We’re all familiar with the idea of solar sails to explore the Solar System, using the
light pressure from the Sun.
But there’s another propulsion system that could harness the power of the Sun, electric
sails, and it’s a pretty exciting idea.
A few weeks ago, I tackled a question someone had about my favorite exotic propulsion systems,
and I rattled off a few ideas that I find exciting: solar sails, nuclear rockets, ion
engines, etc.
But there’s another propulsion system that keeps coming up, and I totally forgot to mention,
but it’s one of the best ideas I’ve heard in awhile: electric sails.
As you probably know, a solar sail works by harnessing the photons of light streaming
from the Sun.
Although photons are massless, they do have momentum, and can transfer it when they bounce
off a reflective surface.
In addition to light, the Sun is also blowing off a steady stream of charged particles - the
solar wind.
A team of engineers from Finland, led by Dr. Pekka Janhunen, has proposed building an electric
sail that will use these particles to carry spacecraft out into the Solar System.
Address the question: To understand how this works, I’ll need
to jam a few concepts into your brain.
First, the Sun.
That deadly ball of radiation in the sky.
As you probably know, there’s a steady stream of charged particles, mainly electrons and
protons, zipping away from the Sun in all directions.
Astronomers aren’t entirely sure how, but some mechanism in the Sun’s corona, its
upper atmosphere, accelerates these particles on an escape velocity.
Their speed varies from 250 to 750 km/s.
The solar wind travels away from the Sun, and out into space.
We see its effects on comets, giving them their characteristic tails, and it forms a
bubble around the Solar System known as the heliosphere.
This is where the solar wind from the Sun meets the collective solar winds from the
other stars in the Milky Way.
In fact, NASA’s Voyager spacecraft recently passed through this region, finally making
their way to interstellar space.
The solar wind does cause a direct pressure, like an actual wind, but it’s incredibly
weak, a fraction of the light pressure a solar sail experiences.
But the solar wind is negatively charged, and this is the key.
An electric sail works by reeling out an incredibly thin wire, just 25 microns thick, but 20 kilometers
long.
The spacecraft is equipped with solar panels and an electron gun which takes just a few
hundred watts to run.
By shooting electrons off into space, the spacecraft maintains a highly positive charged
state.
As the negatively charged particles from the Sun encounter the positively charged tether,
they “see” it a huge obstacle 100 meters across, and crash into it.
By imparting their momentum into the tether and spacecraft, the ions accelerate it away
from the Sun.
The amount of acceleration is very weak, but it’s constant pressure from the Sun and
can add up over a long period of time.
For example, if a 1000 kg spacecraft had 100 of these wires extending out in all directions,
it could receive an acceleration of 1 mm per second per second.
In the first second it travels 1 mm, and then 2 mm in the next second, etc.
Over the course of a year, this spacecraft could be going 30 km/s.
Just for comparison, the fastest spacecraft out there, NASA’s Voyager 1, is merely going
about 17 km/s.
So, much faster, definitely on an escape velocity from the Solar System.
One of the downsides of the method, actually, is that it won’t work within the Earth’s
magnetosphere.
So an electric sail-powered spacecraft would need to be carried by a traditional rocket
away from the Earth before it could unfurl its sail and head out into deep space.
I’m sure you’re wondering if this is a one-way trip to get away from the Sun, but
it’s actually not.
Just like with solar sails, a electric sail can be pivoted.
Depending on which side of the sail the solar wind hits, it either raises or lowers the
spacecraft’s orbit from the Sun.
Strike the sail on one side and you raise its orbit to travel to the outer Solar System.
But you could also strike the other side and lower its orbit, allowing it to journey down
into the inner Solar System.
It’s an incredibly versatile propulsion system, and the Sun does all the work.
Although this sounds like science fiction, there are actually some tests in the works.
An Estonian prototype satellite was launched back in 2015, but its motor failed to reel
out the tether.
The Finnish Aalto-1 satellite was launched in June 2017, and one of its experiments is
to test out an electric sail.
We should find out if the technique is viable later this year.
Now that I’ve explained the technology behind the electric sail, I’ll provide a couple
of ideas proposed that could use this exotic propulsion system, but first I’d like to
thank:
Richard Leis Pontus Lindholm
Todd Wilnechenko
And the rest of our 783 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
It’s not just the Finns who are considering this propulsion system.
In 2015, NASA announced that they had awarded a Phase II Innovative Advanced Concepts grant
to Dr. Pekka Janhunen and his team to explore how this technology could be used to reach
the outer Solar System in less time than other methods.
The Heliopause Electrostatic Rapid Transit System, or HERTS spacecraft would extend 20
of these electric tethers outward from the center, forming a huge circular electric sail
to catch the solar wind.
By slowly rotating the spacecraft, the centrifugal forces will stretch the tethers out into this
circular shape.
With its positive charge, each tether acts like a huge barrier to the solar wind, giving
the spacecraft an effective surface area of 600 square kilometers once it launches from
the Earth.
As it gets farther, from Earth, though, its effective area increases to the equivalent
of 1,200 square km by the time it reaches Jupiter.
When a solar sail starts to lose power, an electric sail just keeps accelerating.
In fact, it would keep accelerating out past the orbit of Uranus.
If the technology works out, the HERTS mission could reach the heliopause in just 10 years.
It took Voyager 1 35 years to reach this distance, 121 astronomical units from the Sun.
But what about steering?
By changing the voltage on each wire as the spacecraft rotates, you could have the whole
sail interact differently on one side or the other to the solar wind.
You could steer the whole spacecraft like the sails on a boat.
In September 2017, a team of researchers with the Finnish Meteorological Institute announced
a pretty radical idea for how they might be able to use electric sails to comprehensively
explore the asteroid belt.
Instead of a single spacecraft, they proposed building a fleet of 50 separate 5-kg satellites.
Each one would reel out its own 20 km-long tether and catch the Sun’s solar wind.
Over the course of a 3-year mission, the spacecraft would travel out to the asteroid belt, and
visit several different space rocks.
The full fleet would probably be able to explore 300 separate objects.
Each spacecraft would be equipped with a small telescope with only a 40 mm aperture.
That’s about the size of a spotting scope, or half a pair of binoculars, but it would
be enough to resolve features on the surface of an asteroid as small as 100 meters across.
They’d also have an infrared spectrometer to be able to determine what minerals each
asteroid is made of.
That’s a great way to find that $10 trillion asteroid made of solid platinum.
Because the spacecraft would be too small to communicate all the way back to Earth,
they’d need to store the data on board, and then transmit everything once they came
past our planet 3 years later.
The planetary scientists I’ve talked to love the idea of being able to survey this
many different objects at the same time, and the electric sail idea is one of the most
efficient methods to do it.
According to the researchers, they could do the mission for about $70 million, bringing
the cost to analyze each asteroid down to about $240,000.
That would be cheap compared to any other method proposed of studying asteroids.
Space exploration uses traditional chemical rockets because they’re known and reliable.
Sure they have their shortcomings, but they’ve taken us across the Solar System, to billions
of kilometers away from Earth.
But there are other forms of propulsion in the works, like the electric sail.
And over the coming decades, we’re going to see more and more of these ideas put to
the test.
A fuel free propulsion system that can carry a spacecraft into the outer reaches of the
Solar System?
Yes please.
I’ll keep you posted when more electric sails are tested.
What do you think?
Are you intrigued by the idea of an electric sail as much as I am?
Let me know your thoughts in the comments.
Time for your playlist, all about electric sails.
First, an explainer from one of the Finnish researchers working on the idea, a video from
Tech Insider about how NASA is considering the concept, an animation from NASA, a longer
lecture from Bruce Wiegmann at NASA, and finally, a lecture from Pekka Janhunen, the pioneer
of the idea.