One of the biggest problems with deep space travel is fuel availability. For a spacecraft traveling millions of miles without returning to earth, running out of fuel is simply an eventuality. With common propulsion systems, such as cold gas propulsion (think squeezing air out of a balloon), a high thrust is achieved but with very little efficiency. While the vacuum of space makes it theoretically possible to use a small amount of thrust and then fly forever in a straight line, this is useless in practice. The necessity of course corrections and inevitability of outside forces like the gravity of large planets or solar radiation means that for long space missions, we simply need more efficient engines. In addition, cold gas thrusters require moving parts, which means they’re eventually bound to break down and need maintenance.
Que the introduction of ion thrusters. These amazingly efficient engines can have an Isp (a measure of how much velocity change a fuel can produce compared to its mass) of up to 10,000 seconds. Typical cold gas thrusters have an Isp of around 220 seconds. An ion engine could effectively run infinitely with only a tiny amount if propellant. These awesome systems have more going for them than just efficiency too. Ion engines depend entirely on electricity to activate propulsion, meaning no moving parts, no fire, and (theoretically) no need for maintenance. That said, there is one major disadvantage to using ion propulsion. By the law of conservation of momentum, we can write the change in an object’s velocity as -vₚmₚ/mₛ, or the velocity and mass of the propellant divided by the mass of the satellite. While ion thrusters can expel material at high velocities (10 times as fast as chemical rockets), the material they expel is just a stream of ions and electrons, which has so little mass that the velocity change of the much larger spaceship is quite small in comparison.
Ion engines work by first expelling a lot of electrons through the negative end of a battery. These electrons run into the fuel atoms (a gas, usually xenon), turning them into negatively charged particles called ions. Sometimes, a magnetic field is used to enhance this process. An electric field at the exit nozzle is used to expel the ionized gas at high velocities, generating thrust. The largest thrust recorded from an ion engine so far is only 5.4 Newtons, which isn’t much. Still, since there’s no air in space to create drag, this can add up to a huge change in speed over time.
Two major examples of ion thrusters are actually already being used in the real world. The most famous example is NASA’s Dawn spacecraft. In an unprecedented mission, the craft was able to go into orbit about two separate celestial bodies (asteroids Ceres and Vesta). The incredibly efficiency of the ion engines on board allowed it to carry enough fuel to escape the gravity of the first body even after dropping the powerful booster stage that launched it.
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In addition, a team at MIT recently created the first ion propelled airplane. In addition to just being really cool, the airplane is completely silent and has no moving parts at all. And since it uses an ion engine, it doesn’t rely on fossil fuels to run and is completely environmentally friendly, since solar power can easily be used to charge it. If a more powerful ion engine could be created, such a plane could completely replace modern commercial jets with an economical, quite, low maintenance alternative.
On the Horizon 