What is electric propulsion?
Electric propulsion (EP) uses electrical energy — generated by solar arrays or, theoretically, nuclear reactors — to accelerate propellant to exhaust velocities far higher than achievable with chemical combustion. The key metric is specific impulse (Isp), which measures how efficiently a propulsion system uses propellant: chemical thrusters achieve 200–450 seconds of Isp; ion thrusters and Hall-effect thrusters achieve 1,500–10,000 seconds. This means EP systems can perform the same orbit change with 5–10× less propellant mass, dramatically extending satellite lifetime or reducing launch mass.
Hall-effect thrusters
Hall-effect thrusters (HETs) are the dominant EP technology in commercial telecommunications satellites. They ionise xenon gas (or increasingly iodine or krypton) using crossed electric and magnetic fields, then accelerate the ions to 10–40 km/s exhaust velocity. A typical 300 mN HET operating on a GEO satellite consumes approximately 4.5 kW of power. SpaceX Starlink Gen1 satellites use Hall-effect thrusters operating on krypton propellant — chosen over xenon for cost reasons. Eutelsat's Konnect VHTS, the Boeing 702SP, and the Airbus Eurostar NEO all use electric propulsion for orbit raising and station-keeping.
Trade-offs vs. chemical propulsion
The principal disadvantage of EP is its extremely low thrust — millinewtons to a few Newtons, compared to kilonewtons for chemical upper stages. Orbit-raising from GTO to GEO with electric propulsion takes 6–12 months of continuous firing rather than the 1–2 days required chemically. This extended transfer phase means the satellite spends months in the Van Allen radiation belts, requiring radiation-hardened electronics. All-electric GEO satellites (Boeing 702SP variants) can be launched as a rideshare pair on Falcon 9, significantly reducing launch cost.