# On sailing space craft.

1. Dec 12, 2008

### _F_

My question is not really technical, just something to mull over. But I figure that if we set a sailing spacecraft from earth, I don't think it will be able to get to another solar system.

Sailing ships use radiation pressure as their "wind," so leaving away from the Sun is no problem, but what about entering back into another solar system? Won't that sun send its radiation pressure in the opposite direction of the ship? What then? After years of sailing will the ship just settle with some median distance between stars?

The only way I can think of to avoid this is if the ship had enough velocity and enough momentum that the target star could only slow down the ship until it reached its destination (hopefully a planet!). Otherwise, I don't see any other way around it.

Thanks.

2. Dec 12, 2008

### D H

Staff Emeritus
You can sail a ship downwind via tacking. In fact, sailing ships can go a lot faster downwind than upwind. The same concept applies to solar sails.

3. Dec 12, 2008

### epenguin

I think it is the only way to go. You can build up speed indefinitely, not carry fuel and not waste any in friction in space.

I always think of this example of tacking against the wind as an example of "practical men". It was obvious to practical men for millenia that you could only sail downwind. Correct me if I am wrong but that is much what they did for millenia and only mastered the opposite a century or two before Columbus? Same as they only developed harnesses allowing horses to pull their full force in late middle ages or later, and Inuit and Amerindian dog harnesses only allowed a fraction of the possible effort? States and Empires lived by sailing and now it is only a sport yet sailing technology has advanced more in the past half century than in several previous? (Hopefully it will make a comeback).

So I remember reading but correct me if wrong. Then to be fair I have to recognise there are practical men and practical men.

4. Dec 12, 2008

### Staff: Mentor

For a water-borne ship, don't you need the force of the water against the keel of the boat in order to tack? If so, how would you accomplish something similar with a spacecraft?

5. Dec 12, 2008

### schroder

I think the idea is to use the force of gravity as the “second force”. To move closer to a sun, the space craft would adjust the tacking angle so the solar force is opposite to the direction that the craft is orbiting. This reduces the velocity of the orbit, allowing the craft to move closer. Just the opposite method allows the craft to move farther. The actual mechanics involved is quite complicated, but this is the basic idea.

6. Dec 13, 2008

### LURCH

I think you might have that backward.

Also, solar sails are not generally thought of as a means for interstellar travel, but only for interplanetary travel within the solar system. And even then, it is mostly an " inner planets" type of thing. As you get further from the sun, out toward the outer planets, the radiation pressure from the sun becomes too weak.

However, if radiation pressure from a star were used to propel a craft to another star, then the radiation from that other star would probably be used for breaking.

7. Dec 13, 2008

### jdstokes

The only realistic' proposals for interstellar spaceflight using solar sails that I'm aware of (near-solar manoeuvres + gravity assist) are just barely capable of attaining 0.01 c.

That puts a lower bound of 430 y to the nearest star, which is not known to harbour any interesting planets.

To travel any further than that, you really need go ultra-relativistic (> 0.1 c). The only semi-realistic way to achieve that is laser light sail propulsion, particle-beam-propelled magnetic sails or more likely, hybrid methods such as Jordin Kare's sailbeam' concept (laser-accelerated beams of dielectric microsails reflected by magnetic sails).

Decelerating is troublesome. Proposals have been put forth by Robert Forward for second-stage' sails which back-reflect laser light to decelerate the first stage, but this proves nearly impossible. Deceleration can be overcome, however, if high-temp superconducting magnetic sails can be constructed to break' against (inter)stellar wind + high energy density propulsion such as direct matter-antimatter annihilation.

Last edited: Dec 13, 2008