Deep Space Travel with Ion Thrusters – an Overview

Electrically powered ion thrusters are one of the most common propulsion systems for the vacuum of space.

Ion Propulsion Key Takeaways:

• Ion thrusters allow spacecraft to travel further, faster, and cheaper than other systems.
• 11.5 times as efficient as chemical propellant.
• Though efficient, the system provides very little thrust, a fraction of 1 newton. The tiny force, over time, eventually results in big changes in velocity.
  • Cannot be used to launch a rocket from Earth.
  • Are excellent for maintaining satellite orbit, sending smaller probes on long distance voyages to asteroids or outer planets.
• Can operate continuously for years. Ion thrusters can be used over very long periods of time. For example, Dawn, the spacecraft that was the first to reach a dwarf planet Ceres, used ion thrusters to reach a speed of 10 km/s.
• Could be used in the near future to power additional missions to Saturn’s moon, Titan, or other places proximate to our solar system, for example.
• Could one day be used to send people on a thousand year voyage to other stars.

Even though the amount of force that an Ion Thruster Engine provides is barely the weight of a piece of paper, these systems allow spacecraft to reach enormous speeds.
Although going from 0-60 takes about 4 days, the compounding acceleration of running these engines for years allows them to cover distances of billions of miles through space.

Ultimately, ion engines are the perfect system for long-distance, deep space travel.

What is an Ion Thruster?

We now know that an ion thruster is a method of powering a spacecraft during flight through outer space.

As ions are ejected from the back of the engine, each ion generates a tiny force which slowly begins accelerating the spacecraft.

In the absence of air, there is no friction or wind resistance to slow down a spacecraft’s trajectory. Thus, the tiny, consistent force generated is successful at increasing the speed of a vehicle in space when maintained for long periods of time – often years.

Ion thruster engines are the most useful known method of movement in space, particularly over long durations.

Why are ion thrusters used?

Ion propulsion is specifically valuable for long distance space travel because less propellant is needed to increase speed.

With traditional rockets, chemical-based fuel like methane or hydrogen is used. In these systems, fuel tends to be such a large fraction of a spaceship’s total mass.

Ion thrusters, on the other hand, are effective in generating the most thrust for a given mass of fuel. A lower fuel mass is analogous to a car getting more miles per gallon.

Traditional chemical rockets release energy stored in molecular bonds of propellant (Methane, CH4 for example) and are limited by their low specific impulse, no matter what type of fuel or design is used.

For missions that require a large acceleration (like deep space travel over millions of miles), chemical propulsion is no good because the low specific impulse requires fuel to take up a larger percentage of the payload.

According to the Tsiolkovsky Rocket Equation, as acceleration increases, the amount of fuel needed increases exponentially.

Specific impulse describes how effectively propellant is converted into thrust.

Key differences between ion and chemical propulsion:

• Compared to chemical engines, ion powered spaceships are able to reach speeds that are more than 11 times as fast.
• Greater efficiency; high specific impulse, which means its able to generate larger force for a given mass of propellant.
• Less powerful
• Ionizes atoms rather than reacting molecules in an exothermic combustion reaction.
• Acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.
• Less propellant is required, which means we can send smaller, cheaper vehicles on missions.

What are the drawbacks of ion thrusters?

There are a few tradeoffs to using an ion thruster engine. Although sustainable for months or even years, ion thrusters produce only a small amount of force, so it takes a long time to reach high speeds.

Additionally, ion thrusters can only be used in the vacuum of space. They don’t work in the presence of air particles, and the tiny force cannot overcome air resistance.

An ion thruster won’t work for launching a rocket from a planet’s surface, but it can be used for steering, orientation and acceleration once you’re in space.

How do Ion Thrusters work?

Instead of igniting propellant, ion propulsion works by taking inert, unreactive gas atoms (such as xenon or krypton), and inducing a positive charge by removing an electron.

An inert gas is used as the propellant because it is unreactive and non-corrosive. Xenon and Krypton are quite unreactive because they contain a full 8 electrons in the valence shell. Elements with a full outer shell are called noble gases.

Xenon is a slightly better propellant than Krypton because it has a larger atomic mass, producing more force.

The process of removing electrons requires electricity. Often, solar panels are a good option to power the ionization. However, for deep space missions where less solar power is available, the energy is too small and acceleration is slowed. Because of this, alternative energy sources are required. Nuclear is one possible option. NASA and Livermore labs working on a nuclear system for electricity production.

Once electrons are ionized (electrically charged), the ion can be accelerated. This is done by applying a voltage to create an electrical field, causing them repel one another (similar to the way magnets behave when you hold them next to each other). The large amount of ions repelling each other produce momentum and force to accelerate the spacecraft.

The voltage causes the ions to leave the engine at up to 90,000 miles per hour. Each individual ion provides a small amount of thrust for the spacecraft.

With a large number of these ions being expelled, a constant force is generated that can move the spacecraft forward, to the left in the image above.

Given the amount of force is small, for reference it takes four days to accelerate from 0 to 60 miles per hour. Though small, when sustained over many years, continual acceleration can cause the spacecraft to reach up to 200,000 miles per hour, fast enough for deep space travel.

Who works with ion thrusters today?

SpaceX Starlink satellites use ion thrusters. Each SpaceX Starlink satellite is able to propel and orient itself to ensure it doesn’t run into other satellites or orbital debris. Starlink uses krypton for the inert gas because it is cheaper (though less efficient) and better suited for their large amount of satellites.

NASA Glenn Research center has done tests on a Hall Effect thruster, known as HERMeS, which is three times as powerful as other systems.

The University of Michigan is developing the X3 Ion thruster, which is also a type of Hall Effect thruster capable of generating 5.4 Newtons of thrust, and has set numerous records.

NASA Dawn Mission has used ion thrusters to travel 4.3 billion miles towards Ceres, a planet in the asteroid belt.

Thanks for reading!

sources

• https://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html
• https://aip.scitation.org/doi/10.1063/5.0010134
• NASA HERMES: https://www.youtube.com/watch?v=_YDj4JwqoU4
• ION propulsion with Dr. Marc Rayman Dawn Mission (good animations at 1:09) https://www.youtube.com/watch?v=5OFgJwdZxRc
• Gridded Ion Thrusters: https://www1.grc.nasa.gov/space/sep/gridded-ion-thrusters-next-c/
• X3 ion thruster: https://www.youtube.com/watch?v=CiWb44VRZGo
• https://core.ac.uk/download/pdf/189597252.pdf