THE TITAN BALLOON

Jacques Blamont

Centre National d'Etudes Spatiales, 2, place Maurice Quentin, 75039 Paris Cedex 01, France ;Jacques. blamontfa),cnes. fr

ABSTRACT

After a présentation of thé major scientifîc objectives ofTitan's exploration, thé montgolfière of thé TandEMproposai to ESA is described, followed by théalternative option of a hydrogen balloon System.

1. INTRODUCTION

After several close Cassini flybys, in January 2005,thé Huygens Probe became thé first robot toperform a descent through Titan's atmosphère ontoits surface during which it returned more than3 hours of useful data.

Water ice constitutes 40 % of thé mass of Titan,Saturn's largest satellite, and is thé dominant featureof thé surface. An atmosphère of 95 % N2 and 5 %CH4 (at thé ground) is maintained in thé gaseous stateby thé greenhouse effect of méthane at a surfacetempérature of 94 K. Méthane to Titan is like water toEarth in thé hydrological cycle. Discovering clouds,lakes, fluvial and dendritic features attributed to CH4 inits three phases, solid, liquid and gaseous, Cassini-Huygens found that thé balance of géologie processes -impacts, tectonics, fluvial, aeolian - is somewhatsimilar to thé Earth's. Titan may well be thé bestanalogue to an active terrestrial planet in thé sensé ofour home planet, albeit with différent workingmaterials : it is unique in thé Solar System with itsextensive, dense atmosphère, which bas turned out to bea complex chemical reactor over an altitude range ofmore than 1,000 km.

Even if thé solar flux is reduced to one hundredth of itsvalue at Earth, thé méthane is irreversiblyphotodissociated into hydrogen, which escapes, andmolecular fragments which form not only higher orderhydrocarbons as ethane, acétylène and even benzène,but also nitriles by combination with nitrogen, whichfall to thé surface. A sea of ethane should exist with adepth of a few hundreds meters, but is not observed.The life time of méthane is 30 million years, a veryshort time by geological scales. The surprising présenceof méthane in thèse conditions and thé absence of théethane sea hâve led to suppose thé existence of anunderground sea of liquid water containing an antifreezeas dissolved ammonia, at a depth of tens of kilomètres.To support or discard this hypothesis is one of thé main

objectives of Titan science, being it understood that thémajor problems to be solved are related to thé cycle ofméthane - as on earth and Mars, they are related to thécycle of water.

An important limitation of Cassini as concerns Titanscience has been thé insufficient spatial coverage of itsorbit. While thé measurements hâve highlighted thécomplexity of Titan's atmosphère and magneticenvironment, thé coverage has been insufficient toactually understand them. The minimum altitude of950 km as well as thé uneven horizontal coverage haslimited thé in situ atmosphère measurements,opportunities for occultation hâve been very rare andbig gaps are remaining in thé magnetosphericdownstream région. Thus, in spite of thé incontestablebreakthroughs, many new questions hâve arisen fromthé discoveries of thé Cassini-Huygens mission.

For thé spécifie science goals, thé Cassini-Huygenspayload and orbital tour were either not optimized oradéquate. Huygens was not developed as a lander but asa descent module. Even its in situ measurements of théatmosphère are limited to just one vertical profile.

The limitations of Cassini-Huygens and thé questionsthat will remain to be answered hâve led a consortiumof Titan and Enceladus experts, led by AthenaCoustenis, to propose a mission to thé Saturnian Systemwhich includes an aerostatic station floating in théatmosphère of Titan [1]. This proposai, first calledTandEM and later TSMM, was presented to ESA in théframe of thé Cosmic Vision program, and rejected.

Its primary science goal was to improve ourglobal understanding of Titan's surface, interior andatmosphère, détermine what kind of pré- and proto-biotic chemistry may be occurring on Titan, and learnabout thé satellite's origin and évolution.

To achieve thèse objectives, thé System shouldperform in situ measurements in thé atmosphère, atvery low altitude for a long period of time, in order to :a) obtain a large body of data on thé atmosphère itself.b) image at thé meter scale, access, sample and

analyze thé varions surface features (such as théhydrocarbon lakes, thé aérosol and organicdeposits, thé dunes, river Systems, mountain

___________________________________________________________________________________ Proc. ‘19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7–11 June 2009 (ESA SP-671, September 2009)

ranges and volcanoes), inciuding thé solid surfaceand subsurface material.

c) caracterize thé chemical and isotopic nature of théatmosphère, of thé aérosols of thé largest numberpossible of thé matter covering thé ground and ofthé liquids constituting thé lakes.

d) détermine whether Titan bas a sub-surface liquidocéan. An explanation is in order hère because ofthé new nature of thé method to be employed.Cassini-Huygens discovered a strong ELFémission at 36 Hz interpreted as thé secondeigen-mode of thé cavity foraied by théionosphère at thé top and a conducting boundarysituated below thé surface. The phenomenon,called Schuman's résonance, could be excited bya plasma instability mechanism associated withthé corotating saturnian magnetosphere. Theelectric field of thé émission would hâve to bemapped in order to verify thé hypothesis andeventually characterize thé "océan" [2].

e) détermine thé value of thé magnetic fîeld, if itexists. The magnetic fîeld is another way to detectan underground océan.

2. WHY A BALLOON ON TITAN ?

Withstanding thé compelling scientific reasons justexposed (thé mapping of thé Schumann résonances andthé détermination of thé magnetic fîeld can only beobtained by instruments placed on a balloon), it happensthat Titan is thé best place for scientifîc ballooning inthé solar System [3,4].

ÎSÛ •

ÊQ ?o ao 9n 103 ito 12& ÎSD uo TSQ ieo iroTempérature (K)

Figure 1 : Température versus altitude is shown forTitan's atmosphère. The solid Une represents thétempératures measured by thé HASÎ instrument on théHuygens probe [5], whereas thé symbols arefrom théVoyager radio occultation data [6]. The horizontalUne shows thé base of thé Huygens-inferred méthanecloua [7],

l. Its atmosphère is cold and dense with a groundtempérature of 94 K and a pressure of 5 kgm"3 at théground level compared to 1 kgm"3 on Earth.

Therefore thé effect of differential molecularmass between thé buoyant gas and thé ambient ismaximized.

2. The low value of soîar radiation (1Q~2 of radiation atEarth) créâtes no diurnal change in thé externalenergy source and opens thé possibility of longduration flights.

3. Because of thé thickness of thé atmosphère, théinflation during descent is easy ; it can be started ata vertical velocity of 5 ms"1 around 30 km ofaltitude (20 mbar pressure) down to 3 km within anumber of hours (compared to a velocity of30 ms"1 for thé Martian balloon).

It was recognized in 1978 at thé Service d'Aéronomiedu CNRS that montgolfières could fly for a long timein thé Earth's atmosphère, and I proposed to extendthis concept to thé exploration of Titan (Tec. Note 675CNES/HC, 7-2-78).

After thé détermination of thé atmosphère model byVoyager I, more studies were made of Titan balloons ;a fîrst proposai was put forward in 1983 - andrepeated on various occasions without success, for ahélium fîlled vinyl fluoride open balloon of 6 meterdiameter. Total System mass was 41 kg.

Since 2000, JPL bas conducted studies, rediscoveredold concepts, introdueed thé idea of modem MMRTGand developed material suitable for low températureaérostats.

3. THE TITAN MONTGOLFIERE

The problems which may be encountered by a balloonas a vehicle in Titan's environment are :

» The resources it carries (lifting power, energy) arenot renewable.• The gas providing buoyancy bas to be broughtinside tanks whose mass is of thé order of ten times thémass of gas (could go down to 5-6 in récentdevelopments).

Thèse two constraints favour thé choice of hot airballoons, or montgolfières which, using thé ambient air,need no tanks to be filled.

This choice is supported by a second reason, thénecessity to use RTG for electric power. Its largethermal loss becomes a benefit since it could constitutethé energy source for heating thé internai gas andproviding free lift.

The physics of thé montgolfière is dominated by beatexchanges with thé ambient atmosphère, essentially dueto convection. Radiation exchanges are negligible.

Gondola

Figure 2 : Principle ofthe RTG heated montgolfière

Ai Titan, significantly less beat is required than on Earthto provide thé same buoyancy with thé same sizeballoon. The cryogénie environment at Titan results inlower convective and radiative beat transfercoefficients, reducing beat loss from thé balloon surfaceand also greater buoyancy for a given températuredifférence between thé balloon internai température andthé ambient température.

Studies bave been made at APL and JPL in thé scope ofa NASA Flagship mission to Titan inciuding a RTG-heated montgolfière [8].

Q.5-meter V§nîAltitude Control

11,5-meterMonîgsîfisra

RPS (2000 watts)

Insyîatîon

2'àxis GlffibaledO.Ô-m&ter psarnetsr

Rotation DiskPayioad

Figure 3 : Titan RPS montgolfière with altitude control(after J. Jones and al) [9]

A montgolfière is an open balloon with an aperture at itsbottom, filled with ambient gas. A venting valve isplaced at thé top. The internai gas is heated andtherefore less dense that thé ambient ; thé températuredifferential provides buoyancy. The drawback ofthe botair balloon is that, because this differential is smail, itsdimensions bave to be very large. Such vehieles, usingas an energy source thé solar radiation during thé day,and thé infrared émission ofthe ground during thé night,

hâve been used since 1979 by CNES in its baîloonprogram, with a launch of 2 to 5 long duration balloonsnearly every year. For a fifty kg payîoad, theîr volumelies in thé range of 50,000 ni3. A large body ofexpérience bas been accumuiated.

As a support for thé TandEM mission Guillaume Masand Jean Marc Charbonnier at thé CST performed in2007 an analysis of options relative to thé configurationof a montgolfière plus a hélium filled auxiîiary balioonsupposed to provide extralift if thé convection with théambient atmosphère would be large and thereforereduce thé buoyancy. Their preferred option had a1,000m3 montgolfière (radius 6.2 m) and a 50m3

ballonet (radius 4.6 m) with 125 kg for thé jettisonablegas tanks. Thèse balloons sustained a 120 kg gondola(80 kg for thé RTG, 25 kg of instruments),corresponding to a total mass of 350 kg.

The JPL solution for overcoming convection losses wasdifférent : it used a double wall for thé montgolfièreenvelope, qualified by a nuinber of flîghts carryingaeronauts. This solution is lighter (276 kg for thémontgolfière) and simpler, and therefore was adoptedby thé TandEM Team for its final proposai. The totalmass charged for thé aerial piatform at iaunch was600kg. Cryogénie balloon material consisting of apolyester film and fabric laminate, was developed byJPL and a prototype bîimp (length 7 m) was built byJeffHallandal[10].

The MMRTG is located inside thé balîoon justabove thé bottom opening, The science payîoaditself is included m thé gondola, which is suspendedbeneath thé balloon and provides unobstructed viewsof Titan's surface and horizon for seientiflcobservations.

The beat generated by thé MMRTG during thé émisephase is dissipated through a radiator with 2.5 m2

surface. The radiator éléments hâve been placed onthé support structure for an optimal view factor tospace and to make use of an already existingstructure. The height was sized such that thé requiredarea (2.5 m2) could be accommodated on thécircumference, taking into account continuity at thépanel edges for thé routing ofthe fluid fines.

The MMRTG needs integrating at thé final stages,The montgolfière system bas therefore been split intothree major sub-assembîies, which are ail connectedat thé three mounting points at thé side of thé mainpiatform :1. front beat shield,2. main piatform, and3. back cover inciuding back-shield, parachutes andballoon.

- Balloon diameter in m x 10

- Baîloon mass in kg

150

Gondola mass in kg

Figure 4 : Relation between balloon diameter/massand gondola mass

In this way thé three assemblies can be supportée!individualiy allowing a iate intégration of théMMRTG, and subsequently relatively simple finalassembly steps. The MMRTG needs to be connectedto thé main platform for support during launch loads,and to thé support cabling of thé balloon, such that itcan be pulled into thé balloon during deployinent.

Table 1Elément

POIS (payload to orbiterinterface System)

EDIS (entry descent andinflation System)

Balloon 132Gondola 144

Montgolfière total

Total launch mass

Launch margin

Allocated mass

Mass in kg

93

202

276

571

29

600

Opérations

The montgolfière will be targeted at about 20°N,where thé zonal wind has a predicted maximum with aspeed of a few ms"1 for thé time of arrivai in thé year2030. The lifetime will be at least six month, whichcorresponds to one circumnavigation around thé globewith winds at 1 ms"1.

The inflation is a criticai phase in any planetaryballoon mission, but in Titan's thick atmosphère itshould présent no major difficulty ; thé modélisationshows a time of about 12 hrs for reaching thé ceiling.

The release of thé main parachute is triggered by adécélération event. For thé montgolfière thé altitudeof this release is at about 130 km ± 20 km

depending on thé choice of thé atmospheric profile.The release of thé montgolfière occurs about 1.5 -2 hours after entry. The diameter of thé mainparachute is 9 m, as required for a clean séparationof thé 2.6 m diameter heat shield. The terminalvelocity of thé parachute is 6.5 m/s, which iscompatible with thé deployment and filling of théballoon.

•a

Montgolfière deployment and fillûiialtitude : 30 km, 5 m/s descenî

Montgolfière filtad1.5 m/s < faîl spssd <

Fîighî altitudecontroled by a

<U piloted valveattitude : 10 km

Time (h)Figure 5 : Titan deployment andflight simulation

(in thé configuration wifh a hydrogen balloneî)

At an altitude of about 40 km (measured by apressure gauge, and using an assumed altitudepressure relation) thé balloon will be pulled out, and théMMRTG will be pulled inside thé balloon at thé sametime. After having achieved sufficient buoyancy,thé float altitude of 10 km will be activelymaintained within a range of ± 2 km by a ventvalve placed at thé top, which will be controlledby a pressure sensor for altitude measurement,

The data are retrieved through a relay satellite. Thebuoyant montgolfière will slowly drift around Titan.During this time thé orbiter is still performing Titan fly-bys during its séquence towards thé finalobservations orbit. The distance to thé orbiter variesbetween 5 x 106 km and a few 1,000 km during thénominal lîfetime of thé montgolfière. The distance tothé orbiter is shown as a fonction of time. In thisfigure, thé total évolution of distance is shown.Periods where thé orbiter is above an élévation of 20°are piotted with full lînes. ït can be seen that théorbiter cornes significantly closer during shortintervais, which provides much higher telemetrycapability.

The orbiterfs télécommunication System includes asteerable 4 m diameter HGA with a multiple frequencycapability, which will allow using thé saine telemetryand telecommand System for thé montgolfière and thélander included in thé Tandem mission. Thecommunications link will be in X-band at 8.45 GHz.

The montgolfière has a 50 cm2 degrees-of-freedomsteerable HGA with an antenna gain of 31 dB. Apointing accuracy of 1° was assumed. The position tothé orbiter will be measured by using a beacon signalthat will be emitted by thé orbiter. A coarse positiondétermination will be performed by a phase based lineof sight measurement, and a fine pointing ;measurement will be performed by a narrow angleantenna scan.

DhlariiV to orhitor

Table 2

VA A

I ÎT I IC in vlu\ ;itu*r enlr>

Figure 6 : Distance between montgolfière andorbiter. The évolution ofthe distance is plotted with a

dashed line; periods when thé orbiter is above 20°élévation (typical useful limitfor

télécommunications) are drawn withfull line.

Vloiiliîolllcf l'rynsnnssion I^ulc

I imc siriec \y idavsj

Figure 7 : Theoretical data transmission ratefrom thémontgolfière to thé orbiter assuming a link margin of

3 dB, and minimum élévation of30°.

The theoretical capability of thé telemetry link to théorbiter ranges from a few 10 kbps to > 100 Mbps. Athigher levels, thé processor and transponder capabilitieswould likely be saturated. To make thé most optimumuse of this large variation of link capability, a variabletransmission data rate will be implemented.

4. THE PAYLOAD

Tab. 2 présents thé characteristics of thé balloon system,ineluding a model payload and thé mass breakdown of thémission. The priority is given to thé GCMS for chemicalanalysis and to thé caméra for ground pictures. However thédétermination of thé magnetic field and of thé Schumannrésonances for thé détection of an underground océan is alsoconsidered as essential.

ITT situ

éléments

Overalldimensions

Interface mas s

Payload mass

Model

Power system

Operationallifetime

Communications

One aerial vehicle (montgolfière)floating at mid latitudes(10 km altitude)

Front shield : 2.6 m 0Balloon : 10.5 m 0Gondola: 1.6 m 0

571kg

21.5kg

* Visible imaging System(0.4-0.7 |um, ineludingstéréo vision)

* Imaging spectrometer(1 - 5.6 jim)

* Chemical analyzer(10 - 600 Da mass spectrometer)

* Atmospheric structureinstrument/ meteorological package

* Electric environment package* Magnetometer• Radar sounder(> 150 MHz)* Radio science using

montgolfière télécommunicationsystem

MMRTG(100Wd)

6 months (baseline)+ 6 months (extended)

X-band HGA 50 cm 0,55 W T W T A

5. SURFACE OPERATION CAPABILITY

No surface opérations were contemplated in thé frame ofTandem. However, since this mission has not beenaccepted by ESA/NASA for a launch before 2020 in théCosmic Vision program, it may be interesting to studyother concepts for thé future exploration of Titan.

The objective of surface measurements would stayunchanged but with more emphasis on thé understandingof thé organic chemistry of thé crust. In order to collectsample, a guide rope similar to thé "snake" ofthe Russian-French Mars balloon seems adéquate. Such a snake wouldmove slowly on thé ground for a while and thé balloonwould climb again in altitude, repeating this cycle with aperiod of a number of hours. The snake would collectsamples and analyze them with its own detector, or hâve away to carry thé samples to thé gondola.

Tether îo Balloon

- Rope Tail

Figure 8 : Baselme guide-rope configuration(thé snake ofthe Mars-94 French-Russian mission)

The problem is thé création of such a periodic motionwhieh would proteet thé montgolfière from any contactwith thé ground. A possible method is thé use of théventing valve whîch could be commanded by a laser rangefinder in order to maintain thé altitude around 100 meters,as has been suggested at JPL.

Another method would be thé addition of a balloon filledwith argon. In Titan's atmosphère, argon goes from thévapour to thé lîquid phase at thé altitude of 3km,providing a change in buoyancy equal to p^ Yb (Patm isthé atmospheric density, Vb is thé volume of thé ballooncontaining argon).

s _( j_,__

Figure 9 : Van 't Hoffdiagramfor argon m Titan 'satmosphère (pressure in ordinates, inverse ofthe

température in abcissae)

For instance a mass of 10 kg argon would fill a ballonetof 1.5 m3 (mass 0.3 kg) at thé altitude of 3 km. Itsvaporization would provide 7 kg of lift.

The main bailoon in a "smali mission" could be ahydrogen filled balloon. On Titan, an open balloonfilled with hydrogen would hâve a lifetime of a fewweeks because ofthe low température. A mass of 10 to20 kg of hydrogen (carried to Titan in a 100 kg tank)

Time (h)Figure 10 : Modélisation ofthe motion ofthe TandEM

montgolfière with an argon ballonet

would sustain a 12 kg gondola in a 30 m3 balloon. Boththé argon and hydrogen baîloons would never be fullyinflated, thé System oscillating between 0 and 10 km ofaltitude with a period of tens of hours. A modificationof thé buoyancy could be obtained by an on/ofifcirculation ofthe hydrogen around thé MMRTG.

Such a mission would be thé culmination of scientificballooning since it was started by Gay-Lussac in 1804.

6. REFERENCES

1. Coustenis A. et al, TandEM : Titan and Enceladusmission, Exp. Astron. (2009) 23:893-946.

2. Beghin C et al, A Schumann-lïke résonance onTitan driven by Saturn's magnetosphere. Icarus,(2007) 191,257-266,

3. Biamont J., Planetary balloons, Exp. Astron.(2008)22:1-39.

4. Lorenz R.D., A review of balloon concepts forTitan, JBIS (2008) 61,2-13.

5. Fulchignoni M. et al, Titan's physicalcharacteristics measnred by thé Huygensatmospheric structure instruments, Nature (2005)438,785-791.

6. Lîndel G,F., The atmosphère of Titan fromVoyager I radio occultation measurements, Icarus(1983) 53, 348-363.

7. Tokano J. et al, Méthane drizzle onTitan, Nature,(2006) 442,432-435.

8. Leary J.C. (study lead), Titan Explorer FlagshipMission Study, NASA, (2007) 07-05735, NASAWashington, D.C.

9. Jones J. and Wu J.J., Montgolfière aerobots forTitan, Internat Planet Probe Workshop, Pasadena(2007).

10. Hall J.C., Kerzhanovich V.Y., Lachenmeier T.,Mahr P., Pauken M., Plett G.A., Smith L., VanLuvender M.L., Yavroman A.H., Expérimentalresults for Titan aerobot thermomechanicalsubsystem development, J. Adv. Space Res. (2007),DOI 10-1016/j.asr 2007.02.060.