WASHINGTON, July 20 — The ion thruster, a form of electric propulsion for spacecraft, has quietly become a workhorse of deep-space exploration, despite generating thrust measured in millinewtons rather than kilonewtons. For all the noise around chemical rockets, the salient point is that these engines offer something no combustion-based system can: sustained, efficient acceleration over years.
The basic principle is straightforward. Ion thrusters ionise a neutral gas — typically xenon — strip electrons from the atoms, then accelerate the resulting positive ions using electricity. Two broad categories exist. Electrostatic thrusters use the Coulomb force to push ions along an electric field, with a neutraliser later reinjecting electrons to keep the exhaust electrically neutral. Electromagnetic thrusters, often called plasma propulsion engines, rely on the Lorentz force to accelerate all species — electrons and both positive and negative ions — in the same direction, irrespective of charge.
Operational ion thrusters, the source notes, typically consume 1–7 kW of power, produce exhaust velocities of 20–50 km/s (specific impulse 2,000–5,000 seconds), and deliver thrusts of 25–250 mN with propulsive efficiency between 65 and 80 per cent. Experimental versions have pushed to 100 kW and 5 N of thrust — enough to lift a small apple, but not a spacecraft off a launch pad.
**Notable Missions**
Two NASA missions stand out. Deep Space 1, launched in 1998, changed its velocity by 4.3 km/s using 73.4 kg of xenon propellant. The Dawn spacecraft, launched in 2007, achieved a velocity change of 11.5 km/s while consuming 425 kg of xenon — though with less efficiency, the source notes.
Applications extend beyond deep space. Some satellites carry dozens of low-power ion thrusters for attitude control and station-keeping. They have served as main propulsion for low-mass robotic vehicles like Deep Space 1 and Dawn. China’s Tiangong space station also uses ion thrusters for propulsion.
**Limitations and a Quirk**
An ion engine cannot generate enough thrust for initial liftoff from any celestial body with significant surface gravity. Spacecraft must still rely on conventional chemical rockets or non-rocket launch technologies to reach orbit. The thrust is so minuscule that ion thrusters are practical only in the vacuum of space — except, notably, for a development by the Massachusetts Institute of Technology. MIT has designed ion-thruster systems using ultra-light materials and low-drag aerofoils capable of flying short distances at low speeds at ground level.
**A Brief History**
The concept dates to 1911, when Konstantin Tsiolkovsky first suggested electric propulsion. Hermann Oberth discussed it in his 1929 book *Wege zur Raumschiffahrt*. Science-fiction writer Jack Williamson popularised the idea in 1947. The first working ion thruster was built by Harold R. Kaufman in 1959 at NASA’s Glenn Research Center, using mercury propellant. Suborbital tests followed in the 1960s, culminating in the Space Electric Rocket Test-1 (SERT-1) in 1964, which operated for 31 minutes on a suborbital flight before falling to Earth. A subsequent orbital test, SERT-2, was conducted in 1970. (The source also notes that Voskhod 1 carried the first multi-crew spaceflight on 12 October 1964, though that mission is not directly related to ion thrusters.)
**What to Watch**
With experimental thrusters already demonstrating 5 N of thrust and 100 kW of power, the technology continues to inch — or rather, millinewton — toward greater capability. For low-mass robotic probes, ion drives will likely remain the propulsion method of choice. The MIT ground-level experiments suggest that terrestrial applications, though limited, are no longer purely theoretical. For all the decades of development, the ion thruster’s real breakthrough may still lie ahead.
