October 2026 update: SR-1 Freedom makes power conversion the next gate On October 20, NASA announced contract actions for the nuclear power module of Space Reactor-1 Freedom. Sierra Lobo will supply a closed Brayton cycle power conversion system, while the Department of Energy’s Idaho National Laboratory is developing the fission reactor. The converter turns reactor heat into electricity for propulsion. NASA is targeting a late-2028 launch; this is a schedule target, not a completed flight milestone. SR-1 Freedom is a Mars-bound nuclear electric demonstration, not a lunar lander and not a nuclear thermal engine. NASA plans for it to deliver the SkyFall mission’s three Mars helicopters. The cislunar relevance is the engineering chain: reactor, power conversion, heat rejection, power electronics and electric thrusters must work together. NASA also links the mission’s technology development to future fission-powered Moon Base systems. That distinction changes how readers should compare propulsion proposals. A nuclear thermal engine heats propellant directly and expels it through a nozzle. A nuclear electric vehicle carries the equipment needed to generate and distribute electricity first. Eliminating dependence on sunlight does not eliminate spacecraft mass, cooling requirements or the low-thrust trajectory problem. A 120-kilowatt test is not a flight-ready megawatt tug NASA JPL reported on April 28 that its lithium-fed magnetoplasmadynamic thruster reached 120 kilowatts during a February 24 ground test, across five ignitions. The technology uses high currents and a magnetic field to accelerate lithium plasma. JPL says this class of thruster has never flown operationally. This experiment is separate from SR-1 Freedom; the announcement does not establish that this prototype will fly on that mission. The research team aims for 500 kilowatts to 1 megawatt per thruster in coming years. Its human-Mars reference case would require 2 to 4 megawatts and more than 23,000 operating hours. Those are development objectives and mission needs, not demonstrated endurance. Electrode temperatures exceeded 2,800 degrees Celsius during the test, making lifetime and erosion central questions. For a lunar logistics buyer, the useful comparison is delivered payload per trip at a stated arrival date. Ask whether quoted power is generated power or power reaching the thruster, whether the mass budget includes radiators and shielding, and whether the lifetime test used the intended operating conditions. High specific impulse reduces propellant demand; it does not, by itself, prove a cheaper transport service. Sources: NASA’s October 20 SR-1 Freedom update ; JPL’s April 28 thruster test report ; NASA’s nuclear propulsion technical overview . Reviewed October 23, 2026. Getting a spacecraft to the Moon is not just about pointing it in the right direction and lighting a match. The journey from Earth orbit to lunar orbit, and every maneuver in between, demands propulsion systems chosen carefully for thrust level, fuel efficiency, time constraints, and payload mass. Pick the wrong engine for the mission and you either run out of propellant or take six months to arrive somewhere you needed to be in three days. As cislunar space fills with commercial landers, Gateway modules, crewed Orion capsules, and future logistics tugs, the industry is converging on three core propulsion families: chemical propulsion , solar electric propulsion (SEP) , and nuclear thermal propulsion (NTP) . Each solves a different problem. None is universally superior. Understanding the tradeoffs is the starting point for understanding why cislunar missions are designed the way they are. AI-generated image Solar electric propulsion spacecraft maneuvering in cislunar space. AI-generated illustration. The Basic Physics: Thrust vs. Efficiency Every rocket engine is defined by two numbers that pull against each other: thrust and specific impulse (Isp) . Thrust is the raw force the engine produces, measured in newtons or pounds-force. Specific impulse is a measure of fuel efficiency. It tells you how long a given weight of propellant can produce a pound of thrust. Higher Isp means less propellant needed for the same velocity change, which translates directly into lower launch mass and lower cost. Chemical engines provide high thrust by releasing chemical energy. Electric engines use electrical power to accelerate propellant to much higher exhaust velocities, trading thrust for propellant efficiency. Nuclear thermal propulsion heats hydrogen directly in a reactor; its performance advantage comes partly from hydrogen’s low molecular mass, not from a blanket claim that reactors exceed all chemical flame temperatures. Nuclear electric propulsion instead converts reactor heat into electricity for electric thrusters. ~450 s Chemical Isp (LH2/LOX) ~900 s Nuclear Thermal Isp 2,000+ s Solar Electric Isp High Chemical Thrust Medium Nuclear Thermal Thrust Very Low SEP Thrust The mission profile determines which combination of those properties matters most. A crewed lunar landing needs to decelerate from trans-lunar injection velocities in hours, not weeks. That demands high thrust. A cargo module delivering supplies to Gateway has weeks or months of schedule margin and cares deeply about how much propellant it burns, because propellant mass is money. An in-space tug that repositions satellites in cislunar orbits wants a combination of both. These different requirements are why the industry is not converging on one engine type, but on a portfolio. Chemical Propulsion: High Thrust, Long Heritage Chemical propulsion has powered every crewed mission to the Moon. It works by combusting a fuel and oxidizer at high temperature and pressure, forcing hot gas through a nozzle to generate thrust. The primary configurations relevant to cislunar missions are liquid oxygen / liquid hydrogen (LOX/LH2) , which delivers the highest Isp of any chemical combination at around 450 seconds, and LOX/methane , which is becoming increasingly relevant for reusable systems. NASA's Space Launch System uses four RS-25 engines burning LOX/LH2 for the core stage. The Interim Cryogenic Propulsion Stage (ICPS) for Artemis I and II, and the more powerful Exploration Upper Stage for later flights, use the same propellant combination to inject the Orion capsule onto a trans-lunar trajectory. For lunar landing, SpaceX's Starship Human Landing System uses methane and liquid oxygen, accepting slightly lower Isp in exchange for propellant that can potentially be produced at Mars using in-situ resources. Why Chemical Still Dominates Crewed Missions The laws of orbital mechanics require large velocity changes to enter and exit lunar orbit on short timescales. A crewed spacecraft approaching the Moon must decelerate hard in a single burn. Solar electric propulsion cannot produce enough thrust to do this in any reasonable timeframe. Chemical engines are the only currently flight-proven option for human-rated lunar landing and ascent maneuvers. The downside of chemical propulsion for cislunar logistics is propellant mass. Getting to the Moon and back requires carrying all that propellant from Earth, which is expensive. For a cargo tug making repeated runs between low Earth orbit and lunar orbit, the propellant bill becomes the dominant cost driver. This is what makes higher-efficiency alternatives so attractive for commercial cislunar logistics, even if they require more time. System Propellants Isp (s) Role SLS RS-25 LOX/LH2 452 Earth ascent Starship HLS LOX/CH4 ~380 Lunar landing/ascent Blue Moon Mk1 LOX/LH2 ~450 Lunar surface delivery IM Nova-D LOX/CH4 ~360 Polar lunar delivery Solar Electric Propulsion: Slow, Efficient, and Getting More Powerful Solar electric propulsion uses photovoltaic arrays to generate electricity, which is then used to ionize a propellant (typically xenon or krypton) and accelerate those ions to high velocities using electromagnetic fields. The e