Prevention of global warming or Venus terraforming

This maybe a rather stupid idea but would it be possible to place a
very large sunshade between Venus and the Sun.
I am imagining an ultra thin reflective surface placed in a stable
orbit position around the Sun. Maybe a mylar disc slowly spinning to
keep it stretched out.
Would the light push this like a solar sail or could it be positioned
so that gravity cancelled this out ?
To kill two birds with one stone this could act as a mirror aiming the
suns rays at Mars.
If this idea is feasible the same technique could be used to prevent
Earth suffering from global warming. The disc would not need to cover
the whole Earth, but just enough to cool key areas possibly reflecting
the light to provide longer hours of daylight in the North of Canada
or Russia in winter.

In article ,
Stephen wrote:
This maybe a rather stupid idea but would it be possible to place a
very large sunshade between Venus and the Sun.
I am imagining an ultra thin reflective surface placed in a stable
orbit position around the Sun. Maybe a mylar disc slowly spinning to
keep it stretched out.
Would the light push this like a solar sail or could it be positioned
so that gravity cancelled this out ?

It can be done, although it is a bit trickier than it looks. See the
McInnes paper on this in the Sept/Oct 2002 JBIS.

To kill two birds with one stone this could act as a mirror aiming the
suns rays at Mars.

No, that is out of the question. For one thing, the constraints on the
position and angle of such a sunshade prevent pointing its reflections at
a moving target. For another, the beam would not hold together well
enough to be useful over that distance. (There are fundamental limits on
how tight a beam of sunlight you can make with lenses and mirrors -- the
Sun is not a point source and that turns out to matter a lot.)

However, you *can* build a solar reflector to hover on light pressure near
Mars. (McInnes again, JBIS March/April 2002.)

If this idea is feasible the same technique could be used to prevent
Earth suffering from global warming.

It's been suggested. (That is, in fact, the topic of the first McInnes
paper cited above.) Big project but not impossible.

The disc would not need to cover
the whole Earth, but just enough to cool key areas possibly reflecting
the light to provide longer hours of daylight in the North of Canada
or Russia in winter.

No, you need to reflect the sunlight away from Earth entirely to have any
impact on global warming. (And again, the system is constrained tightly
enough that you actually don't have much room to fiddle with where the
light goes.)
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

"Stephen" wrote in message
om...
This maybe a rather stupid idea but would it be possible to place a
very large sunshade between Venus and the Sun.
.. . .
If this idea is feasible the same technique could be used to prevent
Earth suffering from global warming. The disc would not need to cover
the whole Earth, but just enough to cool key areas possibly reflecting
the light to provide longer hours of daylight in the North of Canada
or Russia in winter.

It would probably be easier to use millions of helium balloons to reflect
sunlight away from Earth. That way, you don't have to mess with rockets.