An ion thruster produces about the thrust of a sheet of paper resting on your hand. It is not going to lift anything off the ground. But the same thruster can accelerate a spacecraft continuously for months or years, using a fraction of the propellant a chemical rocket would need. That tradeoff — tiny thrust, extraordinary efficiency — makes electric propulsion the dominant technology for a surprising fraction of modern spacecraft.
How Electric Propulsion Works
All rocket propulsion throws mass out the back to push the vehicle forward. Chemical rockets throw a lot of mass, not very fast, using the energy stored in the propellants themselves. Electric propulsion throws a small amount of mass, extremely fast, using energy from solar panels or a nuclear source.
The two most common types in flight today:
- Gridded ion thrusters. Neutral propellant atoms (usually xenon) are ionised inside a chamber, then accelerated through a series of charged grids that produce an electric field. The resulting ion beam exits at 30–50 km/s. A neutraliser adds electrons back to the beam outside the thruster to prevent the spacecraft from charging up. The NSTAR and NEXT thrusters used on Deep Space 1 and Dawn are the best-known examples.
- Hall-effect thrusters. Similar in concept but architecturally simpler: ionisation and acceleration happen in the same chamber, using a radial magnetic field to trap electrons and ionise the propellant they collide with. Hall thrusters produce higher thrust than gridded ion engines at somewhat lower specific impulse. They dominate the commercial market for satellite station-keeping and are increasingly used for orbit-raising.
Both types produce thrust measured in millinewtons to about a newton. To put that in perspective: a Falcon 9 first stage produces about 7.6 million newtons. An ion thruster is about seven orders of magnitude weaker. But it can keep firing for years.
Why It Works
The relevant measure of propellant efficiency is specific impulse (Isp) — how much thrust you get per unit of propellant consumed per second, measured in seconds. A kerosene-oxygen rocket has an Isp around 300 seconds. Hydrogen-oxygen hits about 450. Ion thrusters routinely hit 3,000 to 5,000, with research-grade Hall thrusters pushing higher.
What that means in practice: for a given mission delta-v, an electric-propulsion spacecraft needs a small fraction of the propellant a chemical spacecraft would need. A Falcon 9 upper stage burning chemical fuel to enter low Earth orbit, then using Hall thrusters to spiral slowly to a geostationary orbit, can deliver substantially more payload than the same launcher doing the entire job chemically — at the cost of taking several months rather than hours to reach GEO.
For deep-space science missions, the calculation is even more favourable. Missions that would be impossible with chemical propulsion alone, like asteroid rendezvous or main-belt orbiters, become tractable when the spacecraft can accelerate continuously.
What's Flying
Electric propulsion is no longer experimental. It is routine.
- Dawn (NASA, 2007–2018) used three NSTAR ion engines to visit and orbit both Vesta and Ceres — the first spacecraft to orbit two separate extraterrestrial bodies on a single mission.
- Hayabusa and Hayabusa2 (JAXA) used microwave-powered ion thrusters to rendezvous with asteroids Itokawa and Ryugu and return samples to Earth.
- BepiColombo (ESA/JAXA) is currently using four T6 ion engines on its long, gravity-assisted cruise to Mercury orbit insertion.
- Commercial geostationary satellites. The majority of new GEO communications satellites use Hall thrusters for both orbit-raising from a geostationary transfer orbit and station-keeping throughout their operational life. Boeing's all-electric 702SP platform was the first widely flown example; nearly every operator has a similar product line now.
- Starlink. Each Starlink satellite uses a krypton or argon Hall thruster for altitude maintenance, collision avoidance, and controlled deorbit. The constellation is the single largest deployment of electric propulsion in history.
- Psyche (NASA, 2023) is using four SPT-140 Hall thrusters for its cruise to the metal asteroid 16 Psyche.
The Gateway's PPE
The Power and Propulsion Element of the Lunar Gateway is a purpose-built electric-propulsion module using a set of AEPS Hall thrusters — larger and more powerful than any flown commercially. It is what actually holds Gateway in its near-rectilinear halo orbit around the Moon, with enough thrust to perform the required station-keeping manoeuvres and to reposition Gateway to different lunar orbits for different phases of the Artemis programme.
The PPE matters partly because it is the first application of high-power electric propulsion in a crewed architecture. The Artemis missions themselves use chemical propulsion for the fast transfer between Earth and Moon; electric propulsion handles the slow, efficient work of keeping Gateway where it needs to be. The same basic architecture — chemical for fast transits, electric for sustained manoeuvres — is likely to be the template for future Mars-transit vehicles.
Limitations
Electric propulsion cannot do two things. It cannot launch from the ground — the thrust is simply too low. And it cannot do time-critical manoeuvres, because building up a meaningful delta-v takes weeks or months. A spacecraft that needs to dodge an incoming impactor, perform a powered descent, or catch a departing launch window cannot rely on ion thrusters.
Everything else — satellite station-keeping, orbit raising, deep-space cruise, asteroid rendezvous, sustained station operations — is increasingly electric. See the nuclear thermal propulsion article for the other candidate architecture for crewed deep-space travel, which sits between chemical and electric in the thrust-and-efficiency trade space.