rtgs for voyager

26 N U C L E A R N E W S April 1999

T H E U.S. D E P A R T M E N T of Energyand its predecessors have provided nu-clear power systems for use in space

for about 38 years. These systems have provento be safe, reliable, maintenance-free, and ca-pable of providing both thermal and electricalpower for decades under the harsh environ-ment experienced in deep space.

The unique characteristics of these systemsmake them especially suited for environmentswhere large solar arrays are not practical, andat long distances from the Sun. To date, theDOE has provided radioisotope power sys-tems and heater units for use on a total of 26missions to provide some or all of the space-

craft on-board power and heating of criticalspacecraft components.

Early program historyRadioisotope power systems have been

providing primary spacecraft power for manyunique space missions since 1961. In the mid-1950s, research was started on ways to use nu-clear energy to generate electrical power forspacecraft. This research resulted in the de-velopment of radioisotope thermoelectricgenerators (RTGs), which are nuclear power

generators that convert heat generated by thenatural decay of a radioisotope fuel (plutoni-um-238) into electricity through the use ofthermoelectric couples.

The first two space flights with RTGs werethe Navy’s Transit 4A and 4B navigationalsatellites, launched in June and November1961. A 3-watt RTG, which was called Systemsfor Nuclear Auxiliary Power (SNAP-3), wasflown on each spacecraft to prove the opera-tional capability of the RTGs in a space envi-ronment. On subsequent flights beginning in1963, RTGs provided total electrical power forthe spacecraft. The DOE and its predecessoragencies have provided radioisotope power sys-tems for missions orbiting Earth, on the Moon,and other solar system bodies. Five Apollo mis-sions used RTGs to power the Apollo LunarSurface Experiment Packages. RTGs providedprimary electrical power on the Viking landersand the Pioneer, Voyager, Galileo, Ulysses, andCassini spacecraft. These missions have given

Twenty-six U.S. space missions have usednuclear power systems to take spacecraft to placesscientists otherwise would not be able to study.

U.S. space missions usingradioisotope power systems


BY RICHARD R. FURLONG

AND EARL J. WAHLQUIST

Richard R. Furlong is a Program Manager in the Space and Defense Power Systems Office within the Of-fice of Nuclear Energy, Science and Technology. He is the site Program Manager for heat source fabri-cation activities at Los Alamos National Laboratory, in Los Alamos, N.M., and is responsible for coordi-nating DOE’s contingency planning/emergency response activities at the site of radioisotope power systemslaunches. He previously served as RTG Project Manager for the Cassini program. Earl J. Wahlquist isthe Associate Director for Space and Defense Power Systems within the Office of Nuclear Energy, Sci-ence and Technology. He directs the design, development, and fabrication of space nuclear power sys-tems for use by other federal agencies. He has been involved in space and national security nuclear pow-er systems for over 25 years.

Radioisotope thermoelectric generator (RTG). The length is 44.5 in (113 cm), the diameter is 16.8 in (42.7 cm), and the weight is 124 lb (56.2 kg).(Source: DOE)

scientists throughout the world the opportunityto study and investigate the mysteries of the for-mation of our solar system.

Why use RTGs?Nuclear power sources are very important

for use in space applications for many reasons:� Long life—Nuclear power is the only pow-er source currently available for spacecraft op-erating in deep space for long missions. Ra-dioisotope power systems provide predictablepower levels for mission planners to dependon (RTG power levels reduce about 0.8 per-cent per year based on the decay rate of Pu-238 fuel).� Environment—Nuclear power sources canoperate in extreme environmental conditions,such as the high-radiation belts surroundingJupiter, extreme temperatures experienced onthe moon, and severe dust storms seen on Mars.� Operational independence—RTGs pro-vide power to spacecraft without concern forwhere the spacecraft is in its orbit or how it isoriented in relation to the sun. An RTG beginsto generate electrical power when the ra-dioisotope fuel source is loaded in the con-verter. Therefore, it can be used for spacecraftsystem checkouts prior to launch and be avail-able to provide power to the spacecraft wheninstalled on the launch pad.� Reliability—RTGs have proven to be themost reliable power systems ever flown onU.S. spacecraft. For example, the two Pioneerspacecraft operated for more than two decadesbefore being shut down, and NASA is plan-ning on an extended Voyager mission thatcould last up to 40 years.

How RTGs workRTGs operate on the principle of thermo-

electric generation that converts heat directlyinto electricity. The principle was discoveredin 1821 by a German scientist, Thomas Jo-hann Seebeck. He observed that an electriccurrent is produced in a closed circuit whenthe junctions of two dissimilar metals aremaintained at different temperatures. Suchpairs of junctions are called thermoelectriccouples, or thermocouples.

In the design of the RTGs flown on theGalileo, Ulysses, and Cassini missions, heatgenerated by the natural decay of the Pu-238dioxide fuel is converted to electric power bysilicon germanium (SiGe) thermoelectric cou-ples or unicouples. A heat source module con-tains four Pu-238 fuel pellets, each weighing151 g and encapsulated within a vented iridi-um clad. Eighteen heat source modules pro-vide the total thermal inventory for an RTGwith a heat output of about 4400 watts. Thethermoelectric converter portion of the RTGconsists of 572 thermoelectric unicoupleswired in a two-string, series-parallel electriccircuit. This configuration generates about 300watts of electrical power at initial fuel loading.

Making RTGsA number of DOE facilities, laboratories,

and contractors were used in the development,fabrication, assembly, and testing of the RTGsfor the Cassini program. The DOE facilitiesare located at Savannah River Plant (SRP), in

Aiken, S.C.; Oak Ridge National Laboratory(ORNL), in Oak Ridge, Tenn.; Los AlamosNational Laboratory (LANL), in Los Alam-os, N.M.; the Mound Plant, in Miamisburg,Ohio; and Sandia National Laboratories, inAlbuquerque, N.M.

The Pu-238 processing facilities at Savan-nah River were used to process the Pu-238 ox-ide fuel and package it for shipment to LANL.ORNL was responsible for fabricating iridiumcladding that was used to encapsulate Pu-238fuel pellets. ORNL also fabricated graphitecomponents used in assembling the encapsu-lated fuel pellets into heat sources. At LANL,the Pu-238 was processed and pressed into fuelpellets. The pellets for the RTG heat sourceswere encapsulated in iridium cladding provid-ed by ORNL. The encapsulated fuel pelletswere shipped from LANL to the Mound Plantwhere they were assembled into heat sources.Mound completed the RTG assembly by in-stalling a stack of heat sources into the ther-moelectric converter built by Lockheed Mar-tin Astronautics, of Valley Forge, Pa. Aftercompletion of the RTG assemblies, Moundpersonnel conducted final acceptance tests ofthe units, and then packaged and shipped themto the launch site where they were installed onthe Cassini spacecraft.

RTG safetyMany design improvements have been

made over the four decades that RTGs haveflown on space missions. In addition to im-proving the efficiency of RTGs, the DOE con-ducted extensive safety testing—at SandiaNational Laboratories and LANL—to assurethat the systems would be safe under all acci-

dent conditions, including accidents occurringon or near the launch pad and orbital reentryaccidents. The Pu-238 fuel form was changedfrom a metal to a more stable pressed oxide.During the three mission aborts that did oc-cur, the RTGs performed as designed.

On April 21, 1964, the Transit 5-BN-3 mis-sion was aborted because of a launch vehiclefailure resulting in burnup of the RTG duringreentry, in keeping with the RTG design at thetime. This resulted in dispersal of the plutoni-um fuel in the upper atmosphere. Subse-quently, the RTG design was changed to pro-vide for survival of the fuel modules duringorbital reentry.

A second accident occurred when the Nim-bus B-1 launch on May 18, 1968, at Vanden-berg AFB, Calif., was aborted shortly afterlaunch by a range safety destruct of the vehi-cle. The heat sources were recovered intact inabout 300 feet of water off the California coastwith no release of plutonium. The fuel cap-sules were reworked and the fuel was used ina later mission.

The third incident occurred in April 1970,when the Apollo 13 mission to the moon wasaborted following an oxygen tank explosionin the spacecraft service module. Upon returnto Earth, the Apollo 13 lunar excursion mod-ule with a SNAP-27 RTG on board reenteredthe atmosphere and broke up above the southPacific Ocean. The graphite reentry cask con-taining the heat source plunged into the oceanin the vicinity of the Tonga Trench, where theocean depth is five to six miles. Atmosphericand oceanic monitoring showed no evidenceof release of nuclear fuel.

April 1999 N U C L E A R N E W S 27

Continued

In 1968, a NIMBUS B-1 weather satellite was destroyed after its launch vehicle malfunctioned a fewminutes into its flight from Vandenberg AFB, Calif. The satellite had two SNAP-19B2 RTGs on-board.The plutonium heat sources from the RTGs were recovered intact after five months in 300 feet ofseawater, from the bottom of the Santa Barbara Channel near the California coast. No radioactive fuelwas released. The heat sources were disassembled and repackaged, and the plutonium was used onthe next mission. The photo shows the intact heat sources during the underwater recovery operation.(Source: JPL)

The missionsApollo

Radioisotope power was used on six suc-cessful Apollo landings on the moon. Begin-ning with the Apollo 11 mission in July 1969and ending with the Apollo 17 mission in De-cember 1972, radioisotope power systems werean integral and important part of the lunar ex-ploration program. On the first lunar landing,Apollo 11, radioisotope heater units (RHUs)were used to provide heating of critical com-ponents in a seismic experiment package.

RTGs were employed on the Apollo 12, 14,15, 16 and 17 missions to power the ApolloLunar Surface Experiment Packages(ALSEPs). The SNAP-27 RTG, used in theALSEPS, was designed to supply about 63watts of power at 16 VDC one year afterplacement on the lunar surface. Use of RTGswas a natural choice because of their lightweight, reliability, and ability to produce fullpower during the long lunar night-day cycles.Because the ALSEPs were to be positioned onthe moon by astronauts, the RTGs were de-signed to be assembled after landing. The con-

verter and sealed fuel capsule were kept sep-arate in the Lunar Module and assembled onthe moon.

The SNAP-27 RTGs powering ALSEPsexceeded their mission requirements in bothpower output and lifetime. All five ALSEPswere operating when NASA shut down thestations on September 30, 1977.

PioneerThe Pioneer 10 and 11 missions, launched

in 1972 and 1973, respectively, were spectac-ular successes in the long-range strategy to ex-plore the outer planets. Each spacecraft waspowered by four SNAP-19 RTGs that deliv-ered approximately 165 watts of total electri-cal power at launch.

Pioneer 10 was the first spacecraft to sur-vive passage through the asteroid belt and theintense radiation environment at Jupiter. Afterits investigation of Jupiter, Pioneer 10 beganan escape trajectory from the solar system. OnFebruary 1, 1999, Pioneer 10 was about 72 as-tronomical units (AUs) from the Sun (1 AUequals 93 million miles, the distance from

Earth to Sun), heading away from the Sun at2.6 AU/year. Routine tracking and data pro-cessing operations were terminated on March31, 1997 for budgetary reasons. Pioneer 10,however, is still occasionally tracked for train-ing purposes. Pioneer 10 and its RTGs con-tinue to operate 27 years after launch!

Pioneer 11 was the second mission to in-vestigate Jupiter and the first to explore theplanet Saturn and its rings. During its closestapproach on December 4, 1974, Pioneer 11passed to within 34 000 km (about 21 000miles) of Jupiter and passed by Saturn on Sep-tember 11, 1979, at a distance of 21 000 km(about 13 000 miles). Science operations anddaily telemetry ceased on September 30, 1995,when the RTG power level was insufficient tooperate any experiments. These spacecraftwere the first manmade objects to pass the or-bit of Pluto and enter interstellar space.

VikingNASA’s two Viking missions to Mars,

Viking 1 and Viking 2, each had an orbiter anda lander. Viking 1 was launched on August 20,1975, and Viking 2 on September 9, 1975. Al-though the orbiters were solar-powered, eachlander was powered by two SNAP-19 RTGs,delivering a total of approximately 85 wattsof electrical power. The primary objectives ofthe mission were to obtain high-resolution im-ages of the Martian surface, characterize thestructure and composition of the atmosphereand surface, and search for evidence of life.

After each Viking spacecraft orbited Marsand returned images to Earth, NASA selecteda landing site for each lander. The orbiters andlanders were separated, and the landers en-tered the Martian atmosphere and soft landedat the chosen sites. The orbiters imaged the


Artist’s conception of the Pioneer 10 Jupiter flyby (Source: NASA)

Deployment of the SNAP-27 RTG during theApollo 12 moon mission, on November 19, 1969(Source: NASA)

U.S. spacecraft launches involving radioisotope systems (Source: DOE)

entire surface of Mars. The landers transmit-ted higher resolution images of the landingarea, took surface samples and analyzed themfor composition and signs of life, studied at-mospheric composition and meteorology, anddeployed seismometers.

The Viking 2 orbiter was powered down onJuly 25, 1978, after 706 orbits, and the Viking1 orbiter on August 17, 1980, after more than1400 orbits. The Viking 2 lander shut downoperations on April 11, 1980, and Viking 1 onNovember 13, 1982, after transmitting morethan 1400 images of the two landing sites. Theresults from the two Viking missions gave sci-entists an extensive view of Mars.

VoyagerVoyager 1 and Voyager 2 were launched in

August and September 1977. These spacecraftrequired a significant increase in power overprevious RTG missions and an operational lifeof at least four years after launch to achievemission objectives of exploration of Jupiter,Saturn, Uranus, and Neptune. Mission re-quirements led to the development of the mul-tihundred-watt (MHW) RTG.

The MHW-RTG contained a new heatsource of 24 pressed plutonium oxide fuelspheres. Conversion of the decay heat of theplutonium to electrical power was accom-plished through 312 silicon-germanium(SiGe) thermoelectric couples. The designthermoelectric couple hot junction tempera-ture was 1273 K (1832 °F) with a cold junc-tion temperature of 573 K (572 °F). EachMHW-RTG provided approximately 157watts of power at beginning of mission.

The Voyager missions successfully com-pleted all of their objectives by the end of1989 with the close flyby of Neptune by the

Voyager 2 spacecraft. With the continuinghealthy operation of the RTG power systemand spacecraft scientific instruments, NASAdesigned a significantly extended mission forthe Voyager spacecraft called the Voyager In-terstellar Mission (VIM). VIM has the poten-tial for obtaining useful science data on inter-planetary and interstellar magnetic fields,

charged particles, and plasma waves untilabout the year 2020, when the RTGs’ abilityto generate adequate electrical power forspacecraft operation will come to an end,more than 40 years after launch! To date, theVoyager spacecraft have traveled throughspace more than 15 billion miles.

GalileoThe Galileo spacecraft was launched

aboard the Space Shuttle Atlantis (STS-34) onOctober 18, 1989. The Galileo mission wasdesigned to investigate the Jovian system—the largest planet in our solar system, Jupiter,and four of its major moons: Io, Europa,Ganymede, and Calisto. The spacecraft con-sisted of a Jupiter Orbiter and an Atmospher-ic Entry Probe.

To meet the larger power requirements ofspace missions such as Galileo and Ulysses,the DOE developed the general purpose heatsource (GPHS) RTG. The GPHS-RTG wasdesigned using similar heat-to-electrical con-version technology successfully demonstratedby the MHW-RTGs flown on the Voyagermissions. Using SiGe unicouples and the Pu-238–fueled GPHS, the GPHS-RTGs werebuilt to deliver approximately 300 watts ofelectrical power with a nominal fuel loadingyielding about 4400 watts of thermal energy.

In addition to providing the total electricalpower to operate the spacecraft’s instruments,communications, and other power demands,120 lightweight radioisotope heater units(LWRHUs) were used to provide temperaturecontrol of sensitive electronic components.LWRHUs consist of a 2.68-g Pu-238 dioxidefuel pellet that produces 1 watt of heat output.Each fuel pellet is encapsulated in a platinum-rhodium cladding and encased in a multilay-


R A D I O I S O T O P E P O W E R S Y S T E M S I N S P A C E

Artist’s conception of a Voyager spacecraft during Saturn flyby (Image courtesy of DOE)

Artist’s conception of Galileo, near Jupiter and a Jovian moon (Image courtesy of DOE)

er graphite containment to protect the pelletin the event of an accident.

On its six-year journey from Earth toJupiter, the Galileo spacecraft followed a tra-jectory that used the gravity of planets to ac-celerate the spacecraft onto its final flight pathto Jupiter. Inertial Upper Stage (IUS) rocketmotors that were approved for shuttle flightswere incapable of providing the necessarythrust to inject Galileo on a direct Earth-Jupitertransfer trajectory. Therefore, the trajectory de-signed for the Galileo mission included aVenus flyby in February 1990 and Earth fly-bys in December 1990 and December 1992 toattain the necessary velocity to reach Jupiter.

During the Earth flyby in December 1990,Galileo provided spectacular photographs ofthe Earth and Moon, the first photographs ofthe Earth and Moon together ever taken by anunmanned spacecraft. Galileo also providedthe first closeup photographs of an asteroid,Gaspra, in 1993. On its approach to Jupiter in1994, Galileo was in position to witness andrecord the only direct photographs ever takenof a comet colliding with a planet, the Shoe-maker-Levy 9 comet collision with Jupiter inJuly 1994.

Galileo arrived at Jupiter on December 7,1995. Five months before arriving at the giantplanet, the atmospheric probe was released forits plunge into the Jovian atmosphere. Whenthe Galileo spacecraft began its maneuvers toachieve orbit around Jupiter, it swung by themoon Io and fired its main engine.

Galileo has collected a vast array of scien-tific data about Jupiter and its four majormoons. Examination of some of these data hasindicated that Ganymede is the first moonfound in the solar system to have its own mag-netic field, and Europa may have an ocean be-neath a relatively young surface of ice thatmay be only about 1-km (0.62-mile) thick inplaces.

Originally, Galileo’s exploration of theJovian system was to end on December 7,1997, but since significant discoveries werefound, especially those about Europa, the mis-sion was extended for two years through theend of 1999, and has been named the GalileoEuropa Mission (GEM). The GEM will com-plete its studies of Europa, fly by Calisto fourtimes, and lower its orbit in preparation fortwo flybys of Io.

UlyssesThe Ulysses mission was a joint project of

the European Space Agency, NASA, and theJet Propulsion Laboratory. Launched by theSpace Shuttle Discovery on October 6, 1990,Ulysses flew by Jupiter in February 1992, where

a gravity assist maneuver lifted the spacecraftout of the ecliptic plane and into a polar orbitabout the Sun. Ulysses flew over the south poleof the Sun in 1994, and over the North Pole in1995. The primary studies of the mission re-sulted in a greater understanding of the behav-ior of sunspots, solar flares, solar X rays, solarradio noise, and the region known as the he-liosphere, which is dominated by the solar wind.

Since existing launch vehicles were not ca-pable of boosting the Ulysses spacecraft outof the ecliptic plane, it had to rely on the largegravity assist provided by Jupiter. BecauseUlysses had to travel to Jupiter and attain alarge solar orbit, where the sunlight is about4 percent of that near Earth, solar arrays werenot feasible. A GPHS-RTG of the same de-sign as those flown on the Galileo spacecraftprovided the required power for Ulysses.



General purpose heat source (GPHS) module assembly (Source: DOE)

Lightweight radioisotope heater unit (LWRHU)(Source: DOE)

Artist’s conception of the Ulysses spacecraft after release by the Space Shuttle Discovery (Imagecourtesy of DOE)

Continued

With the primary objectives of the Ulyssesmission successfully accomplished, a secondorbit of the Sun was initiated. This investiga-tion will examine for the first time the high-latitude properties of the solar wind during theSun’s maximum solar activity cycle.

Mars PathfinderThe Mars Pathfinder project was one of the

first missions in the NASA Discovery Program.Launched on December 4, 1996, the MarsPathfinder arrived at Mars and successfullylanded on the Martian surface on July 4, 1997.

The first robotic rover sent to Mars, the So-journer, was aboard the Pathfinder spacecraft.The rover was deployed from the lander to per-form a number of experiments on the Martiansoil and to demonstrate the ability of a rover totraverse the terrain in the vicinity of the lander.

Three LWRHUs were employed in the So-journer Warm Electronics Box to maintain crit-ical electronic component temperatures withintheir operating limits during the Martian nights.The LWRHUs, providing essential heat to theSojourner electronics, were key components ofthe thermal design that enabled the rover to op-erate for 84 days, 12 times its design lifetime.

CassiniThe Cassini mission is an international co-

operative project of NASA, the EuropeanSpace Agency, the Italian Space Agency, andother academic and industrial partners through-out the world. The mission is managed forNASA by the Jet Propulsion Laboratory.

The goal of the Cassini mission is to con-duct extensive studies of Saturn and its rings,moons, and magnetosphere, including a de-

ployment of the Huygens probe to the giantmoon Titan.

The Cassini spacecraft lifted off at CapeCanaveral Air Station on October 15, 1997,aboard a Titan IV/Centaur launch vehicle.Cassini, the largest NASA interplanetaryspacecraft ever launched, will travel on an in-terplanetary voyage lasting nearly sevenyears, arriving at Saturn on July 1, 2004. TheCassini spacecraft and probe weighed about5700 kg (6.3 tons) at liftoff, more than 50 per-cent of which was liquid fuel. The spacecraftmeasured more than 6.7 m (22 ft) high andmore than 4 m (13.1 ft) wide.

Traveling to distant Saturn, the large space-craft and probe required 3 GPHS-RTGs and117 LWRHUs to provide the necessary elec-trical power to operate Cassini’s instrumentsand systems and to maintain temperatures ofcritical equipment at acceptable levels in thesevere cold of deep space. The 3 GPHS-RTGsdelivered approximately 888 watts of totalelectrical power to the spacecraft at time oflaunch.

Cassini is another spacecraft that uses plan-etary gravity assists to attain the velocity andfinal trajectory necessary to complete its jour-ney to Saturn. The spacecraft successfullycompleted a gravity assist from Venus onApril 26, 1998, and will obtain another grav-ity assist from Venus on June 24, 1999, andfrom Earth on August 18, 1999, before gettingits final gravity assist from Jupiter on Decem-ber 30, 2000.

After arrival at Saturn, the Cassini missionwill begin its four-year, closeup study of theSaturnian system. Studying the Saturnian sys-tem will help scientists find out how the plan-et and its rings and moons formed andevolved, and may provide many clues as tothe origin of our solar system.

On November 4, 2004, the orbiter space-craft will release the Huygens probe for itsthree-week trip to Titan. Upon entering Ti-tan’s atmosphere, the probe will deploy itsparachute and slowly descend to the surface.The Cassini orbiter will receive data from theprobe, where it will be stored and eventuallyrelayed to Earth.

During the four-year mission, the Cassinispacecraft will conduct some four dozen closeflybys of bodies of interest, including aboutthree dozen encounters with Titan and sever-al icy satellites. The orbiter will also makemany distant flybys of other Saturnian moons.Changes in orbit inclinations will allow Cassi-ni to study Saturn’s polar regions and equato-rial zone.

The 21st CenturyA new program

The DOE Office of Space and DefensePower Systems has embarked on a new pro-gram to develop an advanced radioisotopepower system (ARPS) that improves the effi-ciency of heat-to-electric-power conversion ina smaller, lighter-weight system. The systemunder development uses a technology knownas alkali-metal-thermal-to-electric conversion(AMTEC). An ARPS generator will consist ofa heat source (GPHS modules), AMTEC cells,and a housing designed for space operation.


The Sojourner rover, part of the Mars Pathfinder mission (Source: NASA)

The Cassini spacecraft’s major components (Source: JPL/NASA)

The AMTEC cell (see figure, page 34) con-verts to electricity the heat delivered by thePu-238 heat source by the following process.Each cell is made up of eight beta alumina sol-id electrolyte (BASE) tubes connected in se-ries. The end of the cell at which the BASEtubes are located is mounted adjacent to thethermally hot end of the heat source. At thisend, liquid sodium (Na) alkali metal is heatedand takes the form of a vapor. As the sodiumatoms in the vapor are driven through thewalls of the BASE tubes, they are stripped ofan electron, thus creating positively chargedsodium ions (Na+). The vapor is cooled downand collected by a condenser at the cold end ofthe cell, and the cycle is repeated as the sodi-um fluid flows through the artery toward thehot surface at the other end of the cell. The cellis designed to use thermal shields located inthe upper portion of the cell to reduce radiativebypass heat losses from the hot-side compo-nents to the cold-side condenser.

Leads are taken from the first and eighthtubes in series as the positive and negativeleads for the cell. The electricity generated isthen used to power the spacecraft systems andinstruments.

Although specific missions for an ARPShave not yet been selected, potential missionsto use these generators include the Outer Plan-ets/Solar Probe Project: Europa Orbiter, Plu-to-Kuiper Express, and Solar Probe missions.

EuropaThe Europa Orbiter is envisioned to circle

Jupiter’s ice-covered moon searching for sub-surface oceans that might support life. Theprimary objectives of the mission are: (1) todetermine if there is a liquid ocean beneath theice, and, if so, how thick the ice is; (2) char-acterize the three dimensional distribution ofany subsurface liquid water and its overlying

ice layers; (3) determine the energy source forthe ocean; and (4) identify potential landingsites for future probes.

Recent images have been relayed back toEarth by the Galileo spacecraft showing de-tails of a surface of water ice on Europa. Manyscientists believe the pictures reveal a rela-tively young surface of ice, possibly onlyabout 1-km (0.62-mile) thick in places. Inter-

nal heating from Europa’s inner core and tidalaction caused by Jupiter could melt the under-side of the ice surface, forming an ocean of liq-uid water beneath the surface. An instrumentcalled a radar sounder would be used to bounceradio waves through the ice to determine thethickness, and other instruments would revealdetails of the surface and interior processes.



Cassini trajectory from Earth to Saturn (Source: JPL/NASA)

Artist’s conception of the Europa Orbiter near Europa, the fourth largest moon of Jupiter (in thebackground) (Source: JPL/NASA)

Continued

An ARPS has the potential for providingpower for the spacecraft systems and instru-ments. It would be designed to deliver ap-proximately 210 watts of electrical power sixyears after launch. The Europa mission has apotential launch date of November 2003, withan arrival at Europa in 2007.

Pluto/Kuiper ExpressThe Pluto/Kuiper Express mission is envi-

sioned to fly by the only planet in our solarsystem not yet visited by spacecraft—Plutoand its moon Charon. Pluto’s elongated andtilted orbit (inclined 17 degrees relative to theEarth’s orbit) is 248 years long. Its thin at-mosphere is expected to freeze and collapsein the next 35 years as it moves out to the out-er reaches of its orbit, making it impossible tostudy for 200 years or more. Beyond Pluto liesthe recently discovered Edgeworth-KuiperDisk of “ice dwarfs,” or minor planets.

Pluto’s tiny size and great distance fromEarth make its study a continuing challengeto planetary astronomers. Most of what isknown about Pluto was learned since the late1970s. Many of the key questions about Plu-to and its satellite Charon await a closeup ob-servation by a space flight mission. Pluto’shistory may be connected with the Earth’s at-mosphere and biosphere.

To gain insight into these connections,NASA is now studying a reconnaissance mis-sion to Pluto-Charon using newly availableminiaturized technologies and advanced soft-ware systems. The current version of the Plu-to/Kuiper Express spacecraft would weighabout 225 kilograms (495 pounds) and has apotential launch date of December 2004. Apotential radioisotope power system would bedesigned to provide about 185 watts of elec-trical power 14 years after launch, with a pro-jected arrival at Pluto in 2012.

Solar ProbeAnother potential mission that might take

advantage of using radioisotope power is So-lar Probe. The current mission design conceptwould involve the spacecraft’s going out toJupiter first, akin to the Ulysses mission, fora gravity-assisted swingby in order to get intothe proper orbit about the Sun. The currentversion of the Solar Probe spacecraft has amass of about 250 kg (550 lb), making it an-other member of the new generation of small-er, smarter, and more efficient spacecraft.

The primary mission objectives are to: (1)measure the birth and acceleration of the so-lar wind; (2) measure the heating of the solarcorona; (3) detect waves and turbulence in-

side the solar corona; (4) view the poles of theSun up close for the first time; and (5) viewthe Sun with the highest spatial resolutionever—20 km (12 miles).

The Solar Probe will approach as close as3 solar radii (about 2 million km or 1.23 mil-lion miles) from the surface of the Sun, witha thermal shield designed to withstand tem-peratures of about 2400 K (3800 °F) to enablescience instruments to perform their studies.An anticipated launch date for the Solar Probemission is 2007, with an arrival at the Sun in2010.

Other potential missionsNASA has identified a number of missions

that address numerous objectives for solarsystem exploration for the years 2000–2015.In the early decades of solar system explo-ration, missions were dominated by simpleflybys and planetary orbiters. Some of the po-tential future missions may require extensiveencounters within the atmospheres or on thesurfaces of planetary bodies, moving aroundwithin these environments, acquiring and an-alyzing samples, and potentially returningthem to Earth. Some of these missions mayalso require survival and operation of equip-ment within harsh thermal and radiationenvironments.

Some examples of potential missions thatcould make use of radioisotope power systemsare the Interstellar Probe, Europa Lander, IoVolcanic Observer, Titan Organic Explorer,and Neptune Orbiter. In order to achieve mis-sion objectives, development of small, effi-cient radioisotope systems may be required topower miniaturized sensors for in-situ mea-surements, penetrators to carry miniature geo-physics/chemistry laboratories, and an effi-cient thermal-to-electric conversion systemwith active cooling. These missions are justsome of the examples of applications of ra-dioisotope power systems that will enable abroader, more detailed investigation of themysteries of our solar system.



How AMTEC works (Source: DOE)

Artist’s conception of the Pluto-Kuiper Express. Pluto is at left and its moon Charon at top right.(Source: JPL/NASA)

rtgs for voyager

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