Radioisotope Heater Units - ESAemits.sso.esa.int/emits-doc/AO5315-RD30.pdf · 2010-01-13 · TECHNICAL ASPECTS OF SPACE NUCLEAR POWER SOURCES VII. Radioisotope Heater Units Leopold

TECHNICAL ASPECTS OF

SPACE NUCLEAR POWER SOURCES

VII. Radioisotope Heater Units

Leopold SummererESA Advanced Concepts Team

18th December 2006(version 1)

ESA-ACT Reference Number: ACT-RPT-2327-RHU

2Leopold Summerer

ESA Advanced Concepts Team , Technical Aspects of Space NPS

18th December 2006 2 ACT-RPT-2327

Contents

Scope and Introduction 5Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5General description of radioisotope heater units . . . . . . . . . . . . 6

1 Technical Aspects of Angel-type Radioisotope Heater Units 91.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.1.1 Soviet/Russian Mars’96-Programme . . . . . . . . . . . . 91.2 Pantera heat source . . . . . . . . . . . . . . . . . . . . . . . . . . 101.3 Mars’96 Angel radioisotope heater unit (RHU) . . . . . . . . . . 111.4 Mars’96 Angel radioisotope thermo-electric generator (RTG) . . 121.5 Safety aspects of Angel radioisotope heater units . . . . . . . . . 13

1.5.1 Normal and accidental operating conditions . . . . . . . 131.5.2 Radiation protection information . . . . . . . . . . . . . . 141.5.3 Isotopic composition . . . . . . . . . . . . . . . . . . . . . 14

1.6 RHU and RTG Angel Photos . . . . . . . . . . . . . . . . . . . . . 15

2 US Radioisotope Heater Units (RHU) 172.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Light-Weight Radioisotope Heater Units . . . . . . . . . . . . . . 17

2.2.1 LW RHU safety . . . . . . . . . . . . . . . . . . . . . . . . 192.3 General Purpose Heat Sources - GPHSs . . . . . . . . . . . . . . 192.4 Recent US space missions using RHU . . . . . . . . . . . . . . . 20

2.4.1 Galileo mission . . . . . . . . . . . . . . . . . . . . . . . . 202.4.2 Mars Pathfinder mission . . . . . . . . . . . . . . . . . . . 222.4.3 Cassini-Huygens mission . . . . . . . . . . . . . . . . . . 222.4.4 MER Rover missions . . . . . . . . . . . . . . . . . . . . . 23

A Acronyms 25

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Scope and Introduction

Scope

This technical note provides information about the latest generation of US andRussian radioisotope heater units (RHU) for space use. The US RHU are of thegeneration that has powered US deep space missions since the 1980s (includingthe ESA Huygens probe). The Russian Angel-type RHU have been developedfor and used on the Mars’96 mission.

The note does not try to provide a complete collection of reported and pub-only most recentUS and Russian RHUdescribed

lished features of these RHU nor does it contain information on other US andRussian / Soviet RHUs, used in previous missions. It furthermore tries to re-strict the information as much as possible on confirmed available data, fromeither official and public documents concerning the US RHUs and from directstudy contracts with competent Russian entities.

The information in this note intends to serve ESA-internally and for Europeanindustry as a starting basis for preliminary assessments of options for andeventual implications of their potential integration into European spacecraftand launchers.

Introduction

The first chapter presents the Russian RHUs, including very briefly the mis-sion for which the Angel RHU and RTG have been designed and produced inthe late 1980s and early 1990s. The heat source is described which constitutesthe basis for the Angel RHU described in the following section and which itselfis the basis for the Angel RTG, dealt with subsequently. The chapter gives fur-ther some information regarding the normal and accidental conditions againstwhich the design was tested, lists the reported ionising radiation dose levelsand isotope composition and concludes with some photos of Angel RHU andRTGs.

The second chapter of the technical note gives some technical informationon the US RHUs as well as a summary description of the heat sources used forthe two most recent generations of US RTGs, which are however not described

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themselves. It also contains a section on some of the recent US missions usingRHUs.

General description of radioisotope heater units

Radioisotope heater units are arguably among the possibly most simplest spacenuclear power sources. They contain no moving parts nor any electronics.RHU are small devices of usually cylindrical shape of the sizes of coins (Figure

Fuel pellets with238PuO2

2.3). In their core, they contain the radioactive isotope (usually 238Pu or 210Po)surrounded by several protective layers destined to contain the radioisotopefuel in any foreseeable accidental condition.

The usually have several safety layers trying to prevent the accidental re-lease of radioisotopes. The first is usually the brick-like ceramic form of thefuel pellets. Others are different materials surrounding the source intending toabsorb all foreseeable energy impacts during accidents.

Characteristics of RHUs include:

• Highly reliable, continuous, and predictable output of heat

• No moving parts

• Compact structure

• Resistance to radiation and meteorite damage

• The heat produced is independent of the distance from the Sun

The natural decay heat of the radioisotope keeps the entire device at a cer-tain temperature, dependent on the heat dissipation of its surface (convection,radiation, conduction).

RHUs are used to keep the temperature of critical parts of the spacecraftThermal control above a certain threshold temperature by usually being physically mounted

close-by. More sophisticated thermal designs based on RHUs include heatpipes and thermal conductors.


Leopold SummererESA Advanced Concepts Team , VII. Radiosiotope Heater Units 7

Table 1: All reported space missions using RTGs and RHUs.

Payload Date Power Source Source TypeTransit 4A 29.06.1961 SNAP-3B7 RTGTransit 4B 13.11.1961 SNAP-3B8 RTGTransit 5BN-1 28.09.1963 SNAP-9A RTGTransit 5BN-2 05.12.1963 SNAP-9A RTGTransit 5BN-3 21.04.1964 SNAP-9A RTGKosmos 84 (Strela-1/24) 03.09.1965 Orion-1 RTGKosmos 90 (Strela-1/29) 18.09.1965 Orion-1 RTGNimbus B 18.05.1968 2 SNAP-19B2 RTGNimbus 3 14.04.1969 2 SNAP-19B2 RTGLEM 5 (Apollo 11) 16.07.1969 RHULEM 6 (Apollo 12) 14.11.1969 SNAP-27 RTGLEM 7 (Apollo 13) 11.04.1970 SNAP-27 RTGLunokhod-1 1970 11-K, V3-R70-4 RHULEM 8 (Apollo 14) 31.01.1971 SNAP-27 RTGLEM 10 (Apollo 15) 26.07.1971 SNAP-27 RTGPioneer 10 02.03.1972 4 SNAP-19 RTGLEM 11 (Apollo 16) 16.04.1972 SNAP-27 RTGTriad 1 (TIP 1) 02.09.1972 Transit-RTG RTGLEM 12 (Apollo 17) 07.12.1972 SNAP-27 RTGPioneer 11 05.04.1973 4 SNAP-19 RTGLunokhod-3 1973 11-K, V3-R70-4 RHUViking 1 Lander 20.08.1975 2 SNAP-19 RTGViking 2 Lander 19.09.1975 2 SNAP-19 RTGLES 8 15.03.1976 2 MHW-RTG RTGLES 9 15.03.1976 2 MHW-RTG RTGVoyager 2 20.08.1977 3 MHW-RTG RTGVoyager 1 05.09.1977 3 MHW-RTG RTGGalileo 18.10.1989 2 GPHS-RTG RTG and RHUUlysses 06.10.1990 1 GPHS-RTG RTGMars 8 (Mars 96) 16.11.1996 Angel RTG and RHU

2 Lander, 2 PenetratorsMars Pathfinder Rover 04.12.1996 LW RHU RHUCassini/Huygens 15.10.1997 3 GPHS-RTG RTG and RHUMER A Rover (Spirit) 10.06.2003 LW RHU RHUMER B Rover (Opportunity) 08.07.2003 LW RHU RHUNew Horizon 19.01.2006 1 GPHS-RTG RTG

ACT-RPT-2327 7 18th December 2006

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Chapter 1

Technical Aspects ofAngel-type RadioisotopeHeater Units

1.1 Introduction

The Angel RHU and RTG were developed for the Russia-led Mars’96 programmein the late 1980 / early 1990.

1.1.1 Soviet/Russian Mars’96-Programme

The preparation for this Soviet/Russian Mars exploration programme beganin early 1989. Initially, it included two missions to Mars: one in 1994 and onein 1996. The 1994 mission was abandoned and rescheduled for launch in 1996due to ecomonic constraints.[22]

The Mars’96 spacecraft consisted of:

• an orbiter for remote sensing studies of Mars and in-situ studies of theplasma environment around Mars;

• two autonomous small stations to land on the surface of Mars;

• two penetrators into the Martian soil.

The spacecraft, small stations and penetrators were developed and manu-factured at the Lavochkin Research and Operational Association (NPO). Formore information on the Mars’96 mission it is referred to [22].

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The surface stations and the penetrators were powered by RTGs and hada total expected lifetime of one year. BIAPOS was the prime contractor forthe design and development of the RHUs/RTGs ”Angel” for Mars-96. It co-ordinated several national scientific and technical institutions as subcontract-ors, among them Mayak, Avangard and Krasnaya Zwezda.

The launch of Mars’96 was a failure and the spacecraft re-entered the Earthatmosphere over the west-coast of South America and the pacific ocean. TheAngel RTGs/RHUs with their Pantera heat sources were never found.

1.2 Pantera heat source

The innermost layer of the Angel RHU and RTG is the radioisotope heat source,sometimes referred to as Pantera heat source. The PuO2 fuel is contained incoated multilayer metal coatings as shown in Figure 1.1, emitting about 8.5 Wth

with a total mass of about 85 g. The heat source has a diameter of about 17 mmand a height of 35 mm. (Table 1.1)

As in the case of the US LW RHU, the fuel is in the heat resistant, ceramicform of plutonium dioxide (PuO2), which reduces its chance of vaporising infire or re-entry environments. This ceramic-form fuel is also highly insoluble,has a low chemical reactivity, and primarily fractures into large, non-respirableparticles and chunks. These characteristics help to mitigate the potential healtheffects from accidents involving the release of radioisotopes.

Figure 1.1: 238Pu-based thermal source Pantera

The numbers in figure 1.1 showing the Pantera heat source design corres-pond to:

1. 238Pu clad by iridium and packed into a casing made of a platinum-basedalloy

2. protective shell case (tantalum-based alloy)



3. protective shell cover (tantalum-based alloy)

4. plug (tantalum-based alloy)

5. three-layer protective coating

6. membrane (tantalum-based alloy)

Table 1.1: Pantera specifications

thermal capacity 8.5 Wth

diameter ≤ 16.7 mmheight ≤ 35 mmmass ≤ 85 g

heat source PuO2238Pu mass 15− 17 g

n flux 1.6× 104 n/(gs)3.5× 105 n/s

The Pantera heat sources cannot be considered independently of the AngelRHU/RTG system, of which it constitutes an integral part. While these Panteraheat sources were specifically mentioned in earlier reports, the most recentstudies are only mentioning as basis element the Angel heater unit. It seemstherefore reasonable to use the information on the assembled Angel RHU asbasis for any technical assessment.

1.3 Mars’96 Angel radioisotope heater unit (RHU)

A Angel radioisotope heater unit consists of a Pantera heat source surroundedby additional shielding and protective layers. The additional layers more thandouble the diameter of the device and almost double its height. The Angel RHUhas the form of is a cylinder with a diameter of 40 mm and height of 60 mm.The total mass of the Angel RHU is approximately 180 g (Table 1.2, Figure 1.2).Each Angel RHU delivered a thermal power of 8.5 Wth.

Table 1.2: Angel RHU specifications (as used on Mars’96)

thermal capacity 8.5 Wth

diameter 40 mmheight 60 mmmass 180 g

heat source PuO2238Pu mass 15− 17 g

n flux 1.6× 104 n/(gs)3.5× 105 n/s

activity ≤ 9.6× 1012 Bq


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Figure 1.2: 238Pu-based Angel RHU as used on Mars-96; 1: Pantera heat source, 2: corrosion resist-ant shell, 3: Outer protective shell, 4: PuO2 pellet, 5: heat shield, 6: multilayer insulation

Table 1.3: Angel RTG specifications

thermal power 8.5 Wth

electric power output 200± 5% mWe

voltage 15± 5 Vdiameter 85 mm

height 125 mmmass ∼ 500 g

activity ≤ 9.6× 1012 Bq

1.4 Mars’96 Angel radioisotope thermo-electric gen-erator (RTG)

This section provides some information on the RHU-Angel-based RTG withthe same official name. These are currently not foreseen to be used on any ofthe two baseline options of the ExoMars mission but since the use of the samename for the RHU and the RTG leads sometimes to confusion, some summaryinformation on the RTG is also included in this technical note. Angel RTG es-sentially consist of Angel RHU at their core, surrounded by thermal protectionand at one side a thermoelectric converter unit, converting part of the thermaloutput of the Angel RHU into electricity.

An RTG equipped with the Angel radioisotope heater unit had a designedoutput power of no less than 0.15 We at the end of its life time delivering anoutput voltage of 15 V . Given the isotopic composition and the thermal poweroutput of 8.5 Wth, one RTG contains about 21 to 23 g of PuO2 correspondingwell with the reported 15− 17 g of 238Pu.

Figure 1.3 shows an outline of the Angel RTG. Its main parameters are listedin Table 1.3.



Figure 1.3: Mars-96 RTG (1: RHU Angel heater unit; 2: Thermoelectric converter; 3: thermal insu-lation; 4: outer casing; 5: mechanical attachment elements; 6: electrical interface)

1.5 Safety aspects of Angel radioisotope heater units

1.5.1 Normal and accidental operating conditions

Table 1.4 lists the normal and accidental conditions, that the RHUs were de-signed to withstand. The parameters of these conditions are usually derivedfrom foreseeable accidental conditions that depend on the specificity of eachmission (launcher, spacecraft design, orbits, etc). Therefore the here quotedvalues and conditions are specific to the Mars’96 mission, launched from Baikonuron a Proton launcher. They should thus only be seen as an example and not asdefinitive absolute values.

Table 1.4: Angel operating conditions

Normal operating conditionstemperature 300◦C

environments inert gas, vacuumlife time 10 years

internal pressure ≤ 60 atm

Abnormal conditionsthermal loads resulted from the explosion of fuel compositions of the space rocket system;thermal effects under aerodynamic deceleration in atmosphere of the Earth (1700◦C/3 min);impact on the Earth surface at a speed of 70 m/s with RHU temperature up to 1100◦C;prolonged hydrostatic (up to 100 atm) and corrosion effects of salt and fresh water;corrosion effects of the Earth atmosphere, when the RHU is on the ground surface or in soil.


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1.5.2 Radiation protection information

According to the design specifications, the neutron flux of a Pantera heat sourceunit is lower than about 1.6 × 104 n/(gs) and the exposure rate at 1 m dis-tance from the source is less than 2.78× 10−9Sv/s, which corresponds to about10 µSv/h, 0.24 mSv/d or 88 mSv/a.

In case of a distance of 10 m, which is sometimes used to evaluate the ra-diation hazard to the general population in case the fuel ampoule re-enteredintact in a populated area and could not be found and retrieved, these valuesdrop to 0.1 µSv/h, 24 µSv/d or 0.88 mSv/a.

1.5.3 Isotopic composition

Russia is producing different batches of 238PuO2, differing in their isotopiccomposition. The main difference is related to its purity (e.g. the proportionalmass fraction of higher plutonium isotopes, oxygen isotopic composition) de-termining the total heat output, the total radiation level and the type of radi-ation.

While the specific power of pure 238Pu is about 0.56Wth/g, the isotopiccomposition of the actually used Pu reduces this value to about 0.33−0.42 Wth/g.

Table 1.5 lists the measured isotopic composition of two Mars’96 RHUs.

Table 1.5: Pu isotope composition two Mars’96 Angel RHUs [15]

Specification RHU No 8 RHU No 10Pu isotope mass fractions in %:

238Pu 88.82 88.39239Pu 8.71 9.23240Pu 2.15 2.04241Pu 0.28 0.30242Pu 0.04 0.04236Pu

mass fractionin % of 238Pu mass

2× 10−5 2× 10−5

neutron fluxof a preparation

containing 1g 238Pu (n/(gs))1.6× 104 1.8× 104



1.6 RHU and RTG Angel Photos

Figure 1.4: Mars-96 Angel RHU (left) and RTG (right) (Photo: Pustovalov, BIAPOS)

Figure 1.5: Mars-96 Angel RHU (Photo: L.Summerer)


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Figure 1.6: Mars-96 Angel RTG (Photo: Pustovalov, BIAPOS)


Chapter 2

US Radioisotope Heater Units(RHU)

2.1 Introduction

Radioisotope power sources were identified as early as the late 1950s and earlyRadioisotopePower Sources asstrategic asset forUS space activities

1960s as of strategic importance for US space activities. Before being used forscientific and exploration missions, they have powered meteorogical (Nimbusseries), communication (e.g. LES series) and navigation (Transit series) satel-lites. Further-on they were used for lunar and Martian surface (Apollo, Viking)and deep space exploration (Pioneer, Voyager) missions.

The development of radioisotope heater units (RHU) was closely related tothe development of radioisotope thermo-electric generators (RTG), even if theheat source of the latter is currently different from the one used in RHUs.

A schematic overview of US missions using RTGs is shown in Figure 2.1.There seems to be no such graph available for all US missions using RHU,though many of the mentioned ones use RTGs as well as RHUs. A list of knownUS missions using RTGs or RHUs is shown in Table 1.

Section 2.2 tries to give an overview of current US radioisotope heater units,followed by a section on General Purpose Heat Sources (GPHSs), the buildingblocks for the US RTGs, which are themselveds not described in this technicalnote. Section 2.4 describes summarily some of the latest US missions usingRHUs.

2.2 Light-Weight Radioisotope Heater Units

The latest version of US RHUs are the so-called ”Light-Weight Radioisotope HeaterUnits” (the LWRHUs), flown on the Galileo, Cassini-Huygens missions as well as

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Figure 2.1: Schematic overview of US radioisotope powered missions. (courtesy NASA)

on the recent US Martian rovers Pathfinder and the two MER rovers.

A LWRHU consists of a small 1 Wth238PuO2 pellet, a clad of Platinum-30%-

40 g LWRHU1 Wth

Rhodium alloy, an insulation system of pyrolytic graphite and an aeroshell andimpact body of fine weave pierced fabric.

The exterior dimensions of LWRHUs are 26 mm diameter and 32 mmheight with a unit mass of 40 g. The LWRHU includes a He-vent allowingfor the release of the gradually produced helium gas. Figure 2.2 shows theschematic design of LWRHUs. The operating temperature of LWRHU is about310K.

Table 2.1: US LW RHU specifications

Heat source 238PuPuO2

Thermal power output 1 Wth

shape cylinderdimensions 26 mm diameter

32 mm heightMass 2.7 g PuO2 / RHU

40 goperating temperature 310 K



Figure 2.2: US LWRHU (image NASA/DoE)

2.2.1 LW RHU safety

The LW RHU are designed to withstand most accidental conditions of a widedesigned for safetyrange of space missions without the release of radioisotopes by using different

protective layers. In case of an accidental release of radioisotopes, the fuel is inthe heat resistant, ceramic form of plutonium dioxide (PuO2), which reducesits chance of vaporising in fire or re-entry environments. This ceramic-formfuel is also highly insoluble, has a low chemical reactivity, and primarily frac-tures into large, non-respirable particles and chunks. These characteristics helpto mitigate the potential health effects from accidents involving the release ofradioisotopes.

Figure 2.3 shows the the brick-like ceramic form of the fuel pellets (a), thecladding (b) and insulator (c) protect the fuel pellet from the extreme heat ofre-entry and from the environment in case of accident. Figure 2.2 shows a cut-through image of US LW RHU.

2.3 General Purpose Heat Sources - GPHSs

The General Purpose Heat Source (GPHS) module is the building block for theBuilding block forthe GPHS RTG:modular assembly

GPHS-RTGs (e.g. used in the US missions from Ulysses to New Horizon).


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Figure 2.3: US Leight-weight radioisotope heater unit delivering 1 Wth. The image shows the fuelpellet (a), the cladding (b) and the insulator (c). (source: DoE)

These modules are not used as RHUs. Their short technical description is how-ever included in this technical note since as “heat sources” for the RTGs, theyare sometimes mixed up with the actual RHUs.

These modules are also based on 238Pu, in form of its dioxide and fabricatedin pellets. The PuO2 is encapsulated in an iridium cladding. Fuelled clads areencased within nested layers of carbon based material and placed within anaeroshell housing to comprise the complete GPHS-module (Figure 2.4). EachGPHS-module contains 4 fuel pellets.

GPHS modules are approximately 5 cm tall with an almost square base ofslightly less than 10 cm side-length. Each GPHS has a mass of approximately1.44 kg and produces a nominal thermal power of 250 Wth from its about 0.6 kgof PuO2. A total of 18 modules are stacked together to provide the heat sourcefor each GPHS-RTG (Figure ??). The new MM RTG is based on 8 GPHS andthe Stirling converter based RTG under development is based on only 2 GPHS.

Table 2.2 provides the isotope composition of the GPHS-RTG used in theCassini-Huygens mission. It seems likely that the isotope composition of eachproduction batch is slightly different. The values in this table should thereforeonly used as an example and in order to give an indication of the relative shareof the different present Pu-isotopes.

2.4 Recent US space missions using RHU

2.4.1 Galileo mission



Figure 2.4: Image of a stack of GPHSs with the different components of a single GPHS module.

Table 2.2: Pu isotope composition (US RTGs)[12][2]

Fuel weight % T1/2 Bq/g Bq/RTGcomponent at launch (years)

236Pu 0.000001 2.858 1.970× 1013 2.14× 1009

238Pu 70.810000 87.74 6.347× 1011 4.89× 1015

239Pu 12.859000 2.411× 104 2.305× 1009 3.22× 1012

240Pu 1.787000 6563 8.449× 1009 1.64× 1012

241Pu 0.168000 14.35 3.819× 1012 6.98× 1013

242Pu 0.111000 3.750× 105 1.456× 1008 1.76× 1009

other 2.413000Oxygen 11.852000 NA NA

total 100 NA NA 4.96× 1015


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Galileo was launched aboard the Space Shuttle Atlantis (STS-34) on October 18,1989. The mission was designed to investigate the Jovian system - the largestplanet in our solar system, Jupiter, and four of its major moons: Io, Europa,Ganymede, and Calisto. The spacecraft consisted of a Jupiter Orbiter and anAtmospheric Entry Probe.

Galileo used 2 GPHS-RTG providing the electrical power and approxim-ately 120 lightweight radioisotope heater units for thermal control of sensitive

2 RTG and 120 RHU electronic components.

Originally, Galileo’s exploration of the Jovian system was to end on Decem-ber 7, 1997, but since significant discoveries were found, especially those aboutEuropa, the mission was extended for two years through the end of 1999, called

Mission extensions:GEM and GMM

the Galileo Europa Mission (GEM). After having completed its studies of Europa,Galileo pursued its research with the Galileo Millennium Mission (GMM), ex-tending into 2001, consisting of 4 flybys of Calisto, and then lowering its orbitin preparation for two flybys of Io.

The Galileo spacecraft’s 14-year odyssey came to an end [5] on September21, 2001, when the spacecraft passed into Jupiter’s shadow and then disinteg-rated in the planet’s dense atmosphere. The spacecraft was purposely put ona collision course with Jupiter because the onboard propellant was nearly de-pleted and to eliminate any chance of an unwanted impact between the space-craft and Jupiter’s moon Europa, which Galileo discovered is likely to have asubsurface ocean. The presence of RTG and RHU were part of this decisionprocess.

2.4.2 Mars Pathfinder mission

The Mars Pathfinder project was launched on December 4, 1996 and success-fully landed on the Martian surface on July 4, 1997. The spacecraft carried theSojourner rover . The rover was deployed from the lander to perform a number

Sojourner roboticrover on Mars

of experiments on the Martian soil and to demonstrate the ability of a rover totraverse the terrain in the vicinity of the lander.

Three LWRHUs were employed in the Sojourner Warm Electronics Box3 LWRHUs (WEB) to maintain critical electronic component temperatures within their op-

erating limits during the Martian nights. The LWRHUs, providing essentialheat to the Sojourner electronics, were key components of the thermal designthat enabled the rover to operate for 84 days, 12 times its design lifetime.

2.4.3 Cassini-Huygens mission

The Cassini-Huygens mission is an international cooperative project of NASA,Joint mission NASA,ESA and ISA

the European Space Agency and the Italian Space Agency with the goal to con-duct extensive studies of Saturn, including a deployment of the Huygens probeto the giant moon Titan.



Cassini, the largest NASA interplanetary spacecraft ever launched, liftedoff at Cape Canaveral on October 15, 1997, aboard a Titan IV/Centaur launchvehicle. On July 1, 2004, after an interplanetary voyage lasting nearly sevenyears, it fired its main engine to reduce its speed and enter orbit around Saturn.

Cassini also made use of planetary gravity assists to attain the velocity andfinal trajectory necessary to complete its journey to Saturn. The spacecraft suc-cessfully completed two gravity assists from Venus (April 26, 1998, and June24, 1999), one from Earth (August 18, 1999) and one from Jupiter (December30, 2000).

The spacecraft is currently exploring the ringed planet, its moons, the ringsand its complex magnetic environment.

The spacecraft and probe used 3 GPHS-RTGs and 117 LWRHUs to providethe necessary electrical power to operate Cassini’s instruments and systemsand to maintain temperatures of critical equipment at acceptable levels.[7, 2]

Figure 2.5: Left: The Cassini spacecraft with the Huygens probe attached in the front.Right: The nuclear aspects of Huygens: 36 RHUs, providing each 1 Wth for thermal control.

The joint NASA, ESA and ASI Cassini/Huygens mission to the Saturniansystem included the deployment of the European Huygens probe to Titan.

In order to keep temperature-critical parts of Huygens (especially the bat-teries) above certain minimal working temperature levels, Huygens contained36 RHUs providing each 1 Wth of heat during the coast phase to and at the

36 RHUsproviding 36 Wth

surface of Titan (Figures 2.5). Together with a multi-layer blancket/radiationwindow and foam inside the descent module these RHUs were an integral partof the thermal control system of Huygens.

2.4.4 MER Rover missions


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The Mars Exploration Rovers (Spirit and Opportunity) were launched respect-ively on June 10 and July 7, 2003. They landed on Mars on January 4 andJanuary 25, 2004. These successors of the Sojourner rover (section 2.4.2), have a

2 successors ofSojourner: Spiritand OpportunityLW RHUs

similar Warm Electronics Box (or WEB) heated by 8 LW RHU each.[4]


Appendix A

Acronyms

ACT ESA’s Advanced Concepts Team

AEC Atomic Energy Commission, former U.S. government commission cre-ated by the Atomic Energy Act of 1946 and charged with the develop-ment and control of the U.S. atomic energy program following WorldWar II, dissolved in 1974, activities integrated into DoE

IAEA International Atomic Energy Agency

ICRP International Commission on Radiological Protection em[INTAS] The In-ternational Association for the Promotion of Co-operation with Scientistsfrom the New Independent States (NIS) of the Former Soviet Union

IRCU International Commission on Radiation Units and Measurements

LEDA Lunar European Demonstration Approach programme

LWRHU Light-weight Radioisotope Heating Unit (US)

MMMB Ministry of Medium Machine Building (Russia)

NASA National Aeronautics and Space Administration (US)

NPS Nuclear Power Source, in this document synonymously used for spacenuclear power sources

RPS Radioisotope Power System (sometimes used synonymously to RTG)

RTG Radioisotope Thermo-electric Generator.The term ”thermo-electric” should not be confounded with thermoelec-tricity, the Seebeck-effect based mechanism used in thermocouples, butdesignates only the thermal to electric conversion, which might be staticor dynamic

RTS Radioisotope Thermal Source, Soviet/Russian term used synonymouslyfor RHU

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RHU Radioistotope Heating Unit

SNM Special Nuclear Material


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[3] Jet Propulsion Laboratory. Mars Exploration: Missions (Mars ScienceLaboratory), 2005. http://marsprogram.jpl.nasa.gov/missions/future/msl.html

(acc. Aug. 05).

[4] Jet Propulsion Laboratory. Mars exploration rover mission, 2005. http:

//marsrovers.jpl.nasa.gov/home (acc. July 2005).

[5] Jet Propulsion Laboratory. NASA’s Solar System Exploration: Galileo Leg-acy Site, 2005. http://galileo.jpl.nasa.gov/ (acc. 8 August 2005).

[6] Jet Propulsion Laboratory. Voyager, 2005. http://voyager.jpl.nasa.gov (acc.July 2005).

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28Leopold Summerer


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Documents

Radioisotope Heater Units - ESAemits.sso.esa.int/emits-doc/AO5315-RD30.pdf · 2010-01-13 · TECHNICAL ASPECTS OF SPACE NUCLEAR POWER SOURCES VII. Radioisotope Heater Units Leopold