People always ask me why we're stuck with chemical rockets.
Seriously, exploding a bunch of hydrogen or kerosene is the best we can do?
Good news, there are other, exotic science fiction-sounding propulsion systems out there
which use electromagnetic fields to accelerate atoms, allowing their spacecraft to accelerate
for months at a time.
I'm talking about ion engines, of course, and several spacecraft have already used these
exotic thrusters to perform some of the most amazing missions in the exploration of the
Solar System.
I know, I know, chemical rockets seem really primitive.
Take tonnes of liquid or solid fuel, light it on fire with an oxidizer, and then use
the speed of the explosive gases to give you a kick in the opposite direction.
Thanks Newton's Third Law.
But chemical rockets do the trick.
Those gases do give a rocket the kick it needs to get into space.
Because they bring their own oxidizer with them, they work in the atmosphere and they
work in the airlessness of space.
The advantage of rockets is that they can deliver enormous amounts of energy in short
periods of time, the kind of reaction you need to blast tonnes of cargo off Earth and
into space.
But they're incredibly inefficient.
A 550 metric tonne Falcon Heavy is carrying almost 400 tonnes of fuel and oxidizer.
The first stage will only burn for 162 seconds, and the second stage will fire for 397 seconds.
That gives you a total burn time of about 9.5 minutes.
Want to make more maneuvers?
Want to accelerate for days, weeks or even months?
Too bad, you're out of fuel.
Of course, these shortcomings from chemical rockets have led scientists to search for
other forms of propulsion, especially when you're out in space, and the one of the
most successful so far is the ion thruster.
When you're working out the rocket equation, an important factor is the velocity that you're
ejecting your propellant.
The most efficient chemical rocket can throw hot gases out the back at 5 km/s.
Ion engines, on the other hand, can eject individual atoms 90 kilometers a second.
This high velocity gives the spacecraft a much more efficient acceleration.
The best chemical rockets see a fuel efficiency of about 35%, while ion engines see an efficiency
of 90%.
So how do ion thrusters work?
It's actually pretty weird, and totally sounds like science fiction.
Instead of hot gases, ion thrusters eject ions.
These are atoms or molecules which have an electrical charge because they've lost or
gained an electron.
In the case of an ion engine, they're emitting positively charged ions which have lost an
electron.
Once you've got ions, you can direct them with a magnetic field, accelerating them into
space at tremendous speeds.
So where do they get all the ions?
The thrusters create them by generating a plasma inside the spacecraft.
They bombard neutral propellant atoms of some gas, like xenon with electrons.
These collisions release even more electrons from the propellant, turning them into positively
charged ions.
This plasma soup of electrons and positively charged ions has an overall neutral charge.
The electrons are held in the chamber, leading to more ionizing events, while the positive
ions are siphoned out through a grid at the end of the chamber.
As they pass through this grid, high voltage accelerates them out of the back of the spacecraft
at speeds of up to 90 km/s.
For each ionized particle that the spacecraft can kick out, it gets a tiny kick in return.
The whole system is powered by solar panels, so the spacecraft itself doesn't need to
carry any kind of battery or power system, minimizing the total weight it has to carry.
The big problem is that that kick really is tiny.
The thrust of ion engines is measured in millinewtons, like, thousandths of a Newton.
Hold a piece of paper in your hand, that's the kind of forces involved.
But they can operate for days, weeks, even months, accelerating and accelerating long
after chemical rockets would have run out of fuel.
So if you're already out of the gravity well of a planet, they're very efficient
engines for dramatic changes in velocity.
NASA and other space agencies have actually used ion engines very successfully in a range
of missions.
They had been developing this thruster concept for decades but were never willing to risk
it on an active mission where a failure could end it.
So NASA gathered up a bunch of risky technologies, and packages them together as the Deep Space
1 mission, which launched in 1998.
Deep Space 1 was equipped with 12 different technologies that NASA wanted to test out,
including low power electronics, solar concentrator arrays, various scientific instruments, and
a solar electric propulsion system.
Its engine was run for enormous lengths of time, allowing it to make close observations
of asteroids and comets, and even Mars.
NASA doubled down on the technology of Deep Space 1, giving its Dawn Mission three redundant
ion engines.
These allowed the spacecraft to go into orbit around the asteroid Vesta, make observations,
then break orbit and travel to asteroid Ceres and make even more observations.
And it could still have fuel in the tank to visit even more asteroids.
Just to give a sense of its acceleration, Dawn can go from 0 to 100 km/h in 4 days of
continuous thrusting.
Ion thrusters were used to carry ESA's Smart 1 spacecraft from Earth orbit to lunar orbit,
and on the Japanese Hayabusa spacecraft.
Ion engines have been tested here on Earth, and successfully operated for more than 5
years continuously.
With these successes, we're going to see even more spacecraft equipped with ion thrusters
in the future, but ion thrusters themselves are getting more powerful and resourceful.
I said that ion engines produce very little thrust, but there are some ideas to boost
their output.
The first is dramatically increase the amount of electricity you're using to accelerate
the ions.
Instead of solar panels, NASA considered creating an ion engine powered by a nuclear reactor.
About 15 years ago, NASA considered a mission known as the Jupiter Icy Moons Orbiter mission.
Powered by the Nuclear Electric Xenon Ion System (or NEXIS) engine, the spacecraft would
be capable of exploring each of Jupiter's icy large moons in sequence: Ganymede, Callisto
and Europa.
The spacecraft would have been launched into orbit in three separate pieces, which would
then be assembled in Earth orbit and launched off to Jupiter.
The spacecraft would use its 8 ion thrusters to study Callisto and then Ganymede for three
months each, and then settle into a final orbit around Europa.
If conditions were right, it could even go into orbit around Io.
Of course, we don't get to have nice things, and the mission was cancelled back in 2005.
There are other ways ion thrusters can be scaled up.
NASA is testing a high thrust version of ion engines known as the X3 hall thruster.
This engine is capable of blasting out ions, and produces 5.4 newtons of force.
Again, not much, but remember that previous thrusters top out in the thousandths of newtons.
At the highest power levels, this could be the technology that will carry human astronauts
to Mars, cutting down the flight times to just a few months.
Engineers are planning to run the X3 for 100 hour tests this year to see if it has the
same kind of long-term operation as the smaller ion engines.
The coolest idea I've heard recently for ion engines is the idea of an air breathing
engine under development by the European Space Agency.
Instead of carrying any propellant at all, engineers at ESA demonstrated that a spacecraft
in low Earth orbit should be able to pull in molecules of air right from the atmosphere,
and then ionize them and blast them back out.
Since the spacecraft would be using unlimited solar electricity for power, and pulling its
propellant from the atmosphere, it could operate without refueling for long periods.
Spacecraft could operated at lower altitudes, and space stations could remain in low Earth
orbit indefinitely without needing to be reboosted.
This is going to be real game changer.
And not only Earth, this technology could be used on Mars or Venus, or Titan.
Anywhere with an atmosphere.
Ion engines have already made an impact on space exploration, and in the next few years,
we're going to see more missions equipped with them.
They could even be the engines that carry human astronauts from Earth to Mars in the
coming decades.
What do you think about ion engines?
Let me know your thoughts in the comments.
Once a week I gather up all my space news into a single email newsletter and send it
out.
It's got pictures, brief highlights about the story, and links so you can find out more.
Go to universetoday.com/newsletter to sign up.
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