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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

A cometary and interplanetary dust experiment on the Vega spacecraft missions to Halley's Comet

The dust counter and mass analyzer on board both the Vega 1 and Vega 2 spacecraft scheduled to encounter Halley's Comet are discussed. The operational modes, data formats, telemetry modes and data acquisition, and scientific measurements planned for this experiment both at encounter and during interplanetary travel are described. The initial postlaunch calibration and experiment status are summarized in an appendix.

Perkins, M. A.↗
NASA NTRS1991

Cometary and interstellar dust grains - Analysis by ion microprobe mass spectrometry and other techniques

A survey of microanalytical measurements on interplanetary dust particles (IDPs) and interstellar dust grains from primitive meteorites is presented. Ion-microprobe mass spectrometry with its capability to determine isotopic compositions of many elements on a micron spatial scale has played a special role. Examples are measurements of H, N, and O isotopes and refractory trace elements in IDPs; C, N, Mg, and Si isotopes in interstellar SiC grains; and C and N isotopes and H, N, Al, and Si concentrations in interstellar graphite grains.

Zinner, Ernst↗
NASA NTRS1985

Trajectories of charged dust grains in the cometary environment

Using a simple model of the particles and fields environment of a comet, the trajectories of the smallest (micron- and submicron-sized) dust grains that are expected to be released from a cometary nucleus are calculated. It is shown that electromagnetic forces play a crucial role in the dynamics of these particles. The present calculations indicate not only the asymmetry of the sunward dust envelopes that have been suggested earlier by other authors, but they also indicate the possible existence of wavy dust features far down the tail, reminiscent of the peculiar wavy dust feature observed in the dust tail of Comet Ikeya-Seki 1965f. The importance of these findings in studying the lower end of the cometary dust mass spectrum during the forthcoming fly-by missions to Comet Halley is underscored.

Horanyi, M.↗
NASA NTRS1989

Role of dust to gas production rate ratio in cometary physics

According to results of Earth based observations and in situ measurements cometary nuclei are intensive sources of both the gaseous and the dusty matter. Investigations of the interaction between gas-dust cometary atmospheres and dust grains of the Zodiacal dust cloud reveals the presence of two principally possible mechanisms: meteor-like and explosive-type ones. The meteor-like mechanism is connected with bombardment of zodiacal dust particles by cometary molecules when the temperatures T = 2000 to 3000 K are developed. This mechanism can create over-saturated vapors of refractory elements (Fe, Si, C etc.) in cometary heads. The explosion-type mechanism is connected with high-velocity impacts between cometary and zodiacal dust particles resulting in the generation of high-temperature plasma (T = 10(exp 5) to 10(exp 7) K) and, hence, of x ray radiation and multicharged ions, i.e., with the processes not yet realized in the laboratory conditions.

Ibadov, S.↗
NASA NTRS1990

Carbon in comet dust

The association of Halley particle results with data from existing meteoritic materials that can be analyzed in the laboratory is discussed. Comet samples must exist in present collections of meteoritic materials and the Halley results provide clues for identifying them. Although it is not presently possible to positively identify cometary meteorites or cometary interplanetary dust (IDP) samples, it is possible to determine which materials are similar to Halley dust and which ones are distinctly unlike Halley. The properties of these existing Halley-compatible samples provide insight into the possible properties of cometary material. Positive identification of meteoritic comet samples or direct samples returned from a comet nucleus would of course revolutionize our ability to study carbonaceous matter in comets. Modern analytical techniques are very powerful and it is possible to perform elemental, chemical, mineralogical and even limited isotopic analysis on micron-size particles. There is an important synergism between the laboratory studies of collected samples and astronomical data from comets and interstellar grains. To fully interpret results there must be convincing methods for associating a particular class or classes of meteoritic material with comets. Ultimately this will be done by direct comet sample return such as the Rosetta mission under development by ESA. At the present time the only links that can be made involve comparison with sample properties and measurable properties of comets. Unfortunately there is at present no known unique property of cometary dust that allows its absolute identification in the laboratory. The results from Halley encounters and observation do provide much new information on cometary grains. The Halley grain compositions, density, size distribution and scattering properties all provide a basis for future investigations. Other Halley properties such as the presence of polyoxymethylene and the 3.4um emission feature could play key roles for making convincing links in the future.

Brownlee, D. E.↗
NASA NTRS1996

Dust and Sputtered Particle Streams in Cometary Atmosphere

Dynamics of dust particles in a comet atmosphere was investigated using Monte-Carlo simulation method taking into account radiation and gravitational forces applied to the particles. The model distribution function found out in VEGA experiments was chosen for calculations. Dust particles with masses more then 10(exp -10) g were considered. The delay effect for the big mass particles is shown. In this paper the boundary distribution function for sputtered particles drifting off the surface was determined using Nochilla's model. General dependences for the normal component of the relative mass velocity of sputtered particles are given as the function of particle energy normalized by Tomas-Fermi energy.

Svirschevsky, Stanislav B.↗
NASA NTRS1989

Atmospheric entry heating - A criterion to distinguish between asteroidal and cometary sources of interplanetary dust

Interplanetary dust samples obtained in the earth stratosphere by NASA sampling aircraft have been analyzed to ascertain the duration of their exposure as small particles in space, and the peak temperature they attained during atmospheric entry and deceleration. A three-dimensional orbital evolution model is then developed from these data which involves asteroidal and cometary sources for the dust as well as the influence of Poynting-Robertson drag. Particles from comets with perihelia greater than 1.2 AU typically undergo 600-800 C heating, while those from comets with smaller perihelia are heated to temperatures in excess of 800 C. Atmospheric-entry velocities inferred are consistent with the major fraction of stratospheric cosmic dust being derived from parent bodies in the main asteroid belt.

Flynn, G. J.↗
NASA NTRS2011

Inferring Sources in the Interplanetary Dust Cloud, from Observations and Simulations of Zodiacal Light and Thermal Emission

Interplanetary dust particles physical properties may be approached through observations of the solar light they scatter, specially its polarization, and of their thermal emission. Results, at least near the ecliptic plane, on polarization phase curves and on the heliocentric dependence of the local spatial density, albedo, polarization and temperature are summarized. As far as interpretations through simulations are concerned, a very good fit of the polarization phase curve near 1.5 AU is obtained for a mixture of silicates and more absorbing organics material, with a significant amount of fluffy aggregates. In the 1.5-0.5 AU solar distance range, the temperature variation suggests the presence of a large amount of absorbing organic compounds, while the decrease of the polarization with decreasing solar distance is indeed compatible with a decrease of the organics towards the Sun. Such results are in favor of the predominance of dust of cometary origin in the interplanetary dust cloud, at least below 1.5 AU. The implication of these results on the delivery of complex organic molecules on Earth during the LHB epoch, when the spatial density of the interplanetary dust cloud was orders of magnitude greater than today, is discussed.

Levasseur-Regourd, A. C.↗
NASA NTRS1985

Preliminary results of the Giotto radio science experiment

Doppler and ranging measurements using the radio signal of the Giotto spacecraft were taken before, during, and after the encounter with Comet Halley on Mar. 13, 14, 1986. The spacecraft velocity was found to decrease by a total of 23.3 cm/s due to impacting gas and (primarily) dust in the cometary atmosphere. A preliminary dust production rate of 1000 kg/s is found to be consistent with this deceleration. Power spectra of the carrier phase fluctuations reveal an increase in level and a flattening of the spectrum just prior to encounter, presumably associated with the enhanced dust impact rate. Finally, simulated Doppler time profiles are computed using the radial dependence of plasma density observed by the Giotto in situ investigations. It is shown that the cometary electron content profile would have been clearly seen if a dual-frequency downlink radio configuration had been available at encounter.

Edenhofer, P.↗
NASA NTRS1986

IR heating of the cometary atmosphere

The transfer of radiation, solar and IR dust, in the cometary atmosphere is analyzed using a six-band radiative transfer model. The first five bands correlate with the strongest IR band of H2O and the sixth corresponds to the bulk of solar energy. The radiation field for each band is calculated as a function of the cometocentric distance using the three-stream approximation. It is observed that the mean intensity of the IR radiation field generated by the dust and the cometary nucleus drops from a very large value of about 40,000 erg/sq cm sec at the surface to 400 erg/sq cm sec at 7 km, rises to a peak value of 500 erg/sq cm sec at 100 km and then falls off monotonically. The causes of these changes in radiation field intensity are discussed. The effect of variation of the IR radiation field on the gas temperature is studied.

Marconi, M. L.↗
NASA NTRS1989

Vega-Giotto flyby missions and cometary cosmogony

The most important implication of the Vega/Giotto flyby missions to Halley's Comet for cometary cosmogony is the opportunity to absorb the results of the in-situ measurements as made onboard the spacecrafts. Unfortunately the exploration of ejecta form the nucleus was unable to provide an unambiguous definition of the chemical-mineralogical nature of the nucleus: it failed to provide information comparable to that which was expected from a sample return mission. However, the obtained results are significant enough to affect and redirect cosmogonical thinking. Accordingly, the understanding of the cometary-matter dichotomy is modified as deduced from the distiction of water-dominated volitiles and silicate-based non-volitiles. Organic carbon compounds emerge as a major constituent of cometary nuclei. Presently, it is likely that the revision of Whipple's classic concept of the icy conglomerate cannot be avoided. Affected by the Vega/Giotto flyby missions to Hally's Comet, cometary cosmogony seems to enter a new conceptual period. The results of the in-situ measurements (mass spectrometric, UV spectroscopic, and IR spectroscopic) appear to be of basic importance. A chemical explanation is employed to explain the occurrence inside the nuclei of the variety of species, as inferred from the mass spectrometric data, to predict the results of the processes possibly involved. A cosmochemical factor is postulated to operate behind the observed cometary phenomena. The chemistry of the interstellar medium, covering the circumstellar and interstellar dust, advances cometary cosmogony.

Lang, Bruno↗
NASA NTRS2007

Interplanetary Dust Clouds Near 1AU Detected by STEREO

For a 21 day interval beginning on Jan 28, 2007, the radio and plasma wave instruments (S/WAVES) on the twin STEREO spacecraft detected intense impulse-like emissions in both the time-domain receivers and the low end of the frequency-domain receivers. These emissions were much more intense on the STEREO Ahead instrument, but were detected by both. During that interval, the two STEREO spacecraft were only separated by about 0.1deg in heliocentric longitude. All attempts to understand the emissions in terms of electrical disturbances onboard the spacecraft were unsuccessful and, since the impulsive events essentially ceased on Feb 18,2007, the entire study was put on lower priority. However, beginning abruptly on July 20, 2007, a very similar and more intense episode of impulses was detected exclusively by S/WAVES on the STEREO Ahead spacecraft, now separated from Behind by more than 20deg of heliocentric longitude. This new episode lasted nearly as long as the Jan-Feb episode, some 20 days, ending on Aug. 9. On Aug. 13,2007, the STEREO Behind spacecraft arrived at the same heliocentric longitude as the Ahead spacecraft was at on July 20 and the S/WAVES instrument abruptly started detecting impulsive events, although at lower intensities than those on Ahead. The individual waveform of the detected impulses is very similar to signals recorded by the Voyager plasma wave (PWS) and radio astronomy (PRA) instruments and more recently by the Cassini radio and plasma wave science (RPWS) instrument when those spacecraft plunged through Saturn's rings. Those Saturnian signals are attributed to micron-sized dust particle impacts on the spacecraft which creates a temporary plasma cloud which, in turn, induces an electrical signal on the antennas. We reanalyzed the STEREO episodes along the lines of the Saturnian ring plane analyses and have concluded that the STEREO impulses are very likely due to similar dust impacts with bands or clouds of dust of cometary or, more generally, zodiacal light material near 1 AU. The distribution of these clouds of small dust particles is significantly different than theoretical models of interplanetary dust distribution

Kaiser↗
NASA NTRS2003

Mid-Infrared Spectrum of the Zodiacal Emission: Detection of Crystalline Silicates in Interplanetary Dust

Within a few astronomical units of the Sun the solar system is filled with interplanetary dust, which is believed to be dust of cometary and asteroidal origin. Spectroscopic observations of the zodiacal emission with moderate resolution provide key information on the composition and size distribution of the dust in the interplanetary space. They can be compared directly to laboratory measurements of candidate materials, meteorites, and dust particles collected in the stratosphere. Recently mid-infrared spectroscopic observations of the zodiacal emission have been made by two instruments on board the Infrared Space Observatory; the camera (ISOCAM) and the spectrophotometer (ISOPHOT-S). A broad excess emission feature in the 9-11 micron range is reported in the ISOCAM spectrum, whereas the ISOPHOT-S spectra in 6-12 microns can be well fitted by a blackbody radiation without spectral features.

Ootsubo, T.↗
NASA NTRS1981

On the dust zoning of rapidly rotating cometary nuclei

The effects of nuclear rotation on the surface of a cometary nucleus (a comet at 1 AU that is H2O dominated and has a radius of 1 km) are considered. It is shown that this dust does not accumulate uniformly on the surface, which here is considered spherical. While dust particles in the two polar cap regions and an equatorial belt remain at rest on the surface, those in two midlatitude bands migrate toward the equator, stopping at the two low latitudes to form dust ridges. As the nucleus spins up, both the polar caps and the equatorial belt shrink in size, and the dust ridges move toward the equator, eventually spinning off the dust from the nucleus when the nuclear rotation period is less than about 3.3 hr. For larger particles for which the gas buoyancy is negligible, migration takes place only if the rotation period is not significantly larger than the critical value of 3.3 hr or if the surface friction is abnormally small.

Houpis, H. L. F.↗
NASA NTRS1996

Kuiper Belt Dust Grains as a Source of Interplanetary Dust Particles

The recent discovery of the so-called Kuiper belt objects has prompted the idea that these objects produce dust grains that may contribute significantly to the interplanetary dust population. In this paper, the orbital evolution of dust grains, of diameters 1 to 9 microns, that originate in the region of the Kuiper belt is studied by means of direct numerical integration. Gravitational forces of the Sun and planets, solar radiation pressure, as well as Poynting-Robertson drag and solar wind drag are included. The interactions between charged dust grains and solar magnetic field are not considered in the model. Because of the effects of drag forces, small dust grains will spiral toward the Sun once they are released from their large parent bodies. This motion leads dust grains to pass by planets as well as encounter numerous mean motion resonances associated with planets. Our results show that about 80% of the Kuiper belt grains are ejected from the Solar System by the giant planets, while the remaining 20% of the grains evolve all the way to the Sun. Surprisingly, the latter dust grains have small orbital eccentricities and inclinations when they cross the orbit of the Earth. This makes them behave more like asteroidal than cometary-type dust particles. This also enhances their chances of being captured by the Earth and makes them a possible source of the collected interplanetary dust particles; in particular, they represent a possible source that brings primitive/organic materials from the outer Solar System to the Earth. When collisions with interstellar dust grains are considered, however, Kuiper belt dust grains around 9 microns appear likely to be collisionally shattered before they can evolve toward the inner part of the Solar System. The collision destruction can be applied to Kuiper belt grains up to about 50 microns. Therefore, Kuiper belt dust grains within this range may not be a significant part of the interplanetary dust complex in the inner Solar System.

Liou, Jer-Chyi↗
NASA NTRS1977

Dust evolution from comets

The studies of the evolution of cometary debris are reviewed. The subject is divided into three major sections: (1) the developments in the immediate vicinity of the cometary nucleus, which is the source of the dust; (2) the formation of the dust tail; and (3) the blending of the debris with the dust component of interplanetary matter. The importance of the physical theory of comets is emphasized for the understanding of the early phase of the evolution of cometary dust. A physico-dynamical model designed to analyze the particle-emission mechanism from the distribution of light in the dust tails is described and the results are presented. Increased attention is paid to large particles because of their importance for the evolution of the zodiacal cloud. Finally, implications are discussed for the future in situ investigations of comets.

Sekanina, Z.↗
NASA NTRS2009

Comet 81p/Wild 2: The Updated Stardust Coma Dust Fluence Measurement for Smaller (Sub 10-Micrometre) Particles

Micrometre and smaller scale dust within cometary comae can be observed by telescopic remote sensing spectroscopy [1] and the particle size and abundance can be measured by in situ spacecraft impact detectors [2]. Initial interpretation of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft [3] appears to show that very fine dust contributes not only a small fraction of the solid mass, but is also relatively sparse [4], with a low negative power function describing grain size distribution, contrasting with an apparent abundance indicated by the on-board Dust Flux Monitor Instrument (DFMI) [5] operational during the encounter. For particles above 10 m diameter there is good correspondence between results from the DFMI and the particle size inferred from experimental calibration [6] of measured aerogel track and aluminium foil crater dimensions (as seen in Figure 4 of [4]). However, divergence between data-sets becomes apparent at smaller sizes, especially submicrometre, where the returned sample data are based upon location and measurement of tiny craters found by electron microscopy of Al foils. Here effects of detection efficiency tail-off at each search magnification can be seen in the down-scale flattening of each scale component, but are reliably compensated by sensible extrapolation between segments. There is also no evidence of malfunction in the operation of DFMI during passage through the coma (S. Green, personal comm.), so can the two data sets be reconciled?

Price, M. C.↗