What Is an Ion Thruster? Electric Propulsion Explained

📘 Definition

An ion thruster works by ionising a noble gas propellant (typically xenon) and accelerating the resulting ions through a high-voltage electric grid to produce thrust. The exhaust velocity of 20–50 km/s gives ion thrusters a specific impulse of 2,000–5,000 seconds — an order of magnitude better than chemical rockets (300 s). The trade-off is extremely low thrust: typically 0.01–1 newton, compared to millions of newtons for a chemical engine. This makes ion thrusters unsuitable for launch but ideal for in-space applications: orbit raising, station-keeping, and deep-space transfers where they can fire for months or years, gradually accumulating enormous delta-v. NASA's Dawn spacecraft used ion propulsion to orbit both Vesta and Ceres. Starlink satellites use a variant called Hall-effect thrusters.

2,000–5,000 s

Specific Impulse

20–50 km/s

Exhaust Velocity

0.01–1 N

Typical Thrust

Xenon (Xe)

Common Propellant

Understanding Ion Thruster

How Ion Thrusters Work

A neutral gas (usually xenon) flows into an ionisation chamber where it is bombarded by electrons, stripping atoms of one or more electrons to create positive ions. These ions are then electrostatically accelerated through a pair of charged grids — the screen grid (positive) and accelerator grid (negative) — reaching exhaust velocities of 20–50 km/s. A neutraliser (electron gun) injects electrons into the exhaust beam to prevent the spacecraft from accumulating a negative charge.

Ion Thrusters vs Chemical Rockets

Property	Ion Thruster	Chemical Rocket
Specific Impulse	2,000–5,000 s	250–450 s
Thrust	Millinewtons to 1 N	Kilonewtons to meganewtons
Propellant	Xenon, krypton	LOX/RP-1, LOX/LH2, hydrazine
Burn Duration	Months to years	Seconds to minutes
Use Case	In-space manoeuvres	Launch, orbit insertion

Notable Missions Using Ion Propulsion

NASA's Dawn mission (2007–2018) used three NSTAR ion thrusters to travel to and orbit two separate bodies in the asteroid belt — Vesta and Ceres — a feat impossible with chemical propulsion alone. ESA's SMART-1 used a Hall-effect thruster to reach the Moon. DART (the asteroid deflection mission) used the NEXT-C ion engine. And Starlink satellites each carry Hall-effect thrusters powered by krypton, used for orbit raising and collision avoidance.

Frequently Asked Questions

Why can't ion thrusters be used for launch? ▼

Ion thrusters produce extremely low thrust — typically less than 1 newton, equivalent to the weight of a sheet of paper on Earth. A rocket needs millions of newtons to overcome Earth's gravity. Ion thrusters only work in the vacuum of space where there is no gravity well to overcome and they can fire for months, gradually building speed.

What fuel do ion thrusters use? ▼

Most ion thrusters use xenon gas because it is inert (non-reactive), dense, has a high atomic mass (efficient momentum transfer), and is easy to ionise. Krypton is increasingly used as a cheaper alternative — Starlink Hall-effect thrusters use krypton. Some experimental systems use iodine or argon.