The Role of Nuclear Power in Space Exploration

Atomic Ambitions

More than fifteen billion miles from Earth, two aging emissaries drift through the silent, star-strewn darkness of interstellar space. Launched in 1977, the Voyager 1 and Voyager 2 spacecraft are the farthest human-made objects from home, relics of a bygone era of exploration. After nearly half a century, their instruments are failing, their systems are being carefully powered down one by one, yet they continue to whisper data back across an unimaginable void. This remarkable feat of endurance isn’t powered by the Sun, which from their vantage point is just another bright star in the cosmic tapestry. It’s made possible by the steady, quiet warmth of decaying plutonium atoms, a faint but persistent nuclear heartbeat that has kept them alive for decades.

The Voyager saga perfectly captures the central challenge of deep space exploration: the need for a reliable, long-lived source of energy. Power is the one thing a spacecraft cannot do without. It runs the computers, operates the scientific instruments, and sends the priceless data back to Earth. For missions that stay close to home, in Earth orbit or traveling to the inner planets, sunlight is an abundant and practical choice. But as humanity’s reach extends outward, into the dim and frigid realms of the outer solar system and beyond, solar panels become impractically large and ineffective. This is where nuclear energy enters the story.

For over sixty years, nuclear technology has been an indispensable, if sometimes controversial, tool for space exploration. It has operated in two distinct but equally important capacities. The first is power: generating the electricity and heat necessary to keep a spacecraft functioning, much like the nuclear “batteries” aboard the Voyager probes. The second is propulsion: harnessing the immense energy of the atom to create thrust, pushing a spacecraft through the vacuum of space faster and more efficiently than any chemical rocket could hope to achieve. These two pillars—power and propulsion—represent fundamentally different applications of nuclear science, each with its own unique history, set of technical challenges, and future potential. The story of nuclear energy in space is a journey through this technological landscape, from the pioneering power sources of the 1960s to the advanced propulsion systems and planetary power stations envisioned for the future. It’s a tale of scientific ambition, engineering ingenuity, and the relentless quest to push the boundaries of what’s possible.

The Nuclear Battery: Powering the Pioneers

The first and most widespread use of nuclear energy in space has been for generating electrical power. For missions venturing into the dark, cold expanses of the outer solar system, a power source that is compact, rugged, and can operate for decades without maintenance or sunlight isn’t just an advantage; it’s a necessity. This need gave rise to a family of devices known as Radioisotope Power Systems, the unsung heroes behind some of humanity’s greatest voyages of discovery.

Radioisotope Power Systems: A Steady Heartbeat in the Void

At its core, a Radioisotope Power System (RPS) is a remarkably simple and elegant device. It’s a type of nuclear battery with no moving parts, designed for unparalleled reliability. Its function relies on the natural, predictable process of radioactive decay. The system is fueled with a special isotope, Plutonium-238 (Pu-238), which as it decays, releases a steady and continuous flow of heat. It behaves much like a charcoal briquette that has been heated to a glow and then left to cool over a very, very long time.

This heat is the key to generating electricity. The fuel is housed in a container whose walls are studded with devices called thermocouples. A thermocouple is made of two different types of electrically conductive materials joined together. A physical principle known as the Seebeck effect dictates that when one junction of these materials is heated and the other is kept cold, a small electric voltage is produced. In an RPS, the heat from the decaying plutonium warms the inner junction of the thermocouple, while the frigid vacuum of space cools the outer junction. This large temperature difference creates a continuous flow of electricity.

There are two main types of Radioisotope Power Systems used in space missions:

• Radioisotope Thermoelectric Generators (RTGs) are the “power plants” of the system. They are designed to convert the heat from Pu-238 decay into a steady supply of electricity to run a spacecraft’s computers, radios, and scientific instruments.
• Radioisotope Heater Units (RHUs) are much smaller devices that function as tiny “space heaters.” Each RHU contains a small pellet of Pu-238 and produces about one watt of heat. They are strategically placed on a spacecraft to keep sensitive electronics, mechanical systems, and scientific instruments warm enough to operate in the extreme cold of deep space.

The origins of this technology trace back to the dawn of the space age. In the 1950s, the U.S. Atomic Energy Commission initiated the Systems for Nuclear Auxiliary Power (SNAP) program. This ambitious effort explored various ways to harness nuclear energy for use in space, on land, and at sea. The program was cleverly divided into two parallel tracks, distinguished by an odd-even numbering system. The “odd-numbered” SNAPs, like the SNAP-3 which became the first nuclear power source to fly in space in 1961, were RTGs that used the heat from radioisotope decay. The “even-numbered” SNAPs, like the experimental SNAP-10A reactor, were compact fission reactors.

This dual-track approach from the very beginning of the space age reveals a significant strategic foresight. Early planners didn’t just think “nuclear”; they immediately recognized that space exploration would present different power challenges requiring fundamentally different solutions. They saw the need for two distinct technological paths: one for low-power, high-reliability applications where simplicity was paramount, and another for high-power, high-complexity needs where more energy was required. This foundational decision shaped the next sixty years of space nuclear development. RTGs became the go-to solution for scientific probes venturing into the unknown, while fission reactors were pursued for more demanding applications like military surveillance and, eventually, high-energy propulsion.

Case Studies in Deep Space Endurance

The true measure of Radioisotope Power Systems lies in the missions they have made possible. These are not just technical demonstrations; they are epic tales of exploration that have fundamentally reshaped our understanding of the solar system.

The Grand Tour: Voyager’s Endless Voyage

The story of the twin Voyager 1 and 2 probes is perhaps the most iconic example of the power of radioisotope systems. Launched just weeks apart in the summer of 1977, they were designed to take advantage of a rare alignment of the outer planets that occurs only once every 176 years. Their mission was a “Grand Tour” of the gas giants: Jupiter, Saturn, Uranus, and Neptune.

To power this unprecedented journey, each Voyager spacecraft was equipped with three Multi-Hundred Watt Radioisotope Thermoelectric Generators (MHW-RTGs). These cylindrical devices, with their distinctive cooling fins, each contained spheres of plutonium-238. Collectively, the three RTGs provided each spacecraft with about 470 watts of electrical power at the start of the mission—roughly enough to run a few household light bulbs.

For over four decades, this steady trickle of power has been the lifeblood of the Voyager mission. It has run the probes’ computers, powered their scientific instruments, and transmitted their discoveries back to Earth. this power source is not infinite. The output of the RTGs diminishes over time for two reasons: the natural decay of the plutonium fuel, which has a half-life of about 88 years, and the slow degradation of the thermocouples that convert heat into electricity.

Today, after more than 45 years in space, the power output on each Voyager probe has dwindled to about 230 watts. This slow decline has created a compelling, decades-long narrative of engineering ingenuity. As the available power has decreased, mission controllers at NASA’s Jet Propulsion Laboratory have had to make difficult choices, carefully managing the spacecrafts’ energy budget. Over the years, they have strategically turned off non-essential systems and heaters to conserve every precious watt. In a recent and remarkable feat of deep-space maintenance, engineers found a way to bypass a voltage regulator circuit on Voyager 2, freeing up a small amount of power that had been held in reserve for safety. This clever workaround is expected to postpone the shutdown of the next scientific instrument for several more years, extending the mission’s life and allowing it to continue its exploration of interstellar space.

Lord of the Rings: Why Cassini Needed Nuclear Power

When the Cassini mission to Saturn was launched in 1997, it was the largest and most complex interplanetary spacecraft ever built. Its destination, the Saturnian system, is nearly ten times farther from the Sun than Earth is. At that distance, sunlight is about 100 times weaker, making solar panels an impractical power source. To generate enough electricity, Cassini would have needed solar arrays the size of a tennis court, which would have been far too large and heavy for the launch vehicle.

The only viable option was nuclear power. The Cassini orbiter was powered by three General Purpose Heat Source Radioisotope Thermoelectric Generators (GPHS-RTGs). These were a more advanced design than the ones used on Voyager. At the beginning of the mission, they provided a combined total of about 880 watts of electricity. This robust power supply allowed Cassini to operate a full suite of twelve sophisticated scientific instruments simultaneously. For thirteen years, from 2004 to 2017, Cassini orbited Saturn, revolutionizing our understanding of the ringed planet and its diverse family of moons.

The abundant power from its RTGs enabled a mission of unparalleled scientific richness. Cassini’s radar pierced the thick, hazy atmosphere of Saturn’s largest moon, Titan, to reveal a world with liquid methane lakes, rivers, and seas. Its instruments detected plumes of water vapor erupting from the south pole of the small, icy moon Enceladus, hinting at a subsurface ocean that could potentially harbor life. To keep its intricate systems functioning in the cold, Cassini also carried 82 small Radioisotope Heater Units, each providing a single watt of warmth to critical components. The Huygens probe, which detached from Cassini and successfully landed on Titan, used 35 similar RHUs to stay warm during its descent to the frigid surface.

A Flyby of the Ninth Planet: New Horizons’ Power Budget

The New Horizons mission, launched in 2006, represented a different kind of challenge. Its goal was a fast flyby of Pluto, the distant dwarf planet at the edge of the solar system. At Pluto’s orbit, sunlight is over 900 times weaker than at Earth, making nuclear power the only choice. by the early 2000s, the United States’ supply of plutonium-238 was dwindling, a consequence of production having ceased in the late 1980s.

This constraint forced a new paradigm in mission design: extreme power austerity. To save mass, cost, and precious nuclear fuel, New Horizons was powered by a single GPHS-RTG, a spare left over from the Cassini mission. At launch, this single generator provided about 250 watts of electricity, which had decayed to around 200 watts by the time it reached Pluto nine and a half years later.

The entire piano-sized spacecraft was a marvel of efficiency, designed from the ground up to operate on less power than a pair of 100-watt light bulbs. Each of its seven scientific instruments used only 2 to 10 watts when turned on. To conserve energy during the long cruise to Pluto, the spacecraft’s electronics were put into “hibernation” for long periods, with its computer programmed to wake up periodically to check its systems and send a status report back to Earth. The spacecraft was also wrapped in a lightweight, gold-colored, multilayered thermal blanket, which acted like a high-tech thermos bottle, trapping the waste heat from the operating electronics to keep the spacecraft warm.

The engineering philosophies behind these three missions reveal a clear evolution driven by the availability and constraints of nuclear power. Voyager and Cassini represent an era of relatively abundant power, which allowed for the design of robust, multi-instrument science platforms. New Horizons, in contrast, was a product of an era of scarcity. The design of the entire spacecraft was dictated by the limited output of its single RTG. This constraint forced engineers to innovate, leading to breakthroughs in autonomy, thermal control, and low-power electronics. It’s a story of engineers learning to do more with less, a lesson that has become increasingly valuable as NASA plans future missions to the outer solar system.

Nuclear Power on Other Worlds

The use of radioisotope power isn’t limited to probes flying through the void of space. These systems have also been essential for operating scientific instruments and robotic explorers on the surfaces of other worlds, from the Moon to Mars.

The Red Rovers: Curiosity and Perseverance

The first robotic rovers to explore Mars, Sojourner, Spirit, and Opportunity, were all powered by solar panels. While remarkably successful, their operations were fundamentally constrained by the Sun. They had to shut down at night to conserve energy, and their long-term survival was threatened by the accumulation of dust on their solar arrays. The planet-encircling dust storm that ultimately ended the Opportunity rover’s mission in 2018 highlighted this vulnerability.

For its next generation of Mars rovers, NASA turned to nuclear power. The Mars Science Laboratory rover, Curiosity, which landed in 2012, and the Mars 2020 rover, Perseverance, which landed in 2021, are both powered by a more advanced nuclear source: the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG).

The MMRTG provides about 110 watts of continuous electrical power. This electricity is used to drive the rover’s wheels, operate its robotic arm and scientific instruments, and charge its two lithium-ion batteries. Just as importantly, the waste heat from the generator is circulated through a network of pipes to keep the rover’s sensitive electronics warm during the frigid Martian nights, when temperatures can plunge below -100° Celsius (-148° Fahrenheit).

This nuclear power source provides a key advantage over solar panels on Mars: operational freedom. The MMRTG allows the rovers to operate around the clock, day and night. They can continue to work through the long, dark Martian winters and even during planet-encircling dust storms that can block out the sun for weeks at a time. This dramatically increases their scientific productivity and allows mission planners to select landing sites based on their scientific potential, rather than their proximity to the equator where sunlight is most reliable.

The use of the MMRTG represents a qualitative leap in surface exploration capability. It transforms the rovers from intermittent field explorers, dependent on the weather and the time of day, into persistent, all-weather scientific outposts. As the power from the MMRTGs slowly wanes over their 14-plus-year design life, engineers at JPL are developing clever software updates to make the rovers even more efficient. By enabling them to perform multiple tasks at once—such as driving while analyzing a rock sample and communicating with an orbiter—they can get more science done each day with less energy, extending the rovers’ operational lives for years to come.

An Astronaut’s Best Friend: Powering the Apollo Moonwalks

Nuclear power also has a distinguished history of supporting human exploration. During the Apollo program, five missions carried a nuclear power source to the surface of the Moon. The Apollo 12, 14, 15, 16, and 17 missions were each equipped with a SNAP-27 RTG.

In a procedure that became a routine part of the lunar surface excursions, one of the astronauts would use a special handling tool to remove the 3.8-kilogram (8.4-pound) plutonium fuel capsule from a protective cask mounted on the side of the Lunar Module. They would then carry it over and insert it into the generator, which was part of a central station for a suite of scientific instruments known as the Apollo Lunar Surface Experiments Package (ALSEP).

Once activated, the SNAP-27 provided about 75 watts of electricity to power the ALSEP instruments, which included seismometers, magnetometers, and particle detectors. These nuclear-powered science stations were a resounding success. They continued to operate and transmit valuable data back to Earth for years after the last astronauts had departed the Moon, providing an unprecedented long-term scientific record of the lunar environment, including data on moonquakes, the solar wind, and the Moon’s internal heat flow.

The Nuclear Engine: The Quest for Speed

While radioisotope systems provide the steady power needed for a spacecraft to function, they don’t provide the push needed to get it where it’s going. For that, space exploration has historically relied on chemical rockets, which generate thrust by burning propellants. But chemical rockets have their limits. To send humans to Mars and beyond, or to move large payloads around the solar system efficiently, a more powerful and efficient form of propulsion is needed. This is the domain of the nuclear engine, a technology that promises to dramatically shorten travel times and open up new possibilities for exploration.

Nuclear Thermal Propulsion: A Hotter, Faster Ride

The most direct way to use nuclear energy for propulsion is through a concept called Nuclear Thermal Propulsion (NTP). In an NTP rocket, the engine’s core is a compact nuclear fission reactor. Instead of using the reactor’s heat to generate electricity, it’s used to directly heat a propellant, typically liquid hydrogen. The liquid hydrogen is pumped through channels in the hot reactor core, where it is heated to extreme temperatures—over 2,500° Celsius (4,500° Fahrenheit). This process instantly turns the liquid into a superheated gas, which is then expelled at very high velocity through a nozzle to produce thrust.

The key advantage of NTP is its remarkable efficiency. In rocketry, efficiency is measured by a metric called “specific impulse” (Isp), which is essentially the engine’s “miles per gallon.” A higher specific impulse means the engine can generate more thrust for a given amount of propellant. The best chemical rockets, which burn liquid hydrogen and liquid oxygen, have a specific impulse of around 450 seconds. An NTP engine, by heating pure hydrogen to much higher temperatures, can achieve a specific impulse of 900 seconds or more—double the efficiency.

This doubling of efficiency has significant implications for mission design. An NTP-powered spacecraft could travel to Mars much faster than one using chemical rockets, reducing the one-way transit time by up to 25%. For a human crew, this is a significant benefit, as it would dramatically reduce their exposure to the hazardous radiation of deep space. Alternatively, an NTP rocket could carry a much larger payload—more scientific instruments, more supplies for a surface base, or a larger crew habitat—for the same amount of propellant.

The idea of a nuclear thermal rocket is not new. In the 1960s, during the height of the space race, NASA and the Atomic Energy Commission embarked on an ambitious joint program to develop this technology. The program began with Project Rover, which focused on basic reactor research, and evolved into the Nuclear Engine for Rocket Vehicle Application (NERVA) program. At a remote test site in the Nevada desert, engineers and scientists successfully built and ground-tested a series of nuclear rocket engines. These tests, with names like Kiwi (after the flightless bird, as these engines were never intended to fly) and NRX (NERVA Reactor Experiment), proved that the fundamental technology was viable. The legendary rocket scientist Wernher von Braun, the architect of the Saturn V moon rocket, saw NTP as the essential next step for sending humans on deep space missions to Mars and beyond.

Despite its technical successes, the NERVA program was canceled in 1973. The Apollo program was winding down, and with no funded, near-term mission that required NTP’s advanced capabilities, the program fell victim to shifting priorities and budget cuts. This cancellation was a pivotal moment in the history of space exploration. It effectively locked human spaceflight into a chemical propulsion paradigm for the next fifty years. The technology had been proven to work, but the absence of a clear “mission pull” made it politically and financially unsustainable. This created a technological inertia that has significantly shaped the architecture of space exploration, forcing mission planners to rely on slower, less efficient trajectories that are dictated by the limitations of chemical rockets. The modern push to develop NTP is, in many ways, an attempt to finally pick up where NERVA left off six decades ago.

Nuclear Electric Propulsion: The Marathon Runner

There is another, entirely different approach to nuclear propulsion: Nuclear Electric Propulsion (NEP). In an NEP system, a fission reactor is used not for direct heating, but to function as a high-output electrical power plant, generating kilowatts or even megawatts of electricity. This substantial electrical power is then used to drive highly efficient electric thrusters, such as ion or Hall-effect thrusters.

These thrusters work by using electric and magnetic fields to accelerate a propellant, typically an inert gas like xenon or argon, to extremely high exhaust velocities. The trade-off is that they produce very low thrust—often described as being equivalent to the force of a piece of paper resting on your hand. unlike chemical or nuclear thermal rockets that fire for minutes at a time, an NEP system can sustain this gentle push continuously for months or even years.

This makes NEP systems the “marathon runners” of space propulsion. Their incredible propellant efficiency allows them to achieve very large changes in velocity over long periods. They are ideal for missions that require moving very heavy payloads, such as cargo missions to Mars, or for sending robotic probes on fast, direct trajectories to the outer solar system. An NEP system could enable orbiters, landers, and even sample return missions from a much greater number of destinations across the solar system.

The history of NEP in the United States includes one notable flight: SNAP-10A. Launched in 1965, this experimental spacecraft was powered by a small fission reactor that produced about 500 watts of electricity. The reactor successfully operated in orbit for 43 days before a failure in one of the spacecraft’s non-nuclear voltage regulators caused it to shut down. SNAP-10A remains the only nuclear reactor the United States has ever operated in space, and it is expected to remain in its high orbit for about 4,000 years.

The limited American experience with space reactors stands in stark contrast to the approach taken by the Soviet Union during the Cold War. This divergence reveals a fundamental difference in the strategic priorities of the two superpowers. While the U.S. focused on developing high-reliability, low-power RTGs to enable its scientific deep-space probes like Pioneer and Voyager, the Soviets invested heavily in operational, high-power fission reactors for military purposes in low Earth orbit. The two nations developed the technologies that best suited their differing objectives, creating two distinct legacies of space nuclear power.

The Soviet Approach: Reactors in Orbit

From 1967 to 1988, the Soviet Union launched a constellation of more than 30 reconnaissance satellites known as RORSATs (Radar Ocean Reconnaissance Satellites). Their mission was to use powerful radar systems to track NATO naval vessels, including aircraft carriers and nuclear submarines. Operating these power-hungry radar systems in low Earth orbit presented a challenge: large solar arrays would create significant atmospheric drag, causing the satellites’ orbits to decay rapidly.

The Soviet solution was to power them with compact nuclear reactors. Most of the RORSATs carried a BES-5 fission reactor, fueled with highly enriched uranium-235, which provided about two kilowatts of electricity. At the end of their missions, the reactor cores were designed to be ejected into a higher, long-lived “disposal orbit” where they could safely decay over hundreds of years.

In the 1980s, the Soviets developed a more advanced and powerful space reactor called TOPAZ. Unlike earlier systems that used thermoelectric or other conversion methods, TOPAZ was a thermionic reactor. It used a process where heat from the nuclear fuel directly “boiled” electrons off a metal surface (the emitter), which then traveled across a small gap to a collector, generating an electric current. This direct conversion within the reactor core itself was a significant technological leap. Two TOPAZ-I reactors, capable of producing 5 to 10 kilowatts of electricity, were successfully flight-tested in 1987 aboard the Kosmos 1818 and Kosmos 1867 satellites. After the collapse of the Soviet Union, the United States’ Strategic Defense Initiative Organization even purchased several unfueled TOPAZ-II reactors for ground testing and study, a testament to the advanced state of the Soviet program.

Challenges and Realities

Despite its immense potential, the use of nuclear energy in space is not without its challenges. The primary concerns revolve around safety, the availability of nuclear fuel, and the complex web of policy and public perception that governs the technology’s use. These earthbound hurdles have often proven to be as formidable as the technical challenges of designing and building the systems themselves.

Safety and the Specter of a Fall

The foremost public concern associated with space nuclear systems is the risk of an accident, particularly one that could release radioactive material into the environment. Addressing this concern has been a central focus of engineering and policy for decades, leading to a continuous evolution in safety design and protocols.

The philosophy behind the safety design of Radioisotope Power Systems has undergone a significant transformation since the early days of the space program. The first generation of RTGs, such as the SNAP-9A, were designed with a “burn-up and disperse” strategy. The idea was that in the event of an accidental reentry into the atmosphere, the heat source would burn up at a very high altitude, dispersing the plutonium-238 fuel as fine particles into the stratosphere, where it would be widely scattered and diluted to supposedly safe levels.

Beginning in the late 1960s, this approach was abandoned in favor of a “robust containment” philosophy. Modern RTGs are designed to contain their nuclear fuel under all foreseeable accident conditions, including a launch pad explosion, a catastrophic ascent failure, or a high-speed reentry through the atmosphere. This is achieved through a multi-layered, defense-in-depth design. The plutonium fuel itself is in a ceramic form (plutonium dioxide), similar to the material in a coffee mug, which is highly durable and resistant to being broken into fine, inhalable particles. These ceramic pellets are then encapsulated in a shell of iridium, an extremely tough and corrosion-resistant metal with a very high melting point. This fueled clad is then encased in multiple layers of high-strength graphite blocks, which serve as an impact shell and a heat shield. The entire assembly is known as a General Purpose Heat Source (GPHS) module.

The history of space exploration includes three significant incidents involving nuclear power sources, each of which provided valuable lessons and shaped the evolution of safety design:

1. SNAP-9A (1964): A U.S. Navy navigation satellite failed to reach orbit. As it reentered the atmosphere, its SNAP-9A RTG behaved as designed for that era: it burned up and dispersed approximately 1 kilogram (2.2 pounds) of plutonium-238 into the upper atmosphere, primarily over the Southern Hemisphere. This event highlighted the potential for global contamination from the burn-up design philosophy and was a major factor in the shift toward robust containment.
2. Kosmos 954 (1978): A Soviet RORSAT satellite malfunctioned in orbit. Its onboard system, designed to boost the nuclear reactor into a high, safe disposal orbit, failed. The satellite made an uncontrolled reentry over northern Canada, scattering radioactive debris from its uranium-fueled reactor across a vast, sparsely populated area of the Northwest Territories. The incident triggered a massive, multi-million-dollar cleanup effort called Operation Morning Light, conducted jointly by Canadian and American teams. It raised international alarm about the risks of operating nuclear reactors in low Earth orbit.
3. Apollo 13 (1970): This incident, though born from a mission failure, stands as a resounding nuclear safety success. When an oxygen tank explosion crippled the Apollo 13 service module, the crew had to abort their lunar landing and use the Lunar Module, Aquarius, as a lifeboat. The LM carried the SNAP-27 RTG that was intended to power the science station on the Moon. Before reentering Earth’s atmosphere, the crew jettisoned the LM. Mission controllers carefully planned the reentry trajectory to ensure the LM would come down over the deep Pacific Ocean. The RTG’s fuel cask, which had been designed with the new robust containment philosophy, performed exactly as intended. It survived the fiery reentry intact and sank to the bottom of the 6-kilometer-deep (20,000-foot) Tonga Trench, where its plutonium fuel remains safely contained. This real-world, high-stakes event provided a powerful validation of the containment approach, which became the cornerstone of the safety case for every subsequent American RPS-powered mission.

Today, any U.S. mission involving a nuclear power source is subject to a rigorous, multi-layered safety analysis and launch approval process. This process is guided by federal policy, most recently updated in a 2019 document called National Security Presidential Memorandum-20 (NSPM-20). Mission planners must conduct a detailed probabilistic risk assessment, which is then reviewed by the Department of Energy and NASA. An independent panel of experts, the Interagency Nuclear Safety Review Board (INSRB), then conducts its own evaluation of the mission’s safety analysis. Only after this exhaustive review process is complete can the final launch authorization be granted, a decision that for higher-risk missions is made at the level of the White House.

The Fuel of the Gods: The Plutonium-238 Story

The remarkable performance of Radioisotope Power Systems is made possible by the unique properties of their fuel, Plutonium-238. This specific isotope of plutonium is nearly ideal for use as a space power source. It has a half-life of 87.7 years, which means it provides a predictable and long-lasting source of heat for decades-long missions. It decays primarily by emitting alpha particles, a type of radiation that is easily stopped by a thin shield (like a sheet of paper or the outer layers of the fuel clad itself), which simplifies the design and reduces the mass of the power system. It also has a high power density, meaning a small amount of the material generates a significant amount of heat. It’s important to note that Pu-238 is a non-fissile isotope; it cannot sustain a chain reaction and cannot be used to make a nuclear weapon.

For decades, the United States produced Pu-238 at the Savannah River Site in South Carolina as a byproduct of its nuclear weapons programs. with the end of the Cold War, the reactors at Savannah River were shut down, and production of Pu-238 ceased in 1988. For the next quarter-century, NASA’s deep space missions had to rely on the dwindling domestic stockpile and on purchases of Pu-238 from Russia.

By the early 2010s, the supply had become critically low, creating a potential bottleneck that threatened the future of U.S. planetary exploration. In response, Congress provided funding for the Department of Energy to restart domestic production of this vital material. The 30-year gap in production meant that this was not a simple matter of turning a switch back on; it was a painstaking effort to reconstruct a unique and highly specialized national capability.

The new production process is a complex, cross-country endeavor that relies on the unique facilities and expertise of three national laboratories:

• Oak Ridge National Laboratory (ORNL) in Tennessee is responsible for the first step. Technicians there create targets made of another element, neptunium-237, and irradiate them in a high-flux nuclear reactor. This process converts some of the neptunium into plutonium-238.
• The irradiated targets are then sent to Los Alamos National Laboratory (LANL) in New Mexico. There, chemists perform a complex series of steps to separate and purify the newly created Pu-238, which is then converted into a ceramic oxide and pressed into fuel pellets.
• Finally, the finished fuel pellets are shipped to Idaho National Laboratory (INL), where they are encapsulated in their protective iridium shells and assembled into the final General Purpose Heat Source modules. INL is also responsible for assembling and testing the complete RTGs before they are delivered to NASA for integration with the spacecraft.

This restarted production line is now successfully producing hundreds of grams of new Pu-238 each year, with a goal of reaching a steady-state production rate of 1.5 kilograms per year by 2026. This effort has broken the supply bottleneck and ensures that NASA will have the fuel it needs for future flagship missions, such as the planned Dragonfly rotorcraft, which will use an MMRTG to explore the surface of Saturn’s moon Titan in the 2030s. The story of Pu-238 is a powerful lesson in the long-term strategic planning required to sustain a deep space exploration program. Access to this specific isotope is not just a matter of manufacturing; it’s a critical national capability that enables our exploration of the solar system.

Policy and Perception: The Earthbound Hurdles

Beyond the technical and logistical challenges, the use of nuclear power in space must navigate a complex landscape of international law, public opinion, and domestic policy.

The foundational legal framework for all space activities is the 1967 Outer Space Treaty. This landmark international agreement, signed by over 100 nations, prohibits the placement of nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies and dedicates the Moon and other celestial bodies to peaceful purposes. More specific guidelines for the use of nuclear power sources were established in a set of principles adopted by the United Nations General Assembly in 1992. These principles state that nuclear power should only be used for missions where other power sources are not reasonably feasible and lay out requirements for safety assessments, notification of potential reentries, and liability in case of an accident.

Public perception of nuclear technology is often complex and polarized. While opinion polls have shown generally increasing support for the use of nuclear energy for terrestrial power generation, the idea of launching radioactive materials into space often faces heightened scrutiny. Anti-nuclear groups have frequently organized protests against missions carrying RTGs, such as Cassini and New Horizons, raising concerns about the potential for a launch accident to release plutonium. Space agencies like NASA work to address these concerns through extensive public outreach and by highlighting the rigorous safety design and review processes that are in place.

Finally, the development of advanced space nuclear systems is an expensive, long-term endeavor that requires sustained political will and stable funding. Programs can be stalled or canceled due to shifting budgets, changes in presidential administrations, or complex regulatory hurdles that involve multiple government agencies, including NASA, the Department of Energy, and the Federal Aviation Administration. Overcoming this institutional fragmentation and maintaining a consistent, long-term strategy has historically been one of the most significant barriers to progress in the field.

The Nuclear Future

As humanity looks toward a future of sustained presence on the Moon and eventual human missions to Mars, the need for more powerful and efficient nuclear systems is becoming increasingly apparent. The technologies that powered the pioneers of the 20th century are now evolving to become the foundational infrastructure for the explorers and settlers of the 21st.

A Power Plant on the Moon: Fission Surface Power

The Artemis program, NASA’s ambitious plan to return humans to the Moon and establish a sustainable presence there, presents a new set of power challenges. To support a permanent lunar base, with habitats, science labs, rovers, and equipment for mining local resources, a power source is needed that is far more capable than the RTGs used by the Apollo astronauts. This power source must be able to operate continuously through the 14-day-long lunar night and in the permanently shadowed craters near the lunar poles, where scientists believe vast quantities of water ice may be trapped. In these locations, solar power is not a viable option.

To meet this need, NASA, in partnership with the Department of Energy, is actively developing the Fission Surface Power (FSP) project. The goal is to design, build, and demonstrate a small nuclear fission reactor on the surface of the Moon. This project marks a pivotal conceptual shift in the use of space nuclear power—from a tool for exploration to a foundational piece of infrastructure for settlement. A reliable, continuous power grid is the first step toward creating a permanent, self-sustaining human presence beyond Earth.

The initial goal of the FSP project is to develop a power system capable of producing at least 40 kilowatts of electricity (kWe), with future plans calling for systems that can generate 100 kWe or more. A 40 kWe system would be enough to continuously power about 30 households on Earth and would be sufficient to support early lunar habitats, scientific experiments, and in-situ resource utilization (ISRU) activities, such as extracting oxygen and water from the lunar soil.

The FSP system is envisioned to be fully manufactured, assembled, and fueled on Earth and then launched to the Moon as a single, integrated package. Once landed, it would be deployed and activated remotely, designed to operate autonomously for at least ten years without maintenance. NASA has awarded contracts to several industry teams, including partnerships led by Lockheed Martin with BWX Technologies, and Westinghouse with Aerojet Rocketdyne, to develop initial design concepts for the reactor, its power conversion system, and its heat rejection system. The current timeline calls for a flight demonstration on the Moon in the early 2030s.

Revisiting the Nuclear Rocket: The Rise and Fall of DRACO

For decades, advocates for human exploration of Mars have pointed to Nuclear Thermal Propulsion as a key enabling technology. The speed and efficiency of an NTP rocket would shorten the long journey to the Red Planet, reducing the risks to the crew and increasing the amount of payload that could be delivered. In the early 2020s, it seemed this long-held dream was finally moving toward reality.

In 2023, NASA and the Defense Advanced Research Projects Agency (DARPA) announced a high-profile partnership on a program called the Demonstration Rocket for Agile Cislunar Operations (DRACO). The goal was ambitious: to build and fly the first NTP rocket in space, with a demonstration planned for as early as 2027. The program would leverage the legacy of the NERVA program but use a modern fuel design based on high-assay low-enriched uranium (HALEU), which presents fewer security and proliferation concerns than the highly enriched uranium used in the 1960s.

in mid-2025, the program was abruptly canceled. The reason was not a failure of technology, but a dramatic shift in the economic landscape of spaceflight. According to DARPA officials, the “precipitous decrease in launch costs,” driven by the success of commercial companies like SpaceX and the anticipated capabilities of its massive Starship vehicle, had fundamentally changed the cost-benefit analysis for NTP.

The primary advantage of NTP has always been its high efficiency, which reduces the amount of propellant a spacecraft needs to carry. This, in turn, reduces the total mass that needs to be launched from Earth, saving money on expensive rocket launches. But when the cost of launching that mass drops dramatically, the economic incentive to spend billions of dollars on research and development to reduce it becomes much less compelling. The massive R&D cost of developing the nuclear engine no longer looked like a positive return on investment compared to the simpler option of just launching more chemical propellant on a cheaper rocket.

The cancellation of DRACO is a stark illustration of how disruptive commercial innovation can reshape government technology strategy. For decades, the high cost of launch was a fixed constraint that made NTP’s efficiency a top priority. The commercial launch revolution shattered that constraint. This event shows that the future development of advanced space technologies must now be weighed not just against their performance benefits, but against the rapidly evolving economics of the commercial launch market. For NTP to move forward, its advocates must now make a case based on its other advantages, such as the speed that reduces crew radiation exposure on long-duration missions, a benefit that remains regardless of launch costs.

The Distant Horizon: Fusion and Beyond

Looking further into the future, scientists and engineers envision an even more powerful form of nuclear propulsion: the fusion rocket. While fission releases energy by splitting heavy atomic nuclei, fusion releases energy by combining light atomic nuclei, the same process that powers the Sun and stars. A fusion rocket could, in theory, achieve a specific impulse and thrust far beyond what is possible with fission-based NTP, enabling rapid transit throughout the solar system.

Various concepts for fusion rockets are being explored, though all remain in the very early stages of theoretical and laboratory research. These include designs based on magnetic confinement, where powerful magnetic fields contain a superheated plasma of fusion fuel, and inertial confinement, where powerful lasers or particle beams are used to compress and ignite small pellets of fuel. The primary advantage of fusion is its potential for unparalleled efficiency and power, while the main disadvantages are the immense technical challenges and the likely enormous mass and complexity of a fusion reactor.

Despite the challenges, a number of government labs, universities, and private companies are pursuing this long-term goal. One notable example is the “Sunbird” concept from the UK-based company Pulsar Fusion. They propose a fusion-powered “tug” that could dock with spacecraft in orbit and propel them to their destinations. The company claims their technology could cut the trip to Mars to just over a month and enable a mission to reach Pluto in four years. While such capabilities are likely decades away from becoming a reality, they represent the ultimate aspiration of space propulsion—a technology that could one day make humanity a truly interplanetary and, perhaps eventually, an interstellar species.

Summary

From the faint, steady power source keeping the Voyager probes alive in interstellar space to the powerful engines envisioned to carry the first humans to Mars, nuclear energy has been and will continue to be a vital technology for space exploration. It is not a single, monolithic entity, but rather a diverse toolkit of systems, each tailored for different eras and objectives.

The journey began with Radioisotope Power Systems, the simple and ultra-reliable nuclear batteries that enabled the first reconnaissance of the outer solar system and powered scientific outposts on the Moon and Mars. These systems, from the early SNAPs to the modern MMRTG, have a proven legacy of success spanning more than sixty years.

In parallel, the dream of the nuclear rocket has persisted since the dawn of the space age. The technical successes of the NERVA program in the 1960s demonstrated the immense potential of Nuclear Thermal Propulsion, a potential that is now being revisited as NASA sets its sights on Mars. More advanced concepts like Nuclear Electric Propulsion promise even greater efficiency for hauling cargo and sending robotic probes on fast trajectories to the farthest corners of the solar system.

Looking to the future, as humanity seeks to establish a permanent foothold on other worlds, nuclear fission reactors are poised to become the foundational infrastructure for lunar and Martian bases, providing the abundant and continuous power needed for settlement and industry. And on the distant horizon, the promise of fusion propulsion offers the tantalizing possibility of rapid transit across the vast distances of our solar system.

Significant challenges related to safety, cost, fuel supply, and public perception remain. These hurdles are not insignificant, and overcoming them will require sustained investment, political will, and a commitment to transparent and responsible engineering. But as we look to the next chapter of humanity’s journey into space, it’s clear that the power of the atom will be an indispensable tool, unlocking new destinations and enabling us to realize our most ambitious goals of exploring and eventually settling the final frontier.

What Questions Does This Article Answer?

• What enables the Voyager spacecraft to continue functioning billions of miles away from Earth?
• How do nuclear power systems benefit spacecraft on deep space missions beyond the reach of solar energy?
• What are the main types of Radioisotope Power Systems used in space exploration, and how do they work?
• What historical space mission first utilized nuclear power, and what was its significance?
• How have the technologies for nuclear power in space developed since the early systems of the 1960s?
• Why was the Cassini spacecraft powered by nuclear energy instead of solar panels?
• In what ways did nuclear energy provide solutions to the limitations of current chemical rockets for deep space travel?
• What critical safety measures ensure the safe use of nuclear power sources in space missions?
• What challenges does the future of nuclear propulsion face in the context of changing space exploration dynamics and commercial spaceflight advances?
• What role will nuclear power technologies play in future lunar and Martian missions in terms of establishing a sustainable human presence?