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NUCLEARENERGY
DOE/NE-0071
U.S. Department of Energy
U . S . D e p a r t m e n t o f E n e r g y
O f f i c e o f N u c l e a r E n e r g y ,
S c i e n c e a n d Te c h n o l o g y
UN
ITE
DSTATES OF AM
ER
ICA
TNEMTRAPE
D
OF ENERGY
NUCLEAR Power in Space
Printed on recycled paper
The Department of Energy produces publications to fulfill a statutory mandate to disseminate information to the public on all energy sources and energy conservation technologies. These materials are for public use and do not purport to present an exhaustive treatment of the subject matter. This is one in a series of publications on nuclear energy.
2 27
Scientists have an intense interest in the Saturniansystem—an interest beyond the planet’s vast andbeautiful system of rings. Saturn’s largest and mostintriguing moon, Titan, has a dense atmosphere thatresembles the Earth’s early atmosphere, and somescientists suggest there may be oceans on Titan.
Cassini consists of an orbiter and a probe. The probe,named Huygens after the Dutch scientist who discov-ered Titan in 1659, will descend by parachute to thesurface of Titan late in 2004. It will beam data to theorbiter during descent and after landing. Once there,it will examine the surface of Titan with threedozen sensors.
After releasing the Huygens probe for descent toTitan, the Cassini orbiter will explore the Saturniansystem for nearly four years, flying some 60 orbits ofthe giant planet during that time. The electricalenergy to power Cassini’s mission equipment, in-cluding all its communications and scientific sensors,will come from three RTG units that provide a total of850 watts of power.
One way the craft uses this energy is to control itsdata-gathering systems. For example, it can pointsensors to accuracies of a 10th of a degree, maintainstability levels over 10 times slower than the motionof a clock’s hour hand, navigate to accuracies of 30kilometers (about 20 miles), and broadcast data toEarth at rates as high as 140,000 bits per second.
Future RequirementsNASA has identified a number of potential missionsthat can best or only be undertaken using radioiso-tope power and/or heat sources. These futuremissions depend upon two important conditions.
On the cover:Launch of the Atlantis Space Shuttlecarrying Galileo into space (October 1989).
1
Nuclear Power in Space
Table of Contents
Introduction3Contemplating the Heavens4The Pioneer Missions7The Voyager Missions8The Ulysses Mission8The Galileo Mission11The Missions - An Overview16Power in Space - The Special Requirements18How Does an RTG Work?18Pu-238 - The Radioisotope of Choice for RTGs19RTGs - The Safety Factor21The Future of RTGs in Space25Conclusion28
U.S. Department of EnergyOffice of Nuclear Energy, Science, and TechnologyWashington, D.C. 20585
28
First, there must be a reliable and continuing supply ofPu-238 fuel from the U.S. Department of Energy. U.S.facilities that could supply Pu-238 are being consid-ered, as are foreign sources such as Russia, England,and France.
Second, smaller and more efficient power systemswill have to be developed consistent with NASA'sneeds.
ConclusionRTGs have made vital contributions to the U.S. spaceprogram from its earliest days to the present. Andthey are slated for service on future projects such asthe 1997 Cassini mission to Saturn, Mars Pathfinder,and Pluto Express.
For many missions on which RTGs were used, therewas no other viable option for providing power.These missions include Apollo, Viking, Pioneer, Voy-ager, Galileo, and Ulysses, which have provided sci-entists with critical information about the origins ofthe solar system.
Nuclear fuel has proven to be an ideal source of energyin space because of its high power, acceptable weightand volume, and excellent reliability and safety whenused in RTGs.
Because of its many advantages, it seems likely thatnuclear energy will continue to provide power onspace missions into the next century, whether in RTGs,other advanced generators, or nuclear reactors.
3
IntroductionIn the early years of the United States space program,lightweight batteries, fuel cells, and solar modulesprovided electric power for space missions. As mis-sions became more ambitious and complex, powerneeds increased and scientists investigated variousoptions to meet these challenging power require-ments. One of the options was nuclear energy.
By the mid-1950s, research had begun in earnest onways to use nuclearpower in space. Theseefforts resulted in thefirst radioisotopethermoelectric genera-tors (RTGs), which arenuclear power genera-tors built specifically forspace and special ter-restrial uses. TheseRTGs convert the heatgenerated from thenatural decay of theirradioactive fuel intoelectricity. The low-power devices weredesigned to supplement a craft’s primary non-nuclearpower source, but as the technology progressed,they soon began shouldering many missions’ entirepower needs.
Today, RTG-powered spacecraft are exploring theouter planets of the solar system and orbiting the sunand Earth. They have also landed on Mars and themoon. They provide the power that enables us to seeand learn about even the farthermost objects in oursolar system.
26
The Cassini Mission to SaturnThe Cassini mission is scheduled to begin in October1997 when a Titan IV launch vehicle lifts its payloadinto orbit. The mission is a joint U.S.-European ven-ture to explore Saturn in detail, a journey that will takenearly seven years. Like Galileo, Cassini will usegravity-assists from other planets to achieve the neces-sary speed to reach Saturn. Cassini will employ fourflybys: two by Venus, one by Earth, and one by Jupiter,to reach its destination in deep space.
✹
Saturn ArrivalJune 2004 Jupiter Flyby
Dec. 2000
LaunchOctober 1997
Earth FlybyAugust 1997
1st VenusFlybyApril 1998
2nd VenusFlybyJune 1999
Cassini Mission. Four flybys will enable the Cassini spacecraft toreach Saturn, its destination in deep space.
4 12 25
The Future of RTGsin Space
The TechnologyOther nuclear generator technologies for spaceapplications have been under investigation. Thesetechnologies involve more efficient conversion of heatinto electricity. Safety and reliability are key factors indetermining the possible value of each technology inspace missions.
One option is the dynamic isotope power systems(DIPS), which are much more efficient in convertingheat into electricity than the RTGs used on recentmissions. The dynamic systems have moving partsthat transform heat into mechanical energy, which isused to generate electricity. One such engine, theStirling engine, contains helium that expands by ab-sorbing heat on the hot side of the engine and rejectingit on the cold side. The rapidly changing pressurecycles cause a piston to move back and forth, drivingan alternator and producing electricity.
The range of technologies under investigation is wide.For instance, a process called Alkaline Metal Thermalto Electric Conversion (AMTEC) converts infraredradiation into electricity using liquid metal ions, whichare charged atoms. By contrast, the thermo-photovol-taic (TPV) converter changes infrared radiation emit-ted by a hot surface into electricity. Design goals forAMTEC and TPV technology call for even more effi-cient conversion of heat into electricity of about 20-30%, or a three-fold increase over RTGs.
The higher efficiencies of these new technologies meanthat future spacecraft may require less Pu-238 thanRTGs typically use. This makes these new spacepower technologies highly attractive due to lowerweight and less radioactive material for the samepower output.
Contemplatingthe Heavens
Many ancient peoples, including the Aztecs, Egyp-tians, and the builders of Stonehenge, were intenselyinterested in astronomy. Their writings and architec-ture indicate they studied the moon’s phases andmovement. They related the position and the per-ceived movement of the sun to Earth and its seasons.Some charted the stars, identifying the constellations.
Through the ages, scholars suggested various expla-nations for the makeup, movement, and relationshipto Earth of these heavenly bodies. Like us today, theywanted to know more about those distant objects, buta lack of technology limited their ability to learn.
Over the centuries, scientists like Galileo and Newtondescribed the structure of the solar system and themovement of the planets. Inventions such as thetelescope permitted them to see the moon’s craters, the“canals” on Mars, Saturn’s rings, and other intriguingdetails. This knowledge increased their curiosity aboutthe moon, sun, and planets, and they longed for moreinformation. They even dreamed of expeditions acrossspace to encounter them firsthand.
513
The Quest for Reliable PowerOn January 16, 1959, a dramatic photograph appeared in a Washington,D.C., newspaper. The headline proclaimed “President Shows AtomGenerator.” The photograph, above, pictured President Eisenhower anda group of U.S. Atomic Energy Commission (AEC) officials in the OvalOffice at the White House. They were gathered around the president’sdesk, staring at a strange grapefruit-shaped object. Dubbed the world’sfirst atomic battery, it was actually one of the earliest models of aradioisotope thermoelectric generator (RTG), a nuclear generator spe-cifically developed by the AEC to provide electric power during spacemissions.
➜
24
Blast - Blast waves greater than those predictedfrom a shuttle fuel explosion were simulated withexplosives; no fuel was released during the tests.
Re-entry - The modules survived the high tem-peratures of simulated atmospheric orbital decayentry, as tested in an arc-jet furnace; no fuel wasreleased.
Earth Impact - Impact at 120 miles per hour (ap-proximate top speed for an aeroshell falling toEarth) on sand, water, or soil produced no releaseof heat source module fuel. Impact on rock andconcrete sometimes produced releases, but muchof the fuel was retained by the surrounding graph-ite module, leaving only small amounts of low-level radioactive material to enter the localizedenvironment.
Immersion in Water - Long-term exposure to thecorrosive effects of seawater showed the iridiumcapsule is corrosion-resistant and the fuel itself ishighly insoluble.
Shrapnel - Researchers used aluminum and tita-nium bullets to simulate the small fragments thatmight be present in a launch vehicle explosion.Speeds of test fragments exceeded those predictedfor an actual explosion. Results indicated thatshrapnel would not cause a release of nuclear fuel.
Large Fragments - In tests representing solid rocketbooster failures, steel plates were fired at simulatedRTGs. Some unlikely events, such as impact withthe edges of steel plates, caused some release in afew fuel capsules. More likely events, such asimpacts with the faces of steel plates, did not pro-duce a release.
The safety tests demonstrated that the RTGs are ex-tremely rugged and capable of meeting the designobjective to prevent or minimize any fuel release.
The Century of FlightDuring the 20th century, fantasies about space flightprogressed toward reality on the wings of the manytypes of aircraft that became commonplace. Peoplefrom various countries contributed to the beginningsof spacecraft propulsion. One of the most importantwas an American, Robert Goddard, who studied bothsolid and liquid propellants. In 1916 he launched a testrocket that traveled 184 feet. Two decades later, Wernervon Braun and his team of German scientists weredeveloping powerful new rockets that could fly hun-dreds of miles.
In 1954 the U.S. began Project Orbiter, the nation’s firstmajor step in developing satellite systems. The U.S.soon became a world leader in this emerging technol-ogy, with the National Aeronautics and SpaceAdministration (NASA) playing a leading role.
GraphiteImpact Shell
EncasedFuel Pellet
Aeroshell
313/16"
21/8"
311/16"
GIS
6
On the moon, Apollo-12 astronaut Gordon Bean prepares toload the plutonium-238 heat source into the SNAP-2 thermo-electric generator (arrow). The generator produced 73 watts ofpower for the Apollo lunar surface experiment package fornearly eight years.
Today, the U.S. Space Shuttle Program is the nation’sprimary space transportation system.It has launchednumerous spacecraft and satellites on research andcommunications missions.
23
GPHS Module
Each of the RTG's 18 modules contains four Pu-238 fuel pellets enclosedin three layers of protection - - the metal encasing the pellets, the graphiteshell, and the aeroshell.
Galileo/Ulysses RTGThe first unmanned vehicle in space was Sputnik,launched by the Soviet Union in 1957. The U.S.followed three months later with its first unmannedflight, Explorer 1. The number of U.S. and Sovietspace missions increased dramatically in the next fewyears. And by the end of 1969, more than 1,000spacecraft were orbiting Earth. Five years later thenumber of these satellites had climbed to nearly 1,700.
Testing the PossibilitiesThe nuclear fuel in the GPHS faces a variety of pos-sible accidents during a space mission. Launch andre-entry pose many types of risks to the spacecraftand its components.
As a result, rigorous testing is conducted to ensure theRTG’s nuclear fuel will survive a launch accident orother mishap, remain intact, and contain the fuel. Thebattery of tests that the GPHS’s fuel modules haveundergone included the effects of:
Fire - Direct exposure to solid propellent fires, suchas the GPHS might encounter in a launchaccident, produced minimal damage and nonuclear fuel release.
7
In 1961, the first RTG used in a space mission waslaunched aboard a U.S. Navy transit navigation satel-lite. The electrical power output of this RTG, whichwas called Space Nuclear Auxiliary Power (SNAP-3),was a mere 2.7 watts. But the important story was thatit continued to perform for 15 years after launch.
Since that initial SNAP-3 mission, RTGs have been anindispensable part of America’s space program. Theyhave been involved in more than 25 missions, orbitingEarth and traveling to planets and their moons bothnearby and in deep space. (Astronauts on five Apollomissions left RTG units on the lunar surface to powerthe Apollo Lunar Surface Experiment Packages.)
In addition to contributing to numerous navigational,meteorological, and communications flights, RTGpower sources have provided all the electrical powerrequired on the Pioneer, Voyager, Galileo, and Ulyssesmissions. This series of unmanned flights has helpedus study the sun and outer planets of the solar system.RTGs should continue to play a pivotal role in majorspace missions in the next century.
The Pioneer MissionsThe Pioneer 10 and Pioneer 11 missions launched in1972 and 1973 were spectacular successes in the long-range strategy to explore the outer planets. Pioneer 10and its RTG survived the asteroid belt and the intenseradiation field around Jupiter and continued to per-form experiments perfectly. Pioneer 10 continued itsjourney and finally left the solar system—the firstmanmade object ever to do so. Pioneer 11 flew byJupiter, going through the peak radiation zone in evenbetter shape than the earlier Pioneer 10. It also visitedSaturn—our first close encounter with the ringedplanet. Both Pioneer spacecraft continued to sendback data to Earth after 17 years, even as they passedbeyond the solar system, because the RTGs providecontinuous power.
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General Purpose Heat SourceThe general purpose heat source (GPHS) is the radio-active fuel package being used in RTGs of current andplanned space missions. GPHSs use Pu-238 fuel justas earlier-model RTGs did, but the unique fuel con-tainment is designed to maximize safety in an acci-dent. The basic heat source unit is the GPHS module.Each GPHS/RTG contains 18 modules.
Design features include:
• Fuel pellets made of hard, ceramic
plutonium-238 oxide that do not dissolve in water. The pellets are also
highly resistant to vaporizing or fracturing into breathable particles
following impact on hard surfaces. Each of the GPHS’s 18 modules
contains four fuel pellets, for a total of 72 pellets per GPHS RTG.
• Iridium, a very stable metal with elastic properties, encapsulates each
fuel pellet. These capsules (about the size and shape of a marshmallow)
would tend to stretch or flatten instead of ripping open if the GPHS
module struck the ground at high speed. This would help keep the
capsules intact and contain the fuel.
• A high-strength graphite cylinder called a graphite impact shell that
holds a pair of fuel pellets. The graphite impact shell is designed to limit
damage to the iridium fuel capsules from free-fall or explosion frag-
ments.
• An “aeroshell,” which encloses a pair of graphite impact shells. It serves
as a shield designed to withstand the heat of
re-entering Earth’s atmosphere in case of an accident.
• Fuel pellets made of hard, ceramic plutonium-238oxide that do not dissolve in water. The pellets arealso highly resistant to vaporizing or fracturing intobreakable particles following impact on hard sur-faces. Each of the GPHS's 18 modules contains fourfuel pellets, for a total of 72 pellets per GPHS RTG.
• Iridium, a very stable metal with elastic propertiesthat encapsulates each fuel pellet. These capsules(about the size and shape of a marshmallow) wouldtend to stretch or flatten instead of ripping open ifthe GPHS module struck the ground at high speed.This would help keep the capsules intact and con-tain the fuel.
• A high-strength graphite cylinder called a graphiteimpact shell that holds a pair of fuel pellets. Thegraphite impact shell is designed to limit damage tothe iridium fuel capsules from free-fall or explosionfragments.
• An "aeroshell," which encloses a pair of graphiteimpact shells. It serves as a shield designed to withstand the heat of re-entering Earth's atmosphere incase of an accident.
8 12
The Ulysses MissionThe Ulysses mission is a joint enterprise of the Euro-pean Space Agency and NASA, with the Jet Propul-sion Laboratory in California providing major sup-port. The craft was launched in October 1990 aboardthe space shuttle Discovery. Its mission is to study thesun, the magnetic fields and streams of particles thesun generates, and the interstellar space below andabove it.
Artist's conception of the Voyager spacecraft as it passed near Saturn.Three multihundred-watt RTGs, shown in the foreground, providedpower for the spacecraft.
21
Smaller and Lighter PowerThe weight and volume of solar panels can causeproblems on some space missions. With RTGs, weightand volume are far less of a concern. Pu-238 has arelatively high power density, and a given volume orweight of Pu-238 can produce a relatively high num-ber of watts of power for long periods of time. Thesequalities lead to smaller and lighter heat sources thancomparable power levels from other sources. Thismakes Pu-238 fuel an efficient power producer for thespace it occupies and the weight it adds to a mission.
The Voyager MissionsVoyager 1 and Voyager 2 were launched within a fewweeks of each other in 1977. Though they useddifferent routes and trajectories (curved paths of flight),both spacecraft visited Jupiter and Saturn. They tookspectacular photographs of Saturn’s rings andobserved 11 of its moons close up. Voyager 2 thencontinued to Uranus and Neptune. The RTGs pow-ered versatile and complex instruments, includingcomputers and communications equipment that madeit possible for the Voyagers to transmit 115,200 bits ofdata per second from Jupiter.
RTGs—The Safety Factor
Any penetrating radiation that escapes a radioiso-tope heat source is of potential concern. RTG safetyefforts revolve around containing the radioactive fuelin case of accident during a critical time in the mis-sion, such as launch or re-entry. Multiple layers ofspecial material enclose the plutonium-238 fuel tocontain it under both normal and accident conditions.Extensive testing and analysis are used to demon-strate that safety design criteria are met. Today wedesign to have the fuel capsule remain intact evenafter Earth re-entry and impact. The Apollo 13accident and subsequent re-entry of the RTG, withoutany material release, provided further proof of thissafety principle.
✹
Orbit ofJupiter
End of mission(late 1995)Ecliptic
crossing(early 1995)
Orbit ofEarth
Jupiterencounter
(Early 1992)
Southtrajectory
First polar pass(mid 1994)
Launch (October 1990)
100
50
01 10 87
5 -YearDesign
Life
% o
f Orig
inal
Pow
er
Years
9920
Safety IssuesBecause the nuclear fuel in RTGs is radioactive, safetyis a critical issue. As it decays, Pu-238 emits radiationmainly in the form of alpha particles, which have avery low penetrating power. Only lightweight shield-ing is necessary because alpha particles cannot pen-etrate a sheet of paper. Radioisotopes producingmore penetrating radiation, such as beta or gammaparticles, would be more difficult to handle safely andwould require heavier shielding, a distinct drawbackon space missions.
Polonium-210, which was used in the early SNAP-3RTG, has a half-life of 138.4 days. At the end of thattime, the amount of radioactive material remaining ishalf of the original amount. This means there isonly half the heat available for conversion intoelectric energy.
Longer space missions require a radioisotope with alonger half-life. Pu-238, with its half-life of 87.7 years,fills the need. For example, after five years, approxi-mately 96 percent of the original heat output of Pu-238is still available.
The 87-year half-life of Pu-238 results in 96% of the original heatoutput even after five years
Second polar pass(late 1995)
Ulysses Mission Trajectory. The mission, which was extended beyond theoriginal five-year period, has provided original data on thesun's poles.
Previous craft launched to study the sun always or-bited the sun’s equator—the plane in which Earthorbits the sun. Ulysses is the first craft to orbit the sun’spoles, giving scientists an entirely new perspectiveand important new data.
They achieved this new positioning by heading thecraft toward Jupiter after launch. The mission calledfor Jupiter’s immense gravity to push the spacecraftout of its ecliptic plane into a polar orbit of the sun. Thisnovel orbit produced original data from areas of thesun not previously studied. It also created a morecomplete understanding of the sun and its effectson Earth.
In June 1994, mission scientists received the first scien-tific data on the south pole of the sun. In June 1995, theyreceived data on the north pole as well. Because theRTGs have a long life, mission planners extended themission to a second orbit cycle, vastly increasingmission data.
10 10 19
This heat-to-electricity conversion occurs through thethermoelectric principle discovered early in the lastcentury. This principle is a way of producing electriccurrent without using a device that has moving parts.It involves two plates, each made of a different metalthat conducts electricity. Joining these two plates toform a closed electrical circuit and keeping the twojunctions at different temperatures produces anelectric current. These pairs of junctions are calledthermocouples. In an RTG, the radioisotopic fuel heatsone of these junctions while the other junction remainsunheated and is cooled by space.
RTGs are reliable because they produce electricitywithout moving parts that can fail or wear out. Thishigh degree of reliability is especially important inspace applications, where the investment is great, andrepair or replacement of equipment is not feasible.
Pu-238—TheRadioisotope of Choice
for RTGsAlthough other radioactive fuels have been consid-ered for RTGs, plutonium-238 (Pu-238) has been usedmost widely. Pu-238 is a radioactive isotope—a formof plutonium that gives off energy as rays and par-ticles. It continues to be the radioactive fuel of choicetoday and in planned future missions.
What qualities make Pu-238 a good choice for fuel in anRTG? Its half-life is one of the most important. Half-life is the time it takes for half of the radioactivematerial to decay.
The spacecraft Ulysses was named for theGreek warrior king who went on a strange,exotic 10-year journey returning from the Trojan War.
A single RTG provides all the power for instrumentsand other equipment aboard Ulysses. It is the onlyavailable power source capable of meeting the mission’spower requirements. Furthermore, the RTG will pro-vide power for many years, enabling mission scien-tists and program managers to extend the life of thespacecraft by several years and reap more scientificbenefits. But since Ulysses orbits the sun, why not usesolar energy instead of an RTG to power it?
Why Ulysses Doesn’t UseSolar EnergyThe closest approach to the sun by the spacecraftoccurred when it was launched from Earth. As Ulyssestraveled to Jupiter, the sun grew more distant. NearJupiter, the sun’s rays are 25 times weaker than nearEarth. A solar panel system large enough to catch this
weak energy would add1,200 pounds, nearly dou-bling the weight of thespacecraft. In comparison,the RTG weighs 124pounds. No rocket boosterexisted that could havesent Ulysses on its missionwith the added weight ofsolar panels. Chemicalpower systems and batter-ies are useful for short mis-sions, but on longer mis-sions, such as the oneUlysses is on, they wouldalso add too much weight.
1111
The Galileo MissionGalileo lifted into space in October 1989 aboard thespace shuttle Atlantis. Its mission involves a sched-uled eight-year, deep-space voyage to the solar system'slargest planet, Jupiter, and its four major moons. Thespacecraft will go into orbit around Jupiter andconduct detailed inves-tigation of this system.Scientists are intenselyinterested in Jupiter. Un-like Earth and otherplanets, Jupiter has keptmuch of its originalcomposition and canenhance our under-standing of the solarsystem’s origins.
Throughout the mission,Galileo’s instrumentshave examined the ce-lestial bodies it haspassed, and camerashave relayed photosback to an internationalteam of scientists onEarth. Infrared systemswill map Jupiter and itslargest moon, andspecialized instrumentswill study the planetaryatmosphere.
Galileo provided the first close-up photographs of anasteroid, Gaspra, in 1993. It also provided the onlydirect photos of the impacts on Jupiter of the cometShoemaker-Levy 9 in July 1994.
Galileo used a technique called gravity-assist to makethe journey to Jupiter, which is nearly 500 million milesfrom Earth.
18
Power in Space—TheSpecial Requirements
Scientific instruments and electronic, photographic,and communication equipment are the heart of ex-ploratory missions because they collect the data andtransmit it back to Earth. Without the technology toreliably power these instruments in space, our knowl-edge of the solar system would be only a fraction ofwhat it is today. RTG technology was developed toprovide that electric power.
Requirements for power in space are highly special-ized. The weight and volume of hardware launchedinto space are carefully considered, including powersources. The generator must meet mission powerrequirements, as well as weight and space limitations.Safety is also a prime consideration because of haz-ards associated with launch, re-entry, and other mis-sion activities.
How Does an RTG Work?An RTG has no moving parts. It produces electricenergy through the interaction of its two maincomponents: the radioactive heat source (fuel andcontainment) and the thermoelectric generator.
Radioactive material can be used for fuel because it isunstable and decays, or spontaneously disintegratesinto a different atomic form. As the material naturallydecays, it produces heat. The other main componentof the RTG, the thermoelectric generator, converts thisheat into electricity.
The Galileo spacecraft was namedafter the 17th-century scientist whodiscovered Jupiter's moons.
Dec. 1991 Jupiter arrivallate 1995
Venus FlybyFeb. 1990
Earth FlybyDec. 1992
Earth FlybyDec. 1990
LaunchOct. 1989
SUN
12 17
Galileo's Trajectory to Jupiter
The Galileo mission is built around two distinct piecesof spacecraft, an orbiter and the separate atmosphericprobe named Huygens. About 150 days before arriv-ing at Jupiter, the probe separated from the orbiter andentered the planet's atmosphere. For a short 60 to 75minutes, its instruments took atmospheric measure-ments and relayed the data to the orbiter for transmis-sion to Earth. The probe descended farther and fartheruntil the increasing temperature and pressure ofJupiter’s atmosphere crushed and eventuallyvaporized it.
- An OverviewAvailable launch vehicles were not powerful enoughto place Galileo on a direct path to Jupiter. Instead,Galileo used the gravity of other planets to arrive at itsdestination. The craft flew by Venus first, and thenmade two passes by Earth. As Galileo flew towardEarth and Venus, the planets exerted gravitationalpull on it, finally increasing its velocity enough for itto complete the journey to Jupiter. However, thesemaneuvers added significantly to the mission’s mile-age and duration. By the time Galileo made the twopasses by Earth, the mission was three years old.
Status
RTG operated for 15 years. Satellite now shut down but operational.
RTG operated for 9 years. Satellite operated periodically after1962 high altitude test. Last reported signal in 1971.
RTG operated as planned. Non-RTG electrical problems on satellitecaused satellite to fall after 9 months.
RTG operated for over 6 years. Satellite lost ability to navigateafter 1.5 years.
Mission was aborted because of launch vehicle failure. RTG burned up on re-entry as designed.
Mission was aborted because of range safety destruct. RTG heat sourcesrecovered and recycled.
RTGs operated for over 2.5 years.
Radioisotope heater units for seismic experimental package. Station was shutdown August 3, 1969.
RTG operated for about 8 years until station was shut down.
Mission aborted on the way to the moon. RTG re-entered earth's atmosphereand landed in South Pacific Ocean. No radiation was released.
RTG operated for over 6.5 years until station was shut down.
RTG operated for over 6 years until station was shut down.
RTGs still operating. Spacecraft successfully operated to Jupiter and is nowbeyond orbit of Pluto.
RTG operated for about 5.5 years until station was shut down.
RTG still operating.
RTG operated for almost 5 years until station was shut down.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, andbeyond.
RTGs operated for over 6 years until lander was shut down.
RTGs operated for over 4 years until relay link was lost.
RTGs still operating.
RTGs still operating.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, Uranus,Neptune, and beyond.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, andbeyond.
RTGs still operating. Spacecraft en route to Jupiter.
RTG still operating. Spacecraft en route to solar polar flyby.
13916
The orbiter is scheduled to make 10 orbits of Jupiterover a 22-month period. It will study the planet, itsmagnetic atmosphere, and the four major Joviansatellites, or moons — Io, Europa, Ganymede,and Callisto.
The Galileo mission involves many challenges, notleast of which is meeting the craft’s instrument,communications, and other power requirements.Without a reliable source of power, the mission’s veryreason for existing—to provide valuable newinformation about our solar system—would bejeopardized.
(c)
(a)(b)
The brick-like ceramic form of the fuel pellets (a) is designed to break intolarge pieces, instead of dust, on impact. This makes it difficult to inhale andminimizes its exposure to the environment. The cladding (b) andinsulator (c) protect the fuel pellet from the extreme heat of re-entry andfrom the environment in case of accident.
Radioisotope Heater Unit
• Heat Output - 1 watt• Weight - 1.4 ounce • Size - 1 inch x 1.3 inch
The MissionsLaunch Date
June 29, 1961
Nov. 15, 1961
Sept. 28, 1963
Dec. 5, 1963
April 21, 1964
May 18, 1968
April 14, 1969
July 14, 1969
Nov. 14, 1969
April 11, 1970
Jan. 31, 1971
July 26, 1971
March 2, 1972
April 16, 1972
Sept. 2, 1972
Dec. 7, 1972
April 5, 1973
Aug. 20, 1975
Sept. 9, 1975
March 14, 1976
March 14, 1976
Aug. 20, 1977
Sept. 5, 1977
Oct. 18, 1989
Oct. 6, 1990
Mission Type
Navigational
Navigational
Navigational
Navigational
Navigational
Meteorological
Meteorological
Lunar surface
Lunar surface
Lunar surface
Lunar surface
Lunar surface
Planetary
Lunar surface
Navigational
Lunar surface
Planetary
Mars surface
Mars surface
Communications
Communications
Planetary
Planetary
Planetary
Planetary/Solar
Spacecraft
Transit 4A
Transit 4B
Transit 5-BN-1
Transit 5-BN-2
Transit 5-BN-3
Nimbus-B-1
Nimbus III
Apollo 11
Apollo 12
Apollo 13
Apollo 14
Apollo 15
Pioneer 10
Apollo 16
“Transit” (Triad-01-1X)
Apollo 17
Pioneer 11
Viking 1
Viking 2
LES 8*
LES 9*
Voyager 2
Voyager 1
Galileo
Ulysses
Power Source
SNAP-3B7
SNAP-3B8
SNAP-9A
SNAP-9A
SNAP-9A
SNAP 19B2
SNAP 19B3
ALRH
SNAP-27
SNAP-27
SNAP-27
SNAP-27
SNAP-19
SNAP-27
Transit-RTG
SNAP-27
SNAP-19
SNAP-19
SNAP-19
MHW-RTG
MHW-RTG
MHW-RTG
MHW-RTG
GPHS-RTG
GPHS-RTG
The Galileo orbiter’s two RTGs provide all the electrical energy the spacecraft requires. The heater units are small, barrel-shaped devices that contain a plutonium-238 dioxide ceramic pellet much like the fuel pellets used in RTGs. The heaterunits are designed and tested to contain their radioac tive fuel during all normal and accident condi-
tions. Eighty-four of these units are placed at various locations on the orbiter to heat its instruments, and 36 are located on the atmospheric probe. Each heater unit produces about 1 watt of
heat—about as much as a miniature Christmas tree bulb. But, it is enough to protect the instruments from the cold.
The Pioneer, Voyager, Ulysses, and Galileo missionshave produced a huge amount of information aboutthe history and makeup of the solar system. Nonecould have been accomplished without RTGs, whichhave played a key role in helping the U.S. establish itsposition as the world leader in outer planetary andspace science exploration.
RTG technology has evolved steadily and dramaticallysince the groundbreaking, low-power units of the 1950s.The first RTGs, which produced only a few watts, haveevolved into today’s generation of RTGs, whichproduce power in the hundreds of watts.
14 15
1 RTG
1 RTG
6 RHUs
5 RHUs
This drawing of the Galileo spacecraft shows locations and quantitiesof radioisotope heater units (RHUs), which provide heat, and RTGs,which provide power. There are120 RHUs and 2 RTGs.
1 RHU
4 RHUs
5 RHUs
12 RHUs
1 RHU30 RHUs 36 RHUs
Electricity and HeatThe effectiveness of Galileo’s instruments dependsnot only on RTG power, but also on heat from radio-isotope heater units (RHU). Because the journey is farfrom the sun, these compact, light, and long-lastingRTG and RHU units are the only effective power andheat sources for the Galileo mission.
Two RTGs provide electrical power to drive the Galileospacecraft and its instruments. Each RTG producesabout 285 watts of electricity at the beginning of themission. One hundred and twenty small RHUs pro-tect the craft’s sensitive instruments from damage inthe cold vacuum of outer space, which can reach-400 degrees Fahrenheit.
16 RHUs
4 RHUs
The Galileo orbiter’s two RTGs provide all the electrical energy the spacecraft requires. The heater units are small, barrel-shaped devices that contain a plutonium-238 dioxide ceramic pellet much like the fuel pellets used in RTGs. The heaterunits are designed and tested to contain their radioac tive fuel during all normal and accident condi-
tions. Eighty-four of these units are placed at various locations on the orbiter to heat its instruments, and 36 are located on the atmospheric probe. Each heater unit produces about 1 watt of
heat—about as much as a miniature Christmas tree bulb. But, it is enough to protect the instruments from the cold.
The Pioneer, Voyager, Ulysses, and Galileo missionshave produced a huge amount of information aboutthe history and makeup of the solar system. Nonecould have been accomplished without RTGs, whichhave played a key role in helping the U.S. establish itsposition as the world leader in outer planetary andspace science exploration.
RTG technology has evolved steadily and dramaticallysince the groundbreaking, low-power units of the 1950s.The first RTGs, which produced only a few watts, haveevolved into today’s generation of RTGs, whichproduce power in the hundreds of watts.
14 15
1 RTG
1 RTG
6 RHUs
5 RHUs
This drawing of the Galileo spacecraft shows locations and quantitiesof radioisotope heater units (RHUs), which provide heat, and RTGs,which provide power. There are120 RHUs and 2 RTGs.
1 RHU
4 RHUs
5 RHUs
12 RHUs
1 RHU30 RHUs 36 RHUs
Electricity and HeatThe effectiveness of Galileo’s instruments dependsnot only on RTG power, but also on heat from radio-isotope heater units (RHU). Because the journey is farfrom the sun, these compact, light, and long-lastingRTG and RHU units are the only effective power andheat sources for the Galileo mission.
Two RTGs provide electrical power to drive the Galileospacecraft and its instruments. Each RTG producesabout 285 watts of electricity at the beginning of themission. One hundred and twenty small RHUs pro-tect the craft’s sensitive instruments from damage inthe cold vacuum of outer space, which can reach-400 degrees Fahrenheit.
16 RHUs
4 RHUs
13916
The orbiter is scheduled to make 10 orbits of Jupiterover a 22-month period. It will study the planet, itsmagnetic atmosphere, and the four major Joviansatellites, or moons — Io, Europa, Ganymede,and Callisto.
The Galileo mission involves many challenges, notleast of which is meeting the craft’s instrument,communications, and other power requirements.Without a reliable source of power, the mission’s veryreason for existing—to provide valuable newinformation about our solar system—would bejeopardized.
(c)
(a)(b)
The brick-like ceramic form of the fuel pellets (a) is designed to break intolarge pieces, instead of dust, on impact. This makes it difficult to inhale andminimizes its exposure to the environment. The cladding (b) andinsulator (c) protect the fuel pellet from the extreme heat of re-entry andfrom the environment in case of accident.
Radioisotope Heater Unit
• Heat Output - 1 watt• Weight - 1.4 ounce • Size - 1 inch x 1.3 inch
The MissionsLaunch Date
June 29, 1961
Nov. 15, 1961
Sept. 28, 1963
Dec. 5, 1963
April 21, 1964
May 18, 1968
April 14, 1969
July 14, 1969
Nov. 14, 1969
April 11, 1970
Jan. 31, 1971
July 26, 1971
March 2, 1972
April 16, 1972
Sept. 2, 1972
Dec. 7, 1972
April 5, 1973
Aug. 20, 1975
Sept. 9, 1975
March 14, 1976
March 14, 1976
Aug. 20, 1977
Sept. 5, 1977
Oct. 18, 1989
Oct. 6, 1990
Mission Type
Navigational
Navigational
Navigational
Navigational
Navigational
Meteorological
Meteorological
Lunar surface
Lunar surface
Lunar surface
Lunar surface
Lunar surface
Planetary
Lunar surface
Navigational
Lunar surface
Planetary
Mars surface
Mars surface
Communications
Communications
Planetary
Planetary
Planetary
Planetary/Solar
Spacecraft
Transit 4A
Transit 4B
Transit 5-BN-1
Transit 5-BN-2
Transit 5-BN-3
Nimbus-B-1
Nimbus III
Apollo 11
Apollo 12
Apollo 13
Apollo 14
Apollo 15
Pioneer 10
Apollo 16
“Transit” (Triad-01-1X)
Apollo 17
Pioneer 11
Viking 1
Viking 2
LES 8*
LES 9*
Voyager 2
Voyager 1
Galileo
Ulysses
Power Source
SNAP-3B7
SNAP-3B8
SNAP-9A
SNAP-9A
SNAP-9A
SNAP 19B2
SNAP 19B3
ALRH
SNAP-27
SNAP-27
SNAP-27
SNAP-27
SNAP-19
SNAP-27
Transit-RTG
SNAP-27
SNAP-19
SNAP-19
SNAP-19
MHW-RTG
MHW-RTG
MHW-RTG
MHW-RTG
GPHS-RTG
GPHS-RTG
Dec. 1991 Jupiter arrivallate 1995
Venus FlybyFeb. 1990
Earth FlybyDec. 1992
Earth FlybyDec. 1990
LaunchOct. 1989
SUN
12 17
Galileo's Trajectory to Jupiter
The Galileo mission is built around two distinct piecesof spacecraft, an orbiter and the separate atmosphericprobe named Huygens. About 150 days before arriv-ing at Jupiter, the probe separated from the orbiter andentered the planet's atmosphere. For a short 60 to 75minutes, its instruments took atmospheric measure-ments and relayed the data to the orbiter for transmis-sion to Earth. The probe descended farther and fartheruntil the increasing temperature and pressure ofJupiter’s atmosphere crushed and eventuallyvaporized it.
- An OverviewAvailable launch vehicles were not powerful enoughto place Galileo on a direct path to Jupiter. Instead,Galileo used the gravity of other planets to arrive at itsdestination. The craft flew by Venus first, and thenmade two passes by Earth. As Galileo flew towardEarth and Venus, the planets exerted gravitationalpull on it, finally increasing its velocity enough for itto complete the journey to Jupiter. However, thesemaneuvers added significantly to the mission’s mile-age and duration. By the time Galileo made the twopasses by Earth, the mission was three years old.
Status
RTG operated for 15 years. Satellite now shut down but operational.
RTG operated for 9 years. Satellite operated periodically after1962 high altitude test. Last reported signal in 1971.
RTG operated as planned. Non-RTG electrical problems on satellitecaused satellite to fall after 9 months.
RTG operated for over 6 years. Satellite lost ability to navigateafter 1.5 years.
Mission was aborted because of launch vehicle failure. RTG burned up on re-entry as designed.
Mission was aborted because of range safety destruct. RTG heat sourcesrecovered and recycled.
RTGs operated for over 2.5 years.
Radioisotope heater units for seismic experimental package. Station was shutdown August 3, 1969.
RTG operated for about 8 years until station was shut down.
Mission aborted on the way to the moon. RTG re-entered earth's atmosphereand landed in South Pacific Ocean. No radiation was released.
RTG operated for over 6.5 years until station was shut down.
RTG operated for over 6 years until station was shut down.
RTGs still operating. Spacecraft successfully operated to Jupiter and is nowbeyond orbit of Pluto.
RTG operated for about 5.5 years until station was shut down.
RTG still operating.
RTG operated for almost 5 years until station was shut down.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, andbeyond.
RTGs operated for over 6 years until lander was shut down.
RTGs operated for over 4 years until relay link was lost.
RTGs still operating.
RTGs still operating.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, Uranus,Neptune, and beyond.
RTGs still operating. Spacecraft successfully operated to Jupiter, Saturn, andbeyond.
RTGs still operating. Spacecraft en route to Jupiter.
RTG still operating. Spacecraft en route to solar polar flyby.
1111
The Galileo MissionGalileo lifted into space in October 1989 aboard thespace shuttle Atlantis. Its mission involves a sched-uled eight-year, deep-space voyage to the solar system'slargest planet, Jupiter, and its four major moons. Thespacecraft will go into orbit around Jupiter andconduct detailed inves-tigation of this system.Scientists are intenselyinterested in Jupiter. Un-like Earth and otherplanets, Jupiter has keptmuch of its originalcomposition and canenhance our under-standing of the solarsystem’s origins.
Throughout the mission,Galileo’s instrumentshave examined the ce-lestial bodies it haspassed, and camerashave relayed photosback to an internationalteam of scientists onEarth. Infrared systemswill map Jupiter and itslargest moon, andspecialized instrumentswill study the planetaryatmosphere.
Galileo provided the first close-up photographs of anasteroid, Gaspra, in 1993. It also provided the onlydirect photos of the impacts on Jupiter of the cometShoemaker-Levy 9 in July 1994.
Galileo used a technique called gravity-assist to makethe journey to Jupiter, which is nearly 500 million milesfrom Earth.
18
Power in Space—TheSpecial Requirements
Scientific instruments and electronic, photographic,and communication equipment are the heart of ex-ploratory missions because they collect the data andtransmit it back to Earth. Without the technology toreliably power these instruments in space, our knowl-edge of the solar system would be only a fraction ofwhat it is today. RTG technology was developed toprovide that electric power.
Requirements for power in space are highly special-ized. The weight and volume of hardware launchedinto space are carefully considered, including powersources. The generator must meet mission powerrequirements, as well as weight and space limitations.Safety is also a prime consideration because of haz-ards associated with launch, re-entry, and other mis-sion activities.
How Does an RTG Work?An RTG has no moving parts. It produces electricenergy through the interaction of its two maincomponents: the radioactive heat source (fuel andcontainment) and the thermoelectric generator.
Radioactive material can be used for fuel because it isunstable and decays, or spontaneously disintegratesinto a different atomic form. As the material naturallydecays, it produces heat. The other main componentof the RTG, the thermoelectric generator, converts thisheat into electricity.
The Galileo spacecraft was namedafter the 17th-century scientist whodiscovered Jupiter's moons.
10 10 19
This heat-to-electricity conversion occurs through thethermoelectric principle discovered early in the lastcentury. This principle is a way of producing electriccurrent without using a device that has moving parts.It involves two plates, each made of a different metalthat conducts electricity. Joining these two plates toform a closed electrical circuit and keeping the twojunctions at different temperatures produces anelectric current. These pairs of junctions are calledthermocouples. In an RTG, the radioisotopic fuel heatsone of these junctions while the other junction remainsunheated and is cooled by space.
RTGs are reliable because they produce electricitywithout moving parts that can fail or wear out. Thishigh degree of reliability is especially important inspace applications, where the investment is great, andrepair or replacement of equipment is not feasible.
Pu-238—TheRadioisotope of Choice
for RTGsAlthough other radioactive fuels have been consid-ered for RTGs, plutonium-238 (Pu-238) has been usedmost widely. Pu-238 is a radioactive isotope—a formof plutonium that gives off energy as rays and par-ticles. It continues to be the radioactive fuel of choicetoday and in planned future missions.
What qualities make Pu-238 a good choice for fuel in anRTG? Its half-life is one of the most important. Half-life is the time it takes for half of the radioactivematerial to decay.
The spacecraft Ulysses was named for theGreek warrior king who went on a strange,exotic 10-year journey returning from the Trojan War.
A single RTG provides all the power for instrumentsand other equipment aboard Ulysses. It is the onlyavailable power source capable of meeting the mission’spower requirements. Furthermore, the RTG will pro-vide power for many years, enabling mission scien-tists and program managers to extend the life of thespacecraft by several years and reap more scientificbenefits. But since Ulysses orbits the sun, why not usesolar energy instead of an RTG to power it?
Why Ulysses Doesn’t UseSolar EnergyThe closest approach to the sun by the spacecraftoccurred when it was launched from Earth. As Ulyssestraveled to Jupiter, the sun grew more distant. NearJupiter, the sun’s rays are 25 times weaker than nearEarth. A solar panel system large enough to catch this
weak energy would add1,200 pounds, nearly dou-bling the weight of thespacecraft. In comparison,the RTG weighs 124pounds. No rocket boosterexisted that could havesent Ulysses on its missionwith the added weight ofsolar panels. Chemicalpower systems and batter-ies are useful for short mis-sions, but on longer mis-sions, such as the oneUlysses is on, they wouldalso add too much weight.
✹
Orbit ofJupiter
End of mission(late 1995)Ecliptic
crossing(early 1995)
Orbit ofEarth
Jupiterencounter
(Early 1992)
Southtrajectory
First polar pass(mid 1994)
Launch (October 1990)
100
50
01 10 87
5 -YearDesign
Life
% o
f Orig
inal
Pow
er
Years
9920
Safety IssuesBecause the nuclear fuel in RTGs is radioactive, safetyis a critical issue. As it decays, Pu-238 emits radiationmainly in the form of alpha particles, which have avery low penetrating power. Only lightweight shield-ing is necessary because alpha particles cannot pen-etrate a sheet of paper. Radioisotopes producingmore penetrating radiation, such as beta or gammaparticles, would be more difficult to handle safely andwould require heavier shielding, a distinct drawbackon space missions.
Polonium-210, which was used in the early SNAP-3RTG, has a half-life of 138.4 days. At the end of thattime, the amount of radioactive material remaining ishalf of the original amount. This means there isonly half the heat available for conversion intoelectric energy.
Longer space missions require a radioisotope with alonger half-life. Pu-238, with its half-life of 87.7 years,fills the need. For example, after five years, approxi-mately 96 percent of the original heat output of Pu-238is still available.
The 87-year half-life of Pu-238 results in 96% of the original heatoutput even after five years
Second polar pass(late 1995)
Ulysses Mission Trajectory. The mission, which was extended beyond theoriginal five-year period, has provided original data on thesun's poles.
Previous craft launched to study the sun always or-bited the sun’s equator—the plane in which Earthorbits the sun. Ulysses is the first craft to orbit the sun’spoles, giving scientists an entirely new perspectiveand important new data.
They achieved this new positioning by heading thecraft toward Jupiter after launch. The mission calledfor Jupiter’s immense gravity to push the spacecraftout of its ecliptic plane into a polar orbit of the sun. Thisnovel orbit produced original data from areas of thesun not previously studied. It also created a morecomplete understanding of the sun and its effectson Earth.
In June 1994, mission scientists received the first scien-tific data on the south pole of the sun. In June 1995, theyreceived data on the north pole as well. Because theRTGs have a long life, mission planners extended themission to a second orbit cycle, vastly increasingmission data.
8 12
The Ulysses MissionThe Ulysses mission is a joint enterprise of the Euro-pean Space Agency and NASA, with the Jet Propul-sion Laboratory in California providing major sup-port. The craft was launched in October 1990 aboardthe space shuttle Discovery. Its mission is to study thesun, the magnetic fields and streams of particles thesun generates, and the interstellar space below andabove it.
Artist's conception of the Voyager spacecraft as it passed near Saturn.Three multihundred-watt RTGs, shown in the foreground, providedpower for the spacecraft.
21
Smaller and Lighter PowerThe weight and volume of solar panels can causeproblems on some space missions. With RTGs, weightand volume are far less of a concern. Pu-238 has arelatively high power density, and a given volume orweight of Pu-238 can produce a relatively high num-ber of watts of power for long periods of time. Thesequalities lead to smaller and lighter heat sources thancomparable power levels from other sources. Thismakes Pu-238 fuel an efficient power producer for thespace it occupies and the weight it adds to a mission.
The Voyager MissionsVoyager 1 and Voyager 2 were launched within a fewweeks of each other in 1977. Though they useddifferent routes and trajectories (curved paths of flight),both spacecraft visited Jupiter and Saturn. They tookspectacular photographs of Saturn’s rings andobserved 11 of its moons close up. Voyager 2 thencontinued to Uranus and Neptune. The RTGs pow-ered versatile and complex instruments, includingcomputers and communications equipment that madeit possible for the Voyagers to transmit 115,200 bits ofdata per second from Jupiter.
RTGs—The Safety Factor
Any penetrating radiation that escapes a radioiso-tope heat source is of potential concern. RTG safetyefforts revolve around containing the radioactive fuelin case of accident during a critical time in the mis-sion, such as launch or re-entry. Multiple layers ofspecial material enclose the plutonium-238 fuel tocontain it under both normal and accident conditions.Extensive testing and analysis are used to demon-strate that safety design criteria are met. Today wedesign to have the fuel capsule remain intact evenafter Earth re-entry and impact. The Apollo 13accident and subsequent re-entry of the RTG, withoutany material release, provided further proof of thissafety principle.
7
In 1961, the first RTG used in a space mission waslaunched aboard a U.S. Navy transit navigation satel-lite. The electrical power output of this RTG, whichwas called Space Nuclear Auxiliary Power (SNAP-3),was a mere 2.7 watts. But the important story was thatit continued to perform for 15 years after launch.
Since that initial SNAP-3 mission, RTGs have been anindispensable part of America’s space program. Theyhave been involved in more than 25 missions, orbitingEarth and traveling to planets and their moons bothnearby and in deep space. (Astronauts on five Apollomissions left RTG units on the lunar surface to powerthe Apollo Lunar Surface Experiment Packages.)
In addition to contributing to numerous navigational,meteorological, and communications flights, RTGpower sources have provided all the electrical powerrequired on the Pioneer, Voyager, Galileo, and Ulyssesmissions. This series of unmanned flights has helpedus study the sun and outer planets of the solar system.RTGs should continue to play a pivotal role in majorspace missions in the next century.
The Pioneer MissionsThe Pioneer 10 and Pioneer 11 missions launched in1972 and 1973 were spectacular successes in the long-range strategy to explore the outer planets. Pioneer 10and its RTG survived the asteroid belt and the intenseradiation field around Jupiter and continued to per-form experiments perfectly. Pioneer 10 continued itsjourney and finally left the solar system—the firstmanmade object ever to do so. Pioneer 11 flew byJupiter, going through the peak radiation zone in evenbetter shape than the earlier Pioneer 10. It also visitedSaturn—our first close encounter with the ringedplanet. Both Pioneer spacecraft continued to sendback data to Earth after 17 years, even as they passedbeyond the solar system, because the RTGs providecontinuous power.
22
General Purpose Heat SourceThe general purpose heat source (GPHS) is the radio-active fuel package being used in RTGs of current andplanned space missions. GPHSs use Pu-238 fuel justas earlier-model RTGs did, but the unique fuel con-tainment is designed to maximize safety in an acci-dent. The basic heat source unit is the GPHS module.Each GPHS/RTG contains 18 modules.
Design features include:
• Fuel pellets made of hard, ceramic
plutonium-238 oxide that do not dissolve in water. The pellets are also
highly resistant to vaporizing or fracturing into breathable particles
following impact on hard surfaces. Each of the GPHS’s 18 modules
contains four fuel pellets, for a total of 72 pellets per GPHS RTG.
• Iridium, a very stable metal with elastic properties, encapsulates each
fuel pellet. These capsules (about the size and shape of a marshmallow)
would tend to stretch or flatten instead of ripping open if the GPHS
module struck the ground at high speed. This would help keep the
capsules intact and contain the fuel.
• A high-strength graphite cylinder called a graphite impact shell that
holds a pair of fuel pellets. The graphite impact shell is designed to limit
damage to the iridium fuel capsules from free-fall or explosion frag-
ments.
• An “aeroshell,” which encloses a pair of graphite impact shells. It serves
as a shield designed to withstand the heat of
re-entering Earth’s atmosphere in case of an accident.
• Fuel pellets made of hard, ceramic plutonium-238oxide that do not dissolve in water. The pellets arealso highly resistant to vaporizing or fracturing intobreakable particles following impact on hard sur-faces. Each of the GPHS's 18 modules contains fourfuel pellets, for a total of 72 pellets per GPHS RTG.
• Iridium, a very stable metal with elastic propertiesthat encapsulates each fuel pellet. These capsules(about the size and shape of a marshmallow) wouldtend to stretch or flatten instead of ripping open ifthe GPHS module struck the ground at high speed.This would help keep the capsules intact and con-tain the fuel.
• A high-strength graphite cylinder called a graphiteimpact shell that holds a pair of fuel pellets. Thegraphite impact shell is designed to limit damage tothe iridium fuel capsules from free-fall or explosionfragments.
• An "aeroshell," which encloses a pair of graphiteimpact shells. It serves as a shield designed to withstand the heat of re-entering Earth's atmosphere incase of an accident.
GraphiteImpact Shell
EncasedFuel Pellet
Aeroshell
313/16"
21/8"
311/16"
GIS
6
On the moon, Apollo-12 astronaut Gordon Bean prepares toload the plutonium-238 heat source into the SNAP-2 thermo-electric generator (arrow). The generator produced 73 watts ofpower for the Apollo lunar surface experiment package fornearly eight years.
Today, the U.S. Space Shuttle Program is the nation’sprimary space transportation system.It has launchednumerous spacecraft and satellites on research andcommunications missions.
23
GPHS Module
Each of the RTG's 18 modules contains four Pu-238 fuel pellets enclosedin three layers of protection - - the metal encasing the pellets, the graphiteshell, and the aeroshell.
Galileo/Ulysses RTGThe first unmanned vehicle in space was Sputnik,launched by the Soviet Union in 1957. The U.S.followed three months later with its first unmannedflight, Explorer 1. The number of U.S. and Sovietspace missions increased dramatically in the next fewyears. And by the end of 1969, more than 1,000spacecraft were orbiting Earth. Five years later thenumber of these satellites had climbed to nearly 1,700.
Testing the PossibilitiesThe nuclear fuel in the GPHS faces a variety of pos-sible accidents during a space mission. Launch andre-entry pose many types of risks to the spacecraftand its components.
As a result, rigorous testing is conducted to ensure theRTG’s nuclear fuel will survive a launch accident orother mishap, remain intact, and contain the fuel. Thebattery of tests that the GPHS’s fuel modules haveundergone included the effects of:
Fire - Direct exposure to solid propellent fires, suchas the GPHS might encounter in a launchaccident, produced minimal damage and nonuclear fuel release.
513
The Quest for Reliable PowerOn January 16, 1959, a dramatic photograph appeared in a Washington,D.C., newspaper. The headline proclaimed “President Shows AtomGenerator.” The photograph, above, pictured President Eisenhower anda group of U.S. Atomic Energy Commission (AEC) officials in the OvalOffice at the White House. They were gathered around the president’sdesk, staring at a strange grapefruit-shaped object. Dubbed the world’sfirst atomic battery, it was actually one of the earliest models of aradioisotope thermoelectric generator (RTG), a nuclear generator spe-cifically developed by the AEC to provide electric power during spacemissions.
➜
24
Blast - Blast waves greater than those predictedfrom a shuttle fuel explosion were simulated withexplosives; no fuel was released during the tests.
Re-entry - The modules survived the high tem-peratures of simulated atmospheric orbital decayentry, as tested in an arc-jet furnace; no fuel wasreleased.
Earth Impact - Impact at 120 miles per hour (ap-proximate top speed for an aeroshell falling toEarth) on sand, water, or soil produced no releaseof heat source module fuel. Impact on rock andconcrete sometimes produced releases, but muchof the fuel was retained by the surrounding graph-ite module, leaving only small amounts of low-level radioactive material to enter the localizedenvironment.
Immersion in Water - Long-term exposure to thecorrosive effects of seawater showed the iridiumcapsule is corrosion-resistant and the fuel itself ishighly insoluble.
Shrapnel - Researchers used aluminum and tita-nium bullets to simulate the small fragments thatmight be present in a launch vehicle explosion.Speeds of test fragments exceeded those predictedfor an actual explosion. Results indicated thatshrapnel would not cause a release of nuclear fuel.
Large Fragments - In tests representing solid rocketbooster failures, steel plates were fired at simulatedRTGs. Some unlikely events, such as impact withthe edges of steel plates, caused some release in afew fuel capsules. More likely events, such asimpacts with the faces of steel plates, did not pro-duce a release.
The safety tests demonstrated that the RTGs are ex-tremely rugged and capable of meeting the designobjective to prevent or minimize any fuel release.
The Century of FlightDuring the 20th century, fantasies about space flightprogressed toward reality on the wings of the manytypes of aircraft that became commonplace. Peoplefrom various countries contributed to the beginningsof spacecraft propulsion. One of the most importantwas an American, Robert Goddard, who studied bothsolid and liquid propellants. In 1916 he launched a testrocket that traveled 184 feet. Two decades later, Wernervon Braun and his team of German scientists weredeveloping powerful new rockets that could fly hun-dreds of miles.
In 1954 the U.S. began Project Orbiter, the nation’s firstmajor step in developing satellite systems. The U.S.soon became a world leader in this emerging technol-ogy, with the National Aeronautics and SpaceAdministration (NASA) playing a leading role.
4 12 25
The Future of RTGsin Space
The TechnologyOther nuclear generator technologies for spaceapplications have been under investigation. Thesetechnologies involve more efficient conversion of heatinto electricity. Safety and reliability are key factors indetermining the possible value of each technology inspace missions.
One option is the dynamic isotope power systems(DIPS), which are much more efficient in convertingheat into electricity than the RTGs used on recentmissions. The dynamic systems have moving partsthat transform heat into mechanical energy, which isused to generate electricity. One such engine, theStirling engine, contains helium that expands by ab-sorbing heat on the hot side of the engine and rejectingit on the cold side. The rapidly changing pressurecycles cause a piston to move back and forth, drivingan alternator and producing electricity.
The range of technologies under investigation is wide.For instance, a process called Alkaline Metal Thermalto Electric Conversion (AMTEC) converts infraredradiation into electricity using liquid metal ions, whichare charged atoms. By contrast, the thermo-photovol-taic (TPV) converter changes infrared radiation emit-ted by a hot surface into electricity. Design goals forAMTEC and TPV technology call for even more effi-cient conversion of heat into electricity of about 20-30%, or a three-fold increase over RTGs.
The higher efficiencies of these new technologies meanthat future spacecraft may require less Pu-238 thanRTGs typically use. This makes these new spacepower technologies highly attractive due to lowerweight and less radioactive material for the samepower output.
Contemplatingthe Heavens
Many ancient peoples, including the Aztecs, Egyp-tians, and the builders of Stonehenge, were intenselyinterested in astronomy. Their writings and architec-ture indicate they studied the moon’s phases andmovement. They related the position and the per-ceived movement of the sun to Earth and its seasons.Some charted the stars, identifying the constellations.
Through the ages, scholars suggested various expla-nations for the makeup, movement, and relationshipto Earth of these heavenly bodies. Like us today, theywanted to know more about those distant objects, buta lack of technology limited their ability to learn.
Over the centuries, scientists like Galileo and Newtondescribed the structure of the solar system and themovement of the planets. Inventions such as thetelescope permitted them to see the moon’s craters, the“canals” on Mars, Saturn’s rings, and other intriguingdetails. This knowledge increased their curiosity aboutthe moon, sun, and planets, and they longed for moreinformation. They even dreamed of expeditions acrossspace to encounter them firsthand.
3
IntroductionIn the early years of the United States space program,lightweight batteries, fuel cells, and solar modulesprovided electric power for space missions. As mis-sions became more ambitious and complex, powerneeds increased and scientists investigated variousoptions to meet these challenging power require-ments. One of the options was nuclear energy.
By the mid-1950s, research had begun in earnest onways to use nuclearpower in space. Theseefforts resulted in thefirst radioisotopethermoelectric genera-tors (RTGs), which arenuclear power genera-tors built specifically forspace and special ter-restrial uses. TheseRTGs convert the heatgenerated from thenatural decay of theirradioactive fuel intoelectricity. The low-power devices weredesigned to supplement a craft’s primary non-nuclearpower source, but as the technology progressed,they soon began shouldering many missions’ entirepower needs.
Today, RTG-powered spacecraft are exploring theouter planets of the solar system and orbiting the sunand Earth. They have also landed on Mars and themoon. They provide the power that enables us to seeand learn about even the farthermost objects in oursolar system.
26
The Cassini Mission to SaturnThe Cassini mission is scheduled to begin in October1997 when a Titan IV launch vehicle lifts its payloadinto orbit. The mission is a joint U.S.-European ven-ture to explore Saturn in detail, a journey that will takenearly seven years. Like Galileo, Cassini will usegravity-assists from other planets to achieve the neces-sary speed to reach Saturn. Cassini will employ fourflybys: two by Venus, one by Earth, and one by Jupiter,to reach its destination in deep space.
✹
Saturn ArrivalJune 2004 Jupiter Flyby
Dec. 2000
LaunchOctober 1997
Earth FlybyAugust 1997
1st VenusFlybyApril 1998
2nd VenusFlybyJune 1999
Cassini Mission. Four flybys will enable the Cassini spacecraft toreach Saturn, its destination in deep space.
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Scientists have an intense interest in the Saturniansystem—an interest beyond the planet’s vast andbeautiful system of rings. Saturn’s largest and mostintriguing moon, Titan, has a dense atmosphere thatresembles the Earth’s early atmosphere, and somescientists suggest there may be oceans on Titan.
Cassini consists of an orbiter and a probe. The probe,named Huygens after the Dutch scientist who discov-ered Titan in 1659, will descend by parachute to thesurface of Titan late in 2004. It will beam data to theorbiter during descent and after landing. Once there,it will examine the surface of Titan with threedozen sensors.
After releasing the Huygens probe for descent toTitan, the Cassini orbiter will explore the Saturniansystem for nearly four years, flying some 60 orbits ofthe giant planet during that time. The electricalenergy to power Cassini’s mission equipment, in-cluding all its communications and scientific sensors,will come from three RTG units that provide a total of850 watts of power.
One way the craft uses this energy is to control itsdata-gathering systems. For example, it can pointsensors to accuracies of a 10th of a degree, maintainstability levels over 10 times slower than the motionof a clock’s hour hand, navigate to accuracies of 30kilometers (about 20 miles), and broadcast data toEarth at rates as high as 140,000 bits per second.
Future RequirementsNASA has identified a number of potential missionsthat can best or only be undertaken using radioiso-tope power and/or heat sources. These futuremissions depend upon two important conditions.
On the cover:Launch of the Atlantis Space Shuttlecarrying Galileo into space (October 1989).
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Nuclear Power in Space
Table of Contents
Introduction3Contemplating the Heavens4The Pioneer Missions7The Voyager Missions8The Ulysses Mission8The Galileo Mission11The Missions - An Overview16Power in Space - The Special Requirements18How Does an RTG Work?18Pu-238 - The Radioisotope of Choice for RTGs19RTGs - The Safety Factor21The Future of RTGs in Space25Conclusion28
U.S. Department of EnergyOffice of Nuclear Energy, Science, and TechnologyWashington, D.C. 20585
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First, there must be a reliable and continuing supply ofPu-238 fuel from the U.S. Department of Energy. U.S.facilities that could supply Pu-238 are being consid-ered, as are foreign sources such as Russia, England,and France.
Second, smaller and more efficient power systemswill have to be developed consistent with NASA'sneeds.
ConclusionRTGs have made vital contributions to the U.S. spaceprogram from its earliest days to the present. Andthey are slated for service on future projects such asthe 1997 Cassini mission to Saturn, Mars Pathfinder,and Pluto Express.
For many missions on which RTGs were used, therewas no other viable option for providing power.These missions include Apollo, Viking, Pioneer, Voy-ager, Galileo, and Ulysses, which have provided sci-entists with critical information about the origins ofthe solar system.
Nuclear fuel has proven to be an ideal source of energyin space because of its high power, acceptable weightand volume, and excellent reliability and safety whenused in RTGs.
Because of its many advantages, it seems likely thatnuclear energy will continue to provide power onspace missions into the next century, whether in RTGs,other advanced generators, or nuclear reactors.
NUCLEARENERGY
DOE/NE-0071
U.S. Department of Energy
U . S . D e p a r t m e n t o f E n e r g y
O f f i c e o f N u c l e a r E n e r g y ,
S c i e n c e a n d Te c h n o l o g y
UN
ITE
DSTATES OF AM
ER
ICA
TNEMTRAPE
D
OF ENERGY
NUCLEAR Power in Space
Printed on recycled paper
The Department of Energy produces publications to fulfill a statutory mandate to disseminate information to the public on all energy sources and energy conservation technologies. These materials are for public use and do not purport to present an exhaustive treatment of the subject matter. This is one in a series of publications on nuclear energy.
