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[ intro ]
When you imagine humans on their way to Mars,
you probably imagine them in a spacecraft with big, explosive, chemical engines.
And that’s totally reasonable.
Humans need to travel with a lot of stuff,
and engines that rely on chemical combustion are currently the only ones powerful enough
to move us through space at a reasonable speed.
Except, chemical engines also have a pretty big downside.
They need to carry a bunch of fuel,
which makes their spacecraft super heavy.
And that leads to more expensive missions that are harder to launch.
Honestly, it would be nicer if we could move humans with a lighter,
more fuel-efficient propulsion system.
And the good news?
We might have already found our best option.
It’s a form of electric propulsion called a Hall-effect thruster,
or a Hall thruster for short.
They’re thrusters that look a bit like a bullseye
and glow with an eerie, colorful light.
And they could be the future of human space exploration.
Unlike some of the ideas we talk about on this channel,
Hall thrusters aren’t theoretical — or even new.
They were invented in the 1960s,
and engineers have spent decades advancing the technology.
On a basic level,
these thrusters work by accelerating charged particles called ions.
First, you start with a circular channel —
or a few channels, depending on how big your thruster is.
Between each channel,
you put some magnetic coils that generate a magnetic field.
Then, at the bottom of your channels,
you add an electrically-charged plate called an anode,
which creates an electric field.
And finally, you add a cathode,
which is located somewhere outside the channel
and can spit out a bunch of electrons.
Now, you’re ready to go.
When you power up the thruster,
the cathode starts releasing those electrons.
The particles are attracted to the anode, so they go flying into the channel.
There, they’re caught up in the magnetic field
and start zooming in circles around and around the thruster.
And that’s where the magic happens.
Once the electrons are zooming around,
Hall thrusters pump a bit of propellant into the channel —
usually a neutral gas like xenon.
The xenon gets hit with all of those incoming electrons,
and that knocks off some of its electrons and turns the xenon atoms into ions.
The electric field inside the thruster then pushes those xenon ions out of the channel
at incredible speeds —
sometimes more than a dozen kilometers per second.
And that’s what generates the thrust to move your thruster and your spacecraft forward.
Now, this basic idea of accelerating ions isn’t unique to Hall thrusters.
Every form of ion-based electric propulsion does something like this.
What makes Hall thrusters special
is that they satisfy three major conditions.
For one, they have among the highest thrust of all forms of electric propulsion.
There are a few reasons for this,
but one is because the propellant ions are created
and accelerated in the same area.
Other thrusters keep these processes separate,
and there’s a limit on how many charged particles they can cram into one spot
before the electric field gets messed up.
Hall thrusters also use their fuel really efficiently.
Since they accelerate their ions to such high speeds,
they generate more thrust for every molecule of propellant they use.
And finally, they can fire for a long time.
Other ion thrusters have components that quickly wear out,
and while Hall thrusters do have their own lifetime problems,
engineers have found ways to solve or mitigate many of them.
So in the end, these thrusters can fire for much longer,
which means they’re a lot more practical for spaceflight.
Since the ‘60s, Hall thrusters have flown on dozens of missions.
Mostly, they’ve been used to adjust satellites’ orbits,
but in the 2020s, they’ll be used on even bigger projects,
like the Psyche mission to investigate an asteroid.
But earlier, I said Hall thrusters could be the future of human space exploration.
And the thrusters we have today…
well, they’re nowhere near strong enough to push around people at a helpful speed.
Because here’s the thing about Hall thrusters —
and about electric thrusters in general:
Their main benefit is that they can fire for a long time.
That means though you might start off slow,
you can gradually build up speed until you’re zooming along faster
than any spacecraft that uses chemical propulsion.
The problem right now is that getting to those speeds takes a long time.
With our current tech, it would take years to get people to Mars.
But someday, that could change.
Because there’s a Hall thruster currently in development
that could become strong enough to move humans.
It’s called X3.
It’s been in development since 2009,
and it gets its name because it has three channels instead of the more common two.
This allows it to accelerate more ions at once.
It’s still nowhere near strong enough to fly humans,
even if we put several of them on the back of a spacecraft.
But, it has generated more thrust in a test than any other Hall thruster.
Now, engineers are working to make X3 more reliable and increase its thrust.
And if they can manage that,
NASA may eventually select the thruster to help send people to Mars.
Even if this doesn’t happen, though…
there’s a good chance this project will inspire other teams to continue the work.
There are a lot of electric propulsion methods out there,
and many of them are already changing spaceflight.
But when it comes to flying humans around,
Hall thrusters might be our best bet.
At the end of the day,
a lot of people just want to see humans walk on the surface of Mars.
But while that will be amazing,
it’s worth remembering the engineering behind this goal, too.
It’s taking a lot of clever, creative work to make something like this feasible.
And the research being done on Hall thrusters is a great example of that.
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