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Showing papers by "Jet Propulsion Laboratory published in 1986"


Journal ArticleDOI
15 May 1986-Nature
TL;DR: The mass spectra of cometary dust particles measured by the PIA dust particle analyzer aboard the Giotto spacecraft showed some unexpected and striking features as mentioned in this paper, such as small particles below 10 to the -14th g are much more abundant than anticipated by models.
Abstract: Mass spectra of cometary dust particles measured by the PIA dust particle analyzer aboard the Giotto spacecraft show some unexpected and striking features. First, small particles below 10 to the -14th g are much more abundant than anticipated by models. Second, most of the particles are rich in light elements such as H, C, N, and O, suggesting the validity of models that describe the cometary dust as including organic material. Third, the light elements specifically seem to have a low ratio of mass to volume. Three examples of original mass spectra showing typical compositions are given; these have been measured, and are compared with a computer-simulated mass spectrum.

411 citations


Journal ArticleDOI
15 May 1986-Nature
TL;DR: The Giotto space probe's ion mass spectrometer has obtained data on the composition and velocity distributions of cometary ions at distances of between 7.5 million and 1300 km from the comet Halley nucleus as mentioned in this paper.
Abstract: The Giotto space probe's ion mass spectrometer has obtained data on the composition and velocity distributions of cometary ions at distances of between 7.5 million and 1300 km from the comet Halley nucleus. Solar wind He(2+) was found throughout the coma, as close as 5000 km, with the He(+) produced by charge exchange being within about 200,000 km. A pile-up of heavy cometary ions was found at about 10,000 km from the nucleus. Inside the contact surface, which was found at about 4600 km, ion temperatures as low as about 340 K and outflow velocities of about 1 km/sec were found.

391 citations


Journal ArticleDOI
01 Jun 1986-Nature
TL;DR: In this article, the authors have studied surface sediments from the Santa Barbara Basin and reported the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra-fine-grained, single-domain magnetite.
Abstract: Remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time-scale, study geomagnetic reversals and secular variation, and measure the rates of seafloor spreading. Although these sediments may contain different magnetic minerals, magnetite is the most commonly observed and magnetically stable phase, and its size, shape and post-depositional fate affect the magnetic remanence of the sediments. Biogenic magnetites are single-domain, with a high natural magnetic remanence (NRM), and have been suggested as a significant source of magnetic remanence in marine sediments. We have studied surface sediments from the Santa Barbara Basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra-fine-grained, single-domain magnetite. Using a novel extraction technique, transmission electron microscopy and SQUID magnetometry, we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments.

218 citations


Journal ArticleDOI
TL;DR: In this article, the relationship between the morphological parameters of lava flows and the process parameters, such as lava composition, eruption rate, and eruption temperature, was investigated using literature data on Hawaiian lava flows.

188 citations


Journal ArticleDOI
20 Mar 1986-Nature
TL;DR: The concept of primordial rubble piles as discussed by the authors was proposed as a modification of the basic Whipple model, not as a replacement for it, and is analogous to the "rubble pile" model suggested for the larger main-belt asteroids, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris.
Abstract: Whipple's icy conglomerate model for the cometary nucleus has had considerable sucess in explaining a variety of cometary phenomena such as gas production rates and nongravitational forces. However, as discussed here, both observational evidence and theoretical considerations suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or even frequent disruptive events. The proposed model is analogous to the 'rubble pile' model suggested for the larger main-belt asteroids, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case. The concept of cometary nuclei as primordial rubble piles is proposed as a modification of the basic Whipple model, not as a replacement for it.

150 citations


Journal ArticleDOI
15 May 1986-Nature
TL;DR: In this paper, the density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system, showing depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses.
Abstract: The density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system. The dust spectrum obtained at 291,000 km from the comet nucleus show depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses. Most of the mass striking Giotto is noted to reside in the few large particles penetrating the dust shield. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact.

123 citations


Journal ArticleDOI
01 Feb 1986-Icarus
TL;DR: In this article, an upper haze layer of finite optical depth and a lower cloud layer of infinite optical depth at discrete altitudes are used to bound the wavelength-averaged phase integrals and bolometric albedos of Uranus and Neptune.

90 citations


Proceedings Article
11 Aug 1986
TL;DR: An implemented computer program which is part of the execution monitoring process for an experimental robot system is described, which analyzes a plan and automatically inserts appropriate perception requests into the plan and generates anticipated sensor values.
Abstract: This paper addresses the problem of verifying plan execution. An implemented computer program which is part of the execution monitoring process for an experimental robot system is described. The program analyzes a plan and automatically inserts appropriate perception requests into the plan and generates anticipated sensor values. Real-time confirmation of these expectations implies successful plan execution. The implemented plan verification strategy and knowledge representation are described. Several issues and extensions of the method are discussed, including a language for plan verification, heuristics for constraining plan verification, and methods for analyzing plans at multiple levels of abstraction to determine context-dependent verification strategies.

84 citations


Journal ArticleDOI
TL;DR: There was a consistent shift in spectral statistics in the course of the three bursts of EMG, and the center frequency and the variance of the spectral energy distribution grew from the first burst to the second burst in the same muscle.
Abstract: We applied short-time Fourier analysis to surface electromyograms (EMG) recorded during rapid movements, and during isometric contractions at constant forces. We selected a portion of the data to be transformed by multiplying the signal by a Hamming window, then computed the discrete Fourier transform. Shifting the window along the data record, we computed a new spectrum each 10 ms. We displayed the transformed data in spectograms or "voiceprints." This short-time technique allowed us to see time-dependencies in the EMG that are normally averaged in the Fourier analysis of these signals. Spectra of EMG's during isometric contractions at constant force vary in the short (10-20 ms) term. Moments of the spectral distribution show this variability. Short-time spectra from EMG's recorded during rapid movements were much less variable. The windowing technique picked out the typical "three-burst pattern" in EMG's from both wrist and head movements. Spectra during the bursts were more consistent than those during isometric contractions. Furthermore, there was a consistent shift in spectral statistics in the course of the three bursts. Both the center frequency and the variance of the spectral energy distribution grew from the first burst to the second burst in the same muscle. We discuss this pattern with respect to the origin of the EMG bursts in rapid movement. We also extend the analogy between electromyograms and speech signals to argue for future applicability of short-time spectral analysis of EMG.

82 citations



Journal ArticleDOI
TL;DR: Biconical diffuse reflectance spectra in the mid-infrared are presented for powder and other solid forms of sulfur, gold, potassium bromide, magnesium oxide, and Halon and recommendations are made regarding the relative usefulness of these materials as reflectance standards in themid-IR.
Abstract: Biconical diffuse reflectance spectra in the mid-infrared are presented for powder and other solid forms of sulfur, gold, potassium bromide, magnesium oxide, and halon. Comparisons are made with previously published results of other investigators, and recommendations are made regarding the relative usefulnees of these materials as reflectance standards in the mid-IR. Sulfur has strong intrinsic bands at wavelengths greater than 7 microns that must be taken into account for its use as a reflectance standard. Some sulfur samples have hydrocarbon contaminants and in powder form may have adsorbed water, both of which produce bands in the 3-4-micron region. Potassium bromide has several weak intrinsic bands and is very sensitive to adsorbed water contamination; otherwise it is a good IR reference material. Magnesium oxide and halon have major bands structure and low reflectivity at wavelengths greater than 2.6 microns and thus are unsuitable as reference materials in the mid-IR. Vapor-deposited gold on fine sandpaper (600 grit) is very bright, spectrally flat, and fairly diffuse, so it is the superior material (of those examined) for reflectance reference material throughout the IR. Fine gold powder, on the other hand, is much less bright than evaporated gold, and its reflectivity at wavelengths greater than its particle size is highly sensitive to particle packing density.

Journal ArticleDOI
01 Jan 1986-Icarus
TL;DR: In this paper, a multispectral mosaic of the Galilean satellite Europa was used to map surface units with similar optical properties, color assignments are indicative of the spectral nature of the given unit.

Journal ArticleDOI
TL;DR: In this article, the mathematical basis for radar stereomapping, along with preliminary results from one of the methods (using a so-called "analytical" stereoplotter) for the Mt. Shasta site were described.
Abstract: One of the goals of the SIR-B experiment was the definition of optimum radar incidence and intersection angles for radargrammetric stereoscopy by comparing the results from three separate data reduction methods. To this end three overlapping images of the prime radargrammetric site (Mt. Shasta in northern California) were obtained, fewer than anticipated. This paper describes the mathematical basis for radar stereomapping, along with preliminary results from one of the methods (using a so-called "analytical" stereoplotter) for the Mt. Shasta site. Height coordinate errors range from 60 to 170 m depending on the density (number per ground area) of ground control points used.

Journal ArticleDOI
TL;DR: On October 5, 1984, the second Shuttle Imaging Radar (SIR-B) was launched into orbit aboard the space shuttle Challenger, providing a means of acquiring multiple incidence angle imagery over specific targets on successive days of the mission.
Abstract: On October 5, 1984, the second Shuttle Imaging Radar (SIR-B) was launched into orbit aboard the space shuttle Challenger. SIR-B is part of an evolutionary radar program designed to progressively develop a multifrequency, multipolarization synthetic aperture radar with a variable Earth-imaging geometry. The SIR-B instrument is an upgraded version of SIR-A, with the additional capability of tilting the antenna mechanically to acquire imagery at variable incidence angles ranging from 15°to 60°.The variable look angle capability provided a means of acquiring multiple incidence angle imagery over specific targets on successive days of the mission. These data are being used to classify surface features by their backscatter signatures as a function of incidence angle and for topographic mapping. In addition to the antenna tilt capability, a digital data-handling system was added to increase the dynamic range, the resolution was improved by a factor of two over SIR-A, and a calibration subsystem was added to improve the radiometric accuracy of the data. The mission had a number of problems, including loss of the primary mary digital data path between the shuttle and the ground. In spite of these problems, approximately 20 percent of the planned digital data were collected over the 8-day shuttle mission corresponding to an areal coverage of about 6.4 million km2.

Journal ArticleDOI
01 Apr 1986
TL;DR: The encounter between the ICE spacecraft and Comet P/Giacobini-Zinner was characterized in the solar wind by intense fluxes of heavy ions, measurable over a region 6 x 10 to the 6th km in extent as mentioned in this paper.
Abstract: The encounter between the ICE spacecraft and Comet P/Giacobini-Zinner was characterized in the solar wind by intense fluxes of heavy ions, measurable over a region 6 x 10 to the 6th km in extent. The ions are observed with highly anisotropic angular distributions, steep energy spectra, and a change in the energy spectrum at around 80 keV, consistent with a composition predominantly of the water group. Flux versus time profiles follow a general fall off with increasing distance from the comet, but with a marked inbound/outbound asymmetry. This asymmetry is due to the higher solar wind velocity on the outbound pass, giving rise to an increased energy gain of the pick-up ions. The flux versus time profiles are strongly modulated by the rapid changes in the direction of interplanetary magnetic field. Correlated observations of energetic ions, the interplanetary magnetic field and the solar wind are presented, and these observations are compared with theoretical predictions of the ion pick-up process.

Journal ArticleDOI
TL;DR: In this article, four overlapping SIR-B radar images were obtained across southern Argentina; these form a total of six stereo models with intersection angles ranging from 5° to 23°.
Abstract: Four overlapping SIR-B radar images were obtained across southern Argentina; these form a total of six stereo models with intersection angles ranging from 5° to 23°. This data set is uniquely suited for experimental evaluation of some basic assumptions on stereo -radargrammetry. Each stereo model was measured on a specially programmed photogrammetric analytical plotter; the resulting coordinates of ground points were compared with those from maps. It is concluded that accuracies are lower than expected at the larger stereointersection angles, amounting to about ±60 m in each coordinate direction. This might be explained by limitations of the quality of stereofusion caused by look angle differences and specular point migration, backscatter differences due to different incidence angles, differences in azimuth directions, and image noise and speckle.

Journal ArticleDOI
15 May 1986-Nature
TL;DR: The plasma-wave probe carried by the spacecraft Sakigake discovered discrete spectra of emissions from Comet Halley in the frequency range 30-195 kHz as discussed by the authors, where the observed cometary kilometric radiation appears to come from moving shocks in the coma region, possibly associated with temporal variations of the solar wind.
Abstract: The plasma-wave probe carried by the spacecraft Sakigake discovered discrete spectra of emissions from Comet Halley in the frequency range 30-195 kHz. The observed cometary kilometric radiation appears to come from moving shocks in the coma region which are possibly associated with temporal variations of the solar wind. Waves due to plasma instabilities associated with the pick-up of cometary ions by the solar wind were observed within a region almost 10 million km from the comet nucleus.

Journal ArticleDOI
TL;DR: In this paper, a method for analyzing arbitrary one-dimensional waveguides is described and applied to find the lateral modes of gain-guided laser arrays directly, without the need for a coupled-mode approximation.
Abstract: A method for analyzing arbitrary one-dimensional waveguides is described and applied to find the lateral modes of gain-guided laser arrays directly, without the need for a coupled-mode approximation. Detailed results are given for a four-element array for which experimental results are available, and the effect of varying device parameters on the relative gains of the modes is considered. The results show differences between gain-guided and real-index guided arrays which are not evident in prior analyses.

Journal ArticleDOI
TL;DR: In this article, a subsolar standoff distance of 4,000 km is computed for a comet gas production rate of 3 x 10 to the 28th molecules/s in accord with ground-based determination of G.
Abstract: Electron heat flux observations and associated plasma phenomena upstream from Comet Giacobini-Zinner show many similarities to observations upstream from the earth's magnetosheath. Two similarities are discussed. Heat flux events are used to compute the transition region boundary location. The computed subsolar standoff distance of 4,000 km is commensurate with a comet gas production rate, G, of 3 x 10 to the 28th molecules/s in accord with ground-based determination of G.

Proceedings ArticleDOI
14 Jul 1986
TL;DR: In this article, a theoretical model is presented which predicts the output response of a CCD to soft X-ray spectra, and compared the results of the analysis to an existing state-of-the-art CCD and project improvements which will be made in the near future.
Abstract: A theoretical model is presented which predicts the output response of a CCD to soft X-ray spectra. The model simulates the four fundamental parameters that ultimately limit CCD performance: Quantum efficiency, charge collection efficiency, charge transfer efficiency, and read noise. Simulated results are presented for a wide variety of CCD structures, and general conclusions are presented about achieving a practical balance of sensitivity, energy, and spatial resolution for an AXAF instrument. We compare the results of the analysis to an existing state-of-the-art CCD and project improvements which will be made in the near future.

Journal ArticleDOI
01 Feb 1986-Fuel
TL;DR: In this paper, the combustion of single droplets of coal-water slurries was studied experimentally by determining the instantaneous size and mass of the droplets and theoretically modelled for the char agglomerate by allowing for the presence of the ash layer as well as the residual volatiles.

Journal ArticleDOI
TL;DR: Sinking rate is an important selective loss process in temperate coastal areas which can be simulated by using this selective loss, semi-continuous culturing technique and the advantages of using this technique to study the effects of sinking rate and its interaction with nutrients on succession in natural phytoplankton assemblages are discussed.

Book ChapterDOI
01 Jan 1986
TL;DR: The first observations of interplanetary scintillations with the VLA telescope are reported in this article, where the solar wind in the accelerating region from 3 to 12 solar radii was observed by scintillation of the radio source 3C279.
Abstract: The first observations of interplanetary scintillations with the VLA telescope are reported. The solar wind in the accelerating region from 3 to 12 solar radii was observed by scintillation of the radio source 3C279. The results obtained outside of 7 solar radii showed good agreement with previous work but observations between 3 and 4.5 solar radii were new and unexpected. Turbulence in the solar wind has a spatially anisotropic structure elongated in the radial direction, the flow direction being also in the radial direction. An abrupt change of both the velocity and the spatial anisotropy of turbulence was found at distances from 3 to 4.5 solar radii. There is a large random velocity component inside of 12 solar radii which is comparable to the bulk flow speed, and it has a spatially anisotropic probability distribution. From the measured cross-correlation functions we found evidence which may be related to the complex structure of the magnetic field.

Proceedings ArticleDOI
01 Jan 1986
TL;DR: The overall optical navigation system and operation at Uranus are outlined in this article, where the formulation of the optical observables are presented, as well as their subsequent integration into the overall orbit determination products.
Abstract: Optical observations were necessary during the Voyager II Uranus approach and encounter in order to meet mission navigational requirements. In this paper, the overall optical navigation system and operation at Uranus are outlined. The formulation of the optical observables are presented, as well as their subsequent integration into the overall orbit determination products. Optical data analysis and some resulting orbit determination results from Uranus are shown. Certain scientifically important results were natural outcomes of the optical data analysis, and these are also reviewed. Finally, certain optical navigation related problems are anticipated at the forthcoming Neptune encounter in 1989; these are briefly discussed as are plans to deal with them.

Journal ArticleDOI
TL;DR: In late March 1986, the International Cometary Explorer (ICE) detected sporadic bursts of strong plasma turbulence with average wave characteristics very similar to those detected six months earlier, during the ICE traversal of the Comet Giacobini-Zinner (G-Z) heavy ion pick-up region as discussed by the authors.
Abstract: In late March 1986 the plasma wave instrument on the International Cometary Explorer (ICE) detected sporadic bursts of strong plasma turbulence with average wave characteristics very similar to those detected six months earlier, during the ICE traversal of the Comet Giacobini-Zinner (G-Z) heavy ion pick-up region. In both cases the observations of enhanced wave levels were generally correlated with simultaneous detection of energetic ions. The 1986 activity is interpreted in terms of plasma instabilities associated with solar wind pick-up of ions produced by heavy neutrals from Comet Halley. On March 25, when the distance between Comet Halley and ICE was 28.1 million kilometers, the ICE-to-comet range was about six times greater than the distance that marked the measured outer boundary of the turbulent heavy ion pick-up region of G-Z. Based on comparison with G-Z data and with earlier Halley observations, plausible arguments suggest that in late March, Halley should have produced detectable levels of energetic ions and associated plasma turbulence in a region with a spatial extent of 30-40 million kilometers.

Journal ArticleDOI
TL;DR: In this article, the nature of the comet-solar wind interaction is studied by analyzing the detailed evolution of the plasma wave spectra of Comet Giacobini-Zinner across the interaction region.
Abstract: The nature of the comet-solar wind interaction is studied by analyzing the detailed evolution of the plasma wave spectra of Comet Giacobini-Zinner across the interaction region. Electron heat fluxes and associated electron plasma waves, steepened low-frequency wave packets, and density fluctuations observed upstream of Giacobini-Zinner shocks are also found upstream of quasi-parallel bow shocks. Downstream, the pulsations in the cometary magnetic field magnitude, in addition to the large density spikes, are usually also found downstream of quasi-parallel bow shocks. Other similarities to interplanetary shocks and terrestrial bow shocks are described.

Journal ArticleDOI
TL;DR: The atmospheric sounder for the Mars Observer mission is described and how it will measure key Martian atmospheric parameters using IR gas correlation and filter radiometry.
Abstract: The study of the atmosphere and climate of Mars will soon be advanced considerably by the Mars Observer mission. This paper describes the atmospheric sounder for this mission and how it will measure key Martian atmospheric parameters using IR gas correlation and filter radiometry. The instrument now under development will provide high-resolution vertical profiles of atmospheric temperature, pressure, water vapor, dust, and clouds using limb sounding techniques as well as nadir observations of surface thermal properties and polar radiative balance.


Journal ArticleDOI
TL;DR: Within 100,000 km of comet Giacobini-Zinner's nucleus, strong plasma wave turbulence was detected by the ICE electric and magnetic field wave instruments as mentioned in this paper, and the spatial profiles of the wave amplitudes are compared with measurements of the heavy ion fluxes of cometary origin, the plasma electron density, and the magnetic field strength.
Abstract: Within 100,000 km of comet Giacobini-Zinner's nucleus, strong plasma wave turbulence was detected by the ICE electric and magnetic field wave instruments. The spatial profiles of the wave amplitudes are compared with measurements of the heavy ion fluxes of cometary origin, the plasma electron density, and the magnetic field strength. The general similarity of the wave and heavy ion profiles suggest that the waves might be generated by free energy in the pick-up ion distribution function. However, the expected parallel streaming instability of electrostatic modes generates waves with frequencies that are too low to explain the observations. The observed low frequency magnetic turbulence is plausibly explained by the lower hybrid loss-cone instability of heavy ions.

Proceedings ArticleDOI
13 Oct 1986
TL;DR: In this article, preliminary findings are presented for a new approach that significantly improves the quantum efficiency of the current generation of high-performance, thinned, backside illuminated silicon CCDs.
Abstract: Preliminary findings are presented for a new approach that significantly improves the quantum efficiency of the current generation of high-performance, thinned, backside illuminated silicon CCDs. Experiments have shown that the application of a less than 4-micron thick layer of metal with high work function to the backside of the CCD can yield 100-percent internal quantum efficiency in the visible, UV, XUV and soft X-ray regions of the spectrum. Theory and solid state models describing the new technique (the 'CCD flash gate'), and a considerable amount of experimental data, are discussed. Specific recommendations for use of the flash gate in present and future CCDs are also reviewed.