LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
planetary.org
planetary.org
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.
The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic (https://en.wikipedia.org/wiki/Inverse-square_law), not exponential.
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.
There's an omni-directional wind turbine (https://newatlas.com/2018-dyson-award-winner-o-wind-turbine/...) which sort of encapsulates what I'm thinking of.
I'm not too sure about how easy it would be to use a non-planar sail to redirect photons in the direction required to constantly build forward momentum - but a photonic "wind" turbine used to power a quantum dot emitter array might be able to do something.
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.
But a bigger sail would increase the atmospheric drag!
> The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft.
That's not to say the solar pressure isn't potentially slowing it down too, but it sounds like the main driver is drag in this case.
EDIT: this matters because full 180° reflection is like "bouncing" the photon to opposite velocity, can up to double exchanged momentum vs absorption. (This is why reflective mylar is nice)
However, I suspect the goal was kept as modest as possible to increase odds of success and lower costs.
This is a proof of concept test vehicle in low earth orbit, it isn't going anywhere.
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?
I wonder if there is more solar wind at apogee, too...
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...
"In the past 4 days, the spacecraft has raised its orbital high point, or apogee, by about 2 kilometers. The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft."
They also say this on the mission page:
"LightSail 2's attitude control system does not have the precision to maintain a circular orbit and continuously fly the spacecraft higher. Therefore, as one side of LightSail 2’s orbit rises, the other side will dip lower, until atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The spacecraft will remain in orbit roughly a year before entering the atmosphere and burning up."
Id imagine that a sail's effectiveness probably increases as its area increases and mass decreases so it should have have a relatively high B* compared to similar objects. Looking at the published TLE's seems to back this up— Lightsail 2's B* is larger than any current cubesat's value (https://celestrak.com/NORAD/elements/cubesat.txt)
[0] https://planetary.s3.amazonaws.com/assets/images/charts-diag...
https://forum.kerbalspaceprogram.com/index.php?/topic/176144...
I have limited KSP experience and an aerospace degree, so I'd defer to someone with more KSP experience.
> 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.
"The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft."
Basically, they're using the sail to boost their apogee while at the same time drag is bringing the orbit as a whole down.
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."
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 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?
[1] https://www.reddit.com/r/askscience/comments/4anry6/does_lig...
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?
This is called a 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). Like a mass driver, it gets around the rocket equation by separating the propulsion system from the spacecraft.
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.
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.
This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.
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).
To add just a tiny amount of nuance - sailing is redirecting airflow with a sail.
Surface and flow effects mean that very little air should be bouncing off the sail. If there is you probably have a lot of turbulence, and a 'bubble' will form around the sail around which most of the wind will simply divert, rather than redirect.
Or to put it another way, think of an entire parcel of air rebounding away from the sail, not individual air molecules.
Which, to bring it back to the origin of this thread of armchair physicists incorrecting each other, means that an LED mounted on a solar sail would not work, because the only force it imparts is collision. Which makes the sail analogy kind of a bad one for the use of solar pressure as a driving force.
Edit: Hmmm, but what if the surface material of the sail were phosphorescent and could gain energy state from the solar pressure, then kick it a photon when dropping energy state? And then what if there were a fly wheel (magnetic axel, in the vacuum of space, extremely low friction) that could act as the keel... Hmmmmmmm
Edit 2: I wonder if a lens that refracted light similarly to how air flows over an airfoil would create a "solar lift" effect? Unfortunately, lenses are heavy.
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.
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.
[1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
There is a whole separate device called an EM Drive, which claims to basically violate conservation of momentum. It is highly controversial, and unlikely to be anything other than a measurement mistake.
In fact, a jet engine could in theory (ignoring things like heat rejection) operate just fine in space if you brought along a massive bag of air and hooked it up to the intake duct. But then you've basically just designed the world's worst rocket engine :)
Either way, an LED is not generating any meaningful thrust, and pointing that thrust at a sail adds no benefit.
A propeller makes thrust like a paddle, by pushing off of something. A jet/rocket engine makes thrust by recoil, using conservation of momentum.
But even if they did not have fans, there is no fundamental difference between using a propeller or fan to speed up the flow of air and heating the air inside a chamber to speed up the flow of air. (The fan is used because at subsonic speeds it can be made more efficient.)
In other words, all 3 means of propulsion work by conservation of momentum.
Using atmospheric oxygen for the early phase of flight is a massive weight savings. If/when SABRE works, the Skylon spaceplane could be a practical reusable SSTO. It could carry a very respectable payload mass (the aim is 11-17 metric tonnes) to LEO, and it could do so quite cheaply if the designers' claims pan out.
Fun fact for Kerbal fans: the RAPIER engine in the game is inspired by SABRE.
If you attach a fan to a derby car and point it backwards, the car will move forwards. If you redirect the fan so it's pointing forward, the car will move backwards. If you put a sail in the way, the car will move backwards less efficiently. If you make a perfect sail that blocks all the air from the fan, it will not move at all.
It should push the fan and the sail apart, which could result in a structural failure.
The more realistic outcome would be that the sail did not disperse the air with perfect efficiency, and the craft would just move backwards.
What you’re describing would require redirecting the flow of air backwards, like a curved exhaust. Which is really just a less efficient version of turning the fan around.
Not really. If a fan was able to blow air in a vacuum it would produce thrust as well. Basically a rocket does that. It's just blowing gasses out the back.
It's Newton's law of motion, "every action has an equal and opposite reaction".
Also, why not just use the fan or laser or led as the means of propulsion directly?
I guess I think of reversers as redirecting the jet blast in a controlled manner, I guess I don't think of the sail as redirecting the wind in a controlled manner.
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?
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.
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.
Gloriously over-engineered.
That being said, the project itself was a disaster and I'm glad to have escaped the class with a decent grade despite it.
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...
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).
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
I'm not sure to what extent the mission will rely on the solar sail though.
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.
https://physics.stackexchange.com/questions/26712/what-is-th...
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.
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...
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.
More likely, the small rock would put a hole into the spaceship. This isn't a disaster. The spaceship residents would notice a gradually decreasing amount of pressure, assess where the leak is, and put a piece of duct tape over the hole.
Remember kids, real life isn't like the movies.
In the same way that an object won't explode when hit by a subsonic 9mm round with ~0.5kJ of muzzle energy ('just' causing a leak and decreasing amount of blood pressure) it will when hit by a 0.50 calibre round with 14-20kJ of muzzle energy.
For two objects in low earth orbit, travelling in opposite directions, the speed of impact can be over 14km/s - or 98000kJ per kg. At that speed even a relatively small object is going to cause a massive explosion when it hits you.
Since a solar sail is just a huge sheet of plastic coated in aluminum, any impact with the sail will tend to leave just a pinhole, or possibly a tear. You do need to design your sail with ripstops, but the total area lost to the pinholes will tend to be quite small.