Re: Radiation Pressure

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

John DaCorte asked:

> What causes radiation pressure?  I have read that this could be used to
> power interstellar spaceships.  If this is true how would the ships
> control their direction?  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?  Thanks in advance for any
> insight.

Paul Camp gave a nice description for the use of space sails for lifting a
spacecraft to a higher orbital radius from the sun for intra-solar system
space travel.  Since John specifically asked about propulsion, steering and
navigation in interstellar space "from a variety of stars" (rather than just
using a sail to blow a craft out of the solar system with greater than escape
velocity with the craft only coasting in interstellar space) I thought that I
would throw out some numbers for the use of radiation pressure for propulsion
in interstellar space.

The sun's radiation pressure at the earth's location is about
4.58 x 10^(-6) Pa (assuming the flux is incident on a black body, otherwise
the reflected photon flux could cause momentum to be imparted to a sail up to
twice this rate).  If we go to an interstellar distance of 1 pc (rather than
1 AU) and recalculate the solar radiation pressure we get about 10^(-16) Pa.
In order for the starlight to produce a thrust/unit mass of 1/10 g ~= 1 m/s^2
we need that for every kg of mass for the spaceship the sail area required is
about 10^16 m^2.  Supposing the sail is made of mylar or some other such
material this requires that the sail thickness be reduced to 0.0005 Angstroms
to prevent the sail's mass from going over the mass limit.  Since the
diameter of an atom is of the order of angstroms we see that no sail can be
made anywhere that thin.  The above calculation ignored all the rest of the
spacecraft's mass, payload, support structure for the sail, etc.

Thus it is apparent that any sail propulsion system is useless for thrust in
interstellar space.  The only way such a sail system could be used is for 
blowing (launching) a spacecraft out away from the planetary system of a star
and for braking as such a craft approached another star system.  In
interstellar space the craft would have to just coast or have some other
means of propulsion.

David Bowman
dbowman@gtc.georgetown.ky.us