
LightSail 2 Spacecraft Successfully Demonstrates Flight by Light - spazz
http://www.planetary.org/blogs/jason-davis/lightsail-2-successful-flight-by-light.html
======
mabbo
My understanding of what's going on here is that the light sail isn't aiming
or changing at all. It's just a big sail, in orbit.

When it's moving away from the sun, the sail is being pushed "forward", which
adds to it's velocity- this increases the apogee on the opposite side of the
orbit. When it's moving towards the sun, the sail is being pushed "backwards",
lowering it's current velocity and reducing the perigee. As the perigee comes
lower, it spends less time in that end of the orbit, where it's gaining
velocity, and when it's apogee is high it spends more time in that end of the
orbit, where it's losing velocity. It's not a winning strategy I'm afraid.

But imagine! Imagine if we had a slightly different solar sail. One that could
retract and expand at will, via solar powered electric motors. Then we could
have it open it's sails when the "wind" is favorable, then close them when
it's not. It could slowly but surely escape earth's gravity well, and
gracefully travel around the solar system. A big enough one might even be able
to accelerate right out of the solar system.

~~~
parsimo2010
I didn’t work on Lightsail, but the article implies that it does aim the sail.
It isn’t expanding/contracting, but it does change orientation using the
reaction wheels. Basically, they turn the sail broadside to catch the sunlight
to raise the orbit, and turn it on edge when they don’t want the extra drag.

This particular satellite won’t ever leave orbit because the sail can’t
overcome even the tiny amount of atmospheric drag. If you want to achieve
escape velocity with a sail you’ll need a bigger sail and a higher orbit to
start out. Right now it’s only feasible for small satellites and we’ve been
able to achieve escape velocity using rockets for decades.

Eventually, I do envision a competition to see who can get a payload to Mars
with the least amount of fuel. Kind of like the fuel efficiency challenges
that college engineering departments participate in today.

~~~
WalterBright
> a bigger sail and a higher orbit

But a bigger sail would increase the atmospheric drag!

~~~
parsimo2010
But the higher orbit would reduce the drag. You could probably get away with
the same size sail, but you’ve got to start high enough that the sail’s tiny
thrust over its 50% (ish) duty cycle can overcome the drag present in 100% of
the orbit. Once you’re high enough a bigger sail would mean lifting the orbit
faster.

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adrianpike
Can someone more familiar with orbital mechanics help me understand why it
looks like the perigree is coming down in opposite to the apogee?

I'm but a mere KSP player, but I've always thought the perigee comes down in a
fairly linear fashion based on drag, which is mostly a function of altitude -
I would assume they'd be using the sail to just "burn prograde" whenever the
solar angle is possible, raising both the apogee and the perigee, but the
apogee coming up way faster?

~~~
rzimmerman
I'd guess the sail orientation is not changing relative to the sun, so at
perigee this provides an acceleration (because it's mostly in the velocity
direction), which raises apogee. At apogee it provides deceleration (because
it's in the anti-velocity direction), which lowers perigee. There are also
probably other effects depending on the sail orientation that muck with the
inclination and other elements. It's a little different than drag which is
always anti-velocity.

I have limited KSP experience and an aerospace degree, so I'd defer to someone
with more KSP experience.

~~~
adrianpike
According to the article though they're already adjusting orientation to
adjust thrust, but possibly they saturate too fast and they can't control it
enough to truly null the deceleration?

> One such refinement involves LightSail 2's single momentum wheel, which
> rotates the spacecraft broadside and then edge-on to the Sun each orbit to
> turn the thrust from solar sailing on and off.

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zeroping
What about
[https://en.wikipedia.org/wiki/IKAROS](https://en.wikipedia.org/wiki/IKAROS)?
The only new part, is that Lightsail is a cubesat, right? (IKAROUS was 315
kg.)

From article: "Our criteria was to demonstrate controlled solar sailing in a
CubeSat by changing the spacecraft’s orbit using only the light pressure of
the Sun, something that’s never been done before. I'm enormously proud of this
team. It's been a long road and we did it."

~~~
Tepix
Yes, they demonstrated putting a light sail in a tiny cheap satellite and
controlling it

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silasdavis
> Most spacecraft use chemical thrusters to desaturate momentum wheels;
> LightSail 2 relies on electromagnetic torque rods, which orient the
> spacecraft by pushing against Earth's magnetic field.

I'm trying to visualise what this process might look like. Having read a bit
about momentum wheels it seems like it's probably not a one way street in
terms of the direction of torque so the wheel at times might speed up and
other times slow down so to avoid saturation. Clearly this is not always
possible so an external energy source is used to provide a balancing torque so
the wheels can be slowed down without turning the craft (too much).

For the lightsail 2 it uses magnetic torque rods, which from
[https://en.m.wikipedia.org/wiki/Magnetorquer](https://en.m.wikipedia.org/wiki/Magnetorquer)
are electromagnets so the energy comes from turning them on for a while.

Can anyone guess a bit more or explain where in the orbit the saturation might
occur and what this desatiration process would look like in terms of the
motion of the craft?

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hairytrog
If a photon hits the sail and is reflected, it must transfer two photon
momentums to the sail. The energy should be less than before it the sail. Is
it redshifted?

~~~
kmm
Correct. Consider an initially stationary sail. Even though this isn't
physically what is happening, you can imagine the reflection from the point of
view of the sail as a photon getting absorbed and a photon of exactly the same
frequency being emitted immediately after in the opposite direction. Now, to a
stationary observer, the sail will have started moving after the absorption,
so the emmitted photon will be redshifted due to the Doppler effect.

It's only be a very tiny amount of course at normal velocities, probably
smaller than the energy wasted due to a mirror not being 100% efficient. To
extract all the kinetic energy of a bundle of light, you'd have to bounce it
between two mirrors many times. Maybe that's a possibility for shorter
distances, perhaps during take off?

~~~
azernik
Reflection can indeed be used! However, it's only useful for pushing _apart_
two objects that you've built - e.g. a large station with a mirror and a
laser, and a small probe with just a mirror.

This is called a photonic laser thruster
([https://en.wikipedia.org/wiki/Photonic_laser_thruster](https://en.wikipedia.org/wiki/Photonic_laser_thruster))
and is a variant of laser propulsion
([https://en.wikipedia.org/wiki/Laser_propulsion](https://en.wikipedia.org/wiki/Laser_propulsion)).
Like a mass driver, it gets around the rocket equation by separating the
propulsion system from the spacecraft.

------
zitterbewegung
In grade school they asked us to make a model spaceship. I made a light sail
spaceship. It’s really neat that we have a working implementation of the
technique .

~~~
claudiawerner
When I was 16 in my physics class the light sale ship (or perhaps just the
idea of it) was part of the section where the physics of photons were being
investigated (along with stuff like lasers and the photoelectric effect).
Fantastic stuff.

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Causality1
What I'm curious about is how fast a solar sailing craft could go before the
thrust of the photons is balanced out by the drag of the interplanetary
medium. Five particles per cubic centimeter isn't much but I suspect it
wouldn't be irrelevant for a square mile sail moving at 400km/s.

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umvi
Very cool! I know nothing about solar sails, but I wonder if the craft could
have LEDs mounted to it so it could bombard different regions of the sail with
photons to make it turn appropriately.

~~~
henryfjordan
If you emit a particle (a photon) and then catch it the net effect will be 0.

~~~
umvi
Net effect is not zero if the photon bounces off of the sail

~~~
comboy
Why use sail then? You can just shoot your photon the opposite way.

~~~
seiferteric
That would be a "photon rocket" which is an existing idea, but requires a lot
of energy, whereas this uses free energy from the sun.

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jaypeg25
Slightly unrelated, but I did a project this past semester in Grad School
looking at the economics of a solar sail de-orbiting mechanism to attach to
satellites. Seeing the successful demonstration here just gives me the
slightest sense of pride and vindication toward the people in the class who
said there's no practical use for something like that.

That being said, the project itself was a disaster and I'm glad to have
escaped the class with a decent grade despite it.

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elkos
Here is a data dashboard from the telemetry collected via SatNOGS stations
tracking LightSail2
[https://dashboard.satnogs.org/d/CBwYeHSZk/lightsail-2?orgId=...](https://dashboard.satnogs.org/d/CBwYeHSZk/lightsail-2?orgId=1&refresh=5m&from=now-30d&to=now)

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JoeDaDude
I'm glad they raised the orbit, it's just that it's not enough for me (in the
northeast US) to be in the radio footprint of the satellite. It was launched
into equatorial orbit with a fair amount of inclination, but elevation angles
from my location never exceed 11 degrees making reception difficult.

I was hoping to receive the satellite's Morse code beacon and/or telemetry
stream [1]. BTW: Does anyone know the downlink frequency for the photos?

[1] [http://www.planetary.org/blogs/jason-
davis/2016/20160609-lig...](http://www.planetary.org/blogs/jason-
davis/2016/20160609-lightsail-2-morse-code.html)

~~~
TenaciousValor
Downlink frequency is 437.025 MHz. You'll be able to download telemetry, but
you won't be able to decode photos because NOAA says no.

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hyperpallium
Can you "tack" a solar sail?

Niven/Pournelle had a cool idea in _The Mote in God 's Eye_ of charging your
craft when traversing a magnetic field to change direction (not specific to
sails, but another passive technique).

~~~
msl
Using Earth's magnetic field for attitude control is already a thing [1].

[1]
[https://en.wikipedia.org/wiki/Magnetorquer](https://en.wikipedia.org/wiki/Magnetorquer)

~~~
apkessl1
In fact, that’s exactly what LightSail is using to control its attitude:
[http://www.planetary.org/blogs/jason-
davis/2016/20160418-lig...](http://www.planetary.org/blogs/jason-
davis/2016/20160418-lightsail-test-success.html)

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stillbourne
Why deorbit it though? Why not try for an EM-Lagrange point?

~~~
cptaj
They are in a low earth orbit and there is still considerable atmospheric drag
in the region. This causes the satellite to deorbit.

This was a technology demonstrator and it has completed its mission
successfully. It simply wasn't launched to an orbit where it can do more
(Cubesats hitch rides on rockets launching larger, expensive payloads)

NASA does have a mission intended to navigate quite a bit more using a solar
sailing cubesat.

[https://www.nasa.gov/content/nea-scout](https://www.nasa.gov/content/nea-
scout)

I'm not sure to what extent the mission will rely on the solar sail though.

~~~
ben-schaaf
As long as the trust produced by the solar sail is more than atmospheric drag
it technically has enough Δv to go anywhere.

~~~
mkl
Solar sail thrust isn't like rocket thrust; you can't just point it wherever
you like, or turn it on and off as effectively, and there isn't nearly as much
thrust available either. This craft can't leave low Earth orbit.

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b_tterc_p
Not really relevant but, if we we had an arbitrarily super fast spacecraft,
how would we prevent it from hitting a small rock and exploding?

~~~
alpha_squared
Ultimately, you can't. Space is really, really, really big and empty, though.
Unimaginably so, which would make an incident be some really bad luck. In
something like the lightsails, you'd just launch multiple simultaneously.

Space is so big and empty that even for something like the asteroid belt you
could pass through any point randomly and on average be thousands of
kilometers from the nearest rock. Perhaps even tens of thousands.

~~~
Gustomaximus
Is it though? To try and quantify risk I looked up how many metoeites hit the
earth each day and its 18,000 to 84,000 that are over 10 grams.

Whiile the earth is obviously much bigger than a space craft, you would think
should there be a bunch of craft flying around in the future, theres a fair
odds of it happening every few years.

Also for example, the ISS has already been hit a couple of times:
[https://www.iflscience.com/space/an-astronaut-used-his-
finge...](https://www.iflscience.com/space/an-astronaut-used-his-finger-to-
plug-that-leak-on-the-international-space-station/)

If we have thousands or 10's of thousands of craft in space in the future I
would suspect this will be a very real issue on a semi regular basis.

Take what I say with a healthy dose of skeptism as not an expert or even well
researched, but logically on above seems quite a reasonable risk.

~~~
ben-schaaf
Things tend to collect around large gravity wells, that's just kinda how that
works. I don't think you can compare the ISS in LEO to something traveling
within or without the solar system.

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inflatableDodo
Would be really interesting now to see a successful experiment with an
electric solar sail to get some idea of how the technologies compare in
practice -
[https://en.wikipedia.org/wiki/Electric_sail](https://en.wikipedia.org/wiki/Electric_sail)

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trilila
We need more of this, and more spending and results in space research. Not
only because we are destroying our own planet, but also because we as a
species have been stagnating for quite some time.

~~~
aetherspawn
Why spend money on leaving our lovely planet when we could spend money on
reducing emissions so we don’t have to.

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bratbag
Awesome!

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mmaunder
Ugh this title. Flight by light? A solar sail is not flying. An airfoil
generating lift is flying.

~~~
inflatableDodo
Balloons, rockets and cannonballs all fly. And the last two do not require an
atmosphere to fly in.

