THE VENUS-IIALLEY CAMPAIGN: 1984

Campaign objectives:

For the Soviets this campaign combined a Venus flyby/entry mission with a flyby of Comet Halley, and it was their first (and thus far only) multiple-target mission. After releasing their entry systems at Venus in June 1985 the two flyby spacecraft were to be re-targeted by the gravity-assist of their encounter with the planet onto a course to intercept Comet Halley in March 1986.

In addition to a lander, the entry system carried an atmospheric balloon. The idea to float a balloon in the atmosphere of Venus grcwr from French-Soviet cooperation initiated after the successful Venera 4 mission in 1967. France and the Soviet Union had come to a rapprochement of sorts in the Cold War, opening a breach in the Iron Curtain by establishing cooperation in space science. In 1974 Dr. Jacques Blamont of CNES and Boris PeLrov, Chairman of the Intercosmos Council, began to discuss a joint mission consisting of an entry probe to deliver a large French balloon into the atmosphere of Venus, and a Soviet orbiter to provide the communications relay. By 1977 a date had been tentatively set for a 1984 launch of the ‘Venera-84’ mission to mark the bicentennial of the Montgolfier brothers’ invention of the hot-air balloon, and the division of work had been established. Jacques Blamont and Mikhail Marov were named as science co-chairs for the mission. The French would supply the two 10 meter diameter balloons with their 50 kg gondolas, including transponders for very long baseline interferometry (VLB!) tracking, and the Soviets would supply the spacecraft, entry systems, and the remaining mission support. But events changed these plans.

In the late 1970s the world’s space science community was beginning to plan for the eagerly awaited apparition of Comet Halley in 1986. The IJS offered to carry a French ultraviolet instrument on one of its spacecraft. When the US withdrew from this effort in 1979 the Soviets offered to fly the French instrument on Venera-84 to enable it to observe the comet from Venus orbit – which would be a more favorable vantage point because although the comet would approach that planet no closer than 40 million km. that was much closer than it would approach Harth. In the process of investigating how to improve observations of Halley from Venus the Soviets found that it would be possible to utilize a gravity-assist during a flyby of Venus to set up an encounter with Halley. The science value of a mission to both Venus and Halley as argued by Jacques Blamont intrigued Roald Sagdeev, Director of 1K1, who set out to have it supersede the Venera-84 mission. The new project w as called ‘Vega’ as a Russian contraction of’Venera’ and ’Galley’, with the name of the comet using a G’ because there is no H’ in the Cyrillic alphabet. Valery Barsukov. Director of the Vernadsky Institute, w as far more interested in Venus than he w as in the comet, but Sagdeev sold the mission to him by including a lander, albeit at the cost of reducing the size of the balloon package to enable both to fit inside the standard entry system. Three years of intensive development of the Venera-84 mission, including partially manufactured hardware, was lost. When the furious French declined to participate further, the small balloon became a Russian project. Nevertheless, Sagdeev managed to coax the French into providing several instruments for the lander and balloon, as well as two key remote sensing instruments for the Halley encounter. And by taking advantage of their bridging position between the East and the West, the French were able to gain the participation of the Deep Space Netw ork in the VLB1 network that would measure the dynamics of the balloons as they drifted in the atmosphere of Venus. For the first time, therefore, the arch rival Americans became a participant in a Soviet planetary mission, albeit by providing tracking resources. The University of Chicago supplied an instrument to investigate dust particles during the Halley flyby, but this was arranged through the science community as a private venture rather than at government level and the principal investigator had to assure the US military that he was using only commercial parts from his local Radio Shack store! He dismissed the military’s concerns with. "Let them [the Soviets] copy this, it will set them back years.’

Sagdeev, by enthusiasm, energy, and personal effort, instituted the new project as a broadly international venture by off ering 120 kg on the spacecraft for instruments originating from countries outside the USSR. This extensive internationalization was unprecedented for the historically closed Soviet space program. And internally the perestroika initiative enabled him to overcome resistance by the Soviet bureaucracy.

But the final credit must go to Chief Designer Vyacheslav Kovtunenko and the NPO-Lavochkin scientists and engineers wTo, by building the most comprehensive and successful deep space mission in their history, created a legacy for Soviet lunar and planetary exploration.

Vega 1 (5VK No. 901)

Venus Flyby/Lander/Balloon and Halley Flyby USSR, NPO-Lavochkin ’ ‘ ‘

Proton-K

December 15, 1984 at 09:16:24 UT (Baikonur) June 11, 1985 March 6, 1986 Successful.

Vega 2 (5VK No.902)

Venus Flyby/Lander/Balloon and Halley Flyby USSR, NPO-Lavochkin ’ ’ ‘

Proton-K

December 21, 1984 at 09:13:52 UT (Baikonur) June 15, 1985 March 9, 1986 Successful.

The Vega missions became an integral part of the International Halley Mission (IHM) organized initially by the European Space Agency to coordinate operations and data analysis for the various Halley missions being planned by Europe, Japan, the IJS and the Soviet Union. An Interagency Consultative Group consisting of high level representatives of the space agencies overseeing the IIIM provided cover for US participation in the midst of the Cold War, effectively ci re um venting the absence of a formal agreement between the US and USSR. Ironically spacecraft were sent to Halley by all these nations except the US, whose formal involvement was ultimately limited to providing tracking and science support.

With lander, balloon, and flyby components the Vega missions were both very ambitious, and by involving a host of international interfaces including a large array of international instruments were extraordinarily complex. The nations participating included Austria, Bulgaria, Czechoslovakia, East Germany, France, Hungary. West Germany, Poland, and the United States. The Hungarians built part of the navigation system and the Czechs supplied the optical system for the automated scan platform. Foreign investigators were allowed into the country to participate fully and actively in the project from beginning to end; not passively as previously by delivering their completed instruments in advance and waiting at home to find out what happened to them. Team meetings were held in the USSR and foreign contributors were allowed into Soviet facilities for development, testing and integration activities. This style of cooperation w ith the USSR w7as unprecedented. An organization called Intercosmos had existed since the 1960s for coordination of cooperation in space research mainly among Eastern Bloc nations and with France, but this was the first time the activity assumed such a large scale and included Western nations to such a degree.

The 1984 launches gave the Soviets enormous influence in the international space
community. With such a bold move to internationalization, leadership in planetary exploration passed to the USSR. After the busy era of Mariner, Pioneer. Viking and Voyager launches in the 1970s, the US launch rate had fallen precipitously to zero in the 1980s. The USSR continued to reap a harvest from its Venera series, and began its transition from a closed program to an open program far more international than any flight project in the US. The Soviets now issued open calls for participation in its science missions. US science missions would not become more international than ”participation by invitation onlv*

The Vega missions were highly successful in meeting all their science objectives, and a major achievement for the Soviet robotic lunar and planetary program. They concluded the run of ten consecutive highly successful heavy-class Venera missions that started with Venera 9 in 1975. and they were the final Soviet missions to Venus after twenty-nine launch attempts since 1961. During this 24 year period only three of sixteen windows for Venus were not used. Nineteen of the twenty-nine launches sent spacecraft on trajectories to Venus, of w hich fifteen successfully delivered three entry probes, ten landers, tw’o balloons, and four or biters. The Soviet scientists and engineers participating in the Vega missions would have dismissed as ridiculous the prospect of there being only two more campaigns in the Soviet planetary exploration program, both of w’hich would be embarrassing failures.

Spacecraft:

Flyby spacecraft:

The flyby spacecraft was nearly identical to Venera 9 to 14 but used the larger solar panels of Venera 15 and 16 to handle the pow er demand and w^as loaded w ith 590 kg of propellant instead of the usual 245 kg. It w’as protected from hypervelocity comet dust impacts by an aluminum shield consisting of an outer multi-layer sheet of 100 micrometers thickness mounted at a standoff distance of 20 to 30 cm.

A data rate of 65 к bits/s was provided for the comet encounter, but a slower mode would be used in the cruise phase. Approximately half of the spacecraft w as devoted to the Halley science instruments and half to the Venus entry system. In making the flyby of Venus in the manner required to set up the Halley encounter, the spacecraft would relay to Earth the transmission from the lander during its descent and surface operations as previously. However, the balloon would transmit its telemetry directly to Earth.

The spacecraft was fitted with an 82 kg articulated scan platform that could rotate from -147 to +126 degrees in azimuth and from -60 to +20 degrees in declination for a pointing accuracy of 5 minutes of arc and a stability of 1 minute of arc per second. Its automated tracking would enable instruments to be continuously pointed at the nucleus of the comet during the rapid flyby while the spacecraft held an orientation that permitted its high gain antenna to point at Earth for real-time transmission. The pointing was controlled either by an eight-element photometer or by using the wide angle camera, and gyroscopic attitude control was provided as a

Figure 18.1 Vega spacecraft (courtesy NPO-Lavochkin). Scan platform folded on left, parabolic antenna on the right, toroidal instrument compartment on the bottom with external instruments.

Figure 18.2 Museum model Vega spacecraft without insulation and dust shields. Front side at right shows solar panels, parabolic antenna, and navigation instruments. Back side at left show’s camera platform hanging down below toroidal instrument section, radiator panels and black disks where helical lander relay antennas were mounted.

precaution against comet dust upsetting the optical sensors. The scan platform carried the narrow and wide angle cameras, an infrared sounder, and a three – channel spectrometer. All other experiments were body-mounted except for two magnetometer sensors and various plasma probes and plasma wave analyzers which were mounted on a 5 meter boom. The total science payload for Halley weighed 130 kg.

Figure 18.3 Vega 1 folded and ready to launch. Note scan platform, insulation and metal shielding.

Entry system:

The entry system was virtually identical to the recent Venera missions, consisting of an insulated sphere 2.4 meters in diameter whose upper and a lower hemispheres were joined non-hermetically. In this case, however, the lander was installed in the lower half and the balloon in the upper half.

Figure 18.4 Entry capsule cross-section (by James Garry): 1. Antenna; 2. Balloon compartment; 3. Helium inflation tank; 4. Lander aerodynamic stabilizer; 5. Gas chromatograph; 6. Spectrophotometer; 7. Entry heat shield; 8. Thermal insulation; 9. Oscillation damper; 10. Battery; 11. Stabilizing vanes; 12. Crushable impact torus; 13. Drill and sample collector; 14. Coolant delivery piping: 15. Balloon aerobrake; 16. Science instrument bay; 17 Parachute.

Lander:

The Vega landers were almost identical to the Venera 13 and 14 landers with some aerodynamic modifications for increased stability while free falling. These included spoke-like blades interior to the landing ring to reduce spinning and a thin collar-like sleeve installed beneath the disk of the aerobrake to minimize the turbulence which would be induced by the externally mounted instruments.

Figure 18.5 (left) show’s the sleeve and the blades. Tn view on the landing ring are the two white hygrometer compartments, the temperature and pressure unit offset to its right, and also the drill. Figure 18.5 (right) shows the large shiny cylindrical gas chromatograph on the ring to the left, the horizontal drill vacuum reservoir, and the penetrometer and the hydrometers on the far right. The impact velocity of 8 m/s was to be cushioned by the shock absorbers that support the main spherical pressurized compartment.

Figure 18.5 Venera 13 and Venera 14 landers during tests at Lavochkin

Strap to balloon

Semi-Directional / Antenna

Straps

Radio Transmitter and Electronics

Temperature Sensors

Photometer Pressure Sensor Lithium Batten" „ Nephelometer Windows

r-3 ——— lb кґ 4 ,—^

Anemometer

Figure 18.6 Gondola diagram (from Don Mitchell) and testing on a short tether.

Balloon:

The balloons were a new component, and were to be carried in and deployed by the upper hemisphere of the entry system. The super-pressure helium aerostat with its attached gondola w as designed to float in the middle layer of cloud at an altitude of 54 km, where the temperature was a mild 32’C and the pressure was 5.35 millibars.

Each radio-transparent balloon had a mass of 11.7 kg and when inflated it wfas 3.4 meters in diameter and held 19.4 cubic meters of helium that had a mass of 2 kg. A 13 meter long tether suspended the 7.0 kg gondola (including 1.6 kg for the tether). The entire system weighed a little over 20.7 kg. The rate of helium diffusion w’as sufficiently low to sustain pressure for about 5 days.

The 1.2 meter long 14 cm wide gondola contained a transmitter with a stabilized oscillator for Doppler tracking, a conical antenna, a vertical anemometer, sensors for ambient temperature and pressure, a light photometer, a nephelometer. a eontrol and ballast system, and sixteen lithium batteries for 300 wratt-hours of power. The 1 kg battery package w as designed for 46 to 52 hours of life. To simplify the task for the network of radio telescopes wdiich would track the balloons, the 4.5 W transmitter operated in the 18 cm astronomical band at 1.6679 GHz. It transmitted direct to Earth via the conical antenna at either 1 or 4 к bits, s. Except for the lightning counter which was sampled every 10 minutes and the photometer twice every 30 minutes, all the other instruments were sampled once every 75 seconds. The data w as stored on a 1,024 bit memory. A 5.5 minute burst of data was sent to Earth every 30 minutes, alternating between two transmission inodes in a predetermined sequence. In the first mode, 852 bits of data collected from the instruments were transmitted in a 270 second burst preceded and followed by 30 seconds of carrier for VLB1 velocity measurements. In the alternative 330 second mode, only two tones were transmitted for VLBI position and velocity.

The balloon system had to be folded up during cruise and entry, survive the forces of deployment, and then withstand the corrosive atmosphere of sulfuric acid aerosol. The envelopes were made using a woven teflon and cloth matrix, the gondola was covered with a white paint resistant to sulfuric acid, and the tethers were made of a type of nylon. Timing, as determined by pressure sensors, was critical to successful deployment: if the envelope were inflated at too high an altitude it would burst in the low pressure; if it were inflated at too low’ an altitude it w ould not gain the necessary buoyancy, would penetrate too deep and be destroyed by the high temperature. The inflation system had 2 kg of helium, and altitude control would be by the release of ballast.

The balloon system was carried in the upper hemisphere of the entry system, in a toroidal canister that surrounded the helical antenna of the lander. In addition to the folded balloon and gondola, this canister contained a 35 square meter parachute and the spheres of pressurized helium to inflate the balloon. The deployment began at an altitude of 64 km by separating the hemispheres while on the drogue parachute. This released the lower hemisphere containing the lander. Separation deployed a braking parachute for the lander, which then performed its own deployment sequence as on previous missions. The upper hemisphere then released the toroidal balloon package

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Scientific instruments

Navigation sensors

Scan platforms

Oust shield

Scientific instruments

Figure 18.8 The Vega spacecraft configured for Halley encounter (courtesy NPO – Lavochkm).

at 62 km, deploying its parachute in the process. At 57 km the package deployed the balloon system. At 55 km the inflation system was activated. By the 53 km level the envelope had inflated and the package on the parachute released the balloon system. At 50 km the balloon system released its ballast, deployed the boom that carried the temperature sensors and anemometer, and then rose to 54 km to travel wherever the prevailing wind took it. Since the temperatures in this altitude range w’ere benign there was no requirement for thermal control.

On Earth, a global distribution of twenty international antennas consisting of two networks was ready to perform Doppler tracking and receive the scientific data from the balloons – one at a time, as they were to arrive at the planel several days apart. One network was led by 1K1 and used six Soviet antennas including a new 70 meter dish that was built for the Vega missions. The second network was led by CNES and used the three 64 meter antennas of the Deep Space Network in the US, Australia and Spain, and astronomical antennas in Brazil, Canada, England, Germany, Puerto

Rico, South Africa, and Sweden. Doppler tracking by each antenna gave the range and velocity along the Earth-Vcnus line, but lateral motion of the balloons required interferometry that combined phase information from antennas located far apart and linked electronically. In addition, the network simultaneously tracked carrier wave signals provided by one or other of the two flyby spacecraft to provide a third leg to greatly increase the precision of distance and velocity measurements in a differential interferometry technique developed by the IJS for the Apollo lunar missions.

Vega spacecraft system mass

Payload:

flyby spacecraft:

Mounted on the scan platform:

1. TV imaging system (TVS, USSR-France-Hungary)

2. Three-channel (ultraviolet, visible and near-infrared) spectrometer (IKS. France-USSR-Bulgaria)

3. Infrared spectrometer (IKS, France)

Body mounted:

1. Dust mass spectrometer (PUMA, FRG-USSR-France)

2. Dust particle counter (SP-1)

3. Dust particle counter (SP-2)

4. Dust particle detector (DUCMA, USA)

5. Dust particle detector (FOTON)

6. Neutral gas mass spectrometer (ING, FRG)

7. Plasma spectrometer (PLASMAG)

8. Energetic particle analyzer (TUNDE-M. Hungary-USSR-FRG-ESA)

9. Energetic particles (MSU-TASPD)

10. Magnetometer (MISCHA, Austria)

11. Low frequency wave and plasma analyzer (APV-N, USSR-Poland-Czecho – slovakia)

12. High frequency wave and plasma analyzer (APV-V, USSR-France-ESA)

Three instruments for remote sensing of Halley were mounted on the ASP-G scan platform; the 32 kg TVS camera, the 14 kg TKS three-channel spectrometer, and the

18 kg IKS far-infrared spectrometer. The camera was Russian and the spectrometers were provided by France. The far-infrared spectrometer was cryogenically cooled by a Joule-Thompson cryostat and operated in the range 2.5 to 12.0 microns. The three – channel instrument operated in the ultraviolet 120 to 290 nm, visible 275 to 715 nm, and near-infrared 950 to 1,200 nm. The flyby range at Ilailey was deliberately large to avoid damaging to the spacecraft, so to obtain the desired view of the nucleus the camera required a narrow angle optical system capable of a resolution of 150 meters at a range of 10,000 km. The computers for the science instruments were enhanced by using Western electronics. Hoivever. because CCD technology was restricted the Soviets had to develop their ота 512 x 512 device for the camera. The optics were built by the French, and comprised a 150 mm f/3 wide angle lens that was limited to the red, and a 1,200 mm f/6.5 narrow angle lens with six filters from the visible to infrared. The Hungarians were responsible for the camera electronics with assistance from the Soviets.

The five instruments to study the dust issued by the nucleus of the comet were the

19 kg PUMA dust particle impact mass spectrometer to measure the composition of

A

individual dust particles, the 2 kg SP-1, 4 kg SP-2, and 3 kg DUCMA dust particle counters to determine the flux and mass distribution of dust particle in different si/e ranges, and the FOTON dust particle detector that was installed to measure the large particles that punched through the standoff shield. In-situ measurements by the 7 kg INC neutral gas mass spectrometer would analyse gas in the space through which the spacecraft was traveling. The composition and energy spectrum of ions would be determined by the 9 kg PLASMAG plasma spectrometer, and the flux and energy of ions would be measured by the 5 kg TUNDE-M energetic particle analyzer. It also had the 4 kg MISCHA magnetometer, two plasma w*ave analyzers, the 5 kg APV-N for ion flux and frequencies below 1 kHz, and the 3 kg APV-V for plasma density, temperature and frequencies in the range 0 to 300 kHz.

Including the scan platform and its supporting structure, the instrument payload of the flyby spacecraft was 253 kg.

Lander:

Entry and descent:

1. Temperature, pressure and wind sensors (МЕТЕО, USSR-France)

2. Hydrometer for water vapor concentrations ( VM-4)

3. Ultraviolet spectrometer for atmospheric SO2 and sulfur measurements (ISAV-S)

4. Optical nephelometer-seatterometer for aerosol size and properties (ISAV-A)

5. Particle si/e spectrometer for aerosols (LSA)

6. X-ray fluorescence spectrometer for aerosol elemental analysis (IFP)

7. Gas chromatograph for aerosol chemical analysis (SIGMA-3)

8. Mass spectrometer for aerosol chemical analysis (MALAKIIIT-V. USSR – France)

9. Doppler experiment for wind and turbulence

The deseent instruments focused on aerosols in particular. There were two particle size instruments for measuring the physical properties of aerosols, two instruments for aerosol chemistry, and one instrument for an elemental analysis of the aerosols. These five instruments had externally mounted components with limited insulation from the ambient temperature and pressure, but since the aerosols were confined to the upper atmosphere they were required to function only above 35 km. The aerosols were carried into the instruments by inlet tubes. Some instruments analyzed the light scattered by the aerosol partieles in these tubes to determine their si/e. The ISAV-A instrument also included a nephelometer to determine the cloud density by shining a beam of light through a window’ in the pressure vessel and measuring the light returned through this window. It shared electronics with the ultraviolet spectro­meter.

The gas chromatograph instrument was specifically designed for Vega to measure sulfuric acid aerosol by trapping the droplets in a carbon saturated filter that reacted with sulfuric acid to produce sulfur dioxide and carbon dioxide.

The x-ray spectrometer was a significant improvement on the ones carried by the

Venera 13 and 14 landers. It distinguished grain size using laser imaging. The mass spectrometer sampling system used an aerodynamic inertial separator to segregate grains into small and large sizes on two separate filters. These were then vaporized and analyzed in the mass spectrometer.

The ultraviolet spectrometer was an active experiment, particularly effective for a descent in darkness. It had an ultraviolet lamp and a 1.7 meter path length absorption cell into which the atmosphere was admitted in order to measure the absorption at 512 points between 230 and 400 nm. The objective was to determine the nature of the mysterious ultraviolet absorber’ deduced from remote sensing measurements. The spectrometer was inside the lander, but there was a pipe through the hull to allow the atmosphere into the instrument. It was operated from 62.5 km down to the surface.

The temperature and pressure instruments were similar to those of the Venera 13 and 14 landers but revised for greater accuracy. They comprised two platinum wire thermometers and three pressure sensors covering the ranges 0 to 2, 0 to 20 and 2 to 110 bar. The hydrometer was also improved.

Surface:

1. Drill and surface sampler (SSCA)

2. X-ray fluorescence speciromcter (BDRP)

3. Gamma-ray spectrometer (GS-15STsV)

4. Dynamic penetrometer (PrOP-V)

As both the gravity-assist to deflect the flyby trajectory for Halley and the mission of the balloons required the Vega entries to occur on the night-side, the landers were not given cameras or optical instruments, and those instruments they did carry w’ere similar to those utilized previously. I he Vega landers were focused mainly on solving mysteries about the atmosphere and rectifying problems with instruments on previous missions that were caused by the hostile atmosphere. The gamma-ray soil spectrometer had been deleted after Venera 9 and 10 in favor of the combined drill and x-ray fluorescence spectrometer; this time they were all carried. And since there were no imagers the penetrometer was upgraded to provide an electrical readout.

Lander instrument mass 117 kg.

Balloon:

1. Temperature and pressure sensors (‘USSR-France)

2. Vertical wind velocity anemometer

3. Nephelometer for density and particle size of local aerosols (USA)

4. Light level photometer and lightning detector

5. Stable oscillator for VLBI measurements

A boom w as deployed from the side of the gondola to expose sensors. One w as a propeller anemometer. It measured vertical winds as fast as 2.0 m/s. The horizontal winds were measured by VLBI analysis of radio tracking. The ambient temperature

was measured by two thin-film resistance thermometers with a range of 0 to 7Cf’C and an accuracy of 0.5°C mounted at separate positions on the boom. Pressure was measured by a vibrating quartz beam sensor with a range of 0.2 to 1.5 bar and an accuracy of 0.25 millibar. The photometer consisted of a silicon PIN diode sensitive in the 400 to 1,100 nm range with a 60 degree field of view at the nadir. It was also designed to detect lightning by counting short bursts of abnormally bright intensity. The ncphelomcter was a simple backscatter instrument similar to those of previous missions.

Mission description:

Flyby spacecraft:

In keeping with the international nature of the project. Westerners were allowed to visit Baikonur and view the launches of Vega 1 on December 15. 1984, and Vega 2 on December 21. This was also the first time that Soviet television showed a Proton launch. And although the US routinely tracked Soviet spacecraft, this w’as the first time that this was done officially. The announcement that an American instrument was onboard prompted a small furor in the US. One of the booms for the plasma wave experiment initially failed to deploy on each spacecraft, but these both sprung out after the first midcourse maneuver.

Vega 1 arrived at Venus in early June 1985, only weeks after Venera 16 had been switched off. The spacecraft released their entry systems 2 days out from the planet,

on June 9 for Vega 1 and on June 14 for Vega 2. The points at which they were to enter the atmosphere were on the night-side in order to enable the spacecraft to head for Halley and to maximize the cruising lifetime of the balloons before they suffered solar heating. After releasing its entry system Vega 1 maneuvered to pass the planet at a range of 39,000 km for the gravitv-assist to llallev and to relay the data from its lander. Vega 2 did likewise at a range of 24.500 km. Each spacecraft turned off-Sun to receive the transmission from its lander at a rate of 33)72 hits/s in the meter band and to relay it to Earth in the centimeter and decimeter bands. They did not conduct any science observations at Venus. On finishing the relay, each spacecraft resumed cruise operations. The gravity-assist of the flyby did most of the work in deflecting the path of each spacecraft toward Halley, but maneuvers were needed to refine the final approach.

Vega 1 flew past the nucleus of the comet at a range of 8.890 km on March 6. 1986. and Vega 2 did likewise at a range of 8,030 km on March 9. Both made highly successful scientific measurements. Two Japanese spacecraft had been observing the comet at extreme distance and Europe’s Giotto was scheduled to arrive on March 13 for a daring close flyby at a range of only 500 km. By combining tracking data with imaging, the Vega spacecraft gave a more precise position for Halley in space than was possible using terrestrial telescopes. This was used to improve the accuracy of Giotto’s terminal maneuvers, both to reduce the targeting error in order to obtain the intended observations and to reduce the potential risk to that spacecraft. Both Vega spacecraft flew through the tail of the comet and were pummeled by small grains impacting at 80 km s. The shields installed on one side of each vehicle protected it from damage. The solar panels suffered both dust impacts and electrical discharges induced by the comet plasma. Vega 1 lost 40% of its power supply and Vega 2 lost 80%. After a circuit around the Sun, both spacecraft passed through the tail again in 1987. providing further data. Vega 1 ran out of attitude control gas on January 30 of that year and then on March 24 contact with Vega 2 was discontinued.

Entry system:

The Vega 1 capsule entered the night-side atmosphere at 01:59:49 UT on June 11. 1985, at a speed of 10.75 kin/s and at an angle of 17.5 degrees. The Vega 2 capsule entered at 01:59:30 UT on June 15, at 10.80 km/s and at an angle of 18.13 degrees. The pilot parachutes were deployed at an altitude of 65 km. Eleven seconds later, at

64.5 km, the capsules split into hemispheres and the pilot parachutes drew the upper hemispheres containing the balloon systems away, in the process deploying the main parachutes of the landers in the lower hemispheres. Hour seconds later, at 64.2 km. the landers shed their hemispheres. Having slowly descended to 47 km. each lander released its parachute in order to free-fall to the surface. The new aerodynamic drag devices successfully reduced both vibration and spin, thereby increasing the stability of the descending landers.

Meanwhile, the balloon packages were released from their hemispheres at 62 km. in the process deploying the pilot parachute of each balloon package. At 57 km the

main parachute was deployed. At 55 km the inflation of the envelope was initiated. With the balloon fully inflated, the main parachute was released at 53 km. At 50 km the inflation system and ballast was released and the balloon system rose to 54 km in the middle of the cloud layers with its gondola deployed to make measurements.

Landers:

The Vega 1 lander settled at 7.11 N 177.48 E. just north of eastern Aphrodite Terra and 0.6 + 0.1 km below the planetary mean radius. It was 03:02:54 IJT on 11 June. 0:24 local time, and the solar zenith angle was 169.3 degrees. The measured surface temperature was 467’C and the pressure was 97 bar. The transmission was curtailed 20 minutes after landing in order to conserve energy on the flyby spacecraft, which was not facing its solar panels at the Sun. and to ensure readiness for the subsequent Halley trajectory maneuver.

At an altitude of 17 km the Vega 1 lander experienced electrical spikes and the Doppler tracking data showed violent upward excursions. This shock triggered the accelerometer that was to indicate contact with the ground, causing a premature start to the surface activity sequence, including deployment and operation of the drill and x-ray spectrometer. As the x-ray soil analysis instrument had failed its pre­launch tests and been flown regardless, this may not have mattered. Venera 11 to 14 and the four IJS Pioneer probes also experienced electrical anomalies in the altitude region 12 to 18 km, but Venera 9 and 10 and Vega 2 did not. The cause of these anomalies remains unknown.

The Vega 2 lander touched down at 7.52°S 179.4 E, 1,300 to 1,500 km southeast of Vega 1 and 0.1 +0.1 km above the planetary mean radius. It was 03:00:50 UT on 15 June, 1:01 local time, and the solar zenith angle was 164.5 degrees. The surface temperature was 462°C and the pressure was 90 bar. The transmission was truncated 22 minutes after landing to preserve energy on the flyby spacecraft. There were no anomalies during the descent and the surface operations were performed nominally.

Balloons:

The Vega balloons were both successfully deployed at the anti-solar point (i. e. local midnight) and drifted with the wind at an altitude of about 53 km where the pressure was about 0.5 bar, right in the middle of the three cloud layers. They were carried longitudinally by zonal winds through the night-side atmosphere for 30 hours before crossing the dawn terminator. No latitude measurements could be made, and it was assumed that the balloons remained at a constant latitude. 8°K in the case of Vega 1 and 7 S for Vega 2. Each balloon transmitted for 46.5 hours until its batteries were exhausted. Loss of signal occurred in the early morning hours on Venus after having traveled some 10.000 km. about one-third the way around the planet. The balloons continued silently into the day-side where they would eventually have succumbed to solar heating and burst their envelopes.

Results at Venus:

Landers on descent:

A telemetry problem prevented Vega 1 temperatures from being transmitted during the descent, but the Vega 2 data indicated the presence of a sharp thermal inversion that reached a minimum temperature of -20°C at an altitude of 62 km. The optical spectrometers operated between 63 and 30 km and reported an atmospheric structure similar to that seen by earlier landers and confirming a three layer cloud deck. But on this mission, as for Venera 8, no sharp lower cloud boundary was observed. Aerosol particle size measurements were taken down to 47 km, and were in general agreement with earlier Soviet results and the data from the Pioneer entry probes and confirmed that there were at least two layers of differing particle sizes. The measurements from Vega 1 and 2 were highly consistent, indicating the cloud layers to be very similar at their entry points except in the uppermost layer where Vega 2 found less dense aerosols than Vega 1. The smallest ‘mode Г particles were speculated to be aluminum and/or ferric chloride. About 80% of the larger ‘mode T particles were shown to be spherical with a refractive index of 1.4, a characteristic consistent with sulfuric acid, wdiile the remaining 20% had a refractive index of 1.7, suggestive of solid sulfur, The highest particle counts wrere in the altitude range 58 to 50 km. The Vega instruments were insensitive to the largest ‘mode 3’ particles reported by Pioneer probes.

The Vega 1 and 2 gas chromatographs and the Vega 1 mass spectrometer were the first to make an in-situ detection of sulfuric acid, confirming remote sensing results and yielding a density for the altitude range 63 to 48 km of about 1 milligram of sulfuric acid per cubic meter. The Vega 1 mass spectrometer heavy particle sample contained sulfur trioxide (sulfuric acid anhydride) and chlorine. Unfortunately, the Vega 2 mass spectrometer failed. The x-ray fluorescence spectrometer on Vega 2 detected sulfur (—1.5 mg/m3), chlorine ( — 1.5 mg/m3), and iron (0.2 + 0.1 mg/’mJ). It also made the first detection of phosphorus (—6 mg/m3), this possibly in the form of phosphoric acid, and explaining the persistence of a small amount of aerosol in the sub-cloud region with a base at 33 km. Iron was also reported by the x-ray

Figure 18.11 Chlorine, sulfur and phosphorus profiles from the descent x-ray aerosol analyzer (from Don Mitchell).

analysis, perhaps as ferric chloride in the aerosols. The Vega 1 x-ray fluorescence instrument failed. The ultraviolet speetrometcrs gave vertical profiles for sulfur dioxide mixing ratios with upper region abundances in general agreement with remote sensing and other sources, and generally deer easing towards /его at the surface. The possibility of elemental sulfur vapor was also noted. Solar ultraviolet was completely absorbed below an altitude of 10 km. although this was probably due to aerosols coating the instrument. The hydrometer reported a water vapor abundance of 0.15% at high altitudes (60 to 55 km) decreasing by a factor of ten at lower altitudes (30 to 25 km). The fact that this large abundance is inconsistent with other measures may indicate that the instrument was confused by other atmospheric constituents. The water vapor profile on Venus remains poorly determined.

Landers on the surface:

The Vega 1 lander conducted a gamma-ray soil analysis but the drill had failed and so no x-ray soil analysis could be performed. The Vega 2 gamma-ray spectrometer, drill, and x-ray fluorescence experiments all worked well.

X-ray fluorescence results from Vega 2 (as oxides):

silicon	47%
titanium	0.2%
aluminum	16%
iron	8.5%
manganese	0.14%
magnesium	11%
calcium	7.3%
potassium	0.1%
sulfur	4.7%
chlorine	<0.3%

These analyses showed rocks poor in iron and magnesium but rich in silicon and aluminum, indicating a composition similar to lunar highland rocks. The fairly high sulfur abundance may be an indicator of older rocks.

Gamma-г а г results:

The potassium, uranium and thorium values were very similar to Venera 9 and 10. in contrast to the Venera 8 results that showed significantly higher concentrations of all three elements.

Balloons:

Even although this was the first attempt at deploying a planetary aerostat, both of the balloons succeeded. They made the first measurements of the horizontal structure of the atmosphere to complement the many vertical profiles from descent probes. The temperature in the Vega 1 air mass was a constant 40°C. It was about 6°C cooler for the Vega 2 balloon. The atmosphere was more turbulent than expected. At times the balloons precipitously plunged in downdrafts of 1 to 3 m/s by hundreds of meters, sometimes several kilometers. The Vega 1 balloon encountered heavy turbulence at the start of its run and then again towards its end. Shortly after sunrise, passing over the Aphrodite Terra highlands, the Vega 2 balloon plunged more than 3 km to a pressure level of 0.9 bar, very close to the lower limit of its buoyant zone, before it rebounded.

atm	‘		km
0.6	–	——-	53
0.7	*	V 1 „	52
o.8	–	__ t___ i____ 1___ I___ i____ l____ I___ i___	5i

Ю 20 30 40 hOlirS

Figure 18.12 Flight profile of the Vega 1 balloon (from Don Mitchell).

The nephelometer on the Vega 1 balloon was hard to interpret due to calibration problems but generally seemed to agree with particle data from the nephelometers on the descent probes, showing the middle cloud in which the balloon drifted to be horizontally homogeneous with no clear regions. Unfortunately, the Vega 2 balloon nephelometer failed. In their cruise to the dawn terminator, the photometers noted some variation in light levels that may have been due to variations in the underlying clouds, and although there were some light flashes there was no strong evidence for lightning. Vega 1 crossed the terminator into daylight 34 hours into the flight, and its photometer registered dawn 2 hours prior to sunrise. The Vega 2 photometer did not
function correctly, but indicated dawn 3 hours before the terminator crossing. The anemometers reported downdrafts of 1 m s. The VLB! Doppler measurements found horizontal winds of up to 240 km/hr, made the first in-situ observations of the ‘super-rotation’ of the atmosphere at this altitude, and made measurements of atmospheric turbulence.

Results at Halley:

The results of the Vega Halley encounter were more than just scientific, they were also cultural and political. The project would be the first to image the nucleus of the world-famous Comet Halley. For the first time, a Soviet mission and its purpose was made known well in advance. The portion of the Vega mission at Venus went barely noticed outside scientific circles, but the whole world was waiting in expectation for the spectacle of the Halley encounter and the Soviets were well aware that this was unlike any other space mission they had ever conducted.

The Vega 1 spacecraft closed in on the comet at the blazing speed of 79.2 km/s in early March 1986. It performed a final trajectory correction on February 10. Its scan platform locked onto the comet on February 14 and began tracking. Far encounter images on March 4 and 5 demonstrated the camera’s performance. On March 6, the day of close encounter, the world’s press was present in the IKI control room for the first time, disturbing the usual professional calm with a bustling jumble of people eager to experience a Soviet mission event as it happened, including US television and media with both Roald Sagdeev and Carl Sagan providing commentary. Sagan as commentator for a Soviet spacecraft encounter in real-time was clear evidence that perestroika had become reality. Vega 1 switched to high rate telemetry 2 hours before closest approach and took over 500 images during the 3 hour encounter. The raw images looked overexposed and fuzzy. It was hard to pick out the nucleus from the obvious dust jets. But the IKI press room was filled with awe and applause. The images and other data streamed in for another 2 days.

Vega 2 closed in 3 days later at 76.8 km s. It did not require a final correction but 30 minutes before the encounter on March 9 it gave its controllers a scare when the computer guidance system failed. However, the spacecraft quickly switched over to the backup system and the observations began as planned. By the time the encounter was over on March 11 the spacecraft had provided over 700 images.

The images of Halley revealed a potato-shaped nucleus 14 x 7 km with a very dark albedo of 4%. a rotation rate of 53 hours, and at least five dust jets that could be counted on its sunward side. The environmental sensors on board the two spacecraft made pioneering measurements of the plasma fields in the vicinity of the comet, and defined the interaction of the solar w ind with the out-flowing eometary gases. Some of the constituents of the gas were identified and measured. The size and flux of dust particles varied enormously as the spacecraft flew through and in between the jets of dust and gas. A number of instruments were lost during the encounter, and the solar panels were extensively damaged by impacts and the electrical discharges that were induced by the cometary plasma.

Figure 18.14 Vega 2 image of Halley (processing by Ted Slryk).

The infrared spectrometer on Vega 2 failed due to a leak in the cryogenic system. The Vega 1 infrared spectrometer was sent an erroneous command which put it into calibration mode during the 30 minutes at closest approach, which was unfortunate, but it did report data taken at greater distances. The C-TT band of hydrocarbons was detected. The fact that the temperature of the nucleus was 300 to 400K meant that it had an insulating layer at its surface. The dust and gas were jetting through fissures in this crust opened by the heating of volatiles contained within. The three-channel spectrometer on Vega 1 was crippled by an electrical fault, and despite its partner on Vega 2 losing the ultraviolet channel this was able to detect water, carbon dioxide, the hydroxyl radical and the cyano radical, various other products of hydrocarbon photolysis, ammonia and other organic materials in the coma. It was concluded that the principal components of the gas were water containing carbon monoxide and carbon dioxide molecules, as well as photo-produced radicals and atomic hydrogen, oxygen and carbon.

Analysis of the dust in the jets revealed grains in the submicron-lo-micron size range of compositions varying from metallic to siliceous to carbonaceous. The dust mass spectrometers returned results showing three families of materials: one very similar Lo the carbonaceous chondrile meteorites which are thought to be the most primitive of Solar System material, another enriched in carbon and nitrogen, and the third enriched with water and carbon dioxide ice.

Thus ended one of the most daring, innovative, complex and successful missions in the history of robotic space exploration to that time. It established the USSR as the leader in the field; a distinction that was sadly short-lived and later forgotten.