***** File VEGA.TXT NOTE: This file was created by scanning the original hardcopy article and only the Figure captions are included. The Vega Missions * R. Grard ESA Space Science Department, ESTEC, Noordwijk, The Netherlands T.I. Gombosi Central Research Institute for Physics, Hungarian Academy of Sciences, Budapest, Hungary R.Z. Sagdeev Space Research Institute, Academy of Sciences of the USSR, Moscow, USSR * Based on a more comprehensive document entitled 'Venus-Halley Mission, Experiment Description and Scientific Objectives of the International Project Vega (1984-1986)', May 1985, prefaced by R. Sagdeev, and published by Imprimerie Louis-Jean, 05502 Cap, France 1. Introduction A unique opportunity to combine missions to Venus and Comet Halley is available in 1985-1986 by employing a two-element space vehicle consisting of a Venus lander and a Halley flyby probe. This mission is being conducted by the USSR with the cooperation of a number of other countries. Two spacecraft, Vega-1 and Vega-2*, have been launched aboard Proton rockets from Baykonour (Kazhakstan), on 15 and 21 December 1984, respectively. The two spacecraft are identical and the 'redundancy' is aimed at increasing the overall reliability of the scientific mission. In June 1985, the landers were separated from the Halley probes in the vicinity of Venus. They were injected into the planet's atmosphere to perform measurements until after surface impact. During their descent, the landers each released a balloon, to drift in the planet's atmosphere. These balloons were tracked with the help of an international Very-Long-Baseline Interterometry (VLBI) network. After relaying the lander telemetries towards the Earth, the Halley probes con- tinued their journey towards the comet, following an orbital correction and a gravita- tional manoeuvre around Venus. The cometary flyby will occur in the period 6-12 March 1986, at distances of the order of 10,000 km from the nucleus and with a relative velocity of 78 km/s. One of the Vega programme's aims was study of Venus' atmospheric composition and circulation, clouds and planetary surface. This article, however, is devoted entire- ly to the second aim of the mission, the exploration of Comet Halley. The scientific objectives of the Vega cometary investigation are: (i) determination of the physical parameters of the nucleus: dimensions, shape, temperature and surface properties (ii) study of the structure and dynamics of the coma around the nucleus (iii) definition of the gas composition in the close vicinity of the nucleus and the nature of the parent molecules (iv) study of the dust particles' composition and mass distribution as functions of the distance to the nucleus, and (v) study of the interaction of the solar wind with the atmosphere and ionosphere of the comet. The Halley probes are three-axis-stabilized and their orientation will be defined with an accuracy of 1deg during Halley flyby. The optical instruments are mounted on a point- ing platform which can track the nucleus with an angular accuracy of the order of 5 arcmin. The other instruments are mounted on the main structure of the probes. The data are transmitted via two independent telemetry links, with capacities of 3072 bit/s and 65 536 bit/s, respectively. Each Halley probe carries a complement of 14 ex- periments, which are listed in Table 1. 2. The scientific payload 2.1 Television system (TVS) The purpose of the television system (TVS) is to detect the cometary nucleus, measure its dimensions and albedo, and study the structure and dynamics of the central part of the coma. This instrument also constitutes the sensor for the servo-system that controls the motion of the pointing platform, on which it is mounted together with the two other optical instruments (IKS and TKS). The television system consists of two telescopes, one narrow-angle camera (TVY) for high-resolution imaging of the nucleus, and one large-angle camera (TDN) for detecting and tracking the comet (Fig. 1). The TVY optics have a reflecting objective with a focal length of 1200 mm and a detector which yields an average angular resolution of 3 arcsec, i.e. a spatial resolution of the order of 150 m at the nominal flyby distance of 10,000 km. The maximum angular dimension of the nucleus and its near environment is expected to be 5 arcmin at closest approach, and the pointing error is estimated to be +/- 5 arcmin. The field of view of the TVY must therefore be not less than 15 arcmin. __________________________________ * The name Vega is a contraction of the Russian words Venera (Venus) and Gallei (Halley) The TDN has a refractory objective with a focal length of 150 mm. It is characterized by an angular resolution of 0.5 arcmin, required for early acquisition of the comet and its nucleus, and a 2deg field of view imposed by the constraints associated with control of the pointing platform. The light collected by each telescope is divided into two paths by a beam splitter. One channel is fitted with a fixed filter, the other has a set of eight filters mounted on a rotating wheel, to yield a spectral analysis of the signal. The images are formed on area CCDs cooled by a passive radiator regulated by a Peltier plate. The commutable filters of the TDN have an additional function, namely to adjust the amount of light collected by the detector. This channel is operated autonomously and performs an independent analysis of the video signal, thus providing redundant information to the platform pointing system for the sake of reliability. The signals delivered by the three other TDN and TVY channels are handled by the same microprocessor system which analyzes the images and generates the commands that control the motion of the platform. The TVS electronics include a 816 kbit memory to store both data and programs. The main characteristics of the TVS are given in Table 2. ---------------------------------------------------------------------------------------------------------------------------- Table 1. VEGA scientific payload Direct telemetry Recorded telemetry Collaborating institutes Acronym Experiment Mass (kg) Power (W) (bit/s) (bit/20 min) (Principal Investigators) ---------------------------------------------------------------------------------------------------------------------------- TVS Television 32 50 32768 LAS, Marseille, France (P. Cruvellier) System Central Research Inst for Physics, Budapest, Hungary (L. Szabo) IKI, Moscow, USSR (G. Avanesov) IKS Infrared l8 18 2048 4320 Observatoire de Meudon, France(M. Combes) Spectrometer IKI, Moscow, USSR TKS Three-Channel 14 30 12288 Observatoire de Besancon, France (G. Moreels) Spectrometer IKI, Moscow, USSR (V. Krasnopolskii) Bulgaria (M. Gogoshev) PHOTON Shield Pene- 2 4 108 USSR tration Detector DUCMA Dust Particle 3 2 100 100 University of Chicago, USA (I. Simpson) Detector MPI, Lindau, W. Germany IKI, Moscow, USSR Central Research Institute Physics, Budapest SP-2 Dust Particle 4 4 1024 2160 IKI, Leningrad, USSR (E. Mazets) Detector SP-1 Dust Particle 2 l 150 2160 IKI, Moscow, USSR (O. Vaisberg) Detector PUMA Dust Mass 19 31 10240 MPI, Heidelberg, W. Germany (J. Kissel) Spectrometer Serv d'Aero, Verrieres, France (J.L. Bertaux) 11(1, Moscow, USSR (R. Sagdeev) ING Neutral Gas 7 8 1024 1080 MPI, Lindau, W. Germany (E. Keppler) Mass Spectro- Central Research Institute Physics, Budapest meter IKI, Moscow, USSR University of Arizona, USA PM-1 Plasma Energy 9 8 2048 15120 IKI, Moscow, USSR (K. Gringauz) Analyser Central Research Institute Physics, Budapest MPI, Lindau, W. Germany ESA Space Science Dept., ESTEC, Netherlands TN-M Energetic Par- 5 6 512 6480 Central Research Institute Physics, Budapest ticle Analyzer Hungary (A. Somogyi) IKI, Moscow, USSR MPI, Lindau, W. Germany ESA Space Science Dept., ESTEC, Netherlands Nuclear Research Institute, Moscow, USSR MISCHA Magnetometer 4 6 512 2160 Space Res Inst , Graz, Austria (W. Riedler) Izmiran, Troitsk, USSR APV-N Wave and 5 8 2048 28080 IKI, Moscow, USSR, (S. Klimov) Plasma Analyser Aviation Institute, Warsaw, Poland Geophysical Scientific Institute, Prague APV-V Wave and 3 2 512 15120 ESA Space Science Dept., ESTEC (R. Grard) Plasma Analyser LPCE, Orleans, France Izmiran, Troitsk, USSR ---------------------------------------------------------------------------------------------------------------------------- Figure 1. The telescopes of the television system (TVS): (a) high-resolution camera, (b) low-resolution camera ------------------------------------------------------------------------------ Table 2. Television System characteristics Camera system High resolution Low resolution ------------------------------------------------------------------------------ Objective Reflector Refractor Focal distance 1200 mm 150 mm Aperture 240 mm 50 mm Relative aperture 1:5, effective 1:6.5 1:3 Channel Multispectral Integral Multispectral Integral ------------------------------------------------------------------------------ Spectral range, micro m 0.4-1.1 0.63-0.76 (1) 0.4-1.1 0.63-0.76 (2)0.63-0.76 ------------------------------------------------------------------------------ Number of filters 8 1 Range (1):1 1 Range (2):7 ------------------------------------------------------------------------------ Field of view 26.4'x39.6' 211'x316' 211'x158' Resolution 3.1"x4.1" 24.75"x33" 99"x132" ------------------------------------------------------------------------------ Shutter Mechanical Electronic ------------------------------------------------------------------------------ Exposure time range 0.01-163 s 6-800 ms ------------------------------------------------------------------------------ Detector area 5l2x512 pixels 512x256 pixels ------------------------------------------------------------------------------ Data compression Floating window of 128 X 128 pixels Full image of around brightest point 128x128 pixels after integration of 4x2 pixels ------------------------------------------------------------------------------ 2.2 The infrared spectrometer (IKS) The infrared spectrometer is designed to study the radiation from the inner coma, in the wavelength range 2.5-12 micro m. The instrument includes two high-resolution spectral channels for the chemical analysis of the cometary matter, gas and dust, and an image modulation channel for determination of the size of and thermal emission from the nucleus. The light that exits from the telescope is divided into three by means of two beam- splitters. The secondary beams are then focussed on a wheel that rotates at the rate of 8 rev/sec and carries three rings, namely two circular variable filters and one image modulator. Behind the encoding wheel, the beams enter a cryostat which cools the three detectors (Fig. 2). The two filters cover the ranges 2.5-5 micro m and 6-12 micro m, which include the emission bands of the parent molecules. The long-wave channel can detect water ice and the short-wave channel can be used to identify a number of minerals, such as silicates. The imaging channel does not resolve details of the nucleus, the objective being to derive its most important parameters, size, shape, temperature and optical properties in the infrared. To accommodate the nucleus pointing uncertainties, a field of view of 1deg is judged necessary and an angular resolution of 1 arcmin satisfactory. More instrument characteristics are given in Table 3. Since the sensitivity of the instruments is degraded by four to five orders of magnitude between 77 K and room temperature, it appears mandatory to cool the detectors during the two measuring sequences using the Joule-Thomson expansion of nitrogen. For that purpose, 2 l of nitrogen are stored in four tanks at a pressure of 350 atm. The detector temperatures reach stability within 25 min (to +/- 0.1deg) and can be maintained so for 3 h. 2.3 The three-channel spectrometer (TKS) The objectives of the three-channel spectrometer are to define the chemical composition of the cometary coma and tail, to identify the polarization and spectrum of the light component diffused by the dust, to detect the primary molecules, and to obtain the spectral signature of the nucleus and its environment. Figure 2. Optical system of the infrared spectrometer (IKS) The TKS instrument includes a Cassegrain refractory telescope. The secondary mirror can be tilted about two axes in increments of 8 arcmin. It covers a field of 2deg x 1.5deg, equivalent to an area of 350x260 km**2 at a distance of 10**4 km. The spectral map is made up of 7 lines and each line consists of 15 locations. The measurement cycle at each location lasts 5 s, so that a complete spectral map is taken every 8 min 45 s. Three slits located in the focal plane of the objective form the beams which enter the three channels of the spectrometer, as illustrated in Figure 3. The visible and ultraviolet channels are similar and symmetrically arranged. A holographic diffracting array makes a spectral image of the entrance slit on a photocathode system. The electron flows delivered by the elements of the photocathode are first amplified by four orders of magnitude and subsequently accelerated to an energy of several keV before impacting on a luminescent screen. Optical fibres then transfer the spectrum from the screen to a linear CCD. Each channel delivers a spectrum of 700 points every 5 s. The UV measurements provide information about the fluorescence spectra of a number of atoms, radicals and ions, whereas the visible channel gives the spectrum of the light diffused by the dust and the nucleus. The infrared and polarization channel makes use of an interferential circular filter, which sweeps the spectrum as it rotates. Two sectors of this wheel are occupied by narrow-band filters, which are transparent for the wavelengths 560 and 920 Nm; each of these two filters is itself divided into three zones covered with various polaroids. The filter wheel is associated with a modulator, which rotates at a faster rate, allowing the detected signal to be differentiated more easily from the superimposed noise. A fraction of the light entering the infrared polarisation channel is deflected by a beam -------------------------------------------------------------------------------- Table 3. Infrared Spectrometer characteristics Objective Ritchey-Chretien Primary mirror aperture 140 mm Secondary mirror aperture 56 mm Focal distance 538.1 mm Field of view 1deg Resolution Diffraction limited -------------------------------------------------------------------------------- Channel Imaging Short wavelength Long wavelength -------------------------------------------------------------------------------- Wavelength. micro m 7-14 2.5-5 6-12 Spectral resolution, 2.5 50 50 lambda/delta lambda Optical transmission 0.10 0.39 0.33 Detector HgCdTe In Sb HgCdTe Chip area, mm**2 2x2 2x2 2x2 Geometrical factor, cm**2 sr 0.046 0.045 0.038 IR background, W 1.2x10**-4 3.5x10**-6 9.7x10**-5 Photon noise, W 1.3x10**-12 4x10**-13 1.4x10**-12 Nominal NEP, WHz**-1/2 4x10**-12 2x10**-12 1x10**-11 at 6 kHz at 200 Hz at 200 Hz -------------------------------------------------------------------------------- Figure 3. Schematic of the three-channel spectrometer (TKS) splitter and focussed on a detector which yields the integral signal received over the whole infrared range. The infrared signal contains spectral components that characterize the vibration modes of a number of parent molecules. The map of the integral infrared flux will be used to locate the position of the nucleus. since the flux is proportional to the columnar dust density integrated along the line of sight. Polarization measurements will complete this information by giving indications of the size of the dust particles. The main characteristics of the TKS instrument are summarized in Table 4. -------------------------------------------------------------------------------- Table 4. The Channel Spectrometer characteristics Objective Cassegrain Primary mirror aperture 100 mm Secondary mirror aperture 43 mm Focal distance 350 mm Field of view 2.5deg x 1.5deg (using tiltable secondary minor) -------------------------------------------------------------------------------- Channel Ultraviolet Visible Infrared Polarimeter -------------------------------------------------------------------------------- Spectral range, Nm 115-290 280-700 900-1800 560-920 Spect res, lambda/delta lambda 700 700 100 100 Sensitivity, Rayleigh 200 200 10**6 10**6 Spatial res., km**2, 10**4 km 75x3 75x3 300x30 300x30 Detector Photocathode Photocathode Ge, photo- Ge, photo- & linear CCD & linear CCD diode diode -------------------------------------------------------------------------------- 2.4 The shield penetration detector (PHOTON) The objectives of this experiment are: (i) to measure the flux density of dust particles in the high-mass range, (ii) to understand the mechanism of high-velocity impacts on the spacecraft surface, and (iii) to establish the performance of the meteoroid shields protecting a number of subsystems. The impact surface is a circular nickel sheet, 0.1 mm thick, which makes an angle of 52deg with the Sun's direction and 60deg with the dust flow direction (Fig. 4). A piezo- electric element and an optical system which consists of a silicium photoemissive diode and a parabolic mirror with a focal distance of 7.5 cm are mounted on the back side of this foil. The whole detector is mechanically decoupled from the structure. Figure 4. View of the shield penetration detector (PHOTON) -------------------------------------------- Table 5. Shield Penetration Detector characteristics Target material Nickel Target thickness 0.1 mm Sensor effective area 137 cm**2 Dust mass range 10**-10 -10**-5g Volumic mass density range 0.8-3.5 g/cm**3 -------------------------------------------- The dust angle of incidence is such that the impact ejecta are collected by the side wall of the chamber, thus minimizing the degradation of the mirror surface. The acoustical signal is recorded by the piezo-electric element and the luminous flash associated with the impact and the increment in solar illumination due to the perfora- tion are measured with the optical system. The technical parameters of this experiment are given in Table 5. ----------------------------------------------------- Table 6. Dust Particle Detector and Mass Analyser characteristics Impact detector 28 micro m**2 PVDF foil Detector area 75cm**2 Maximum count rate 10**5/s Differential dust mass 1.5x10**-13 -9x10**-13 g range 9x10**-13 -9x10**-12 g 9x10**-12 -9x10**-11 g Integral dust mass range >9x10**-11 g Integration time for flux measurements at encounter 2 s ------------------------------------------------------ 2.5 The dust-particle counter and mass analyzer (DUCMA) This instrument measures the count rate and mass distribution of dust particles in the cometary environment. The detector is a 28 micro m thick film of polarized polyvinylidene fluoride (PVDF), covered on each face with a metallic conducting coating. A dust particle impacting the detector will displace a small volume of polarized material in the bulk of the detector, which then results in a fast depolarization signal whose amplitude is a known function of the particle mass and velocity. Electronic circuits measure the pulse height and ac- cumulate pulse events above four different threshold levels. Since the relative impact velocity is known (i.e. the comet-spacecraft velocity), the mass is determined direct- ly from the known mass/velocity relationship for these detectors. The detector assembly is shown in Figure 5. It consists of the dust detector (M), and a small anti-coincidence detector (V), mounted perpendicular to the direction of arrival of the dust particles. The purpose of the small detector is to detect very large mechanical shocks on the spacecraft which might trigger the most sensitive level of the dust detector. Further technical details are given in Table 6. The detector also includes an anticoincidence system mounted in a direction perpen- dicular to the direction of arrival of the dust particles. This instrument was selected one year before launch and could still be considered at this late stage because it did not require any direct telemetry or telecommand inter- face with the spacecraft. Its inclusion was facilitated because it was allowed to share the data format and telecommands initially allocated to the ING instrument. Figure 5. The detector of the dust-particle analyzer (DUCMA) 2.6 The dust-particle impact detector (SP-2) The objective of this instrument is to measure the flux and the mass distribution of the dust particles. The counter makes use of acoustical and plasma detectors (Fig. 6). The acoustical detector consists of three piezo-electric elements mounted on a mem- brane (3), two of these sensors (4) are connected to two identical recording circuits in order to improve the overall reliability of the system; the third element (5) is used as a stimuli source for calibration purposes. The membrane is mounted in a frame (l) which damps the mechanical oscillations and improves the counting rate. The detector assembly is fixed to the electronics box (6) by means of three acoustical insulators (2). Each piezo-electric element delivers a signal which is fed into a narrow-band amplifier working at a frequency of about 160 kHz. The output signal is split into l6 channels, which have their sensitivity thresholds logarithmically distributed across the whole dynamic range. Four identical impact plasma detectors are mounted at the periphery of the acoustical detector. They are associated in pairs and connected to identical electronic circuits. The entrance to each detector is protected against the environmental plasma by a system of deflecting electrodes and grids (7,8). The ions and electrons generated by each particle impact are separated and collected by a grid (9) and a target (10), bet- ween which a potential difference of 2 kV is applied. The electron pulse detected by the lower electrode constitutes the input signal, which is analyzed in several channels with different sensitivity thresholds. The characteristic features of SP-2 are listed in Table 7. --------------------------------------------------------------------- Table 7. Dust Particle Impact Detector characteristics Detector Piezo-electric Impact plasma --------------------------------------------------------------------- Total sensor area 500 cm**2 40 cm**2 Maximum count rate 4095 s**-1 65500 s**-1 Integration time 1 s 1 s Mass resolution,m/deltam 2.82 10 Number of channels 16 6 Dynamic range 2x10**-6 - 3x10**-3 g 3xl0**-16 - 3x10**-11 g --------------------------------------------------------------------- Figure 6. General view and schematic of the dust-particle impact experiment (SP-2). See text for details 2.7 The dust-particle impact plasma detector (SP-1) The main scientific objective of the SP-1 instrument is similar to that of SP-2, name- ly to measure the flux and mass distribution of the dust particles. This instrument detects the electric charges contained in the plasma cloud generated by a solid particle impacting on a gold target. The principle of this technique is similar to that adopted for the impact plasma detectors of SP-2. The magnitude of the positive and negative charges is proportional to the mass of the particle Q=Am, where A= 10**3C/s for an impact velocity of 78 km/s. The SP-1 system includes two similar detectors (Fig. 7). Each unit is made up of a base plate (1) covered by a gold target (2) at zero potential, perpendicular to the dust flow, and an array of strip collectors (4). The collectors are parallel to the dust flow; they are mounted on an insulator (3) and their edge is protected from impacts by a shield (5) connected electrically to the structure. Adjacent collectors are biased at potentials of 30 V, with opposite polarities. The two sets of electrodes in each detector detect a positive and a negative current pulse, which are analyzed and recorded by the electronics unit. The entrance to one detector is covered by a plastic foil, to obtain additional information on density and/or calibration factor. The characteristic parameters of SP-1 are given in Table 8. ------------------------------------------------------------------------------------------ Table 8. Dust Particle Impact Plasma Detector characteristics Target material Gold (0.1 mm) Total sensor area 160 cm**2, one sensor covered with 0.6 and 2 micro m thick plastic foil on Vegas-1 and 2, resp. Impact charge range 3x10**-14 - 10**-8 C Estimated mass range 3x10**-17 - 10**-11 g Integration time 2 s (high data rate) 2.5 min (low data rate) ------------------------------------------------------------------------------------------ Figure 7. General view of the dust-impact plasma instrument (SP-1) and details of the detector. Dimensions are given in mm. See text for details 2.8 The dust mass spectrometer (PUMA) The dust mass spectrometer measures the chemical composition, the size and the spatial density of solid particles using a time-of-flight technique, with particular em- phasis on the determination of the Li, C and B isotopic ratios. The operating principle of PUMA, illustrated in Figure 8a, is similar to that of the PIA instrument flown on Giotto. The dust particles enter through a baffle and impact on a silver target (M) at a speed of 78 km/s. The particles and a certain amount of the target material are vaporized and partly ionized. The two Vega spacecraft have different targets (Fig. 8b); one type is mounted in a cartridge as in the PIA, the second has a corrugated surface such that a larger amount of projectile ions enter the analyzer. The target is at + 1020 V; the ions are accelerated by a grid (1), which is held at a potential of -2000 V, and enter the field-free drift tube at zero potential (4). These charged particles are sent by the electrostatic reflector (5) into the second drift tube (6) and on towards the detector (7). The ions trajectories are focused by the lenses (9), (10) and (11). A set of three elec- trodes (12), consisting of an inner grid at +1000 V between two grids at zero poten- tial, prevents ions with energies less than 1 keV from reaching the detector. The geometry of the reflector is designed in such a way as to bunch ions of the same species (particles with energies E>E(0) travel a larger distance than those with energies E= 2) 3.2-13 MeV/nucleon Integral flux of protons and nuclei Energy >= 13 MeV/nucleon Geometrical factor 0.2 cm**2 sr Field of view 25deg half cone Time resolution 4 s (encounter); 10 or 20 min (cruise) ---------------------------------------------------------------------------------- ---------------------------------------------------------------------------------- Table 13. Magnetometer characteristics Dynamic range +/- 100 nT Resolution 0.05 nT Noise level 0.01 nT Hz**-1/2 Zero drift +/- nT/month Bandwidth 10 Hz Time resolution 10 vector/s (high-speed telemetry) 1 spectrum/25 s Number of frequency points 128/spectrum ---------------------------------------------------------------------------------- The experiment consists of two sensors, one dipole made of two meshed spheres with integral pre-amplifiers and a Faraday cup, and an electronics box (Fig. 13). The dipole detects electric fields and the Faraday cup measures ion-flux fluctuations with frequencies of up to 1 kHz. The electric sensors are mounted on a Y-shaped 5 m long boom and the Faraday cup is located on the spacecraft body (Fig. 14). The main parameters of the APV-N experiment are given in Table 14. 2.14 The electric-field and Langmuir-probes experiment (APV-V) The primary objectives of the plasma and wave measurements performed by APV-V are: (i) to measure the density of the solar wind just before it is influenced by cometary constituents, thereby establishing a reference for understanding the subsequent solar. wind-comet interaction, (ii) to observe the mass loading of the solar wind by com- etary ions, either directly or through the associated wave instabilities, (iii) to obtain plasma-density and temperature profiles, as well as wave-frequency spectra during the cometary transit, and (iv) to search for the signatures of collision-free shocks and con- tact surface. The sensors are mounted on two symmetrical stubs attached to the outer solar panels (Fig. 15). Two spheres, 10 cm in diameter (Fig. 14a) and separated by 11 m, form a dipole for measuring electric fields. The Langmuir probes are fixed at mid length; they are cylindrical and their collecting area is 4.4 cm**2; they are oriented in such a way that their symmetry axis is parallel to the gas-flow velocity vector during the flyby; conical elements fixed at their tips protect them from the direct impact of corn- etary gas and dust particles (Fig. 14b). The main characteristics of the APV-V instrument can be found in Table 15. -------------------------------------------------------------------------------- Table 14. Plasma-Wave and Ion-Trap Experiment characteristics Electrical antenna Base line 2m Frequency range 0.01-1000 Hz Sensitivity 1 micro V/Hz**1/2 at 1kHz Dynamic range 60 dB Faraday cup Collector area 5 cm**2 Frequency range 0.01-1000 Hz Sensitivity 10**-3 A/cm**2 at 25 Hz Dynamic range 60 dB Data analysis (for both instruments) Signal waveform 10**-2 - 10**2 Hz Spectral analysis in range 10-1000 Hz 10 frequency points/s Passband filters (10-100 Hz and 100-10O0 Hz) 1 sample/s -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- Table 15. Electric-Field and Langmuir-Probe Experiment characteristics Electrical antenna Base line 11m Frequency range 0-300 kHz Sensitivity 3 micro V/m Hz**1/2 at 1 kHz Dynamic range 70 dB Waveform analysis 0-8 Hz Passband filters in range 8 Hz - 300 kHz 16 filters Langmuir probes (two units) Collector area 4.4cm**2 Frequency range and waveform analysis 0-4 Hz Sensitivity 2x10**-12 A/cm**2 Dynamic range 60 dB -------------------------------------------------------------------------------- Figure 13. The plasma-wave and ion-flux experiment (APV-N): electronic box (back left), ion trap (back right) and meshed spheres (front) Figure 14. The sensors of the electric field and Langmuir probe experiment (APV-V): (a) electric-field sensor, (b) Langmuir probe with its protective cone 3. The spacecraft The Vega spacecraft was composed of a Halley flyby probe and a Venus descent module; the whole system weighed about 4.5 t. The Halley probe is shown in Figure 15 in its nominal flyby configuration: the orientation of the vehicle velocity relative to the comet is also indicated. The spacecraft has a wingspan of the order of 10 m, and it carries 120 kg of scientific instrumentation. On its trajectory to Venus, the probe was still surmounted by the descent module (not shown in Fig. 15), which was a spherical object with a diameter of 2.5 m and a mass of approximately 2 t. The Vega vehicle is derived from the Venera series of spacecraft. A number of modifications improve the reliability of the probe; for example, 5 m**2 of shield have been added in order to protect the most essential subsystems against the bombardment of dust particles with masses of less than 0.1 g. A dual-sheet bumper shield has been adopted; it is composed of a thin metallic front sheet (0.4 mm) and a thicker rear sheet, separated by several centimetres. Figure 15. The Vega spacecraft in cometary flyby configuration after release of the Venus lander. The orientation of the relative velocity vector ('relative' in the comet frame of reference) is defined by its projections in the XY- and ZY-planes of the spacecraft coordinate system The spacecraft structure resembles a cylindrical body connected to two conical skirts. The lower skirt houses a motor for orbital manoeuvres and a toroidal pressurized utility instrument bay; the cylindrical compartment contains the fuel tanks and the upper skirt is the interface that held the Venus lander. Two pairs of deployable solar panels are mounted on each side of the cylindrical section: the solar array has a total area of nearly 10 m**2. The spacecraft is three-axis-stabilized during the cometary flyby by a gyroscopic system and a number of gas nozzles, most of which are mounted on the solar panels. The telemetry system consists of a high-data-rate channel (BRL) and a low-data-rate channel (BTM). The BRL channel is used for real-time transmission only. Its capacity of 65536 bit/s can be reduced by half if required by propagation conditions; that of the BTM channel is 3072 bit/s. The scientific data can also be stored by onboard magnetic tape recorders (capacity 5 Mbit) and subsequently telemetered through the BTM channel, once every 20 days during the interplanetary transit and once every 20 min around the time of cometary flyby. The high-gain antenna must be directed towards the Earth whenever data are transmitted via the BRL channel. The scientific instruments can be classified into three categories, characterized by common objectives: (i) The electromagnetic field sensors (MISCHA, APV-N and APV-V) are mounted on booms, as far as possible from the spacecraft to achieve the best degree of electromagnetic cleanliness. (ii) The dust, gas and plasma detectors have pointing directions generally related to the spacecraft velocity relative to the comet. (iii) The optical systems that observe the nucleus (TVS, IKS, TKS) are located on the automatic pointing platform. The platform is shown in Figure 16; it has a mass of 82 kg and carries 64 kg of in- strumentation. It can scan an angular sector of 110deg in the ecliptic plane, and 60deg in a plane perpendicular to the ecliptic. Figure 16. General view of the pointing platform without thermal blanket 4. The Mission The Vega spacecraft were launched from the Tyuratam pad area of the Baykonour Cosmodrome by two three-stage Proton rockets fitted with six strap-on boosters. The space vehicles were injected into their trajectories towards Venus before completing their first revolution around the Earth (Fig. 17a). During most of the transit towards Venus, the solar panels were oriented towards the Sun. The spacecraft's attitudes were not otherwise controlled, except during possi- ble periods of a few hours when three-axis-stabilization was required for high-bit-rate data transmission. Orbital corrections were performed during the first two weeks after launch and was repeated during the last two weeks before arrival in the environment of Venus. Three further orbital manoeuvres were foreseen on the second leg of the journey to Comet Halley: the first, two to four weeks (Fig. 17b) after the Venus flyby, the second midway between the planet and the comet, and the last two to four weeks before the Halley flyby. Once the last manoeuvre has been performed, the pointing platform will be oriented and the camera can then take its first look at the comet. Fig- 17. The paths of the Vega probes (a) from Earth to Venus, and (b) from Venus to Halley. The projection of the inner portion of the trajectory of Halley on the plane of the ecliptic is a1so shown. The Comet and the Earth are orbiting in planes that intersect at an angle of 18deg. The nodes are the points where Halley's orbit intersects the ecliptic All experiments are to be switched on two days before closest approach. The direct high-speed telemetry will be transmitted from -48 h to -45 h at a cometary distance of 14x10**6 km, from -24 h to -22 h at a cometary distance of 7 x 10**6 km and from -2 h to + 1 h during the cometary flyby. Two other high-speed telemetry transmission sequences are also foreseen, one and two days after flyby. A limited amount of data can also be stored and transferred every 20 min to the onboard tape recorder, in order to cover the 22 h gaps when the high-speed telemetry is switched off (Table 1). A number of other telemetry modes are available. They are being used for, for example, transmitting low-bit-rate information from a number of selected experiments during the interplanetary cruise. Acknowledgements The author wishes to thank A. Ammar, P. Cruvellier, E. Keppler, J. Kissel, S. Klimov, G. Moreels, W. Riedler, J. Runavot, J. Simpson, A. Somogyi, and O. Vaisberg for providing useful information and their critical reading of this paper.