Feature | August 21, 2018

A Brief History of Ion Propulsion

By Zoë Webb-Mack

Dawn is a mission of many firsts. The first mission to orbit two interplanetary bodies, the first to orbit a body in the asteroid belt, and the first to orbit a dwarf planet, Dawn has accomplished many feats in its decade of flight. It has shed light on giant asteroid Vesta and dwarf planet Ceres, providing new perspectives on the two worlds’ fascinating evolutionary paths and the history of our solar system. Dawn was also the first mission devoted exclusively to science to be enabled by an ion propulsion system, an incredible technological accomplishment without which Dawn’s multi-world mission would have been impossible.

Ion propulsion isn’t new. In fact, ion propulsion, a type of electric propulsion, was originally conceived of in the early 1900s. Despite its relatively early conception, its journey from being the subject of intellectual curiosity to being used on spacecraft was a long one.

Though a few dedicated rocket scientists, particularly the famed Robert Goddard and Konstantin Tsiolkovsky, studied the technology during the early 1900s, ion propulsion was considered more science fiction than reality until the 1950s, when interest in rocketry following World War II and leading into the Cold War took off (no pun intended). And since ion propulsion requires near-vacuum conditions and does not produce enough thrust to lift off, its technological requirements outpaced the chemically propelled rocketry needed to launch such a spacecraft. Finally, however, in 1964, NASA’s Space Electric Rocket Test I (SERT I) tested this technology in space for the first time, operating its ion thruster for just over half an hour.

But what exactly is ion propulsion? Ion propulsion systems ionize (charge) atoms and then exploit their non-neutral charge to expel them from the spacecraft, creating thrust. Dawn’s gridded ion thrusters achieve this by accelerating xenon, ionized by an electron beam, through a voltage between two charged grids. Other ion thrusters, like Hall-effect thrusters, which use magnetic fields to ionize the propellant, work slightly differently.

Ion propulsion requires less propellant than traditional chemical systems, so ion propulsion systems add less mass to the spacecraft, making them far more efficient. And though the thrust they produce is low (Dawn’s system, for example, produces a thrust equivalent to the weight of a piece of paper), gradually ion propulsion systems can achieve incredibly high speeds. Their efficiency makes ion propulsion suitable for missions requiring a great deal of maneuverability for trajectory changes. Though ion propulsion’s potential to serve as the primary propulsion system for deep space missions was not fully realized until recently, it is also used to adjust spacecraft orientation, shift orbit, and combat drag for near-Earth missions and commercial satellites. The spacecraft must operate within vacuum-like conditions, since the thrust generated by ion propulsion is too low to overcome significant air resistance.

Early ion thrusters, like those of SERT I and II, used mercury as a propellant. Though mercury is heavy and therefore able to provide greater momentum to the spacecraft, its toxicity makes it cumbersome to use and can sometimes contaminate the spacecraft. Xenon, used on geosynchronous satellites as well as interplanetary missions like Deep Space 1 and Dawn, is now a common propellant for ion propulsion systems since it is both heavy and inert, meaning that it won’t react with any of the spacecraft’s materials and can provide sufficient momentum.

In 1998, NASA’s Deep Space 1 (DS1), a mission that tested various advanced technologies for future interplanetary missions (and later, during in its first extended mission, acquired NASA’s first close-up pictures of the nucleus of a comet), became the first spacecraft to use ion propulsion as its primary propulsion source. Drawing heritage from DS1’s demonstration of the feasibility of ion propulsion use in deep space missions, Dawn’s three ion thrusters are based on DS1’s system. Dawn’s mission, to explore Vesta and dwarf planet Ceres in the asteroid belt, would not have been possible without it. Not only would a conventional chemical system have been costly, but it also would not have provided the maneuverability critical to Dawn’s dual target exploration.

Dawn has proven ion propulsion to be effective and sustainable for interplanetary exploration, allowing Dawn to orbit two targets, an unprecedented and immensely valuable accomplishment. With renewed interest in ion propulsion for orbital control of satellites and for propulsion of interplanetary spacecraft, NASA is investing in the future of ion propulsion. Both NASA’s Evolutionary Xenon Thruster (NEXT) and X3 – new systems under development—are more powerful than Dawn’s thrusters. These thrusters would be able to provide greater acceleration, decreasing the flight time to the distant worlds that their spacecraft explore. One day, these thrusters may fly deep into space and may even be able to propel human missions.

Dawn’s legacy as the first exclusively science-focused mission to employ such innovative and efficient technology will leave a great impact on the future of spaceflight. As we expand our understanding of the universe with even more complex missions to destinations near and far, it’s likely that ion propulsion, pioneered by DS1 and advanced by Dawn, will bring us there.