Electric propulsion has been around for several decades. In fact, the idea dates back to 1906, when Robert Goddard made an entry in his personal notebook. Five years later, Konstantin Tsiolkovsky published the idea.

The late 1950s and early 1960s saw a flurry of research on electric propulsion devices. By 1962, technical papers that addressed the use of these new gadgets for controlling the orbits of geostationary satellites started to appear. The first in-space demonstration of an ion engine was carried on board the SERT-1 (Space Electric Rocket Test) spacecraft, launched in 1964.

Advances have continued through the last several decades, and as a result, we now see many spacecraft applications for electric thrust devices. For example, many of the latest geostationary communications satellites use some form of electric propulsion for station keeping and orbit adjustment functions.

Large electric device are being considered for planetary probes and other applications. One might go so far as to claim this technology is mature and ready for many missions.

How do these devices work? Electrically powered spacecraft use electrical energy to change the velocity of a space vehicle. Most of these propulsive devices create thrust by electrically expelling propellant mass at high speed.

When compared to chemical thrusters, electric propulsion devices offer much higher propellant exhaust velocities, i.e., higher specific impulse. This leads to much more efficient use of propellants for space missions.

However, thrust levels are much lower than for chemical rockets, preventing their use on launch vehicles. Thus, applications of electric devices are limited to in-space maneuvering.

There is one electric thrust device that does not use propellant, the electrodynamic tether. This works by interacting with the Earth's magnetic field. Such devices have not yet been widely used and will be limited to near-Earth missions.

Do not confuse space electric thrusters with electric devices used on ground vehicles. Electrically-propelled cars use batteries for power and expel no exhaust products. In space, electric thrusters can derive power in one of two ways, from solar arrays or from nuclear power generators. To date, we have used only solar power sources for these applications.

Today, electric thrusters are used on geostationary satellites and a number of exploration missions. There is talk of using these devices in connection with future human planetary missions.

For example, earlier this month, Mars mission advocates suggested the use of both chemical and electric propulsion devices for a human mission.

Since chemical rockets can provide rapid flights to Mars, these would be used to propel a crew vehicle to the red planet. A habitat, supplies and equipment would be sent ahead of the crew on electrically propelled vehicles. Robotic cargo ships would leave Earth about 2.5 years ahead of the human explorers.

Clearly, electric thrust devices have found important space applications and will continue to be used in increasing numbers and applications.