NASA Technical Reports Server (NTRS) 20000052454: Venus Aerobot Multisonde Mission

10 Pages·0.64 MB·English

by  NASA Technical Reports Server (NTRS)

w
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 asufficiently 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
are a more practical solution. However, to be
(2) NASA Headquarters, Washington, D.C.
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 1Comparison 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 adirect 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
receivingantennoantheaerobotT.hesedataare
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 afactor 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 aVenus 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
;II ',
I
I
Z
flu
t,'
i_,
.,..°
It
Fig3:(a)The sondesupport ringsuspended from theVEVA gondola keepstheweightofsondes
centeredon thegondola aftereach sonderelease(b)thegondola provide structuresupport for
thesclenceInstrumentsand thecommunlcations antennae toenablecollectionand returnof
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
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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 106km 200 km
• The baseline proposed for VEVA is a
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
Xmit antenna gain 66.9 dBi 1dBi
been tested and shown to tolerate the
Receive antenna 20dBi IdBi
sulfuric acid effects as described in the
gain
companion paper.
Data Rate 125BPS 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.IdBi 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
ADVANCE within range of the gondola are transmitting on
Mass of Balloon 94kg 121ks different frequencies. JPL is currently
System prototyping low power space-qualified
Floating mass 63kg 21kg
transceivers meeting these requirements in the
Payload to 0.65 0.33 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
Fraction inflation to minimize polarization loss.
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
Modem (STM) currently under development is
data from each imaging sonde and then to relay
now expected to be ready for this missions and
6
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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 atthe 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
altitude. The overall descent time for the large
5O . , . , , • , • . , "; , ., ,
probe is about the same but it does not use a
gliding parachute to brake its descent rate near
the surface.
_i[_ / _ " i,a,l ico)l":
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 thatmission with the 38cm outer
stabilizing tail and fins foreach 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 tosurvive 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 I11W 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 closeto the aerobot, which would circumnavigate Titan. It
VGA performancetargetof 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
vacuumML[ concept.The lightweightprototype the atmosphere and traverse the ethane lakes or
wasalsosuccessfully pressuretestedto 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 thatof
test, which was completely successful with post- Earth)and modest low-level winds should permit
test inspection revealing no damage or plastic aTitan 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 fordetailed 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 JPLimaging 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 Sondeconceptalso hasapplicationsto a burstof 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 isplanned inthe near future.
isan 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 acontract withthe
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 aGondola for
URL: http://eis.jpl.na.ut.gov/roadmap/site/ Venus Deep Atmosphere Operations by Jeffery
L.Hall, PaulD. MacNeal, Moktar A. Salama,
(2) Venus Geoscience Aerobot, 1996, Published JackA. Jones andMatthew Kuperus Heun -
on the Interact at URL:
AIAA Journalof Spacecra_ (in press)
http://eis.jpl.ng._.gov/roadmap/site/na._/eatthen
vironment.Cearth12.html
(3) Venus Balloon Discovery Study by Venus
Balloon Study Team, JPL InternalReport,
September 23, 1997
(4) The VEVA Mission: Exploration of Venus
Volcanoes and Atmosphere presented at the
Lunar and Planetary Scientific Conference March
1999.
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American Institute of'Aeronautics and Astronautics