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The Challenge

The distances involved in space travel are literally astronomical. An “Astronomical Unit” or AU – the average distance between our Earth and our Sun – clocks in at around 150 million kilometres. Such colossal numbers are hard to imagine for a mind evolved for hunting animals and gathering plants. In some sense, our explorations of space remain rooted in their own Stone Age. Thousands of years from now, we may look back and describe this period of history as the Rocket Age.

One of the greatest challenges we face in space flight is the exorbitant cost of lifting objects into space from Earth. Huge rockets shaking the Earth in their ponderous rise towards the heavens demonstrates the challenge faced by the space industry. Weight of fuel is the most prohibitive attribute of any spacecraft; during the years that the Space Shuttle was in service, the weight of the fuel was twenty times greater than that of the payload itself.

While there are a few solutions that have been proposed to resolve this issue, there is a more urgent question. If so much fuel is used just to launch space craft into orbit, how can they be efficiently manoeuvred once in space? In this article we will focus on the design, advantages and applications of Ion thrusters.

Physics of the Ion Thruster

An Ion thruster uses the momentum of accelerated ions in order to produce the thrust required to accelerate a spacecraft. Imagine firing a cannon – as the cannonball accelerates, the cannon is forced backward, in the opposite direction. This is a consequence of Newton’s Third Law, which states that every action has an equal and opposite reaction.

Ion thrusters use the same principle on an atomic scale. Rather than launching a cannonball, they use ions to provide the acceleration. An ion is an atom that is missing some negatively charged electrons, making it positively charged. As a consequence, an ion is responsive to electrical fields. Once the ion stream leaves the craft, its electrical charge is neutralized by a stream of electrons from the craft. This stops the spacecraft itself from becoming negatively charged and attracting the ions back, which would negate the entire process.

So, how are the ions accelerated? There are a few methods which can be used but all are based upon either the Coulomb or Lorentz forces. To understand these forces we need the concept of potential difference, more commonly referred to as Voltage. Voltage may be visualized as a field of ghostly springs that only interact with electrically charged matter. As the potential difference increases, the springs become increasingly coiled and hold more ‘potential’ to cause a ‘difference’ to the environment. Day to day this may involve running an electrical motor, a smart phone or a light bulb.

In the case of the ion thruster, this force is used to accelerate ions at speed. If the thruster uses the Coulomb force, it creates a powerful, negative electrical field and our ghostly springs respond to the positively charged ion by forcefully expelling it from the thruster. The Lorentz force, though similar, responds to ions that are already moving. In this version the springs are “motion sensitive” and alter the direction of motion from across the thruster to out of it. While this metaphor has its limitations, it serves to give a feel for how an ion thruster works.

Advantages,  Disadvantages and Uses

The advantage of the ion thruster is its long lifetime. Although little thrust is produced per second, an ion thruster may be active for thousands of hours due to efficient use of propellant. Whereas a traditional chemical rocket may only accelerate propellant to around 1km/s, an ion thruster can accelerate ions to 20-50km/s, giving significantly more thrust per kilogram of propellant, incredibly important given the prohibitive costs involved lifting propellant into space.

Rocket thrusters are heavily damaged by the high heats they are exposed to. In contrast, ion thrusters suffer very little wear and tear. In addition, solar energy can be used to provide the power required to operate the thruster. There is a pay-off – ion thrusters give little acceleration and are only operable in the vacuum of space where there are no other atoms or ions to complicate matters. The propellant also requires consideration. Most ion thrusters use xenon, expensive and rare on Earth, although alternatives are being developed.

So how are ion thrusters used? With their long lifetime and low thrust they are generally used in situations where small adjustments are required. This varies from maintaining the height of a low-Earth orbit, to making minute changes in attitude and positioning for scientific experiments. They are also used for interplanetary and deep-space missions. Their long lifetime allows the velocity of a craft to be gradually increased, reaching high speeds in an extremely efficient manner. In fact, the Deep Space 1 craft accelerated by 4.3km/s with the use of under 74kg of xenon gas and energy from solar panels. You read that correctly, 4.3 km per second.

Stay tuned for our next foray into propulsion systems – Solar Sails.

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