This video is sponsored by MagellanTV.
Welcome back to Launch Pad, I’m 
Christian Ready, your friendly  
neighborhood astronomer. In our last 
video about the Sun's Gravitational  
Lens, we talked about a plan to send 
solar sails 650 astronomical units  
from the Sun. That’s the distance where 
the Sun effectively becomes a telescope  
with a magnifying power of 100 billion.
It’s an ambitious project, but 
one that’s becoming feasible  
thanks to advances in small cubeSats 
and lightweight solar sails.
In fact, scientists are developing 
an advanced sail called the SunVane  
that will accelerate to 35 km/s (~22 
mi/s), or 7 astronomical units per year.
Compare that to the New Horizons 
spacecraft. When it was launched in  
2006, it had an initial speed of more 
than 16 km/s (~10 mi/s). That it the  
fastest spacecraft ever launched. It got 
a gravity assist from Jupiter a little  
over a year later, and reached Pluto 
in 2015, just nine years after launch.
That’s an amazing achievement, but 
it’s pretty much the fastest we  
can go with chemical rockets. But a 
sailcraft with an escape speed of 7  
AU per year could reach 
Pluto in just 5 1/2 years!
In other words, sailcraft have 
the potential to be the fastest,  
and most economical, method of 
exploring the outer solar system.
We’re going to talk about 
how solar sails work,  
and the kinds of exploration missions we 
may very well see in the coming decades.
But first I’d like to thank Magellan TV,  
who are very kindly 
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Sailing the solar system is hardly a new 
idea. In 1607, Johannes Kepler noticed  
that the tails of comets always pointed 
away from the Sun. He speculated - quite  
correctly, in fact - that the Sun must 
be heating the comet and somehow blowing  
the liberated material into a tail. In 
his letter to Galileo in 1610, Kepler  
wrote that it might one day be possible 
to "Provide ships or sails adapted to  
the heavenly breezes, and there will 
be some who will brave even that void."
Although Kepler was also correct in 
his speculation about the solar wind,  
solar sails are actually propelled 
by the photons of sunlight.  
Even though photons don’t 
have any mass, they still  
have momentum. That means when light 
strikes a surface, some of its momentum  
is transferred to the object. The 
more reflective the surface, the  
more momentum is transferred. So, the 
object feels a tiny force acting on it.
How tiny? Well, if you go outside on 
a bright sunny day, the total pressure  
of sunlight is, roughly, less than a 
small paperclip resting in your hand.  
Even the slightest breeze 
exerts far more pressure.
But in space, it’s a 
different story. Granted,  
it’s the same tiny pressure we 
feel here on Earth. In fact,  
it works out to about 9 Newtons 
or 2 pounds per square kilometer!  
That won’t produce a very strong 
acceleration, even on a lightweight  
sail. But as long as the sail faces the 
Sun, it keeps accelerating. If you wait  
long enough, the sail will build up a 
lot of speed, with no fuel required.
Sunlight will of course push a sail 
away from the Sun. But a sail can also  
tack inward towards the Sun, like 
a sailboat tacking into the wind.  
This is done by angling the sail so 
sunlight pushes against the direction  
of the spacecraft’s orbit. This causes 
the spacecraft to lose angular momentum,  
and fall inward toward the Sun.
Solar sails can control their 
direction in other ways.
For example, Mariner 4 wasn’t a 
dedicated sail, but it was equipped with  
vanes mounted on the tips of its four 
solar panels. The vanes used sunlight  
pressure to stabilize the spacecraft as 
it flew past Mars. And that was in 1965!
In the early 1970’s, the Mariner 
10 mission to Venus and Mercury  
prematurely ran out of fuel for its 
thrusters. Engineers saved the mission  
by adjusting the angle of its solar 
panels to maintain attitude control.
In the mid-1970’s, the Jet 
Propulsion Laboratory designed  
a solar sail to rendezvous 
with Halley’s comet in 1986.  
Carl Sagan appeared on the Tonight Show 
with Johnny Carson to promote the idea.
Although Sagan discussed a 
traditional kite sail design,  
NASA opted for a second design called 
the Heliogyro. Instead of using struts  
to deploy a kite, the Heliogyro would 
spin up and extend its twelve blades  
by centrifugal force. In order to 
achieve the required surface area,  
each blade would be 8 m (26 ft) 
wide and more than 6 km (4 mi) long.
The spacecraft's attitude and 
direction would be controlled by  
changing the pitch angle of the blades,  
just like the collective 
pitch control on a helicopter.
The Heliogyro would tack into a 
slingshot trajectory around the Sun,  
and then set off for Comet Halley;  
the same comet that captured Kepler’s 
imagination nearly 400 years earlier.
Although it could have been manufactured 
for less cost than a traditional  
spacecraft, the Heliogyro was a large 
and complicated design. In fact,  
it was so large it could only be 
launched on the Space Shuttle.
But the Shuttle was behind schedule 
and over budget. So, Congress canceled  
funding for new planetary missions in 
1980 and the Heliogyro was scrapped.
It wouldn’t be until thirty years later, 
when the Japanese Space Agency, JAXA,  
finally demonstrated the 
first solar sailcraft.
The Interplanetary Kite Accelerated 
by Radiation Of the Sun,  
or IKAROS, was launched in May 2010  
as a secondary payload to the 
Venus Climate Orbiter, Akatsuki.
With a launch mass of 315 kg, IKAROS 
deployed its sail by centrifugal force.  
It was made of polyamide sail 
20 meters (66 feet) diagonal  
but only 7.5-microns (0.00030 in) thick.
IKAROS used thrusters for attitude 
control during normal operations.  
But it also tested the use of 
liquid crystal displays for attitude  
control. By varying the reflectivity 
of individual LCDs, they could vary  
the amount of sunlight pressure on 
different parts of the sail. This  
allowed IKRAOS to control its attitude 
without the use of any propellant!
NASA finally demonstrated solar 
sailing the following year,  
when it launched the 
NanoSail-D2 cube sat.  
At just 4 kg, it was much smaller than 
IKAROS. Because it was a secondary  
payload to low-Earth orbit, it only 
flew for 240 days before burning up  
on re-entry. But it was the first 
demonstration of a CubeSat sailcraft.
In 2019, the Planetary Society, 
which by the way was co-founded  
by Carl Sagan, launched LightSail 2 
by hitching a ride on a Falcon Heavy.  
After deploying its mylar sails, 
it faced the Sun through perigee  
and flipped edge-on to 
the Sun through apogee.  
The extra push from sunlight allowed 
the spacecraft to raise its orbit.
Even though it orbits higher than 
the International Space Station,  
LightSail 2 still feels the 
drag of Earth's atmosphere.  
As a result, its orbit has 
been gradually decaying.
The Planetary Society are 
still operating LightSail 2,  
and are learning a lot about how 
to optimize the sail’s performance.  
This in turn will help the development 
of the next generation of sailcraft.
To that end, NASA is developing 
the Near-Earth Asteroid Scout,  
or NEA Scout. NEA Scout’s mission is 
to demonstrate the ability for a small  
14 kg CubeSat sail to make a slow 
flyby of a near-Earth asteroid.
Like all sail missions to date, NEA 
Scout will be another technology  
demonstrator launched as a 
ride share. In this case,  
on the first Artemis 
I mission to the Moon.
Once it reaches cis lunar space, NEA 
Scout will separate from Artemis I,  
deploy its 86 square meter (930 sq 
ft) sail and set out for the asteroid.
The target asteroid hasn’t been 
announced yet because it will depend on  
when Artemis I actually launches. It’s 
currently scheduled for November 2021.  
We’ll see. But if it goes as planned,  
it could reach its target asteroid 
within 3 years after launch. 
Another mission NASA is considering 
is Solar Cruiser. This would be a much  
larger sail at nearly 1700 square 
meters (18,000 square feet).
Solar Cruiser would be launched 
as a - you guessed it - secondary  
payload to the Interstellar Mapping and 
Acceleration Probe, or IMAP mission.  
It’s currently slated for 
launch in October 2024.
If selected, Solar Cruiser will 
study the Sun from a halo orbit  
around the Sun-Earth L1 
Lagrangian point. This is  
the same orbit used by the Solar 
Heliophysics Observatory, or SOHO.
However, solar sails aren’t confined 
to the inner solar system where  
there’s plenty of sunlight. On 
the contrary, they’re our best  
chance for reaching the farthest 
regions of the solar system fast!
Remember, it took New Horizons 
nearly a decade to reach Pluto,  
and that was because it was 
launched on the most powerful  
rocket available at the time. But it 
will need another century to reach the  
Sun’s gravitational lens, or the 
hypothesized orbit of Planet 9.
On the other hand, a sail craft cruising 
at 300 km/s could reach Pluto within a  
year, the heliosphere in 2 years, 
and interstellar space in 5 years.
So how is it that a sail, which gains 
such low thrust from sunlight pressure,  
can outrace a traditional 
spacecraft launched by a rocket?
Well, one of the Solar Gravitational 
Lens' Phase III objectives  
is to demonstrate this vey capability 
in a Technology Demonstration Mission.
Traditional kite sails are difficult to 
package, deploy, and control at large  
scales. That’s why the TDM spacecraft is 
using a new approach called the SunVane.  
The SunVane design breaks up the large 
sail area into smaller vanes that are 
distributed along a lightweight truss.  
This simplifies their 
construction and deployment.
Not only that, but the Vanes 
can be individually articulated,  
making the sail highly maneuverable. 
This allows the sail to rapidly  
tack inwards toward the Sun, while 
minimizing its cross-section as it  
spirals in. At perihelion, the Vanes are 
turned face-on to the Sun for maximum  
acceleration. Afterward, the Vanes make 
fine adjustments to its trajectory.
Trajectory...tra-jec-tor-y...anyway.
By the time it reaches Jupiter’s 
orbit, the sunlight pressure is too  
weak at this point to accelerate 
the spacecraft any further,  
and the Sun’s gravity is 
too weak to slow it down.  
The spacecraft’s momentum 
is more or less constant.
The spacecraft can jettison 
the Vanes to lower its mass  
and gain an additional acceleration.
Now keep in mind, all of 
this requires zero fuel.  
And we haven’t even considered 
possible flybys of planets yet!
The Technology Demonstration Mission 
is still early in the planning stages,  
but its goal is to come to within 
10 solar radii and achieve an exit  
velocity of around 7 AU per year. That 
would make TDM two to three times faster  
than Voyager 1. At that speed, it will 
reach Jupiter in less than a year,  
Saturn in two years, and Voyager 
1’s distance in 20 years.
As impressive as those 
speeds are, they’re still  
not fast enough to reach the solar 
gravitational lens within, say,  
20 years. Going faster with a solar sail 
means getting even closer to the Sun.
Materials such as silicon nitride and 
silica are lightweight, reflective,  
and have high melting temperatures. A 
sail made of such material could come  
as close as ~2−5 solar radii.  
That would allow them reach cruising 
speeds of 300 km/s, or ~25 AU per year.
One of the SunVane’s design goals is to  
be easy to manufacture at low cost. 
That could lead to mass production,  
economies of scale, and open up the 
outer solar system for exploration.
Imagine sending a swarm of sailcraft 
to fly through the water plumes of  
Europa at Jupiter and Enceladus at 
Saturn. They could sample the water  
and test for the presence of microbes 
blasting out from the subsurface oceans.
Water plumes on Europa were 
first discovered in 2016.  
The Europa Clipper will fly through 
these plumes when it launches in 2024.
However, nothing is planned 
for Enceladus at Saturn,  
even though Cassini discovered 
its water geysers in 2005.
And what about the nitrogen 
geysers of Neptune’s moon Triton?  
They were discovered in 1989!
There’s even evidence of 
water geysers on Pluto!
My point is that since the sail 
craft are, by their very nature  
fast and inexpensive. Missions could be 
launched to investigate new phenomena  
shortly after they're discovered. We  
can always follow up with a large 
flagship mission later if needed.
Then there’s the rest 
of the Kuiper belt.
In 2019, New Horizons flew 
past Arrokoth, the first  
pristine Kuiper Belt Object. But 
that was only possible because  
it happened to lie along 
New Horizon’s flight path.
More than 10,000 KBOs larger than 100 km 
are thought to exist. Among them are the  
dwarf planets Haumea, Makemake, Eris, 
and Quaoar, all waiting to be explored.
These destinations could be 
reached by small sail craft  
in less time it took New 
Horizons to reach Pluto.
Sailcraft would also be 
the fastest way to explore,  
or maybe even search for, 
the hypothetical Planet 9.
Right now, our best hope of 
finding it is with the Vera  
Rubin observatory's 10-year 
survey of the southern sky.
But what if Planet 9 is so far 
away it's beyond the reach of  
even the Rubin survey? Or maybe 
it’s closer but is unusually dark?  
Or what if Rubin just can’t find it  
because of all of those Starlink 
satellites ruining the night sky?
Well, one way to find it would be to fly 
probes to where Planet 9 is thought to  
exist, and measure any gravitational 
deflections along their flight paths.  
That would allow us to 
triangulate Planet 9’s  
location and even constrain its mass.
We could then bring our space telescopes 
to bear and get our first view of the  
planet. Then we could send a follow-up 
sail craft to make a direct flyby.
We could even use sailcraft to chase 
down and explore interstellar objects.
In 2017, ‘Oumuamua became the first 
known asteroid of interstellar origin.  
Two years later, comet Borisov became 
the first known interstellar comet.
It’s thought that as many as 10,000 
interstellar objects - or ISOs - may  
be passing inside Neptune’s orbit on 
any given day. A fast-moving sail craft  
could intercept these ISOs and allow 
us to answer unsolved mysteries about  
their nature. For example, after 
swinging past the Sun, ‘Oumuamua  
accelerated instead of slowing down. 
Was this due to outgassing like a comet,  
solar radiation pressure, or some other 
mechanism? There would be no better way  
to find out than with a rendezvous and 
flying along side to see how it changes.
If ‘Oumuamua had been detected early 
enough, a 100-gram sail craft could  
accelerate to 60 km/s and rendezvous 
with ‘Oumumua at its closest approach  
in about 8 days. Of course, that assumes 
‘Oumuamua had been spotted early enough;  
when it was actually discovered, it had 
already swung around the Sun and was on  
an exit trajectory. If a pursuit 
had been initiated at the time of  
‘Oumuamua's discovery, a sailcraft could 
have caught up with it a month later.
Imagine trying to do that with 
our conventional approach.  
We’d spend years building a 
dedicated spacecraft before  
launching it on the most powerful 
rocket we could build, and then wait  
years, or decades to intercept? Even 
then, it would only be possible if  
‘Oumuamua was discovered before 
it entered the solar system.
Sail craft can also help us map the 
shape and structure of the Heliosphere.
The solar wind races outward from 
the Sun at supersonic speeds,  
until it collides with the collective 
interstellar wind of the Galaxy.  
This sets up a bubble called the 
Heliosphere. Since the Sun is  
moving through the interstellar 
medium, it’s long been thought  
that the Heliosphere should have 
a giant comet-shaped structure,  
with a long tail in the opposite 
direction of the Sun’s motion.
But recent simulations predict that 
the Sun’s extended magnetic field  
would instead channel the solar wind 
into a more or less bipolar outflow.
This new model predicts that as 
the Sun moves through the ISM,  
interstellar hydrogen atoms slam 
into the concentrated solar wind,  
and create a population 
of fast-moving ions.
A fast sail mission could test for the 
presence of these ions at roughly 150  
AU. Their presence or absence would 
prove or rule out this new model.
Multiple fast sails could be 
launched into different directions  
and begin to map the true 
shape of the heliosphere.
Eventually they’d reach beyond 
500 AU, and sample the pristine,  
undisturbed interstellar medium.
Directly sampling the ISM would allow 
us to better understand the abundance  
of the elements available for building 
stars and their planetary systems.  
Right now, we do this with 
surveys, but interference from dust  
along our line of sight reduces 
the certainty of our measurements.  
Direct sampling of the ISM would let 
us calibrate our surveys and account  
for interference. In other words, we’d 
know how much starstuff is out there.
Of course, the ultimate sail mission 
would be to another planetary system.  
The Breakthrough Starshot initiative 
aims to reach the Alpha Centauri system  
with tiny nano sails 
weighing just 1 gram each.
Instead of a slingshot around the 
Sun, an array of powerful lasers  
would accelerate the nano sails to 20% 
the speed of light. In other words, 100  
million miles per hour! At 
that speed, the sails could  
reach the Alpha Centauri system 4.3 
light-years away in just 20 years.
Obviously, there’s a lot of engineering 
challenges to overcome. Such as,  
ow to make a fully functional spacecraft 
that weighs just one gram. How to build  
lasers that can beam them out of 
the solar system, and many others.
But the sail craft missions we’ve 
discussed so far, including those  
to the solar gravitational lens, 
would be the proving grounds for  
Starshot. Think of what we would learn 
about mass production, miniaturization,  
navigation, communication, interstellar 
spaceflight, and high-speed sailing.
As the Starshot laser 
system is built out,  
they can be used to support 
these missions as well.  
This leads to yet another economy of 
scale that further lowers mission costs.
Now keep in mind that one of the cost 
savers is that most of the technologies  
needed are agnostic of the mission, 
whether it’s exploring the Kuiper belt  
or imaging exoplanets at the 
Sun’s gravitational lens.  
The Solar Gravitational 
Lens mission will allow  
us to map the coastlines 
and oceans of an exo-Earth!
In fact, I made a video about that very 
topic, so if you haven’t seen it yet,  
I invite you to check it 
out when we’re done here.
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