Vega Spacecraft Research Articles

Influence of the magnetic field on the density distribution of solar wind protons and cometary ions in the shock layer ahead of cometary ionospheres

The “mass loading” of the solar wind by cometary ions produced by the photoionization of neutral molecules outflowing from the cometary nucleus plays a major role in the interaction of the solar wind with cometary atmospheres. In particular, this process leads to a decrease in the solar wind velocity with a transition from supersonic velocities to subsonic ones through the bow shock. The so-called single-fluid approximation, in which the interacting plasma flows are considered as a single fluid, is commonly used in modeling such an interaction. However, it is occasionally necessary to know the distribution of parameters for the components of the interacting plasma flows. For example, when the flow of the cometary dust component in the interplanetary magnetic field is considered, the dust particle charge, which depends significantly on the composition of the surrounding plasma, needs to be known. In this paper, within the framework of a three-dimensional magnetohydrodynamic model of the solar wind flow around cometary ionospheres, we have managed to separately obtain the density distributions of solar wind protons and cometary ions between the bow shock and the cometary ionopause (in the shock layer). The influence of the interplanetary magnetic field on the position of the point of intersection between the densities with the formation of a region near the ionopause where the proton density is essentially negligible compared to the density of cometary ions is investigated. Such a region was experimentally detected by the Vega-2 spacecraft when investigating Comet Halley in March 1986. The results of the model considered below are compared with some experimental data obtained by the Giotto spacecraft under the conditions of flow around Comets Halley and Grigg–Skjellerup in 1986 and 1992, respectively. Unfortunately, our results of calculations on Comet Churyumov–Gerasimenko are only predictive in character, because the trajectory of the Rosetta spacecraft, which manoeuvred near its surface for several months, is complex.

From the Vega mission to comet Halley to the Rosetta mission to comet 67/P Churyumov–Gerasimenko

The data acquired by the Vega and Giotto spacecraft, while investigating comet 1Р/Halley in 1986, are compared to the results of the first phase of exploration of the nucleus of comet 67P/Churyumov–Gerasimenko performed with the Rosetta and Philae modules. The course of the Rosetta mission activity and the status of the modules after the Philae probe landing on the comet’s nucleus are overviewed. Since some elements of the touchdown equipment failed, a number of in-situ experiments on the comet’s nucleus were not carried out.

Identification of anthracene in Comet 1P/Halley

Context. Polycyclic aromatic hydrocarbons (PAHs) have been detected in many interstellar medium (ISM) sources. They are considered among the most abundant organic compounds in these environments. Aims. We aim at identifying the carriers of the near UV fluorescence bands that appear in the spectra of 1P/Halley’s inner coma. Methods. Near UV spectra of 1P/Halley were recorded on March 9, 1986 by the three-channel spectrometer onboard the Vega2 spacecraft at projected distances of between 421 and 932 km. We compare these data to laboratory spectra obtained under laser-induced fluorescence conditions in a jet-cooled molecular beam. Results. The cometary spectral features are found to be consistent with the laboratory fluorescence spectrum of anthracene. Four main peaks coincide at 363, 367.5, 373 and 382.5 nm. We then report the identification of anthracene, a three-ring PAH in the inner coma of 1P/Halley at projected distances of less than thousand km. We derive an abundance relative to water of 5 × 10 −5 to 1 × 10 −4 for this molecule. This new detection adds to the previously reported UV identifications of phenanthrene and pyrene in the innermost coma of 1/P Halley. Conclusions. Recently, three small PAHs, namely naphtalene, phenanthrene and pyrene, were identified in dust grains collected by the Stardust probe in the environment of Comet 81P/Wild2. These findings, together with the present identification of anthracene in 1P/Halley’s near UV spectra, confirm that similarities exist between the composition of comets and that of the ISM.

Structure of the Venusian atmosphere from surface up to 100 km

The goal of this paper is to summarize the experimental data on the atmosphere of Venus obtained after 1985, when the VIRA (Venus International Reference Atmosphere) or COSPAR model was published. Among the most important results that have appeared since then are the following: measurements of the vertical temperature profile by the VEGA spacecraft with high precision and high altitude resolution; measurements made with balloons of the VEGA spacecraft; radio occultation measurements of Magellan, Venera-15, and Venera-16; and temperature profiles derived from the data of infrared spectrometry obtained by Venera-15. The new result as compared to VIRA is the creation of a model of the atmosphere in the altitude range 55 to 100 km dependent on local time. This model is presented in our paper in tabulated form.

On the distribution of pickup ions as observed by the Vega spacecraft at comet Halley

Abstract Two electrostatic analysers of the PLASMAG-1 plasma instrument package detected energy/charge spectra of ions (without mass separation) aboard the Vega spacecraft when encountering comet Halley. They could observe only two relatively small sections of velocity space showing a mixed effect of decelerated solar wind particles and cometary pickup ions downstream of the bow shock. The separation of the different ion components was attempted by comparing the energy spectra with simple plasma distributions. The effect of cometary pickup ions observed by Vega-1 was different from that observed by Vega-2 at cometocentric distances >0.5 million km. These differences could be interpreted when using the bispherical shell distribution based on magnetic field vectors measured by the magnetometer.

Dust Spatial Distribution and Radial Profile in Halley's Inner Coma

Mosaic images of the brightness distribution of solar radiation scattered by dust in Halley's coma are constructed using the spectra obtained by the three-channel spectrometer TKS during the approach phase of the Vega 2 spacecraft. They cover a sector having a radius of 40,000 km centered at the nucleus with an angular extent of 50°. The dust scattered brightness is plotted as a function of the distance pbetween the nucleus and the line of sight. This distance pis also called impact parameter. The brightness varies as the inverse of pin the inner coma when pis less than 3000 km and larger than 7000 km. In the 3000–7000 km distance range, the brightness varies as p −1.52. At distances larger than 7000 km, two dust jets are clearly visible with a contrast comparable to the gaseous jets which appear in the OH, NH, CN, C 2, and C 3images (Clairemidi, J., G. Moreels, and V. A. Krasnopolsky, 1990, Astron. Astrophys.231, 235–240; 1990, Icarus86, 115–128). In the inner coma, the spatial distribution of dust seems to be more isotropic and less contrasted than the distribution of gaseous emissive species. A model is developed to calculate the scattered intensity integrated along a line of sight at a projected distance pfrom the nucleus. The model is based upon Mie theory and uses the data of the impact particle counter SP-2 on board the Vega spacecraft (Mazets, E. P., R. Z. Sagdeev, R. L. Aptekar, S. V. Golenetskii, Yu. A. Guryan, A. V. Dyachkov, V. N. Elyinskii, V. N. Panov, G. G. Petrov, A. V. Savvin, I. A. Sokolov, D. D. Frederiks, N. G. Khavenson, V. D. Shapiro, and V. I. Shevchenko, 1987, Astron. Astrophys.187, 699–706), extrapolated from 8030 to 440 km. The model takes into account the fountain effect due to the competition between solar gravitation and radiation pressure, the variation of phase function with scattering angle, and fragmentation processes. A simple method is used to simulate fragmentation: a particle in a mass decade class splits into particles of the lower mass decade, assuming that the mass conservation law is fulfilled. A single α( R) fragmentation coefficient is introduced, Rbeing the distance between the dust particle and the nucleus. A good agreement with the measured dust-scattered intensity radial profile can be reached if an efficient fragmentation process at R< 1000 km with α( R) = 0.87 is introduced, with the fragmentation coefficient decreasing to zero at R= 6000 km. Different values of the density ρ and complex index ( n− ik) of the grains are used. Two good fits with the model results are obtained for ρ = 2.2 − 1.4 a/( a+ 1) g cm −3, where ais expressed in micrometers (Lamy, P. L., E. Grün, and J. M. Perrin, 1987, Astron. Astrophys.187, 767–773) and both indices 1.387 − 0.031 i(Mukai, T., S. Mukai, and S. Kikuchi, 1987, Astron. Astrophys.187, 650–652) or 1.7 − 0.02 i(Khare, B. N., C. Sagan, E. T., Arakawa, F. Suits, T. A. Callcott, and M. W. Williams, 1984, Icarus60, 127–137). However, if a color index variation with pis considered, the agreement is much better when the complex index of tholin (1.7 − 0.02 i) is adopted.

Detection of the inner plasma pileup region at comet Halley during the Vega 1 flyby by the radio sounding experiment

The two Vega spacecraft sounded the ionosphere of Comet P/Halley during their flybys at two coherent one‐way downlink frequencies. The frequency recordings were processed with software adapted from a software code used for processing Ulysses radio science data. An electron density model was developed in order to model the electron content profile observed during the Vega 1 flyby ±1500 s (±120,000 km) about the closest approach (8900 km). The electron density for distances larger than 11,000 km followed a modified r−2 law, while the electron density in the “pileup” region between 8900 km and 11,000 km increased with distance according to a single power law r4. The inferred electron density is a factor of 2 lower than that measured by the Vega BD 3 in situ experiment during inbound flight and agrees with the outbound observations. The electron density profile inferred by the radio science experiment is an average over all inhomogeneities along the ray path and is probably a better representation of the large‐scale density profile originating from the comet nucleus than the in situ observations, which are affected by inhomogeneities along the trajectory.

Plasma flow in the cometosheath of comet Halley

Plasma parameters are investigated in the regions upstream and downstream of the bow shock of comet Halley based on the ion spectra measured by the Plasmag-1 instrument aboard the Vega-2 spacecraft. Proton velocities observed by the Solar Direction Analyser and by the Cometary Ram Analyser are compared with the simulated bulk velocity profile provided by a three-dimensional multiscale MHD model (Gombosi et al., 1996). Disregarding the effects of presumably interplanetary disturbances, the simulated and measured values are in good agreement at cometocentric distances larger than about 0.5 million km.

Disintegration of Dust Aggregates as Origin of the Boundaries in Halley's Coma: Derivation of the Sublimation Parameters

This paper is focused on the pair of best pronounced boundaries (enhanced gradients of dust flux) which have been observed by the SP-1 and -2 detectors, one inbound and the other outbound, during both Vega encounters of Comet Halley. First, it is shown that the V-detector of the DUCMA instrument, also aboard the Vega spacecraft, served as an additional dust sensor, recording large particle impacts on the spacecraft and its$jytarget area (0.2–0.1 m2), as well as its two mass thresholds, VL ∼5 × 10−8g, and VH ∼10−6g, are derived. An extensive analysis of the boundaries based on the SP-1, SP-2, and V-detector data has led to the conclusion that the two dust boundaries: (1) are common for a very wide mass range, from ∼10−17g up to ∼10−7g; (2) are observed at nearly equal distances, inbound and outbound; (3) during the Vega-1 flyby were closer to the nucleus (∼60,000 km) than during the Vega-2 flyby (∼84,000 km). It is shown that none of these properties can be interpreted in terms of the fountain model. On the other hand, the first two can be easily explained as a consequence of disintegration of dust aggregates if only the aggregates were confined to a limited region around the nucleus. Assuming that the disintegration was due to sublimation of the volatile component gluing together dust particles, the condition for the aggregate confinement is formulated and a set of equations describing the formation and expansion of a dust boundary is derived. The boundary expansion speed drawn from the observations allows estimates for two sublimation parameters. In particular, the latent heat is determined to beL≈ 16 ± 1.5 kcal/mole. Using the derived parameters, some model calculations are made. They show that the dust boundary was first formed about 20 days after the comet perihelion at a distance of ∼45,000 km from the nucleus and expanded with a speed increasing from ∼35 m/sec to ∼140 m/sec during the next 20 days. The size of aggregates (at the time of emission from the nucleus) reaching the boundary was decreasing rapidly, by an order of magnitude in a 10-day interval, but the aggregate lifetime varied weakly. The expansion of the aggregate domain combined with the decrease of disintegration rate resulted in a rather fast decline of the boundary. The DID4 data from the Giotto encounter of Comet Halley give evidence in support of the model predictions.

Interaction of an expanding plasma cloud with a simple antenna: Application to anomalous voltage signals observed by Voyager 1, Voyager 2, ICE and Vega spacecraft

High‐velocity impacts of interplanetary dust grains with spacecraft can give rise to transient plasma clouds from the spacecraft bodies. It is believed these plasma clouds can affect spacecraft instruments. Laboratory results are presented demonstrating the interaction of small expanding plasma clouds with a simple antenna. Results corroborate the hypothesized origin of anomalous impulsive voltage signals recorded by Voyager 1 and 2 spacecraft during flybys of Saturn, Uranus, and Neptune, the International Cometary Explorer (ICE) during its flyby of comet Giacobini‐Zinner, and Vega during its flyby of comet Halley. Results suggest that preflight calibration of antenna‐plasma interactions may extend the range of spacecraft diagnostics.

Mass-spectrometric in situ studies of cometary organics for p/Halley and options for the future

When the VEGA and GIOTTO spacecrafts flew by comet p/Halley in 1986 the mass-spectrometers Puma and PIA measured the composition of cometary dust particles impacting at speeds of well above 65 km/s. Ion formation upon impact lead to mostly atomic ions. However, a small fractions of the ions measured could be related to molecules. A sophisticated analysis allowed for the first time to point to the chemical nature of cometary organics based on actual mass spectra. With the instrument CoMA for the NASA-BMFT mission CRAF much higher mass-resolution and molecule masses become accessible for in situ measurement, and will yield complementary information to the gas chromatograph CIDEX also on-board CRAF.

Temperature evolution of porous ice samples covered by a dust mantle

The existence of nonvolatile dust mantles covering the ices of cometary nuclei is suggested both from the behavior of many cometary lightcurves and from the direct observations of Comet Halley's nucleus by the Giotto and VEGA spacecraft in March 1986. In this paper we present a systematic study which sheds some light on the evolution of such dust-ice systems when they are irradiated by the sun. Hereby we assume that the dust mantle as well as the underlying ice are grainy, porous structures permeable to gases. It is found that both the surface temperature of the dust mantle and the temperature at the dust-ice interface are strong functions of the average pore radius of the materials and of the thermal conductivity of the mantle. For moderately insulating dust mantles (coal dust) with grain sizes in the micrometer range, high ice temperatures (up to the triple point of water ice) may be reached in response to irradiation with one solar constant. If the dust mantles are extremely bad heat conductors (assuming radiative heat transfer only) the ice temperature remains below 200 K in all cases considered. An increase of the dust mantle thickness may result either in a higher or in a lower ice temperature, depending upon the parameters chosen.

The effect of electron collisions on rotational populations of cometary water

The e-H2O collisional rate for exciting rotational transitions in cometary water is evaluated for conditions found in Comet Halley during the Giotto spacecraft encounter. In the case of the 0(00)-1(11) rotational transition, the e-H2O collisional rate exceeds that for excitation by neutral-neutral collisions at distances exceeding 3000 km from the cometary nucleus. The estimates are based on theoretical and experimental studies of e-H2O collisions, on ion and electron parameters acquired in situ by instruments on the Giotto and Vega spacecraft, and on results obtained from models of the cometary ionosphere. Thus, the rotational temperature of the water molecule in the intermediate coma may be controlled by collisions with electrons rather than with neutral molecules, and the rotational temperature retrieved from high-resolution IR spectra of water in Comet Halley may reflect electron temperatures rather than neutral gas temperatures in the intermediate coma.

Comet Halley remote plasma tail observations and in situ solar wind properties: VEGA- [formula omitted] IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986

Properties of the interplanetary magnetic field and solar wind plasma are analysed from continuous observations from 1 December 1985 to 1 May 1986 made by the Vega spacecraft (Riedler et al., 1986, Nature 321, 288; Gringauz et al., 1986, Nature 321, 282). Several high speed streams and flare-related shocks are found; the magnetic field sector boundaries are also seen. Many Earth-based observations of disconnection events in the plasma tail of comet Halley are now available and several mechanisms [changes in the solar wind (Jockers, 1985, Astron. Astrophys. Suppl. Ser. 62, 791; 1986, Asteroids, Comets and Meteors II, Reprocentralen HSC, Uppsala), interplanetary magnetic field features, e.g. day-side reconnection (Niedner and Brandt, 1978, Astrophys. J. 223, 665) or night-side reconnection (Russell et al., 1986, J. geophys. Res. 91, 1417)] have been proposed to explain the occurrence of such events. We investigated the correlation between the IMF sector boundaries measured by Vega and the observed disconnection events; such a correlation would support the day-side reconnection explanation. It is found that only in 50% of the considered events does a correlation between these two phenomena exist. For the other cases, a sector boundary of the IMF sweeping over the comet cannot explain the occurrence of disruptions of the main plasma tail. An indication is also seen that density enhancements (e.g. as they occur for high speed streams) in the solar wind are connected with large tail events.

Surface Morphology of Cometary Nuclei

Abstract.The first in situ observations of a cometary nucleus by imaging techniques on board the VEGA and Giotto spacecraft in 1986 opened the possibility of directly studying the properties and structures of a cometary surface. This new information, combined with results from other experiments made by these spacecraft and with results from laboratory experiments and Earth-bound radar observations, gave a better basis for coming from speculation to more precise knowledge in describing properties of cometary surfaces and related activity. It is the aim of this paper to discuss the different views developed to understand properties of cometary surfaces and to propose a consistent model for understanding topological surface properties and related cometary activity. We accomplish this by extending former hypotheses and approaches and making them more precise in the light of the new and more complex data.

P/Halley, the Model Comet, in View of the Imaging Experiment Aboard the Vega Spacecraft

Abstract.In this paper, those results of the VEGA imaging experiments are summarized that probably have general validity for any comet. Shape, size, surface structure, jet activity, and rotation pattern are considered in this respect. It is pointed out that imaging data provide indispensable information for an understanding of cometary activity.

Attogram dust cloud a million kilometers from Comet Halley

view Abstract Citations (57) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Attogram Dust Cloud a Million Kilometers from Comet Halley Utterback, Nyle G. ; Kissel, Jochen Abstract Particle-impact ion mass spectrometers carried on the Giotto and two Vega spacecraft missions to Comet Halley in 1986 produced unexpected signals almost one million kilometers from Halley which can be attributed to high number densities of dust particles with masses of order 10 to the -18 g. The data imply that up to several percent of the total mass lost during the entire apparition was present as such dust within a half-million kilometer radius if radial symmetry is assumed. The total volume mass density of particles observed varied with r to the -5 at large distance, and the mass of the largest individual particles observed varied with r to the -3, while the number density varied as the inverse of r. The mass distribution was also determined. The results imply strong fragmentation processes. The spectrometers were not designed to operate with such small dust particles, and the signals were long thought to be spurious noise. Although atomic mass spectra (composition) of the very small dust could not be extracted from the data, a number of requirements on the particle properties are implied. The close detailed agreement among data from all three missions is used to verify that the signals were indeed produced by dust particles. Publication: The Astronomical Journal Pub Date: October 1990 DOI: 10.1086/115599 Bibcode: 1990AJ....100.1315U Keywords: Chemical Composition; Halley'S Comet; Interplanetary Dust; Spatial Distribution; Giotto Mission; Mass Distribution; Mass Spectrometers; Vega Project; Astrophysics; COMETS full text sources ADS |

Clusters and packets of grains in comet Halley and the fragmentation of dust

Clusters and packets of dust grains, detected by the VEGA spacecrafts in the coma of comet Halley, are interpreted either as due to microjets arising directly from the nucleus or as due to fragmentation processes of larger conglomerates. After a summary of the characteristics of the particle swarms observed and of the conclusion from their interpretation as microjets, some aspects from the fragmentation scenario of this phenomenon are outlined. From the spatial widths and the total number of grains in a particle swarm the mass of the parent conglomerate can be estimated. For the clusters and packets parent bodies of about 1 to several 100 g can be expected. Their travel time in the coma after release from the nucleus range from 10 to about 100 days before the spacecraft encounters. Finally, possible fragmentation processes are discussed and some comments for further investigations, both in theory and for in-situ data, are given.

On the temperature and gas composition in the region of comet formation

The findings of the Giotto and Vega spacecrafts on the gas composition of comet Halley, together with an experimental study on the trapping of gas mixtures in amorphous water ice, enable estimation of the gas composition and temperature in the region of comet Halley's formation: If Halley was formed in the solar nebula by condensation of water vapor in the presence of gas, in the region of its formation the CO/CH 4 ratio had to be at least 100 and the temperature about 48 K. The ice particles that formed the comet could not have condensed at a higher temperature and subsequently cool down because then the 7% CO found as a parent molecule could not have been trapped in the ice. A ∼48 K formation temperature implies that the ice was in amorphous form. This temperature is surprisingly close to the temperatures observed by IRAS for the circumstellar dust shells around α PsA (55 K) and ε Eri (45 K) and supports the suggestion that short-period comets were formed outside the region of planet formation. The CO content of comet Halley and the high sensitivity to explosion of irradiated, ice-coated, interstellar grains seem to exclude the possibility of their direct incorporation into comets. Yet, they might have provided the condensed organics—the “CHON” materials.

On charge exchange effect in the vicinity of the cometopause of Comet Halley

view Abstract Citations (38) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Charge Exchange Effect in the Vicinity of the Cometopause of Comet Halley Ip, W. -H. Abstract In order to explore the physical nature of the cometopause observed at Comet Halley by the Vega spacecraft and by the Giotto probe, the chemical compositional changes and variations of the thermal-energy distributions of the water-group ions are examined, adopting a two-dimensional cometary-plasma flowfield model based on three-dimensional MHD simulations of Fedder et al. (1986). The charge-exchange loss of hot cometary ions and the solar-wind protons could be used to explain the observed number-density profiles quantitatively. The resulting exponential depletion of the hot-ion populations with a scale length of about 10,000 km occurs near 60,000-80,000 km along the trajectory of Giotto, as indicated by both theoretical computations and the ion-mass-spectrometer measurements. The formation of the cometopause located at about 140,000 km is therefore not necessarily as closely related to the charge-exchange process. Publication: The Astrophysical Journal Pub Date: August 1989 DOI: 10.1086/167764 Bibcode: 1989ApJ...343..946I Keywords: Charge Exchange; Halley'S Comet; Magnetohydrodynamic Flow; Solar Wind; Chemical Composition; Energy Distribution; Flow Distribution; Giotto Mission; Vega Project; COMETS; HALLEY; CHARGE EXCHANGE; COMETOPAUSE; SPACECRAFT OBSERVATIONS; VEGA MISSIONS; GIOTTO MISSION; CHEMISTRY; THERMAL PROPERTIES; STREAMLINES; ENERGY; DISTRIBUTION; IONS; MODELS; MAGNETOHYDRODYNAMICS; DEPLETION; IMS INSTRUMENT; PLASMA; SOLAR WIND; CALCULATIONS; OXYGEN; WATER; PROTONS; THEORETICAL STUDIES; Astrophysics; Comets; COMETS; HYDROMAGNETICS; PLASMAS; SUN: SOLAR WIND full text sources ADS |