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. The relationship between thrust and Isp is not accidental. Chemical engines burn propellants at high temperatures to create high-pressure exhaust, producing lots of thrust but at relatively low exhaust velocities (low Isp). Electric engines accelerate propellant using electromagnetic fields to extremely high velocities (high Isp), but the power required to do that limits how much mass per second can be expelled, producing very low thrust. Nuclear thermal sits between the two: reactors heat hydrogen to temperatures no chemical flame can match, yielding Isp roughly double that of chemical engines at usable thrust levels. ~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, TLI assist 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 exhaust velocity is far higher than any chemical reaction can achieve, producing specific impulse values between 1,500 and 3,000 seconds depending on power level and thruster type. The tradeoff is thrust. A 12-kilowatt Hall thruster produces roughly 600 millinewtons of thrust. For context, a garden hose with moderate pressure produces more force than that. What saves SEP is that cislunar space is largely a gravitational gradient. A spacecraft does not need to slam on the brakes in one burn to reach the Moon. Given enough time, even millinewton-level thrust can raise an orbit from low Earth orbit to lunar distance through a series of slow, continuous spirals. For cargo missions without strict timelines, this is a significant economic advantage. Burning 1 kilogram of xenon in an SEP system achieves the same velocity change that would require 5 to 10 kilograms of chemical propellant. AI-generated image Hall effect thrusters ionize xenon propellant and accelerate it electromagnetically to produce thrust at extremely high efficiency. Credit: AI-generated illustration. NASA's Lunar Gateway depends on SEP for its power and propulsion. The Power and Propulsion Element (PPE) built by Maxar Technologies is equipped with two 12-kilowatt Advanced Electric Propulsion System (AEPS) Hall thrusters developed by Aerojet Rocketdyne under contract to NASA's Glenn Research Center. Together they produce 60 kilowatts of solar power, enough to run station systems and keep the Gateway in its Near-Rectilinear Halo Orbit with regular station-keeping burns. The AEPS thrusters completed more than 5,000 hours of testing at Glenn before the PPE program progressed to launch readiness. SEP Trade Space • Transit time: Months vs. days for chemical; not viable for time-critical crewed missions • Propellant mass: Typically 5-10x more propellant-efficient than chemical for equivalent delta-v • Power dependency: Performance drops with distance from Sun; solar panels add mass and area • Xenon cost: Increasing demand is driving prices higher; krypton alternatives being explored • Best mission fit: Cargo delivery, station-keeping, satellite servicing, orbit raising Several commercial operators are developing SEP-equipped cislunar vehicles. Orbit Fab has proposed xenon refueling depots to extend mission life. Momentus designed a propellant-efficient transfer vehicle for small satellite deployment. The Gateway PPE is the largest and most powerful SEP system ever planned for deep space, but it will not be the last. As solar array technology improves and Hall thruster lifetime increases, the efficiency ceiling for SEP continues to climb. Nuclear Thermal Propulsion: High Pe