From: Doug Jones <randome@lightspeed.net>
Newsgroups: rec.arts.sf.science,rec.arts.startrek.tech,sci.space.policy
Subject: Re: Space Terrorizism
Date: Wed, 20 Jan 1999 23:47:01 -0800

Damien wrote:

> The sun being a parallel source, the image reflected from the earth
> would indeed be a pinpoint, when viewed on the target off axis
> of the incident ray.

You are mistaken- the sun is a blackbody source of noncoherent
radiation, with an angular size of about 0.5 degree.  That angular
dispersion carries forawrd through any concentrating optics.

> But why is it not possible to produce 10-1000x sun concentration on
> a target 100-200kilometer away? Are you saying that we cannot make
> a mirror with a focal length of 100kilometer?

Certainly- but the image of the sun that it would produce at 100 km
range would be just under a kilometer across, *regardless of the area of
the mirror*.  To get 100x concentration at 100 km range, the mirror
would need to be almost 10 km across.

Visualize it from the target's point of view- a concentrating mirror
looks like an extended object at the temperature of the surface of the
sun.  if the mirror is small and far, it occupies less area than the sun
itself, and provides less light.

Low f-numbers give poor concentration.

--
Doug Jones, Rocket Plumber
Rotary Rocket Company

Newsgroups: sci.space.science
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Stability of the Sun
Date: Tue, 16 May 2000 05:04:53 GMT

In article <wM1U4.173$m26.3915@news.tufts.edu>,
Kirk Is <kisrael@andante.cs.tufts.edu> wrote:
>> ...However, little tweaks of the thermostat up and down are not
>> precluded, and they could make things a trifle exciting.
>
>...do the models go into detail about what those tweaks might entail?

No, the basic fusion and heat-transfer processes of the interior are more
or less understood, but nobody understands the dynamics of the Sun's
surface well enough to model them.

It's noteworthy that the Sun was essentially devoid of sunspots from about
1715 to 1845, coinciding with the bitterest period of the "Little Ice Age"
(a period of several centuries when climate in Europe and North America
was unusually chilly and Alpine glaciers grew substantially).

Several other lines of evidence -- all of them, mind you, rather indirect
and unsatisfying, due to very limited records before the 20th Century --
also suggest that the Sun may be slightly variable on longer time scales.
For example, even after compensating for better observing and recording in
recent times, conspicuous auroral displays seem to have become much more
common in the last century or so than they were for several centuries
earlier.
--
Microsoft shouldn't be broken up.  It   |  Henry Spencer   henry@spsystems.net
should be shut down.  -- Bruce Schneier |      (aka henry@zoo.toronto.edu)

From: "Paul F. Dietz" <dietz@interaccess.com>
Newsgroups: sci.military.naval,rec.aviation.military,alt.war.nuclear,
	sci.space.policy
Subject: Re: Global Warming and Economic Defelompment (was Right-Wing 
	Republicans
Date: Sun, 07 Mar 1999 06:35:09 -0600

> Um... I think what the last person was getting at is the Sun grows brighter
> over geologic times.  In fact its about twice as bright as it was when the
> Earth formed.  That's because as time goes in the Sun burns Hydrogen and
> Helium builds up at its center.  This Helium starts to burn into heavier
> stuff (this process is called nucleosynthesis) which burns at a higher
> temperature than Hydrogen. Note: by the word "burn" I mean nuclear
> transformations, not chemical ones.


This is mistaken.  The sun is not getting brighter because helium
is fusing -- the conditions for that to happen are more extreme
than now occur in the center of the sun, because the rate goes as
the cube of density.  What really happens is that the accumulation
of helium "ash" in the core of the sun is making the gas there
have a lower specific heat.  Since the thermal kinetic energy
content of the gas in the sun equals 1/2 the gravitational binding
energy (by the virial theorem) this means the sun must, overall,
be getting hotter.  Viewed another way, as hydrogen is converted
to helium there are fewer gas particles bouncing around to provide
pressure, so to maintain pressure equilibrium the gas must become
hotter.

	Paul