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LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

640 points by spazz 7y ago

umvi 7y ago
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 7y ago
  If you emit a particle (a photon) and then catch it the net effect will be 0.
  - moopling 7y ago
    Exactly, it is much like using a fan on a boat to blow a sail.
    - redorb 7y ago
      mythbusters were able to show a weird effect where the fan on a boat thing worked - I think they chalked it up to some blown wind from the fan drawing in more than from around the fan.. https://youtu.be/uKXMTzMQWjo?t=75 https://youtu.be/uKXMTzMQWjo?t=75
      - walrus01 7y ago
        From a physics point of view it's not like they created an infinite motion machine, the stored energy in the liquid petroleum is being burned, heat from the internal combustion engine radiated into the atmosphere, etc. It's just converting stored energy (some amount of kJ or kWh in a fuel tank, or battery) into an aerodynamically weird highly inefficient thrust method.
  - umvi 7y ago
    Net effect is not zero if the photon bounces off of the sail
    - comboy 7y ago
      Why use sail then? You can just shoot your photon the opposite way.
      - seiferteric 7y ago
        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.
    - gitgud 7y ago
      If it bounces off in the opposite direction then the net effect would be actually be negative.
      - deleted 7y ago
        [deleted]
  - simplicio 7y ago
    But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that direction. So having a directional lightsource fire into the sail wouldn't work, but having one that emits light in every direction on one side of the sail would.
    - bsmith0 7y ago
      So just use a directional light source pointing away from the craft...
- Rychard 7y ago
  I would assume the number of photons coming from the sun is many orders of magnitude higher than any LED could ever hope to produce. This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.
- cgh 7y ago
  That's like trying to propel your sailboat by holding up an electric fan.
  - umvi 7y ago
    I mean, it's only the size of a boxing ring, not a football field. I think LEDs could produce enough light to have a net effect.
    - bmdavi3 7y ago
      I don't know if the strength of an LED is enough to move the craft significantly, but if it is you'd need to point it at anything _but_ the ship itself, otherwise the two forces would cancel out. Just shine it in the opposite direction of where you want to go.
      - umvi 7y ago
        No, the two forces don't cancel out, I can't remember exactly why but elastic collisions don't work like that. That's why you can point a fan into your sailboat's sail and it will still go forward, mythbusters proved it.
        mkl 7y ago
        Mythbusters demonstrated something different, and something much less efficient than simply pointing the fan backwards like a normal fan boat (no sail required). Here: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail
    - Skunkleton 7y ago
      No. Think of the photons as producing little pushes. When they are emitted by the LED, they produce a little push in one direction, and then the sail pushes a little bit back in the other direction. If it were possible to create a significant volume of thrust from photons emitted by LEDs, then you would be better off just pointing the LEDs away from where you wanted to go (like a rocket).
- omarhaneef 7y ago
  That would be like taking fan and blowing it into your own sails. It wouldn't work.
  - umvi 7y ago
    It does work though, depending on the material your sail is made of. Try it: attach an electric fan to a pinewood derby car and point it so it is blowing into a tin foil sail also attached to the pine wood derby car. The car will move forward.
    - omarhaneef 7y ago
      I thought you want the LED mounted on the spacecraft itself. That would be the equivalent of the fan being on the sailboat. Yes, if you had another power boat next to the sailboat you could blow the boat, but it would be transferring some energy from the engine of the powerboat. So if you had another spacecraft near the space sail, and that spacecraft was shooting light, it would have some miniscule effect, but I didn't think that is what you meant. Edit: I suspect you're kidding but I can't be sure.
      - umvi 7y ago
        That's what I'm talking about. A handheld fan[1] is on the pinewood derby car, same as the sail. I built one before so I know it works. [1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Rechargeable/dp/B07QXHT67B/ https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
        omarhaneef 7y ago
        Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.
    - deleted 7y ago
      [deleted]
    - Skunkleton 7y ago
- comboy 7y ago
  This kind of technology had been demonstrated many years ago and many people around the world had been watching it at the time[1]. 1. https://i.imgur.com/VL0Q9hT.gif https://i.imgur.com/VL0Q9hT.gif
- blacksmith_tb 7y ago
  Not a sailor (solar or otherwise) but my instinct is you could turn by "furling" parts of the sail (say, if there were vanes that could be turned perpendicularly to the flow of photons). But lots of designs seem to call for propulsion from a laser, so maybe that could just be angled away slightly. At astronomic distances that might be tough, though.
  - hinkley 7y ago
    Supposedly, if you are orbiting the sun, you can turn a solar sail in such a way that you can brake against the solar wind, thereby moving in-system rather than out. But the sailing analogy breaks down for turning. A boat is pushing against the water and the wind. That gives it the ability to do things that you can’t in a vacuum, like move faster than the wind. For ‘turning’ you have only thrust vectoring, right?
    - db48x 7y ago
      There are several control methods available to solar sails that can work. They were studied quite extensively by NASA during the 60s. The thrust generated by a solar sail is proportional to the angle between it and the sun, as is the direction of that thrust. If any part of your sail can twist, then you have steering vanes that are quite useful. You can twist the vanes to angles where the vane on one side of the craft is reflecting sunlight while the one on the other side is not in order to get a rotational acceleration. If your sail is carrying a payload, then that payload will likely be in the center of the craft, with the sail spread out around it symmetrically. If you can shift the payload side to side relative to the sail, then that will shift the center of mass of the whole craft. With more of the sail to one side of the center of mass you will again get a rotation. If you want more information, find a copy of the book Space Sailing by Jerome Wright; it's quite good.
- walrus01 7y ago
  https://en.m.wikipedia.org/wiki/Photon_rocket https://en.m.wikipedia.org/wiki/Photon_rocket It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.
  - mkl 7y ago
    There are key differences with that: either the photons are shot backwards (like a rocket), or the source of the photons is not on the ship itself. ("The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft.")
- tfolbrecht 7y ago
  I'm curious if you could selectively turn off the reflective effect in different regions of the material and steer it that way.
  - andygates 7y ago
    IKAROS used this technique. The bird was spin-stabilised and a section of the sail had LCD shades that could be dark or light. It could darken its, say, venus-side segments as they went round. Gloriously over-engineered.
- JshWright 7y ago
  Changing orientation is easily done using spinning masses (this is how most satelites maintain their orientation)
stillbourne 7y ago
Why deorbit it though? Why not try for an EM-Lagrange point?
- cptaj 7y ago
  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 7y ago
    As long as the trust produced by the solar sail is more than atmospheric drag it technically has enough Δv to go anywhere.
    - mkl 7y ago
      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.
adrianpike 7y ago
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?
- TeMPOraL 7y ago
  My "mere KSP player" guess (without having access to my desktop on which I'd check that, in KSP) is that the angle at which they're reflecting the Sun amounts to burning prograde and radial-in (i.e. forward and down), which when done near perigee should both rise the apogee (the prograde component) and lower the perigee (the radial-in component).
  - adrianpike 7y ago
    Aha, that does make sense. Thanks for helping me wrap my mind around it. :)
  - hoseja 7y ago
    Yes, that would be my intuition as well, they seem to be "shifting" the orbital trajectory "sideways".
- hinkley 7y ago
  Another armchair dweller here, but you can use low thrust to make an orbit more elliptical or more circular. With the earth in the way part of the day, making it more circular is probably not in the cards. I wonder if there is more solar wind at apogee, too...
- sdenton4 7y ago
  (another ksp'er here...) I think the problem is atmospheric drag, which increases when the sail is open. If you're looking to increase apogee, the most efficient place to do so is at the perigee, which also happens to be where the atmospheric drag is the worst. So setting the mission, you get to choose the orientation of the sun relative to your orbital ellipse. If the sun direction to the sun is perpendicular to the long axis, then you (presumably) want your sail wide at the perigee, where drag will also be greatest, but will also provide the most efficient raising of the apogee. OTOH, if the sun's direction is parallel to the long axis, you can try to avoid drag at the perigee, and then rotate into position to get more thrust during the journey from perigee to apogee. But this is a less efficient way to raise the orbit, and will deform (circularize?) the orbit. I might have to fire up KSP with 'cubesat' with an ion thruster to simulate this...
zitterbewegung 7y ago
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 7y ago
  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.
b_tterc_p 7y ago
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?
- simlevesque 7y ago
  Maybe they we will be able to detect a marble from hundreds of thousands of miles in the empty space with new radar/lidar tech.
  - ztjio 7y ago
    LIDAR can't even see the moon.
- alpha_squared 7y ago
  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.
  - ceejayoz 7y ago
    High hundreds of thousands, if not a million or two, actually. https://physics.stackexchange.com/questions/26712/what-is-the-average-distance-between-objects-in-our-asteroid-belt https://physics.stackexchange.com/questions/26712/what-is-th... Space is BIG.
  - Cogito 7y ago
    Space is big. It's extremely unlikely to get close to any object while randomly passing through its orbit. Unfortunately, space becomes a lot smaller once you want to park yourself in a similar orbit to another object. Don't get me wrong, it's still massive, but since orbits trace out a path the problem becomes figuring out if two paths intercept rather than if two points intercept. This is why access to 'space' will not really be impacted in the case of a Kessler cascade; you can go through low earth orbits on the way to somewhere else, but if you tried to stay in low earth orbit you'd almost certainly be hit by something.
hairytrog 7y ago
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?
- mdorazio 7y ago
  Yes, but by a tiny amount. This reddit comment [1] explains in better detail than I can. [1] https://www.reddit.com/r/askscience/comments/4anry6/does_light_lose_energy_when_reflected/ https://www.reddit.com/r/askscience/comments/4anry6/does_lig...
  - Ductapemaster 7y ago
    Thanks for sharing this! Fascinating.
- kmm 7y ago
  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 7y ago
    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.
Causality1 7y ago
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.
elkos 7y ago
Here is a data dashboard from the telemetry collected via SatNOGS stations tracking LightSail2 https://dashboard.satnogs.org/d/CBwYeHSZk/lightsail-2?orgId=1&refresh=5m&from=now-30d&to=now https://dashboard.satnogs.org/d/CBwYeHSZk/lightsail-2?orgId=...
mmaunder 7y ago
Ugh this title. Flight by light? A solar sail is not flying. An airfoil generating lift is flying.
- inflatableDodo 7y ago
  Balloons, rockets and cannonballs all fly. And the last two do not require an atmosphere to fly in.
mabbo 7y ago
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.
- c3534l 7y ago
  Gravity is exponential, which makes lifting fuel off of Earth a losing strategy for long-distance travel. Anything that is fueled by something already in space seems incredibly promising.
  - yarg 7y ago
    Newton's approximation of gravitational force is: G = gMm/d^2 The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic (https://en.wikipedia.org/wiki/Inverse-square_law https://en.wikipedia.org/wiki/Inverse-square_law), not exponential.
    - foota 7y ago
      I suspect they were referring to the tyranny of the rocket equation.
- adrianpike 7y ago
  > retract and expand at will I think LightSail 2 already does just that, though, but possibly not very efficiently. From the article: > 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.
inflatableDodo 7y ago
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
JoeDaDude 7y ago
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-lightsail-2-morse-code.html http://www.planetary.org/blogs/jason-davis/2016/20160609-lig...
- TenaciousValor 7y ago
  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.
Haga 7y ago
Fuelscooping from atmo?
zeroping 7y ago
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 7y ago
  Yes, they demonstrated putting a light sail in a tiny cheap satellite and controlling it
- andygates 7y ago
  One of the limits of cubesats is that they're so small they have very limited propulsion, if any. Prop levels are correlated to mission lifetime; this presents a nice way to extend missions.
silasdavis 7y ago
> 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?
hyperpallium 7y ago
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 7y ago
  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 7y ago
    In fact, that’s exactly what LightSail is using to control its attitude: http://www.planetary.org/blogs/jason-davis/2016/20160418-lightsail-test-success.html http://www.planetary.org/blogs/jason-davis/2016/20160418-lig...
trilila 7y ago
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 7y ago
  Why spend money on leaving our lovely planet when we could spend money on reducing emissions so we don’t have to.
bratbag 7y ago
Awesome!
jaypeg25 7y ago
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.

640 points by spazz 7y ago

umvi 7y ago

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 7y ago
If you emit a particle (a photon) and then catch it the net effect will be 0.
- moopling 7y ago
  Exactly, it is much like using a fan on a boat to blow a sail.
  - redorb 7y ago
    mythbusters were able to show a weird effect where the fan on a boat thing worked - I think they chalked it up to some blown wind from the fan drawing in more than from around the fan.. https://youtu.be/uKXMTzMQWjo?t=75 https://youtu.be/uKXMTzMQWjo?t=75
    - walrus01 7y ago
      From a physics point of view it's not like they created an infinite motion machine, the stored energy in the liquid petroleum is being burned, heat from the internal combustion engine radiated into the atmosphere, etc. It's just converting stored energy (some amount of kJ or kWh in a fuel tank, or battery) into an aerodynamically weird highly inefficient thrust method.
- umvi 7y ago
  Net effect is not zero if the photon bounces off of the sail
  - comboy 7y ago
    Why use sail then? You can just shoot your photon the opposite way.
    - seiferteric 7y ago
      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.
  - gitgud 7y ago
    If it bounces off in the opposite direction then the net effect would be actually be negative.
    - deleted 7y ago
      [deleted]
- simplicio 7y ago
  But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that direction. So having a directional lightsource fire into the sail wouldn't work, but having one that emits light in every direction on one side of the sail would.
  - bsmith0 7y ago
    So just use a directional light source pointing away from the craft...
Rychard 7y ago
I would assume the number of photons coming from the sun is many orders of magnitude higher than any LED could ever hope to produce. This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.
cgh 7y ago
That's like trying to propel your sailboat by holding up an electric fan.
- umvi 7y ago
  I mean, it's only the size of a boxing ring, not a football field. I think LEDs could produce enough light to have a net effect.
  - bmdavi3 7y ago
    I don't know if the strength of an LED is enough to move the craft significantly, but if it is you'd need to point it at anything _but_ the ship itself, otherwise the two forces would cancel out. Just shine it in the opposite direction of where you want to go.
    - umvi 7y ago
      No, the two forces don't cancel out, I can't remember exactly why but elastic collisions don't work like that. That's why you can point a fan into your sailboat's sail and it will still go forward, mythbusters proved it.
      - mkl 7y ago
        Mythbusters demonstrated something different, and something much less efficient than simply pointing the fan backwards like a normal fan boat (no sail required). Here: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail
  - Skunkleton 7y ago
    No. Think of the photons as producing little pushes. When they are emitted by the LED, they produce a little push in one direction, and then the sail pushes a little bit back in the other direction. If it were possible to create a significant volume of thrust from photons emitted by LEDs, then you would be better off just pointing the LEDs away from where you wanted to go (like a rocket).
omarhaneef 7y ago
That would be like taking fan and blowing it into your own sails. It wouldn't work.
- umvi 7y ago
  It does work though, depending on the material your sail is made of. Try it: attach an electric fan to a pinewood derby car and point it so it is blowing into a tin foil sail also attached to the pine wood derby car. The car will move forward.
  - omarhaneef 7y ago
    I thought you want the LED mounted on the spacecraft itself. That would be the equivalent of the fan being on the sailboat. Yes, if you had another power boat next to the sailboat you could blow the boat, but it would be transferring some energy from the engine of the powerboat. So if you had another spacecraft near the space sail, and that spacecraft was shooting light, it would have some miniscule effect, but I didn't think that is what you meant. Edit: I suspect you're kidding but I can't be sure.
    - umvi 7y ago
      That's what I'm talking about. A handheld fan[1] is on the pinewood derby car, same as the sail. I built one before so I know it works. [1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Rechargeable/dp/B07QXHT67B/ https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
      - omarhaneef 7y ago
        Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.
  - deleted 7y ago
    [deleted]
  - Skunkleton 7y ago
comboy 7y ago
This kind of technology had been demonstrated many years ago and many people around the world had been watching it at the time[1]. 1. https://i.imgur.com/VL0Q9hT.gif https://i.imgur.com/VL0Q9hT.gif
blacksmith_tb 7y ago
Not a sailor (solar or otherwise) but my instinct is you could turn by "furling" parts of the sail (say, if there were vanes that could be turned perpendicularly to the flow of photons). But lots of designs seem to call for propulsion from a laser, so maybe that could just be angled away slightly. At astronomic distances that might be tough, though.
- hinkley 7y ago
  Supposedly, if you are orbiting the sun, you can turn a solar sail in such a way that you can brake against the solar wind, thereby moving in-system rather than out. But the sailing analogy breaks down for turning. A boat is pushing against the water and the wind. That gives it the ability to do things that you can’t in a vacuum, like move faster than the wind. For ‘turning’ you have only thrust vectoring, right?
  - db48x 7y ago
    There are several control methods available to solar sails that can work. They were studied quite extensively by NASA during the 60s. The thrust generated by a solar sail is proportional to the angle between it and the sun, as is the direction of that thrust. If any part of your sail can twist, then you have steering vanes that are quite useful. You can twist the vanes to angles where the vane on one side of the craft is reflecting sunlight while the one on the other side is not in order to get a rotational acceleration. If your sail is carrying a payload, then that payload will likely be in the center of the craft, with the sail spread out around it symmetrically. If you can shift the payload side to side relative to the sail, then that will shift the center of mass of the whole craft. With more of the sail to one side of the center of mass you will again get a rotation. If you want more information, find a copy of the book Space Sailing by Jerome Wright; it's quite good.
walrus01 7y ago
https://en.m.wikipedia.org/wiki/Photon_rocket https://en.m.wikipedia.org/wiki/Photon_rocket It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.
- mkl 7y ago
  There are key differences with that: either the photons are shot backwards (like a rocket), or the source of the photons is not on the ship itself. ("The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft.")
tfolbrecht 7y ago
I'm curious if you could selectively turn off the reflective effect in different regions of the material and steer it that way.
- andygates 7y ago
  IKAROS used this technique. The bird was spin-stabilised and a section of the sail had LCD shades that could be dark or light. It could darken its, say, venus-side segments as they went round. Gloriously over-engineered.
JshWright 7y ago
Changing orientation is easily done using spinning masses (this is how most satelites maintain their orientation)

stillbourne 7y ago

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

cptaj 7y ago
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 7y ago
  As long as the trust produced by the solar sail is more than atmospheric drag it technically has enough Δv to go anywhere.
  - mkl 7y ago
    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.

adrianpike 7y ago

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?

TeMPOraL 7y ago
My "mere KSP player" guess (without having access to my desktop on which I'd check that, in KSP) is that the angle at which they're reflecting the Sun amounts to burning prograde and radial-in (i.e. forward and down), which when done near perigee should both rise the apogee (the prograde component) and lower the perigee (the radial-in component).
- adrianpike 7y ago
  Aha, that does make sense. Thanks for helping me wrap my mind around it. :)
- hoseja 7y ago
  Yes, that would be my intuition as well, they seem to be "shifting" the orbital trajectory "sideways".
hinkley 7y ago
Another armchair dweller here, but you can use low thrust to make an orbit more elliptical or more circular. With the earth in the way part of the day, making it more circular is probably not in the cards. I wonder if there is more solar wind at apogee, too...
sdenton4 7y ago
(another ksp'er here...) I think the problem is atmospheric drag, which increases when the sail is open. If you're looking to increase apogee, the most efficient place to do so is at the perigee, which also happens to be where the atmospheric drag is the worst. So setting the mission, you get to choose the orientation of the sun relative to your orbital ellipse. If the sun direction to the sun is perpendicular to the long axis, then you (presumably) want your sail wide at the perigee, where drag will also be greatest, but will also provide the most efficient raising of the apogee. OTOH, if the sun's direction is parallel to the long axis, you can try to avoid drag at the perigee, and then rotate into position to get more thrust during the journey from perigee to apogee. But this is a less efficient way to raise the orbit, and will deform (circularize?) the orbit. I might have to fire up KSP with 'cubesat' with an ion thruster to simulate this...

zitterbewegung 7y ago

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 7y ago
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.

b_tterc_p 7y ago

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?

simlevesque 7y ago
Maybe they we will be able to detect a marble from hundreds of thousands of miles in the empty space with new radar/lidar tech.
- ztjio 7y ago
  LIDAR can't even see the moon.
alpha_squared 7y ago
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.
- ceejayoz 7y ago
  High hundreds of thousands, if not a million or two, actually. https://physics.stackexchange.com/questions/26712/what-is-the-average-distance-between-objects-in-our-asteroid-belt https://physics.stackexchange.com/questions/26712/what-is-th... Space is BIG.
- Cogito 7y ago
  Space is big. It's extremely unlikely to get close to any object while randomly passing through its orbit. Unfortunately, space becomes a lot smaller once you want to park yourself in a similar orbit to another object. Don't get me wrong, it's still massive, but since orbits trace out a path the problem becomes figuring out if two paths intercept rather than if two points intercept. This is why access to 'space' will not really be impacted in the case of a Kessler cascade; you can go through low earth orbits on the way to somewhere else, but if you tried to stay in low earth orbit you'd almost certainly be hit by something.

hairytrog 7y ago

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?

mdorazio 7y ago
Yes, but by a tiny amount. This reddit comment [1] explains in better detail than I can. [1] https://www.reddit.com/r/askscience/comments/4anry6/does_light_lose_energy_when_reflected/ https://www.reddit.com/r/askscience/comments/4anry6/does_lig...
- Ductapemaster 7y ago
  Thanks for sharing this! Fascinating.
kmm 7y ago
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 7y ago
  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.

Causality1 7y ago

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.

elkos 7y ago

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

mmaunder 7y ago

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

inflatableDodo 7y ago
Balloons, rockets and cannonballs all fly. And the last two do not require an atmosphere to fly in.

mabbo 7y ago

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.

c3534l 7y ago
Gravity is exponential, which makes lifting fuel off of Earth a losing strategy for long-distance travel. Anything that is fueled by something already in space seems incredibly promising.
- yarg 7y ago
  Newton's approximation of gravitational force is: G = gMm/d^2 The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic (https://en.wikipedia.org/wiki/Inverse-square_law https://en.wikipedia.org/wiki/Inverse-square_law), not exponential.
  - foota 7y ago
    I suspect they were referring to the tyranny of the rocket equation.
adrianpike 7y ago
> retract and expand at will I think LightSail 2 already does just that, though, but possibly not very efficiently. From the article: > 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.

inflatableDodo 7y ago

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

JoeDaDude 7y ago

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-lightsail-2-morse-code.html http://www.planetary.org/blogs/jason-davis/2016/20160609-lig...

TenaciousValor 7y ago
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.

Haga 7y ago

Fuelscooping from atmo?

zeroping 7y ago

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 7y ago
Yes, they demonstrated putting a light sail in a tiny cheap satellite and controlling it
andygates 7y ago
One of the limits of cubesats is that they're so small they have very limited propulsion, if any. Prop levels are correlated to mission lifetime; this presents a nice way to extend missions.

silasdavis 7y ago

> 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?

hyperpallium 7y ago

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 7y ago
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 7y ago
  In fact, that’s exactly what LightSail is using to control its attitude: http://www.planetary.org/blogs/jason-davis/2016/20160418-lightsail-test-success.html http://www.planetary.org/blogs/jason-davis/2016/20160418-lig...

trilila 7y ago

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 7y ago
Why spend money on leaving our lovely planet when we could spend money on reducing emissions so we don’t have to.

bratbag 7y ago

Awesome!

jaypeg25 7y ago

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.