The next step for solar sails

[Vultur]

So, now that IKAROS has flown, and Sunjammer and LightSail are built and waiting for launches...

...what are solar sails going to be used for?

Given that they get more thrust closer to the sun, I think it would make sense to use a solar sail to send a spacecraft to Mercury or very close to the Sun itself, using the radiation pressure to counter the Earth-derived orbital velocity and dip inward towards the Sun.

[GClark]

Lunar Flashlight and NEA Scout in '18.  Sunjammer may yet fly in some iteration.

One of the proposals for SPRINT-C is a Plasma-Magnetosail demonstrator.

The Russians are talking about a sail craft for a Intergelio-Zond follow-on.

[Stormbringer]

just going to drop this off here...

http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=4274

Synopsis. one molecule thick material.

I would think something like this is of interest for a potential solar sail material.

[Nilof]

As far as new materials go, Graphene will likely be a very good alternative for solar sails in the decades to come. It has an exceptionally high absorbtion factor for white light given by 𝜋𝛼 per layer where 𝛼 is the fine structure constant. This works out to ~2.4%, roughly half of which is reflected. It's also fairly cheap to manufacture(the price is still falling) as long as you aren't too picky about it having extreme mechanical and electronic properties, though even cheaper graphene is still very resillient.

It's likely to be used either by itself for lighter stuff, or as a component in a thicker sail design if the sail diameter rather than the mass is bounded.

[sghill]

As far as new materials go, Graphene will likely be a very good alternative for solar sails in the decades to come. It has an exceptionally high absorption factor for white light given by 𝜋𝛼 per layer where 𝛼 is the fine structure constant.

That's backwards from what you want for a solar sail.  If it absorbs the light, it isn't being pushed as hard as if it reflects light because the entire sail would radiate the absorbed light back out as heat on both sides of the sail. With a reflective surface, the sail doesn't heat up, so the imparted energy/momentum is reflected back out on one side away form the direction of travel.

Otherwise graphene is an interesting choice of materials. 

Durability is the limiting factor IMHO.  I don't see anything lasting long before shredding due to micrometeors and cosmic rays makes the thing unusable.  For that reason, I think solar sails will be very limited in size, and/or very limited in the time they are required for doing work- such as an initial push out, or as breaks on the way back in.

[Jet Black]

As far as new materials go, Graphene will likely be a very good alternative for solar sails in the decades to come. It has an exceptionally high absorption factor for white light given by 𝜋𝛼 per layer where 𝛼 is the fine structure constant.

That's backwards from what you want for a solar sail.  If it absorbs the light, it isn't being pushed as hard as if it reflects light because the entire sail would radiate the absorbed light back out as heat on both sides of the sail. With a reflective surface, the sail doesn't heat up, so the imparted energy/momentum is reflected back out on one side away form the direction of travel.

Otherwise graphene is an interesting choice of materials. 

Durability is the limiting factor IMHO.  I don't see anything lasting long before shredding due to micrometeors and cosmic rays makes the thing unusable.  For that reason, I think solar sails will be very limited in size, and/or very limited in the time they are required for doing work- such as an initial push out, or as breaks on the way back in.

F=MA though. If the mass of the sail was such that the mass of the total assembly is half (or less) than an equivalent assembly with the the lightest reflective sail, it could still be better.

[Nilof]

Absorbtion in my previous post only means "not transmitted", it includes reflection. Graphene provides more of that than any other known single molecular layer material. As far as reflection goes, of the light that does not tunnel through roughly half is absorbed and half is reflected, which is quite good for visible light(comparable to gold).

In practice, Solar sails are likely to be thicker than that to prevent tunneling so as to reduce their total area. So you'd probably have some form of graphene-aluminum composite to maximize reflectivity.

[JasonAW3]

Odd question here;

    Does anyone know if it is possible to produce a solar sail that can also be used as a photovoltaic array?  This would tend to kill 2 birds with one stone.

[sghill]

Odd question here;

    Does anyone know if it is possible to produce a solar sail that can also be used as a photovoltaic array?  This would tend to kill 2 birds with one stone.

Graphene would be the place to start, but again, you don't want to absorb sunlight, you want to reflect it and turn it into momentum for your spacecraft.  Solar panels absorb the energy (yes, I know they also reflect some of it) that could be used for increasing your ship's speed.

[Soralin]

I've thought before, that if you have something like a solar sail in structure, that can act like a photovoltaic array, it seems like it would be very useful for high power electric propulsion, ion drives, vasimr, etc.

Basically, instead of using solar panels that are mechanically deployed, turned, etc. with a rigid structure, you use very thin photovoltaic material that you can deploy and handle like you would a solar sail.  So that you can minimize mass and gather MWs of power.

[hop]

I've thought before, that if you have something like a solar sail in structure, that can act like a photovoltaic array, it seems like it would be very useful for high power electric propulsion, ion drives, vasimr, etc.

You're not the only one. IKAROS had thin film photovoltaics in the sail and JAXA has a follow-on Jupiter trojan mission proposal which would combine it with ion propulsion. See http://www.jspec.jaxa.jp/e/activity/ikaros.html

[Vultur]

As far as new materials go, Graphene will likely be a very good alternative for solar sails in the decades to come. It has an exceptionally high absorption factor for white light given by 𝜋𝛼 per layer where 𝛼 is the fine structure constant.

That's backwards from what you want for a solar sail.  If it absorbs the light, it isn't being pushed as hard as if it reflects light because the entire sail would radiate the absorbed light back out as heat on both sides of the sail. With a reflective surface, the sail doesn't heat up, so the imparted energy/momentum is reflected back out on one side away form the direction of travel.

Otherwise graphene is an interesting choice of materials. 

Durability is the limiting factor IMHO.  I don't see anything lasting long before shredding due to micrometeors and cosmic rays makes the thing unusable.  For that reason, I think solar sails will be very limited in size, and/or very limited in the time they are required for doing work- such as an initial push out, or as breaks on the way back in.

F=MA though. If the mass of the sail was such that the mass of the total assembly is half (or less) than an equivalent assembly with the the lightest reflective sail, it could still be better.

The more it absorbs the hotter it gets though, which would limit getting a really big push by a very close Sun flyby as the sail would be destroyed.

[scienceguy]

anybody know if a sheet of metal such as gold can reflect if only one molecule thick? How thick are the gold layers on JWST?

[Nilof]

The more it absorbs the hotter it gets though, which would limit getting a really big push by a very close Sun flyby as the sail would be destroyed.

Well, if we're talking about single-molecular layer stuff, the front and the back side of the sail will be identical. So whether you go for a reflective or an absorbing material, the equilibirum temperature will be the same. Increased absorbtion also means that the same material will be more effective at radiating heat.

[Hanelyp]

Vacuum vapor deposit aluminum is used for first class astronomy mirrors, and should be a good finish for a solar sail.  As for keeping a sail cool, high reflection sun side and dark back side works best.

[scienceguy]

The more it absorbs the hotter it gets though, which would limit getting a really big push by a very close Sun flyby as the sail would be destroyed.

Well, if we're talking about single-molecular layer stuff, the front and the back side of the sail will be identical. So whether you go for a reflective or an absorbing material, the equilibirum temperature will be the same. Increased absorbtion also means that the same material will be more effective at radiating heat.

What about 1 molecule thick carbon fiber with gold or aluminum vacuum deposited onto it for the reflective side? This would make it 2 molecules thick, but could the 1 molecule thick gold or aluminum still reflect?

[Nilof]

As long as we're talking about materials that thin most of the light will tunnel through. Having a difference between the two layers in a two-molecule thick sail isn't going to affect the temperature by much, though you could perhaps reduce it by percent or so.

As for reflection on few-layered materials, one should keep in mind why reflections work like they do in the first place. The real underlying process is that the electrons and to a lesser extent atomic nuclei in the material scatter light. With many atoms, you get constructive interference for some scattering angles, and destructive interference for others. In the limit of a large number of layers, you get constructive interference when the reflection angle equals the incidence angle. Reflection doesn't happen at the surface of a material, it happens inside the material and destructively interferes beyond a certain depth of a couple wavelengths.

The "reflection" of the sun on a single layer sail will have interference fringes which will produce sidelobes like phased array antennas. For white light you'll get a rainbow pattern. The scattering pattern will be symmetric along the plane of the sail, with equal ammounts of scattered light being reflected and scattered towards the other side of the sail, since there is nothing to break the symmetry. In multiple layer materials, the mirror symmetry is broken by the fact that part of the incoming light was scattered by the previous layer so there is less to scatter.

The light scattered so that it's direction is parallel to the incoming light may destructively interfere with the incoming light. This is the mechanism that reduces the intensity of the incoming light as it continues through the material, and is the essence of the optical theorem.

[Solman]

Odd question here;

    Does anyone know if it is possible to produce a solar sail that can also be used as a photovoltaic array?  This would tend to kill 2 birds with one stone.

 If the sail is actually a parabolic concentrator mirror it can use highly efficient concentrator type PV to generate electricity.
Such a sail would be a little less efficient than a flat sail since the reflected photons will not be traveling perpendicular to the sail/mirror but power can be generated at great distance from the Sun making a solar thermal or electric thrust at Jupiter practical for a close perihelion.
modified to add: The solar cell array would have to have the ability to be moved into and out of the focus of course so that the cells don't intercept the reflected light when in solar sail mode.

[Vultur]

The more it absorbs the hotter it gets though, which would limit getting a really big push by a very close Sun flyby as the sail would be destroyed.

Well, if we're talking about single-molecular layer stuff, the front and the back side of the sail will be identical.

I was thinking of nearer term sails, micrometers thick like IKAROS.

[TrevorMonty]

See the 21 May talk.

 http://spirit.as.utexas.edu/~fiso/archivelist.htm