, P.↗
NASA NTRS1986

The orbit properties of colliding co-orbiting bodies

It is generally assumed that an ensemble of small bodies located in similar Keplarian orbits will, because of collisions, tend to disperse into more and more dissimilar orbits. For example, it is thought that the asteroids may represent the remnants of a few larger bodies that broke up or failed to fully accrete. A proposed experiment using the space station consists of an ensemble of small bodies or particles released gently from a central location in a large chamber, much like the breaking of billiard balls. The particles would them co-orbit and occasionally collide. Their subsequent behavior could be monitored by several video recorders, their linear and angular velocities before and after collision calculated, and their general behavior studied. This experiment could yield results of fundamental importance for theories of the origin of the planets, asteroids, comets, and probably ring systems.

Freeman, J. W.↗
NASA NTRS1982

OH fluorescence in comets - Fluorescence efficiency of the ultraviolet bands

The present investigation is concerned with the calculation of the fluorescence efficiency of OH over a wide range of heliocentric radial velocities. Three vibrational states in each of the two lowest electronic states are included in these calculations, producing a total of nine vibrational bands in the ultraviolet. The attempt is made to reproduce observed, ultraviolet, cometary spectra of OH by utilizing the Swings fluorescence mechanism. The inclusion of four rotational states within each of the three vibrational states and a detailed solar atlas make it possible to match observations very well, although there remain some discrepancies. This ability to reproduce observations provides confidence that the more general results obtained for the fluorescence efficiency as a function of heliocentric radial velocity can be used in interpreting both previous and future observations of OH in comets.

Schleicher, D. G.↗
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.↗