United States General Accounting Office

GAO Report to the Honorable

Barbara Boxer, U.S. Senate

May 1998 SPACE EXPLORATION

Power Sources for

Deep Space Probes

GAO/NSIAD-98-102

GAO United States

General Accounting Office

Washington, D.C. 20548

National Security and

International Affairs Division

B-279348 

May 29, 1998

The Honorable Barbara Boxer

United States Senate

Dear Senator Boxer:

The National Aeronautics and Space Administration (NASA) launched its

nuclear-powered Cassini spacecraft on October 15, 1997, on a 12-year

mission to Saturn. You asked that we review the use of nuclear power

systems for Cassini and other space missions. As agreed with your office,

this report (1) describes the processes NASA used to assess the safety and

environmental risks associated with the Cassini mission, (2) describes

NASA’s efforts to consider the use of a non-nuclear power source for the

Cassini mission, (3) identifies the federal investment associated with the

development of non-nuclear power sources for deep space missions, and

(4) identifies NASA’s planned future nuclear-powered space missions. On

December 11, 1997, we briefed your staff on the results of our work. This

report discusses and updates the information presented at that briefing.

Background The Cassini Program, sponsored by NASA, the European Space Agency, and

the Italian Space Agency, began in fiscal year 1990. NASA’s Jet Propulsion

Laboratory (JPL), which is operated under contract by the California

Institute of Technology, manages the Cassini Program. The spacecraft is

expected to arrive at Saturn in July 2004 and begin a 4-year period of

scientific observations to obtain detailed information about the

composition and behavior of Saturn and its atmosphere, magnetic field,

rings, and moons. Power for the Cassini spacecraft is generated by three

radioisotope thermoelectric generators (RTG) that convert heat from the

natural radioactive decay of plutonium dioxide into electricity. The

spacecraft also uses 117 radioisotope heater units to provide heat for

spacecraft components. The spacecraft carries 72 pounds of radioactive

plutonium dioxide in the RTGs and 0.7 pounds in the heater units. The

Department of Energy (DOE) provided the RTGs and their plutonium

dioxide fuel, and the Department of Defense (DOD) provided the Titan

IV/Centaur rocket to launch the spacecraft.

According to NASA and JPL officials, most deep space missions beyond

Mars, including the Cassini mission, must use RTGs to generate electrical

power. The only proven non-nuclear source of electrical power for

Page 1 GAO/NSIAD-98-102 Spacecraft Power
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spacecraft are photovoltaic cells,1 also called solar arrays. However, as

distance from the sun increases, the energy available from sunlight

decreases exponentially. Thus, existing solar arrays cannot produce

sufficient electricity beyond Mars’ orbit to operate most spacecraft and

their payloads.

Before launching a spacecraft carrying radioactive materials, regulations

implementing federal environmental laws require the sponsoring agency,

in this instance NASA, to assess and mitigate the potential risks and effects

of an accidental release of radioactive materials during the mission. As

part of any such assessments, participating agencies perform safety

analyses in accordance with administrative procedures. To obtain the

necessary presidential approval to launch space missions carrying large

amounts of radioactive material, such as Cassini, NASA is also required to

convene an interagency review of the nuclear safety risks posed by the

mission.

RTGs have been used on 25 space missions, including Cassini, according to

NASA and JPL officials.2 Three of these missions failed due to problems

unrelated to the RTGs. Appendix I describes those missions and the

disposition of the nuclear fuel on board each spacecraft.

Results in Brief Federal laws and regulations require analysis and evaluation of the safety

risks and potential environmental impacts associated with launching

nuclear materials into space. As the primary sponsor of the Cassini

mission, NASA conducted the required analyses with assistance from DOE

and DOD. In addition, a presidential directive required that an ad hoc

interagency panel review the Cassini mission safety analyses. The directive

also required that NASA obtain presidential approval to launch the

spacecraft. NASA convened the required interagency review panel and

obtained launch approval from the Office of Science and Technology

Policy, within the Office of the President. While the evaluation and review

processes can minimize the risks of launching radioactive materials into

space, the risks themselves cannot be eliminated, according to NASA and

JPL officials.

1

Photovoltaic cells are electronic devices that convert sunlight into electricity. In locations with

sufficient sunlight, large numbers of interconnected cells are mounted on panels and used to provide

electrical power for spacecraft.

2

Two of the missions were launched on a single rocket.

Page 2 GAO/NSIAD-98-102 Spacecraft Power
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As required by NASA regulations, JPL considered using solar arrays as an

alternative power source for the Cassini mission. Engineering studies

conducted by JPL concluded that solar arrays were not feasible for the

Cassini mission primarily because they would have been too large and

heavy and had uncertain reliability.

During the past 30 years, NASA, DOE, and DOD have invested over

$180 million in solar array technology, the primary non-nuclear power

source. In fiscal year 1998, NASA and DOD will invest $10 million to improve

solar array systems, and NASA will invest $10 million to improve

nuclear-fueled systems. According to NASA and JPL officials, advances in

solar array technology may expand its use for some missions; however,

there are no currently practical alternatives to using nuclear-fueled power

generation systems for most missions beyond the orbit of Mars.

NASA is studying eight future deep space missions between 2000 and 2015

that will likely require nuclear-fueled power systems to generate electricity

for the spacecraft. None of these missions have been approved or funded,

but typically about one-half of such missions are eventually funded and

launched. Advances in nuclear-fueled systems and the use of smaller, more

efficient spacecraft are expected to substantially reduce the amount of

nuclear fuel carried on future deep space missions. Thus, NASA and JPL

officials believe these future missions may pose less of a health risk than

current and prior missions that have launched RTGs into space.

Safety, Environmental

Impact, and Launch

Approval Processes

for the Cassini

Mission

The processes used by NASA to assess the safety and environmental risks

associated with the Cassini mission reflected the extensive analysis and

evaluation requirements established in federal laws, regulations, and

executive branch policies. For example, DOE designed and tested the RTGs

to withstand likely accidents while preventing or minimizing the release of

the RTG’s plutonium dioxide fuel, and a DOE administrative order required

the agency to estimate the safety risks associated with the RTGs used for

the Cassini mission. Also, federal regulations implementing the National

Environmental Policy Act of 1969 required NASA to assess the

environmental and public health impacts of potential accidents during the

Cassini mission that could cause plutonium dioxide to be released from

the spacecraft’s RTGs or heater units.3 4

 In addition, a directive issued by

the Executive Office of the President requires an ad hoc interagency

3

42 U.S.C. 4321 et. seq., as amended.

4

14 C.F.R. part 1216, Environmental Quality—Procedures for Implementing the National

Environmental Policy Act, and 40 C.F.R. parts 1500 to 1508, Council on Environmental Quality.

Page 3 GAO/NSIAD-98-102 Spacecraft Power
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Nuclear Safety Review Panel.5 This panel is supported by technical experts

from NASA, other federal agencies, national laboratories, and academia to

review the nuclear safety analyses prepared for the Cassini mission. After

completion of the interagency review process, NASA requested and was

given nuclear launch safety approval by the Office of Science and

Technology Policy, within the Office of the President, to launch the

Cassini spacecraft.

In addition to the risks associated with a launch accident, there is also a

small chance that the Cassini spacecraft could release nuclear material

either during an accidental reentry into Earth’s atmosphere when the

spacecraft passes by Earth in August 1999 or during the interplanetary

journey to Saturn. Potential reentry accidents were also addressed during

the Cassini safety, environmental impact, and launch review processes.

RTG Safety Assessment

Process

DOE originally developed the RTGs used on the Cassini spacecraft for NASA’s

previous Galileo and Ulysses missions. Figure 1 shows the 22-foot,

12,400-pound Cassini spacecraft and some of its major systems, including

two of the spacecraft’s three RTGs.

5

Presidential Directive/National Security Council-25, paragraph 9, as amended on May 8, 1996.

Page 4 GAO/NSIAD-98-102 Spacecraft Power
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Figure 1: Cassini Spacecraft’s Major Components

4m High-gain

antenna

Low-gain

antenna (1 of 2)

Radar bay

Fields and

particles pallet

Huygens

Titan probe

Propellant tank

Thruster

cluster (4)

445 N engine (2)

Reaction

wheel (4)

Radioisotope

thermoelectric

generators (3)

Remote

sensing pallet

11m Magnetometer

boom

Source: NASA/JPL.

DOE designed and constructed the RTGs to prevent or minimize the release

of plutonium dioxide fuel from the RTG fuel cells in the event of an

accident. DOE performed physical and analytical testing of the RTG fuel

Page 5 GAO/NSIAD-98-102 Spacecraft Power
B-279348 

cells known as general-purpose heat source units, to determine their

performance and assess the risks of accidental fuel releases. Under an

interagency agreement with NASA, DOE constructed the RTGs for the Cassini

spacecraft and assessed the mission risks as required by a DOE

administrative order.6 DOE’s final safety report on the Cassini mission,

published in May 1997, documents the results of the test, evaluation, and

risk assessment processes for the RTGs.7

The RTG fuel cells have protective casings composed of several layers of

heat- and impact-resistant shielding and a strong, thin metal shell around

the fuel pellets. According to NASA and DOE officials, the shielding will

enable the fuel cells to survive likely types of launch or orbital reentry

accidents and prevent or minimize the release of plutonium dioxide fuel.

In addition to the shielding, the plutonium dioxide fuel itself is formed into

ceramic pellets designed to resist reentry heat and breakage caused by an

impact. If fuel is released from an impact-damaged fuel cell, the pellets are

designed to break into large pieces to avoid inhalation of very small

particles, which is the primary health risk posed by plutonium dioxide.

Cassini Environmental

Impact Assessment

Process

Federal regulations implementing the National Environmental Policy Act

of 1969 required NASA to prepare an environmental impact statement for

the Cassini mission.8

 To meet the requirements NASA conducted

quantitative analyses of the types of accidents that could cause a release of

plutonium dioxide from the RTGs and the possible health effects that could

result from such releases. NASA also used DOE’s RTG safety analyses and Air

Force safety analyses of the Titan IV/Centaur rocket, which launched the

Cassini spacecraft.

NASA published a final environmental impact statement for the Cassini

mission in June 1995. In addition to the analyses of potential

environmental impacts and health effects, the document included and

responded to public comments on NASA’s analyses. NASA also published a

final supplemental environmental impact statement for the Cassini mission

in June 1997. According to NASA officials, NASA published the supplemental

statement to keep the public informed of changes in the potential impacts

of the Cassini mission based on analyses conducted subsequent to the

publication of the final environmental impact statement. The supplemental

6

Department of Energy Order 5480.23, April 30, 1992.

7

GPHS-RTGs in Support of the Cassini Mission: Final Safety Analysis Report, Updated Executive

Summary, Department of Energy, May 1997.

8

See footnote 4.

Page 6 GAO/NSIAD-98-102 Spacecraft Power
B-279348 

statement used DOE’s updated RTG safety analyses to refine the estimates of

risks for potential accidents and document a decline in the overall

estimate of risk for the Cassini mission.9

The environmental impact assessment process for the Cassini mission

ended formally in August 1997 when NASA issued a Record of Decision for

the final supplemental environmental impact statement. However, if the

circumstances of the Cassini mission change and affect the estimates of

accident risks, NASA is required to reassess the risks and determine the

need for any additional environmental impact documentation.

Cassini Launch Approval

Process

Agencies planning to transport nuclear materials into space are required

by a presidential directive to obtain approval from the Executive Office of

the President before launch. To prepare for and support the approval

decision, the directive requires that an ad hoc Interagency Nuclear Safety

Review Panel review the lead agencies’ nuclear safety assessments.

Because the Cassini spacecraft carries a substantial amount of plutonium,

NASA convened a panel to review the mission’s nuclear safety analyses.

NASA formed the Cassini Interagency Nuclear Safety Review Panel shortly

after the program began in October 1989. The panel consisted of four

coordinators from NASA, DOE, DOD, the Environmental Protection Agency,

and a technical advisor from the Nuclear Regulatory Commission. The

review panel, supported by approximately 50 technical experts from these

and other government agencies and outside consultants, analyzed and

evaluated NASA, JPL, and DOE nuclear safety analyses of the Cassini mission

and performed its own analyses. The panel reported no significant

differences between the results of its analyses and those done by NASA, JPL,

and DOE.

The Cassini launch approval process ended formally in October 1997 when

the Office of Science and Technology Policy, within the Executive Office

of the President, gave its nuclear launch safety approval for NASA to launch

the Cassini spacecraft. NASA officials told us that, in deciding whether to

approve the launch of the Cassini spacecraft, the Office of Science and

Technology Policy reviewed the previous NASA, JPL, DOE, and review panel

analyses and obtained the opinions of other experts.

9

These analyses were part of an ongoing executive branch review process for the Cassini mission.

Page 7 GAO/NSIAD-98-102 Spacecraft Power
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Estimated Probabilities of

Accidents and Potential

Health Effects

NASA, JPL, and DOE used physical testing and computer simulations of the

RTGs under accident conditions to develop quantitative estimates of the

accident probabilities and potential health risks posed by the Cassini

mission. To put the Cassini risk estimates in context, NASA compares them

with the risks posed by exposure to normal background radiation. In

making this comparison, NASA estimates that, over a 50-year period, the

average person’s risk of developing cancer from exposure to normal

background radiation is on the order of 100,000 times greater than from

the highest risk accident for the Cassini mission.

Estimated Probabilities of

Launch Accidents and Potential

Health Effects

For the launch portion of the Cassini mission, NASA estimated that the

probability of an accident that would release plutonium dioxide was 1 in

1,490 during the early part of the launch and 1 in 476 during the later part

of the launch and Earth orbit. The estimated health effect of either type of

accident is that, over the succeeding 50-year period, less than one more

person would die of cancer caused by radiation exposure than if there

were no accident.

Estimated Probabilities of

Earth Swingby and

Interplanetary Trajectory

Accidents and Potential Health

Effects

Although the Titan IV/Centaur rocket is the United States’ most powerful

launch vehicle, it does not have enough energy to propel the Cassini

spacecraft on a direct route to Saturn. Therefore, the spacecraft will

perform two swingby maneuvers at Venus in April 1998 and June 1999, one

at Earth in August 1999, and one at Jupiter in December 2000. In

performing the maneuvers, the spacecraft will use the planets’ gravity to

increase its speed enough to reach Saturn. Figure 2 illustrates the Cassini

spacecraft’s planned route to Saturn.

Page 8 GAO/NSIAD-98-102 Spacecraft Power
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Figure 2: Cassini Trajectory From Earth to Saturn

Venus Swingby

26 April 1998

Orbit of

Earth

Deep Space

Maneuver

3 December 1998

Orbit of

Venus

Jupiter Swingby

30 December 2000

Saturn Arrival

1 July 2004

Launch

Earth Swingby 15 October 1997

18 August 1999

Venus Swingby

24 June 1999

Orbit of

Saturn

Orbit of

Jupiter

Source: NASA/JPL.

NASA estimates that there is less than a one in one million chance that the

spacecraft could accidentally reenter Earth’s atmosphere during the Earth

swingby maneuver. To verify the estimated probability of an Earth

swingby accident, NASA formed a panel of independent experts, which

reported that the probability estimates were sound and reasonable.

If such an accident were to occur, the estimated health effect is that,

during the succeeding 50-year period, 120 more people would die of

cancer than if there were no accident. If the spacecraft were to become

unable to respond to guidance commands during its interplanetary

journey, the spacecraft would drift in an orbit around the sun, from which

Page 9 GAO/NSIAD-98-102 Spacecraft Power
B-279348 

it could reenter Earth’s atmosphere in the future. However, the probability

that this accident would occur and release plutonium dioxide is estimated

to be one in five million. The estimated health effect of this accident is the

same as for an Earth swingby accident.

Due to the spacecraft’s high speed, NASA and DOE projected that an

accidental reentry during the Earth swingby maneuver would generate

temperatures high enough to damage the RTGs and release some plutonium

dioxide. As a safety measure, JPL designed the Earth swingby trajectory so

that the spacecraft will miss Earth by a wide margin unless the

spacecraft’s course is accidently altered. About 50 days before the

swingby, Cassini mission controllers will begin making incremental

changes to the spacecraft’s course, guiding it by Earth at a distance of

718.6 miles. According to NASA and JPL officials, the Cassini spacecraft and

mission designs incorporate other precautions to minimize the possibility

that an accident could cause the spacecraft to reenter during either the

Earth swingby maneuver or the interplanetary portion of its journey to

Saturn.

NASA’s Consideration

of a Non-Nuclear

Power Source for Its

Cassini Mission

NASA regulations require that, as part of the environmental analysis,

alternative power sources be considered for missions planning to use

nuclear power systems. JPL’s engineering study of alternative power

sources for the Cassini mission concluded that RTGs were the only

practical power source for the mission.10 The study stated that, because

sunlight is so weak at Saturn, solar arrays able to generate sufficient

electrical power would have been too large and heavy for the Titan

IV/Centaur to launch. The studies also noted that, even if the large arrays

could have been launched to Saturn on the Cassini spacecraft, they would

have made the spacecraft very difficult to maneuver and increased the

mission’s risk of failure due to the array’s uncertain reliability over the

length of the 12-year mission. Figure 3 compares the relative sizes of solar

arrays required to power the Cassini spacecraft at various distances from

the sun, including Saturn.

10Cassini Program Environmental Impact Statement Supporting Study, Alternate Mission and Power

Study, Jet Propulsion Laboratory, July 1994, Vol. 2.

Page 10 GAO/NSIAD-98-102 Spacecraft Power
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Figure 3: Relative Solar Array Sizes for the Cassini Spacecraft at Saturn, Jupiter, Mars, and Earth

Saturn

Earth

Jupiter

Mars

Source: NASA/JPL.

Investments in

Advanced Power

Generation Systems

Since 1968, NASA, DOE, and DOD have together invested more than

$180 million in solar array technology, according to a JPL estimate. The

agencies are continuing to invest in improving both solar and nuclear

spacecraft power generation systems. For example, in fiscal year 1998,

Page 11 GAO/NSIAD-98-102 Spacecraft Power
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NASA and DOD will invest $10 million for research and development of

advanced solar array systems, and NASA will invest $10 million for research

and development of advanced nuclear-fueled systems.

NASA officials in charge of developing spacecraft solar array power systems

said that the current level of funding is prudent, given the state of solar

array technology, and that the current funding meets the needs of current

agency research programs. The fiscal year 1998 budget of $10 million for

solar array systems exceeds the estimated 30-year average annual funding

level of $6 million (not adjusted for inflation).

According to NASA and JPL officials, solar arrays offer the most promise for

future non-nuclear-powered space missions. Two improvements to solar

array systems that are currently being developed could extend the range of

some solar array-powered spacecraft and science operations beyond the

orbit of Mars.11 New types of solar cells and arrays under development will

more efficiently convert sunlight into electricity. Current cells operate at

18 to 19 percent efficiency, and the most advanced cells under

development are intended to achieve 22 to possibly 30 percent efficiency.

Although the improvement in conversion efficiency will be relatively

small, it could enable some spacecraft to use solar arrays to operate as far

out as Jupiter’s orbit. Another improvement to solar arrays under

development will add lenses or reflective surfaces to capture and

concentrate more sunlight onto the arrays, enabling them to generate

more electricity. NASA’s technology demonstration Deep Space-1

spacecraft, scheduled for launch in July 1998, will include this new

technology.

Over the long term, limitations inherent to solar array technology will

preclude its use on many deep space missions. The primary limitation is

the diminishing energy in sunlight as distance from the sun increases. No

future solar arrays are expected to produce enough electricity to operate a

spacecraft farther than Jupiter’s orbit. Another key limitation is that solar

arrays cannot be used for missions requiring operations in extended

periods of darkness, such as those on or under the surface of a planet or

moon. Other limitations of solar arrays, including their vulnerability to

damage from radiation and temperature extremes, make the cells

unsuitable for missions that encounter such conditions.

11Some spacecraft use solar array power systems beyond the orbit of Mars (e.g., the Near Earth

Asteroid Rendezvous mission) on their way to perform science operations at targets closer to the sun.

When traveling beyond Mars, solar-powered spacecraft operate in only a low-power coasting mode

and perform few or no science operations, which does not demand much power.

Page 12 GAO/NSIAD-98-102 Spacecraft Power
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NASA and DOE are working on new nuclear-fueled generators for use on

future space missions. NASA and DOE’s Advanced Radioisotope Power

Source Program is intended to replace RTGs with an advanced

nuclear-fueled generator that will more efficiently convert heat into

electricity and require less plutonium dioxide fuel than existing RTGs. NASA

and DOE plan to flight test a key component of the new generator on a

space shuttle mission. The test system will use electrical power to provide

heat during the test. If development of this new generator is successful, it

will be used on future missions.

Future

Nuclear-Powered

Space Missions

NASA is currently studying eight future space missions between 2000 and

2015 that will likely use nuclear-fueled electrical generators. These

missions are Europa Orbiter, Pluto Express, Solar Probe, Interstellar

Probe, Europa Lander, Io Volcanic Observer, Titan Organic Explorer, and

Neptune Orbiter. On the basis of historical experience, NASA and DOE

officials said that about one-half of such missions typically obtain funding

and are launched. In addition, several planned Mars missions would carry

from 5 to 30 radioisotope heater units to keep spacecraft components

warm.12 Each heater unit would contain about 0.1 ounces of plutonium

dioxide.

In accordance with NASA’s current operating philosophy, spacecraft for

future space science missions will be much smaller than those used on

current deep space missions. Future spacecraft with more efficient

electrical systems and reduced demands for electrical power, when

coupled with the advanced nuclear-fueled generators, will require

significantly less plutonium dioxide fuel. For example, the new

nuclear-fueled generator that NASA studied for use on the Pluto Express

spacecraft is projected to need less than 10 pounds of plutonium dioxide

compared with 72 pounds on the Cassini spacecraft. According to NASA

and DOE officials, spacecraft carrying much smaller amounts of radioactive

fuel will reduce human health risks because it is anticipated that less

plutonium dioxide could potentially be released in the event of an

accident.

NASA and JPL officials also pointed out that planned future missions may

not need to use Earth swingby trajectories. Depending on the launch

vehicle used, the smaller spacecraft planned for future missions may be

12The Mars 2001 and 2003 missions will carry between 5 and 8 heater units each, and the Mars 2004

mission will carry approximately 30 heater units.

Page 13 GAO/NSIAD-98-102 Spacecraft Power
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able to travel more direct routes to their destinations without the need to

use Earth swingby maneuvers to increase their speed.

Agency Comments In written comments on a draft of this report, NASA said that the report

fairly represents NASA’s environmental and nuclear safety processes for the

Cassini space mission (see app. II). In addition, NASA and DOE also provided

technical and clarifying comments for this report, which we incorporated

as appropriate.

Scope and

Methodology

To obtain information about the processes used by NASA to assess the

safety and environmental risks of the Cassini mission, NASA’s efforts and

costs to develop non-nuclear power sources for deep space missions, and

future space missions for which nuclear-fueled power sources will be

used, we interviewed officials at NASA Headquarters in Washington, D.C.;

JPL in Pasadena, California; and DOE’s Office of Nuclear Energy, Science,

and Technology in Germantown, Maryland. We reviewed the primary U.S.

legislation and regulations applicable to the use of nuclear materials in

space and NASA, JPL, and DOE documents pertaining to the safety and

environmental assessment processes that were used for the Cassini

mission. We reviewed the Cassini Safety Evaluation Report prepared by

the Cassini Interagency Nuclear Safety Review Panel. We also reviewed

NASA and JPL documents on the development of improved non-nuclear and

nuclear electrical power sources for spacecraft and studies for future

nuclear-powered space missions. We did not attempt to verify NASA and

DOE estimates of risks associated with the Cassini mission or the financial

and other data provided by the agencies.

We performed our work from September 1997 to February 1998 in

accordance with generally accepted government auditing standards.

We are sending copies of this report to the Director of the Office of

Management and Budget, the Administrator of NASA, the Secretary of

Energy, and appropriate congressional committees. We will also make

copies available to other interested parties on request.

Page 14 GAO/NSIAD-98-102 Spacecraft Power
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Please contact me at (202) 512-4841 if you or your staff have any questions

concerning this report. Major contributors to this report are Jerry Herley

and Jeffery Webster.

Sincerely yours,

Allen Li

Associate Director

Defense Acquisitions Issues

Page 15 GAO/NSIAD-98-102 Spacecraft Power
Contents

Letter 1

Appendix I 

Past Nuclear-Powered

Space Missions

18

Appendix II 

Comments From the

National Aeronautics

and Space

Administration

21

Figures Figure 1: Cassini Spacecraft’s Major Components 5

Figure 2: Cassini Trajectory From Earth to Saturn 9

Figure 3: Relative Solar Array Sizes for the Cassini Spacecraft at

Saturn, Jupiter, Mars, and Earth

11

Figure I.1: RTG Fuel Cells During Underwater Recovery

Operation

19

Abbreviations

DOD Department of Defense

DOE Department of Energy

JPL Jet Propulsion Laboratory

NASA National Aeronautics and Space Administration

RTG radioisotope thermoelectric generators

Page 16 GAO/NSIAD-98-102 Spacecraft Power
Page 17 GAO/NSIAD-98-102 Spacecraft Power
Appendix I 

Past Nuclear-Powered Space Missions

Since 1961 the United States has launched 25 spacecraft with radioisotope

thermoelectric generators (RTG) on board. Three of the missions failed,

and the spacecraft reentered Earth’s atmosphere. However, none of the

failures were due to problems with the RTGs.

In 1964, a TRANSIT 5BN-3 navigational satellite malfunctioned. Its single

RTG, which contained 2.2 pounds of plutonium fuel, burned up during

reentry into Earth’s atmosphere. This RTG was intended to burn up in the

atmosphere in the event of a reentry.

In 1968, a NIMBUS-B-1 weather satellite was destroyed after its launch

vehicle malfunctioned. The plutonium fuel cells from the spacecraft’s two

RTGs were recovered intact from the bottom of the Santa Barbara Channel

near the California coast. According to National Aeronautics and Space

Administration (NASA) and Department of Energy (DOE) officials, no

radioactive fuel was released from the fuel cells, and the fuel was recycled

and used on a subsequent space mission. Figure I.1 shows the intact fuel

cells during the underwater recovery operation.

Page 18 GAO/NSIAD-98-102 Spacecraft Power
Appendix I 

Past Nuclear-Powered Space Missions

Figure I.1: RTG Fuel Cells During Underwater Recovery Operation

Source: Jet Propulsion Laboratory.

Page 19 GAO/NSIAD-98-102 Spacecraft Power
Appendix I 

Past Nuclear-Powered Space Missions

In 1970, the Apollo 13 Moon mission was aborted due to mechanical

failures while traveling to the moon. The spacecraft and its single RTG,

upon return to Earth, were jettisoned into the Pacific Ocean, in or near the

Tonga Trench. According to DOE officials, no release of radioactive fuel

was detected.

Page 20 GAO/NSIAD-98-102 Spacecraft Power
Appendix II 

Comments From the National Aeronautics

and Space Administration

(707327) Page 21 GAO/NSIAD-98-102 Spacecraft Power
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