Newsgroups: sci.physics.plasma,sci.physics
From: Fred McGalliard <frederick.b.mcgalliard@boeing.com>
Subject: Re: ??? Plasma mirror propulsion...
Organization: Boeing D&SG


Steven wrote

> Is the "kilometer sized" based on a calculation or rhetorical?

A rough idea of a light sail large enough to accelerate a few KG of mass
to some really usefull velocity, if you have a really bright source.
Actually, I think that this is not the best light sail, since the
photons from a bright source used to accelerate the ship are not
reflected but absorbed. This would be a good sail if you were sailing
toward a bright source from a dim one. Since being close enough to the
target to care about it's light intensity is pretty much the same as it
being time to rotate the ship and start deacceleration, I just don't see
the point. When would such a plasma mirror pay off?

> And I'm not certain what you mean by "free space", either. I was
> thinking along the lines of a low pressure gas contained by a physical
> structure, glass or plastic etc.

Yes, but to use your mirror I have to place it out in "free space" and
then the energy of my plasma radiates away quite rapidly.

> Additionally, while light is one good source, one could
> presumable use different velocities to utilize (with the proper shift
> proportions) whatever frequencies of energy are available, RF, IR or visible
> or UV or whatever. The universe appears to be full of sources of these
> :-).

Yeah. It is, but not much of this energy is concentrated enough to move
a ship very fast. Figure the lower the concentration of energy, the
bigger the mirror. The bigger the mirror, the more mass per unit thrust
and the lower the acceleration. Simple economics really. You get what
you pay for, sort of.