As humanity pushes further into the cosmos, the environmental impact of rocketry on Earth and in the upper atmosphere can no longer be ignored. Traditional propellants such as kerosene (RP-1), hydrazine, and solid ammonium perchlorate composites produce exhausts rich in carbon dioxide, soot, chlorine compounds, and toxic byproducts that contribute to ozone depletion, local air pollution, and ground contamination. With launch cadences accelerating toward weekly or even daily operations, the imperative to develop cleaner alternatives has never been more urgent. Recent years have witnessed significant progress in environmentally friendly propellants—from cryogenic liquid hydrogen and high-performance “green” monopropellants to hybrid systems and electric thrusters. These innovations promise to reduce toxicity, lower greenhouse gas emissions, and enable more sustainable spaceflight without sacrificing the reliability and thrust needed for a wide range of missions.

The Environmental Toll of Conventional Propellants

To appreciate the importance of greener alternatives, one must first understand the legacy of traditional rocket fuels. Kerosene-based rockets (e.g., the Soyuz, Falcon 9, and Atlas V) burn RP-1 refined kerosene with liquid oxygen, producing carbon dioxide, water, and a small amount of soot. While not as immediately toxic as some hypergolic fuels, kerosene combustion releases up to 2.7 kg of CO₂ per kg of propellant—plus black carbon particles that absorb solar radiation in the stratosphere. Solid rocket boosters, used on the Space Shuttle and Europe’s Ariane 5, contain ammonium perchlorate oxidizer, which generates chlorine compounds that deplete stratospheric ozone. Ground contamination around launch sites from perchlorate salts has also been documented, raising health concerns for nearby communities.

Even more problematic are hypergolic propellants like hydrazine (N₂H₄) and its derivatives (monomethylhydrazine, unsymmetrical dimethylhydrazine). Used extensively in satellite propulsion, upper stages, and interplanetary probes, these compounds are highly toxic, carcinogenic, and require specialized handling in bulk. Spills during transport or pre-launch processing have led to evacuations and environmental cleanups. As the space industry expands, the cumulative burden of these traditional propellants becomes unsustainable.

Categories of Eco-Friendly Propellants

Researchers and agencies around the world are pursuing several distinct families of greener propellants, each offering a different balance of performance, cost, and environmental benefit. Below we examine the most promising categories.

Liquid Hydrogen and Liquid Oxygen (LH2/LOX)

Among the cleanest options available, the combination of liquid hydrogen and liquid oxygen produces virtually no harmful exhaust—its primary combustion product is water vapor. Used in the upper stages of the Space Launch System, the Ariane 5’s Vulcain engine, and Japan’s H-IIA rocket, LH2/LOX eliminates CO₂, soot, and toxic residuals. However, hydrogen’s low density (70 kg/m³ as a liquid) requires large, well-insulated tanks, and its cryogenic storage ( −253 °C) adds complexity. Production of liquid hydrogen also has a carbon footprint unless the hydrogen is generated via electrolysis using renewable energy. Despite these challenges, LH2/LOX remains a cornerstone of clean propulsion, especially for heavy-lift and deep-space stages.

Green Monopropellants: AF-M315E and LMP-103S

Traditional monopropellant thrusters (e.g., hydrazine) rely on a catalyst bed to decompose the fuel. Green monopropellants are designed to be non-toxic, stable at ambient temperature, and easier to handle, while delivering higher specific impulse. The two leading formulations are AF-M315E, developed by the U.S. Air Force Research Laboratory, and LMP-103S, developed by ECAPS (Sweden). AF-M315E is a hydroxylammonium nitrate-based compound that achieves a specific impulse of ~250 s (compared to ~220 s for hydrazine). NASA’s Green Propellant Infusion Mission (GPIM), launched in 2019, successfully demonstrated AF-M315E on a small satellite, validating its performance and safety. LMP-103S, a blend of ammonium dinitramide (ADN) and fuel, has been used on several commercial satellites, including the PRISMA mission and the SkySat constellation, and is now qualified for ESA programs. Both propellants eliminate the need for hazardous handling suits and scrubbers, drastically reducing ground operations costs and environmental risks.

Ammonium Dinitramide-Based Formulations

While AF-M315E uses hydroxylammonium nitrate, a parallel stream of research focuses on ammonium dinitramide (ADN) as the oxidizer. ADN-based propellants, such as those developed under the EU’s HYPROGEO project, combine ADN with water, methanol, or other fuels. These formulations are insensitive to shock and have lower freezing points than hydrazine, allowing simpler spacecraft thermal management. In laboratory tests, ADN-based propellants have shown comparable or better performance than hydrazine, with zero plume toxicity and reduced environmental persistence. The challenge lies in optimizing the catalyst bed to ensure complete and stable combustion, especially for pulse-mode operations in attitude control thrusters.

Hybrid Rocket Propellants

Hybrid rockets, which use a solid fuel grain (often a polymer like hydroxyl-terminated polybutadiene, HTPB) with a liquid oxidizer (typically N₂O or LOX), occupy a middle ground between solid and liquid systems. They offer throttle-ability, shutdown capability, and simpler design compared to liquid engines, while eliminating the catastrophic explosion risk of solid boosters. Recent developments focus on “green” oxidizers like nitrous oxide (N₂O), which decomposes to non-toxic gases, and on wax-based or paraffin-based fuels that burn more cleanly than HTPB. Experiments with cryogenic oxygen and non-toxic gelling agents have further reduced emissions. The key advantage of hybrid propulsion is its inherently safer handling and reduced environmental contamination at launch sites. Private companies like Virgin Galactic and Rocket Lab (with its now-retired Electron’s hybrid stage) have demonstrated operational viability, though scaling to larger vehicles remains a technical hurdle.

Electric Propulsion: Ion and Hall Thrusters

Though not chemical propellants in the traditional sense, electric propulsion systems provide an extremely clean and efficient method for spacecraft maneuvering and deep-space missions. Ion thrusters and Hall-effect thrusters use inert gases such as xenon or krypton as propellant. The gas is ionized and accelerated by electric fields, producing exhaust velocities far higher than chemical rockets. Because the propellant is a noble gas, there is zero toxicity and no combustion byproducts. Solar arrays or nuclear reactors provide the power, enabling long-duration, low-thrust operations. NASA’s Dawn mission to Vesta and Ceres, as well as the upcoming Psyche mission, rely on Hall thrusters. The main environmental drawback is the energy required to produce high-purity xenon, but the overall lifecycle emissions are much lower than chemical alternatives.

Recent Breakthroughs and Notable Missions

The past few years have seen the green propellant shelf transition from laboratory curiosity to flight-proven hardware. NASA’s GPIM (2019) was a landmark: the 180-kg spacecraft demonstrated AF-M315E on a 1-N thruster, executing orbit adjustments and attitude control while downlinking telemetry on propellant performance. Data showed no degradation of the catalyst bed over the mission lifetime, confirming the propellant’s readiness for operational use. ESA’s ongoing LMP-103S qualification has led to its adoption on the European Small Geostationary Satellite (SmallGEO) platform and on the upcoming Hera asteroid mission.

Another breakthrough came in 2023 when a team at the University of Stuttgart successfully tested a 500 N ADN-based thruster, the largest green monopropellant engine ever fired, achieving 97% of theoretical performance. Such scaling demonstrates that green propellants can replace hydrazine not only in small reaction control thrusters but also in main propulsion for orbital transfer vehicles. Additionally, the U.S. Department of Defense has begun transitioning satellite propulsion systems to green alternatives, citing both safety and logistics benefits.

On the hybrid front, the company Virgin Galactic flew its SpaceShipTwo (now VSS Unity) using a hybrid engine that burns hydroxyl-terminated polybutadiene with nitrous oxide. While not entirely emission-free (N₂O has a global warming potential ~300 times CO₂), the hybrid approach eliminated chlorine and perchlorate contamination. Research into N₂O decomposition catalysts aims to further reduce the oxidizer’s greenhouse effect.

Challenges to Widespread Adoption

Despite the clear environmental and safety advantages, the road to replacing legacy propellants is strewn with obstacles. Cost is perhaps the most immediate. Green monopropellants like AF-M315E and LMP-103S are currently more expensive per kilogram than hydrazine, partly due to smaller production volumes and the need for specialized synthesis processes. However, when factoring in the savings from simplified handling, reduced personal protective equipment, and lower insurance premiums, the total lifecycle cost can be competitive.

Infrastructure also poses a challenge. Launch sites and satellite fueling facilities are designed around hydrazine and other hypergolic fuels; converting to new propellants requires modifications to storage tanks, transfer lines, and safety protocols. Certification for flight is a multi-year process involving extensive ground testing to validate performance across all expected mission scenarios. Regulatory bodies such as the U.S. Environmental Protection Agency and the European Chemicals Agency are still establishing guidelines for the new propellants.

A third issue is performance trade-offs. While green monopropellants offer specific impulse improvements over hydrazine, they do not match the density impulse of some hypergolic families, and their freezing points may be higher, requiring heaters. For electric propulsion, the low thrust (sub-Newton levels) makes it unsuitable for launching from Earth’s surface or for rapid orbit insertions. Therefore, a hybrid architecture—using chemical boost for launch and green monopropellants or electric propulsion for orbit—is often necessary.

The Role of Regulation and International Cooperation

Regulatory pressure is accelerating the shift toward greener rocket propulsion. The European Space Agency’s Clean Space initiative, launched in 2013, explicitly includes the development of non-toxic propellants as one of its key technology priorities. ESA’s Future Launchers Preparatory Programme funds projects like HYPROGEO and the green thruster qualification for LMP-103S. In the United States, NASA’s Space Technology Mission Directorate has invested heavily in AF-M315E and supports the development of green alternatives through Small Business Innovation Research (SBIR) awards.

International cooperation is essential because many launch complexes are shared or border waterways (e.g., Guiana Space Centre, Cape Canaveral, Baikonur). The International Association for the Advancement of Space Safety (IAASS) and the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) have called for the reduction of toxic propellant usage. With the rise of large constellations (e.g., Starlink, OneWeb) that require hundreds of satellites with propulsion systems, the cumulative environmental impact of hydrazine-based thrusters becomes non-negligible, driving further regulatory interest.

Future Outlook

The next decade will likely see green propellants become the norm for new satellite platforms, with AF-M315E, LMP-103S, and ADN variants capturing a growing share of the market. NASA’s planned use of green monopropellants on the Lunar Gateway and on landers for the Artemis program signals confidence in their reliability. The development of “green” solid rocket motors—using environmentally benign binders and oxidizers such as ammonium nitrate combined with novel catalysts—is still in its infancy but shows promise for launch vehicle boosters. For deep space, electric propulsion powered by nuclear reactors could eventually eliminate the need for chemical propellants altogether beyond low Earth orbit.

Continued investment in production scale-up will drive down costs, while advanced catalyst materials (e.g., including iridium-free formulations) will improve thruster longevity. The emergence of reusable rockets will also benefit from greener fuels: engines that burn RP-1 produce soot that contaminates engine components and complicates reusability; transparent exhaust from LH2/LOX or green monopropellants eases refurbishment.

Ultimately, the transition to environmentally friendly propellants is not just a matter of regulatory compliance—it is a necessary evolution if spaceflight is to become a truly sustainable industry. The technical hurdles are real but surmountable; the payoffs include safer launch pads, reduced atmospheric pollution, and a smaller chemical footprint on Earth and beyond. The rocket engines of the coming decades will be cleaner, safer, and just as powerful as those of the past—if the commitment to innovation holds strong.

For further reading, see the NASA GPIM page; an overview of green monopropellant performance from ESA’s Clean Space; and technical details on AF-M315E in a SpaceNews article. Additional insights on LMP-103S can be found in the ECAPS technical literature.