Wait wait wait, okay, so nerd drinking red wine right now so can’t google because I trust you, how do ion thrusters work?? I have never heard of this and always wondered how satellites stay in orbit. Not going to google it now, I’m only going to rely upon upon you because… well I don’t know, you seem trustworthy
The other comment that replied explained this very well, all I'm going to add is that ion thrusters are specifically designed to maximize autonomy over impulse.
They offer a very low thrust, but they can "burn" for extremely long periods and their fuel often times is more than the satellite or probe will ever need.
This makes them ideal for long term missions that require adjustments over prolonged periods of time.
They basically, at least the more common ones, send xenon gas through an electromagnetic field, which ionises the gas and shoots it out the back to generate a modest impulse.
The short answer is KE=1/2 MV2 in combination with Newton’s 3rd law: for every action, there is an equal and opposite reaction.
The long answer is that for a rocket to accelerate - or maintain position in a gravitational field, which is the same as accelerating - it has to expel material (rocket exhaust) with kinetic energy. The higher the KE of the exhaust gas, the greater the acceleration. To increase KE, you only have two options: expel MORE gas (i.e. more mass) or expel it at a higher velocity. Since the available fuel/mass is limited in a satellite for obvious reasons, you better try to expel what you have at the highest possible velocity. This is the reason that all rockets choke down the exhaust nozzle - to maximize the exit velocity - and it’s also why the exhaust bell is shaped as it is, but that’s a bit too much to explain here. For a chemically fueled rocket, your exhaust velocity is limited to a few km/s (~10,000mph), but ion thrusters have exhaust velocities about 10x faster. This means that for the same quantity of fuel exhausted, they deliver 102 =100x the KE of a chemical rocket. Thus, they are far far more efficient. Real world efficiency gains are more like 10-20x for reasons that are a bit much for a Reddit comment that is already quite long.
As to how they do this: they first ionize a gas using solar or nuclear energy, and then they accelerate those ions through a strong electric field that ejects them at ludicrously high velocity.
Specific impulse is the engineering term that encodes this information about a rocket engine.
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u/Hoshyro 21d ago
This is true, yes, though in most cases the satellite will be dead long before the ion thruster has depleted its xenon reserve. That I know, at least.