- #1

tony873004

Science Advisor

Gold Member

- 1,751

- 142

## Main Question or Discussion Point

Formula for solar radiation pressure

I'm looking for a formula that gives the amount of force in Newtons that is produced from the pressure of solar radiation.

I've googled it and found:

[tex] F_R = C_R \frac{I}{c}S[/tex]

where I is the radiation intensity, c is the speed of light, and S is the cross-sectional (sun-facing) area of the object being pushed, and Cr is the solar radiation coefficient which equals 1.0. (Does this value have units? I'm assuming it doesn't. And why clutter up the formula by multiplying it by 1?)

I don't quite know what I (intensity) is. Googling it, I find that its units are the Candela (CD). But that it is also a measure of energy which is expressed in Watts. (aka Joules / second, or Nm/s)

So:

[tex]N = \frac{Nm/s} {m/s} m^2 [/tex]

[tex]N = N m^2[/tex] which makes no sense. Unless the solar radiation coefficient has units of /m^2. Maybe that's why it's thrown in?

Furthermore, Intensity should change with distance. So I'm guessing that for Intensity I would use:

Luminosity of the Sun (3.9e26 watts) * (area of a sphere of 1 solar radii / area of a sphere of radius = distance), which would give me Intensity in Watts.

Ultimately, I'm trying to figure out how much solar radiation pushes against small objects at any given instant, causing their orbits to spiral outward.

Any thoughts? And any idea which is stronger: pressure from solar radiation or pressure from solar wind? Am I correct in assuming that pressure from solar radiation is basically constant, while pressure from solar wind varies with solar activity? Which is more responsible for creating the tails on comets?

I'm looking for a formula that gives the amount of force in Newtons that is produced from the pressure of solar radiation.

I've googled it and found:

[tex] F_R = C_R \frac{I}{c}S[/tex]

where I is the radiation intensity, c is the speed of light, and S is the cross-sectional (sun-facing) area of the object being pushed, and Cr is the solar radiation coefficient which equals 1.0. (Does this value have units? I'm assuming it doesn't. And why clutter up the formula by multiplying it by 1?)

I don't quite know what I (intensity) is. Googling it, I find that its units are the Candela (CD). But that it is also a measure of energy which is expressed in Watts. (aka Joules / second, or Nm/s)

So:

[tex]N = \frac{Nm/s} {m/s} m^2 [/tex]

[tex]N = N m^2[/tex] which makes no sense. Unless the solar radiation coefficient has units of /m^2. Maybe that's why it's thrown in?

Furthermore, Intensity should change with distance. So I'm guessing that for Intensity I would use:

Luminosity of the Sun (3.9e26 watts) * (area of a sphere of 1 solar radii / area of a sphere of radius = distance), which would give me Intensity in Watts.

Ultimately, I'm trying to figure out how much solar radiation pushes against small objects at any given instant, causing their orbits to spiral outward.

Any thoughts? And any idea which is stronger: pressure from solar radiation or pressure from solar wind? Am I correct in assuming that pressure from solar radiation is basically constant, while pressure from solar wind varies with solar activity? Which is more responsible for creating the tails on comets?