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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