BBC Focus Magazine

[jpYB3Gq]

http://www.newscientist.com/article/mg2 ... steer.html

T'S the trailblazer of solar sailing. Japan's IKAROS spacecraft has used "smart glass" technology to steer using only the pressure of sunlight – a first for solar sails.

IKAROS launched in MayMovie Camera and soon after became the first solar sail to be fully propelled by sunlight. Now liquid crystal devices along the outer edge of the sail have been used to steer the craft. The devices control the reflectivity of the outer sections of the sail; switching one on creates a mirror-like effect, allowing sunlight to push more on those parts of the sail.

"With this we can control both the orbit and the attitude using only sunlight," says Yuichi Tsuda of the Japan Aerospace Exploration Agency.

Still, the new technology may not win solar-sail races any time soon. Compared with thrusters, which remain the main method of steering the spacecraft, the effect of the reflecting devices is slight. The sail can only change its attitude by about 1 degree per day, Tsuda says, and it gets less effective the faster the sail spins.

[Hyrulian Outlaw]

I'm sure MPL will disagree with the statement it uses sunlight and not solarwind, but the article seems confused to me. I don't understand how changing the reflectivity of a section of the sail will affect the solarwinds push against it so maybe there is something to photonic mass afterall.

[M Paul Lloyd]

Well you can point a really powerful searchlight at a thin fabric screen and the only difference reflectivity will make is the amount said screen heats up by, and when something heats up it emits tiny amounts of gaseous material creating minute amounts of thrust.

Light on its own doesn't have any effect whatsoever... sorry. I've tried and it simply doesn't work.

[Hyrulian Outlaw]

um well, I thought that for something to heat up from light it would want to absorb the light to capture its energy, not reflect it?

[M Paul Lloyd]

Which is why the idea that light imparts any kinetic/mass/thrust is flawed Hylurian, photons do not have mass and therefore no kinetic energy to give up so if the surface is reflective then particles that make up the solar wind are presumably going to transfer their energy more efficiently than if the surface were dull but the photons will be reflected away with no net gain.

To test whether photons create thrust a simple terrestrial experiment can be devised by pointing a beam of light at a sensitive pressure gauge held in a vacuum chamber, fact is, apart from a little localised heating, nothing happens.

[jpYB3Gq]

To test whether photons create thrust a simple terrestrial experiment can be devised by pointing a beam of light at a sensitive pressure gauge held in a vacuum chamber, fact is, apart from a little localised heating, nothing happens.

First you need to find out the intensity I of the light that you are using and also the wavelength x.
From x, you can get the momentum p of the photon using de broglie relation p = h / x.
From x, you also can obtain the frequency f of the photon using c = f x where c is speed of light.
From f , you can obtain the energy of a photon E = h f where h is Planck constant.
From E and I you can obtain n, the number of photon hitting the surface per second, using the equation I = n E.
From n and p you can calculate the force on the surface using Newton 2nd and 3rd Law: np = F.
Using A, the area of the surface and F , you can calculate the pressure P = F/A
Now MPL, lets calculate P and this is what you should expect to measure in the lab if the momentum theory is correct.

(rate of change of momentum is force)

[M Paul Lloyd]

Fine, but enough of the fine theory, now go try the experiment.

[Hyrulian Outlaw]

Sorry MPL, think we may of got crossed wires. The OP says that a reflective area changes the direction of the craft but a more reflective area would just reflect the photons which you insist have no mass. So if they have no mass how can they impart thrust?

If the reflective bands were dark wouldn't they absorb the radition and re-emit it as heat which would give a tiny amount of thrust?

[M Paul Lloyd]

But how can anyone tell if it is the photons that are doing the actual work here?

We have a spacecraft exposed to and being pushed along by the solar wind, which is composed of small particles being constantly emitted at tremendous velocity by the sun, these actually have mass and can push the craft along, therefore any supposed effect caused by light/photon pressure in negated.

You need a control experiment whereby you can eliminate the solar wind from the equation, such as shining a bright light onto a fine balance in a vacuum chamber.

And yes, if you had a black surface it would absorb more radiation and get hotter, quite a lot hotter infact, and it would radiate energy but also mass in the form of gas and it would be the expelled mass that would impart the thrust not the radiated heat.

This is rocket science.

[M Paul Lloyd]

But yes, the Original Point. Got distracted there sorry.

A smooth surface is subject to a greater perpendicular transfer of energy whilst a rough surface is subject to a degree lateral force factor.

You can illustrate this by comparing how rain falls onto a corrugated surface as opposed to a flat one.
On a corrugated surface a portion of the rains kinetic energy is transfered into lateral force (which are opposing and thus cancel each other out) whilst the flat surface transfers a greater amount of kinetic energy at right angles to the surface.

[jpYB3Gq]

My theory implies your pressure meter is not sensitive enough.

[M Paul Lloyd]

Yes but that is very much the point isn't it jpYB3Gq, it is a theory and that is all, no actual experimental evidence just an observed anomaly that photons behave both as particles and waves.

Wouldn't it make more sense to try and find out why this is the case rather than mere theorizing?

[Shadowwolf]

My theory implies your pressure meter is not sensitive enough.

Indeed that would be quite possible, surely though some one has a sufficiently sensitive sensor?

[M Paul Lloyd]

Here is an example of light being used to move a membrane but please note this is the result of gas flowing through it when one color of light is shined on its surface but shows no indication that light itself exerts pressure.
http://www.medicaldaily.com/news/201008 ... s-stop.htm