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I revised the introduction, beginning of a major overhaul suggested by several viewers. This will be done at an irregular pace, mostly top to bottom. Readers and editors are asked to comment, especially where I might violate Wiki standards. SpaceSailor 22:04, 19 May 2012 (UTC)

The history of concept section was replaced. A paragraph was added to the science fiction section covering earlier works. SpaceSailor 12:42, 27 May 2012 (UTC)

The physical principles section was replaced._SpaceSailor 21:27, 22 June 2012 (UTC)

The section currently titledMathematical survey is overly verbose and does not follow the tone of an encyclopedia. Further, the example presented as optimal is in fact not optimal--it is a steering strategy no one would actually use. I propose replacing this section with one having a more balanced set of useful examples. Someone obviously put a lot of effort into the current section, but it just does not fit. Comments invited. SpaceSailor 15:12, 25 June 2012 (UTC)

The section has been removed. In addition to the above, the section is a recap of a paper by Vulpetti, and thus goes against the Wiki rule against including research results. SpaceSailor 15:54, 18 July 2013 (UTC)— Precedingunsigned comment added byJerry Wright (talk •contribs)

The page heading has a "cleanup" banner which says "and majority effects from solar winds (gas) have been completely wiped." That sounds as though it is making the same mistake discussed on this page in several places, namely the misapprehension that the solar wind is significant compared to photon pressure. The fact that the photon pressure is ~1000 times greater than the gas pressure is noted so that part should be maintained. A further point is that for a fast sail leaving the Solar System, once it's speed exceeds that of the wind, the gas actually produces a drag rather than assisting, but again it is of negligible magnitude.— Precedingunsigned comment added byGeorgeDishman (talk •contribs)21:56, 19 January 2013 (UTC)[reply]

Right. At the time I write this, the wind's dynamic pressure is 0.8 nPa, so solar radiation pressure to solar wind pressure is ~11,000. Solar sails are driven by photonic pressure. Period. The solar wind has no discernible effect on performance. Telling readers that the wind is effective is a disservice. If a design effort yields a characteristic acceleration of exactly 1. under radiation pressure, that is the value used in performance analyses and simulations, not 1.00009. SpaceSailor 22:28, 30 June 2013 (UTC)

The operations section is essentially completed now. Much remains to be done with the missions section. A lot of non-solar sail material remains in the article, which will be removed or greatly reduced. Most of this relates to forms of beam sailing, which have their own articles.SpaceSailor (talk)13:02, 12 January 2014 (UTC)[reply]

There is an error in the history section, two different people are given credit for a single quote.Épargnez le visage (talk)01:01, 12 August 2016 (UTC)[reply]

Can someone please do the appropriate math and come up with a maximum speed for a solar sail using the sun only (no laser)? Assume a mass, let's say, of 18,000 metric tonnes for the ship itself (manned spaceflight)...what percentage of the speed of light could one reach using the sun only? Could you do 10%? 90%?71.80.232.252 (talk)06:16, 4 March 2012 (UTC)[reply]

With a hypothetical high-performance solar sail, which we cannot now build and don't really know how to build, it would leave the Solar System at about 0.001c, probably less than you were hoping for--with a very small payload at that. That is why Bob Forward went to sails pushed by giant lasers, which we really, really don't know how to build. Its hard to fully appreciate just how difficult interstellar travel actually is. SpaceSailor 20:18, 18 May 2012 (UTC)

The effect of a payload is to reduce the speed by a simple formula based on the mass of the sail, the terminal speed is proportional to the square root of the ratio of the total mass to the sail mass, thus if the sail has a mass of M and the payload is 3M, the speed will be half that of the sail alone. The speed can therefore be made arbitrarily close to that of the bare sail by making the sail arbitrarily large so that its mass is much greater than that of the payload. The bare sail speed depends on how close it can get to the light source which in turn depends on its maximum temperature. That in turn depends on how much heat is absorbed from the source and the emissivity of the dark side. The dark side can be assumed to be close to a black body and the heat absorbed depends on the reflectance and hence the chosen material. It also depends on the orientation to the source. The sail can get much closer at grazing incidence, the limit probably being where the plane of the sail is aligned with the limb of the sun or star. I agree, it would be useful to include the equations that lead to your quoted maximum and perhaps a table of values for different materials and normal/grazing incidence but is there a third-party source for that? My own calculations suggest the limit speed would be closer to 1%c for aluminium than the 0.1% you quote, but your figure is probably more practical while mine disregards engineering limitations.George Dishman (talk)21:41, 19 January 2013 (UTC)[reply]

When I get back to work on the mission section, I will include some hard data on escape speeds and the physics of solar swingbys. There have been a lot of distractions for me, but I didn't realize I had not done any work in a year. SpaceSailor 22:45, 30 June 2013 (UTC)

Vulpetti's very detailed simulations of fast solar sails show escape speeds of about 0.0003 c with areal densities approaching 1 g/m², close to some of my early trials, but better quality. Those are very difficult design challenges. Payload has to be included in that value. So, I think my estimate of 0.001 c might be an essentially unachievable upper limit. SpaceSailor 13:59, 13 July 2013 (UTC)

I ran a few quick simulations and found areal density of 0.02 g/m² gives about 300 km/s = 0.001 c = my earlier guess on best possible. SpaceSailor 15:56, 16 July 2013 (UTC)

Below 40nm, aluminium starts to transmit rather than reflect so a practical limit is around 0.11gsm for solid sheet. The perihelion distance and hence the terminal speed is then set by your choice of maximum sail temperature, what are you assuming?George Dishman (talk)10:15, 6 October 2013 (UTC)[reply]

I agree, an Al sail 'bottoms out' around 0.1 g/m2, while lithium bottoms out around 0.02 g/m2, something I have long been interested in. I recently added a bit on lithium in Materials. I generally assume 0.20 AU for close approach for Al sail, but much farther out for Li. An all-Al sail has no emission coating and suffers a lower emissivity value, affecting its temp (I do not know what e values might be for this case, hence don't know temp values). Usable emissivity values are empirical because thin-film effects dominate; bulk emissivity values do not hold up in these cases because material thickness is much thinner than the emitted wavelengths. Radiation degradation is always a serious issue (sail can darken), and must be considered along with temperature and structural loads in a close swingby.Radiation and loads may be more limiting than temperature. So the closest practical approach remains a big unknown--I consider 0.20 AU a reasonable estimate for the present for Al, recognizing it might be off substantially one way or the other. Li is highly reactive with oxygen and has a low melt point, so it must be fabricated in orbit above oxygen levels, and might not be usable much inside of 1 AU.SpaceSailor (talk)16:27, 7 October 2013 (UTC)[reply]

One of the key advantages of aluminium is that it retains its reflectivity to below 200nm. I haven't been able to find a reflectivity graph for lithium, it would be useful if that could be added to the graph on thereflectivity page.George Dishman (talk)15:31, 30 January 2014 (UTC)[reply]

I think perhap we should modify the initial sentence to state that solar sails are a proven technique of spacecraft propulsion. I need to do some research and get a cite to verify that existing test data is considered adequate to prove the concept as applicable to spacecraft.

There was early experience with the Echo balloons being pushed off orbit bysolar wind. I do not know if the light pressure was influential (reflection provides 2 times the photon mass thrust adjusted for vector dot products) or negligable compared to the momentumm of absorbed ions in the solar wind.

There was an asteroid or comet rendezvous proposed. I do not remember if it was scrubbed while still conceptual or failed on deployment.

Anyway, the issue is manufacturing technology and spacecraft design. Nobody argues AFAIK that the method will not work provided our spacecraft design and manufacturing technology gets good enough to deploy and use the sails.

There have been tests with laser light pressure:

http://spaceflightnow.com/news/n0007/06lasersail/http://www.eps.org/aps/meet/SHOCK01/baps/abs/S250.html

Planetary society site may have latest informtion:

http://www.space.com/businesstechnology/technology/solar_sail_update_010723.html

http://www.planetary.org/solarsail/suborbital.html

This articlehttp://www.quinion.com/words/weirdwords/ww-sol1.htmthat NASA used light pressure on Mercury Mariner probes in approximately 74 timeframe to extend usefulness of the probe 's data collection period. I have been unable to verfiy this from an independent reputable source.

If true, IMHO, this is clearly successful solar sailing. Just as a crippled power boat or raft will drift or "sail" with ambient wind. A sail boat is designed to generate and use the thrust more efficiently but it exists in the more primitive application of technology.

Opinions, insight, queries or sources anybody?user:mirwin

The revised intro should take care of this issue. Doubters overlook the fact that every successful interplanetary spacecraft is another demonstration of solar pressure predictions.Mariner 10 experience was written up in AvWeek and described inSpace Sailing. I met with the guys who did it and got to review their telemetry printouts. SpaceSailor 13:31, 20 May 2012 (UTC)

My understanding is that not all the criticism of solar cells is empty or based on misunderstandings of the technology. Not so the article's. Can someone clean up the section, so it sounds less gung-ho about the technology it is criticizing? -173.3.112.55 (talk)00:26, 10 November 2009 (UTC)[reply]

The information about the charged-wire "solar wind sail' ought to move to the magnetic sail article. It's out of scope.Ray Van De Walker (talk)01:23, 26 June 2008 (UTC)[reply]

I'm apparently having difficulty with the wording of this bit:

The radiation pressure from the Sun against an absorbing sheet with a mass of 0.8 g/m² is equal to its weight with respect to the Sun's gravity, independent of its distance to the Sun. This mass/area ratio is doubled in the case of perfect reflection.

"doubled" in this case seems to mean that the radiation pressure from the Sun against a perfectly reflecting sheet of1.6 g/m² is equal to its weight with respect to the Sun's gravity, whereas it should actually be0.4 g/m² since reflecting photons gives twice the momentum that absorbing them would. So I changed it to "halved", and Patrick reverted with an explanation that seems to be in complete agreement with what led me to make the change in the first place. So I plead confusion. :)Bryan 00:45, 16 Sep 2004 (UTC)

The radiation pressure on an absorbing sheet, regardless of mass, is equal to the gravity on a mass of 0.8 g/m², so there is a balance if that is the actual mass. If the radiation pressure is doubled then it can support a larger weight.--Patrick 07:01, 2004 Sep 16 (UTC)

Ah, lightbulb. That was a very dumb mistake for me to make, but once I made it it really buried itself deeply in my head. Sorry about that. :)Bryan 15:11, 16 Sep 2004 (UTC)

No problem, that can happen to everybody.--Patrick 19:51, 2004 Sep 16 (UTC)

I would like to see another category. Earth orbiting light sail craft or using other energy from the sun to provide maneuvering power for garbage collection duties. While extra solar exploration is the correct direction to go as one component of our quest for basic science, the junk in place must be considered or all near Earth space will become unusable in the centuries to come. By using a sail powered orbiter to collect and de-orbit the material and clear near Earth space of this clutter. I think this might be more cost efficient than something that requires refueling at times. This will only work if the price is right. I think we have reached a point where such a craft could be self directed most of the time. When needed a ground controller could maneuver the craft to collect a specific object. I will try to work on the math and materials. I'm just an engineer so I may not have the math and or science needed.BillWilliam (talk)23:03, 10 September 2010 (UTC)[reply]

Solar sails cannot operate at those low altitudes because aero drag > solar pressure. Break even is typically around 800 km. SpaceSailor 13:54, 20 May 2012 (UTC)— Precedingunsigned comment added byJerry Wright (talk •contribs)

The section on 'current progress' mentions the Japanese prototypes launched in August. Could somebody add something about what happened to them, and whether they were successful?The Singing Badger 17:10, 30 Sep 2004 (UTC)

They tested deployment, not propulsion, so I imagine after popping out, they re-entered. Added a ref.SeanCollins (talk)07:51, 24 April 2008 (UTC)[reply]

This must be the umpteenth time I have had to change "altitude" back into "attitude". Yes, "attitude" is perfectly correct, it controls the direction the spacecraft is turning. There is little need for "altitude" control in space, for one, because there isn't really a "height" in space - its all subjective, but certainly attitude is needed because direction is far more important. Solar sails aren't really meant to be manoeuvred through the atmosphere, where altitude applies, either.Can you people please stop doing this? Its VERY VERY EXASPERATING. "Attitude" IS a valid word for spacecraft. --Natalinasmpf 14:14, 22 Jun 2005 (UTC)

How about putting some HTML comments <!-- like this --> into the text at places where "attitude" keeps getting changed, explaining that the usage is correct? That way any editor who goes in to change it will definitely see it. I've used this approach before in other articles and it seems to work pretty well.Bryan 06:34, 23 Jun 2005 (UTC)

Hmm, I'll see where I could place it. I just don't want the comments to be too invasive, but it seems worth it anyway.

if, as the article states, a reflected photon provides thrust to a physical object; then does this not suggest that the photon has mass? In other words, since force is created from the change in momentum, then in order for a photon to create some sort of change in momentum of the solar sail, the photon must posses a non-zero mass.

J. Crocker 18:01, Jun 23, 2005 (UTC)

I see you have never heard of Arthur Compton. p=mv does not apply to photons. p=E/c does, and therefore p=h/λ. Mass does not enter into it, and anyone who says this comes from relativistic mass is wrong. Compton scattering dictates that a photon imparts momentum on an electron as a function of the angle by which the photon is deflected in the reaction, which defines its change in wavelength: Δλ = h(1-cos θ)/mc. Wikipedia is not a classroom. If you're too ignorant to evaluate an article, don't. --70.131.90.87 (talk)16:11, 17 December 2007 (UTC)[reply]

You don't have to be condescending when explaining a photon's momentum, especially when what you're saying isn't true. Yes,${\displaystyle p=E/c}$ applies, but${\displaystyle p=mv}$ ALSO applies, if you takem to be the relativistic mass, or${\displaystyle \gamma m_0}$, wherem₀ is the invariant mass and${\displaystyle \gamma }$ is the Lorentz factor. In fact, it DOES "come from relativistic mass" in the sense that your equation can be derived from mine.${\displaystyle E=\gamma m_0{c}^2}$, so${\displaystyle p=Ev/c^2}$. In the case of a photon,v=c, so${\displaystyle p=E/c}$. Therefore given the energy or momentum equation one can derive the other. If you really want to take it back to definitions, look at the equations for Noether charge and report back to me. Basically, it's stupid to state which reason photons have momentum is fundamental, they just do.

One important thing to notice on an intuitive level is that the reason photons have momentum is that their speed isc, which means that from at least one perspective they do have mass.—Precedingunsigned comment added byEebster the Great (talk •contribs)01:58, 8 November 2008 (UTC)[reply]

SeeTalk:Cosmos 1 for parallel discussion. Apparently the photons are absorbed and re-reflected at a lower energy (lower frequency). I'd like some references, though. -Omegatron 20:37, Jun 23, 2005 (UTC)

Seephoton or, for the classical model,Poynting vector. Using conservation of energy and momentum you can easily calculate the energy of the reflected photon. For experimental verification see P. Lebedev, 1901, "Untersuchungen über die Druckräfte des Lichtes", Annalen der Physik, 1901.Icek17:07, 8 October 2006 (UTC)[reply]

The correct title of Lebedev's paper should be "Untersuchungen über die Druckkräfte des Lichtes" (note the double k).Icek00:29, 2 November 2006 (UTC)[reply]

It's basic physics that photons have momentum. Photons are both particles and waves. You can apply de Broglie's famous theorem to photons to calculate the momentum of photons.

It's fairly easy to measureradiation pressure using aNichols radiometer. And radiation pressure is all that is needed for a solar sail. If we assume conservation of momentum, then radiation pressure implies that photons have momentum.—Precedingunsigned comment added by68.0.124.33 (talk)05:50, 14 November 2008 (UTC)[reply]

"Another false claim is that solar sails capture energy from the solar wind. The solar wind, composed of charged particles, would indeed apply a small amount of pressure to a solar sail, but this is small compared to the pressure exerted by light that would be reflected from the sail."

Everyone seems to think it's the solar wind. Can we have some references?

What is the mechanism by which photons impart momentum if they have no mass? (In detail, with references) -Omegatron 20:37, Jun 23, 2005 (UTC)

Albeit the photons have zero mass at zero velocity, they posses a momentum, equal to h.nu (Planck's constant multiplied by frequency). Therefore, in the case of total absorption (i.e. albedo = 0), this momentum is transferred totally to the sail. In the case of total reflection (albedo = 1), the momentum of photon is reversed, and therefore t w i c e the value of its momentum is transferred to the sail.

Of course, the solar wind (particles from Sun, mainly protons) are imparting also some pressure on sail, but this is in order of 0.1 nanopascals (compare it with 4.6 micropascals of solar radiation pressure), cf. McDowells site.

Also, if my memory is right, the Pioneer 10 space probe is not using the "solar sail" or sola pressure at all. On other side, Mariner 10 (probe to Mercury), was using a solar vanes for controlling the attitude (ie. orientation, not for changing its path). Probably confusion.Antonin Vitek, (avitek) Czech wiki

Some physics can be found e.g. at:

http://www.inspacepropulsion.com/tech/sails_physics.html

(This link is dead.) In my last quote, the value 4.6 micropascals is for totally absorbing sail (black-body-like), for an totally reflective one is twice as big, i.e. 9.2 micropascals.

The quotes about levelling the weight of 0.8 grams per meter squared (black-body) or 1.6 grams per m.sq. ("white-body") by solar pressure are valid for interplanetary space only(without perturbing forces of planets). It is interesting, that this value (mass per area) isindependent on distance from Sun, as the pressure diminishes with the square of distance from Sun as well as the radiation pressure, i.e. both forces remains in an equilibrium for such sail.

Antonin Vitek, Czech wiki

"The craft would have been gradually accelerating during each orbit as a result of the radiation pressure of photons colliding with the sails."

I thought it was from thesolar wind.

Aha."Another false claim is that solar sails capture energy from the solar wind. The solar wind, composed of charged particles, would indeed apply a small amount of pressure to a solar sail, but this is small compared to the pressure exerted by light that would be reflected from the sail."

"Photons are the sub-atomic particles that make up light, and travel at the speed of light. As photons reflect off the surface of the sails, they transfer momentum to the object."

How does this work, anyway? The photons would have to lose velocity if they transferred momentum, no? Are they absorbed and re-reflected with lower energy (frequency)? -Omegatron 18:07, Jun 22, 2005 (UTC)

They are indeed reflected with lower frequency. The easiest way to think about it is to assume it's something similar to an elastic collision, but with the photon moving at C before and after impact. These assumptions are realistic under most conditions. --Christopher Thomas 19:28, 22 Jun 2005 (UTC)

Careful! Even if the reflected photon had the same energy, a net momentum is imparted on the sail. As long as the photons are reflected from their original direction, the law ofconservation of momentum will "enforce" this.Awolf002 19:57, 22 Jun 2005 (UTC)

The change in momentum of the sail, however, results in a change in kinetic enery of the sail. This is balanced by a change in energy of the photon. It isn't always redshifted; for a decellerating sail, for instance, it would be blueshifted.--Christopher Thomas 20:05, 22 Jun 2005 (UTC)

Well, read theLaser cooling article, where all photons have the same energy. You still change the momentum of the object interacting with those photons.Awolf002 20:13, 22 Jun 2005 (UTC)

I am very familiar with laser cooling. The photons are absorbed and emitted at the transition frequency in theatom's rest frame. The frequency of the lasers is actually tuned to slightly below the transition frequency, with the extra energy for the low-to-high transition supplied by the atom's kinetic energy. From the lab's rest frame, it looks like the atom emitted a photon of higher energy than it absorbed. This is why laser cooling is effective.--Christopher Thomas 20:22, 22 Jun 2005 (UTC)

(moved fromTalk:Spacecraft propulsion)

I heard that solar sails are propelled by photons hitting the 'sails'. But photons have no mass and therefore no momentum, so in a collision they provide no impulse?-- sodium

Actually, photons do have a momentum. The momentum of a photon equals &#8462;/&lambda;, orPlanck's constant divided by the photon'swavelength.

Thanks, yes. Since I asked the question we have covered it in my A level course.

Photons have norest mass. Photons have energy, however, so their "mass" is E/c**2, so their momentum is E/c. This momentum is imparted to the sail upon impact. -RjLesch.

It may be interesting to note that if you make the sail reflect the photons, you actually get 2E/c per photon. --BlackGriffen

yes the photons do provide the impulse to the sails. According to the Planetary Society who have built the first solar sail powered craft 99% of its propulsion should come from reflecting photons, only 1% from the solar wind. See alsohttp://www.physlink.com/ae270.cfm andhttp://www.u3p.net/tipe/phot_jp.htm --rmhermen

Would it be possible tosail upwind with a solar sail? --EnSamulili 28 June 2005 21:13 (UTC)

Not really, at least not in the sense that sailboats do. Sailing into the wind at all is only possible because a boat is in water. In space, the sail is only pushed directly away from the source of the incoming photons, and can't be redirected or caught in any other way without some other force acting on it.siafu 28 June 2005 21:43 (UTC)

Yes, you *can* "tack" with a solar sail, pretty much exactly the same way sailboats do.

• To accelerate *away* from the sun, you point the sail so it bounces photons directly back towards the sun. (Much like when sailing downwind, you point the sail so air particles bounce directly back from where they came from, like a parachute).

• Assuming you're in orbit around the sun, to accelerate *forward* in your orbit, you tilt the sail about 45% so that it bounces photons *behind* you. (Much like when the wind is blowing from the west, and I accelerate *forward* to the North by tilting my sail about 45% so that air particles bounce off my sail heading *behind* me, heading South). (This indirectly leads to a *larger* orbit, *further* from the sun).

• Assuming you're in orbit around the sun, to accelerate *backward* in your orbit, you tilt the sail about 45% the *other* way, so it bounces photons *ahead* of you. (Much like when I turn my sail so that air particles bounce off heading North, pushing me to the South). (This indirectly leads to a *smaller* orbit, *closer* to the sun).

• To *move* directly towards the sun, use one of the above techniques to accelerate *backward* in your orbit for some time, until you are no longer moving relative to the sun. Then fold up the sail (or turn it edge-wise) and drop straight down towards the sun.

• Much like the counter-intuitive fact that sailboats can go much faster *across* the wind than in the *same* direction the wind blows, the "forward in orbit" and "backward in orbit" maneuvers work even with craft that are much too heavy (more than1.6 g/m²) for the "accelerate away from the sun" maneuver to do anything useful.

--DavidCary03:16, 29 July 2005 (UTC)[reply]

Folding up the sail and dropping straight towards the sun is not using the sail to move upwind. It's using gravity; one might as well say, "Sure, just come to a stop and fire the rockets."siafu21:59, 1 August 2005 (UTC)[reply]

Ah, you are exactly right. I said a lot of words, but I never actually answered the original question.

• Say you turn the sail nearly edge-wise to the sun. Tilt it just a little, so that photons from the sun bounce at a glancing angle, deflected slightly to the right. Then the net force of the photons is almost entirely to the left. The net acceleration on the sail (including gravity) is towards a point just to the left of the sun. (Much like when the wind is blowing from the west, and I turn my sail almost edge-wise to the wind, but deflecting the wind slightly to the north. Then the force of the wind is almost entirely to the south. The *combination* of the force of the wind in the sail and the pressure of the water on thekeel /centreboard moves the entire sailboat in a direction just to the south of directly west, just to the south of directly upwind).

• If I turn everything around, I get ... photons deflected slightly to the left ... force ... almost entirely to the right ... net acceleration ... towards a point just to the right of the sun ... moves the entire sailboat ... just to the north of directly west, just to the north of directly upwind.

• Alternating back and forth between left and right allows a sailboat to "tack", reaching a point directly upwind of its initial position.

The combination of the force of the wind on the sail, and the force on the keel/centreboard, allows a sailboat to accelerate/move in (nearly) any direction.

The combination of the force of photons on a solar sail, and the force of gravity, allows a solar sail to accelerate/move in (nearly) any direction.

If there's any way tonot use gravity, I would sure like to know :-).

There's several ways to think about the forces on a solar sail. All give the same results.

One way:

• Whenever a photon bounces off a mirror, the net force on the mirror is from the silver side towards the dark side. Imagine a spike pointing directly out from the dark side of the sail. The force of photons bouncing off the sail is always in the direction of that spike, no matter which direction the photon came from. We have one force directly towards the sun (gravity), and one force we can point half of all directions (reflection). (By turning the sail, we can point that spike almost any direction we choose, as long as we keep the spike in the dark).

A different way:

• There are 2 parts to the force of a photon bouncing off a solar sail. The first part is the force of the photon hitting the solar sail. Since the photon always comes directly from the sun to hit the sail, this part of the force is always directly away from the sun. (This would be the *only* force if the solar sail were black, but no one wants that -- everyone's solar sail design is reflective / mirrored). The second part of the force comes from the photon shooting out of the sail. We control exactly which direction the photon goes when it leaves. We can tilt the sail to send a beam of photons practically any direction (except *directly* away from the sun). Shooting out those photons in *that* direction pushes the sail in the exact opposite direction. We end up with one force directly away from the sun (absorption (optics)), one force directly towards the sun (gravity), and one force that we can point in nearly any direction we choose (emission).

If we put one or the other of these ways of explaining it into the main article, which one do you think would be less confusing? Even better, could you write an even less confusing way of explaining it?

--DavidCary03:11, 15 August 2005 (UTC)[reply]

"We can tilt the sail to send a beam of photons practically any direction (except *directly* away from the sun)"

What if you bounce it off another solar sail? :-) -Omegatron 03:20, August 15, 2005 (UTC)

The reason I objected to your well-reasoned explanation is because a sailboat using only the force of the wind to move upwind, taking advantage of the resistance of water to pressurization, or more generally, the resistance to lateral motion provided by the keel which forces the boat forward (mostly) rather than directly away from the wind, and not because your explanation was wrong in any direct way. Outside of a gravity well with only one source of photons (obviously a purely hypothetical situation) it wouldn't be possible to move "upwind" with a solar sail. However, just about any situation wherein a solar sail would be useful requires a powerful light source, which in our universe is also almost always a powerful source of gravity at the same time. The only situation that I can think of, practically, where such would be eliminated is with a craft in interstellar space being struck with a carefully aimed maser/laser/whatever that is focused on a very narrow angle, allowing it to stay intense despite great distance.

I'd say your second explanation is more accessible to the lay audience, personally, but it might be helpful to refer to and/or explain the nature of vectors, but that's just a suggestion.

In response to Omegatron, if you manage to reflect the photon directly away from the sun again, the net force transferred to the object holding the sails (assuming it's one rigid object) would be zero, given that the photon receives the same force in being reflected out on the same vector it was originally on, so you COULD do that, but it would defeat the purpose of using a solar sail to begin with.siafu04:03, 15 August 2005 (UTC)[reply]

I agree with Siafu that calling the process of moving a solar sail toward the source of light "tacking" is not accurate or useful to the layperson. Technically, tacking is simply the matter of changing direction from one heading to another by passing the bow of the boat through the wind, using the momentum of the boat to successfully reach the other side of the dead zone. Using tacking as means of traveling upwind is simply the process of traveling along successive vectors whose sum cancels out lateral travel and results only in net movement toward the wind source.

• In comparison, traveling "upwind" in a solar sail is a very different process, namely: reducing orbital speed until it is insufficient to maintain escape velocity and the craft spirals into the solar body on account of gravity. To move toward the solar body, the solar sail's angle needs only be adjusted once to apply the photonic pressure against the direction of orbit, and then adjusted again when the desired lower altitude is reached. This process is much more akin to a powerboat's method of decelerating (in a current of gravity, to continue the metaphor) than it is about sailing upwind.
  • And of course, the process described by DavidCary could never move the solar sail "upwind" if the light source had no associated gravity, such as a massive laser light source. Changing the solar sail's angle slightly off from parallel to the light source would only result in the solar sail moving laterally from side to side as it continued to move away from the light source.
• I think the lay person knows enough about tacking as a means of traveling upwind that making a comparison between the two processes is more confusing that it is helpful.

--Sethumme (talk)01:06, 30 June 2008 (UTC)[reply]

I removed the following:

"Aspiring engineer Kyle Caskey has proposed to use designs of manned spacecraft with solar sails as the main means of propulsion for a small pleasure crafts, somewhat likeyachts. These solar sail yachts could move small groups of family and friends between the Moon and Earth and other destinations at varying speeds."

Text from an aspiring engineer shouldn't really be included in this case.kmccoy(talk)21:08, 13 July 2005 (UTC)[reply]

What kind of acceleration are we talking about, here? It would be great if we could translate some of the mechanisms, etc, above into measures that the average sci-fi fan can grok. Do solar-sailed craft whiz about the solar system, or just kinda putter? If HAL had been sail-powered, how long would it have taken Dave to get to Jupiter? Thanks!

I agree that it would be nice to put acceleration, earth-to-mars time, earth-to-jupiter time, etc. into the article. --70.189.75.14800:54, 29 July 2005 (UTC)[reply]

I enthusiastically also agree.Therealhrw00:23, 29 December 2005 (UTC)[reply]

Specific acceleration would depend on the design of the individual spacecraft and its payload/mass and sail area. Perhaps some detailed design discussions at http:\\www.wikiversity.org school of engineering would be appropriate. In general the current state of the art with solar sails implies "puttering" not performance records compared to other propulsion methods.Lazyquasar05:44, 25 September 2007 (UTC)[reply]

Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. However, when in Earth orbit, the majority of mass on most interplanetary missions is taken up by fuel. A robotic solar sail could therefore multiply an interplanetary payload by several times by reducing this heavy fuel mass, and create a reusable, multiimission spacecraft. Most near-term planetary missions involve robotic exploration craft, in which the directness of the course is simply unimportant compared to the small fuel mass and fast transit times of a solar sail. For example, most existing missions use multiple gravitational slingshots to reduce fuel mass, because even though these are terribly indirect they save years of transit time.

This entire first paragraph in the second section, excepting the first sentence, is very unclear. The second sentence appears to begin dealing with one situation ("in Earth orbit") then suddenly move to another ("interplanetary missions") without addressing the first. I replaced "heavy" with "significant" in the third sentence since it seemed redundant; multiimission in the same sentence is spelled incorrectly, but I do not know the correct spelling. I tidied up the second last sentence, as it sounded awkward, but the last sentence I could not decide how to improve, though it currently bothers me.—Precedingunsigned comment added byMywikinick (talk •contribs)21:32, 15 September 2007 (UTC)[reply]

I'm just expressing concern that this article isn't neutral. I didn't add a POV tag because it isn't that bad yet but the article seems to have a "criticism" section but it's only called "misunderstandings", it shouldn't be called that because that's only what PR people write when they want to change people's mind about things and an encyclopaedia's job is to just mention it and not necessarily to change people's minds.Jeffrey.Kleykamp21:11, 22 September 2007 (UTC)[reply]

The section "Misunderstandings" actually appears to attempt to correct common misunderstandings about how a solar sail would work. The theoretical basis for solar sails is well understood. I did a college paper on solar sails about twenty years ago. Critics of the approach are unconvinced the engineering and manufacturing details provide a better approach than other alternatives, the science is uncontroversial. A section for critics should cite some technical engineering studies and describe why solar sails were not used in specific missions. Usually technical development risk but probably some schedule, sail development and travel too long for mission profile. I deleted the last paragraph in this section because it claimed it was a misunderstanding that a solar sail would have to tack and then proceeded to describe how it would tack. Perhaps the paragraph could be restored and revised for useful placement elsewhere.Lazyquasar06:03, 25 September 2007 (UTC)[reply]

I cannot find any reference online regarding any "doppler shift" related to Dr. Gold's contention that solar sails will not work because of application of the Carnot Rule. Further, I cannot find any serious work, analysis, reference, etc. that indicates space scientists and engineers take his contention seriously enough to refute it with published mathematical physics. Perhaps we should delete references to Dr. Gold's contention solar sails will not work? Too bad Cosmos-1 crashed that would settle it definatively. Personally I find the too refutations I have added both convincing.Lazyquasar07:38, 25 September 2007 (UTC)[reply]

I wonder if this section should be moved a separate article/stub similar tohttp://en.wikipedia.org/wiki/Force_field_%28science_fiction%29 and then linked. To me it adds little to a serious understanding of Solar Sail technology towards which the rest of the article is aimed.Lazyquasar13:33, 25 September 2007 (UTC)[reply]

Fair question, but there is at least one fictional instance that is relevant - Robert L. Forward, one of the designers of the concept, gives a fairly detailed description inRocheworld and its sequelReturn to Rocheworld, including the use of the sail for braking.Philcha (talk)11:42, 2 January 2008 (UTC)[reply]

I think I'm right when I say that there should be a continuous variation possible between what we call a solar sail and a solar cell. If you have a perfectly efficient solar cell and use its power to produce a beam of light, that's the same as a mirror. Half the pressure comes with the absorption of the light and half with its emission. Even an inefficient solar cell, or one powering a load, should work almost the same way, if you can cunningly design your heat sinks to send most of the infrared radiation a certain way. If you have power to spare I suppose you can even use refrigeration methods to concentrate all the heat in a very defined way. It would seem to follow, then, as one considers a technologically advanced future, that eventually the ships should use vast membranous solar cells as their solar sails, arranging to do something with all that power before releasing it into space from structural elements designed for the purpose.70.15.116.59 (talk)17:21, 19 December 2007 (UTC)[reply]

Should a wheel be considered a both an electric motor and a generator?75.45.71.133 (talk)05:27, 14 February 2008 (UTC)[reply]

To add to the thrust, the heatsink has to emit the radiation back towards the source, but it will also absorb heat from the source hence you get into complex questions of thermodynamic balance. Basically any energy you extract as useful work detracts from the kinetic energy gained by the craft.George Dishman (talk)09:29, 14 August 2010 (UTC)[reply]

NASA is to launch NanoSail-D in a month:http://science.nasa.gov/headlines/y2008/26jun_nanosaild.htm?list1097511 (at the moment I just put it in "External Links". —Eickenberg (talk)00:37, 28 June 2008 (UTC)[reply]

This section of the article needs to be made easier to read. Solar sailing dynamics is my own area of expertise and I found this section difficult to read. If this article is going to discuss the mathematic of solar sails I think a good place to start would be to clearly and carefully define the three main performance indicies for solar sails: The lightness factor, the sail loading and the characteristic accelration. With these defined the section could then talk about the mathematics from a greater knowledge base. Ideally such a discussion should start with the eqautions of motion being explicitely written out, probably in polar coordinates.Bob The Tough (talk)09:17, 10 September 2008 (UTC)[reply]

Thanks, I agree that it is confusing. But it is also lacking references, so if someone could at least point to reliable sources of information we could more easily bridge the gap --NealMcB (talk)21:06, 21 November 2009 (UTC)[reply]

Just logged back onto this page for "old times sake" and this issue has still not been resolved. If I can find time between job and family I will write out the text as I suggested. Also dig out references to relevant scientific papers as requested by NealMcB.

Another note - there is a line in this section which (I think) is unfounded, namely "In addition, solar-sail spacecraft has been supposed to be able to reverse its motion (in the solar system) provided that its sail were sufficiently light that sailcraft sail loading (σ) is not higher than 2.1 g/m²". I think this is wrong, and that angular momentum reversal can be achieved with any quality of sail it just takes (a lot) longer (I have to check the maths again about this, I've been away from solar sails too long). And I'm not ignoring the radius of the Sun. Before I dig out my old notes, anyone have any conclusive evidence either way?Bob The Tough (talk)12:42, 26 January 2010 (UTC)[reply]

You are basically correct--almost any solar sail should be able to reverse its orbital direction. This 'survey' section will eventually come out. See 'Overhaul of article' above. SpaceSailor 01:30, 9 July 2013 (UTC)

ive heard of one mission that is to launch next year. its a polish nanosatelite. and according to the info i found, it's a solar sail so i sugest that some of the imfo i found could be added to this article.^[1]--Nrpf22pr (talk)23:43, 20 December 2008 (UTC)[reply]

The Interstellar Flight profiles needs reworking. I'm trying to build a coherent summary of Forward's proposals. And after spending an obsessive day over the first sentence in the article, I decided I would have to spend some quality time with the whole article first before deciding how to work this section.Ecualegacy (talk)02:13, 2 April 2009 (UTC)[reply]

Somebody should link this article to a list of science fiction novels dealing with solar sails.Dexter Nextnumber (talk)22:13, 27 November 2009 (UTC)[reply]

_____

The article has this quote in it-

"Also in early 20th century literature, Pierre Boulle's Planet of the Apes novel starts with a couple floating in space on a ship propelled and maneuvered by light sails."

-according to the Wikipedia article about it, that novel was published in 1963. I'm not sure if it's mid 20th Century, or late 20th Century, but it isn't early. That should be changed.— Precedingunsigned comment added by2602:304:CFE1:79C9:952E:1E9F:76AA:9012 (talk)07:14, 27 November 2014 (UTC)[reply]

Jack Vance has written a novellette about using Solar Sails: "Gateway to Strangeness". It was published in 1962. It should be mentioned here. Ciao --Pentaclebreaker (talk)11:21, 8 December 2020 (UTC)[reply]

There are two sources of solar forces. The first is photonic pressure which is small but relatively consistent, and the second is due to solar winds which at times are less predictable, but far more powerful.

It's time to remove the last part of this statement, which was in the section "Physics". You can look it up anywhere you want, the pressure due to photons is much larger than the pressure due to massive particles. For example: The sunlight amounts to 1368 watts per square meter outside the Earths atmosphere. See for instanceSun, section "Sunlight", or, for more detail, seeSunlight, section "Solar Constant".

Consider the force exerted by this radiation on a black body with a perpendicular areaA during a time period of durationt. You can compute the radiation pressure like this:

P=F/A; p=F*t soF=p/t; p=E/c; E=I*t*A

whereP=pressure,F=force,A=area,p=momentum,E=energy,c=velocity of light,t=time duration, andI="irradiation" in the sense of (absorbed) energy per unit time and per unit area - this is the given number 1368 W/m². Substitute these equations into each other and getP=F/A=(p/t)/A=((E/c)/t)/A=((I*t*A/c)/t)/A=I/c, or

P_photons =I/c = 1368 / 3*10⁸) Pa = 4.6*10^-6 Pa.

(If you feel unsure about the relationshipp=F*t, just consider a body of massm, subject to a forceF during a timet. Apply Newton's second law,F=m*a, wherea is the body's acceleration due to the force. Since acceleration is velocity change per unit time,a=v/t, the body gains a velocityv=a*t assuming we start at rest. Then we getp = m*v = m*a*t = m*(F/m)*t = F*t.)

You may also look upRadiation Pressure, which gives both numbers in the lead: 1370 W/m² and 4.6 μPa.

On the other hand, the material particles flowing from the Sun are usually given as a particle density of 8.7 protons per cubic centimeters, seehttp://pluto.space.swri.edu/image/glossary/solar_wind.html. Since the flow is 95% protons by mass, we may disregard other particles in a first approximation. The same source gives the average velocity as 468 km/s. Consider a volumeV that is 468000 m long times 1 m² cross section. This volume flows across a surfaceA of one square meter in timet one second. Each cubic meter has 100*100*100 cubic centimeters, so the particle density becomesn = 8.7*10⁶/m³. Use theproton massm_proton=1.673*10^-27 kg. The computation becomes

P_wind =F/A; F=p/t; p=m*v; m = N*m_proton;N=n*V; V=v*t*A,

P_wind =n*v²*m_proton = 8.7*10⁶ * 468000² * 1.673*10^-27 Pa = 3.2 nPa.

assuming the particles are absorbed, not reflected.N is the total number of protons in the volumeV.

The numbers in this article are not that different: 6.7 billion ton ejected from the Sun per hour, assuming American billions 10⁹, velocities 400km/s for two thirds and 750km/s for a third of the flow (see section Components in this article), a distance of 150 million km (Sun), and using the surface of a sphere A=4*π*r² gives 3.4 nPa.

Conclusion: photonic radiation pressure is more than 1000 times stronger than non-photonic wind pressure.Cacadril (talk)02:13, 26 April 2010 (UTC)[reply]

That solar wind is stronger is a common layman's misconception, so feel free to rewrite and expand the Physics section as you have outlined. Including the math would also be valuable, in which case you should probably translate it to LaTeX (seeHelp:Displaying a formula). And don't forget to properly reference the secondary sources to prevent reversion. --IanOsgood (talk)15:30, 26 April 2010 (UTC)[reply]

I just removed the Criticism section as it does not contain anything of merit. The claims that solar sailing breaks Carnot rule, etc, is a kind of misunderstanding that rather indicates lack of normal intelligence or more likely willful misunderstanding. If you like, you may write about it in an article about the New Scientist magazine or about how to fill magazine pages with pointless spin or fake controversy, making the reader feel clever because the misunderstanding is so stupid. As a real misunderstanding it is implausible.

(To exaggerate a little: it is like rebutting in an article about Cleveland, Ohio, the belief that the Earth is flat and America's discovery was a hoax.)

The section also contained some explanations that may be included in other sections, such as the redshift/blueshift of reflected radiation. This illuminates the principle of conservation of energy, and may have some merit, but should be rewritten to fit in an appropriate section, perhaps the physics section. This is perhaps better handled in a more general physics article.

It shows in this talk page that a couple readers were surprised to discover that light can have momentum. This is quite natural, but the proper place to write about that is in articles about momentum, about light (photons, electromagnetic radiation, etc), or other basic articles. Of course the talk pages are always OK.Cacadril (talk)11:00, 26 April 2010 (UTC)[reply]

To the sectionH-reversal sun flyby trajectory I have added a request for{{expert}} attention, mainly because the sail angles appear reverse of required, but also to improve the explanations given. The sail angle appears to be wrong at "flight start" in the diagram: such an angle needs explanation because it contradicts other examples such as are found at page 4 of Figure 2 on page 4 of AIAA 2004-5406Optimal Solar Sail Trajectories for Missions to the Outer Solar System by Bernd Dachwald, url=http://www.spacesailing.net/paper/200408_Providence_Dachwald_Sail.pdf. Otherwise, the style of language used for the section needs improving to make it clearer, see the html comments and "clarify" tags for examples. -84user (talk)21:42, 1 July 2010 (UTC)[reply]

I'm certainly not an expert and as such I agree the text could be clarified. For example an introduction could explain that the strategy is that, for a very effective sail for which a single pass is already optimum, you get the best terminal speed by getting as close to the Sun as the temperature limit of the material will allow. That requires eliminating the orbital velocity (angular momentum or "H") and then minimising the unwanted outward thrust as the craft falls inwards (if you want to freewheel down a valley then coast up the other side on a bike, you don't use your brakes when going downhill). As the temperature limit is approached, rotate the sail to start increasing the orbital speed and then thrust directly away from the Sun after perihelion. I don't believe "W" is a maximum for speed but perhaps it is for acceleration.

In general the angles are roughly right though I think the detail is slightly in error. The "flight start" point diagram is virtually identical to the "spiralling inwards" diagram in Dachwald which you cite. The Incident light is reflected anti-clockwise ahead of the craft (see the fourth bullet in the earlier comments on this page about "Sailing upwind") so the green arrow showing the total thrust has both an outward radial component and a clockwise tangential component. Overall, the Wiki diagram is just a more extreme version of the "Neptune Flyby" at the top-left of figure 6 in the same paper. At point "Q", the orbital speed has been reduced to zero so the craft is falling directly towards the Sun. However, to maximise the escape speed, at that point you want to minimise the outward thrust so I would expect the optimum sail orientation to be virtually tangential to the Sun-craft line (the blue line lying along the red line). You don't want to heat the back of the sail so specifically it is probably best to align the surface of the sail to the limb of the Sun as shown just after point "S". At point "S", IMHO the optimum orientation should be similar to that at the start point and from "W" onwards the sail should be nearly horizontal on the diagram. Without writing some software to work it out though, that's just a gut feel. Given the description of the method used by Dachwald in Section IV, that's not trivial.

Incidentally while mentioning the Dachwald paper, note that assuming a rear emissivity of 0.55 is not optimum, a matt black coating would minimise the temperature by radiating the absorbed heat over the black body spectrum. They also don't allow for the improvement of reflectance at near-grazing angles which becomes important with a thermal limit (it couldn't be applied in their analysis as they used a point source for the Sun).Limb darkening would also influence the temperature and thrust in a complex but probably minor way. Spinning the craft about the Sun-craft line for a time after point "Q" can also be used to keep the angular momentum at zero but the modelling would then become much more complex than the 2D approximation.

George Dishman (talk)20:22, 13 August 2010 (UTC)[reply]

Thissurvey section has at least 2 errors (retrograde swingby isnot faster than prograde; it is not necessary to reduce energy to approach the sun, just to increase eccentricity) and some misconceptions. See 'Overhaul of article' above. SpaceSailor 01:38, 9 July 2013 (UTC)

There has been a new development in this field, and it is proposed that theoptical lift discovery may be used to help on self-alignment and steering of solar sails. It will be nice for someone familiar with the solar sail article to incorporate this. Cheers, --BatteryIncluded (talk)04:27, 9 December 2010 (UTC)[reply]

The spacecraft Helios 2 has (had?) a top speed record of 67.042 km/s, making it faster than Voyager 1 at 17.06 km/s.

Thangalin (talk)09:20, 15 July 2011 (UTC)[reply]

In the sail parameters section...is the size some sort of length (edge or diagonal) or an area? As it appears now the units of size are not clear.— Precedingunsigned comment added by128.158.1.164 (talk)22:44, 30 August 2012 (UTC)[reply]

Explanations of the dimensions have been added. Thanks. SpaceSailor 01:43, 4 July 2013 (UTC)

Some topics need to be moved out of this section and into the previous section.IKAROS appears to be mentioned twice.Kortoso (talk)19:13, 2 July 2014 (UTC)[reply]

Deeds a precise ref plz might consider removing this ...How do we know that Verne had specifically light pressure on a spacecraft in mind?—172.56.35.133 (talk)17:57, 26 November 2014 (UTC)[reply]

The Summary states "The total force exerted on an 800 by 800 meter solar sail, for example, is about5 newtons (1.1 lbf) at Earth's distance from Sol," 5 newton meters is equal to 3.7 (rounded) foot pounds. Can someone please check the reference and determine which value is actually correct? 5/3.7 or 1.5/1.1 newtons/lbf respectively?EAGreene (talk)00:40, 17 December 2014 (UTC)[reply]

The reference cited (1925) is, according to the Wikipedia biography of F. Zander, and just as its title suggests, about the problems with using rockets for interstellar travel. It is clear to me that it is not about using solar sails, but I don't know if solar sails are mentioned in it. This would need to be verified, I am challenging it. Robert Forward (1984) cites Zander's 1924 paper on solar sails. It is NOT the same paper, I believe the 1925 citation is wrong. I searched google scholar for Zander for the period '23 -'26 and one paper I found say this about Zanders 1924 paper:"There is an analogy between the RPDA mechanism and the “Light Sail” scheme for spacecraft propulsion. This scheme, which uses the photon momentum transfer to the light-sail, has been proposed by F. A. Zander in 1924 [5]." (from arxiv.org pdf of paper). The citing paper is:Unlimited Energy Gain in the Laser-Driven Radiation Pressure Dominant Acceleration of Ions; S. V. Bulanov, E. Yu. Echkina, T. Zh. Esirkepov, I. N. Inovenkov, M. Kando, F. Pegoraro, and G. Korn,Physics of Plasmas (1994-present) 17 (6), 063102, 2010. {note: a similar article in the same year is behind a pay wall:Unlimited Ion Acceleration by Radiation Pressure, Phys. Rev. Lett. 104, 135003 – Published 2 April 2010 but is in a main stream (more accessible) journal.}
and Zander's paper is: [5] F. A. Zander, Technika i Zhizn, No. 13, 15 (1924) [in Russian]. or the Russin citation: Ф. А. Цандер. Перелеты на другие планеты. Техника и жизнь. 1924. No 13. C. 15-16. which seems to translate (I read zero Russian) to "F.A.Zander, Flights to other planets. Tech and Life (maybe Tech Life(?), its the journal's name)
I propose changing the reference to this but I don't know which to cite...Oh, I think we should add the year 1924 to the prose in the article (and perhaps correct the spelling of his last name, ie Zander (aka Tsander)).Abitslow (talk)20:44, 11 January 2015 (UTC)[reply]

CNUSAIL-1, a 3U CubeSat from Korea, was launched on 12 January 2018 on a PSLV rocket together with other 30 nanosatellites. It is a solar sail cubesat intended to fly for at least 3 months.BatteryIncluded (talk)15:11, 24 January 2018 (UTC)[reply]

Why there is no mention of the Dr. Gold / Carnot debacle? There has not been any solar sail empirically verified to be working as intended in an actual space mission thus far, so we cannot know for sure if they work (the poor Cosmos sail prototype burned up when the rocket booster failed). Yet the article has a total lack of mention for critics, visitors notice that and conclude there must be a conspiracy going on as skeptical voices are silenced and so they put on tin foil hats. That's counter-productive.80.99.11.157 (talk)09:34, 26 June 2019 (UTC)[reply]

https://en.wikipedia.org/wiki/Talk:Solar_sail#Removed_%22Criticism%22_section

This person removed it in 2010, they thought it didn't add anything.SqueakSquawk4 (talk)08:10, 27 August 2023 (UTC)[reply]

• Would it be enough to note there was some initial scepticism ? Now we know it works - don't we ? -Rod57 (talk)10:24, 24 January 2024 (UTC)[reply]

A small table to summarise deployed sails by the delta-V they gained ?

Name, date deployed, end of mission, delta-V achieved, notes ?

IKAROS, 2010, still operating?

Lightsail-2, 23 July 2019 to 17 November 2022.

-Rod57 (talk)10:51, 24 January 2024 (UTC)[reply]

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