Using nuclear technology to change how we explore icy Ocean Worlds: examples from nuclear powered mission concept designs

1 Introduction

The potential for Ocean Worlds to answer one of the most fundamental questions, whether we are alone in the universe, has driven increased interest in their exploration. Recent mission selections, including Europa Clipper, Jupiter Icy Moons Explorer (JUICE), and Dragonfly, along with the prioritization of future missions targeting confirmed or candidate icy ocean worlds for exploration in both the 2023-2032 Planetary Science and Astrobiology Decadal Survey and Voyage 2050 demonstrate that exploring these worlds is a community priority (ESA Voyage, 2050 Senior Committee, 2021; Committee on the Planetary Science and Astrobiology Decadal Survey et al., 2023).

However, icy ocean world exploration presents several significant logistical challenges (Sherwood et al., 2018; Hendrix et al., 2019).

• Distance - Long transit times to outer solar system targets increase mission costs and limit opportunities within a researcher’s career timespan.
• Low Solar Radiance - Limited sunlight near Jupiter (3.7% of Earth’s) and Saturn (1% of Earth’s) constrains solar power generation, especially for surface operations on targets with thick atmospheres (e.g., Titan).
• Mission Cadence - Unlike the Mars Exploration Program, which pursued successive small-to-large missions that incrementally produced significant scientific gains, the cadence of outer solar system exploration is limited by long transit times and may require planning subsequent missions before data can be returned.
• Radiation Environment - Particularly in the Jovian system, radiation protection requirements add mass that limits other payload elements such as science instruments or propellant.
• Subsurface Access - Reaching potentially habitable oceans beneath kilometers of ice presents significant technological challenges and power demands.
• Inflexible Launch Schedules - Trajectories requiring gravity assists to efficiently reach outer planets and their moons create limited launch windows and reduced flexibility in mission design.

Nuclear fission systems could potentially mitigate many of these challenges due to their high energy density. Both nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) offer significantly higher specific impulse (I_sp) compared to chemical propulsion, enabling larger payloads, more complex mission architectures, or faster transit times. Here, we discuss the fundamentals of NEP and NTP, provide an overview on past and present technology developments, and review several mission concepts that demonstrate how nuclear propulsion and power systems can be leveraged to meet mission and science objectives unique to icy ocean world exploration.

2 Nuclear propulsion fundamentals

2.1 Nuclear electric propulsion

NEP systems function similarly to nuclear power plants, where heat from the fission reaction is converted to electrical power. The electrical power is then used to accelerate an ionized propellant gas, generating thrust. NEP systems are generally composed of five subsystems, which include (1) a reactor to generate thermal energy and shielding to protect people or materials from radiation exposure, (2) a power conversion system to convert thermal energy into electrical energy, (3) a power management and distribution system to transfer electrical power from the power conversion system to the electric propulsion subsystems, (4) the electric propulsion subsystem to convert electricity into thrust, and (5) heat rejection radiators that serve as the cold side of the power conversion cycle (Figure 1) (National Academies of Sciences, Engineering, and Medicine, 2021).

NEP systems achieve the highest specific impulse of the propulsion types considered, ranging from 2,000 to 8,000 s, which significantly reduces the propellant mass required for a given mission. This propellant efficiency allows a greater fraction of delivered mass to be allocated to science instrumentation, radiation shielding, or other mission elements. Perhaps the most significant advantage of NEP in the context of icy ocean world exploration is that the reactor serves a dual purpose, providing power for both the electric thrusters and spacecraft systems. This enables continuous, moderate power throughout the mission duration, supporting uninterrupted observation, data collection, and communication without consuming propellant. For specific impulse values in the 1,500–4,000 s range, existing thruster technology, demonstrated on NASA’s Dawn, DART, and Psyche missions, can be directly applied, reducing the technology development burden for mission design. Potential disadvantages of NEP include the low thrust levels that require significant time to complete velocity changes in gravity wells, which can increase overall mission duration. Additionally, NEP systems comprise multiple interdependent components, and failure of any single element without redundancy can be mission-ending (Bennett, 2021).

2.2 Nuclear thermal propulsion

An NTP system uses the heat generated by the fission reaction to convert a liquid propellant into a gas. The heated gas is expanded through a nozzle, generating thrust. NTP systems are typically composed of a (1) reactor to generate thermal energy and associated shielding to protect people or materials from radiation exposure, (2) propellant pumps to deliver propellant to the reactor, and (3) a rocket nozzle to convert thermal energy to kinetic energy, providing thrust (Figure 2) (Calomino et al., 2025). For many applications there is a fourth system (4): cryocoolers for keeping the cryogenic propellant from boiling off before it can be used.

NTP systems provide high thrust levels, making them well suited for missions prioritizing fast transit times or requiring orbit insertion or fast escape maneuvers. There is potential to pair NTP and NEP propulsion systems on a spacecraft to combine the advantages of both high- and low thrust propulsion. In addition, NTP systems are roughly twice as efficient as chemical systems, with I_sp values between 850 and 1,000, and most modern systems targeting 900 or higher with low enriched uranium. These systems also have a relatively straightforward design with fewer interdependent subsystems and are currently at a higher technology readiness level compared to some elements of NEP (Calomino et al., 2025). Potential disadvantages of NTP include two distinct propellant management challenges. First, liquid hydrogen propellant is subject to boil-off losses over the course of a mission, which can constrain mission duration and may require active cryogenic fluid management systems to maintain propellant at sufficiently low temperatures. Second, following NTP engine operation, propellant must be expended to remove decay heat from the reactor, a less propellant-efficient process that represents an additional draw on the available propellant budget. Both effects can be limiting for NTP system performance and mission design.

3 Nuclear vs. conventional propulsion and power

Nuclear propulsion and power systems are most usefully evaluated against the technologies they would displace or complement, including chemical propulsion, solar electric propulsion (SEP), and radioisotope thermoelectric generators (RTG/GPHS). Table 1 summarizes the performance regimes, technology readiness, and key restrictions of each.

TABLE 1. Comparison of power and propulsion technologies.

Chemical propulsion’s high thrust and heritage at TRL nine make it an ideal option for discrete, high-ΔV maneuvers such as orbit insertion and trajectory corrections, but its specific impulse constrains the ΔV available for a given propellant mass (Frisbee, 2003). Chemical power systems, including batteries and fuel cells, lack the lifetime to serve as a primary power source and are appropriate only for peak-power supplementation of nuclear or solar systems (Bennett, 2021). SEP typically offers a specific impulse in the 1,500–4,200 s range and is well-demonstrated on the Dawn, DART, and Psyche missions, but its power output falls off with the square of heliocentric distance, limiting its use beyond Saturn. Very high I_sp SEP systems are rarely envisioned because their primary use case in high Δv missions are typically targeting outer planets, where nuclear is a more appropriate technology. Additionally, at megawatt-class power levels the required solar array area can present serious challenges in structures, testing, and materials sourcing (Koppel et al., 2004; Bennett, 2021; Davis et al., 2005). SEP is well-matched to inner solar system missions requiring large in-space ΔV, particularly small-body rendezvous and sample return (Anderson et al., 2011).

RTGs and GPHS modules provide proven, passive power over decade-scale missions, with flight heritage on Voyager, Cassini, New Horizons, and Curiosity, but output is constrained to roughly 100 W to a few kWe and degrades at approximately 4% per year (Lee and Bairstow, 2015), limiting available power at destination following long transits. For missions with higher power demands, fission power systems become more mass-competitive than RTGs around 8 kWe (Xia et al., 2021).

NTP approximately doubles the specific impulse of chemical propulsion (850–1,000 s) while retaining high thrust, enabling direct transfer trajectories that reduce cruise times to Uranus and Neptune by 30%–50% and substantially increase delivered payload mass relative to chemically-propelled missions (Kumar et al., 2022). However, NTP is fundamentally a propulsion system rather than a power source, it does not provide sustained electrical power to the spacecraft, and available ΔV is further reduced by liquid hydrogen boiloff during cruise and propellant expended during post-burn decay heat removal. Full NTP system readiness remains at TRL 4, with continued maturation required before planetary flight (Calomino et al., 2025).

NEP achieves the highest specific impulse of the technologies considered, ranging from 2,000 to 8,000 s, at the cost of very low thrust levels that extend gravity-well escape and require long powered arcs to accumulate large ΔV. Its defining advantage for icy ocean world exploration is the reactor’s dual function: it simultaneously drives the electric thrusters and provides continuous, solar-independent power at tens of kilowatts for science instruments, communications, and onboard systems throughout all mission phases. This enables mission architectures that are inaccessible to any of the alternatives at comparable heliocentric distances, including multi-moon tours, sustained in situ operations, and continuous high-data-rate downlink. The principal risks are the complexity of integrating multiple interdependent subsystems over multi-decade mission lifetimes and the current low TRL of the full NEP system (Bennett, 2021; Calomino et al., 2025). Figure 3 compares the propulsion systems.

4 Past and current development nuclear fission systems for space exploration

Multiple attempts to integrate nuclear propulsion into space exploration have been made through various US programs since the 1950s. The Rover project and the Nuclear Engine for Rocket Vehicle Applications (NERVA) program developed and ground tested multiple nuclear thermal propulsion engines, notably achieving an NTP-produced thrust of 250,000 pounds of force, before both programs ended in the early 1970s (Gunn, 1989). Simultaneous development efforts also occurred through the Systems for Nuclear Auxiliary Program (SNAP), which sought to develop lightweight, compact nuclear electric systems for a variety of applications (Voss, 1984). These culminated in the SNAP-10A flight which flew in 1965, though short-lived due to a spacecraft failure (∼43 days), it demonstrated both nuclear power (500We) and ion propulsion, becoming the first on-orbit NEP demonstration (NASA/TM-20210016968). Later efforts in the 1980s through the SP-100 Program further attempted to develop long-lived, compact nuclear reactor space power systems (Truscello, 1983) and attempts to integrate this technology into a mission to observe Jupiter’s moons with 100 kW class NEP vehicle through the Jupiter Icy Moons Orbiter (JIMO) concept were pursued in the early 2000s by the Prometheus Project (Taylor, 2005). JIMO would have used NEP to place a single spacecraft in successive orbits around Europa, Ganymede, and Callisto to investigate their potential subsurface oceans and characterize the Jovian radiation environment (Greeley et al., 2004). Ultimately, these programs did not result in a nuclear-powered planetary science mission due to changing national priorities, budget issues, and significant technical barriers that may be mitigated today with the availability of superheavy launch vehicles.

Currently, there is renewed interest in integrating nuclear systems into spacecraft and surface operations on the Moon and Mars that could be leveraged to enable NEP or NTP vehicles for outer solar system exploration. Several U.S. government programs are actively working towards developing nuclear subsystems for varied space applications including: NASA’s Fission Surface Power (FSP) program which aims to produce a nuclear power system to support long-term human missions on the Moon and Mars, NASA and the Defense Advanced Research Projects Agency’s Demonstration Rocket for Agile Cislunar Operation (DRACO) program to demonstrate the performance of nuclear thermal propelled rocket in space, and the Air Force Research Laboratory’s Joint Energy Technology Supplying On-Orbit Nuclear Power (JETSON) program to develop a high-power nuclear system to generate electricity for use on satellites and other spacecraft (Reuter et al., 2023). The technology advances achieved by these ongoing efforts will help pave the way for future nuclear propelled planetary science missions, lowering the upfront cost for the technology developments needed. In addition, the commercial development of large, refuellable launchers promise to deliver larger masses at lower costs, further increasing the potential affordability of a nuclear-powered mission to icy worlds.

Given these recent advancements, it is worthwhile to evaluate how integrating nuclear propulsion systems into mission architectures can fundamentally change the approach to the exploration of icy Ocean Worlds. Nuclear propulsion can offer many benefits, including increased flexibility, power, mass, speed and complexity. The advantages of nuclear propulsion systems can be traded to meet specific mission objectives or tailored based on the destination. Several mission concepts evaluated by NASA Glenn Research Center’s Compass team (Turnbull et al., 2020) and NASA Marshall Flight Center’s Advanced Concept Office are reviewed here to demonstrate the capabilities of nuclear propulsion systems and argue for their integration into future icy ocean world exploration.

5 Review of mission concepts using nuclear fission systems

5.1 SERENE: Saturn Enceladus return explorer with nuclear electric propulsion

Mission at a glance profile given in Table 2. The Enceladus Sample Return and Saturn Moon Tour would leverage a 44 kWe HALEU (High Assay Low Enriched Uranium) heat pipe Brayton cycle power system integrated with NEP technology to perform a 4-stage mission that combines comprehensive orbital science at 5 Saturnian moons, an in situ investigation of surface and plume materials at Enceladus, and the return of Enceladean plume material to Earth.

TABLE 2. SERENE mission at a glance.

5.1.1 Transit to Enceladus

After a super heavy launch (Starship representative), the mission begins with an 8-year transit to the Saturn system, utilizing continuous low thrust propulsion to gradually build velocity while maintaining power for onboard systems. During transit, the mission conducts opportunistic science observations through strategic flybys of Titan, Rhea, Dione, and Tethys before spiraling into Enceladus’ orbit.

5.1.2 Enceladus orbit

At Enceladus, the NEP vehicle begins a 6-month orbital phase to identify optimal landing zones near active plume sources and perform orbital observations.

5.1.3 Lander deployment and surface operations

Once a landing site has been selected, the lander/ascent vehicle, an Orbilander-derivative (MacKenzie et al., 2021), is deployed from the NEP carrier to the surface of Enceladus. The Orbilander-derivative science payload is capable of comprehensive in situ analysis and modified to enable sample acquisition for Earth return. High level science objectives for this phase include characterizing the chemical composition of plume and surface materials, searching for potential biosignatures in plume material, determining the physical and chemical environment of the ocean, and characterizing active geologic processes (MacKenzie et al., 2021). Following surface science operations and sample acquisition, the collected material is loaded into a dedicated Earth entry capsule aboard the ascent stage.

5.1.4 Lander earth return

The ascent stage then launches from the surface and the Orbilander vehicles departs the Saturn system independently using chemical propulsion, executing an Earth-return burn for a transit of approximately 9.6 years; the entry capsule is released and recovered at Earth arrival. The lander and entry capsule designs are based on published concepts developed for the Enceladus Orbilander and Titan sample return mission (MacKenzie et al., 2021; Oleson et al., 2022) studies, respectively, as the focus of the SERENE concept was the NEP carrier vehicle.

5.1.5 NEP vehicle moon tour

Concurrently with surface operations and the Orbilander’s return transit, the NEP carrier conducts 1-year orbital science campaigns at Dione, Rhea, and Tethys and a 2-year orbit of Titan before final disposal. High level science objectives for NEP orbital observations include constraining crustal structure and subsurface ocean dynamics, mapping chemical and physical surface features, and studying atmosphere or plume properties where applicable. This parallel operation of two independent vehicles defines the split architecture and drives the total mission duration of approximately 18.8 years. An alternative architecture uses the NEP vehicle itself to return the sample, reducing the total mission duration to approximately 16 years, but does not enable an in-depth moon tour.

This mission architecture directly addresses multiple high-priority science objectives identified in the Planetary Science and Astrobiology Decadal Survey, building upon and extending concepts from the Enceladus Orbilander and Titan Orbiter and Probe mission studies. The dual approach of in situ analysis combined with Earth sample return creates a powerful framework for biosignature detection and validation. Surface measurements provide context and environmental characterization, while Earth-based laboratory analysis enables detection limits and analytical sophistication impossible to achieve with spacecraft instrumentation. Further, the multi-moon tour phase enables unprecedented comparative studies across Saturn’s diverse moon system, investigating the range of evolutionary pathways, internal structures, and potential habitability across icy worlds with varying characteristics.

The 44 kWe power system enables transport of both the substantial lander system and comprehensive orbital science payload. The NEP system also provides continuous, reliable power enabling simultaneous operation of multiple instruments throughout all mission phases. This capability is expected to return a minimum of 3 Tb of science data. While this baseline concept emphasizes increased mission complexity and enhanced payload capacity, alternative NEP configurations offer different mission optimization approaches. Recent work demonstrated that emphasizing transit speed over payload mass could reduce Saturn system arrival time to as little as 4 years, depending on payload, launch vehicle, and system power output (Duchek et al., 2025). These trades allow mission designers to tailor nuclear propulsion benefits to specific science priorities and programmatic constraints. The Enceladus Sample Return and Saturn Moon Tour concept primarily maximizes mission complexity and power, leveraging the continuous energy output of the NEP system to enable a multi-destination architecture combining orbital science, in situ surface operations, and sample return within a single mission (Figure 3).

5.2 Europa tunnelbot

Mission at a glance profile given in Table 3. The Europa Tunnelbot mission would search for evidence of life and assess habitability in Europa’s ice shell and subsurface ocean. The mission would use a 43 kW_th nuclear reactor (Poston et al., 2019) to power a probe’s 3-year descent through an estimated 20 km of ice. Communication between the Tunnelbot and surface lander would be maintained through a fiber optic cable connected to three radioisotope heater unit-powered repeaters, each carrying minimal science payloads. If the cable breaks or experiences issues due to refreezing, RF communications via the repeaters would serve as backup. This mission design enables comprehensive in situ analyses of both Europa’s ice shell and subsurface ocean.

TABLE 3. Europa tunnelbot mission at a glance.

After delivering the Tunnelbot and lander to Europa’s surface, a radioisotope thermoelectric generator (RTG) powered counter-rotating auger penetrates the top regolith layer to position the Tunnelbot at the solid ice interface. The RTG then melts ice to fill the reactor’s shield chambers with water before the nuclear reactor is activated. Once operational, the reactor powers a pumped loop system, transporting waste heat from the reactor to the tip and body of the vehicle, allowing the Tunnelbot to melt/tunnel through the ice at approximately 0.5–1 m/h while continuously collecting data on meltwater temperature, conductivity, pressure, and depth. The Tunnelbot pauses at seven predetermined temperature points during descent to conduct detailed science operations throughout the ice shell and ultimately at the subsurface ocean. More extensive details on this concept are available in the full report (Oleson et al., 2019).

The mission’s primary science goals, derived from the Europa Lander report (Hand et al., 2017), focus on assessing habitability and searching for evidence of life. The science payload addresses four key analytical objectives: (1) analyzing potential organic biosignatures, (2) detecting amino acids and their enantiomeric ratios, (3) visualizing the ice/ocean interface, and (4) assessing physicochemical parameters for habitability evaluation.

The baseline science payload includes a mass spectrometer for organic material characterization, a vibration spectrometer for detecting salts, volatiles, metals, and silicates, and cameras for visualization. Additional instrumentation for sample preparation, e.g., a liquid chromatograph, may be required depending on final payload selection. Although not required to meet science goals, seismometers could be distributed across the lander, repeaters, and Tunnelbot to create a linear array for studying ice shell dynamics through seismic event analysis. The mission is projected to generate approximately 10.5 Gb of science data and meets several objectives highlighted in the 2023 Decadal Survey.

A comparative study examined an alternative general purpose heat source (GPHS) Tunnelbot concept using 54 GPHS ²³⁸Pu bricks to provide 12 kW_th of melting power. Both reactor and GPHS designs can penetrate 20 km of ice in approximately 3 years while carrying 30 kg of science payload. The reactor design requires a larger minimum Tunnelbot diameter, but the reactor-based design also produces significantly more total thermal power, which offers commensurately greater electrical power (400 We vs. 50 We for the GPHS option), enabling more sophisticated instrumentation or continuous/simultaneous instrument operation, and would not require sourcing significant amounts of plutonium. Further, the reactor design uniquely leverages Europa’s icy environment by using meltwater for both radiation shielding and heat rejection, eliminating the mass penalty of dedicated shielding or radiators (Oleson et al., 2019). The Europa Tunnelbot concept primarily maximizes power, with the reactor’s continuous high-power output being the enabling capability, providing the thermal energy required for ice penetration and the electrical power needed for sustained in situ science operations at depths unreachable by any other means.

5.3 Uranus and Neptune systems enabled by NTP

5.3.1 NTP-enabled mission profiles for ice giant exploration

Mission at a glance profile given in Table 4. Outer planet missions using conventional chemical propulsion are constrained by gravity-assist dependencies and infrequent planetary alignments, resulting in long cruise durations and narrow launch windows. Nuclear Thermal Propulsion offers an alternative by enabling direct transfer trajectories to Uranus and Neptune, reducing transit times and broadening launch opportunities which enables a more responsive and sustainable cadence of ice giant missions.

TABLE 4. Uranus and neptune NTP orbiters mission at a glance.

The NTP performance cited in this section refers to the reference system modeled by Kumar et al. (2022): a 330MW_th class engine producing 15,000 lbf of thrust at a vacuum specific impulse of 900 s, flown as an expendable injection stage on a single commercial heavy-lift vehicle (ULA Vulcan Heavy/Blue Origin New Glenn representative). In the presented architecture, the NTP stage performs only Earth departure and the deterministic deep-space maneuver, while planetary orbit insertion is executed by the spacecraft’s storable chemical system. This eliminates the need to store liquid hydrogen for many years in deep space, significantly reducing system complexity and mission risk while preserving the performance benefits of nuclear propulsion where they are most impactful. Trip time reductions are reported relative to the gravity-assist chemical reference mission for the same destination and a comparable delivered-spacecraft class, and payload gains are reported relative to the post-insertion delivered mass of the Uranus/Neptune reference missions.

5.3.2 Uranus mission profile

For Uranus rendezvous, the NTP architecture enables delivery of a 3,000 kg spacecraft on a direct Earth–Uranus trajectory with a time of flight of approximately 8.2 years, including flyby opportunities of both Ariel and Titania. This represents a significant reduction relative to chemically powered Uranus missions, which typically require 12–15 years of cruise due to gravity-assist dependencies (Simon et al., 2021). Here the delivered payload is the 3,000 kg spacecraft placed into Uranus rendezvous on a direct, gravity-assist-free trajectory, whereas the 12–15 years chemical baseline is the gravity-assist-dependent Uranus Orbiter and Probe reference (Simon et al., 2021). On this common basis, the ∼8.2 years direct NTP transfer corresponds to a ∼30–45% cruise-time reduction. Mission concepts were designed for launch opportunities between 2025 and 2034. Alternative mission architectures targeting potential ocean worlds within the Uranus system are also feasible with a comparable cruise duration.

NTP missions to Uranus have additionally been evaluated under a payload-maximization objective, maintaining timelines consistent with reference missions, such as the Uranus Orbiter and Probe (National Research Counsel, 2011). In this variant the trajectory and timeline of that reference are fixed, a New Glenn launch with Venus–Earth–Earth–Jupiter gravity-assist trajectory profile, and a ∼12 years flight where the figure of merit is delivered mass rather than transit time. Using this profile, a hydrogen-based NTP vehicle could deliver approximately 15 mT into Uranian orbit within 12 years. This large payload capability, combined with direct orbit insertion rather than aerocapture or flyby, suggests that a single NTP vehicle could serve as a carrier platform capable of deploying multiple landers and orbiters across various moons and orbits within the Uranian system.

5.3.3 Neptune mission profile

For Neptune, the same NTP architecture enables delivery of a 2,500 kg spacecraft on a direct transfer trajectory with a trip time of approximately 13 years. Conventional chemical missions to Neptune require 12–16 years depending on launch window (Campagnola et al., 2015; Rymer et al., 2021), and non-nuclear missions require a Jupiter gravity assist to achieve comparable cruise durations, significantly impacting flexibility and increasing mission risk. For a like-for-like comparison, the Neptune references are: (i) the Ariane 5 ECA solar-electric concept, which requires two Earth and one Jupiter gravity assist and delivers spacecraft with transfer times of ∼15–20 years and (ii) the Neptune Odyssey flagship class spacecraft uses an SLS Block 2 with a Centaur kick stage on a 16 years direct (or 12 years Jupiter-gravity-assist) cruise, delivering 3,816 kg to Neptune orbit. Against these baselines, the ∼13 years direct NTP transfer removes the mandatory Jupiter gravity assist and shortens the cruise. An alternative NTP concept for Neptune (Polzin et al., 2020) evaluated configurations in which transit times were held comparable to conventional options while maximizing delivered payload mass. Using a notional Triton Orbiter/Lander reference mission (Hofstadter et al., 2017; Rymer et al., 2021) as the baseline, several NTP configurations were assessed assuming a baseline thrust of 12,500 lbf.

Both liquid hydrogen and liquid ammonia propellant architectures were considered. In these payload-maximization cases, transit time is held equal to the conventional reference, the baseline mission is the notional Triton Orbiter/Lander of the Ice Giants Pre-Decadal Survey and payload is defined as the mass delivered to Triton’s surface (lander cases) or to Neptune orbit (orbiter cases). An optimized hydrogen-based NTP vehicle on an advanced SLS launch configuration was found to deliver over seven times the payload mass of the reference mission within the same timeframe, sufficient to deliver a payload mass equivalent to two Mars Perseverance rovers to the surface of Triton within 12 years. An ammonia-based NTP configuration delivered over three times the baseline payload mass within the same transit time. Commercial launch options (New Glenn representative) using hydrogen-based NTP vehicles demonstrated payload delivery improvements exceeding 20% relative to the baseline. In all cases, use of the NTP system eliminated the need for aerocapture at the destination, freeing mass originally allocated to aeroshells for additional payload. Finally, the improved mission feasibility attributed to NTP is defined by three specific, mass- or cadence-quantified effects rather than a qualitative judgment: (i) direct transfers remove dependence on scarce planetary alignments, widening launch windows toward a near-annual cadence; (ii) direct spacecraft insertion eliminates the aerocapture aeroshell, returning that mass fraction to science payload; and (iii) the use of single commercial heavy-lift launcher removes the in-orbit assembly requirement.

5.3.4 RTG power availability and science capability

Transit duration has important implications for electrical power availability throughout the mission. Radioisotope Thermoelectric Generators experience approximately 4% power degradation per year, making cruise duration a critical determinant of available electrical power at arrival and during orbital operations (Lee and Bairstow, 2015). For Uranus, an 8.2-year NTP transfer retains approximately 71.6% of initial RTG power at arrival, compared to 57.9% for a 13.4-year chemically propelled mission—representing roughly 24% higher available power. Higher power margins support advanced instruments, higher data-rate communications, simultaneous multi-instrument operations, and extended mission lifetimes.

5.4 Nuclear electric propulsion for Uranus

Mission at a glance profile given in Table 5. Nuclear Electric Propulsion was also evaluated for Uranus missions. Following launch on a heavy-lift launch vehicle directly to Earth departure, with a single Earth flyby 2 years after launch to augment energy, an NEP system can deliver a spacecraft with 20 kW of available electric power to Uranus orbit in as few as 10 years without the need for a Jupiter gravity assist. Carrying an atmospheric entry probe and science instrument package totaling 375 kg, the spacecraft could arrive at Uranus in under 12 years with 375 kg of additional propellant providing approximately 2 km/s of electric propulsion maneuvering capability for science orbit targeting around Uranus and its moons. The 20 kW power system enables high data rate communications and a power-rich spacecraft bus for science instrument accommodation (Boylston et al., 2025).

TABLE 5. NEP uranus mission.

The combined benefits of higher delivered payload mass and greater available electrical power enable substantially more capable science missions. For Uranus, the delivered spacecraft mass supports a flagship-class orbiter and atmospheric probe, enabling comprehensive investigation of atmospheric composition, interior structure, and magnetospheric dynamics. More broadly, the payload delivery flexibility of NTP-based systems also suggests possibilities for deploying persistent space infrastructure, such as deep-space communications relays, in ice giant orbits, facilitating future science missions to the outer solar system. The ice giant mission concepts demonstrate that nuclear propulsion benefits can be selectively prioritized: the NTP architecture can be optimized toward increased speed or redirected toward increased mass delivery, enabling flagship-class payloads or carrier platforms deploying multiple assets across a moon system.

6 Remaining challenges

Realizing the mission concepts reviewed here requires resolving pre-flight challenges that remain open. No orbital nuclear propulsion system has been flight-qualified for a planetary science mission since SNAP-10A in 1965, and prior development programs including Rover, NERVA, SP-100, and Prometheus did not result in a flight system. Current NEP and NTP subsystems span a wide range of readiness levels: electric thrusters are TRL 9, but fission reactors and power conversion systems remain at lower readiness, and system-level integration has not been demonstrated (Calomino et al., 2025). Launching a fission reactor additionally requires a multi-agency safety review with no established precedent for a planetary science mission. NASA’s Space Reactor-1 (SR-1) Freedom mission, targeting launch in late 2028, is designed to address both gaps: the mission will fly a 20 kWe HALEU-fueled Brayton cycle system with a 12 kW Hall thruster on a Mars flyby, providing the first in-space demonstration of nuclear electric propulsion while setting regulatory and launch precedent for future fission systems (NASA, 2026). SR-1 is explicitly framed as the first in a deliberate sequence toward higher-power systems, though it operates at roughly half the power level of the SERENE concept reviewed here and on a substantially shorter duration; further in-space demonstration will be required before committing to an outer solar system science mission.

Once in flight, nuclear systems face engineering challenges that scale with mission duration. NTP architectures must manage liquid hydrogen stored below 20 K over multi-year cruise phases, with boiloff losses accumulating throughout and additional propellant consumed during post-burn decay heat removal (Calomino et al., 2025). NEP systems must reject substantial waste heat through radiators whose mass scales with power level; for Brayton cycle conversion, rotating mechanical components introduce failure modes not present in RTGs, and radiator degradation from micrometeorite flux over decade-scale missions reduces system efficiency (Bennett, 2021). More broadly, no space fission reactor has demonstrated sustained operation at the power levels and durations these mission concepts require, and ground testing cannot fully replicate the combined effects of the deep-space radiation environment and long-duration thermal cycling on reactor materials and power conversion hardware.

Icy ocean worlds also present planetary protection constraints that are uniquely challenging for nuclear missions. These bodies are designated COSPAR Category V restricted targets, and reactor thermal output could create localized melt pathways through ice that can introduce contamination or disrupt environments, while impact scenarios involving an intact reactor must be assessed for containment failure. The Europa Tunnelbot represents the most demanding case, requiring full sterilization of all hardware contacting ice or ocean and robust containment under all credible failure modes (Oleson et al., 2019), while sample return architectures such as SERENE carry additional requirements for the reentry vehicle. Establishing acceptable protocols for reactor-powered missions to restricted bodies will require coordination with NASA’s Office of Planetary Protection and adds both mass and schedule to mission development.

7 Conclusion

The mission concepts discussed here illustrate that nuclear propulsion does not offer a single fixed advantage, but rather a flexible design space in which speed, mass, power, and complexity can each be emphasized depending on the science objectives of a given mission. A mission prioritizing rapid sample return may favor the transit time reductions enabled by NTP, while a mission requiring sustained in situ operations at a distant target may instead leverage the continuous power output of an NEP system. Missions targeting multiple destinations within a single system can leverage both the mass delivery and power capabilities of nuclear systems to achieve a level of architectural complexity that is not accessible through conventional propulsion. Nuclear propulsion offers a versatile toolkit that can be configured to match the ambition of the science defined by the next-generation of icy ocean world exploration. As ongoing technology development programs, including the recently announced Space Reactor-1 Freedom mission, continue to mature these systems, the planetary science community has an opportunity to plan future missions with ambitions commensurate with the profound scientific questions that icy ocean worlds present.

Generative AI statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI was used to review and edit the manuscript, it was not used for drafting. Generative AI was used to polish the final version of Figure 3.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Footnotes

Edited by: Olga Prieto-Ballesteros, Center for Astrobiology, Spanish National Research Council (CSIC), Spain

Reviewed by: Heng Xie, Tsinghua University, China

Weichao Pan, Shandong Jianzhu University, China