Ada. Space Rae. Vol.2,No.12, pp.83—90, 1983 0273—1177/83/120083—08$04.00/0

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COMETARY PROBE OF THE VENERA-HALLEY MISSION

R. Z. Sagdeev,G. A. Avanesov,A. A. Galeev,V. 1. Moroz,B. S. Novikov andG.A. Skuridin

SpaceResearchInstitute, Academyof Sciencesof theU.S.S.R.

ABSTRACT

Venera—Halley mission is to be launched to Venus in Dec. 1984. It will flyby Venus in June 1985. Separation of the cometary probe and Venera descendmodule will take place at that time. The gravitational swing—by at Venuswill provide the encounter with the Halley comet in March 1986. The remotesensing of the inner coma (TV—imagery, spectrometry in the region from1200 A to 12j~m, polarimetry) and of the nucleus, direct measurementsofdust fluxes, dust composition, plasma and magnetic field are planned in theframework of multinational cooperation.

Next apparition of the Halley’s comet is probably to become one of the mostexiting astronomical events of the near future. The 1986 geometry of Halley’scomet ground observations will be relatively unfavorable. This is one of thereasons for preparing several missions for Halley’s comet investigations.Be—low we shall briefly describe one of them, that is of the Venera—Halley pro-ject (or ‘Vega’ which is the abbreviation of the Russian name of the project).The Vega project has two objectives: (1) Venus, (2) Halley comet. In 6 monthsafter the start (December 1984) the interplanetary automatic station Vegawill fly by Venus, a planet lander will be jettisoned from it to later landon Venus. The main module with an instrnment package to be used in the co-met’s study will approach the latter in about 7 months.

This cometary probe design uses, as a basic unit, the station that many tim-es delivered “Venera” landers on Venus. In the past the interplanetary plas-ma studies and other passengerexperimentshave been conductedduring theflight to Venus but that was a kind of additional program. The different si-tuation occurs with the Vega mission: now the ‘non-Venus—related’ part ofscientific program has the priority.

Several comments on the international status of the project. The spacecraftitself is under full responsibility of the USSR. The design and developmentof scientific payloads and related support systems involve considerable ef-fort of scientists and technicians not only of the USSR but also of many co-untries in Western and Eastern Europe. The International Scientific and Tech—nical Committee (CIST) with the representatives from 8 member—countries isthe managerof the project. This report is preparedon the requestof theCIST. This is the first large—scaleinternational space project initiatedby the USSR and we hope that the experience gained in the course of its de-velopment will be useful for international cooperation in other future fi-elds. At the same time ESA started another international project for Halley’scomet study (“Giotto”). These two space projects have different capabilitiesand both programs complement one another. The Japanese mission for Halley’scomet investigations will contribute to the same fi~.d.

Tables 1 and 2 summarize some basic characteristics of the Halley’s comet asa celestial body (orbital and physical ). Their comparison makes it clearthat the orbital parameters of the comet are very well known (though we sho-uld know them even better for the project to be a success), while littleis known about its physical structure (which is true about other comets aswell). Almost all 01’ the cometary mess i~ concentrated in its nucleus; how—

84 ~ R.Z. Sagdeev et al.

ever, never have an,yof the cometary nuclei been observed, though some mO—dels seemrather probable those which are based on the analysis of eventsobserved in the coma and tail. On the whole, physics of comets is less ad—vaiiced than for instance, physics of planetary atmospheres. Books and re—views on comets L6] to L1O] demonstrated many white spots in the cometaryphysics.

In our view, of all comets the Halley comet is an optimal goal for the firstcometary flight despite its retrograde orbit. Two serious arguments favorthat viewooint 1 ) comet Halley belongs to the class of short—period cometsand only comets of this class have orbits well enough deterained for spacenavigation; ~2) among short—period c,omets it is the only comet that has pity—sical characteristics of a young active comet.

As usual, scientific objectives of the Vega mission are determined by thebalance between the scientific importance of the problems and realistical’y—assessed capabilities. Duet hazard at an encounter rate of about 80 km.sis a serious danger and the closer we will approach the nucleus the largerthe mass of the dust—protection screen will be at the expense of a smallermass of the payload. Besides there is a risk of passing the nucleus nearits dark rather than lit side It ~ias specified that a nominal fly—by dis—tance of 10,000 km is ootimal. It is sufficiently short to see and follow‘the nucleus with a TV camera and other optical instruments and sufficientlylong for a spacecraft to have a chance to survive during its flight throughthe dust coma.. ~

Table 3 presentssome basic mission characteristics An essential typicalfeature is the three—axis orientation of the probe which allows optics po-inting to the near-nucleusregion of the comet. Optical instruments are mo-unted on a special moving platform controlled by the optical pointing sys-tem. Most information is sent to the Earth in real time, it is rather es-sential in view of the enhancedlevel of meteoroid hazard during the enco-unter with the planet

Basic scientific objectives of the project are:

1 Estimates of physical cnaracteristics of the cometary nucleus (size,shape, surface properties, temperature),

2. Studies of tile structure of the coma’s near—nucleus region (p< 1000km);

3. Dete~minationof the gas composition in the near—nucleus region (prob-lem of parent colecules),

4. Investigatio’~iof the composition of gas particles and their distributi—on over the mass at different distances from the nucleus.

5 studies of the solar ~~i1ia interaction with the atmosphereand ionosphere

of the comet.

Table 4 lists the scientific instrwnents to be used.

The spacecraft will have the following scientific equipment.

TV system includes t’io TV cameras and an on—board microprocess-or. TV provides multiscale black—and—white and color ima~es of tne nucleusana central cuma region. The fooal lengths of ~EVoptical systems are 150mmana 1200 mm, uit~i 512y576 CCD—matrices they ensure 3 5x5.2° and O.4x0 6°fields of viett, respectively The spatial resolution on the nucleus surfaceprovicied oy tbe lon~,—focal—len~,tb camera for the passage at 10,000 km is180 in 8 filter.~ in the range from 0 45 to l.35

1t,av, ijill oe used

The TV aata are also used ‘to control the platform which oints the osticalinstruments to the comet.

The TV—information may be used to solve the “roblems (‘1) and (2) 3pccia—lists from the U~R,Hungaryend Prance participate in designing and manu-facturing the system.

Three—channel spectrometer will give, ~iiti a specified resolution,spectra of cometary erniselonc in the three regions: UV (1200—3500A), via—

C~rnetaryProbe of the Venara—llaIIcy Mission 85

ibie (3500—9000A) and IR (0.9 — 1.7 ~ The snectronieter data are import—ant for objectives (1) to (‘D ~~pectrorne’ter viii orovidc

— investigations of the near—nucleus part of the coma (for which no anydata have ever been available) and may be nucleus itself,

— studies of the dust coma (spectra and polarization),

— spectral maps of coma.

Chemical composition in different parts of the coma, physical propertiesof particles, escape velocities of different gases can be extracted fromthese data.

The spectrometer is being jointly developed by specialists from the USSR,Bulgaria and France.

Infrared spectrometer has three optical channels, two of which (2.5 to5j.~m end 6 to 42 ~m) are to operate in a spectroscopic mode, and the thirdis intended to cdnstruct an IR nucleus image in two colours (7—10 and 10—

In the context of the objectives (1 to 4) this spectrometer is supposedto determine some essential characteristics of the nucleLs and inner coma,that is, to estimate

— the size, emissive power, and temperature of the nucleus;

— the composition and temperature of dust particles;

— the nature, fraction and temperatureof parent molecules.

The device is being built by French participants of the project.

Dust mass—spectromete~ is designed to estimate the mass and ohe—,nical composition of particles in the dust coma. After lutting a slAvertarget set perpendicularly to the vector of the relative velocity (8Okm.s )dust particles are evaporated and partially ionized The number and massof ions thus formed carry valuable inform~ion. The in~rument is sensitiveto dust particles in the mass range 3.10 to 510’~’ g.

This spectrometeris intended ‘to solve the objective (2). USSR, FRG and

France are responsible for it.

Two types of dust—particle counters

:

The acoustic—typ~ counter consists of metal plates with three detectorsmade of piezoelectric elements on eacn plate. Impacting dust particles pro-duce elastic waves in metal plates, which are recorded by piezoelec’tricelements. The delay time between signals from various detectors and vibra-tion amplitudes may be used to determinethe impact point and the momentumproportional to the mass of dust par’ticl~s The latter ma~, also be deter-mined if the relative velocity (80 km.s ) is taken into account The in-strument oriented along the relative~elocity vector may count dust par-ticles with a mass of more than 10— g.

The plasma detector is designed to operate in the particle range io—12to 10-Th g as a dust—impact plasma counter. Dust counters are being builtin the USSR.

Neutral—gasmass—spectrometer. The principle of operation is simple, how-ever, it is very complicated technologically Neutral gas molecules enter-ing the instrument are ionized, ions thus produced are acceleratedto acertain energy. A small (about 10 cm) flight—time analyzer is used to mea-sure the velocity of an ion with that energy, then its mass is a.eterminedby simple calculations. Liass range is 1—80 a a m , i’1/~M 25 The instru-ment is a cooperative venture of Hungary, the PRG and the USSR.

ion mass—spectrometerand electron analyzer

It is a packageincluding 5 detectors and electronics. The first detectormeasuresthe total ion flux from the Sun, the second— the numberdensityof cometary ions; the third measures the solar wind ion energy spectrum in

86 R.Z. Sagdeev et czl.

the range 50 eV to 25 KeV, tiic fourth deteraines the spectrum of electionenergy in the energy range 3 eV to 5 KeV. The fifth detector is used tomeasure -the energy specirun of cometary ions from which1an ion mass spect—rum may be estimated Lf the relative velocity (78 km.s ) is taken into ac—count. —

The USSR and hungary. are responsible for this experiment.

High—energy particle counter made of tv~osemiconductor detector, anti—coin—cidence scintillators reriovinE~ ‘the particles coming from ‘the sides, and da—ta processing electronics. The instrument is required to study particlesaccelerated near the comet, suprathermal ions and electrons emitted fromthe Sun as well as galactic radiation. The detector measures ions in therange from 20 KeV to tens of LIeV and electrons from 175 KeV to several MeV.The instrument is intended for the objective (5) and is being designed andmanufactured by specialists from Hungary, the FRG and the USSR.

Plasmawaves analyzers are designed to measureEM waves propagating inplasma end recorded with two antennas. The VLF ante~na receives waves inthe band 0.1 — 1000 Hz, its responsivity is 10 V•m . Plasma flow fluctua-tions are measuredwith the Faradaycylinder. The other antenna should re-ceive electro~iagneti~waves in the frequencyband 0 to 300 kllz, its respon—sivi’ty oeing 3 V in The global parametersof the cometaryplasma (tem-perature, coiicen’tration) are measuredwith the Langmuir probe These ana-lyzers help implement the objective (5), specialists from France, Polandand the USSR participate in their development.

iilagneiometers have two sets of sensors and measure the constant magneticfield component to an accuracy of 10~ Gauss They are also used to imple-ment the objective (5). They are being built by Austrian specialists.

The spacecraft has three housekeeping systems, they support payload opera—ticn 1) turntable for optical experiments, 2) payload control system,3) scientific data acquisition system. They are also being developedby theinternational eftort the Chechoslovakianspecialists are preparing the Ir~ov~ing platform, wnile the s~stems (2) and (3) are a 3oint responsibility ofHungary and the USSR.

It should be noted in closing that the mission to the comet is a challengein space research In certain aspects it is more complicated than flightsto planets. The cometary mission faces three basic specific problems:

1. Navigation. The comet’s orbit should be estimated to a high degree ofaccuracy to ensure the given geometry of encounter There are discrepanciesas -to what trajectory can be optimal for the Halley’s comet. From this view-point a fly—by distance of 10,000 km obviously seems a sufficiently cautiouschoice.

2, Meteoroid hazard. Characteristics of the dus~t we shall meet within thecomet are not xnown reliably enough. Even less is known about the impactat high velocities. Calculations show that the technical solution nay be nottoo complicated (two Al layers about 1 mm thick with a spacing of 100 to200 mm), experimental check, however, would be of use.

3. Pointing of optical instruments to the nucleus.

A priori it seems possible that tile nucleus will be screened by an opaqueciust layer The detailec analysis of the problem reveals that it is barelyprobable, at least for the ~ialley comet Hardly, however, shall we have anyfinal answeruntil we are within its coma.

The flight to comet Halley will be the first true step in the space studiesof small bodies in the Solar system ~e may hope that the knowledgewe shallgaixi in that mission could be valuable to ~et a better insight into the pro-cesses of formation and early evolution of the Solar system.

Cometary Probe of the Venera—lialicy Mission

TABI~ 1 Orbit of halley’s Comet {1]

1~1ajor semiaxis, a 17.93591 a.u.

Perihelion distance 0.5871 a.u.

8ccentricity, e 0.967267

- 0inclination 1o2 .2378

Revolution period 75.96 yr

Perihelion passage time 9.7 11.1986

TABLE 2 Physical characteristics of Comet Halley

(predicted)

Nucleus: radius, km ~ 10 12, 3]

density, g.cm3 ~i 1

mass, g -‘ 5.1018

rotation period, hr ~10 [4~

albedo 0.2+0.2—0.1

stellar magnitude at 106 km5~2m

temperatureon the lit side

(°K) 220+30*

Coma: visual stellar magnitude:

at 1 a.u.

at 1o6 km Gin

neutral gas:

hydrogen a-tom flux, ~ (5+3).1029

gas mass flow, g 6.106

mass of gas, g 3.1011

gas density (cm3) at

a distance of ~ km 2•10~

—1 6flux of dust mass, g s 10

mass of dust, g 2.1010

*at 0.82 a.u. from the Sun

**at 0.82 a.u. from the Sun after the perihelion

Prevailing molecules are assumedto be H20 (at distances

1O4 kin) and OH ( p ~ 10~kin). The prediction is based

on the relationship between the stellar nagnitude and 4, fora few large comets [5~

88 RZ. Sagdeev ~t

TABLE 3 Basic characteristics of the Vega cometarymission

Launch 15_28.XII.1984*

Fly-by near Venus,descender jettisoning 11—22.VI.1985

Passage through the Halleycomet coma 6—12.111.1986

Nominal distance from **

the nucleus, kul 10.000

—~1Encounter velocity, km.s 78Orientation three-axis

Information rate i~basicchannel, bit.s’ 65536

Cometary probe payload mass, kg 120

*Launch windows are given within which the dates are to be

determined

**Fly_by over the sunlit side is planned.

TABI~ 4 Cometary probe payload

Nos Instrument Oojective and oasic cha— Participatingracteristics cu ries

2 3 4

A. Qotical experiments

1. TV system Inner coma and nucleus ima-gery Two cameras. USSR(3.5x5.2° and O.4x0.6°)resolution 180 m at distance Hungary10,000 km with filter wheel Prance

2. Three—channel Inner coma emission c~ectrum USSR,spectrometer from 1,200 to 16,00u X. Bulgaria,

Resolution = 700 Prance.

3. IR spectrometer Coma emission 2.5—12(%tm spect-

rum; IR radiometry of the nu-cleus (7—15~tm)~measurement Franceof nucleus size (0.7 km resolu-tion)

B~ In Situ dust—study experiments

4. Dust mass—spectro—Dust particle Conmpsition U°SRmeter (mase range 3.10_10 to

—10510 g~ Prance.

Cometary Probe of the Vencra—Halley Mission 89

TABLE 4 (continued)

1 2 3 4

5. Dust counter Particle impacts (i~ss USSRof particles ~10~ g)

6. Dust plasma co— The same as above but forunter massesin the range 10—12 USSR

to io_18 g

C. Direct anal~ysisof neutralgas and plasma

7. Neutral gas Gas compositionmass—spectro— M Hungary,meter Nonrange and —~— USSR,

FRG

8. Ion mass—spectro— Ion flux compositionmeter and electron (solar and cometary ions) uu aranalyzer energy spectra of ions us~~

and electrons numbers

B. Particles and. fields

9. High—energy par— Recording of ions andtide counter electrons accelerated in

the vicinity of the comet,and of energetic particles Hungary,of different origin (ions PRG,from 20 KeV to 30 MeV, USSR.electrons from 175 Key to3MeV)

10. Plasmawave ana— a) VLF radio emission in CzSSR,lyzers the range 0.1 to 1000 Hz Prance,

and 0 to 300 kflz Poland,b) flux fluctuation measu- USSR.rements with Faradaycylin-der

11. Magnetometer Liagnetic field up to lo_6 Austria.

MSR 2/Il-C

90 R.Z. Sagdeev at ml.

References

1. D.K. leomans, Astron. J., 82, 435 (1977)

2. R.L. Neuburn, Comet Halley micrometeoroid hazard workshopESA SP153, 35, (1979)

3. G.K. Nazarchuk, Report for Symposium “Ph~y~ics of comets”,Kiev, 22 Dec. 1981

4. F. ;thipple, Cirg. Cent. Bur. Astron. Telegrams IAU (1978)

5. H.U. Keller and C.F. Lillie, Astron. and Astrophys. 62,143 (1976)

6. 0.B. Dobrovo].sky, Comets, Nauka, Moscow (1966) (in Russian)

7. P. Vlhipple. In: The Moon, meteorites and comets, ed. byB.Li. Liiddlehurst and G.P. Kuiper, Chicago (1~3)

8. S.K. Vsekhsvyatsky, Physicê.l properties of Comets,Pizmatgiz, Moscow (1958) (in Russian)

9. L.M. Shulman, D~amics of cometary atmosphere — neutral gas,Naukova Dumka, iiiev (1972) (in Russian)

10. V.P. Konopleva, O.K. Nazarchuk, L.Id. Shulman, Surface ~hotometr.yof comets, Naukova Duinka, Kiev (1977) (in Russian)