Vega (1984-1986)

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Overview

The only USSR mission to comet Halley was that of Vega - a mission involving two probes (Vega 1 and 2). They were identical crafts whose purpose was to carry probes to Venus and then intercept Halley in March of 1986.

The craft were launched December 15 and 21, respectively, and arrived near Venus in June of 1985. They launched the probes and then used Venus' gravity to get speed boosts for Halley the following year.

Vega 1 encountered Halley on March 6, 1986, and Vega 2 reached that locale on March 9. The flyby took place at 77.7 km/sec.

The craft were able to be targeted to a precision of 100 km, but the positions relative to the comet are known to only within a few thousand km. This, in combination with dust, led to estimates of 10,000 km for Vega 1 and 3000 km for the Vega 2 closest approaches. Data were taken from 2.5 hours before through 0.5 hours after the closest approaches, with several periods of data taking before and after, each lasting about 2 hours.

Spacecraft Details

Each craft's main features were large solar panels, a high-gain antenna dish, and an automatic pointing platform for carrying experiments that required directional pointing at the comet's nucleus.

The automatic platform could rotate ±110° and ±40° in two perpendicular directions with a pointing accuracy of 5 arcminutes and stability of 1 arcminutes/sec. On it were the narrow- and wide-angle camera, three-channel spectrometer, and infrared sounder. All other experiments were on the body, with the exceptions of two magnetometer sensors on a 2 m boom and various plasma probes and plasma wave analyzers on a 5 m boom.

The spacecraft was shielded from hypervelocity dust impacts by a shield consisting of a 100 µm multilayer sheet 20-30 cm from the spacecraft, and a 1 mm Al sheet 5-10 cm from the spacecraft. Approximately half of the Vega craft were devoted to the Halley module, and half to the Venus lander package.

Venus Descent Module

The Venus package was a sphere 240 cm in diameter, which separated from the Vega craft two days before Venus arrival. The package entered Venus' atmosphere on an inclined path and without active maneuvers.

The lander was identical to Venera9-14 and had the objectives of studying the atmosphere and superficial crust. It had temperature and pressure sensors; the descent probe had a UV spectrometer for minor atmospheric constituent measurement, H2O measurement instrument, and others for chemical composition of the condensed phase determination - a gas-phase chromatograph, x-ray spectrometer for observing fluorescence of grains or drops, and mass spectrograph measuring the chemical composition of grains or drops.

The x-ray spectrometer separated grains according to their sizes using a laser imaging device, and the mass spectrograph separated them according to their sizes using an aerodynamical inertial separator.

After landing, a small surface sample near the probe was analyzed by gamma spectroscopy and x-ray fluorescence. The UV spectrometer, mass spectrograph, and pressure- and temperature-measuring instruments were developed in cooperation between French and Soviet investigators.

Balloon Aerostat

Besides the lander probe, a constant-pressure instrumented balloon aerostat was deployed immediately after entry into the atmosphere at an altitude of about 54 km. The balloon's diameter of 3.4 m supported a total mass of 25 kg. The payload was 5 kg and hung suspended 12 m below the balloon.

The balloon floated at approximately a 50 km altitude in the middle, most active layer of Venus' three-tiered cloud system. Data from its instruments were transmitted directly to Earth for the 47 hours of life of the mission (the batteries had a lifetime of 60 hours. Instruments measured the temperature, pressure, vertical wind velocity, and visibility (via density of local aerosols).

Very long baseline interferometry was used to track the motion of the balloon to provide the wind velocity in the clouds. The tracking was done by a six-station network on Soviet territory and 12 stations distributed world-wide (organized by France and the NASA Deep Space Network).

After two days and 9000 km, the balloon entered the day side of Venus and expanded, bursting in the solar heat.

Experiments

Television System (TVS):

• This consisted of a wide-angle camera (WAC), a narrow-angle camera (NAC), and an electronics block.
• The WAC was to be used for large-scale coma imaging and as a guide for the NAC. The basic task of the NAC was imaging the nucleus and the surrounding area of Comet Halley; that of the WAC was directing the pointing platform and its instruments to the object of examination.
• Both cameras used CCDs with 512x576 pixels each as detecting devices in the focal plane. The combined data rate for the two cameras was 48 kbps, which was not sufficient to transmit the full contents of the CCDs. Only a "window" one-tenth of the area of the CCD, around the center of brightness, was transmitted.
• The exposure time had to be kept short to keep image blur to a minimum, but it could not be less than 0.01 sec if good sensitivity was to be achieved. The narrow-angle camera could resolve nucleus surface structures down to 200 m from a distance of 10,000 km.
• A set of filters (500-1050 nm) with a relatively wide (80 nm) passband was used in the NAC. The WAC filter covered the range 630-760 nm. The NAC had a focal length of 1200 mm, an f-number of f/6, and a 0.5° field of view; for the WAC these parameters were 100 mm, f/2, and 4°, respectively.
• In addition to the purely scientific objectives of imaging the nucleus, the cameras also had the task of providing the information needed to determine the spacecraft's trajectory relative to the nucleus.

Three-Channel Spectrometer (TKS):

• This was intended for spectral and polarization studies of the dust, spectral mapping of the coma, and determination of the outflow rates of various gases from Comet Halley and their content.
• The instrument had a Cassegrain telescope with a focal length of 500 mm and an objective diameter of 140 mm. The light flux passed through three 1° slits located in the focal plane to three independent spectroscopic channels in the UV, visible, and infrared.
• The UV channel covered the range 120-350 nm, with spectral resolution of 0.5 nm, spatial resolution of 3 x 6 arc-min, and sensitivity of 3 rayleighs. The visible channel covered 350-900 nm, with spectral resolution of 1 nm, spatial resolution of 3x6 arc-min, and sensitivity of 10 rayleighs.
• The infrared channel covered 900-2000 nm, with spectral resolution of 10-12 nm, spatial resolution of 6x60 arc-min, and sensitivity of 3E4 rayleighs. The UV and visible channels used micro-channeltrons for detectors, while the infrared channel used a germanium photodiode.

Infrared Spectrometer (IKS):

• The objectives for this were to determine the size, radiation capacity, and temperature of the nucleus; the nature, density, distribution, and temperature of the dust; and the nature, relative content, and temperature of the parent molecules of Comet Halley.
• The instrument had a Cassegrain telescope with a focal length of 500 mm, diameter of 140 mm, and field of view of 1°. The radiation flux was separated into three beams, each of which passed through its own filter located on a wheel spinning at up to 20 rpm. Two of the channels were devoted to the spectroscopic mode in the wavelength intervals 4000-8000 and 8000-16,000 nm. The third channel was devoted to nucleus imaging at 7000-14,000 nm. Three Hg-Cd-Te photoconductors cooled to 80 K by liquid nitrogen were used as detecting devices.

Dust Mass Spectrometer (PUMA):

• This was designed to estimate the mass and chemical composition of particles in the dust coma. A similar instrument was flown on the Giotto spacecraft.
• The instrument was mounted parallel to the relative velocity vector and analyzed the chemical and isotopic composition of individual dust particles. Impact of a dust particle on the instrument's silver target area caused a plasma to be formed consisting of dust and target material, from which ions were extracted by a 1.5-kV electric field. The ions traveled through a time-of-flight tube (actually two tubes with an electrostatic reflector between, with total length of 1 m) where they were separated according to their mass before being recorded by an electron multiplier.
• The ion mass range was 1 to 100 u, with resolution M/(ΔM) of 200. The instrument observed the spectra of the most common dust particles, which were expected to be in the size range 100-10,000 nm. The instrument was sensitive to dust particles in the mass range 3E-16 to 5E-10 g.

Dust Particle Counter (SP-1):

• This consisted of three piezo-element detectors mounted on a special metallic plate to measure the amplitude of the wave generated by dust particles heavier than 1E-10 g impacting on the plate. The amplitude was proportional to the mass of the dust particle.
• From the arrival time of the pulse at the three detectors, the coordinates of the impact point could be determined. The dead time of the instrument depended on the acoustic decay of the signal in the piezo-elements and could turn out to be significant. The counter was oriented in the direction of the relative velocity vector.

Neutral Gas Mass Spectrometer (ING):

• This measured the elemental and isotopic compositions of the neutral gases in the coma. The instrument was comprised of two mass spectrometers. These were known as EIS and FIS, according to their ionization sources.
• The EIS had an electron impact source of ionization, followed by an electrostatic analyzer, and covered the mass range 1 to 28 u, with resolution of approximately 4%. The FIS had a field ionization source, followed by a time-of-flight velocity analyzer, and covered the mass range 1 to 80 u. Its resolution was a function of the mass.

Plasma Energy Analyzer (PLASMAG):

• This was designed to answer four main questions:
  1. How do the solar wind parameters change as the comet is approached?
  2. Does a near-cometary shock exist in the solar wind, and, if so, where is it and how do the plasma parameters change across it?
  3. Where is the "contact surface" (the cometary ionosphere boundary) and what are the number density and chemical composition of the ions in the cometary ionosphere?
  4. What is the chemical composition of the ions produced by photoionization of cometary neutral particles outside the contact discontinuity and even outside the bow shock and picked up by the solar wind?
• The instrument was composed of five detectors:
  1. An ion spectrometer consisting of a hemispherical electrostatic energy analyzer with a quadrupole electrostatic lens at the aperture was pointed towards the sun, to measure solar wind ions at 30 energy levels logarithmically spaced between 50 eV and 25 keV. Energy resolution was 4%. The field of view was approximately a cone with a half-angle of 25°, and the flux range of the detector was from 5E4 to 5E9/(sq cm-sec).
  2. A similar ion spectrometer was oriented along the spacecraft-comet relative velocity vector and covered the energy range from 15 eV to 3.5 keV at 120 levels. Energy resolution was 4%. The field of view was approximately a cone with a half-angle of 6°. I the thermal velocities of the cometary ions were considerably lower than the encounter velocity, a mass spectrum in the range 1 to 100 u could be obtained. Mass resolution was 4%. The ion density measurements covered the range 1E-3 to 1E5/cc. In this detector the sensitivity could be decreased by a factor of 1000, and this was done for one full spectrum (1 sec) every 4 sec.
  3. An electron detector with a cylindrical electrostatic analyzer was oriented with its aperture normal to the spacecraft-sun line and measured electrons in the energy range 3 to 5000 eV with energy resolution of 5%. The angular aperture was approximately ±5°. This was used both for the measurement of solar wind electrons ahead of and behind the near-cometary shock and for the measurement of energetic electrons inside the cometary ionosphere. To determine the degree of degradation of the channeltron, a separate analyzer (with a tritium isotope particle source) using the same channeltron was operated for a short time once per day. To provide a larger dynamic range of measurements, an additional regime of measurements with sensitivity reduced by a factor of 100 was introduced for 0.5 sec duration every 4 sec, for the measurements of energies up to 30 eV.
  4. An integral plane multigrid retarding potential analyzer (RPA) was directed toward the sun. A short honeycomb in front of the aperture protected it against impacting dust particles, and the field of view was ±45°.
  5. A similar RPA looked along the relative velocity vector, with a field of view of ±8°. This RPA had no honeycomb, but the grids were replaced by relatively thick diaphragms with holes. This detector could be operated in four modes: (a) total ion flux was counted, including cometary ions, local environment, and background; (b) the same, but with ions of the local plasma environment retarded; (c) background only; and (d) the negative suppressor grid potential was replaced by positive 40 V, so that the collector current was due mainly to secondary electrons from the collector produced by cometary neutrals and dust particles.
• Detectors (1) through (3) yielded one spectrum per second, while the RPAs yielded eight current measurements per second. This was true for the encounter mode (3 hours). Beginning 48 hours before the encounter mode, the measurements were slower and sensitivities greater by a factor of 150. In the third mode, used during cruise, only the electron analyzer and the ion sensors pointed toward the sun were operated, and two spectra were measured by each spectrometer during 10 sec every 20 min.

Energetic Particle Analyzer (TUNDE-M):

• The objective of this was to study the solar wind interaction with the atmosphere and ionosphere of the Comet Halley.
• The instrument consisted of two semiconductor detectors plus an anti-coincidence scintillator to filter out particles from lateral directions. The device measured accelerated cometary ions in the energy range 20 keV to 20 MeV, and electrons from 175 keV to several MeV. The field of view was 30°, and the detector was oriented in the ecliptic plane.

Magnetometer (MISCHA):

• This instrument, MISCHA (Magnetic fields in Interplanetary Space during Comet Halley's Approach), was designed to measure the constant component of the magnetic field and its low-frequency fluctuations in the cometary and solar wind interaction zone and in interplanetary space.
• The instrument consisted of two fluxgate sensor units mounted 1.5 m apart on a 2 m boom. One unit was oriented along the spacecraft axis, which was perpendicular to the solar panels and pointing toward the sun.

Wave and Plasma Analyzer (APV-N)

• This was designed to study the mechanism of anomalously high ionization of cometary gas, the shock-front structure, and the phenomena in the region of the contact surface (ionopause).
• The analyzer had a frequency range of 0.1 to 1000 Hz, and was designed to monitor waves excited in the cometary environment, in particular the lower hybrid waves (10 Hz) and ion cyclotron waves (1 Hz).
• A twin-probe technique was used to measure the potential difference between two probes placed on the 5-m boom isolated from the spacecraft. The plasma flow fluctuations were measured with a Faraday cup at the boom's tip.

Wave and Plasma Analyzer (APV-V):

• The primary objectives of this investigation were:
  1. 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
  2. to observe the mass loading of the solar wind by cometary ions either directly or through the associated wave instabilities
  3. to obtain plasma density and temperature profiles, as well as wave frequency spectra during the cometary transit
  4. to search for the signatures of collision-free shocks and contact surfaces
  5. to monitor the electric potential of the spacecraft during the flyby of the nucleus
• Electric fields were measured in the frequency range 0 to 300 kHz; the low frequency fluctuations were transmitted after direct sampling of the waveform; and the upper part of the spectrum, from 8 Hz to 300 kHz, was analyzed with a set of 16 adjacent and logarithmically spaced filters.
• The dynamic range of the electric field measurements was about 70 dB. The plasma density and temperature were measured with two Langmuir probes. The polarization voltage of one probe was kept at a fixed value of 5 V and the low frequency fluctuations of the electron current were analyzed in the frequency range 0 to 4 Hz by direct sampling of the waveform. The potential of the second probe was swept by a sinusoidal voltage with a period of 32 sec superimposed on a DC voltage; the duration of the sweep, its amplitude and its average level could be modified by telecommand. The range of the density measurements was typically 10 to 1E4 per cc for a nominal electron mean kinetic energy of 1 eV. It was foreseen, however, that the effect of plasma emission from the spacecraft surface by impact of gas and dust would have to be carefully evaluated when interpreting the plasma measurements.

Dust Particle Detector (DUCMA):

• This was designed to measure the fluxes of impacting dust particles with masses above the four threshold values of 1.5E-13, 9E-13, 9E-12, and 9E-11 g.
• The dust detector consisted of 75 cm² of a 28 µm polarized polymer film, with conducting surfaces. This film, of polyvinylidene fluoride, had a permanent electric dipole moment per unit volume ("electret") with the polarization direction normal to the film surface. Destruction of the polarization along the path of the impacting particle produced a fast current pulse which was detected by a charge-sensitive amplifier. The magnitude of the signal was a known function of the mass and the velocity of the impacting particle. Because all of the dust particles impact with the flyby speed of 78 km/s, the electronically set thresholds corresponded to known mass values.
• There was a second similar detector, called the veto detector, which was protected from direct dust impacts and responded only to acoustic noises generated by activities of the spacecraft. This veto detector had two output channels, set at a low and a high threshold value. The ten counting rates of the instrument also included four single counting rate channels from the four thresholds of the dust detector, and the same four channels in anti-coincidence with either the low or the high level of the veto detector.
• The instrument was self-calibrating either periodically or upon command, and operated normally even when a portion of the detector was destroyed by large dust particles. It was calibrated at the Heidelberg dust accelerator up to velocities of 12 km/s.
• The counting rates for dust particles required no correction up to 1E4 particles/sec, and only a small correction for 5E4 particles/sec. The instrument was to be powered throughout the mission, so that after Venus encounter it was to search for interplanetary dust particles during the cruise mode.
• The detector was described by J. A. Simpson and A. J. Tuzzolino in Nuclear Instruments and Methods, in press, 1985. A description of the entire instrument is to be published in Nuclear Instruments and Methods, 1985 or 1986.

Dust Particle Counter (SP-2)

• This detected impacting dust particles in the mass range E-12 to E-18 g by observing the plasma clouds produced by the impacts.

Energetic Particles (MSU-TASPD):

• This experiment was one of several provided by the Theoretical and Applied Space Physics Division of the Skobeltsyn Institute of Nuclear Physics of Moscow State University.
• The experiments flew on the Soviet Zond, Luna, Mars, Venera, Vega, and Phobos interplanetary/planetary missions and measured energetic (MeV) particles in the interplanetary medium.
• The earliest missions (Zond 1 and 3) measured protons above 30 MeV. Later missions measured lower energy protons and some also measured fluxes toward and away from the sun.

Craft Data