Grail Mission to the Moon: Does Light Exert Pressure?

[RJ Emery]

In a NY Times article about the Grail mission to the moon (see http://tinyurl.com/84dc5ew), the last paragraph states:

But if part of the core is still molten, as is currently thought, the sloshing will delay the deformation to slightly after the closest approach. The effect is so tiny that the scientists will first have to account for effects like the pressure of light from the Sun pushing on the Moon.​

Does light (photons) exert a pressure?

Sunlight consists of much more than pure light (photons). It is a stream of particles which have mass that can understandably exert a pressure. Therefore, is the writer mistaken?

 

[Doc Al]

Does light (photons) exert a pressure?

Sure. See: Radiation pressure

Despite being massless, photons carry momentum and energy and can exert pressure.

 

[BobG]

Does light (photons) exert a pressure?

Sunlight consists of much more than pure light (photons). It is a stream of particles which have mass that can understandably exert a pressure. Therefore, is the writer mistaken?

As Doc Al said - yes it does.

And sunlight consists of only the electromagnetic radiation (i.e - photons).

The Sun does emit a lot more than just sunlight - i.e. the solar particles in the solar wind, etc. - but those would be considered separately in their own right.

Solar pressure is significant enough that one has to take it into consideration when pushing an old satellite out of the geosynchronous belt to make room a new satellite. It may be small, but it's one of the few perturbations that doesn't eventually cancel itself out, meaning the orbits of old dead satellites can become more elliptical, bringing the old dead satellite back into the geo belt at perigee.

The affect of solar radiation pressure depends on the area to mass ratio of the object. Because of the solar arrays, satellites have a relatively high area to mass ratio. The Moon would have a very small area to mass ratio, meaning they're accounting for very small effects, indeed!