Uranus and Neptune

--- In spacesettlers@yahoogroups.com, Ian Woollard wrote:
>
> On 15 May 2012 21:28, brooksn wrote:
>
> > So wouldn't that mean that the closer you are to the Sun the more
> > difficult it would be to make the rays parallel? So further out, I would
> > think, you could straighten out the lines 'more'. But it also seems
> > distance is relevent, (according what people have said) in other words the
> > further out you 'shoot' your straight lines, the less able you are to keep
> > them straight by then end of your 'shot'.
> >
> No, for solar thermal you want a BIG angle on something! You want as much
> of the sky, as seen by the thing you're trying to heat to be full of Sun,
> as possible (within reason). That's the problem. When you're a long way
> away, the angle is very tiny. That means that the disk of the sun is very
> small, so you get a lot less light and heat.

My thought is that even though you have a small 'disk' in the sky, it is more intense as its rays are reflected from near the sun. But you're saying the different angles are so great that it overwelms any attempt at 'straightening' the diverging rays. So really the Sun's rays are doing two things, they are diverging from each other over distance and the Sun is so big that they have so many different angles from the Sun's surface that 'straigtening' the 'diverging' part of the Sun is essentially ineffective. Ok, I understand. It would still seem that at some distance you could change that, but the Sun in our sky is still as big as the moon, so I guess by the time you get far enough away that the diverging outweighs the 'scattered' angles that the energy is less meaningful anyway and your back to just mirrors near your target instead of mirrors near your source.

Though I guess you could still use concave mirrors near your target body. Increasing the temperature in a much smaller region.

> Interesting, so I'm wondering, if you're out to Venus (in a polar orbit of
> > the Sun though, so you always face Mars from the same distance), then use
> > concave mirrors to redirect and 'straigten' the suns rays to Mars.
>
> That's what I'm saying, they don't straighten AT ALL.
>
> Think about this in terms of mirrors. At any flatt-ish point on a mirror,
> the angle of incidence is equal to the angle of reflection, so if two sides
> of the sun hit the mirror at different angles, they will still be the same
> angle after they've bounced off.
>
> And you don't care if the lines diverge some by the time they reach Mars,
> > since the whole planet is a target, if it would produce a useful effect.
> > The advantage being that the mirrors could be comparably smaller then
> > mirrors in Mars orbit.
> >
> No, the mirrors would have to be stupendous!!! At Mars orbit they only have
> to be twice as big as at Earth (so if you had a window on Earth 1m^2 you
> would need ~2m^2 of mirror at Mars). If they were at Venus pointing at mars
> they would have to be the same apparent diameter as the Sun as seen from
> Mars; which means that their actual size would be freaking enormous!
>
> > I also wonder what the limits are, how close to the sun can you get and
> > how far into the solar system can you 'shoot'? Or is Venus to Mars already
> > too much?
> >
> The smallest mirrors you can make are the ones stuck right next to the
> habitat, with the habitat at the focus. (Unless you use lasers).

Ah, so the distances are that limiting? Thanks.
Brooks