Consequences of cometary aurora on the carbon chemistry at comet P/Halley.

Various experimental data acquired during the visit of Halley's comet in 1986 have shown that the amount of carbon produced due to photodissociation of parent carbon bearing species is not ample enough to explain the observations. This requires the presence of an additional source of atomic carbon. One of the possible source could be auroral-type activities resulting from the precipitation of high-energy "auroral electrons" of solar wind origin, the evidence of which have been inferred from many observations at comet Halley. We have developed a coupled chemistry-transport model to study the role of auroral and photoelectron impact as well as of chemistry on the modelling of carbon in the inner coma (< or = 10(4) km) of comet Halley. Our study suggest that electron impact dissociation of CO is the major source of carbon production in the inner coma, not the recombination of CO+ as suggested by earlier workers, while transport is the main loss process.     

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.     

Dust environment models for comet P/Halley: Support for targeting of the GIOTTO S/C

In March 1985 ESA's GIOTTO spacecraft will fly by P/Halley's nucleus at a distance of a few hundred kilometres. The near nucleus dust environment the probe will traverse poses a hazard with respect to physical damage as well as to attitude disturbance with the possible loss of ground station contact. To predict S/C survivability and dust impact rates for the experiments, a model of the spatial distribution of the dust in the nucleus' vicinity is required. In the ‘dynamic’ model, the local spatial dust density is derived from exact expressions for the dust particle dynamic motion. The model has been implemented in a software system which allows for fast simulations of a cometary fly-by.     

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.     

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.     

Solar wind interaction with comet Halley

In March 6 and 9, 1986 the spacecrafts ‘Vega-1’ and ‘Vega-2’ have flown through the coma of comet Halley and have carried measurements of plasma, energetic particles, magnetic field and plasma waves along its trajectory. A short review of these measurements and its comparison with theoretical models of solar wind interaction with comets are given.

The spacecrafts ‘Vega-1’ and ‘Vega-2’ have studied the solar wind loading by cometary ions, the structure of cometary bow shock and the processes in the inner coma of comet Halley. Exactly in this sequence we discuss the results of measurements and compare them with the theory.     

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.     

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.     

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.     

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.     

Near infrared observations of comet Halley from Vega 2

Spatial distribution of the continuum radiation in the range of 0.95–1.9 μm presumes total dust production rate of the comet of 10ρ tonne s⁻¹ (ρ is the dust material density) and its angular distribution proportional cos . Observations of the water vapor band at 1.38 μ m reveal strong jets, their time shift from the dust jet measured in situ is consistent with gas velocity of 0.82±0.1 km s⁻¹ and dust velocity of 0.55±0.08 km s⁻¹. The OH vibrational-rotational bands observed are excided directly via photolysis of water vapor. Water vapor production rate deduced from the H₂O band and OH band intensities is 8×10²⁹ s⁻¹. Intensity of the CN(0,0) band result in the CN column density of 9×10¹² cm⁻², i.e. larger by a factor of 3 than given by the violet band.     

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.     

Hydrogen cyanide polymers from the impact of comet P/Shoemaker-Levy 9 on Jupiter.

Hydrogen cyanide polymers--heterogeneous solids ranging in color from yellow to orange to brown to black--may be among the organic macromolecules most readily formed within the Solar System. The non-volatile black crust of comet Halley, for example, as well as the extensive orange-brown streaks in the atmosphere of Jupiter, might consist largely of such polymers synthesized from HCN formed by photolysis of methane and ammonia. Laboratory studies of these ubiquitous compounds point to the presence of polyamidine structures synthesized directly from hydrogen cyanide. These would be converted by water to polypeptides which can be further hydrolyzed to alpha-amino acids. Other polymers and multimers with ladder structures derived from HCN would also be present and might well be the source of the many nitrogen heterocycles, adenine included, detected by thermochemolytic analysis. The dark brown color arising from the impacts of comet P/Shoemaker-Levy 9 on Jupiter could therefore be mainly caused by the presence of HCN polymers, whether originally present, deposited by the impactor or synthesized from freshly formed HCN. Spectroscopic detection of these predicted macromolecules and their hydrolytic and pyrolytic by-products would strengthen significantly the hypothesis that cyanide polymerization is a preferred pathway for prebiotic and extraterrestrial chemistry.     

High spectral resolution line profiles and images of comet Halley

High-spectral-resolution line profiles and images of comet Halley were obtained in 1986 at the National Solar Observatory McMath telescope, using a dual-etalon Fabry-Perot spectrometer. The spectrometer was designed to obtain data in four distinct modes: (1) high-resolution (R = 200,000) scanning, (2) high-resolution imaging, (3) moderate resolution (R = 30,000) scanning, and (4) moderate resolution imaging. This paper describes the instrument and some examples of data obtained in the high-resolution scanning mode.     

Cometary ion observations at and within the cometopause-region of comet Halley

Three distinct boundaries are identified from the PICCA cometary ion observations within the innermost part of the coma of comet Halley: (1) the 'cometopause' at a cometocentric distance R_c 1.5×10⁵ km, characterized by the appearance of water-group ions well above background; (2) the 'cold cometary plasma boundary' at R_c 3×10⁴ km, characterized by a sudden and simultaneous decrease in the temperatures of all cometary ions, and (3) the 'ionopause' at R_c 6000 km, characterized by a fast decrease in the intensity of all cometary ions by a factor 3–5. Between the first two boundaries only ions with masses less than 50 amu are present, showing distinct maximum intensities at 18, 32 and 44 amu at the second boundary. Downstream of the second boundary also ions of mass 12, 64, 76, 86 and 100 amu are detected.     

First results of plasma and neutral gas measurements from Vega-1/2 near comet Halley

Based on the ion, electron and neutral gas observations, performed by five of the six sensors comprising the PLASMAG-1 experiment on board VEGA-1 and -2, the following results are discussed: (1) the existence of the bow shock and its location at 1.1×10⁶ km for VEGA-1 inbound; (2) the existence of a cometopause and its location at 1.6×10⁵ km for VEGA-2 inbound; (3) the plasma dynamical processes occurring inside the cometosheath; (4) the phenomena taking place within the cometary plasma region including mass-spectroscopy of cometary ions at distances 1.5×10⁴ km; (5) the existence of keV electrons near closest approach to the nucleus; and (6) the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of 2×10⁶ km and an overall production rate of 1.3×10³⁰ molecules s⁻¹ for VEGA-1 inbound. The results are also discussed in the context of the other, both remote and in-situ, observations, performed on board the VEGA- and GIOTTO-spacecraft.     

Interplanetary scintillations of PKS 2314+03 during occultation by comet Halley

Strong interplanetary scintillations (IPS) of the quasar 2314+03 were recorded at 103 MHz at Thaltej-Ahmedabad, India with a transit type correlation interferometer on 18, 19 and 20 December 1985, as the radio source was predicted to be occulted by the ion tail of the comet Halley.

On 18th through 20th very strong scintillations, with periodicities of 1 sec average were observed, their amplitude progressively decreasing as the source approached the tail-end. The rms variations of scintillating flux of the source on 18, 19 & 20 were about 18, 11 & 4.7 Jy, as against 3.3 Jy on control days 17 and 21 December for solar elongation of 87°.

Assuming Gaussian irregularities with weak scattering, the rms density variations, ΔN, of 10, 6, 3 and 1 elec./cm³ on 18 through 21 December, from the comet nucleus towards its tail-end, varied as (ΔN) ∝ r^−3.3 as against (ΔN) ∝ r⁻² in the solar plasma.

Quasi-periodic modulations of the enhanced scintillating flux possibly imply 10⁴ km scale-size ion condensations and mean electron density of 10³ to 10⁴ electrons/cm³ in the Halley's plasma tail.     

Imaging of comet Halley from Catania observatory using a CCD and Schmidt plates

Observations of comet Halley through CCD and Schmidt plates have been performed at Catania Observatory (Italy) from October 1985 through April 1986. Preliminary results are presented concerning the spatial structures for different spectral ranges of cometary light.     

Scientific instrumentation of PLANET-A VUV imaging of the hydrogen coma of Halley

The PLANET-A spacecraft to fly by Comet Halley is equipped with a VUV imaging camera which will take pictures of the hydrogen coma of the comet. The camera is composed of a telescopic mirror lens, a VUV image intensifier, two dimensional CCD, and controlling electronic circuits with a microprocessor. In order to eliminate the blur in the image due to the spinning motion of the spacecraft, a special technique called “spinsynchronized charge swift” is used in the CCD driving.     

Modelling the neutral gas environment of comets with special application to P/Halley

A technique has been developed which allows relatively accurate modelling of cometary gas production from nothing more than a visible light curve. Application to P/Halley suggests the production rate of parent molecules will be about 2.6 × 10²⁹ per second on March 10, 1986, for example. The uncertainties and intrinsic limitations in this approach are outlined. The theory is then extended to predictions of abundance of other gaseous species, and a photometric model of these gases provided. Combined with the dust model of N. Divine, preliminary predictions of the luminance of P/Halley as seen in any direction from inside the coma or outside can be provided for λλ3000–7000.