Re: Radiation Pressure

["Paul Camp" <pjcamp@coastal.edu>]

> What causes radiation pressure?  

Conservation of momentum. For light, p = E/c. I sometimes use this on 
exams together with the question of whether the sail should be shiny 
or dark (to see if they remember that p is a vector).

> I have read that this could be used to
> power interstellar spaceships.  If this is true how would the ships
> control their direction?  

The following information was ulled from a short search on Alta 
Vista:

It might seem at first that the optimal configuration for a solar sail
is one in which the light hits the sail at normal incidence
(perpendicular to the surface). This doesn't turn out to be the case,
though. A sail oriented this way exerts all its thrust along the line
away from the sun. Because the intensity of the light from the sun
falls off as the square of the distance, the magnitude of this outward
thrust must fall off also as the square of the distance. In this way
it is exactly like gravity. In fact, putting the sail at normal
incidence to the sun has the same effect as would have reducing the
mass of the sun. It places the sail into an elliptical orbit. This is
not a particularly useful configuration. The only way to avoid this
with a sail at normal incidence is for the solar pressure to exceed
the force of gravity, so that the sail goes into a hyperbolic escape
from the solar system. In order to do this, for the power output and
mass of our sun, the sail would have to mass no more than one kilogram
for every 600 square metres of sail area, including the mass of
payload and electronics. 

So, putting the sail at normal incidence to the sun is not the best
configuration. It is better to angle the sail in such a way as to
maximize the component of the thrust which is parallel to the
direction of travel. This turns out to be when the angle between the
sun and the perpendicular to the sail is about 35.3 degrees. In this
configuration the spacecraft is being pushed along the direction of
travel, and so it climbs the gravity well, slowing down the whole 
time. 

Well, if the only important thing is the component of the thrust along
the orbital velocity vector, it can clearly be aligned the other way 
to oppose the velocity vector. Rotate the sail 90 degrees. This pushes 
against the direction of travel, dropping the sail down the gravity 
well, causing it to speed up the whole time. 

> Would it be possible to orient a large "sail" so
> that you could steer in any direction regardless of how strong
> the radiation pressure from a variety of stars?  

The radiation pressure would be negligible from stars due to the very 
great distances involved and the inverse square law for the 
radiation intensity.

> Thanks in advance for any
> insight.
>
> John

Paul J. Camp                   "The Beauty of the Universe
Assistant Professor of Physics  consists not only of unity
Coastal Carolina University     in variety but also of 
Conway, SC   29528              variety in unity.
pjcamp@coastal.edu                    --Umberto Eco
pjcamp@postoffice.worldnet.att.net    The Name of the Rose
(803)349-2227                         
fax: (803)349-2926