# LightSail 2 Meets the Photons



## REBerg (Jul 24, 2019)

LightSail 2 Unfurls, Next Step Toward Space Travel by Solar Sail (Published 2019)
					

The Planetary Society deployed LightSail 2, aiming to further demonstrate the potential of the technology for space propulsion.




					www.nytimes.com
				








*This image of Earth, with lens flares bottom left and upper right, was captured by LightSail 2 on July 7.
To the left of Earth, a piece of material that was used to hold the spacecraft's solar panels closed is visible.*​*                                                            The Planetary Society *​


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## RJM Corbet (Jul 24, 2019)

REBerg said:


> LightSail 2 Unfurls, Next Step Toward Space Travel by Solar Sail (Published 2019)
> 
> 
> The Planetary Society deployed LightSail 2, aiming to further demonstrate the potential of the technology for space propulsion.
> ...


Ok. Nice. But another one exaggerated by the media, imo. The article talks of inexhaustible power source for interstellar travel, etc. But this sail is only going to work with a plentiful supply of solar photons? Will it work even beyond Mars?


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## REBerg (Jul 24, 2019)

RJM Corbet said:


> Ok. Nice. But another one exaggerated by the media, imo. The article talks of inexhaustible power source for interstellar travel, etc. But this sail is only going to work with a plentiful supply of solar photons? Will it work even beyond Mars?


It is my understanding (subject, of course, to correction by those with more accurate information), that the photon force exerted on a solar sail would be cumulative, accelerating it toward the speed of light.
Absent an equal or greater counter-force, the speed of such a device theoretically should not slow as distance from a photon source increases. As photon force diminishes, only the rate of acceleration should decrease.


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## RJM Corbet (Jul 24, 2019)

Cool. The ion drive works the same principle, of incremental acceleration, I believe? But there aren't many photons in deep space? It's all about discussion and learning


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## REBerg (Jul 24, 2019)

Some interesting numbers here:









						Can I derive a combined equation for velocity of solar sail?
					

In this article on The Planetary Society website, I read this about solar sails:  At an acceleration rate of 1 millimeter per second per second (20 times greater than the expected acceleration for ...




					space.stackexchange.com
				




Seems like gaining photon-powered speed heading away from the Sun wouldn't be a problem. Heading back to Earth from an outer planet, would be another story.


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## RJM Corbet (Jul 24, 2019)

REBerg said:


> Some interesting numbers here:
> 
> 
> 
> ...


Ok. I believe you. That's way out of my league, lol. It seems to accelerate faster than an ion drive?


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## farntfar (Jul 24, 2019)

REBerg said:


> Heading back to Earth from an outer planet, would be another story.


According to the original article the Japanese test, Jaxa, passed Venus, which surely means travel towards the sun is possible, presumably by tacking in much the same way as you do with a sailing boat.


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## REBerg (Jul 24, 2019)

RJM Corbet said:


> Ok. I believe you. That's way out of my league, lol. It seems to accelerate faster than an ion drive?


 Way out of my league, too. Equations hurt my brain.
The acceleration speed of the light sail-powered spacecraft, in addition to the density of the photons available, would depend on the size of the sail and the mass of the craft it's towing.
Maybe it could accelerate faster than an ion drive-powered craft of equal mass. It seems like a more promising means of propulsion for round-trip missions than the solar sail. I see the sail as better suited for one-way unmanned deep space probes than human missions, unless another star system is at the other end of the journey.


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## REBerg (Jul 24, 2019)

farntfar said:


> According to the original article the Japanese test, Jaxa, passed Venus, which surely means travel towards the sun is possible, presumably by tacking in much the same way as you do with a sailing boat.


Changing the angle of the sail relative to the Sun, I believe, is how they steer the craft.
I think photon pressure drops off by an inverse square ratio heading away from the sun, which might make solar sail tacking back to Earth from an outer planet a very long process.


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## tinkerdan (Jul 24, 2019)

Assuming photons would not interfere with each other. Considering the solar sail itself.
Would or should I ask when would other photons begin to interfere with the solar sails present trajectory and possibly begin buffeting it around.


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## REBerg (Jul 25, 2019)

tinkerdan said:


> Assuming photons would not interfere with each other. Considering the solar sail itself.
> Would or should I ask when would other photons begin to interfere with the solar sails present trajectory and possibly begin buffeting it around.


That might be among questions the LIghtSail 2 team seeks to have answered in this pioneer effort. I would think that a relatively rudimentary computer could handle trajectory corrections necessitated by varying photon pressures on the sail.


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## Dave (Jul 25, 2019)

I understand the idea of tacking when sailing on the ocean, but this comparison with using a solar sail doesn't quite add up. You can never sail directly upwind but by beating to windward you can make slow progress. However, the solar wind is constantly in the same direction, never calms, but most importantly its direction would have the appearance of coming outward from a point source. On the other hand, while sailing among the inner planets their elliptical orbits would place their apsides in very different directions, so I can see that for travel between Earth, Venus and Mars you would more than likely have the solar wind at your back or, at least, off to one side. When you add in moving targets, the navigation would become very complicated. An equipment failure could mean you miss the target, which means that it won't be back in the same place for another planetary year.


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## hitmouse (Jul 25, 2019)

Most sailing relies on a Bernouille-type effect rather than purely wind-jamming (please excuse my inexact terminology.) Solar sailing relies on sun-jamming alone. Tacking also requires a keel, something which is not very helpful in space.


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## REBerg (Jul 25, 2019)

Outwardbound, crewless probes do seem like the best application of the technology, assuming it proves viable. I'd like to see just how fast a solar sail can go.


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## Dave (Jul 26, 2019)

I'm sounding incredibly negative, but it is only in response to that unmitigated, over-hyped enthusiasm in the article for travel by solar sails.


> There is a limitless supply of solar pressure


True, there is, and it's totally free and simple to make use of, but doesn't the radiation pressure fall off quite rapidly with distance from the Sun? It is 60.6 μN/m2 at Mercury, but only 9.08 μN/m2 at Earth (AU) and only 0.34 μN/m2 at Jupiter. Most of the acceleration could only take place close to the Sun.

Nevertheless, I agree that it will work best for an extra-solar journey. However, on such an extra-solar journey, in order to stop, presumably it reverses orientation and uses another Star to slow it down. That means it is accelerating for half the journey, and decelerating for half the journey. I realise that is the same with any spacecraft but it means that it is unlikely to ever reach the maximum velocity possible. That would require a kind of star-hopping.


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## REBerg (Jul 26, 2019)

Dave said:


> star-hopping.


I like the sound of that. For the sake of setting a speed record, however, I hope they send it toward a relatively starless part of the galaxy.
Massive Sail + Tiny Instrument Package = Giddy-up Go!


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## Daysman (Jul 27, 2019)

Falling close to the sun then swinging out is your best bet... 
No need to sail to the sun, just deploy at closest point... 
High temperatures and solar wind... problematic... 
Possible exit speed around a tenth of one percent of light... 
Cheap and swift...
Centauri in a few of millennia... 
The target star decelerates the sail...
We could drop something on a world there...
But what do we send?


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## Dave (Jul 27, 2019)

Daysman said:


> We could drop something on a world there...
> But what do we send?


Well, any presents made of glass or china would be out of the question!


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## Lumens (Jul 27, 2019)

Dave said:


> Well, any presents made of glass or china would be out of the question!


Yeah... What if they drink neither beer nor tea?


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## StilLearning (Jul 27, 2019)

Having followed this mission for some years: The excitement is not so much over what this solar sail, or solar sails of this sort of design, could do but over where this still-infant technology might lead. A fairly cheap and crude solar sail like Lightsail 2, or Japan's IKAROS space craft, could travel around the inner solar system more or less indefinitely, although not spectacularly fast. It could station keep in an orbit or position that is not naturally long-term stable indefinitely, and it this is the application that most near term uses for it focus on: Things like a long term mission holding a position between the Earth and Sun to provide early warning of solar storms that can damage electronics on Earth and satellites near it, or a re-useable asteroid belt surveyor that will, eventually, visit dozens of asteroids.

Looking to where the technology might lead: As Daysman points out a more advanced (and more heat proof) version of a solar sail could do a 'Sun diver' mission that passed very close to the Sun and picked up a huge speed boost by deploying the sail so close to it. Such a mission, IIRC, would still need centuries or more to reach the nearest star, but could dramatically cut travel time to the outer solar system - imagine if NASA's New Horizons mission to Pluto had needed only one year to get there, instead of nearly ten. Probes to study the plasma, magnetic fields, and dust of interstellar space in under ten years would become feasible this way too (the Voyager probes have only just reached this region of space after decades in flight, and aren't that well designed to study it although they're better than nowt), as well a missions to study very distant comet cores out there. 

In the long term researchers are considering things like the base line mission of the Project Starshot research and design effort: A very lightweight, very tough and heat proof, sail that is not pushed by the Sun but by an Earth based array of powerful lasers. Such a system would still be far more fuel efficient than any system that carried it's energy source with it, and could accelerate a one-way flyby probe up to interstellar speeds (5% to 20% of lightspeed) and be able to reach the nearest stars, although not stop at them (probably - potential breaking systems are being investigated too) within a few decades to a half century. This would not be the U.S.S Enterprise, it'd be more like an interstellar Sputnik, but it would technically be a true starship, that people alive today might live to see launch. This is way out of our engineering reach at the moment, although the point of Starshot is to identify, seriously and systematically for the first time, what development would be needed and what sort of time frame would be realistic to aim for.

Lastly the real reason to be excited is that the planetary society is a private, non profit, space advocacy group that has succeeded in launching it's own space mission to test an experimental design of engine, on a budget such a group can scrape together - and it's worked. Their success proves how much the miniaturization of in-space technology over the last fifteen years has actually increased the accessibility of space - the quite 'cubesat revolution', that has seen spacecraft no bigger than a fist become a common thing, has broken the back of the idea that space is for only the most ultra wealthy governments and corporations. It's still not cheap, or easy, but a university, a private group, or even a well funded school, can now put things into space.  

Anyway, my two cents.


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## hitmouse (Jul 28, 2019)

I read, many years ago, an article which considered the use a giant space-based fresnel lenses to maintain the sun's photon flux on receding solar sails. Might have been in New Scientist, 1980s.


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## StilLearning (Jul 28, 2019)

Me too. From what I've read applying solar sailing tech to launch anything bigger than an Xbox sized probe to another star system is going to mean scaling the sail and everything associated with it up to giant size.


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## StilLearning (Jul 28, 2019)

An interesting read on the subject: Planetary Society Deploys LightSail 2’s Solar Sail. What Does The Future Hold For Solar Sails? - Universe Today


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## Vertigo (Aug 9, 2019)

One or two people have questioned sailing towards the sun. I suspect you are thinking incorrectly about how you navigate around a star system. Though please correct me if I'm wrong.

We tend to get this image of battlestar galactica type spaceships whizzing around all over the place. however when in a star system I believe you don't sail away from the sun by simply pointing in the direction you want to go and blasting away (unless you have truly awesomely powered thrusters). Everything in a star system is permanently falling towards the star; if in a stable orbit then it never hits the star and instead orbits like a planet, if it were 'stationary' ie. not orbiting the star (though not really possible) then it will simply fall directly towards the star either hitting it or just missing it and settling into a highly elliptical orbit. So if you want to move farther from the star you simply increase your orbital speed and you inevitably move to a higher orbit farther from the star. If you want to go closer you decelerate and you fall to a lower orbit. Looked at in those terms, so long as your sail is trimmed to be pulling slightly tangential to and against your direction of orbital travel then you will slow and fall to a lower orbit closer to the star. That should be perfectly possible with a light sail.

In other words you don't get closer to the star by actually sailing towards it, but rather slow down and let the star's gravity draw you closer spiralling in towards the star. And you don't fly out of a star system in a straight line towards your destination star (that would require an enormous thrust) but instead steadily increase your velocity and spiral out from the star. Along the way you might use some of the planets to give your speed an extra sling shot boost. I guess eventually, having achieved the star system escape velocity, you will then be able head direct for your destination but you wouldn't start off this way.

Or I've got it all hopelessly wrong!


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## Venusian Broon (Aug 9, 2019)

Vertigo said:


> One or two people have questioned sailing towards the sun. I suspect you are thinking incorrectly about how you navigate around a star system. Though please correct me if I'm wrong.
> 
> We tend to get this image of battlestar galactica type spaceships whizzing around all over the place. however when in a star system I believe you don't sail away from the sun by simply pointing in the direction you want to go and blasting away (unless you have truly awesomely powered thrusters). Everything in a star system is permanently falling towards the star; if in a stable orbit then it never hits the star and instead orbits like a planet, if it were 'stationary' ie. not orbiting the star (though not really possible) then it will simply fall directly towards the star either hitting it or just missing it and settling into a highly elliptical orbit. So if you want to move farther from the star you simply increase your orbital speed and you inevitably move to a higher orbit farther from the star. If you want to go closer you decelerate and you fall to a lower orbit. Looked at in those terms, so long as your sail is trimmed to be pulling slightly tangential to and against your direction of orbital travel then you will slow and fall to a lower orbit closer to the star. That should be perfectly possible with a light sail.
> 
> ...



Excellent post @Vertigo. I get where your coming from. I got a tad confused with 'slow down' and 'decelerate' (acceleration in physics is complicated by the fact that an object in a circular orbit at a constant linear velocity is still constantly accelerating! ) which threw me as I know that object's orbit closer to the sun is faster that what it would be if it was further out. *But* that increase in speed comes from the spacecraft falling towards the sun, converting graviational potential energy into kinetic energy. 

The point about solar sail craft is that, pragmatically, they will always start in orbit around the earth. For it to change orbit to make it 'go higher' it will angle the solar sail so that it boosts the force on the craft only when it is in the direction of it's orbital motion. (When the craft comes round so that it's pointing the other way, you just pull the sails in so that the opposite force is not applied) This will put it into a larger elliptical orbit, which depending on it's mass and force applied to it, may get escape velocity (or it could do a number of passes, increasing the size of the ellipical till it does). It could then change it's orbit to take into account other planets and use them to accelerate or deceleate the craft using gravitational assists to alter the course onto one you want. (see for example Parker Solar Probe - Wikipedia for the orbital dynamics of the Parker Solar Probe that uses Venus to brake and bring it closer to the Sun in it's orbit. It's not a solar sail but the type of orbit it was given I found instructive)

Or you could use sails as well to decelerate by using them at the correct points of its orbit. 

Yes I think it would be quicker to send solar sails out from Earth, as one could always organise the orbit of the solar sail to actually use the Sun as a focal point of the ships orbital ellipse, so that when you wanted to send it off to Jupiter/Neptune, or to a star, say, you start it with the correct sail set up, right next to the Sun where it gets maximum thrust. And returning to Earth from far away will be awkward. But it's a form of transport that doesn't require fuel and may be a useful technology if we don't have more efficient thrust engine. (And we haven't looked into boosting the thrust artifically by shooting a laser at it, say.)


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## Vertigo (Aug 10, 2019)

Venusian Broon said:


> I get where your coming from. I got a tad confused with 'slow down' and 'decelerate'


Yeah wasn't quite sure how that should've been worded. Possibly decreasing your orbital... speed, rather than velocity might be more correct as your velocity is constantly changing anyway.


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