From: Steve Leffler <leffler@physics.ubc.ca>
Newsgroups: sci.physics,sci.physics.plasma
Subject: Re: ??? Plasma mirror propulsion...
Date: Tue, 29 Jul 1997 11:59:24 -0700
Organization: University of British Columbia, Physics Department


aufsj@IMAP2.ASU.EDU wrote:

> : : Is there some big fundamental flaw I'm missing? Has this idea
> : : been fleshed out before?

> If there were such a relationship, couldn't one simply ensure
> that more energy is 'behind' it than in 'front' of it? Put a powerful
> energy source behind and head in a direction with less energy "in the
> way" and you should get the unbalanced force with the results as
> promulgated by Newton. There seems to be a very clear analogy to sailing

If you're going to do this, you don't need to resort to using a
plasma mirror. A big sail made of aluminum foil will work nicely.
Solar sails have been proposed long ago. They are slow, but have the
major advantage that the craft doesn't need to carry any fuel. You would
only want to use one for interstellar travel, where the absence of fuel
compensates for the weak propulsion.

> A very hypothetical spacecraft, for example, going from earth
> might
>
> #1. Get boosted by traditional technology and orbital swings.
> #2. Reach high speeds. Using the relatively nearby sun as an
> energy source sail towards some distant star (tacking if need be).
> #3. Craft accelerates to high speed and eventually reaches point
> where the Sun's energy from behind is distant and weak, and the target
> star's energy is strong (you can get close by tacking).
> #4. Pull in the sails. Use your momentum to complete the journey.
> In addition, since you are now probably close to your target, gravity
> from it will help "reel you in".

You've got it right, except for #4. If you do that, you'll overshoot
your target and never come back. What you have to do is this: once you
pass the halfway point you reel in the sails, turn the ship around (180
degrees), and redeploy the sails. You then can use the light from your
destination star to decellerate the ship, so that you arrive in the
destination stellar system with relatively low velocity. Then you can
use conventional means to navigate within that system.

I think that the proposed plasma mirror drive isn't worthwhile. The
energy density of the cosmic background radiation, etc. is very low, and
the mirror would be very energy-inefficient. If you have a star at
your starting point and at your destination, a solar sail would always
be more efficient, since you don't need to expend energy to maintain
the "sail", and your sail can be very big.

BTW, the sails for these types of craft would have to be tens or
hundreds of square kilometers in area, even when making use of light
from a nearby star. A sail to catch background radiation in open space
would be *much* bigger. This would probably make the plasma mirror
drive impossible anyway, unless you take another poster's suggestion and
build a terawatt continuous-wave laser.

--
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