FAQ

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 capabilities for space science, and enable safe and successful future space exploration missions.

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

RPS are ideally suited to provide power for missions that need autonomous, long-duration operations in the most extreme cold, dusty, dark, and high-radiation environments, either in space or on planetary surfaces.

NASA works with the DOE to maintain the capability to produce the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which saw its first use on the Curiosity Mars rover and it invests in new technology, such as more efficient thermoelectric materials and Stirling engines to produce electricity.

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

NASA is developing new RPS with multi-mission capability that would 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 and Pluto/New Horizons missions were designed to operate only in space.

Radioisotope Power Systems are selected for potential use in space 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 other electrical power sources, and for missions to locations with very limited sunlight, such as a permanently shadowed crater on the Moon or on an asteroid.

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

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

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.

Potential future missions under study are discussed in the 2011 decadal survey of planetary science by the National Academy of Sciences, and in the annual strategic plan for the NASA Science Mission Directorate.

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

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 the Curiosity Mars rover, 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 for more than 50 years since their first launch in 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 Curiosity rover was launched carrying an MMRTG as its source of electrical power on November 26, 2011, and landed successfully on August 6, 2012.

Each MMRTG uses eight General Purpose Heat Source (GPHS) modules, containing a total of 10.6 pounds (4.8 kilograms) of plutonium oxide, of which Pu238 represents 71% by weight. 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 fly-by mission to Pluto on July 14, 2015.

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 operate additional scientific equipment reliably in cold environments.

A Stirling engine based power convertor would convert heat from the natural radioactive decay of plutonium-238 into electrical power using the back-and-forth motion of a piston utilizing an alternator, rather than the static metallic thermocouples used by an RTG. This technology converts heat to electricity at a higher efficiency than static technologies. With this advantage, Stirling Radioisotope Power Systems can make more efficient use of limited Pu-238, and produce less waste heat for missions where this is an issue.

NASA continues to conduct tests with currently available hardware, and has issued a request for information from industry on the current state-of-the-art. NASA is developing a plan with the goal to mature dynamic conversion based technologies that would prioritize reliability, robustness, and total lifecycle costs over efficiency and mass, looking toward a system that could be ready to fly on space missions in the late 2020s.

It is possible to reclaim some of the waste heat produced by an RPS to supply heat for spacecraft systems and instruments. And, depending on a mission's thermal needs, a spacecraft could utilize Radioisotope Heater Units (RHUs) for additional thermal control.

NASA and the DOE jointly announced in December 2015 that the first small amount of new plutonium-238 (Pu-238) fuel for future deep-space mission Radioisotope Power Systems had recently been produced, after a gap of nearly 30 years. Oak Ridge National Laboratory (ORNL) produced 50 grams of new Pu-238, toward an initial annual production capability of 300-400 grams within a few years.

The Pu-238 Supply Project is currently demonstrating and validating the processes required for steady production, and then will begin scaling up toward the full average production rate of 1.5 kg plutonium oxide/yr.

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 range 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 has its own power requirements, based on its need 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 potentially hinder the spacecraft's ability to maneuver and point accurately at the sun.

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

The latest NASA technology research roadmaps include plans for investments in 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, particularly new Stirling-based 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."

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 lasted years longer than NASA thought they would.

Because Spirit and Opportunity relied 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 designed and built to operate for 90 days, but Spirit lasted for more than six years (20 times longer than its design life) and Opportunity kept going for more than 14 years. Both rovers found significant evidence of ancient Martian environments where wet and habitable conditions existed intermittently, and Spirit remained active for more than six years.

On each of the rovers, the RHUs supplied 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 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.

Yes. Radioisotope Power Systems have been used safely and successfully by NASA to explore the solar system for more than 50 years. These power systems 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 MMRTG have additional protective material that would provide enhanced protection for potential ground impact and accidental reentry scenarios.

The Multi-Mission Thermoelectric Generator (MMRTG) has a design life of 14 years plus three years of pre-launch storage life. 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 10 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.