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[ ♪ Intro ]
While traveling in space, one of the hardest things to do is, stop. Or change direction.
Without anything to push against or friction to slow things down, spacecraft need to do
all the hard work of changing their speed or path.
And sometimes they do that in ways you’d never expect: like by vaporizing Teflon.
They’re called pulsed plasma thrusters, and they can use the same stuff that’s on
your frying pan to make spacecraft zoom around the universe. And they’ve been doing it
since the 1960s.
To make basically any move in space, satellites rely on Isaac Newton’s famous Third Law
of Motion, which is probably on a poster in every high school physics classroom: For every
action, there’s an equal and opposite reaction.
Put another way: throw stuff backwards and you’ll go forward.
In fact, you can boil down every rocket design, no matter how complicated, to this basic idea.
When thinking of a rocket, you might normally imagine what’s called chemical propulsion.
That’s the “fire-coming-out-the-end” kind, which uses a controlled explosion to
hurl material out the back of the rocket.
And once in space, another kind, electromagnetic -- or EM -- propulsion, also becomes available.
They aren’t strong enough to get rockets off the ground, but they are great once you’re
past most of Earth’s atmosphere.
These rockets work kind of like railguns, accelerating charged particles, or ions, out
the back with electric or magnetic fields.
Today, we have all kinds of EM thrusters, but pulsed plasma thrusters, or PPTs, were
the first ones ever flown in space. They were used in 1964 on the Soviet Zond 2 mission to Mars.
Like some other engines, PPTs specifically use plasma to generate thrust, instead of
a random collection of ions. Plasma is a super hot substance made of charged ions, and it’s
the fourth state of matter.
In some ways, it behaves kind of like a gas, because its atoms are pretty spread out. But
unlike the other states of matter, plasmas can be shaped and directed by electric and magnetic fields.
To generate its plasma, PPTs eat Teflon! Which is pretty awesome.
A pulsed plasma thruster places a block of Polytetrafluoroethylene -- what we know as
Teflon -- between a pair of metal plates.
Then, connected wires charge up those plates with electricity until it arcs through the
Teflon block, set off by a spark plug.
That arc delivers thousands of volts into the block, vaporizing the nearby Teflon and
ionizing it into a plasma.
The sudden burst of plasma effectively creates a circuit connecting the metal plates, which
allows electricity to flow like it’s traveling through a wire.
One neat side effect of flowing electricity is that it generates a magnetic field. And
everything in the thruster is already arranged so that this field pushes the plasma out into space.
At this point Newton’s third law springs into action, pushing the spacecraft in the
opposite direction of the departing particles.
And, huzzah, motion!
Well, the tiniest bit of motion.
A pulsed plasma thruster deployed by NASA in 2000 produced an amount of force equal
to the weight of a single Post-it Note sitting on your hand. Which might not seem that exciting,
but it has some big implications.
Like other forms of electromagnetic propulsion, these engines require a lot of electricity
to run, but in exchange they offer incredible efficiency with their fuel.
Pulsed plasma thrusters can produce up to five times more impulse -- or change in momentum
-- for every gram of fuel than a typical chemical rocket.
They do it very, very slowly, but they get the job done.
PPTs also offer exceptional simplicity and safety.
The only “moving part” is a spring that constantly pushes the Teflon block forward
and, without the need to store pressurized liquid or gas fuel, there’s no chance of explosion.
So it makes sense then that pulsed plasma thrusters were so useful back in the 1960s.
Since then, their lack of power has meant that most spacecraft main engines have remained chemical.
And when companies really need some kind of EM drive -- like for the Dawn mission to the
asteroid belt -- they’ll tend to choose more sophisticated designs.
But that doesn’t mean we’re done with these thrusters just yet.
Recently, their extreme simplicity has made them a natural fit for the most up-and-coming
field of exploration: CubeSats.
CubeSats are tiny, shoebox-sized satellites designed for simple missions and built on
the smallest of budgets -- often by research labs or universities.
Earth-orbiting CubeSats seem almost tailor-made for the strengths of pulsed plasma thrusters.
Lots of sunlight gives them ample electric power, but since they’re so small, space
and weight are at an absolute minimum. And right now, most CubeSats typically don’t
have any kind of propulsion system of their own.
So one solution is micro pulsed plasma thrusters, which can weigh just a few hundred grams and
measure under 10 centimeters on a side.
That might not sound like much, but even a tiny amount of thrust could double the useful
life of some kinds of CubeSats.
They’ll likely need to undergo more testing and development before they’re ready for
primetime, but someday, we could have a whole fleet of Teflon-eating satellites.
Not bad for the same stuff that coats our kitchen pans!
So I joked about Newton’s third law of motion being etched into our brains earlier, but
when’s the last time you really thought about good ole’ Newton critically?
Brilliant.org has a whole section on Newton’s Laws as part of their Classical Mechanics
course. And learning about pulsed plasma thrusters made me want to test how well I remember using
Newton’s third law. So, imagine you’re on a beautiful, frictionless lake. In your
own little boat. The sun is shining, there are birds in the air, there might be lilly
pads around you. And then you spot another boat, just like yours, a little bit bigger
across the lake, 30 meters away. And you know that your boat is 60 kilograms and actually
that’s your friend, so you know that that boat weighs 90 kilograms. And you want to
bring the boats together so you can have a picnic. Luckily they’re connected by a string.
So if your string is pulled with a constant force and your two boats meet up after 20
seconds, how far did your boat move? You probably don’t even need to get into a boat to solve
this problem. I believe in you. You can go to Brilliant.org to check out this quiz and
a bunch of others like it and the first 777 people to sign up at brilliant.org/scishowspace
will get 20% off of their annual Premium subscription AND support SciShow Space - so thank you!
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