What is green propulsion?
Green propulsion refers to spacecraft propulsion technologies that offer environmental and safety advantages over conventional hydrazine (N₂H₄) monopropellant — historically the dominant thruster propellant for attitude control and small manoeuvres on satellites. Hydrazine is highly toxic (IDLH: 50 ppm, suspected carcinogen), vapour-pressure sensitive, and requires elaborate safety protocols for handling and launch vehicle integration. 'Green' propellants either eliminate the toxicity concern or replace chemical combustion entirely with non-toxic alternatives.
High-performance green propellants
Two ionic liquid-based green propellants have reached commercial deployment. LMP-103S (ammonium dinitramide / methanol / water mixture, ECAPS HPGP thruster): Isp of 252 seconds — 12% better than hydrazine. First used on PRISMA technology demonstration satellite (2010); now qualified on dozens of commercial satellites. AF-M315E (hydroxyl ammonium nitrate fuel-oxidiser mixture, Aerojet Rocketdyne AF thruster): Isp of 256 seconds; demonstrated on NASA's GPIM (Green Propellant Infusion Mission) satellite (2019). Both propellants are classified as 'low-hazard' — simplifying launch integration and ground handling.
Electric propulsion as the ultimate green option
Electric propulsion (Hall-effect thrusters, ion engines) using inert noble gas propellants (xenon, krypton, argon) is the most environmentally clean option: no toxic materials, no combustion products, and the propellant itself is harmless. The performance advantage (Isp of 1,500–3,000 seconds) dramatically reduces propellant mass, benefiting launch economics even before considering environmental factors. The trend toward electric propulsion for station-keeping and orbit raising on GEO and LEO satellites is inherently aligned with green propulsion objectives.