The vast distances of space travel are a formidable obstacle to mission planners that aim to reach destinations in the solar system within reasonable time frames. A manned mission to Mars powered by conventional chemical rockets would take about six months to reach the red planet.

However, a different type of engine known as an ion thruster could slash that travel time significantly. Ion thrusters accelerate ions (usually xenon) to high speeds and expel them to generate thrust. This electric propulsion technique has tremendous speed potential, with top speeds around 40 km/s, compared to 5 km/s for chemical rockets.

Origins

SERT-1’s program manager, Raymond J. Rulis, stands next to the spacecraft before pre-flight testing. The spacecraft’s mercury-bombardment ion engine, visible on the right side, successfully fired for 31 minutes during the mission, while the cesium-contact ion engine, seen on the left side, failed to operate. Source: NASA

The concept of expelling positively charged particles to generate thrust is not new. American engineer Robert H. Goddard expounded on the idea in his notebook as early as 1906 and first experimented with ion thrusters in 1916. But it wasn’t until 1964 that an ion engine was fired successfully in space for the first time, aboard NASA’s Space Electric Rocket Test (SERT-1). The mission was followed by a second test, SERT-2 in 1970, that operated its ion thruster continuously for 161 days.

The early ion engines employed aboard the SERT spacecraft used cesium and mercury propellants, which presented a number of difficulties. They needed to be heated to gaseous form and after exiting the engine the ions tended to condense and deposit on the spacecraft’s exterior.

NASA switched to xenon propellant for its later ion thrusters. An inert gas, xenon does not present an explosion risk. It is also relatively easy to ionize, can be stored at high density and features a high atomic mass to provide a non-trivial amount of thrust per ion.

In 1998, NASA launched Deep Space 1, debuting its NASA Solar Technology Application Readiness (NSTAR) xenon gridded electrostatic ion thruster. The spacecraft was the first to reach another planetary body with ion propulsion, flying by asteroid 9969 Braille and comet 19P/Borrelly.

The United States wasn’t the only nation investigating electric propulsion. Russia conducted its own research into the field, focusing on a type of ion engine known as the Hall effect thruster. These propulsive devices have operated on Russian spacecraft since the early 1970s.

An artist’s representation of the Dawn spacecraft, which had an unprecedented total delta-v capability of 10 km/s following launch thanks to its three ion thrusters and 425 kg of xenon propellant. Source: NASA

In 2007, NASA followed up Deep Space 1 with Dawn. The design for Dawn’s three, redundant ion engines stems from the same NSTAR technology used on Deep Space 1. Dawn’s ion propulsion technology enabled it to become the first spacecraft to orbit two targets, orbiting the asteroid Vesta before departing for a trip to Ceres to orbit around the dwarf planet.

This impressive feat was possible because of the ion thrusters’ superior efficiency. A comparable mission using a chemical rocket propulsion system would consume ten times more fuel mass, requiring a spacecraft with much more fuel storage and a more powerful booster to launch it from Earth.

The Little Engine That Could

Ion thruster technology is one of the most efficient methods of spacecraft propulsion, characterized by large specific impulses. In other words, relative to other forms of propulsion, the amount of thrust produced by ion engines is high compared to the rate at which they consume propellant. This means spacecraft with ion thrusters don’t need to launch with large fuel reserves, leaving much more room for useful payload like science instruments.

Ion thrusters have specific impulses (an equivalent measure of the “miles per gallon” of a spacecraft) ten times higher than traditional chemical rockets. Source: NASA

The drawback is that the magnitude of thrust produced by ion engines is low, making them unsuitable as launch vehicles to boost spacecraft into orbit. Dawn’s ion thrusters each produced only 91 mN of thrust, equivalent to the force needed to keep a sheet of paper from falling to the ground on Earth.

Once outside of our planet’s large gravity well, however, ion thrusters shine. They can operate continuously over many months, slowly sipping fuel to incrementally build up a spacecraft’s speed to extraordinary heights. Since maximum spacecraft speed is limited by the velocity at which propellants exit their thrusters, and ion thrusters eject propellant at very high velocities, spacecraft equipped with ion thrusters can reach very high speeds.

On the other hand, the thrust produced by ion engines is limited by the amount of power available on the spacecraft. The size of the power supply required to drive a large ion thruster can outweigh the mass savings that result from ion propulsion’s fuel efficiency. As energy generation and power conversion technology advances, light-weight, high-power systems will enable higher-thrust ion engines with the capability to propel large spacecraft.

Early in the development of ion thrusters, a major weakness which limited their operational lifespan was thought to be erosion of their channel walls and acceleration grids by the impact of ions. Design and material improvements have limited the effect of this issue, as evidenced by the success of long-term space missions like Dawn. Indeed, NASA successfully fired an advanced high-thrust ion engine for over 50,000 hours in a long duration test, proving that ion thrusters have the durability required for deep space missions that last years.

Stay tuned for part 2 of this series for a look at how ion thrusters work, as well as some of their future applications.