Singh N. (ed.) Radioisotopes - Applications in Physical Sciences
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Radioisotope Power: A Key Technology for Deep Space Exploration
449
Fig. 25. Cassini spacecraft. Pre-launch checkout of RTG on left. Artist concept of vehicle on
right.
The most recent mission to use a GPHS-RTG is the New Horizons mission to Pluto (Fig. 26),
which was launched on 19 January 2006 aboard an Atlas V 551. The spacecraft is currently
on a 9.5-year transit to Pluto and Charon. At encounter, which is expected in July 2015,
New Horizons will characterize and map the surfaces of Pluto and Charon and their
atmospheres. From 2016 to 2020, the spacecraft will continue to conduct encounters with
one or two Kuiper Belt Objects. So far, it is anticipated that the RTG will exceed its power
and lifetime requirements.
Fig. 26. New Horizons spacecraft. Pre-launch integration with spacecraft on left. Artist
concept of New Horizons flyby of Pluto and Charon on right.
4.9 Multi-mission RTG (MMRTG)
Although the GPHS-RTG served well on Ulysses and Galileo and continues to meet
requirements for Cassini and New Horisons, it is not suitable for future missions on Mars and
other planetary bodies with atmospheres. The GPHS-RTG was only designed to function
effectively in a vacuum environment. Furthermore, its relatively large size and power level
limit its modularity and ease of integration on future small to mid-size spacecraft.
DOE and NASA are currently developing a new generation of RPS generators that could be
used for a variety of space missions. One is the Multi-Mission RTG (MMRTG), which has
Radioisotopes – Applications in Physical Sciences
450
been designed to operate on planetary bodies with atmospheres, such as Mars, as well as in
the vacuum of space. The MMRTG’s smaller size of about 110 We is more modular in
design and flexible in meeting the needs of a broader range of different missions as it
generates electrical power in smaller increments. The design goals for the MMRTG include
ensuring a high degree of safety and reliability, optimizing power levels over a minimum
lifetime of 14 years, and minimizing mass.
The MMRTG (Fig. 27) is designed to use a heat source consisting of eight Step 2 GPHS
modules. These Step 2 modules have additional material in the GPHS aeroshell that
improves structural integrity and performance. Although the Pb-Te/TAGS thermoelectric
materials are the same as those used on SNAP-19, and represent a thoroughly flight proven
technology, the physical dimensions and material changes to improve performance have
resulted in different degradation compared to the SNAP-19. The MMRTG generator has a
fin span of 64 cm, a length of 66 cm, and a mass of about 45 kg. Its BOM power level of
approximately 110 We yields a specific power that is less than the SNAP-19. However, the
purpose in pursuing this unit is not to advance state-of-the-art in specific power, but to
minimize development risk, while providing an RPS capable of operating in different
mission environments.
Fig. 27. Multi-Mission RTG (MMRTG). Cutaway schematic of power unit on left. MMRTG
Qualification Unit undergoing tests on right.
The MMRTG is being developed to serve as the primary power source on the Mars Science
Laboratory (MSL), a concept of which is shown in Fig. 28. This mission is currently planned
for launch in 2011, and is anticipated to land on Mars in 2012.
Fig. 28. Mars Science Laboratory.
Radioisotope Power: A Key Technology for Deep Space Exploration
451
MSL is considerably larger than the Mars Exploration Rovers that landed on the planet in
2004. It will carry more advanced scientific instruments than any other Mars mission to
date, including analysis of samples scooped up from the soil and drilled powders from
rocks. It will also investigate the past and present ability of Mars to support life. The MSL
rover will use power from an MMRTG to supply heat and electricity for its components and
science instruments. A coolant loop and heat exchanger coupled with the MMRTG
radiators will transport waste heat to the electronics, thus extending operation of the rover
into the Martian night and winter season. The goal is to operate for at least one Martian
year (i.e., two Earth years) over a wide range of possible landing sites.
The MMRTG could be used on a number of other potential missions in the future. One
exciting prospect is to use the MMRTG as the principal electrical power and heat source for
a Titan aerobot/balloon mission (Fig. 29). In this scenario, the considerable waste heat
produced by the MMRTG would be used to heat a gas and generate buoyancy for a balloon
carrying a long-lived payload, in addition to providing electrical power to onboard
instruments.
Fig. 29. Titan Aerobot.
4.10 Advanced Stirling Radioisotope Generator (ASRG)
When the potential of radioisotope power became apparent in the 1950s, the original focus
was on development of dynamic power conversion systems. Most of these activities
concentrated on applying the high efficiencies achievable with Brayton and Rankine cycles,
in expectation that systems would evolve to larger power levels in the future.
Although thermoelectric technology supplanted this approach and became the dominant
power conversion option for every RPS flown in space, work on Dynamic Isotope Power
Systems (DIPS) continued at various times throughout the intervening decades. The
principal focus of these efforts was on eventual development of power systems capable of
producing up to tens of kilowatts of power. These higher power technologies would be
used in conjunction with the ambitious crewed missions anticipated in the future. The
studies of DIPS pointed to its excellent suitability for lunar and planetary surface
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452
exploration, particularly surface rovers, remote science stations and backup power supplies
to central base power.
Interest in DIPS was particularly high during the Space Exploration Intitiative (SEI) of the
early-1990s. However with the demise of that effort in 1992, the focus shifted to
determine how dynamic power conversion could benefit radioiosotope power systems in
the multi-hundred watt range. During the 1990s, several advanced dynamic and static
conversion technologies were researched and evaluated. Several technologies that had
appeared promising initially proved to be ill-suited for the unique demands of deep space
missions. In the end, it became apparent that the free-piston Stirling engine offered the
best hope of advancing the efficiency of future generators, while offering lifetimes up to a
decade or two. Unlike previous DIPS designs, which featured turbomachinery-based
conversion technologies (e.g. Brayton), small Stirling DIPS could be advantageously
scaled down to multihundred-watt unit size while preserving size and mass
competitiveness with RTGs.
In 2002, NASA and DOE began a Stirling Radioisotope Generator (SRG) project focused on
evaluating and demonstrating a unit for flight development. The work was initiated to
provide a back-up RPS for the MSL mission. The unit used Stirling convertors built and
tested under a technology development effort funded by DOE. Although the SRG could
achieve a four-fold reduction in fuel requirements for the same power, the final system
specific power of the unit was only slightly better than the MMRTG.
In less than two years, it became apparent that the MMRTG would be selected by NASA’s
Mars program, so that the rover could make use of the significant waste heat produced by
that unit. Finally, a small business technology project initiated in the early 2000s with
Sunpower Technologies in Athens, Ohio, indicated that convertors with much better mass
performance could be developed and substituted into an SRG-based design. Such a unit
could potentially achieve specific powers of about 7 We/kg. With the advancement in
Stirling generator heater head materials and with improved temperature margin and
higher temperature operation, units with specific powers greater than 8 We/kg may be
possible..
In 2005, the decision was made to redirect efforts toward development of an Advanced SRG
(ASRG) technology demonstration Engineering Unit (EU). The effort drew upon the work
that had gone on previously with the controller, housing and insulation systems for the
SRG, but incorporated use of the higher specific power Sunpower generators. In addition to
high specific power, the ASRG would likely achieve an efficiency over 30%. This is four to
five times higher than that from a GPHS-RTG, and is particularly important for conserving
the very limited worldwide supply of Pu-238 fuel.
The ASRG, which is shown in Fig. 30, is being developed under the joint sponsorship of
the U.S. Department of Energy (DOE) And NASA. The eventual flight units are expected
to produce over 130 We in a space environment and to have a mass of 32 kg or less. The
prime contractor is Lockheed-Martin Corporation of Valley Forge, PA, with Sunpower,
Inc. of Athens, Ohio as the main subcontractor. NASA Glenn Research Center (GRC) is
supporting the technology development, along with evaluation and testing of the Stirling
convertors used in the device. In addition to improving fuel utilization efficiency over
previous RPS, the ASRG is being designed for multi-mission use in deep space, and
within the atmosphere of Mars and possibly Titan.
Radioisotope Power: A Key Technology for Deep Space Exploration
453
Fig. 30. Advanced Stirling Radioisotope Generator (ASRG).
Activities are focused on developing and testing the ASRG-EU in thermal and vibrational
environments that closely approximate qualification-level tests (Fig. 31).
The ASRG-EU uses two axially-opposed Advanced Stirling Convertors (ASCs), operating at
a hot-end temperature of 650 deg C, producing about 140 We. Sunpower is developing the
ASC under a 2002 NASA Research Announcement (NRA) with GRC. The low mass of the
ASC is key to the ASRG’s high overall system specific power.
The ASRG has achieved a TRL 6 (system demonstration in a relevant environment) with
operation at qualification level thermal and dynamic environments. Tests on the ASRG-EU
were completed in June 2008 at the Lockheed-Martin Space System Company in King of
Prussia, PA. These evaluations included thermal balance, thermal performance, mechanical
disturbance, sine transient, random vibration, simulated pyrotechnic shock and
electromagnetic interference and magnetic field emission tests. Over 1,000 hours of successful
EU operating time with numerous startup and shutdown cycles were accumulated during the
testing at Lockheed-Martin. The ASRG-EU is now undergoing extended/multi-year duration
testing at NASA GRC. It has achieved over 11,000 hours of successful operation as of April
2011, and is expected to exceed 14,000 hours of operation by the end of 2011.
Ongoing ASRG-EU tests use electrical resistance heaters that simulate the heating
characteristics of the actual GPHS module. Avoiding use of nuclear materials during
early phases of development greatly facilitates testing and evaluation of the ASRG
subsystems.
Radioisotopes – Applications in Physical Sciences
454
Fig. 31. ASRG Engineering Unit.
5. Other potential applications
MMRTG and ASRG should satisfy most RPS mission requirements well beyond 2010,
particularly for those applications involving several hundred watts of power. However,
there will likely be a demand for additional types of units in the future. One potential need
identified by the space science community is for small RPS units ranging in power from ~10
milliwatts (mW
e
) to ~20 W
e
. These so-called ‘milliwatt’ and ‘multiwatt-class’ power
supplies could extend the capability of small, low cost missions supported through NASA’s
small to mid-size programs, and augment human missions involving deployment of
monitoring stations and autonomous devices. They would likely utilize the GPHS or other
existing heat sources. Although flight-qualified systems in this size range do not presently
exist, the promise of RPS has led NASA and DOE to evaluate the possible development of a
small RPS unit in the future.
Radioisotope Power: A Key Technology for Deep Space Exploration
455
Nuclear Electric Propulsion (NEP) has been studied since the early 1960’s because of its
potential for future high-energy space missions. Almost all NEP assessments to date have
assumed fission as the nuclear energy source. Unlike solar-powered electric propulsion
(SEP) systems, NEP operation is generally independent of distance and orientation with
respect to the Sun. Over the last decade, several studies have pointed to Radioisotope Power
Systems (RPS), instead of reactor power sources, as the best way of implementing NEP.
Radioisotope-based NEP, also known as Radioisotope Electric Propulsion (REP), has been
evaluated before, but has not been seriously considered for flight due to the low specific
power range of traditional RPS (e.g., 3 to 5 We/kg). However, the prospects for REP have
improved substantially with the advent of the ASRG and its likely improvement in specific
power.
In this capacity, REP would principally be used as an interplanetary stage for long-duration
deceleration and acceleration in deep space. At remote destinations, REP would perform
deceleration, orbit insertion and maneuvers around outer planets and other planetary
bodies. REP-based spacecraft could also provide ample power at destination for
sophisticated science instruments and communications, but it would fit better within the
relatively modest kilowatt-scale power requirements of the space science community.
6. Conclusion
Radioisotope power systems will continue to play an important role in NASA’s exploration
efforts. These systems also have the potential for use in a variety of new applications, which
would benefit from the technology’s versatility in a broad range of space and planetary
environments. In the near-term, the MMRTG will expand the capability for conducting
science on the surface of Mars. The ASRG will enable even higher performance missions.
These units will also enable more ambitious exploration of other planetary surfaces and
provide a reliable means of powering spacecraft in deep space. Current activities would
also allow the potential development of new systems that could expand application of RPS
to smaller science missions. The key to successful implementation of RPS is to maintain
close ties with potential users and the science community at large. With these
advancements, radioisotope power systems and technology will offer tremendous benefits
for future exploration endeavors.
7. References
Angelo, J.A. and Buden, D., Space Nuclear Power, Orbit Book Co., Malabar, FL, 1985, pp. 133-
157.
Bennett, G.L., “Space Nuclear Power: Opening the Final Frontier,” AIAA-2006-4191, 4th
International Energy Conversion Engineering Conference, June 2006.
Bennett, G.L., and Skrabek, E.A., “Power Performance of U.S. Space Radioisotope
Thermoelectric Generators,” in proceedings of 15th International Conference on
Thermoelectrics, June 1996, pp. 357-372.
Chan, J., Hill, D., Hoye, T., and Leland, D., “Development of Advanced Stirling
Radioisotope Generator for Planetary Surface and Deep Space Missions,” AIAA-
2009-5768, 6
th
International Energy Conversion Engineering Conference, July 28-30,
2009.
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Engler, R.E., “Atomic Power in Space – A History,” U.S. Department of Energy Report DE-
AC01-NE32117, March 1987.
Furlong, R.R. and Wahlquist, E.J., “U.S. Space Missions Using Radioisotope Power
Systems,” Nuclear News, April 1999, pp. 26-34.
Hammel, T.E., and Osmeyer, W.E., “The Selendie Isotope Generators,” AIAA-1997-498,
Conference on the Future of Aerospace Power Systems, March 1977.
Hammel, T.E., Bennett, R., Otting, W., and Fanale, S., “Multi-Mission Radioisotope
Thermoelectric Generator (MMRTG) and Performance Prediction Model,” AIAA-
2009-4576, 7
th
International Energy Conversion Engineering Conference, August 2-
5, 2009.
Lange, R.G. and Mastal, E.F., “A Tutorial Review of Radioisotope Power Systems,” in A
Critical Review of Space Nuclear Power and Propulsion, edited by M.S. El-Genk,
American Institute of Physics, Melville, New York, 1994, pp. 1-20.
National Research Council, New Frontiers in the Solar System – An Integrated Exploration
Strategy, National Academies Press, Washington, DC, 2003a.
National Research Council, The Sun to the Earth – and Beyond: Panel Reports, National
Academies Press, Washington, DC, 2003b.
Schmidt, G., Wiley, R., Richardson, R. and Furlong, R., “NASA’s Program for Radioisotope
Power System Research and Development,” in proceedings of Space Technology and
Applications International Forum (STAIF-2005), edited by M.S. El-Genk, American
Institute of Physics, Melville, New York, 2005.
Schmidt, G., Abelson, R., and Wiley, R., “Benefit of Small Radioisotope Power Systems for
NASA Exploration Missions,” in proceedings of Space Technology and Applications
International Forum (STAIF-2005), edited by M.S. El-Genk, American Institute of
Physics, Melville, New York, 2005.
Surampudi, R., Carpenter, R., El-Genk, M., Herrera, L., Mason, L., Mondt, J., Nesmith, B.,
Rapp, D. and Wiley, R., Advanced Radioisotope Power Systems Report, Jet Propulsion
Laboratory, Pasadena, CA, March 2001.
Wiley, R. and Carpenter, R., “Small Radioisotope Power Source Concepts,” in proceedings
of Space Technology and Applications International Forum (STAIF-2004), edited by M.S.
El-Genk, American Institute of Physics, Melville, New York, 2004.
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Inc. under Contract DE-AC01-NE32117, 2
nd
ed., Springer-Verlag, New York, 1983,
Chaps. 7, 14.
21
Radioisotope Power Systems
for Space Applications
Antonio Sanchez-Torres
Universidad Politécnica de Madrid, Escuela Técnica Superior de Ingenieros Aeronáutcos,
Departamento de Física Aplicada,
Spain
1. Introduction
At the beginning of the Space Age, both propulsion and power generation in the spacecraft
has been the main issue for consideration. Considerable research has been carried out on
technologies by several Space Agencies to reach outer planets and generate electric power
for the systems and subsystems in the spacecraft (SC). Various types of power source such
as solar photovoltaic, Radioisotope power systems (RPS) have been used by Space Agencies.
New technology such as reactor based, electric solar sail and electrodynamic bare tethers
might be used in the future for both propulsion and power generation. Mainly, both NASA
and Russian Agency worked separately using nuclear technology to obtain more efficiency
in their systems for deep space exploration.
Radioisotope Power Systems (RPS), is a nuclear-powered system to generate electric power
to feed communication and scientific systems on a spacecraft. Radioisotope Thermoelectric
Generators (RTGs), a type of Radioisotope Power System, were used in the past as electric
power supplies for some navigational and meteorological missions, and most outer-planet
missions. Radioisotope power systems use the natural decay of radionuclides produced by a
nuclear reactor. The expensive, man-made Plutonium-238 (
238
Pu) is the appropriate source
of energy used in RPS fueling; its long half-life (~87 years) guarantees long time missions.
The limited avability of Plutonium-238 is inadequate to support scheduled NASA mission
beyond 2018. After the Cold War, throughout the Non-Proliferation of Nuclear Weapons
Treaty, the production and processing of these resources have been severally reduced. There
is a high-priority recommendation to reestablish production to solve the severe
238
Pu
demand problem (National Reseach Council, 2009).
The isotope initially selected for terrestrial and space power applications was Cerium-144
because it is one of the most useful fission products available from nuclear reactor (Furlog,
1999; Lange, 2008). Its short half-life (about 290 days) made Cerium-144 compatible with a
possible short-time mission. However, the high radiation associated with a powerful
beta/gamma emission produces several problems with the payload interaction and safety in
the case of reentry orbit. The development of RTGs was assigned to The Atomic Energy
Commission in 1955. The first system developed for space situation was the System for
Nuclear Auxiliary Power (SNAP). The Cerium-144 fueled SNAP-1 power system was never
used in space. The first flight with a RTG was SNAP-3 in 1961 delivering 11.6 kW over a 280
days period, using as fueling Polonium-210 (Po-210) isotope. Po-210 is an alpha emitter with
Radioisotopes – Applications in Physical Sciences
458
a very high power density and low radiation emissions. Since Po-210 has short half-life (138
days), space missions are highly limited. The early RTGs developed a specific power slightly
larger than 1 W/kg. SNAP-9A system reached 20 W/kg whereas later systems such as
Galileo developed 5.4 W/kg (Brown, 2001; Griffin, 2004). Several past missions have used
RPS as it shown in Table 1. Table 2 shows several future missions that will use RPS as main
power system.
Power source (number) Spacecraft Mission type Launch
SNAP-3 RTG (1) Transit 4A Navigational 1961
SNAP-3 RTG (1) Transit 4B Navigational 1961
SNAP-9A RTG (1) Transit 5BN-1 Navigational 1963
SNAP-9A RTG (1) Transit 5BN-2 Navigational 1963
SNAP-9A RTG (1) Transit 5BN-3 Navigational 1964
SNAP-10A Reactor Snapshot Experimental 1965
SNAP-19B RTG (2) Nimbus B-1 Meteorological 1968
SNAP-19B RTG (2) Nimbus III Meteorological 1969
ALRH Heater Apollo 11 Lunar 1969
SNAP-27 RTG (1) Apollo 12 Lunar 1969
SNAP-27 RTG (1) Apollo 13 Lunar 1970
SNAP-27 RTG (1) Apollo 14 Lunar 1971
SNAP-27 RTG (1) Apollo 15 Lunar 1971
SNAP-19 RTG (4) Pioneer 10 Planetary 1972
SNAP-27 RTG (1) Apollo 16 Lunar 1972
Transit-RTG (1) Triad-01-1X Navigational 1972
SNAP-27 RTG (1) Apollo 17 Lunar 1972
SNAP-19 RTG (4) Pioneer 11 Planetary 1973
SNAP-19 RTG (2) Viking 1 Planetary 1975
SNAP-19 RTG (2) Viking 2 Planetary 1975
MHW-RTG (4) LES 8, LES 9 Communication 1976
MHW-RTG (3) Voyager 2 Planetary 1977
MHW-RTG (3) Voyager 1 Planetary 1977
GPHS-RTG (2) RHU Heater Galileo Planetary 1989
GPHS-RTG (1) Ulysses Planetary 1990
RHU Heater (3) Mars Pathfinder Planetary 1996
GPHS-RTG (2) RHU Heater Cassini Planetary 1997
RHU Heater (8) Mars MER Spirit Mars rover 2003
RHU Heater (8)
Mars MER
Opportunity
Mars rover 2003
GPHS-RTG (1) New Horizons Planetary 2006
Table 1. US spacecraft with RPS
The RTG fuel must be produced in adequate quantities with appropriate nuclear safety
requirements for space missions. There are only a limited number of radioisotopes available
for space power system applications. Using isotopes with pure low-energy beta emission
would eliminate the requirements to shield against gamma radiation. Low energy particles