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I have not been able to find a clear or decisive answer about this. On Earth, a ship sails close hauled when the ship is, to some degree or another, facing into the wind. This is made possible with sails that are rigged along the length of the ship rather than perpendicular to the length. Is a similar thing practically or at least theoretically possible with a solar sailing vessel, to sail towards a star in this manner? Why or why not?

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    $\begingroup$ The only way to sail into the solar wind is to furl the solar sails and let gravity take you $\endgroup$ – Jim Mar 11 '15 at 17:37
  • $\begingroup$ Okay, my above statement isn't completely true. A sail could be made that is a near perfect absorber/emitter in the visible spectrum but a poor emitter in infrared on the sunward side and a near perfect emitter in infrared on the dark side. This would have the net effect of making the sail absorb incident light and reradiate it as IR away from the star. Probably not enough to really be effective though. I'd have to do the math on that $\endgroup$ – Jim Mar 11 '15 at 17:58
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    $\begingroup$ A sailboat has a keel which can provide the "opposite reaction" for an "equal but opposite" equation involving close-hauled sail and wind. No such luck with outer space (unless there really is "ether" out there). $\endgroup$ – Hot Licks Mar 11 '15 at 20:12
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    $\begingroup$ @BenVoigt That's still letting gravity take you back to the Sun. $\endgroup$ – Jim Mar 11 '15 at 20:50
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    $\begingroup$ Quite simply no because a solar sailship has no keel. (And indeed there is no substance one could use a keel in.) $\endgroup$ – Fattie Mar 12 '15 at 3:04
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A sailboat can make headway against the wind because of the sum of force vectors due to the wind interacting with the sail and, due to the keel interacting with the water. A sailboat without a keel can not make headway into the wind.

There is no "water" out there into which a solar sailer could dip its keel.


http://newt.phys.unsw.edu.au/~jw/sailing.html

http://web.mit.edu/2.972/www/reports/sail_boat/sail_boat.html

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    $\begingroup$ Further details for interested parties: The action of the keel against the water is necessary. In space, the solar pressure puts an outward force on the spacecraft proportional to the cross-sectional area of the sail that is normal to it. Angling the sail only reduces the force pushing you away from the star; it does not change the direction. $\endgroup$ – Jim Mar 11 '15 at 17:34
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    $\begingroup$ @JimdalftheGrey Your last statement is not true if you are using mirrors. The force exerted is directed perpendicular to the mirror surface. $\endgroup$ – Rob Jeffries Mar 11 '15 at 17:50
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    $\begingroup$ I would add to your (correct) basic point a couple additional facts. First, in land-based sailcraft there are of course no hydrodynamic effects producing straight-line motion upwind, but friction between the wheels and the surface is an analogous force. There has to be something you're pushing against. Second, a lot of the benefits of sailing any course other than a dead-downwind run take advantage of apparent wind; for sailboats it is easy to have an over-ground velocity that is a large fraction of the true windspeed; for spacecraft moving at nonrelativistic speeds that's not the case. $\endgroup$ – Eric Lippert Mar 11 '15 at 23:00
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    $\begingroup$ There is no "water" out there Citation needed! $\endgroup$ – TaW Mar 12 '15 at 21:21
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    $\begingroup$ Downvoted since the answer fails to mention any solar sailing techniques. Rob Jeffries' and dfeuer's answers fix this and are much more informative. $\endgroup$ – Deer Hunter Mar 13 '15 at 3:40
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The question is about solar sails - I thought I would add this (too long for a comment) to clarify some confusion that is apparent in answers and comments.

Solar sailing uses photon momentum, not the solar wind.

The radiation force on a perfectly reflective sail of area $A$ is roughly $$ F_{rad} = 2\times\frac{L A}{4\pi r^2 c}$$ where $r$ is the distance from the Sun, $L$ is the solar luminosity and we assume the light is normal to the sail.

For the solar wind, the force is $$F_w = \frac{\dot{M} v_w A}{4\pi r^2},$$ where $\dot{M}$ is the mass loss rate (assumed isotropic), with a value of about $2\times 10^{-14}\ M_{\odot}$/year and $v_w$ is the solar wind speed of about 400 km/s.

Thus $$\frac{F_{rad}}{F_w} = \frac{2L}{\dot{M}v_w c}$$ Numerically, for the Sun this ratio is $\simeq 5000$, so you wouldn't design a sailship that used the wind! (The wind would be more effective near red giants or possibly very young low-mass stars).

As solar radiation travels radially away from the Sun, it is not possible to exert a force on a solar sail radially inwards. It is however possible to angle a reflective solar sail so that the force is almost tangential to the Sun, since the force is directed at right angles to the mirror surface. Navigating towards a star would largely involve slowing down the sailship, and allowing gravity to do its thing (see picture from https://directory.eoportal.org/web/eoportal/satellite-missions/n/nanosail-d ).

Angling a solar sail

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  • $\begingroup$ "possible to angle a reflective solar sail" .. but how?? do you mean with an active rocket system, which continually, well, keeps it in position? So: imagine a kite, with no string, in a firm breeze, at a height of 10m. (Now, for the argument imagine there is no gravity.) How do you "angle" it (let's say, 30 degrees left of the line of wind). ??? $\endgroup$ – Fattie Mar 12 '15 at 8:32
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    $\begingroup$ @JoeBlow Well, by turning the sail (or the entire ship) of course. There are lots of ideas on the engineering; you could use microthrusters, you could use materials where the reflectivity/absorptivity of different parts of the sail can be controlled electrically and use the solar radiation to do some or all of the work. See en.wikipedia.org/wiki/Solar_sail#Attitude_control $\endgroup$ – Rob Jeffries Mar 12 '15 at 11:19
  • $\begingroup$ Solar-powered gyroscopic reaction wheels should cut it. $\endgroup$ – John Dvorak Mar 12 '15 at 16:21
  • $\begingroup$ @JanDvorak - gyros are much too heavy to be of any use. One would still need attitude control authority to offload the momentum from the gyros. $\endgroup$ – Deer Hunter Mar 13 '15 at 3:46
  • $\begingroup$ If you can articular the sail to any degree you can use the sail to provide the torque needed for tacking. I don't believe that any of the test-bed devices have tried this to-date, however. $\endgroup$ – dmckee Dec 12 '17 at 19:02
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Yes, sort of, sometimes. As others have indicated, it is not possible for a solar sail to produce a force in the direction of the Sun. This does not, however, mean a solar sail cannot take a spacecraft to the Sun. If your spacecraft is in solar orbit, you can tilt the sail "backwards", reducing the orbital energy and lowering the orbit. The potential energy of the spacecraft decreases, its kinetic energy increases (by a smaller amount), and the rest of the energy is given to the photons, making them bluer.

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  • $\begingroup$ Only "sort of" in a very general sense. (One might as well say "well if you also had a rocket engine that is 'sort of' like having water and a keel!") The only possible value of this question is pedagogic, and the absolutely key thing is for the reader to understand what a keel is, and why space {other than in scifi books!} has no substance. $\endgroup$ – Fattie Mar 12 '15 at 3:05
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    $\begingroup$ @JoeBlow But it does mean that a solar sail ship can make arbitrary adjustments to its orbit (since such adjustments generally require a force along or against the orbit rather than straight "down" towards the star) and is not forever being pushed away from the star. Gravity provides the opposing force. The takeaway is that it is possible to "sail towards a star", with a ship with only a sail and not a rocket. $\endgroup$ – Random832 Mar 12 '15 at 5:41
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    $\begingroup$ "sail" in the sense of journey, sure $\endgroup$ – Fattie Mar 12 '15 at 8:32
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    $\begingroup$ @JanDvorak I meant "arbitrary" in terms of what orbit you ultimately end up in. The maneuvers to get in a different orbit might just end up a little more complex. Anyway, the point is the fact that you can lower your orbit. $\endgroup$ – Random832 Mar 12 '15 at 16:20
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    $\begingroup$ @MooingDuck If without the sail, you'd be in a stable orbit forever, never moving closer to the star, then it is definitely helping you. $\endgroup$ – Random832 Mar 13 '15 at 17:14
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Yes, but not the way you think. It's not analogous to a sail boat. A solar craft has no keel to resist movement in one direction, and I doubt that solar wind is dense enough to cause a noticeable wing effect with a sail.

But stars have their own complexities you can exploit instead.

Every answer thus far considers a static model where the star sits in space and you try to approach it using headwind. In this simplified model it's hard to approach the star using headwind. In reality, the star is almost certainly moving perpendicular to your path to some degree. This suggests that the solar wind will approach your sail from some oblique angle. Other factors will also alter the direction of the solar wind, such as the stars magnetic field, and any gravity the solar wind was exposed to.

While you may not be able to head upwind, you can certainly head downwind at any angle you choose with an appropriate sail. Use the apparent wind direction and your sail to intercept the star where it will be in the future. As you accelerate the apparent wind direction may well improve, allowing you to plot a more direct intercept course.

In the end, this is more of an engineering problem than a physics problem. Be creative.

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  • $\begingroup$ Wait -- this is completely wrong isn't it? If you angle a solar sail, you'll simply get less force from it. You will still be pushed precisely exactly outwards form the star. $\endgroup$ – Fattie Mar 12 '15 at 3:07
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    $\begingroup$ @JoeBlow Light comes in at one angle, light goes out at another angle. The entire principle of a solar sail is that photons have momentum. How is momentum conserved? You're not absorbing the photons. $\endgroup$ – Random832 Mar 12 '15 at 5:35
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    $\begingroup$ @JoeBlow The force on a sail is perpendicular to the sail if it is a perfect reflector. However, the above answer appears to think that solar sails use the solar wind rather than solar radiation. I think there are few stars in the galaxy where that would make sense. $\endgroup$ – Rob Jeffries Mar 12 '15 at 7:00
  • $\begingroup$ Rob thanks, But wait -- HOW can you "angle" a solar sail? imagine holding up a kite with the normal to the wind. now, yaw it 30 degrees to the left. ok, now make it sit like that, without your hand holding it :) it makes no sense - it's impossible and wrong. there's no keel. (and indeed, it IS NOT even like a parachute (where you have gravity down, and two "fixed" lines, so you can use "spilling" effects) $\endgroup$ – Fattie Mar 12 '15 at 8:27
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    $\begingroup$ @JoeBlow Why shouldn't my hand be able to hold it? The "keel" is the friction between my feet and the ground, not my own momentum, and it'll work just fine without that. You're simply wrong about it not being possible to angle a solar sail, and your analogy is faulty. $\endgroup$ – Random832 Mar 12 '15 at 15:18

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