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AIAA Balloon Technology Conference 1999

Venus Aerobot Multisonde Mission

By: James A. Cutts ('), Viktor Kerzhanovich o_ j. (Bob) Balaram o), Bruce Campbell (2), Robert Gershman o), Ronald Greeley o), Jeffery L. Hall ('), Jonathan Cameron o), Kenneth Klaasen v) and David M. Hansen o)

ABSTRACT requires an orbital relay system that significantly Robotic exploration of Venus presents many increases the overall mission cost. The Venus challenges because of the thick atmosphere and Aerobot Multisonde (VAMuS) Mission concept the high surface temperatures. The Venus (Fig 1 (b) provides many of the scientific Aerobot Multisonde mission concept addresses capabilities of the VGA, with existing these challenges by using a robotic balloon or technology and without requiring an orbital aerobot to deploy a number of short lifetime relay. It uses autonomous floating stations probes or sondes to acquire images of the (aerobots) to deploy multiple dropsondes capable surface. A Venus aerobot is not only a good of operating for less than an hour in the hot lower platform for precision deployment of sondes but atmosphere of Venus. The dropsondes, hereafter is very effective at recovering high rate data. This described simply as sondes, acquire high paper describes the Venus Aerobot Multisonde resolution observations of the Venus surface concept and discusses a proposal to NASA's including imaging from a sufficiently close range Discovery program using the concept for a that atmospheric obscuration is not a major Venus Exploration of Volcanoes and concern and communicate these data to the Atmosphere (VEVA). The status of the balloon floating stations from where they are relayed to deployment and inflation, balloon envelope, Earth. In this paper, we describe the VAMuS communications, thermal control and sonde mission concept and discuss a proposal to deployment technologies are also reviewed. NASA's Discovery program to apply this concept for the Venus Exploration of Volcanoes INTRODUCTION and Atmospheres (VEVA) mission. A primary Despite a number of successful missions to goal of the paper is to describe the progress in observe the surface and interior of Venus, a validating the key technologies needed for the number of fundamental questions about the mission. planet have yet to be resolved. The Venus Geoscience Aerobot (VGA), identified in VENUS AEROBOT MULTISONDE NASA's Roadmap for Solar System Exploration MISSION CONCEPT (Fig I (a) and Ref 1), would make multiple The environment of Venus present major excursions from high altitude in the Venus challenges for scientific exploration. A thick atmosphere to conduct observations at or near the atmosphere of carbon dioxide, with a surface surface in order to address these questions. The pressure of 90 bars and clouds and haze in the VGA mission requires a number of new upper reaches totally obscures the planet's technologies (Ref 2) including high temperature surface from remote observation from orbit balloon materials, gondola thermal control except using radar imagery. The surface systems and reversible fluid altitude control that temperature is about 460°C and therefore long will require a significant investment and at least duration surface vehicles for in situ investigation five years of development. The VGA also require radioisotope powered refrigerators or high temperature electronics or both and become Aff'diations: complex and extremely costly. Short duration (1) Jet Propulsion Laboratory, California observations in the lower atmosphere using small Institute of Technology, Pasadena California expendable sondes that survive for a few hours (2) NASA Headquarters, Washington, D.C. are a more practical solution. However, to be effective as an exploration tool, these sonde (3) Department of Geology, Arizona State

1 American Institute of Aeronautics and Astronautics Concepts for Venus Surface Exploration

Earth

. v.,._ .,_. i....l_ .,. _ ¸ • s _,... -,..._ .=du_.=,m,,,w ,,,_,.. --A_lli_ t..lul, lbr I,,m_e

Fig 1 Comparison of Venus Geoscience Aerobot and Venus Aerobot Multisonde

must be able to communicate large amounts of data during their limited lifetime. In particular, Sonde Deployment: Aerobot deployment makes the acquisition of high-resolution aerial imaging it possible to sequence deployments and target from near the surface of the planet for a number sondes based on what has already been learned of targets of high scientific interest is of high from earlier missions. A radar map of Venus was priory. obtained by the U.S. Magellan mission in the 1980s and allows important scientific target to be The innovative feature of the VAMuS mission identified. From a float altitude of about 60-kin, concept is to use aerobots (balloons) as both a the sondes can be dropped with an accuracy of a delivery system and a high data rate few kin, which is much better than could be communications relay for multiple low cost achieved from a direct entry. sondes. By using an aerobot for sonde delivery and data relay it is possible to deliver more After deployment from the aerobot, each sonde sondes with higher accuracy and acquiring more will descend rapidly towards the surface. At an data than when sondes are delivered to Venus by altitude of about 3-km, the descent is arrested by direct entry. Capture at Venus takes place by a parachute or gliding device that permits an low-tech aeroentry eliminating the need for a extended data acquisition near the Venus surface. costly propulsion or aerocapture stage. There is Carried by the fast counter- rotating high altitude no requirement for the use of a relay spacecraft winds the aerobot will be carried rapidly to the or orbiter at Venus. west of the probe. The descent speed of the sonde, the thermal survival time and the period Each aerobot upon emplacement consists of" for which the floating station remains within gondola and sondes suspended from a range of the sonde are roughly commensurate, superpressure balloon. The balloon maintains permitting about 15-30 minutes of high rate the vehicle on a surface of"constant atmospheric imaging data near the surface. density in the Venus atmosphere except for perturbations caused by downdrafts and probe Communications Since the distance from the deployments. Superpressure balloon deployment sondes to the aerobot is measured in tens of kms, and inflation was successfully demonstrated in high data rate communications (from 100kbps to 1985 in the Venera Vega balloon mission. Each 2 Mb/sec) is possible with compact low power of two Venera Vega balloons was tracked for two UHF (400MHz) transmitters and omnidirectional days in the Venus atmosphere. antennas on the probes and an omnidirectional

2 American Institute of Aeronautics and Astronautics receivingantennaontheaerobot.Thesedataare Volcanoes and Atmospheres (VEVA) mission stored on the aerobot for later transmission to was proposed to the Discovery program in June Earth. Since the Venus-Earth distance is 1998 by a team led by Professor gonald Greeley measured in tens of millions of kilometers, the of Arizona State University (4). The VEVA free space communications loss is a factor of 10 _2 team included participants from JPL, Lockheed higher. However, the characteristics of the Martin Corporation, Goddard Space Flight aerobot platform makes it possible to bridge this Center and several universities. The VEVA gap and return the data recovered by the probes proposal incorporated many of the featm:eS of the in their short life time. Higher power transmitters VAMuS mission concept. can be deployed on the aerobot than on the probe and the relative stability of the platform and the VEVA Science Theme: The goal of VEVA is to benign thermal environment permits the use of a characterize the surface and lower atmosphere of directional antenna for communication with Venus to determine if the planet has suffered a Earth. The large directional antennas of the Deep periodic global catastrophic resurfacing. The Space Network are used to pick up the signal. A specific objectives of VEVA involve acquiring data rate of I0 to 20 kb/sec appears feasible the first-ever-aerial photography of the surface of permitting return to Earth of all the data from the Venus and definitive in s/tu determination of the probes in • few tens of hours well within the composition of Venus in its lower scale height.

- Take Other Data I -- ___'._._%1 , Eauinox --- i t [3irection of balloon dr, ' _ t_ t (and planet rotation) is t _ __.../_/, ] I clockwise • _i _ _"._Y / , / P-inalTransmd . \', ___%2(_")/_. ' to Earth

Balloon Drift __ . ,.'_"_ _.i • -,, _ - __D .'_ Earth • ..... _" _ Incoming Sun _awae=_ Hyperbola

Fig 2: The geometry of Venus at the time of the VEVA mission. Atmospheric winds carry the balloon Gondolas •round the planet in 7 days serving as • science platform and radio relay for the drop sondes

design lifetime of a Venus superpressure balloon. The VEVA system consists of: • Two balloon platforms floating at 60 km VENUS EXPLORATION OF VOLCANOES altitude AND ATMOSPHERE (VEVA) MISSION • Two atmospheric chemistry sondes deployed NASA's Solar System Exploration Program immediately after entry at Venus includes a program of "core missions" • Eight small imaging sondes released to formulated by scientific advisory groups targeted sites during balloon traverses reporting to NASA and the competitive Discovery Program, in which missions are Mission Overview: The VEVA mission involves conceived and led by individual scientific the delivery of two identical payloads to Venus investigators. The Venus Exploration of with a launch in 2004 and arrival and entry at

3 American Institute of Aeronautics and Astronautics .i

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Fig 3: (a)The sonde support ringsuspended from the VEVA gondola keeps the weight of sondes centered on the gondola aftereach sonde release(b) the gondola provide structuresupport for the sclenceInstruments and the communlcations antennae to enable collectionand return of sciencedata.

Venus about six months later (Fig 2). Each entry • The eight smaller sondes flee falls to 5 km vehicle consists of a large sonde designed for above the surface where a small gliding atmospheric measurements and a balloon parachute opens to slow the descent and gondola system (Fig 3). An aerodynamic heat provide horizontal offsets between shield encloses the payload and provides initial successive images. These imaging sondes g-load protection and support for the entry phase. also carry an integrated atmospheric physics After entry, the heat shield is jettisoned, the sensor suite to measure ambient conditions. atmospheric sonde (Fig 4 (a) begins its fall to the Each sonde will be equipped at a minimum surface and the balloon inflates arresting its with an imaging system. Measurements of descent and providing lift to maintain a 60-km the chemistry in the low atmosphere is float height. The balloon carries an instrumented another key objective and micro- gondola with battery power for 7 days as it miniaturized mass spectrometers are being circles Venus (Fig 3). Each gondola carries four examined for this application small (imaging) sondes (Fig 4 (b)) for release at • The gondola also carries an atmospheric different times during the mission. Several kinds physics suite to measure ambient conditions of science measurement are made at the 60 km altitude for a period of up to 7 * The two large sondes are released from their days an will use a magnetometer to search respective gondolas on entry and fall to the for solar wind produced and intrinsic surface in about 37 minutes. They measure magnetic fields Radio tracking with the composition from below 20-kin altitude to Deep Space Network will be used for precise the surface and an integrated suite of measurements of horizontal winds instruments provides a detailed characterization of the atmosphere. The primary communications mode for both

Fig 4: Miniaturization of instruments and supporting subsystems allow packaging in low mass vessels for thermal and pressure protection during the descent to Venus' surface.

4 American Institute of Aeronautics and Astronautics s._!meuoJlsV pue sogneuoJoV jo alnt!lsu! u_a!JamV

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".mo_ S[qlJale I 'ldaauoa apuos!llnlN loqoJav snuaA at 0 uo paseq OSle 'l_sodmd aau e jo uo!ss!mqnsa_ Joj £_Japtm a_ staid lnq uo!ss!mqns ls_ U sl! u] paloalaS lou se_ I_sodoJd VA3A aCLL "uo!leuuoju! leUO!l!sod ftuunseatu JOj :_u![ o!peJ s!ql asn Ol SUeld ou aJe aJaql atu!l s!ql IV "UOOlleq aql q_no.lql s! SOpLIOSII_tUS pue a_Je I VEGAballoonswereheavyandover-designed Table 2: Telecom Parameters for VEVA minion for that application, use of"an improved material for VEVA has clear advantages. TELECOM DEEP SONDE TO Two concepts for such a balloon have been PARAMETERS SPACE NET GONDOLA devised. Both seem feasible and the choice will TO be an engineering decision: GONDOLA Maximum 1.46 x 106 km • The baseline proposed for VEVA is a 200 km Distance bilaminated Mylar (23 um x 23 urn) balloon UpHnk with a Teflon sheath. This approach has Power transmitter 4 kW iW heritage from the heavy Teflon cloth Frequency 7.17 GHz 400MHz impregnated balloon flown on VEGA. It has 66.9 dBi 1 dBi been tested and shown to tolerate the Xmit antenna gain Receive antenna 20 dBi IdBi sulfuric acid effects as described in the gain companion paper. Data Rate 125 BPS 500 KBPS • An alternative balloon material is Turnaround Yes polyethylene/mylar laminate. Recent tests Ranong indicate that polyethylene is resistant to Xmit antenna gain 24 dBi sulfuric acid. Relative to the Teflon sheath Dowailak balloon, this approach has several Power transmitter 153 W NA advantages: it is a proven technology and a Frequency 8.4 GHz NA number of laminate balloons have been NA made: it is easier to pack and deploy: and an Receive antenna 68. I dBi NA additional weight saving is possible since PE gain (density 0.95gm/cc) is much less dense than Data Rate I0 KBPS NA TFE (density 2.15 gm/cc). those data almost 150 million km from Earth. Technology work is continuing to further The parameters for the Earth-Gondola and the characterize these alternatives. Based on use of Sonde-Gondola communications links are shown the bottom inflation technology and either of in Table 2. these approaches to balloon fabrication, the VEVA goals for payload fraction (Table 1) Sonde-Gondola Communications: The appear readily achievable. system (Table 2) allows for transmission of data from two sondes simultaneously to their gondola Table 1: Comparison of VEVA and VEGA at 500kbps. Each gondola carries a preamplifier Payload Fraction and two UHF receivers tuned to frequencies 10 to 15 MHz apart plus Viterbi encoders. Sondes PARAMETER VEVA VEG/I BASIS FOR will be released so members of any pair that is AD VANCE within range of the gondola are transmitting on Mass of Balloon 94 kg 121 ks different frequencies. JPL is currently System prototyping low power space-qualified mass Floating 63 kg 21 kg transceivers meeting these requirements in the 0.65 0.33 Payload to Lightweight Microcommunications and Avionics System Floating Mass envelope (MCAS) program. The sonde's transmit antenna Fraction materials and the gondola receiving antenna are mounted Payload/Balloon 0.44 0.06 Lightweight vertically to ensure their relative alignment and System Mass bottom to minimize polarization loss. Fraction inflation system Gondola-Earth Communications: This X-band system will be using standard deep space transponder technology developed for the solar Communications Technology: The VEVA mission uses the aerobot as a communications system exploration program and used on relay to temporarily store about 300 Mbits of planetary missions. The Small Transponding data from each imaging sonde and then to relay Modem (STM) currently under development is now expected to be ready for this missions and

6 American Institute of Aeronautics and Astronautics will provides range/doppler data on the gondola the volcanic plains of" Rusalka Planitia. Eight by phase coherent turnaround of the uplink sites have been identified along parallel traverses signal sent from Earth. separated by 4° of latitude (0.7_N, 4.8°N).

The X band antenna on the gondola is 20 cm in Each balloon/gondola is deployed at 60km diameter and must maintain pointing at the Earth altitude and drifts west at about 75m/see with the with an accuracy of about 5°. At insertion, the Venus wind. They are launched just upwind of Earth is very close to the zenith, and no search the prime targets and those for which targeting would be needed for the initial acquisition. accuracy is most critical. These are the volcanoes Servo control for the antennas pointing system is Ozza Mons and Maat Mons. Aider drit_ing for provided by using the AGC signal from the about 2 hours, the balloon/gondolas are each transponder's receiver as a measure of beacon over their first target and a timer is used to signal strength. VEGA experience shows that we initiate the drop of the first imaging sonde. can expect long periods of stable flight with Studies of the Venus atmosphere indicate that, at occasional periods of atmospheric downflow and 60 km altitude, the winds are quite predictable turbulence. Control authority for and that the mean wind speed is 0 m/s N/S and telecommunications (10°/see) is expected to 75m/s E/W and the wind uncertainty is 2 m/s N/S

Fig 6: Magellan Radar image of the region on Venus 5.8S to ll.SN, 140 to 206 E. The two basefine mission traverse paths are shown as white arrows extending from the East. The drop sonde targets are shown as circles corresponding to acceptable are for uncertainty in impact point.

suffice for residual gondola rotation (l°/sec) and and 10m/see E/W. The balloon dispersion at the small angle wobbles (5 ° with oscillation periods first target due to the winds is +/-14 Ion N/S and of 30 to 60 sees to an accuracy of 2°. In the event =/-72 km E/W. The total control dispersion of larger attitude disturbances, re-acquisition will includes the entry error. be facilitated using an on-board attitude estimator. The possibility of using a sun sensor The E/W uncertainty can be reduced by using to achieve more rapid reacquisition of ranging and phase observations of the radio communications with Earth is also being signal from the gondola and the altitude of the explored as part of JPL's navigation and control gondola giving a positional accuracy of about 10 research program. km. This technique was demonstrated on the VEGA mission. Balloon position is used to Deployment of Imaging Sondes: The primary update the drop time to update the control error. goal of the VEVA mission is to investigate This refinement is much more critical for targets evidence for global catastrophes that have further downwind and sonde control accuracy for dramatically modified the surface of Venus. The these targets is expected to be 100 to 200 km. observational strategy that VEVA uses is to The error is mainly in the N/S direction and observe some key diagnostic landforms in a therefore cannot be reduced further just by target area (Fig 6) that is almost twice the area of controlling the time of deployment. Continued the United States and extending from the work on path planning and prediction in NASA's volcanic highlands of Atla Regio to the east to Cross Enterprise Technology Development

7 American Institute of Aeronautics and Astronautics Program is directed at ways of improving temperature of 30°C. An optical window in each targeting control and prediction and optimizing imaging sonde allows the camera a nadir view. science data recovery from the imaging sondes. The calculated descent profile and internal However, projected accuracy with current temperature for the imaging (small) probe is technology is acceptable for the success of the shown in Fig 7. Note the constant internal mission. temperature of 30°C due to melting of the phase

The balloons continue the westward drift, dropping imaging sondes at the other science targets. For the nominal arrival date at Venus of 9/26/2004, sun elevation angle at the preselected sonde sites is at least 20°. All imaging data will be recovered before the balloon drift behind the limb of Venus and are out of communication contact with the Earth. The balloons continues to drift and take data on the far side of Venus emerging in view of Earth again at arrival + 5 days. Science data acquired on the side of the planet away from Earth are dewnlinked and Fig 8: Prototype pressure vessel incorporating position fixes obtained. The mission ends at thermal insulation applicable to VEVA arrival + 7 days.

Sonde Technology: Both the small and large change material, plus the "knee" in the descent sondes consist of spherical pressure vessels 32 profile due to the parachute deployment at 5 km

5O . , . , , • , • . , "; , ., , altitude. The overall descent time for the large probe is about the same but it does not use a gliding parachute to brake its descent rate near

_i[_ / _ " i,a,l ico)l": the surface. t,.t- ,,j Thermal and mechanical technologies for

!tO compact vehicles for exploration of the deep Venus atmosphere have been under development at JPL for several years. Development of technology for short duration trips to the near surface of Venus was originally motivated by the Fig. 7: VEVA Imaging Sonde Drop needs of the Venus Geoscience Aerobot (VGA) mission that involved repeated descents to near the Venus surface. A concentric sphere design cm and 18.4 cm in diameter respectively with was selected for that mission with the 38cm outer stabilizing tail and fins for each sonde. The small sphere taking the external pressure load, and the sonde also features a streamlined aerodynamic payload and phase-change material housed inside fairing to decrease the drop time to the primary a 30-cm diameter inner sphere. The concentric picture-taking altitude of 5 km. The sonde's titanium sphere design originally was selected to pressure vessel and thermal protection permit vacuum multilayer insulation (MLI) components allow survival to 92 bars and 460 °C between the two spheres. However, although for the desired lifetime of each type of sonde. vacuum MLI theoretically offers the best Instrument operating temperatures are insulation performance, implementation maintained during descent using 15 mm difficulties associated with the need to maintain thickness of xenon-gas filled porous fiberglass 10s tort vacuum in the presence of flanges, material as insulation inside the vessel walls, electrical feedthroughs and the hot, high pressure plus a heat sink comprised of lithium nitrate venusian environment subsequently prompted trihydrate phase-change material (also used by consideration of the more robust, reduced the Venera landers) which has a melting performance alternative of gas-filled fiberglass

8 American Institute of Aeronautics and Astronautics performance alternative of gas-filled fiberglass has been developed in collaboration with insulation. A prototype unit was built (Fig 7) Georgia Institute of Technology and the Aurora and tested to evaluate the gas-filled fiberglass Flight Sciences Company for a future Mars concept and the structural integrity of the Micromission. The vehicle would not be lightweight vessel under pressure and high designed to survive impact. deceleration loading (7). This prototype did not include a window. Measured heat leaks on the Titan Aerobot with Sonde: The Titan Explorer prototype were I 11 W for xenon gas and 256 W mission, now under definition by NASA, is for nitrogen at a Venus slh-face temperature of likely to include an aerial platform, probably an 460 °C. The xenon result is very close to the aerobot, which would circumnavigate Titan. It VGA performance target of 100 W, suggesting would cross Titan's icy continents, which may be that it is a viable alternative to the original coated with organic materials precipitating from vacuum ML[ concept.The lightweight prototype the atmosphere and traverse the ethane lakes or was also successfully pressuretested to 92 bar, oceans that are believed to exist on this large both at room temperature and Venus surface satellite of Saturn. Titan's dense nitrogen temperature. A final test was a 250-g centrifuge atmosphere (almost five times as dense as that of test, which was completely successful with post- Earth) and modest low-level winds should permit test inspection revealing no damage or plastic a Titan Aerobot to make safe, controlled descents deformation of gondola components. All of these to within several hundred meters of the Titan demonstrated technologies are applicable to the surface. Microsondes deployed from the aerobot VEVA drop sondes, althoughthe internal sphere would make it possible to characterize these of the concentric design will not be needed in the potentially hazardous land surfaces before absence of vacuum MLI. landing a larger and more expensive instrumentation for detailed investigations of the On the basis of these tests, designs for both the surface chemistry and the search for prebiotic large sonde and the small sonde were developed. chemicals. A prototype thermal test model of the imaging (small) sonde is now being built which will CONCLUSION: The golden age of robotic incorporate improvements permitting more balloon exploration of the planets is now compact packaging. Experience with window beginning. The role of these aerobots will not be technology from a JPL imaging probe which has limited to carrying instrument payloads. been designed for photography of hot volcanic Aerobots that are used as carriers for small, vents at the undersea Hawaiian volcano Lo'ihi is instrumented sondes that are capable of making also being incorporated in the design of this measurements of short duration in hazardous system. areas is one such application. The aerobot can deliver the sondes precisely and serve a critical OTHER MULTISONDE MISSIONS: The communications relay function. After collecting Aerobot Sonde concept also has applications to a burst of high rate data from the sondes, the data Mars and Titan - the two other solar system are stored on the aerobot and communicated bodies with atmospheres substantial enough for across the vast reaches of interplanetary space balloon flight. However, the motivations for over a period of hours to days. A Venus using the sonde are somewhat different than at Multisonde mission is particularly attractive Venus and the sonde design will be driven by because of inability of obtain aerial imaging different factors. from orbit or high in the Venus atmosphere and the severe temperature environment near the Mars Aerobot with Microplane: At Mars, Venus surface which makes long duration deployment can be used to perform close up vehicles with present technology too costly. The observations of scientifically important but Venus Exploration of Volcanoes and hazardous terrains such as strata exposed in the Atmosphere (VEVA) proposal to NASA's steep slopes of the deep canyons and rift systems Discovery program submitted in June 1998 of that planet. Rather than using a parachute to incorporated many of these ideas. A second extend the duration of flight close to the surface proposal, embodying many of the same concepts, of Mars, a glider, possibly with a rocket booster is planned in the near future. is an attractive approach (Ref 6). Such a concept,

9 American Institute of Aeronautics and Astronautics Acknowledgements: (5) Mars Aerobot Validation Program by V. The research described in this paper, was carried Kerzhanovich et.al. AIAA Balloon Technology out by the Jet Propulsion Laboratory, California Conference !999 Institute of Technology, under a contract with the National Aeronautics and Space Administration. (6) Evaluation of Materials for Venus Aerobot Contributions from the other members of the Applications by A. Yavrouian, O. PIER, S.P.S. VEVA team are gratefully acknowledged. Yen,_ J.. CuRs and D. Baek ALAA Balloon References: Technology Conference 1999 "" (!) NASA's Roadmap for Solar System Exploration, 1996. Published on the Internet at (7) Technology Demonstration of a Gondola for URL: http://eis.jpl.na.ut.gov/roadmap/site/ Venus Deep Atmosphere Operations by Jeffery L.Hall, Paul D. MacNeal, Moktar A. Salama, (2) Venus Geoscience Aerobot, 1996, Published Jack A. Jones and Matthew Kuperus Heun - on the Interact at URL: AIAA Journal of Spacecra_ (in press) http://eis.jpl.ng._.gov/roadmap/site/na._/eatthen vironment.Cearth 12.html

(3) Venus Balloon Discovery Study by Venus Balloon Study Team, JPL Internal Report, September 23, 1997

(4) The VEVA Mission: Exploration of Venus Volcanoes and Atmosphere presented at the Lunar and Planetary Scientific Conference March 1999.

10 American Institute of' Aeronautics and Astronautics