Have you ever wondered what space travel might
be like in the future? In many science fiction stories, in the future humanity has spread out
across the solar system, colonising planets and asteroids. Given the hundreds of millions of km
between us and even the closest orbital bodies, this is not easy to do in real life – at least
not with our current level of technology. NASA predicts that it will take 7 months to
make it to even our closest neighbour, Mars. This is why sci-fi writers often invent powerful
engines on their spacecrafts – warp drives, Epstein drives and hyperdrives – that
allow humans to cross those distances in days or minutes, rather than months or years.
These conventional flight times occur because of the limitations in conventional rocketry. But new
technology is arising, something that feels like it’s straight out of sci-fi, that might one day
completely replace conventional rockets. With its greater efficiency, those month-long flight times
could become mere days. And while the technology is still under development, there are examples of
it being used in outer space missions right now. What is this technology? Ion engines. And with
them, the future might be a lot closer than you think. I’m Alex McColgan, and you’re
watching Astrum. Join with me as we learn more about this developing technology,
and learn more about these devices that may well be the future of space travel. To
begin with, for those who are unfamiliar, what is an ion engine? And how are they different
from the conventional rockets we know today? All rocketry works under the principle of
conservation of momentum. If you want to go up, you must send something else flying down, with
enough momentum to equal the upward momentum you wish to achieve. Conventional chemical rockets
do this by burning rocket fuel. Oxidiser mixes with a chemical like liquid methane, heating it
and causing it to expand. By sending out this stream of highly energised exhaust from the bottom
of the rocket, the top is sent flying upwards; kind of like releasing the air from inside a
balloon to send it whizzing around the room. Momentum is conserved in these cases. In our
example with the balloon, the momentum of the air leaving the balloon equals the momentum
of the balloon flying around. With the rocket, the momentum of the exhaust equals
the force of the rocket going upwards. In theory, you could travel around in space by
simply having a very large balloon and releasing its air. However, you would run into a problem
with this method. You would run out of air very quickly, and then would not be able to produce any
more thrust. Balloons are not very efficient forms of rocket propulsion. To a degree, this is also
the problem with our current chemical rockets. Although burning the fuel does give it more
kinetic energy than simply squeezing it out of a balloon, chemical rockets are still not that
efficient, as there is an upper limit to how fast you can accelerate exhaust material by burning
fuel. Rather than burning it hotter, if you want to go faster with such a rocket, the only solution
is to burn more fuel, which means you need to carry more fuel, which means your rocket has to
be bigger and heavier, requiring even more fuel. And once you run out of said fuel, that
is it – you can produce no more thrust. Conserving their fuel is the reason the NASA trip
to Mars will take 7 months. There’s no way they could have a large enough rocket that could carry
enough fuel to accelerate passengers all the way to Mars. Consider the over 60m size of some of
the rockets being launched currently, such as the Artemis 1 SLS rocket that got a spacecraft
to the Moon recently – a much closer target. Its main core stage was filled to the brim
with 2.8 million litres of fuel. That fuel was all burned up in just the first 10 minutes after
launch. To carry enough fuel to accelerate all the way to Mars would need a ridiculously large ship,
which would need a monstrous amount of thrust simply to get it off the ground. It's just not
efficient. Momentum is equal to mass x velocity. Chemical rockets try to go faster by simply
throwing more mass out the back of their thrusters. But what if instead we increased the
velocity at which that mass was thrown? That would also increase momentum, giving you more
thrust. And this is where ion engines come in. Ion engines attempt to give thrust electrically
to their propellant. Rather than burning fuel to cause rapid expansion, they attempt to
create ions – or charged particles – that then are accelerated along electromagnetic
fields – sometimes to speeds of 146,000km/h, depending on the model. The more electricity
you have, the more momentum you could impart to such a particle. And the faster it leaves the
back of your rocket, the more momentum that your rocket gains to move forward. This means that you
could get away with using far less fuel on a trip, provided that you could create enough electrical
energy to accelerate your particles. The takeaway is that ion engines are much more efficient
than chemical rockets. Chemical rocket fuel efficiency could achieve up to 35% efficiency,
while ion engines could manage 90%. Different models vary in their efficiency, but all require
far less propellant to achieve acceleration. So much so, that they can literally accelerate
for years. And this acceleration adds up – NASA space shuttles have top speeds of 29,000km/h.
Ion thrusters can achieve speeds that are 11 times that. The upper cap is how much electricity
you can produce, not how much fuel is in the tank. So, if ion engines are so superior, why
haven’t we already started using them? That question is a little misleading. We have
been using them. The recent NASA DART mission was equipped with a NEXT gridded ion thruster, ready
to be used in the event that its conventional thrusters failed. Deep Space 1 visited distant
comets while using a NSTAR ion engine. For a period between 1972 and the late 1990’s, Soviet
satellites made use of Hall-effect thrusters, a type of ion propulsion, as stabilisers on their
satellites. This functionality is still being used on satellites today. SpaceX’s Starlink satellites
also use Hall-effect thrusters. Even entire space stations have been propelled by these thrusters.
The Chinese Tiangong space station is moved by propellant but also 4 Hall-effect thrusters, which
are used to adjust and maintain the station’s orbit. These thrusters have reportedly been firing
continuously for 8,240 hours with no problems. But, as you might have intuited, there is also
a problem with current-generation ion thrusters, which means they’re not yet ready to replace all
conventional rockets. They have a fatal flaw, an Achilles’ heel. Ion-thrusters on the market today
have terrible “oomph”. To illustrate this point, if you were to take an ion thruster, and were to
hold out your hand to try to stop it moving, the force you would feel would be roughly comparable
to the weight of a single piece of paper. That is the trade-off. Ion thrusters can accelerate for
years. They usually use chemically inert gases as their fuel source, so are very safe. They
can accelerate particles up to huge speeds… but the number of particles being accelerated
is small, so the force of this thrust is tiny. An ion engine cannot produce the large-enough
thrust needed to get a spacecraft out of Earth’s powerful gravity well by itself. Of course, in
space, with no air resistance to fight against and with enough time, this tiny thrust can add up.
Even a gentle acceleration can get you to where you want to go if nothing opposes it. For point
of reference, some Ion engines in space can take a couple of days to accelerate a spacecraft up
to about the speed of a moving car. This means that ion thrusters have a niche on long-distance
missions, ones that can get away with only gentle force to maintain orbits, or for moving very small
things like tiny satellites. But they are a long way away from being able to carry humanity a long
way away. There are other problems to overcome. Ion engines work by creating circuits – moving
patterns of electrons that can carry charge and create electrical and magnetic fields. However,
ions from the atmosphere can interfere with the delicate balance of these circuits. If the circuit
breaks down because extra negative charges are coming in when they shouldn’t, or are bleeding
out unexpectedly, the engine loses its ability to create the right fields, which means it can’t
accelerate reliably. Not only that, but the best fuel source for ion engines – the chemically inert
xenon – is very rare and expensive. $1000/kg. Ion engines will need to overcome all of these
problems if they are to become the primary form of space transportation in the future. That said,
there are some efforts being made to do just that. Helicon thrusters are a new type of ion
thruster that are being developed by the European Space Agency in collaboration with
the Australian National University. They are making breakthroughs that improve thruster
efficiency even further, decreasing the wear on parts and making it so ion thrusters are
even better suited to those long space voyages. In terms of fuel source, some ion thrusters under
development are being built in ways that allows them to use a much wider range of fuel sources.
The complexly named Magnetoplasmadynamic thruster has configurations that allow it to use Hydrogen,
argon, ammonia or nitrogen as propellant. In certain settings it can even use the
ambient gas in low Earth orbit. Imagine having a spaceship whose fuel source was literally
air, whose only waste exhaust was that same air? This is a trait shared by the ever-improving
VASIMR (Variable Specific Impulse Magnetoplasma Rocket), which is particularly intriguing as
it can use almost anything as a fuel source, although it has a preference for argon. Argon
is 200 times cheaper than its competitor, xenon, making it a much more viable fuel source. VASIMR
also has more “oomph” than other ion thrusters. The designers of VASIMR claim that it could take
astronauts to Mars in just 39 days. However, the technology still has some kinks to work out. It
is extremely power-hungry (It is designed to heat plasma inside it to 1,000,000°C, or 173 times the
temperature of the Sun’s surface). We do not yet have power sources efficient enough to feed this
engine at the levels necessary for that 39 day trip; and even when we do, unsurprisingly, getting
rid of the excess heat this creates is problematic as in space, there is nothing to transfer the
excess heat to. These up-and-coming lines of ion thrusters all still have a long way to go before
they will be able to totally replace conventional rockets. While their efficiency is incredible,
their poor thrust leaves much to be desired. But even if an ion engine is never developed with the
thrust necessary to get out of a planet’s gravity well, this capability to significantly reduce the
travel time to distant planets, and its advantages as a way of efficiently moving satellites means
that ion thrusters already have their niche. Scientists keep searching for solutions to
ion-thrusters’ technological challenges. For now, conventional chemical rockets remain the only
option for short-burn, high thrust journeys. But one day, if those challenges are overcome,
this may no longer be true. Ion engines might become the only type of engine worth using.
Then, the solar system as a whole will open up to us like never before. It might one day
be possible to pop over to Mars for a holiday. Perhaps this is one more example of where
science fiction one day becomes science fact.
