Everyone remember sound and light coming out from engines of TIE fighters from Star Wars. But now it is possible to see it in reality.
Probably most of ordinary people consider ion propulsion as George Lucas fantasy. Nothing more wrong. Conception of ion thruster was first announced to public in 1911 by Russian scientist of Polish decent Konstantin Tsiolkovsky. First experiments with ion thrusters were carried out by Robert Goddard at Clark University from 1916. First working ion thruster was designed and set in motion in 1959 NASA engineer, Harold Kaufman. It was called after his surname as "Kaufman ion source”. As we can see ion propulsion is just as old as modern rocket science. Since seventies ion propulsion was utilized in Soviet satellites for maneuvering. At presence ion thrusters are often utilized in satellites - for example Boeing 702SP satellite platform which is designed to use ion propulsion for not only maneuvering but also for reaching correct orbit.
How ion propulsion works
Ion propulsion works in a different way than rocket engine. It utilizes fuel (for example xenon gas) for obtaining ions and accelerating (using Coulomb in electrostatic thrusters or Lorentz force in electromagnetic thrusters) them into beam. Thrust force depends on power given to engine - it is in opposition for rocket engine where thrust depends on amount of fuel. Ion thrusters are reliable and long lasting. For example ion thrusters utilized in Deep Space 1 (launched in 1998) were designed to work for 200 hours but last fifty times longer. It sounds amazing - thrust impulse in rockets during start is counted in seconds, number of maneuvers in space in case of utilizing rocket propulsion is always reduced to minimum. Rocket propulsion has low possibility of throttle which is significant during landing on planet surface and it could not be easy turn off and on again. Should the question arise if ion propulsion is currently being used why it is not being utilized in manned spacecrafts? Answer is simple - modern ion thrusters offers little thrust, enough for small spacecrafts but not sufficient for heavy manned spaceships, not to mention about launching and rising into outer space. As we know from physics to defy Earth gravitation, rocket should reach second cosmic velocity - required speed is 11.19 km per second. It is described by physical formula by Konstantin Tsiolkovsky:
Where V is ideal end speed of the rocket, M0 is starting mass, M1 is final mass and Io is specific impulse of the engine (This formula is describing situation in vacuum and with no Earth gravitation). Velocity of over 11 km per second could not be provided by any modern ion thruster. But formula shows clearly another fact: increasing speed is connected with decreasing mass of the rocket. Highest speed should be reached just before emptying fuel tanks. It means that rocket will be useless after reaching outer space. For further flight spacecraft should be equipped with own propulsion. Modern rockets engines are very inefficient. For example, in modern launching vehicles, fuel to starting mass ratio is 96% and more. This means, that after start and leaving Earth gravity only 4% or less percent of starting mass will be in outer space. So for further propelling, spacecraft will have only a fraction of starting fuel to utilize. Due this fact modern spacecrafts are using inertial force to travel in space. But if it is consider continuing space flight, spacecraft should be able to defy gravitation force of passed during the flight other astral bodies. Unmanned spacecrafts like probes, are using their gravitation to increase speed. They are reaching astral body, entering its orbit and after circling exiting with increased speed. It is solution suitable for probes, but for manned mission where time is significant factor it becomes a problem. Manned spaceship should be able to take enough fuel for performing necessary maneuvers and for keeping appropriate speed during passing near astral bodies (to avoid being put into orbit). In physics factor describing space vehicle's ability for changing trajectory is called delta V described with formula:
To simplify, if mass of space vehicle is decreasing, delta V is increasing and resulting in enhancement of range and maneuvering capability. To sum up, for now rockets are only way to lift spacecraft into outer space. But as further flight is concerned ion thrusters seems promising solution.
Ion thrusters present projects
As engine of the future, ion propulsion is developed by countries possessing advanced space technologies. USA and Russia are predecessors of ion thrusters. In 1964 SERT-1 was launched as ion thruster technology demonstrator. In USSR since 1964 various satellites were equipped in electrical propulsion. First electromagnetic drive was utilized in Zond-3 Mars probe. In the following years, they were joined by other countries: Japan in 1994 with ETS-6 equipped with Xenon ion propulsion XIPS (in 1974 with experimental L-4SC-3 equipped with pulsed plasma thrusters, Japan started developing its own electrical propulsion program), China in 2003 with AsiaSat-4 powered by XIPS designed in cooperation with Hughes and Boeing (first China used electric propulsion in MDT-2A ICBM in 1981). First European ion propulsion adopted satellite was EURECA equipped with RIT-10 ion engine RITA and launched in 1992. At present list of spacecrafts utilizing ion propulsion is quite long. Boeing offered it as an option in their 702 series satellites since 2005. International Space Station is going to be utilized as test field for VASIMR ion engine designed by Ad Astra. It has finished tests on Earth under agreement signed between NASA and Ad Astra is going to be attached to ISS for LEO tests. VASIMR is able to generate 200 kW of thrust but for serious objectives it should provide 200 mW (such powerful ion engine would to shorten flight to Mars to thirty nine days). NASA project called NEXT is still under developing. After five and half years of testing, it runs for over 48000 hours without any problems. During test it used only 870 kg of Xenon fuel. Japan successfully used electric propulsion in their probe Hayabusa sent to return with material from 25143 Itokawa near-Earth asteroid. Russia is still developing their technology and puts hope in Dvina orbital tug with propulsion designed from scratch by NPO Lavochkin. Another attempt of creating modern ion propulsion in Russia was made by KBKhA design bureau. Project by KBKhA was awarded with subsidy of the Ministry of Education and Science in 2013 and is still under development. China after launching first experimental satellite with ion propulsion in 2012 is still making progress. At the moment Chinese ion thrusters are developed in China Academy of Space Technology (CAST). As CAST states thrusters are able to generate 5 kW, but in 2020 CAST is going to create engine providing power at level at 50 kW and use in broadcasting satellite. Electric propulsion combined with conventional launch vehicles seems future of space exploration in both unmanned and manned missions. It is confirmed by many experts. According to Wang Min, deputy chief designer of the communication satellite (CAST) (announced on 8th June 2015):
"Electric propulsion technology will play an important role especially in manned deep space exploration,"
Also Robert Lock, Senior System Engineer at NASA-Jet Propulsion Laboratory said on 24th February 2015 during NASA panel:
“Getting to Mars and then getting back again with sample return is turning out not to be so much of a challenge if we start off with a solar-electric propulsion type basis,”
It seems that we would wait for ion propulsion in manned spacecrafts not very long.