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RPS: Radioisotope Power Systems
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Explore with radioisotope-powered missions in 3D. EYES on the SOLAR SYSTEM.
Before visiting this website, did you know NASA uses nuclear power for some missions that explore the solar system?


Frequently asked questions about NASA's use of radioisotope power systems

What is the Radioisotope Power Systems Program?

The Radioisotope Power Systems (RPS) Program, managed by NASA, is an ongoing strategic investment in nuclear power technologies that seeks to maintain NASA's current space science capabilities, and enable safe and successful future space exploration missions.

NASA, working in collaboration with the Department of Energy (DOE), is designed to enable more capable future space missions by supporting the development of advanced technologies for producing power using heat from the natural decay of plutonium-238.

RPS units are ideally suited to provide electrical power for missions involving autonomous, long-duration operations in the most extreme cold, dusty, dark and high-radiation environments found in space and on planetary surfaces.

NASA works with the DOE to maintain the capability to produce the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). In addition, the RPS Program works to develop higher-efficiency energy conversion technologies, such as more efficient thermoelectric converters (thermocouples) and Stirling converters.

The program also invests in developing effective and efficient processes to support RPS mission launch approval and compliance with the National Environmental Policy Act.

Why did NASA cancel development of ASRG flight units?

For several years NASA invested in Advanced Stirling Radioisotope Generator (ASRG) technology and hardware with the intent to fabricate flight units for deep space missions. Advanced Stirling technology is promising because of its increased efficiency over existing power conversion technology.

However, given a budget constraint faced by the NASA Science Mission Directorate (NASA SMD) and the desire to focus the available resources on developing and launching new missions, NASA SMD decided in late 2013 to terminate work on ASRG flight hardware.

Why develop new RPS technologies?

NASA is considering demanding space science missions that may require new RPS capabilities. In addition, new RPS technologies could make more efficient use of limited resources of plutonium-238.

The new RPS are being developed for multi-mission capability, with the ability to operate in the vacuum of deep space as well as on Mars or other planetary bodies that have an atmosphere. The radioisotope thermoelectric generators (RTGs) used for the Cassini mission to Saturn and Pluto/New Horizons mission to Pluto were designed to operate only in space.

Radioisotope power systems are selected for flight only when they enable or significantly enhance the ability of a mission to meet its scientific and operational goals. As new science goals are identified, the RPS program assesses the required power capabilities to identify new technology needs or development efforts.

An RPS has clear potential benefits for outer solar system missions, where environmental conditions may preclude the use of solar power or other electrical power sources, and for missions to locations with very limited sunlight, such as a permanently shadowed crater on the Moon.

Why do RPS use plutonium-238?

Several radioisotopes could theoretically be used as a heat source for RPS. Plutonium-238 (used in the form of plutonium dioxide) has been selected by the Department of Energy for several important reasons:

  1. Compared to other isotopes, it produces alpha-particle radiation (which is relatively easy to shield against) and little gamma radiation, making it safer to handle and work around.

  2. It has properties that allow it to be used safely, and in a form that is not easily absorbed biologically in the event of a release of material.

  3. The relatively short half-life of Pu-238-though several times a typical space mission duration-means it naturally produces a manageable power degradation over the length of a mission.

  4. It has what engineers call good power density (watts of heat produced per gram of material), which allows for small, lightweight heat sources.

  5. It is available, and able to be produced in sufficient quantities.

  6. It has a minimal and easily mitigated effect on spacecraft systems.

What kinds of missions would need an RPS?

Potential future missions under study are discussed as part of the 2010 Science Plan for NASA's Science Mission Directorate and the 2013-2022 Planetary Science Decadal Survey by the National Academy of Sciences.

Nuclear power enables missions where sunlight is infrequent, obscured, or dimmed by distance, making solar power impractical.

Current concepts for missions that could be enabled or significantly enhanced by the use of radioisotope power include missions to Mars, Venus, Jupiter, Europa, Saturn, Titan, Uranus, Neptune, the moon, asteroids and comets. A number of these missions could be enabled or significantly enhanced by the use of radioisotope power systems.

Visit the homepage for the recent Decadal Survey to learn more about NASA's space science and solar system exploration strategies >

What is an RTG? What is an MMRTG?

A Radioisotope Thermoelectric Generator, or RTG, is a type of power system for space missions that converts heat from the natural radioactive decay of plutonium-238 into electricity using devices called thermocouples, where heat is applied across a circuit that includes dissimilar metals. This produces an electric current via the Seebeck effect. This process involves no moving parts. Essentially a nuclear battery, an RTG provides power to a spacecraft and its science instruments. On some missions, such as Mars Science Laboratory, the excess heat from the RTG can also be used to keep spacecraft systems warm in cold environmental conditions.

The Multi-Mission RTG, or MMRTG, is the eighth generation of such power systems, which have been used safely and successfully by the United States since 1961. It is based on the earlier SNAP-19 RTG design used on the two Viking lander missions, and is capable of working both in space and in a planetary atmosphere (such as on the surface of Mars), hence the name "multi-mission" RTG. NASA's Mars Science Laboratory mission was launched carrying an MMRTG as its source of electrical power on November 26, 2011, and landed successfully on August 6, 2012.

How much plutonium is in the MMRTG? How much power does it supply?

Each MMRTG uses a heat source composed of eight General Purpose Heat Source (GPHS) modules, containing a total of 10.6 pounds (4.8 kilograms) of plutonium dioxide. This compares to 18 GPHS modules in the previous generation of radioisotope thermoelectric generators (the GPHS-RTG, which was able to produce a larger amount of power). The last GPHS-RTG was launched in 2006 on NASA's New Horizons mission to Pluto.

The MMRTG is designed to supply about 110 watts of electrical power at the beginning of a mission. Depending on the power requirements for a mission, multiple MMRTGs could be combined on one spacecraft to provide higher power levels or increased amount of excess heat to keep spacecraft electronics warm enough to operate reliably in cold environments.

Does an RPS supply spacecraft heating in addition to electrical power?

It is possible to reclaim some of the heat produced by an RPS in order to provide heating for spacecraft systems and instruments. In addition, depending on a mission's thermal needs, a spacecraft could utilize radioisotope heater units (RHUs) for additional thermal control.

How many more missions can be flown given the current limited U.S. supply of plutonium-238?

The number of missions that can be supported with current inventories depends on the power required by the proposed missions and the planned power sources. Based on current plans, there will be enough plutonium to fuel the Mars 2020 mission and a Discovery class mission every five years. New Frontiers or Flagship missions could be supported on a less frequent basis in place of Discovery missions. The Department of Energy indicated that missions beyond the 2020 timeframe require new supplies of plutonium-238.

More recently, the latest Decadal Survey for planetary science conducted by the National Academy of Sciences (released in March 2011) strongly recommended several future missions that could be powered by a radioisotope power system. In light of this major study, NASA is re-assessing its plans for solar system exploration missions over the next decade, and will likely have a revised mission planning set for DOE to consider.

NASA is working with DOE to obtain a renewed supply of plutonium-238 fuel for future missions, as advocated by many experts, including a 2009 study by the National Research Council ("Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration").

Instead of nuclear space systems, why not invest in better solar cells for use on space missions?

No single type of power system can supply the array of electrical power needs for the wide array of NASA's missions, so the space agency invests in research and development in a variety of power system types, from batteries and fuel cells to solar cells and radioisotope power systems.

The solar power system for NASA's Jupiter-bound Juno spacecraft, launched in 2011, was developed out of research into improving the performance of so-called Low-Illumination, Low-Temperature (LILT) solar cells for operation at the distance of Jupiter. In recent years, NASA has coordinated with the space photovoltaic technology, vendor, and user communities to define technology research and development paths for more efficient and robust LILT solar cells for deep space applications. Even so, the Juno LILT solar arrays degrade in the charged particle radiation environment around Jupiter and its moons, limiting the mission's lifetime and the regions through which the spacecraft can fly.

Additional Information:

Each spacecraft mission has its own power requirements, based on the need of the spacecraft to function within the specific environment called for by its mission and the minimum science goals of each mission. In addition to some of NASA's most ambitious and successful past and present missions, some future NASA missions would not be possible without an RPS.

Solar power is not practical where sunlight is infrequent or obscured, such as craters at the lunar poles, and becomes less practical as missions travel farther distances from the Sun. Because of the diminished intensity of sunlight, solar panels can become impractically large and hard to point accurately at the sun.

NASA research and development dollars are invested to meet the needs of its strategic plans, such as the 2010 science plan for NASA's Science Mission Directorate and the 2011 decadal survey for planetary science conducted by the National Academy of Sciences. The decadal survey calls for NASA to consider a variety of solar, battery and nuclear power systems to enable a wide range of demanding missions.

The latest Draft NASA Technology Roadmap [November 2010] includes a chapter on Space Power and Energy Storage, which forecasts NASA technology development efforts in chemical systems (fuel cells and batteries), solar energy (photovoltaic and thermal systems), radioisotope power systems, fission power, and fusion power. The high priority of radioisotope power systems was affirmed in early 2012 by the National Academy of Sciences report "NASA Space Technology Roadmaps and Priorities: Restoring NASA's Technological Edge and Paving the Way for a New Era in Space."

What do taxpayers receive for investments in space nuclear research and technology development?

Investing wisely in space nuclear power enables continued cost-effective exploration of our solar system. Such investment can revolutionize our ability to explore the solar system, which contains important clues about the origin and evolution of the planets and the beginnings of life, including possible life beyond Earth.

Money invested by NASA in space nuclear technology can make future missions more effective and more efficient, and thus produce a significant return on investment. Technology research creates good jobs here on Earth in science and technology. Both of these aspects can also inspire future generations to pursue careers in science and engineering.

NASA''s funding priorities are established annually by the President and the U.S. Congress, based on consideration of national needs, the agency's capabilities, and its proposed new activities. NASA in turn develops its budget proposals in accordance with its program goals and priorities, based on its charter to explore space for the peaceful benefit of all humankind, and an open strategic planning process that includes extensive external input and review by the scientific community.

The Mars Exploration Rovers Spirit and Opportunity have lasted years longer than NASA thought they would. Does NASA really need to launch RPS when solar power seems to work better and longer than predicted for the rovers?

Because Spirit and Opportunity rely on solar energy, they were limited in the latitudes, terrain, and seasons that they could land and operate in. Future missions to Mars may have more demanding goals that could require a wider operating range.

In addition to their solar panels, the two Mars Exploration Rovers each were designed to carry and use eight radioisotope heater units (RHUs) as part of their thermal control system. These RHUs have contributed directly to the long and productive lives of the rovers by conserving battery energy that would otherwise have been diverted for heating, especially during unforeseen mission events. The rovers were built to operate for 90 days, but Opportunity continues operating and returning valuable scientific data today, and Spirit remained active for more than six years.

On each of the rovers, the RHUs supply an equivalent of 112 watt-hours of energy during nighttime operations. Solar power supplemented by RHUs was the best fit for Spirit and Opportunity's initial three-month mission operation requirement and power needs.

Without RHUs, the rovers probably would not have continued operating long enough to complete their three-month prime mission, because of the extra draw on the battery that would have been required for heating.

In addition to the RHUs providing critical thermal energy, the rovers have benefitted from random weather events on Mars. For example, local winds and dust devils have unexpectedly boosted declining solar power to the rovers by clearing Martian dust off of the solar arrays. While these fortuitous winds are welcomed, such natural dust-clearing events are highly unpredictable, and cannot be relied upon in the design process for future missions.

Is it safe to launch and use RPS?

Yes. Radioisotope power systems have been used safely and successfully by NASA to explore the solar system for more than 40 years. They even went to the moon with the Apollo astronauts.

Several layers of safety features in an RPS help minimize the release and dispersal of nuclear material under a wide range of possible accident conditions.

The General Purpose Heat Source modules, which contain the nuclear fuel, provide protection for potential ground impact and accidental reentry scenarios. As part of the engineering/design process that aims to improve each new generation of RPS, the heat-source modules in the latest system, the MMRTG, have additional protective material that would provide enhanced protection for potential ground impact and accidental reentry scenarios.

What safety review process does NASA go through for a mission with nuclear materials?

NASA complies with an extensive launch approval process for any space mission planning to utilize Radioisotope Heater Units (RHUs), Radioisotope Power Systems (RPS), or nuclear reactors. Though the primary responsibility for launching and operating the mission safely belongs to NASA, many organizations have responsibilities and/or interests in a spacecraft that uses nuclear systems.

Any NASA mission that proposes to use an RPS, RHU, or nuclear reactor undergoes a comprehensive multi-agency environmental review, including public meetings and open comment periods during the mission planning and decision-making process as part of NASA's compliance with the National Environmental Policy Act (NEPA). Additionally, any such mission proposed by NASA would not launch until formal approval for the mission's nuclear launch safety is received from the Office of the President.

What would NASA do if there were an accident involving an RPS?

NASA works closely in advance of launches with federal, county, and state public information services and emergency management organizations to prepare to respond to any accident in an effective manner, including timely flow of information to the public. Such coordination is part of a comprehensive Radiological Contingency Plan to ensure that NASA, DOE, EPA, and state and local authorities are fully prepared.

Before any launch of a mission carrying an RPS, monitoring teams with a variety of technical expertise and the necessary support equipment would be deployed around the launch site and the surrounding communities to assess any possible release of nuclear material in the event of a launch incident.

To avoid potential exposures, local emergency response officials might ask the public to take precautionary measures following an accident with a mission carrying a fueled RPS or nuclear heat source, such as remaining indoors for a limited period.

What hazards do RPS pose in case of a mission accident?

The fuel in an RPS is plutonium dioxide, which is a radioactive material. In the unlikely event of a mission accident, there is a potential for the release and dispersal of the fuel into the environment, and subsequent human exposure, which could possibly lead to cancer or other detriments to health.

Several layers of safety features designed into an RPS help minimize this potential, including the ceramic form of the fuel and the rugged design of an RPS. NASA and DOE have demonstrated that potential mission risks are very small, through ground-based testing and modeling, the NEPA process, and related launch-approval studies.

Have there been any previous failures with space nuclear systems?

The United States has flown 27 missions with radioisotope power systems and one reactor system over the past five decades.

Three missions carrying spacecraft using radioisotope power systems failed in accidents unrelated to the power system. In each instance, the radioisotope power system performed as it was designed.

  • The April 1964 launch of the Transit 5-BN-3 navigational satellite was aborted during its ascent to orbit. Its RTG burned up upon reentry and, as intended by its design, dispersed its plutonium fuel in the upper atmosphere.
  • The May 1968 launch of the Nimbus B-1 weather satellite was aborted during ascent; its RTG contained the plutonium fuel as designed, the generator was retrieved intact, and the fuel was used on a subsequent mission.
  • An RTG intended to operate science instruments on the surface of the Moon as part of Apollo 13 returned to Earth in April 1970, following the aborted Apollo mission. The Apollo 13 lunar module was used successfully as a "lifeboat" for the three astronauts following unrelated damage to their command module on the way to the Moon. The lunar module carrying the RTG was targeted to fall in the Pacific Ocean near the Tonga Trench. Extensive reconnaissance of the area detected no release of the power source's plutonium-238.

Does NASA have any plans to recover the SNAP-27 RPS from the Apollo 13 lunar lander at the bottom of the Pacific Ocean?


Why should the public believe NASA can manage the safety of a nuclear power system's development given its Space Shuttle safety record?

NASA has an outstanding record of safety in launching spacecraft carrying nuclear power systems, with 17 successful launches and no failures over the past three decades.

We continue to study, plan and practice how to improve the performance of these processes. We apply this extensive experience to every launch, and we prepare for any outcome. This includes extensive radiological contingency planning, with related resources in place for each launch in the unlikely event of an accident.

NASA and the Department of Energy place the highest priority on assuring the safety of their workers and the general public during activities that utilize radioactive materials, and at related facilities.

How hot is the fuel clad inside an RPS?

Depending on the RPS design and operating conditions, the temperature of a fuel clad could range from 1382 to 2012 degrees Fahrenheit (750 to 1100 degrees Celsius).

What is meant by the Pu-238 dioxide having a "ceramic form"?

The plutonium dioxide fuel used in an RPS is a specially refined, fire-resistant ceramic material that is manufactured and used in the form of pellets to reduce the possibility of dispersing the fuel in a launch or reentry accident.

This ceramic resists being dissolved in water and reacts little with other chemicals. If fractured, the ceramic-like a broken coffee cup-tends to break into relatively large particles and chunks that pose significantly less hazard than inhalable microscopic particles.

What is the material the Pu-238 fuel is "clad" in?

Multiple layers of protective material protect and contain the fuel and reduce the chance of a release of the plutonium dioxide. The plutonium dioxide pellets are first clad in iridium, a strong, ductile, corrosion-resistant metal with melting point greater than 4820 degrees Fahrenheit (2660 degrees Celsius).

How long will an RPS continue to produce power?

The General Purpose Heat Source module at the heart of every RPS is targeted to produce heat for at least 14 years with less than a 25 percent power loss. This loss of power over time occurs due to the natural radioactive decay of the radioisotope fuel and the gradual degradation of the thermoelectric junctions inside an RPS.

However, every U.S. planetary exploration mission that has used RPS over the past four decades and counting has been extended beyond its original period of operation, permitting years-and sometimes even decades-of additional scientific data to be returned to Earth.

The longest-operating RPS-powered spacecraft are Voyager 1 and 2, the most distant human-made objects in space. Launched in 1977, these hardy spacecraft are more than 9 billion miles from the sun but, thanks to the power still produced from their RPS, they continue to return valuable data to scientists on Earth about the distant edge of the sun's influence, where our planetary system meets interstellar space.

Is it safe to perform an Earth flyby gravity-assist with a spacecraft carrying nuclear material?

Yes. NASA has flawlessly directed spacecraft to perform several Earth flybys and dozens of similar gravity-assist flybys of other bodies in the solar system (in conditions where navigation is much more challenging), with extremely high accuracy.

In general, Earth flyby maneuvers have been designed to meet a probability of less than one in one million that a flyby accident would occur. These precision maneuvers use proven techniques such as keeping the orbital path of a spacecraft pointed well away from Earth until a carefully planned navigation adjustment close to the date of the flyby.

In addition, the enhanced GPHS modules are designed to contain their Pu-238 fuel in the unlikely event of an Earth re-entry during a flyby maneuver.

What would happen to an RPS in an Earth flyby accident?

All RPS include several layers of protective features, which are designed to contain their plutonium dioxide fuel in a wide range of potential accidents, verified through both a long history of safety testing and advanced supercomputer simulations.

RPS are designed to separate into free-flying GPHS modules in a reentry or flyby accident. The response of each module to thermal and mechanical stresses would vary depending on its specific entry angle and motion. Modules that exhibit any tumbling motion during reentry would not be expected to release any plutonium dioxide. All of the modules contain their fuel inside several layers of hard, heat-resistant material that is highly resistant to impact damage and is designed to contain the fuel upon its landing on the ground or in the ocean.

Extensive safety analysis is performed for each individual mission that proposes to use an RPS, and the probabilities and possible consequences of many different accident scenarios are calculated and cross-checked by independent groups. NASA would not propose to fly a mission that has been determined to be unsafe for the public or its workers.

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Last Updated: 10 Feb 2014