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Advances in Space Research 42 (2008) 1648–1655

Prototype design and testing of a Venus long duration, highaltitude balloon

J.L. Hall a,*, D. Fairbrother b, T. Frederickson c, V.V. Kerzhanovich a, M. Said b, C. Sandy c,J. Ware c, C. Willey c, A.H. Yavrouian a

a Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United Statesb NASA Wallops Flight Facility, Wallops Island, VA, United States

c ILC Dover Inc., One Moonwalker Road, Frederica, DE 19946-2080, United States

Received 30 August 2006; received in revised form 6 March 2007; accepted 7 March 2007

Abstract

This paper describes the design, fabrication and testing of a full scale prototype balloon intended for long duration flight in the upperatmosphere of Venus. The balloon is 5.5 m in diameter and is designed to carry a 45 kg payload at an altitude of 55 km. The balloonmaterial is a 180 g/m2 multi-component laminate comprised of the following layers bonded together from outside to inside: aluminizedTeflon film, aluminized Mylar film, Vectran fabric and a polyurethane coating. This construction provides the required balloon func-tional characteristics of low gas permeability, sulfuric acid resistance and high strength for superpressure operation. The design burstsuperpressure is 39,200 Pa which is predicted to be 3.3 times the worst case value expected during flight at the highest solar irradiancein the mission profile. The prototype is constructed from 16 gores with bi-taped seams employing a sulfuric acid resistant adhesive on theoutside. Material coupon tests were performed to evaluate the optical and mechanical characteristics of the laminate. These were fol-lowed by full prototype tests for inflation, leakage and sulfuric acid tolerance. The results confirmed the suitability of this balloon designfor use at Venus in a long duration mission. The various data are presented and the implications for mission design and operation arediscussed.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Balloon; Aerobot; Venus; Mobility

1. Introduction

The Soviet Union conducted the only planetary balloonflights to date in 1985 when they successfully flew twohelium superpressure balloons in the Venusian atmosphereas part of the VEGA mission (Kremnev et al., 1986). Bothballoons flew for nearly 2 Earth days at an altitude thatranged from 53 to 55 km. That altitude provided a rela-tively benign pressure and temperature environment forthe balloon of approximately 0.6 bar and 30 �C. Thisenabled the use of a compact 3.4 m diameter spherical bal-loon to carry the 7 kg payload without the need for extreme

0273-1177/$34.00 � 2007 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2007.03.017

* Corresponding author.E-mail address: jlhall@pop.jpl.nasa.gov (J.L. Hall).

temperature balloon materials. The actual balloon materialused was a coated fabric of approximately 300 g/m2 arealdensity in which both the fabric and coating were fluoro-polymer materials capable of tolerating the sulfuric acidaerosols found in the Venusian clouds at the float altitude.Although there were indications of gas leakage during theflight, both balloons performed very well and continuedto fly nominally at the 2 day mark when the payload sys-tems exhausted their batteries and fell silent.

Current science objectives for Venus have been articu-lated by the NRC Decadal Survey (NRC, 2003). The toppriority objectives consist of understanding the origin andevolution of Venus which requires making highly accuratecomposition measurements of the atmosphere, with partic-ular emphasis on noble gases, noble gas isotope ratios and

rved.

J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655 1649

trace gases including sulfur compounds and water. Suchmeasurements can be made with a gas chromatograph-mass spectrometer (GCMS) instrument either carried ona balloon or on a short-lived probe that falls to the surfacein the same manner as the Pioneer Venus or Venera mis-sions from the 1970s and 1980s. Two key advantages of aballoon borne instrument are longer lifetimes (days insteadof hours) and the ability to simultaneously perform atmo-spheric dynamics investigations as was done with VEGA.It is expected that the atmosphere is well mixed and GCMSmeasurements will not be sensitive to the altitude at whichthe samples are collected, allowing for the use of the same53–55 km altitude as VEGA.

However, a key obstacle to performing such a new bal-loon mission at Venus is the poor capability of the VEGAballoon-type material for scaling to the larger sizes neededto float GCMS instruments and their attendant avionicsand power subsystems. The use of fluoropolymer fabricsresults in a relatively low strength to weight ratio that ispoorly suited to the superpressure requirements of largeballoons that must fly through the night-to-day transitionsassociated with long duration flight at Venus. Also, thehigh permeation rate of helium through the VEGA mate-rial will contaminate the mass spectrometer data. This indi-cates the need for a different kind of balloon design thatmaintains the VEGA balloon tolerance to sulfuric acidwhile providing the strength and mass efficiency requiredfor GCMS-compatible payload sizes of 40+ kg. This paperdescribes the design, construction and testing of a new, fullscale prototype balloon that solves these problems and canthereby enable the desired new balloon mission to Venus.

2. Balloon requirements and design

The effort to develop a new balloon for Venus wasjointly performed by a team of engineers from the Jet Pro-pulsion Laboratory (JPL), the NASA-Wallops FlightFacility (WFF) and ILC Dover, Inc. We started by synthe-sizing a list of functional requirements and resulting designfeatures for the balloon as derived from JPL Venus missionstudies centered around GCMS-based payload concepts.

Table 1Venus balloon design requirements

Functional requirement Design features

Carry a 44-kg payload Sets the size of the balland balloon mass (5.5

Hold a near-constant altitude of 55.5 km throughoutthe mission

Use a superpressure datmospheric density) lballoon mass. Inject he

Fly for a minimum of 6 days (Earth days) Balloon will circumnavTolerate 85% concentration sulfuric acid aerosols Use Teflon film and suFly balloons at 25�N and 60�N Base the balloon desigLow-risk design that is highly tolerant of Venus

atmosphere model uncertaintiesUse 1.5 multiplier on ecompared to worst cas

Minimize mass per unit area of balloon material Use a high strength VAerial deployment and inflation of the balloon upon

arrival at VenusUse of Vectran fabricdeployment and inflati

These requirements are listed in Table 1. As with VEGA,requiring a near-constant float altitude drives the choiceof a mass-efficient spherical superpressure balloon thattracks a constant atmospheric density level. The listed mis-sion lifetime of 6 days is sufficient for one circumnavigationof Venus during which the balloon will see the full range ofday and night environmental conditions.

Much of the balloon design problem revolved arounddeveloping a new balloon material. It quickly became clearthat a laminate material would be needed to meet all of therequirements for strength, mass, gas retention, sulfuric acidresistance and radiative heat transfer. The final product isgraphically depicted in Fig. 1. It is a single shell material inwhich each element of the laminate serves a different pur-pose: the Teflon outer layer protects from sulfuric acid aero-sols, the metallization on the Teflon minimizes solar heatingvia a second-surface mirror effect (low solar absorption to IRemissivity (a/e) ratio), the metalized Mylar film minimizeshelium permeation, the Vectran fabric gives high strengthto tolerate steady-state superpressure and transient deploy-ment forces, and the urethane inside coating facilitates fabri-cation of interior gore-to-gore seams. The net result is amaterial predicted to have approximately four times thestrength to weight ratio of the VEGA material. This was laterconfirmed with testing (see Section 3).

Table 2 summarizes the final balloon design parametersbased on the functional requirements in Table 1, the lami-nate material described in Fig. 1 and thermodynamic mod-eling of the balloon in the Venus atmosphere at 55.5 kmaltitude. Table 2 shows metrics for both a 2 day and a 6day mission because of some mission interest in the easier2 day mission. The balloon is 5.54 m in diameter and issized for a 44.1 kg payload at 55.5 km altitude. A key resultis that we predict a peak balloon temperature in the fullnoontime sun of only 62 �C based on using second-surfacesilverized Teflon outer material (low a/e = 0.19) and aVenus atmosphere radiation model (Meadows and Crisp,1996) with a 1.5 multiplier for worst-case uncertainty. Weestimate a worst-case pressure excursion of +8400 Pa in 2days and 11,000 Pa during a full 6-day mission, based onthis conservative solar heating model and a maximum ver-

oon and mass of helium inflation gas in conjunction with the float altitudem diameter for 55.5 km altitude and 0.87 kg/m3 atmospheric density)

esign to minimize altitude excursions around the 55.5 km (0.87 kg/m3

evel. Altitude sets the balloon size in conjunction with the payload andlium sufficient to keep positive superpressure during maximum downdraftsigate planet; design must include solar heating effectslfuric acid resistant fitting materials and adhesives on all exposed surfacesn on the worst case environmental conditions at both latitudesxpected solar flux. Require a P2.0 safety factor on material strengthe combined loading of solar and IR fluxes and vertical winds

ectran fabric to take the tensile loads due to superpressureprovides adequate strength to tolerate transient loads during theon process

0.160 mm

Vectran fabric

1-3-1 Ripstop weave25 yarns/cm warp x 25 yarns/cm fill58 g/m2

100 denier

Aliphatic Urethane17 g/m2

Adhesive17 g/m2

Polyester film with 30 nm aluminum on fabric-facing side

17 g/m2

Adhesive9.9 g/m2

54.3 g/m2

Total areal density = 173.2 g/m 2

25.4 um

12.5 um

FEP Teflon with 30 nm aluminum on fabric-facing side

Fig. 1. Balloon laminate material.

Table 2Balloon design metrics

Metric Value

Diameter 5.54 mSurface area 96.4 m2

Volume 89.0 m3

Total balloon mass (including contingency) 25.1 kgMeasured envelope areal density 176 g/m2

Nominal float altitude 55.5 kmNominal float atmospheric density 0.87 kg/m3

Payload mass 44.1 kgPredicted max balloon temperature (6 days) 62 �CPredicted burst superpressure (2 days) 39,200 PaStarting nighttime superpressure 3000 PaPredicted max flight superpressure (2 days) 8400 PaPredicted max flight superpressure (6 days) 11,800 PaSuperpressure safety factor (2 days) 4.6Superpressure safety factor (6 days) 3.3Helium gas mass 7.79 kgPredicted max altitude excursions ±800 m

1650 J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655

tical wind of 3 m/s as measured by VEGA balloons. Ascurrently designed, the balloon has a very robust �4.6structural safety factor for the baseline 2-day mission inworst-case conditions, and �3.3 for a 6-day mission thatmust tolerate more severe solar heating. Despite thesehealthy margins, we anticipate providing further robust-ness by using a pressure relief valve on the balloon to ventgas and avoid an overpressure situation during flight.

Prototyping and testing for this balloon design consistedof five steps:

1. Fabrication of the laminate material in sufficient quan-tity to build a full scale balloon.

2. Development of an approach for making gore-to-goretaped seams.

3. Material coupon testing of the finished laminate andseams to quantify mechanical and thermal properties.

4. Detailed mechanical design and fabrication of a fullscale 5.5 m diameter spherical balloon.

5. Testing of the full scale balloon.

These steps will be described in detail in the followingsections.

3. Material fabrication and seaming

The aluminized polyester and Teflon films were pro-cured from Dunmore Corp., while the Vectran fabric wasacquired from the NASA Mars Exploration Program inthe form of unused surplus material from the Mars Explo-ration Rover (MER) airbag contract. The multi-step lami-nation processes required to manufacture this compositewas performed at Uretek, Inc. under ILC technical direc-tion and oversight. The yield from the laminate productionwas a continuous piece of material 1.52 m wide and 450 mlong. Actual construction of the prototype consumedapproximately half of this material, as was expected.

ILC developed a gore-to-gore seaming approach basedon a butt and taped seam as shown in Fig. 2. A 63.5 mmwide tape of urethane-coated polyester fabric structurallyjoins the spherical gore panels. The tape is thermallywelded to the urethane coated side of the laminate. Thispolyester fabric-based tape is not optimal for this Venusballoon application, but was adopted for reasons of sche-dule compliance during this rapid prototyping activity.Future versions will use stronger fabrics that are bettermatched to the Vectran used in the parent laminate. Theouter joint uses a cover tape to prevent sulfuric acid pene-tration into the gore-to-gore joint. This cover tape uses aspecial ILC-developed adhesive that is itself resistant tosulfuric acid. The Teflon film was subjected to an extra pro-cessing step to prime it for bonding with this cover tape.

4. Material testing

ILC, Wallops and JPL conducted a joint materials testprogram on this laminate using samples cut from the finalproduct. It consisted of mechanical, thermal and chemicaltests to help finalize the prototype manufacturing approachand to assess how well the prototype would fare in theactual Venusian flight environment. The room temperaturemechanical and thermal test results are summarized inTable 3, which is a synthesis of Wallops and ILC work thatcross-checked and largely agreed with each other. Note inparticular the fill direction tensile strength of 57 kN/mwhich for a 5.5 m diameter spherical balloon corresponds

63.5 mm

38.1 mm

Structural Tape

Adhesive

Cover Tape

Envelope GoreEnvelope Gore

Urethane-Coated Polyester Fabric

Aluminized Teflon Film

Thermally Welded to Urethane Side of Gore

Inside

Outside

Fig. 2. Design of gore-to-gore taped seam.

Table 3Mechanical and thermal test results

Parameter Specification Value

Areal density (g/m2) ASTM D3776 176Thickness (mm) ASTM D1777 0.17Tensile strength (warp) (kN/m) ASTM D5035a 71Tensile strength (fill) (kN/m) ASTM D5035a 57Elongation at break (warp) (%) ASTM D5035a 5.3Elongation at break (fill) (%) ASTM D5035a 7.0Helium permeability (liters/m2/day) ILC STP 029 �0Seam tensile strength (warp) (kN/m) ASTM D5035a 45Seam tensile strength (fill) (kN/m) ASTM D5035a 57Seam elongation (warp) (%) ASTM D5035a 16Seam elongation (fill) (%) ASTM D5035a 19Heat capacity (J/g K) 1.22Solar absorptivity 0.17Infrared emissivity 0.56

a Specimen Type 1C.

J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655 1651

to a burst superpressure of 41.4 kPa, a value slightly higherthan the predicted balloon design value of 39.2 kPa listedin Table 2. The seams did not achieve all of this parentmaterial strength value (45 vs 57 kN/m) because of previ-ously discussed use of sub-optimal polyester-based tape.

0

50

100

150

200

250

300

350

400

0.00 0.02 0.04 0

St

Str

ess

(Mp

a)

#1 (23 C) #8 (50 C)

Temperature Dependant H Warp Direction

Fig. 3. Stress–strain curve

The effect on temperature on the tensile strength of thelaminate is shown in Figs. 3 and 4 for the warp and fill direc-tions respectively. This Wallops data is plotted as a materialstress in which a nominal Vectran thickness of 0.17 mm isused to convert the force data. As can be seen, the materialonly loses approximately 13% and 20% for the warp and filldirections respectively going from 23 �C to 75 �C, which isslightly above the maximum balloon temperature of 62 �Cpredicted during Venus flight under the full noontime sun.

Table 4 summarizes the results of sulfuric acid testing ofthe laminate material and seams. Although the expectedsulfuric acid concentration at Venus is expected to be inthe range of 75–85%, tests were conducted up to 96% tounderstand our design margin. The material itself wasfound to be unaffected by prolonged exposure to all con-centrations of sulfuric acid up to a temperature of 70 �C.Adhesively taped seam samples did show some discolor-ation at the tape edges, but there was no evidence of acidpenetration into the joint using blotter paper on the insidesurface. This test confirmed that the cover tape adhesivewas indeed resistant to sulfuric acid as predicted and cansupport a long duration balloon mission at Venus. Tests

.06 0.08 0.10 0.12

rain

#11 (75 C) #18 (100 C)

ull Tensile Test Comparison, Instron 4400R

s for warp direction.

0

50

100

150

200

250

300

350

400

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Strain

Str

ess

(Mp

a)

#1 (23 C) #10 (50 C) #13 (75 C) #18 (100 C)

Temperature Dependant Hull Tensile Test Comparison Fill Direction, Instron 4400R

Fig. 4. Stress–strain curves for fill direction.

Sulfuric Acid Concentration

Component

TestTemp(ºC)

TestDur.(day) 75% 85% 92% 96%

Balloon material 25 7 Balloon material 70 7 Balloon material 70 2 Balloon material bi-fold 25 14 Balloon material bi-fold 70 2 Balloon seam 25 6 Balloon seam 70 2 Vectran fabric 25 7 Vectran fabric 70 2 Mylar film 25 14 Mylar film 70 2 Mylar film 25 0.02 Mylar film 70 0.01 Green: negligible effect, Yellow: external discoloration and/or wrinkling but no evidence of acid penetration, Red: material damaged, White: not tested, Grey: not applicable.

Table 4Sulfuric acid test data

1652 J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655

were also done on bi-folded material samples to show thatsulfuric acid resistance was not compromised when thematerial is severely creased. Bi-folds were made by foldinga second time perpendicular to the first fold and pressingwith 34 kPa (5 psi) of pressure for one day to set the creaseand create a sharp corner at the fold intersection point.This is a conservative test in the sense that a real flight bal-loon would not be folded so severely when packaged forthe trip to Venus.

Additional sulfuric acid tests were conducted on Mylarfilm and Vectran fabric separate from the overall lami-nate. The purpose was to assess the impact on the lami-nate if a pinhole or large defect allowed sulfuric acid to

penetrate the protective Teflon outer layer. As seen inTable 4, both the Mylar film and Vectran fabric toleratedup to the expected Venus maximum concentration of85%. However, Mylar film was very quickly damaged atconcentrations of 92%. We conclude from this that evenif the Venus balloon gets a small hole that allows sulfuricacid to penetrate past the Teflon layer, the Mylar layerunderneath that will tolerate the expected sulfuric acidconcentration of 85% and enable the balloon to continueflying until the gas loss through the hole produces nega-tive buoyancy.

5. Balloon fabrication

One full scale prototype balloon was constructed withthis laminate material using the design parameters of Table2. Fig. 5 shows the finished balloon undergoing testing inthe JPL Spacecraft Assembly Facility. The prototype isconfigured as a spherical assembly of 16 identical goreswith circular end cap assemblies at its poles. Each endcap assembly employed as reinforcement the same ure-thane-coated polyester fabric used in the seam structuretapes because the balloon laminate itself can only be struc-turally attached on one side.

The end fittings include a two-piece metallic assemblythat sandwiches the balloon material to provide a strongjoint. The outer part is made from Hastelloy, a highstrength, sulfuric acid resistant nickel alloy. It contains a100 NPT port for attaching the helium tube, and four lugson its outer perimeter with B3.2 mm holes for tetherattachments. Facing the balloon envelope is an O-ringgroove that receives a Viton O-ring for an H2SO4 resistantseal against the Teflon surface of the material. On the sameface, inboard of the O-ring groove, is an array of 12 blindthreaded holes for attaching the fitting to the envelope andthe inner diffuser/clamp ring. Because these holes areinboard of the O-ring, and because they do not break

Fig. 5. Prototype balloon ungoing testing.

J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655 1653

through the outer surface of the fitting, the attachmentscrews will not be exposed to H2SO4

On the inside of the end cap is the diffuser/clamp ring, amachined part made of 7075-T6 aluminum. Aluminum wasselected for this part to minimize mass; because it will notbe exposed to H2SO4, it was not necessary to select a mate-rial resistant to it. The primary function of this part is toprovide the load path from the external Hastelloy fittingto the VALOR envelope. The 12 high strength (A286)screws carry the load into the aluminum part, which isbonded to the inner urethane surface of the end cap rein-forcements using a structural epoxy.

Fig. 6. Application of ILC prim

For the actual construction, all patterned parts (gores,end caps, end cap reinforcements and end cap cover tapering) were marked and cut on ILC’s automated laser cut-ting table. Gores and end caps then had ILC primer andadhesive manually applied (painted on) to a band approx-imately 23 mm wide around their perimeters (see Fig. 6).Cover tapes and end cap cover tapes were completelypainted on their aluminized sides with primer and adhesive.Gores were then joined by thermal welding (creating theseam section described in Fig. 2). This operation not onlythermally welded the structure tape to the fabric laminate,but also activated the cover tape adhesive. The gore sealingoperation is shown in Fig. 7.

The end cap assemblies were fabricated by thermallywelding their constituent layers. Once sealed together, thecenter hole pattern for the end fitting was marked and lasercut. End fitting assemblies were then installed, starting withthe epoxy bonding of the Diffuser/Clamp Ring using theTeflon bonding fixture. The Hastelloy inflation fitting withViton O-ring was then attached to each end cap with twelve4–40 UNC A286 flathead screws, each installed with anapplied torque of 1.0 N m and thread lock adhesive. Onceinstalled, the fittings were covered in padding to ensure thatthey did not cause damage to the envelope’s Teflon surfaceduring subsequent assembly operations. With only a por-tion of the final gore seam left open, the end cap assemblies(with hardware) were thermally welded into the assembledgores. The final length of gore seam was then thermallywelded, completing the envelope.

6. Balloon testing

The preliminary test results of the prototype balloon arevery encouraging and suggest that it will be able to meetthe requirements for long duration flight at Venus. Threetypes of tests have been conducted so far:

1. Careful weighing of the balloon and its components.2. Inflation of the balloon to low superpressure to evaluate

structural behavior.3. Long duration monitoring of buoyancy force while

under superpressure to deduce leak rates.

er and adhesive to Gores.

Fig. 7. Gore sealing operation.

Table 5Prototype mass

Component Mass (kg)

Balloon envelope 16.70End caps reinforcement 0.97Structure and cover tapes 5.32End cap metallic fittings 0.72

Total mass 23.71

1654 J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655

The tests will now be described.Table 5 gives the mass of the prototype broken down by

key components. The envelope mass corresponds to the0.176 kg/m2 areal density quoted above in the balloondesign (Table 2). The 5.32 kg for the structure and covertapes is an usually large fraction of the overall mass

51.500

52.000

52.500

53.000

53.500

54.000

54.500

0 24 48 72 96 120 144 1

Tim

Mas

s o

f g

as, k

g

Fig. 8. Mass of gas inside balloon during leakage test. (Top solid line is the actincrease superpressure.)

(22%). The switch to stronger Vectran-based structuretapes in the future will provide mass savings in this regard.

Substantial wrinkling of the material is evident in theuninflated material due primarily to handling during man-ufacture. The prototype was packaged up for shipmentfrom ILC to JPL, but care was taken to avoid hard foldsin the material. Once inflated, the prototype takes on theexpected spherical shape as shown in Fig. 5 and most ofthe wrinkles disappear at a low level of superpressure.The residual wrinkles are primarily located on the gore-to-gore seams near the equator and did not disappear atsuperpressures of up to 2500 Pa.

A 13.5 day leak test was conducted on the prototype inthe controlled environment of JPL’s cleanroom SpacecraftAssembly Facility. It consisted of filling the balloon withan approximately 50:50 mixture of helium and nitrogen

68 192 216 240 264 288 312 336 360

e, hours

ual amount of gas, the lower dashed line corrects for gas addition added to

J.L. Hall et al. / Advances in Space Research 42 (2008) 1648–1655 1655

such that the net buoyancy was able to lift a 24 kg mass.This buoyancy force, along with the atmospheric pressure,temperature and humidity, were monitored continuouslythroughout the test for the purpose of quantifying the inte-grated leakage of helium out of the balloon. Fig. 8 showsthe mass of gas in the balloon as a function of time calcu-lated from the experimental data. One complication in thedata is that three times during this test 700 g of nitrogenwas added to increase the superpressure from the initialvalue of 500 Pa to a final value of 2500 Pa. The lowerdashed line in Fig. 8 subtracts out this effect so that theleakage trend can be seen. The data does not show anynoticeable leakage duration the entire 13.5 duration ofthe test. This bodes well not only for supporting long mis-sion lifetimes, but also for not contaminating the GCMSmeasurements.

7. Conclusions

A prototype balloon for long duration, high altitudeflight at Venus has been successfully designed, fabricatedand tested. This balloon could be used in a new Venus mis-sion that would follow in the footsteps of the 1985 VEGAballoon mission and acquire important atmospheric com-position and meteorological data. The prototype balloonis sized for a 44 kg payload, but can in principle be adjustedsmaller or larger. Preliminary testing indicates good toler-ance to the sulfuric acid found in the Venusian clouds atthe float altitude of 55 km. In addition, the measured leakrate on this prototype is very low and would support a mis-sion lifetime of at least 13 days provided that the sulfuricacid tolerance was maintained. A flight of this duration

would circumnavigate Venus more than two times if flownin the equatorial regions and even more if flown in temper-ate or polar areas. Further testing is required to verify thepackaging, deployment and inflation behavior of the bal-loon, but the envelope material is sufficiently strong andpliable that no show stoppers are expected.

Acknowledgments

The research described in this paper was funded by theJet Propulsion Laboratory, California Institute of Technol-ogy, under a contract with the National Aeronautics andSpace Administration. Additional test support funds wereprovided by NASA Wallops Flight Facility and ILC Do-ver, Inc. The authors thank the following individuals fortheir assistance with the work reported therein: Gary Plett,Mike Pauken and Earl Scott of JPL and Dave Puckett andMolly Powell of NASA GSFC. We also thank the JeffreyCornish, Richard Williamson and Rob Manning fromthe Mars Exploration Program for their help in gettingus the flight surplus Vectran fabric used in the prototype.

References

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Meadows, V.S., Crisp, D. Ground-based near-infrared observations of theVenus nightside: the thermal structure and water abundance near thesurface. J. Geophys. Res. 101, 4595–4622, 1996.

NRC. New Frontiers in the Solar System: An integrated explorationstrategy, Solar System Exploration Survey Committee, Space StudiesBoard, Division of Engineering and Physical Sciences, NationalResearch Council, National Academies Press, Washington, DC, 2003.