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Plasma density and plasma wave measurements aboard the "Vega-2" spacecraft discovered two plasma density enhancements in the inner coma of comet Halley that are accompanied by bursts of plasma waves in the lower-hybrid frequency range. These events are explained by the critical ionization velocity (CIV) effects of Alfven. The persistent bursty plasma wave activity and the intermittent plasma density growth toward the cometary nucleus indicate that the CIV effect contributes to the ionization of the cometary gas.

One of the most striking features discovered during the encounter of space probe ICE and comet Giacobini-Zinner was the presence of intensive MHD turbulence even at large distances from the comet. In this paper it is shown that the mass loading of the solar wind with cometary ions can cause such a phenomenon.

This paper considers the evolution of a flat svarm of cometary bodies (under the effect of the passage of stars), initially moving in one direction along the circular orbits with radii 1.4×104

The fact that the small bodies as Comets and asteroids have been submitted to a very weak evolution turn them into witness materials of the early stages of formation of the solar system. After the successful multi—spacecraft missions to the Halley comet a great interest is focusing now on asteroids. The soviet mission to Phobos, launched in July 1988 is a peculiar opportunity to study such a small body which could reasonably be a capturated asteroid. The peculiarity is also in the very low and very slow flyby that can be performed in the case of Phobos ; it has been taken advantage of that to use methfds of active remote chemical analysis derived from the laboratory techniques SIMS and LAMMA

RECENT observations1 from Phobos 2 added about one hundred γ-ray bursts to the several hundred detected over the past 20 years. One of the most promising explanations of these bursts2 is that they originate from the activation of the magnetospheres of old neutron stars3, for example by encounters with small bodies. Here we use estimates of the number density of small bodies, either interstellar or in circumstellar Oort-like clouds, to predict the recurrence times of γ-ray bursts from individual sources, on the assumption that they are produced from neutron star/comet encounters (direct impacts4,5, captures6,7 or passages8). We find that 'classical' (non-repeating) bursts may be accounted for by passages of interstellar small bodies through the magnetosphere, whereas the small number of repeating bursts is consistent with the interaction of neutron stars with small bodies within Oort-like clouds. In the latter case, the best agreement with observational statistics is found by adopting the recent model of the Oort cloud9 derived from the study of comet Halley by the Vega spacecraft10.

The survey of elemental composition of P/Halley dust component according to the data acquired by the PUMA-2 instrument installed on the board of the VEGA-2 spacecraft is presented. Possible mineralogical composition of dust particles is considered. Some reasons for possible presence in comet’s dust particles of virgin organic compounds are presented.

Time-of-flight dust-impact mass-analysers PUMA-1,2 aboard VEGA spacecrafts were intended to investigate the properties of Halley comet dust component by means of direct measurements in the nucleus vicinity. More then 2000 mass-spectra were obtained by PUMA-1 and more then 500 ones were obtained by PUMA-2. Masses of studied particles are in the range 5 10-17 – 10-12 g. The spectra measured in time analyzer mode (zero mode) were the most convinient for data treatment; the number of these spectra was about 4–5% from the total number of PUMA spectra. The principal elements (H, C, N, O, Na, Mg, Al, Si, S, CI, Ca and Fe) and their most abundant isotopes were found. The isotopic composition of C, Mg, Si, S, CI and Fe is similar to mean cosmic isotopic abundance.The cometary dust particles contain a significant amount of light elements (H, C, N and 0). Their presence can be easily explained by the presence of organic compounds similar to observed in carbonaceous chondrites. The results of cometary dust composition measurements shows to a possible similarity of interstellar and cometary dust and can be explained by a model of cometary nuclei as aggregates of interstellar dust particles with no substancial chemical evolution.

The spacecraft Vega 1 and Vega 2 encountered comet Halley on 6 and 9 March 1986. Their scientific payload comprised 14 instruments, which collected data concerning the comet's optical characteristics, dust emission, and neutral gas, plasma and electromagnetic field environment. The main features of the encounters are described and highlights of the first scientific results are presented.

One of the most striking features discovered during the encounter of space probe ICE and comet Giacobini-Zinner was the presence of intensive MHD turbulence even at large distances from the comet. In this paper it is shown that the mass loading of the solar wind with cometary ions can cause such a phenomenon.

Measurements from the Ducma dust counter and mass analyzer on the Vega spacecraft are compared with those from the SP-1 sensor on the same spacecraft. It is shown that the sequence of pulses recorded by the SP-1 instruments show deviations from a random distribution. Some of the nonrandom sequences observed by SP-1 are in the same time intervals as the clusters observed by the Ducma instrument. The results support the analysis of Ducma data which showed highly structured variations of dust intensity through the coma and large fluxes of low mass particles extending to great distances beyond the coma (Simpson et al., 1986).

The orbits of comets from the Oort cloud have eccentricities very close to unity. These orbits are highly elongated, which provide one more possibility of their delivery into the planetary zone, besides collisions of comets with stars which are effective near the aphelions. Weak but repetitive perturbations being produced by the great planets, Jupiter and Saturn, on comets with perihelious < 20 AU can cause chaotization of dynamics of comets at orbits with major axesa ≳ 103 AU. A comet may suffer as large as 100/1000 such « weak » collisions before it is expelled onto a hyperbolic orbit. The dynamic chaos mechanism provides filling of the loss cone, diffusion of comets from inner parts of the Oort cloud into the outer one (the halo) and fluctuatory comings of comets into the inner part of planetary zone. A diffusive evolution of eccentricity can serve as one of the mechanisms of formation of the Oort cloud. A similar role can be played by both the Galactic gravitational field and periodic perturbations from the hypothetical Sun’s companion, Nemesis. The dynamic chaos allows us, along with star encounters, to fill the loss cone in the outer part of the Oort cloud. In the inner part of the Oort cloud the dynamical chaos can be a primary mechanism of the loss cone filling between consecutive rare (although strong) collisions with stars.

Dust-impact PUMA mass-analysers aboard the spacecrafts VEGA-1 and VEGA-2 allow to conduct the first direct measurements of mass spectra of comet Halley’s dust envelope particles with masses higher than 10 -17 g [1,2]. The analysis of spectra measured by the PUMA instruments showed that unindentified peaks in this spectra could be associated with very small particles (mass 10-17–10-20 g).

Venera-Halley mission is to be launched to Venus in Dec. 1984. It will fly by Venus in June 1985. Separation of the cometary probe and Venera descend module will take place at that time. The gravitational swing-by at Venus will provide the encounter with the Halley comet in March 1986. The remote sensing of the inner coma (TV-imagery, spectrometry in the region from 1200 A to 12 μm, polarimetry) and of the nucleus, direct measurements of dust fluxes, dust composition, plasma and magnetic field are planned in the framework of multinational cooperation.

The critical velocity triggering anomalous ionization of the neutral gas by plasma flow is calculated for the model based on the lower hybride instability. It depends strongly on the plasma and gas parameters, denning the instability development of the ionized atoms beam in the counter-streaming plasma. In particular, the possible role of the critical ionization mechanism for Halley's comet is examined. The fulfilment of both Townsend's condition for the selfustained beam plasma discharge and Alfven's condition for the critical velocity mechanism indicates that this mechanism may operate only within 104 km from the cometary nucleus and give an ion production rate close to that observed for Kohoutek's comet.

The VEGA mission aims at an encounter with the comet Halley after a “fly by” over Venus, during which part of the spececraft enters the atmosphere, divided in a Lander and Cruising Balloon. The paper gives a short description of both, as well as the re-entry, landing and cruising programs. After a definition of the preliminary ground tests, the main results obtained by the two VEGA spacecraft are listed.

An important finding of the Vega and Giotto missions to comet Halley was evidence that the cometary nucleus is a consolidated body1–5. The shape, size and rotation of the nucleus were studied as well its albedo1–3 and surface temperature6–8. In determining the nature of the cometary nucleus, a value for the average density ρ would be informative. In this paper we estimate ρ to be 0.6 +0.9−0.4g cm−3 compatible with the 'snow-ball' models of the nucleus, although a more compact ice structure could correspond to such a density value.

The 'Pathfinder' project uses the resources of NASA, ESA and Intercosmos to support the Giotto space probe flyby of comet Halley. The required 500-km approach to comet Halley on the sunward side was so uncertain as to preclude the trajectory's realization with sufficient accuracy on the basis of ground-based astronomical observations alone. Spaceborne observations by the two Vega satellites were accordingly conducted; these, in combination with a very accurate determination of the Vega trajectories using VLBI, allowed a flyby distance of 600 km to be achieved with only + or - 40 km uncertainty.

Measurements of dust particles in comet Halley's coma from Vega spacecraft made with instruments using a new principle of dust detection and having a high time resolution over a large range of dust fluxes and masses are reported. The dust coma, whether quiescent (as seen by Vega 2) or containing a major jet structure, (as seen by Vega 1) displays large, short-term variations throughout which are at times quasi-periodic. The integral mass spectra increase in intensity to the lowest masses measured, and the flux levels lie approximately in the ranges estimated previously from ground-based observations. The coma is highly dynamical on all spatial and temporal scales, suggesting a complex structure of localized regions of dust emission from the nucleus.

The SP-2 experiment carried by the Vega spacecraft during their encounter with comet Halley was designed to determine the spatial distribution of cometary dust particles and their mass distribution over the range ∼10−16–10−6 g. The first infrequent dust particle impacts recorded by the SP-2 sensors indicated that Vega 1 entered the cometary dust coma at a distance of ∼280,000 km from the nucleus. Although the subsequent rapid and even spectacular developments were not unexpected and the results of the in situ measurements agree well with the general predictions of cometary dust environment models1,2, a number of remarkable features were revealed, such as the high density of sub-micrometre-sized particles, considerable variations in the particle mass distribution observed in different regions of the coma and particularly in jets, and the asymmetry relative to the comet–Sun line.

Energetic cometary ion fluxes measured between 2.5106 km and 106 km from the cometary nucleus along the inbound trajectory of s/c VEGA-1 are used to derive the temperatures of the ion distributions in the solar wind frame. The increase of the temperature is modelled by the temperature change derived from a Fokker-Planck type equation with a source term and a stochastic acceleration term. The temperature increase predicted by theory is about 3 keV, higher than the observed one (≃ 1.4 keV). The difference may be due to the approximations applied. The second order Fermi mechanism is thus capable of producing the temperature increase observed.

Measurements of the element composition of the dust particles in Comet Halley were made by the PUMA-1 and PUMA-2 instruments onboard the VEGA spacecraft. The timeof-flight analysis of ions produced by the hypervelocity particle impact on the target was applied with a wide (W = 100 eV) and a narrow (W = 20 eV) energy window of the instruments. The analysis of measured spectra shows that the mass resolution for “small” particles (Q < 10−13 C) was high enough in both modes to separate individual mass lines. For large particles the resolution did not change for the narrow window, it decreased, however, for the wide one (to about 50). A relative transmission Q W /Q n dependence on the mass number for the wide and narrow window made has been found.

The importance of Alfven wave generation in interacting plasmas is discussed in general and illustrated by the example of solar wind interaction with cometary plasma. The quasi-linear theory of Alfven wave generation by cometary ions at distances far from the cometary nucleus is reviewed. The incorporation of a diabatic plasma compression effects into this theory modifies the spectrum of Alfven waves and the integral intensity of magnetic field fluctuations previously published. These results are in quantitative agreement with the in situ observations near the comets Giacobini-Zinner and Halley. However, the polarization of quasi-linearly excited waves needs further detailed comparison with observations.

Direct measurements of the dust particle spatial and mass distributions in comet Halley have been carried out on Vega spacecraft over the mass range ~10−16−10−6g with the SP-2 dust particle detector consisting of a set of acoustic and impact plasma sensors. These measurements have revealed a large-scale, time-variable structure of the dust coma, the position and characteristics of its boundaries, and a strongly pronounced angular directivity of dust emission from the cometary nucleus. An unexpectedly high concentration of tiny dust particles with masses below 10 −4 g and strong systematic variations of the shape of the particle mass distribution as a function of distance to the cometary nucleus have been observed. The mean dust production rate by the nucleus has been estimated at (10–13)106 g s−1 (Vega 1) and (5–7) 106 g s −1 (Vega 2).

International audience; The television system (TVS) aboard the spacecraft Vega 1 and Vega 2 observed comet Halley from 4 to 11 March 1986, and transmitted ∼1,500 images to Earth. The best images of the cometary nucleus were obtained from distances of 8,000–9,000 km. The nucleus is an irregular potato-shaped body, ∼14 × 7.5 × 7.5 km in size, which rotates with a period of 53±3 h. The surface is rather dark; the measured albedo is ∼0.04−0.01+0.02. Active emission of matter was observed on the sunlit side of the nucleus.

One important physical characteristic of comet P/Halley is dust jet activity, seen from Earth as changing coma patterns. At spacecraft resolution it becomes possible to precisly locate the sources of active jets on an irregularly shaped nucleus. The data set employed in this analysis consists of 11 full-format (512 × 512 px) Vega-2 images taken over a 15 min interval starting approximately 370 s before closest approach. The distance to the nucleus was always less then 45,000 km and became as close as 8030 km. Most of the active zones formed at least one quasi-linear structure on the surface. We found evidence of the activity on the dark hemisphere, at least one of the faint but active jet sources is there.

A unique opportunity to combine missions to Venus and comet Halley was available in 1985/86 by employing a two-element space vehicle consisting of a Venus lander and a Halley flyby probe. The mission was called Vega, a contraction of the Russian words Venera (Venus) and Gallei (Halley) and was conducted by the USSR in cooperation with a number of other countries.

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.

The spatial, temporal and mass distributions of coma dust particles for masses > 10−13 g, which we measured with the DUCMA instruments and reported earlier (Simpson et al., 1986a), have been confirmed and extended to reveal additional properties of the dust coma. We have analyzed the inverse square dependence (∝ R −2) of flux with distance R from the nucleus and show that there are well-defined envelope boundaries for the measured range of masses. Regions of enhanced fluxes above these R −2 ‘baselines’ inside these boundaries are identified as dust jets. Of special interest is the giant flux enhancement near closest approach for Vega-1, for which we show that the mass spectrum is similar to the pre-encounter spectra and that there is a smooth rate of rise of intensity over more than four seconds leading into the peak. This is a flux gradient similar to that found for several other much smaller flux enhancements inbound and outbound. The mass dependence of terminal velocities for some jets is consistent with recent predictions, and recurring jets observed by both spacecraft—when traced back to the nucleus—correlate with the active regions predicted by Sekanina and Larson (1986). We have discovered particles with a wide range of masses arriving in clusters or ‘packets’ (i.e., a non-Poisson distribution of dust particles) throughout the regions beyond the envelope boundaries — namely, the fringe regions of the dust coma. These observations suggest the emission from the nucleus of large conglomerates of small particles which gently disintegrate as they travel outward to account for the 10−13 g particles observed beyond the envelope boundaries. Also, dust fluences over the missions have been determined. Finally, we report that both the DUCMA electronics and dust sensors maintained their calibrations and performance throughout the encounters and during the post-encounter interplanetary missions.

Preliminary analyses of Vega 1 and Vega 2 images of the comet Halley nucleus have led to a derived rotation period of 53.5±1 hours (2.23±0.04 days) about an axis approximately perpendicular to the long axis of the nucleus,1–3. Images obtained by Giotto4–6 were also found to be consistent with the Vega-derived rotation period. Recently, however, a longer rotation period of 7.4 days has been proposed7, based on an apparent double-peaked periodicity in the production rates of CN, C2 and dust. Although we acknowledge possible photometric evidence for time-variable production rates and appreciate the assumption that this periodicity should somehow be related to the rotation of the nucleus, we conclude, nevertheless, that the imaging observations from the three spacecraft provide a compelling refutation of the proposed 7.4-day rotation period. Of crucial importance to this conclusion are: (1) the non-symmetrical shape of the Halley nucleus; and (2) the independent observations of this irregular body by Vega 1, Vega 2 and Giotto over an interval of 7.7 days. We note, however, that a rotation period of 53.5 hours and a longer period of torque-free precession with the angular momentum vector inclined somewhat to the spin axis might be consistent with both the spacecraft and ground-based observations.