A preliminary look at the dust in comet Halley

Highlights of infrared observations of the dust are discussed and compared with first results from the space probes. An emission feature was detected at 3.4 μm; the 10 and 20 μm silicate features were well-observed; and far-infrared data out to 160 μm were obtained. Organic material seems to be abundant in grains and may explain the 3.4 μm emission. Calculations are presented for one example of organic material. A component of the grains may volatize at temperatures around 300 K.     

Ground-based observations of the dust emission from comet Halley

A preliminary analysis of the dust emission from comet Halley is presented based on large scale observations of its dust tail. Selected images obtained between February 22 and May 10, 1986 are compared to synchrone-syndyne graphs to infer the history of the dust production and the properties of the dust, at least qualitatively. Quantitative modeling of the dust tall has also been initiated and preliminary results are shown for the cases of isotropic and anisotropic (jet) dust production.     

Coma morphology and dust emission pattern of comet Halley

This contribution starts with a short overview on cometary dust modelling and then focuses on the application of coma modelling with respect to in-situ measurements of cometary dust and ground based observations. The fountain model, valid for the dynamics of small cometary dust particles, is discussed. Models using Keplarian theory for the motion of the dust particles are outlined and the ESOC coma model is presented. Some direct applications of this model to analyse the results of the recent spacecraft flybys of comet Halley, as dust flux profiles, particle ground tracks and envelope positions, are shown. To compare the model with ground-based astronomical observations, the utilization of the ESOC coma model for the generation of synthetic images is demonstrated and some future prospects of this technique are outlined.     

Charges on dust particles

We discuss the potential (charge) on dust particles in various environments. We first consider the classical case of a single isolated dust particle. In conditions which apply to planetary dust rings, the exact value of the dust potential depends critically on several effects (e.g. secondary electron emission, photoelectric efficiency) which are not well known for small dust particles of relevant material and surface conditions. In dust clouds of high dust densities the classical approach fails to give the correct value of the dust potential due to the neglect of collective effects. In terms of an ordering parameter P = a_μN_d0/n₀ (dust radius in microns × cloud dust density/exterior plasma density) the collective effects on the dust potential become apparent at P ～ 10^?6. For increasing values of P the collective effects increase, whence the dust potentials decrease and eventually approach zero.     

Giotto/Cometary dust interaction event near the comet Halley ionopause

Plasma and magnetic field disturbances accompanying dust particle impacts are explained by means of creation of a secondary cloud around the spacecraft. Cold cometary ions impinging upon the cloud are scattered by atoms of the cloud. This scattering changes initial angular distribution of cometary ions. Magnetic field perturbation is created by the friction between the electron component of the cometary plasma flow and the cloud.     

Entry of dust particles into planetary magnetospheres

While interplanetary dust constitutes a primary source of cosmic particulate matter in planetary magnetospheres, the debris produced by its impact with small satellites and ring material provides an important secondary source. Internal processes, such as volcanic activity, particularly in the smaller satellites, could result in a third source. In the case of the terrestrial magnetosphere there are also artificial (internal) sources: 1–10μ sized A?₂O₃ particles injected by solid rocket mortar burns between near earth and geosynchronous orbit constitute one such source, while the fragments of larger bodies (artificial satellites) due to explosions (e.g., “killer satellites”) and collisions constitute another. Finally, if we include the purely induced cometary magnetosphere among planetary magnetospheres, the injection of cometary dust into it due to entrainment by the outflowing gases constitutes another source.As a result of being immersed in a radiative and plasma environment these dust grains get electrically charged up to some potential (positive or negative). Particularly in those regions where the magnetospheric plasma is hot and dense and their own spatial density is low, the dust grains could get charged to numerically large negative potentials.While this charging may have physical consequences for the larger grains, such as electrostatic erosion (“chipping”) and disruption, it also can effect the dynamics of the smaller grains. Indeed, the small but finite capacitance of these grains, which leads to a phase lag in the gyrophase oscillation of the grain potential, could even lead to the permanent magneto-gravitational capture of interplanetary grains within planetary magnetospheres in certain situations. Here we will review the sources of dust in planetary magnetospheres and discuss their physics and their dynamics under the combined action of both planetary gravitational and magnetospheric electromagnetic forces.     

In-situ photopolarimetric measurements of dust and gas in the coma of Halley''s comet

The Halley Optical Probe Experiment (HOPE) on board the Giotto spacecraft has provided the first in-situ measurements, both of the dust and of some gaseous species, from inside the coma of the comet. The instrument has already been described /1/, together with first results /2/. The purpose of this note is to show how optical measurements can lead to in-situ information, how those were obtained during the 13–14 march 1986 Halley fly-by, and what is the status of the data analysis.     

A European probe to comet Halley

A European probe to comet Halley is proposed. The probe's model payload consists of 8 scientific instruments, viz. neutral, ion and dust impact mass spectrometers, magnetometer, medium energy ion and electron analyzer, camera, dust impact detectors and plasma wave experiment. Fly-by of the comet Halley nucleus will take place on November 28th, 1985, at about 500 km miss distance. The main spacecraft serves as relay link to transmit the observed data to Earth. As probe, a modified ISEE 2 design is proposed. Because of the cometary dust hazard expected in the coma a heavy dust shield (27 kg) is required, consisting of a thin front sheet and a 3 layer rear sheet. The probe is spin-stabilized (12 rpm), has no active attitude and orbit control capability and uses battery power only to provide about 1000 Wh for a measuring phase. A despun antenna transmits up to 20 kbit/s, in X-band. The total probe mass is estimated at 250 kg. The 3 model development programme should start in mid 1981 with Phase B.     

A capacitor impact sensor (CIS) on board Giotto for detection of cometary dust particles

A low power high reliability impact sensor based on the discharge of a parallel plate capacitor is described. The choice of a surface area of about 1000 cm² and a penetration thickness of 50 micrometers will provide data on the flux density of cometary dust particles in the 5 micrometers diameter range (10⁻¹⁰g). A high noise immunity promotes excellent reliability under conditions of heavy spacecraft bombardment and high plasma densities in the late stages of the 500 km approach distance. Self-limiting of the event rate compression system also provides flux data at arbitrarily high impact rates. The capacitor sensor will be located on the external face of the outer dust shield of Giotto Spacecraft and it will be a part of the DIDSY experiment.     

A quantitative model for comet nucleus topography

Topography of the surface of a comet nucleus is likely rough at all scales smaller than the mean effective radius. We present a flexible and easily scalable model for quantitative calculations simulating the effects of comet nucleus topography on gas release and dust mantle evolution. The topographic features we describe must be large enough (typically> 10 m) so that they will not erode in one orbit of the nucleus around the Sun. The maximum effective size of a hill is about 1/√2 times the effective radius of the nucleus. If it is larger, then an ellipsoidal shape of the nucleus is more appropriate. The procedure described here also permits for inhomogeneous composition of the topographic features, leading to locally different rates of gas production (e.g., jet-like features and filaments) or different thicknesses of the dust mantle. It also can give rise to different temperature patches, locally varying albedos and emissivities, and may explain the formation of permanent dust mantles.     

Nucleus studies of comet Halley

Early results from, and research initiatives warranted by, the Earth-based observations of Halley's near-nucleus and related phenomena are reviewed. Where appropriate, this information is combined with spacecraft data obtained by the various flight projects. The basic objective is to gain a greater insight into the nature of the comet's nucleus and its environment. Among topics are the brightness variations at large heliocentric distances along the inbound leg of the orbit; the bulk and rotational properties of the nucleus, including possible precession; the surface morphology and the formation of dust jets; subfemtogram dust particles and their presence in a sunward spike and relation to CN jets; comparison of the hydrogen coma's “pulsation” pattern with the surface distribution of major dust vents; the events causing Giotto's wobbling near its closest approach to the comet; and the recent developments in theoretical modeling of the icy-conglomerate nucleus.     

Hydrogen coma of comet P/Halley

The main molecular processes to produce the hydrogen comae of comets are now well known: Water, the main constituent of cometary atmospheres, is photodissociated by the solar ultraviolet radiation to form the high (20 km s⁻¹) and low (8 km s⁻¹) velocity components of the atomic hydrogen. The hydrogen clouds of various fresh comets have been observed in 1216Å by a number of spacecrafts. Ultraviolet observations of short period comets are, however, rather rare. Consequently Comet P/Halley in this apparition is a good object to obtain new physics of the hydrogen coma. Strong breathing of the hydrogen coma of this comet found by “Suisei” provides just such an example. The rotational period of Comet Halley's nucleus, its activity in the form of outbursts alone, and the position of jet sources etc. are determined from the breathing phenomena. Atomic hydrogen from organic compounds with a velocity of 11 km s⁻¹ play an important role in that analysis. The time variations of the water production rate of Comet Halley during this apparition observed by various spacecrafts appear to be in agreement with each other and are about 1.5–2 times larger than the standard model. The difficulty of the calibration problem was emphasized.     

The infrared synthetic spectrum of comet Halley

In order to prepare infrared sounding of comet Halley from the flyby VEGA probes, we have computed the synthetic spectrum between 2.5 and 15 μ of a typical comet at a heliocentric distance of ～ 0.8 AU. The present paper is particularly devoted to the contribution from the cometary gases. For a selection of 20 possible parent molecules, the most efficient excitation process is resonant fluorescence by the solar radiation field. The H₂O, CO, CO₂, CH₄, NH₃ and H₂CO molecules are the best candidates for detection by the IKS infrared spectrometers aboard the VEGA probes. For the water molecule, collisions are too rare to ensure thermal equilibrium in the whole coma ; therefore a limited number of fluorescence lines are expected to be present in the H₂O vibrational bands.     

Silicate core-organic refractory mantle particles as interstellar dust and as aggregated in comets and stellar disks.

The principal observational properties of silicate core-organic refractory mantle interstellar dust grains in the infrared at 3.4 microns and at 10 microns and 20 microns are discussed in terms of the cyclic evolution of particles forming in stellar atmospheres and undergoing subsequent accretion, photoprocessing and destruction (erosion). Laboratory plus space emulation of the photoprocessing of laboratory analog ices and refractories are discussed. The aggregated interstellar dust model of comets is summarized. The same properties required to explain the temperature and infrared properties of comet coma dust are shown to be needed to account for the infrared silicate and continuum emission of the beta Pictoris disk as produced by a cloud of comets orbiting the star.     

细水雾灭火的突变分析

为了认识细水雾灭火机理,寻找细水雾作用下的火焰熄灭边界,采用非线性突变理论分析了细水雾灭火过程,推导出火焰能量势函数及火焰临界熄灭温度的表达式,得到了细水雾扑灭正庚烷油盘火的火焰熄灭边界.进行了细水雾针对正庚烷油盘火的灭火实验,实验结果验证了火焰熄灭边界的正确性.相比吸热过程,火焰熄灭温度受氧浓度的作用更为迅速.     

Thermal modeling of Halley's comet

The comet thermal model of Weissman and Kieffer is used to calculate gas production rates and other parameters for the 1986 perihelion passage of Halley's Comet. Gas production estimates are very close to revised pre-perihelion estimates by Newburn based on 1910 observations of Halley; the increase in observed gas production post-perihelion may be explained by a variety of factors. The energy contribution from multiply scattered sunlight and thermal emission by coma dust increases the total energy reaching the Halley nucleus at perihelion by a factor of 2.4. The high obliquity of the Halley nucleus found by Sekanina and Larson may help to explain the asymmetry in Halley's gas production rates around perihelion.     

Space observations of comets during solar flares: A possible explanation for comet brightness outbursts

Problems connected with mechanisms for comet brightness outbursts as well as for gamma-ray bursts remain open. Meantime, calculations show that irradiation of a certain class of comet nuclei, having high specific electric resistance, by intense fluxes of energetic protons and positively charged ions with kinetic energies more than 1 MeV/nucleon, ejected from the Sun during strong solar flares, can produce a macroscopic high-voltage electric double layer with positive charge in the subsurface zone of the nucleus, during irradiation times of the order of 10–100 h at heliocentric distances around 1–10 AU. The maximum electric energy accumulated in such layer will be restricted by the electric discharge potential of the layer material. For comet nuclei with typical radii of the order of 1–10 km the accumulated energy of such natural electric capacitor is comparable to the energy of large comet outbursts that are estimated on the basis of ground based optical observations. The impulse gamma and X-ray radiation together with optical burst from the comet nucleus during solar flares, anticipated due to high-voltage electric discharge, may serve as an indicator of realization of the processes above considered. Multi-wavelength observations of comets and pseudo-asteroids of cometary origin, having brightness correlation with solar activity, using ground based optical telescopes as well as space gamma and X-ray observatories, during strong solar flares, are very interesting for the physics of comets as well as for high energy astrophysics.     

Rocket observation of the ultraviolet spectrum of comet Halley

An ultraviolet sounding rocket telescope/spectrograph experiment observed Comet Halley on 26 February 1986, 17 days after perihelion. From the long-slit spectra, the production rates of O, C, and CO are calculated. The derived water production rate is a lower limit of 5.0 × 10²⁹ s⁻¹ and the volume mixing ratio of CO to H₂O is 21%. The predicted brightness distribution from a radial outflow model with H₂O and CO as parent molecules are in accordance with the measured spatial profiles of OI and CO emissions. The ratio of the production rates of CO to C is 2.7 which is consistent with the carbon source being the photodissociation of CO. However, the radial outflow model which best fits the CO data predicts significantly weaker CI emissions than was observed. A better fit to the carbon data is found when an inner coma source of C at a rate of 3% of the water production rate is included in the model.     

A comparison of the dust properties in recent periodic comets

Measurements of the thermal emission from the cometary dust coma can be used to derive the rate of dust production from the nucleus as well as the size distribution of absorbing grains. More than ten short-period comets have now been observed in the infrared over a wide range in heliocentric distance. Dust production rates are derived for these comets based on theoretical models of the thermal emission from small absorbing grains and calculations of dust grain velocities. The mean size and albedo of the dust grains is similar in these comets, with the exception of Comet Crommelin, which seems to have had larger, darker grains.     

Spectroscopic observations of comet Giacobini-Zinner during the ice encounter

A set of 5 sequential spectra of Comet Giacobini-Zinner was obtained during the encounter of the ICE spacecraft with that comet on September 11, 1985. The observations were initiated 20 min before the encounter and were conducted in 10 min-long intervals until approximately 40 min after the event. The spectra were taken along a direction nearly coincident to the axis of the cometary tail and cover a distance of approximately 74 000 km at the comet. The preliminary analysis of the data indicates that the measured brightness intensity of CN at 3884 Å and that of the continuum at 5215 Å was nearly constant throughout the period of observation.