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. And finally, here’s a playlist.
