Subject: [NSIAD-98-102] Space Exploration: Power Sources for Deep Space Probes 

From: info@www.gao.gov 

Date: 4 Jun 1998 06:42:59 -0400 

Organization: US General Accounting Office 

Reply-to: gov-us-fed-congress-gao-reports@news.govnews.org 
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Space Exploration: Power Sources for Deep Space Probes (Letter Report, 
05/29/98, GAO/NSIAD-98-102). 

Pursuant to a congressional request, GAO reviewed the use of nuclear 
power systems for the Cassini spacecraft and other space missions, 
focusing on: (1) the processes the National Aeronautics and Space 
Administration (NASA) used to assess the safety and environmental risks 
associated with the Cassini mission; (2) NASA's efforts to consider the 
use of a non-nuclear power source for the Cassini mission; (3) the 
federal investment associated with the development of non-nuclear power 
sources for deep space missions; and (4) NASA's planned future 
nuclear-powered space missions. 

GAO noted that: (1) federal laws and regulations require analysis and 
evaluation of the safety risks and potential environmental impacts 
associated with launching nuclear materials into space; (2) as the 
primary sponsor of the Cassini mission, NASA conducted the required 
analyses with assistance from the Department of Energy (DOE) and the 
Department of Defense (DOD); (3) in addition, a presidential directive 
required that an ad hoc interagency panel review the Cassini mission 
safety analyses; (4) the directive also required that NASA obtain 
presidential approval to launch the spacecraft; (5) 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; (6) 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 Jet Propulsion 
Laboratory (JPL) officials; (7) as required by NASA regulations, JPL 
considered using solar arrays as an alternative power source for the 
Cassini mission; (8) engineering studies conducted by JPL concluded that 
the solar arrays were not feasible for the Cassini mission primarily 
because they would have been too large and heavy and had uncertain 
reliability; (9) during the past 30 years, NASA, DOE, and DOD have 
invested over $180 million in solar array technology, the primary 
non-nuclear power source; (10) in FY 1998, NASA and DOD will invest $10 
million to improve solar array systems, and NASA will invest $10 million 
to improve nuclear-fueled systems; (11) 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; (12) NASA is planning eight future deep space 
missions between 2000 and 2015 that will likely require nuclear-fueled 
power systems to generate electricity for the spacecraft; (13) none of 
these missions have been approved or funded, but typically about 
one-half of such planned missions are eventually funded and launched; 
(14) 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; and (15) thus, NASA 
and JPL officials believe these future missions may pose less of a 
health risk than current and prior missions that have launched radio 
isotope thermoelectric generators into space. 

--------------------------- Indexing Terms ----------------------------- 

REPORTNUM: NSIAD-98-102 
TITLE: Space Exploration: Power Sources for Deep Space Probes 
DATE: 05/29/98 
SUBJECT: Aerospace research 
Space exploration 
Nuclear energy 
Energy research 
Environmental impact statements 
Hazardous substances 
Safety standards 
Radioactive wastes 
IDENTIFIER: Titan IV Rocket 
NASA Cassini Saturn Probe Program 

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Cover 
================================================================ COVER 


Report to the Honorable 
Barbara Boxer, U.S. Senate 

May 1998 

SPACE EXPLORATION - POWER SOURCES 
FOR DEEP SPACE PROBLEMS 

GAO/NSIAD-98-102 

Spacecraft Power 

(707327) 


Abbreviations 
=============================================================== ABBREV 


Letter 
=============================================================== LETTER 


B-279348 

Letter Date Goes Here 

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 
------------------------------------------------------------ Letter :1 

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 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 a spacecraft and its payload. 

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. 


-------------------- 
\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. 


RESULTS IN BRIEF 
------------------------------------------------------------ Letter :2 

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. 

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 planning 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 planned 
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 
------------------------------------------------------------ Letter :3 

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 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. 


-------------------- 
\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. 

\5 Presidential Directive/National Security Council-25, paragraph 9, 
as amended on May 8, 1996. 


RTG SAFETY ASSESSMENT 
PROCESS 
---------------------------------------------------------- Letter :3.1 

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. 

Figure 1: Cassini Spacecraft's 
Major Components 

(See figure in printed 
edition.) 

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 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. 


-------------------- 
\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. 


CASSINI ENVIRONMENTAL IMPACT 
ASSESSMENT PROCESS 
---------------------------------------------------------- Letter :3.2 

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 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. 


-------------------- 
\8 See footnote 4. 

\9 These analyses were part of an ongoing executive branch review 
process for the Cassini mission. 


CASSINI LAUNCH APPROVAL 
PROCESS 
---------------------------------------------------------- Letter :3.3 

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. 


ESTIMATED PROBABILITIES OF 
ACCIDENTS AND POTENTIAL 
HEALTH EFFECTS 
---------------------------------------------------------- Letter :3.4 

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 
-------------------------------------------------------- Letter :3.4.1 

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 
-------------------------------------------------------- Letter :3.4.2 

Although the Titan IV/Centaur rocket is the U.S.'s 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. 

Figure 2: Cassini Trajectory 
From Earth to Saturn 

(See figure in printed 
edition.) 

Source: NASA/JPL. 

NASA estimates that there is less than a 1 in 1 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 analyses and 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 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 1 in 5 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 
------------------------------------------------------------ Letter :4 

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. 

Figure 3: Relative Solar Array 
Sizes for the Cassini 
Spacecraft at Saturn, Jupiter, 
Mars, and Earth 

(See figure in printed 
edition.) 

Source: NASA/JPL. 


-------------------- 
\10 Cassini Program Environmental Impact Statement Supporting Study, 
Alternate Mission and Power Study, Jet Propulsion Laboratory, July 
1994, Vol. 2. 


INVESTMENTS IN ADVANCED POWER 
GENERATION SYSTEMS 
------------------------------------------------------------ Letter :5 

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, 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. 

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. 


-------------------- 
\11 Some 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. 


FUTURE NUCLEAR-POWERED SPACE 
MISSIONS 
------------------------------------------------------------ Letter :6 

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 able to travel more direct routes to their 
destinations without the need to use Earth swingby maneuvers to 
increase their speed. 


-------------------- 
\12 The 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. 


AGENCY COMMENTS 
------------------------------------------------------------ Letter :7 

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 
------------------------------------------------------------ Letter :8 

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.; the Jet Propulsion Laboratory 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. 


---------------------------------------------------------- Letter :8.1 

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 

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 


PAST NUCLEAR-POWERED SPACE 
MISSIONS 
=========================================================== Appendix I 

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 beneath the Pacific Ocean 
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. 

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

(See figure in printed 
edition.) 

Source: Jet Propulsion Laboratory. 

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. 



(See figure in printed edition.)Appendix II 

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