
space travel
Sir Isaac Newton’s third law states that every action has an equal and opposite reaction (imagine air escaping from the end of a balloon and propelling it forward). This is the exact same law used to allow rockets to escape Earth’s gravity and travel across space.
Traditionally, manned spacecrafts have used heavy chemical rockets, which burn liquid or solid fuel into high-temperature and high-velocity gas via a chemical reaction. This gas is then fired out of the nozzle of the rocket, creating thrust and allowing the rocket to move (remember Newton’s third law).
Chemical rockets can accelerate an object very quickly, allowing it to escape gravity. However, they have a very low fuel efficiency of around 35%. This means for longer journeys (let’s say to Mars) a lot of fuel would need to be taken to constantly propel the spacecraft forward at speed. Therefore, chemical rockets quickly become extremely expensive and unsustainable for space travel beyond the Moon.


This is where ion thruster engines come in. They can achieve fuel efficiency of 90% and are currently used to keep communication satellites in the correct orbit around the Earth and for deep space probes.
Ion engines work very differently from chemical engines. Instead of burning fuel, they ionise a neutral gas (such as argon, krypton or xenon) by extracting some electrons out of the gas’s atoms. This creates a cloud of positive ions [1] which uses the principle of the Coulomb force [2] and the atom’s electric field [3] to transform electromagnetic energy into thrust. The temporarily stored electrons are then reinjected by a neutraliser into the cloud of positive ions to become neutral atoms again before being dispersed into space.
[1] An ion is an atom with a net charge, as by removing an electron the atom is no longer neutral.
[2] The Coulomb force is the amount of force between two electrically charged particles at rest.
[3] An electric field surrounds electrically charged particles and applies force on all other charged particles in the field, either repelling or attracting them.
It is this Coulomb force that creates momentum for the spacecraft. However, the acceleration is very slow and takes a long time to build up high speeds, with acceleration continuing throughout the journey. This is one of the reasons why ion engines cannot be used to put a spacecraft into space because they cannot produce the thrust quick enough to escape Earth’s gravity (this is why the Heimdall spacecraft stays in orbit).
However, tiny, constant amounts of thrust over a long time add up to much higher speeds, shorter travel times and much less fuel used if the destination is far away (such as Mars). Deep Space 1 used less than 72kg of fuel in over 16,000 hours of thrusting (that’s very efficient). Since much less fuel must be carried into space, smaller spacecrafts and cheaper space journeys can be made.
