# Light reflection?

You know when people say they have a craft out in space and use the sun light to move it by bouncing it off mirrors . so I take it you would never be able to get the craft back to where you launched it using light from the sun. Or would this work to get it back
rotate the mirrors with an electric motor then the light from the sun if it goes through a half circle plastic pipe so that the light hits its critical angle and then hits the mirror from the other side would this move the craft back to where it came from , or does this violate some laws of physics .

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Cleonis
Gold Member
You know when people say they have a craft out in space and use the sun light to move it by bouncing it off mirrors . so I take it you would never be able to get the craft back to where you launched it using light from the sun.
I wonder whether it's possible to orient the mirroring surfaces at such an angle that the momentum transfer of the reflecting light decreases orbital velocity.

If you can decrease orbital velocity then gravity will make the spacecraft descend in the gravity well. Hence if the starting point of the journey is an inner planet (of some solar system) then I expect that it's possible to first ascend (say, to an outer planet) and later descend again.

Cleonis

Cragar, when light is reflectively bent as you described, the net force would still push it away from the original vector of incident light.
I know of no arrangement of mirrors, fiber optics, etc... that would do otherwise than this.

Granted, you could "brake", but not reverse direction with the incoming light being the sole source.

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HallsofIvy
Homework Helper
Sail boats can "tack" against the wind because their keel prevents sideward movement and only the component of the wind in the direction of the bow is effetive. But in space there is no way to make a "keel" that will prevent sideward movement so the only way you can use the "solar" wind is to move in the direction away from the sun.

And, no, I don't know of any people who "say they have a craft out in space and use the sun light to move it"! I know of a few people who have written science fiction about that but they, as I recall, only use "solar sails" to move between stars.

Yeah, as Halls said. Also, now that I have given it more thought, braking would not work at all !!. My bad, sorry.
The net force vector under any circumstances of this type will always be directly in-line with the incident light vector.

HallsofIvy was actually incorrect in a couple ways. The force vector is always in a direction perpendicular to the reflector for the portion that is reflected, and in the direction of the incident light for the portion that is absorbed. A sailcraft doesn't have a keel, but HallsofIvy failed to consider that it is falling through the sun's gravity well. A reflective sail canted at an angle can tack sunward perfectly well by angling its sail to reduce its orbital velocity around the sun. Also, it is the sunlight that provides the acceleration, the momentum of the solar wind is negligible in comparison.

For trips between the inner and outer system, the spiral trajectories would take a hopelessly long time, however. A far more effective approach would be to use a "H-reversal" solar flyby trajectory: on the way out, tack back and out before dropping in toward the sun, and then turn the sail to face the sun as you fly by...essentially using the sail to maintain the speed achieved at perihelion (the optimal trajectories actually do a little better than this). The reverse is possible, but requires an initial sunward trajectory that would take too long to achieve with a solar sail in the outer system...a gas giant flyby or more conventional rocket would have to be used to start the return, with the sail only being used for braking into the inner system.

And you can reflect light on the back of the sail to brake on the outbound trip, but this requires jettisoning the larger part of the sail to act as both a reflector and a concentrator to direct the light onto the smaller remaining portion of the sail. The jettisoned portion will of course continue to accelerate on away from the craft. This is fairly useless for solar sails, the braking acceleration will be too low to be useful, but could be of some use with laser sails. Robert Forward described such a system in his Rocheworld novels.

Thanks for you responses , maybe if we were next to a black hole and the light was bent around the black hole enough so that it could hit our mirrors from the other side maybe we could go back .

Thanks for you responses , maybe if we were next to a black hole and the light was bent around the black hole enough so that it could hit our mirrors from the other side maybe we could go back .
Black holes are in short supply, and gravitational lensing would only pull a tiny fraction of light into a useful direction. You've got a solar sail, so any star that emits light would be infinitely more useful than a black hole. Ideal would be a high-luminosity, high-mass star. The speed limitations of solar sails make them impractical for reaching other stars, though, at least from our solar system.