# How is does the solar sailing concept work?

This wiki page states the following about solar sailing, "a form of spacecraft propulsion using a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to high speeds."

I understand the part where ejected gasses bump into the sail pushing the spacecraft.
But I dont understand how light can do this since light has no mass.

How does that work?
Does this mean that if I have a mirror balancing on a needle I would be able to push it over with my flashlight?

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–  John Rennie Mar 20 at 15:02

I think you are really asking "how can light deliver an impulse to the sail". The answer is that although light has no mass it does carry momentum. When light is reflected off the sail, conservation of momentum requires that the sail changes momentum by twice the momentum of the light. The extra kinetic energy of the sail comes from the red shift of the reflected light.

This question has several answers that discuss the momentum of light in some detail.

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So this would mean I would be able to push the mirror off balance with a flashlight? –  user17615 Mar 20 at 15:15
Yes, though the mirror would need to be exquisitely finely balanced! –  John Rennie Mar 20 at 15:34
@MichielT: in principle, yes. In practice, the idea of a solar sail is to produce a small force over a long period of time, so you'd have to shine that flashlight for quite a while to have a measurable effect. –  Jerry Schirmer Mar 20 at 15:34
@MichielT: given the link I mentioned in my answer you should be able to estimate the force produced by your flashlight. –  John Rennie Mar 20 at 15:37
@John Rennie: I dont have a background in physics, so all the formulae are jibber jabber to me. But find the concept very interesting! –  user17615 Mar 20 at 15:40