**3. The early telluride-based RTGs**

The initial and current thermoelectric material of choice is based on telluride technology alloyed with lead (Pb-Te) that, to a first approximation, can be used from room temperature to about 900 K before materials properties become an issue. Above 900 K, the U.S. has had great success with a silicon-germanium alloy (Si-Ge) that has operated exceedingly well at temperatures of about 1300 K.

For the upcoming Mars Science Laboratory (MSL) mission, the U.S. will use a telluridebased thermoelectric material because it meets the requirements of being able to operate both in space on the way to Mars and on the surface of Mars with its dusty, cold, carbon dioxide atmosphere (see Section 8). The successes of the earlier (1976 era) Viking Mars Landers 1 and 2 using SNAP-19 telluride-based technology support this decision.

#### **3.1 SNAP-3B RTGs**

476 Radioisotopes – Applications in Physical Sciences

Generator (ASRG) (see Section 10). Depending on the design, dynamic conversion systems can provide double, triple and even quadruple the efficiency of state-of-practice thermoelectric conversion systems which means much less radioisotope fuel would be used to achieve the same electrical power (or, conversely, much more electrical power can be

Fig. 3. SNAP-1 turbomachinery package with the shaft assembly shown separately, ruler

In parallel with the SNAP-1 program a series of radioisotope power sources were studied under the umbrella of the SNAP-3 program that was based largely on using thermoelectric elements in the converter. The early SNAP-3 generators were to use 210Po as the fuel but by the late 1950s it was clear that sufficient quantities of 238Pu would be available to provide the fuel for small RTGs. Plutonium-238 provided a number of features that made it more attractive than 144Ce or 210Po, including a longer half-life (87.7 years) and a more benign radiation emission (alpha particles, which can be stopped by material as thin as a sheet of

Safety is the principal design requirement in the use of RPS, so the heat source is designed to contain or immobilize the fuel throughout a range of postulated accidents such as explosions and atmospheric reentries. Over the years this safety design work has led to the development of the general-purpose heat source (GPHS) module, which is the basic

All of the U.S. RPS that have flown have been either RTGs or RHUs, (see Fig. 4).

dimensions are in inches (TRW via Corliss and Harvey, 1964).

paper) (Corliss and Harvey, 1964).

building block of U.S. RPS (Bennett, 1995).

produced for the same quantity of radioisotope fuel used in an RTG).

The SNAP-3B RTG evolved out of the overall SNAP-3 program with the goal of providing 2.7 We to the U.S. Navy's Transit 4A and Transit 4 B navigational satellites. In particular, the SNAP-3B RTGs were to provide power to the crystal oscillator that was the heart of the electronic system used for Doppler-shift tracking, a precursor of today's global positioning system (Dick and Davis, 1962, JHU/APL, 1980). Both RTGs provided power to their respective spacecraft for over 10 years (Bennett, et al., 1983). Figure 5 shows models of the SNAP-3B RTG and the successor SNAP-9A RTG.

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 479

Fig. 5. Nobel Laureate Glenn T. Seaborg, Chairman of the U.S. Atomic Energy

comparison between the SNAP-9A and its predecessor the SNAP-3B.

SNAP-19 RTGs which were the first use of nuclear power in space by NASA.

weeks into the mission because of solar array degradation (Bennett, et al., 1984).

(Image credit: AEC)

**3.2 SNAP-9A RTGs** 

**4. SNAP-19 RTGs** 

**4.1 Nimbus III** 

Commission, with his hands over a model of the SNAP-9A RTG and program manager Major Robert T. Carpenter holding a model of the SNAP-3B RTG (circa 1963), (AEC). The SNAP-3B RTG produced 2.7 We in a package 12.1-cm in diameter and 14-cm high with a mass of 2.1 kg. The SNAP-9A RTG produced over 25 We at beginning of mission (BOM) within a mass of 12.3 kg and a main body that was 22.9 cm in diameter and 21.3 cm high.

The success of the SNAP-3B RTGs on Transits 4A and 4B gave the Johns Hopkins University Applied Physics Laboratory (JHU/APL) confidence to select the next-generation RTG, known as SNAP-9A, to provide all the power for its Transit 5BN-1 and 5BN-2 navigational satellites. The objective for each SNAP-9A was to provide 25 We at beginning of mission (BOM) at a nominal 6 V for five years in space after one year of storage on Earth. The two SNAP-9As showed that RTGs could be easily integrated into a spacecraft to provide all of the electrical power (Bennett, et al., 1984, JHU/APL, 1980). Figure 5 provides a size

The development work on the SNAP-9A RTG provided the technology that led to the

In 1969, NASA successfully launched the Nimbus III meteorological satellite powered by two SNAP-19 RTGs and solar arrays. The two SNAP-19 RTGs, which produced 56.4 We at launch, provided about 20% of the total power of the spacecraft. Had the SNAP-19 RTGs not been onboard Nimbus III, the power would have fallen below the load line about two

#### **Transit Navy Navigational Satellites**


### **SNAPSHOT Space Reactor Experiment**

SNAP-10A nuclear reactor (1965) (≥500 We)

### **Nimbus-B-1 Meteorological Satellite**

*\*SNAP-19B RTGs* (1968) (2 @ 28We each)

#### **Nimbus-3 Meteorological Satellite**

SNAP-19B RTGs (1969) (2 @ 28 We each)

#### **Apollo Lunar Surface Experiments Packages**

 Apollo 12 (1969**),** *\*13 (1970)*, 14 (1971), 15 (1971), 16 (1972), 17 (1972) SNAP-27 (>70 We each)

#### **Lincoln Experimental Satellites (Communications)**

LES 8 and LES 9 (1976) MHW-RTG (2/spacecraft @ ~154 We each)

#### **Interplanetary Missions**


(Spacecraft/Year Launched/Type of Nuclear Power Source/Beginning-of-Mission Power) Note: SNAP is an acronym for Systems for Nuclear Auxiliary Power

MHW-RTG = Multi-Hundred Watt Radioisotope Thermoelectric Generator GPHS-RTG = General-Purpose Heat Source Radioisotope Thermoelectric Generator

\* Denotes system launched but mission unsuccessful

Table 1. Uses of Space Nuclear Power By The United States

Fig. 5. Nobel Laureate Glenn T. Seaborg, Chairman of the U.S. Atomic Energy Commission, with his hands over a model of the SNAP-9A RTG and program manager Major Robert T. Carpenter holding a model of the SNAP-3B RTG (circa 1963), (AEC). The SNAP-3B RTG produced 2.7 We in a package 12.1-cm in diameter and 14-cm high with a mass of 2.1 kg. The SNAP-9A RTG produced over 25 We at beginning of mission (BOM) within a mass of 12.3 kg and a main body that was 22.9 cm in diameter and 21.3 cm high. (Image credit: AEC)

#### **3.2 SNAP-9A RTGs**

478 Radioisotopes – Applications in Physical Sciences

Apollo 12 (1969**),** *\*13 (1970)*, 14 (1971), 15 (1971), 16 (1972), 17 (1972) SNAP-27 (>70 We

**Transit Navy Navigational Satellites** 

Transits 4A and 4B (1961) SNAP-3B (2.7 We)

Transit TRIAD (1972) Transit-RTG (35 We)

SNAP-10A nuclear reactor (1965) (≥500 We)

*\*SNAP-19B RTGs* (1968) (2 @ 28We each)

 SNAP-19B RTGs (1969) (2 @ 28 We each) **Apollo Lunar Surface Experiments Packages** 

**Lincoln Experimental Satellites (Communications)** 

Galileo (1989) GPHS-RTG (2 @ 287 We each)

 Cassini (1997) GPHS-RTG (3 @ >290 We each) New Horizons (2006) GPHS-RTG (1 @ 245.7 We)

\* Denotes system launched but mission unsuccessful

Note: SNAP is an acronym for Systems for Nuclear Auxiliary Power MHW-RTG = Multi-Hundred Watt Radioisotope Thermoelectric Generator GPHS-RTG = General-Purpose Heat Source Radioisotope Thermoelectric Generator

Table 1. Uses of Space Nuclear Power By The United States

Ulysses (1990) GPHS-RTG (282 We)

**SNAPSHOT Space Reactor Experiment** 

**Nimbus-B-1 Meteorological Satellite** 

**Nimbus-3 Meteorological Satellite** 

each)

**Interplanetary Missions** 

Transits 5BN-1, 5BN-2 (1963) and *\*5BN-3 (1964)* SNAP-9A (>25 We)

LES 8 and LES 9 (1976) MHW-RTG (2/spacecraft @ ~154 We each)

 Pioneer 10 (1972) and Pioneer 11 (1973) SNAP-19 (4/spacecraft @ ~40 We each) Viking Mars Landers 1 and 2 (1975) SNAP-19 (2/Lander @ ~42 We each) Voyager 1 and Voyager 2 (1977) MHW-RTG (3/spacecraft @ >156 We each)

(Spacecraft/Year Launched/Type of Nuclear Power Source/Beginning-of-Mission Power)

The success of the SNAP-3B RTGs on Transits 4A and 4B gave the Johns Hopkins University Applied Physics Laboratory (JHU/APL) confidence to select the next-generation RTG, known as SNAP-9A, to provide all the power for its Transit 5BN-1 and 5BN-2 navigational satellites. The objective for each SNAP-9A was to provide 25 We at beginning of mission (BOM) at a nominal 6 V for five years in space after one year of storage on Earth. The two SNAP-9As showed that RTGs could be easily integrated into a spacecraft to provide all of the electrical power (Bennett, et al., 1984, JHU/APL, 1980). Figure 5 provides a size comparison between the SNAP-9A and its predecessor the SNAP-3B.

#### **4. SNAP-19 RTGs**

The development work on the SNAP-9A RTG provided the technology that led to the SNAP-19 RTGs which were the first use of nuclear power in space by NASA.

#### **4.1 Nimbus III**

In 1969, NASA successfully launched the Nimbus III meteorological satellite powered by two SNAP-19 RTGs and solar arrays. The two SNAP-19 RTGs, which produced 56.4 We at launch, provided about 20% of the total power of the spacecraft. Had the SNAP-19 RTGs not been onboard Nimbus III, the power would have fallen below the load line about two weeks into the mission because of solar array degradation (Bennett, et al., 1984).

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 481

The success of the Viking SNAP-19 RTGs was a key factor in the selection of the telluridethermoelectric-based Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) for

The successful use of the SNAP-9A RTGs on the Transit 5BN series of Navy navigational satellites led JHU/APL to use a new telluride-based RTG called the "Transit RTG" on its TRIAD navigational satellite. The Transit RTG was based on the SNAP-19 radioisotope heat source design although in this case radiatively coupled to a telluride-based thermoelectric converter instead of being conductively coupled as in the SNAP-19 and

The Transit RTG, which was designed to be modular, produced over 35 We at BOM within a mass of about 13.6 kg. The use of a lower hot-junction temperature (~674 K for the Transit RTG versus ~790+ K for the SNAP-19 RTGs) in a vacuum environment eliminated the SNAP-19 practice of using hermetic sealing and a cover gas to inhibit the sublimation degradation that could cause a reduction in cross section and subsequent increase in electrical resistance of the thermoelectric material. (Lowering the hot junction temperature is also one of the strategies adopted for the MMRTG.) While the TRIAD spacecraft had various problems, the Transit RTG operated well beyond its five-year requirement (Bennett,

For the Apollo missions to the Moon, RTGs were a natural choice to power scientific instruments during the long (14-Earth-day) lunar night. To provide this power, the U.S. Atomic Energy Commission (AEC) provided NASA with SNAP-27 RTGs built by General Electric (GE, now part of Lockheed-Martin). The SNAP-27 RTGs were designed to provide at least 63.5 We at 16 V one year after lunar emplacement. (In the case of Apollo 17, the requirement was 69 We two years after emplacement). Figure 7 shows Apollo 12 astronaut Alan L. Bean removing the SNAP-27 fuel-cask assembly from the Lunar Module on 19 November 1969. This was the first use of electricity-producing nuclear power on

All five SNAP-27 RTGs (Apollo 12, 14, 15, 16, 17) exceeded their mission requirements in both power and lifetime thereby enabling the Apollo Lunar Surface Experiment Packages (ALSEPs) to gather long-term scientific data on the internal structure and composition of the Moon, the composition of the lunar atmosphere, the state of the lunar interior, and the genesis of lunar surface features (Pitrolo, et al., 1969, Bates, et al., 1979). On Apollo 11 the experiment package deployed on the lunar surface was named Early Apollo Scientific Experiments Package (EASAP) and consisted of the laser ranging retro-reflector (LRRR, also deployed on each following Apollo mission, and are still in use today) and the passive seismic experiments package (PSEP). The PSEP utilized 2 RHUs called the Apollo Lunar Radioisotopic Heater (ALRH) for thermal control (Apollo 11 Lunar Landing Mission Press Kit, 1969*)* and also had a solar array power system that lasted three weeks. The ALRHs contained ~34 gm of 238Pu producing 15 Wth each. The subsequent PSEP stations utilized

the upcoming MSL mission (see Section 8).

**5. Transit RTG** 

SNAP-9A RTGs.

et al., 1984).

the Moon.

**6. SNAP-27 on Apollo** 

power from the SNAP-27 RTGs.

### **4.2 Pioneers 10 and 11**

In 1972, NASA began its exploration of the outer Solar System with the launch to Jupiter of Pioneer 10 powered by four SNAP-19 RTGs which produced a total of 161.2 We at BOM. The next year Pioneer 10 was followed by the Pioneer 11 spacecraft which was also powered by four SNAP-19 RTGs. In the cold, dark, radiation-rich environment of the Jovian system, nuclear power was the only viable option at that time. Because the SNAP-19 RTGs performed so well, NASA was able to retarget Pioneer 11 to go to Saturn after its flyby of Jupiter. Again, the RTGs performed very well, providing steady power to the spacecraft and its scientific instruments, thus allowing scientists their first close-up measurements of the second largest planet in the Solar System (Bennett, et al., 1984).
