Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v10_4/v10_421.txt

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Subject: SPACE Digest V10 #421

SPACE Digest                                     Volume 10 : Issue 421

Today's Topics:
         Re: Red Shifts through Random Media
            Re: Nuclear Reactors in Space
            Re: Greenhouse Effect
           Plans for a solar eclipse viewer (long)
----------------------------------------------------------------------

Date: 12 Jan 90 21:42:11 GMT
From: zaphod.mps.ohio-state.edu!uwm.edu!csd4.csd.uwm.edu![e-mail]  (michael morris)
Subject: Re: Red Shifts through Random Media

In article <[e-mail]> [e-mail] (Mark S Madsen) writes:
>
>This shows the danger of careless terminology. What you are talking
>about (and Wolf, too) is REDDENING. The original discussion was
>talking about REDSHIFTING. The former is when the intensity peak is
>shifted towards the red, the latter is when the *individual* spectral
>lines are all shifted towards the red.
>
>Astronomers have known about the reddening caused by the interstellar
>medium since last century, and there are well-known and highly
>developed methods for correcting for reddening.

Dear Mark,

    I'm not sure that I understand the distinction between
redshift and reddening in practice (or mainly what Wolf is talking
about). That is, it seems to me that a spectral
line can also appear to shift because of interference between
coherent sources.
The point is that any line has a finite width stretching over
a band of frequencies. If we shine the line
through a double slit, say, then different spectral components
will give different interference patterns on an observer's screen
distantly placed. So, in particular, if one's telescope were placed
in the center of the screen, one would see a different lineshape
than that of the source. The interference at the centre
of the screen would spread the
red end of the lineshape (and lower its contribution
to the intensity) more than the blue end, yielding in this case
an apparent blueshift (I think), although different locations along
the screen would see different shifts and (once again, I think) there
would tend to be redshifting for observers at locations away
from the centre of the screen.
    Once upon a time a friend of mine (working in quantum optics) thought that
he understood Wolf's effect
in terms of this sort of interference redshift. He wrote to
Wolf about it and Wolf said that no, that wasn't what was going on,
although my friend remained convinced that that was in fact what was
going on. I would be interested in learning if this effect has anything
to do with Wolf's and whether one should call it reddening or something
like an interferometric redshift.
                              For what it's worth,
                      Mike

----------------------------------------------------------------
 Michael S. Morris                             ``What I tell you
 1900 E. Kenwood Blvd.                              three times
 Department of Physics                               is true.''
 University of Wisconsin-Milwaukee
 Milwaukee, WI 53201             ([e-mail])
-----------------------------------------------------------------

------------------------------

Date: 12 Jan 90 23:15:24 GMT
From: zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!jarvis.csri.toronto.edu!helios.physics.utoronto.ca!physics.utoronto.ca![e-mail]  (Christopher Neufeld)
Subject: Re: Nuclear Reactors in Space

Sorry to take so long, my newsposter failed.

In article <[e-mail]> [e-mail] (Tim Maroney) writes:
>In article <[e-mail]> [e-mail]
>(Kieran A. Carroll) writes:
>>Tim, your statements here assume that such high specific-energy-
>>capacity batteries are physically possible to produce. If there's a
>>physical principle waiting to be exploited that could result in such
>>a thing, then billions of dollars of research would likely be
>>justified. If there's >no< such principle, however, then no amount
>>of research spending will result in such batteries. Right?
>
>Right.  And I freely admit that this is an issue of basic research,
>and not of mere technical fiddling.
>
>>Now, have you any reason to believe that it is possible to produce
>>batteries with much higher specific energy capacity (i.e. energy
>>capacity divided by mass) than those currently available? Say, two
>>orders of magnitude better than NiCd batteries? I know that a >lot<
>>of money has been spent on battery research in the last decade (by
>>NASA, by spacecraft battery manufacturers, by the DOE, by electric-
>>car proponents; even some by the electric utility companies, who
>>would love to have cheaper load-leveling devices), and as far as I
>>have heard that research has not turned up any physical principles
>>that could be exploited to produce the sort of improvements that you
>>suggest.
>
>That research may have not.
>
   Well, let's look to the military research for an answer. Despite all the
ulterior motives you may assign to people funding the research, the
military really would like a big non-nuclear explosive. Tacticians are
worried about what would follow even a small nuclear explosion in combat
against a nuclear-capable opponent. There are no chemical explosives which
come close to the energy per unit mass which can be delivered by a nuclear
explosive for the same reason that there is no chemical battery which can
come within orders of magnitude of a nuclear battery such as an RTG. The
military spends a lot of money on chemical explosives, and they've not come
up with a yield of more than hundreds of eV per atom. Nuclear processes
release up to millions of eV per atom.

>However, superconductivity research has
>pointed in that direction.  If the "giant flux creep" problems can be
>solved in the new higher temperature superconductors, then such
>batteries may be feasible.  Or may not.  The point is, a large-scale
>basic research program of the magnitude of fusion research would have
>followed up these leads and many others, and I believe would probably
>have delivered some very useful results by now after several decades.
>
   Now we're on my territory. Let's assume that we can manufacture an
energy storage device with high-Tc superconductors which can equal the
storage of conventional superconductors. For some of the conventional
superconductors the limitation on magnetic field produced in a coil is not
the critical current (or critical field) but the magnetic pressure on the
walls of the vessel. Any coil of wire carrying a current has a tendency to
explode which has to be countered by the strength of the wire. High-Tc
superconductors are not going to display superior ductility to conventional
superconductors in the foreseeable future, since they are brittle ceramics.
But, we'll assume for the moment that we can do as well as modern small
scale SMES.
   Now, let's say an interplanetary probe spends four years operating at
0.1kW and then two months at 1kW. This amounts to some 15GJ of energy. This
is the energy equivalent of 3.25 tons of tnt. Now, let's look at some real
numbers. In 1967, Brookhaven National Laboratories built a magnet for the
bubble chamber there. It is described in detail in J.Appl.Physics 39, 2518
(1968). Briefly, the magnet without crogenic support had the following
characteristics:
   Volume 12.86 m^3 cylinder
   Central field 2.5T
   Energy in the field 45MJ
   Maximum current 4500A
   Weight of conductors 12 tons
   Cost of conductors $178000 in 1968 US dollars
   Reason for maximum energy: coil deformation and conductor breaking for
      currents in excess of 4600A.
Conventional superconductor magnets were a mature field, and you couldn't
expect to do much better now, twenty years later. If this magnet were used
to store energy for the hypothetical space probe we would need 330 of them,
occupying over 8000m^3 of volume (you can't close pack these) and weighing
some 4000 tons. I don't know (it wasn't clear to me from the article)
whether the 12 ton figure includes mechanical support against the magnetic
pressure (over 1000 atmospheres in this magnet), but if it doesn't then
I've severely underestimated the weight of this space probe's power
supply. You can see that even if you could do a hundred times better, the
resulting probe would be far too heavy and too bulky to be practical. High
temperature superconductors aren't anywhere near this, and won't be for
quite some time. Being ceramics they are more susceptible to the magnetic
explosive force in the coil, and would be able to hold a correspondingly
lower energy.
   Compare all this to putting two 25kg cannisters on the space probe.

>--
>Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, [e-mail]

--
 Christopher Neufeld....Just a graduate student  |
 [e-mail]              | The meek can have the
 [e-mail]                    | earth, I want the stars.
 "Don't edit reality for the sake of simplicity" |

------------------------------

Date:         Fri, 12 Jan 90 21:01:33 EST
From: Paul Klinkman <[e-mail]>
Subject:      Re: Greenhouse Effect


>>The Washington Post reports that scientists have scaled back
>>predictions of a significant sea level rise in the next 100 years
>>because of global warming.  The latest prediction is based on a
>>noticeable accumulation of snow and ice in Greenland and
>>Antarctica.  Data from Greenland was gathered by scientists at
>>NASA's Goddard Space Flight Center using satellites.  A
>>University of Wisconsin polar research center study indicated
>>similar snow and ice accumulation in Antarctica.

>Ha! I knew it! Everybody's worried about the greenhouse effect, and we
>could be heading for another ice age! Here in Maryland, our last two
>summers were significantly cooler than usual (though nobody but the
>meteorologists believe it), and this past December was the coldest on record.

Humans simultaneously put more smoke particles and more CO2
into the atmosphere.  The CO2 causes the tropics to retain heat.  Not
much sunlight reaches the poles, so heat retention isn't as big an issue
at the poles.  Smoke particles in the stratosphere screen out a bit of
solar radiation.  At the poles the solar radiation goes through the
atmosphere at a low angle.  The light has to pass through much more
smoke.  Imagine the dimness of the sun at sunset through smog.

In theory the tropics get hotter and the poles
get colder.  One might think that everything balances out, but it
doesn't.  Hotter tropics and colder poles can mean bigger storms and
more tornados.  The average wave height in the Atlantic has been rising,
a sign of greater average wind speed.

One cold month is no ice age, nor is a set of two cool summers.  Decem-
ber was nothing compared to 1814, when Krakatoa smoked up the earth's
atmosphere.  Still, twice a year the reporters get an itch to announce
that the greenhouse effect or ice age may already be arriving.  Sure!

--Paul Klinkman                   Boy was it wet on New Year's Eve!  I
                                  took two pairs of socks to First Night
                                  and every hour it was "Wring out the
                                  old, wring in the new."

------------------------------

Date: Fri, 12 Jan 90 01:52:20 EST
From: John Roberts <[e-mail]>
Disclaimer: Opinions expressed are those of the sender
    and do not reflect NIST policy or agreement.
Subject: Plans for a solar eclipse viewer (long)


>From: mephisto!eedsp!chara![e-mail]  (Donald J. Barry)
>Subject: Electronic Journal of the ASA, Vol. I, No. VI

>                        THE ELECTRONIC JOURNAL OF
>                THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
>                    Volume 1, Number 6 - January 1990
>...
>                     EXPLAINING SOLAR AND LUNAR ECLIPSES
>                               by Brent Studer
>...
>        An important note about eclipse observing:  Observing a solar
>    eclipse can be dangerous to your eyes - NEVER look directly at the
>    Sun, particularly through unfiltered telescopes or binoculars.  One
>    alternate method for observing a solar eclipse is to project the image
>    of the Sun onto a piece of white cardboard, either through a telescope
>    or through a small hole cut into another piece of cardboard, but it is
>    highly suggested that even this viewing method should be done with
>    caution and experience.  Lunar eclipses, by comparison, are quite safe
>    to observe directly, either with the unaided eye or through optical
>    instruments.

>                         Copyright (c) 1990 - ASA

*****************************************************************************

This is the "camera obscura" technique, which is the principle behind the
earliest (lensless) photographic cameras. I have developed and used a
refined approach, which I hereby offer freely for general use (unless
by some chance it's covered by a previous patent):

Select a good-sized cardboard box with the top flaps intact, which is not
too heavy to hold comfortably, and a cardboard tube. (The wider the tube is,
the easier it will be to aim the device.) The box will be upside-down in use,
so what was the bottom will be referred to as the top. Near the top of one
end, centered horizontally, cut a round hole the same size as the outside
diameter of the tube. Insert the tube a short distance, and attach it (tape,
glue, support struts, etc.) so that it is rigidly attached to the box and
is perpendicular to the face of the box. Over the other (far) end of the
tube, fasten a piece of opaque cardboard, with a very small round hole of
a carefully-selected diameter centered in the end of the tube. (You can try
various sizes - a sixteenth of an inch might be good as a first approximation.)
This hole is called the aperture of the viewer.

Turn the box over, and attach a piece of white paper to the inside of the side
opposite the tube, to serve as a viewing screen. Now close up the flaps of
the box and tape them shut, then cut a hole through the flaps near the
tube end, large enough to stick your head through with room to spare.

Here is a side view of the assembly:

     +------------------------------+
     ||                             |
     ||white                        +-----------------------------+
     ||paper                                                      |aperture
     ||(screen)                     +-----------------------------+
     ||                             |
     |                              |
     |                              |
     |                     hole     |
     +-------------------  for   ---+
                           head

Use:
The first step is to align the tube with the sun. (Do *not* look through
the tube at the sun. You'll injure your eye.) Either prop the box up,
or get an assistant to help you. Point the tube approximately at the sun,
then move the assembly around until the shadow of the tube exactly covers
the base. The tube is now pointed at the sun. Next, stick your head in
the hole and look at the screen. What you should see is an image of the
sun, far superior to what can be produced using the "two pieces of cardboard"
trick. Make sure you have sufficient ventilation to breathe. With a little
practice, you can keep the tube aligned with the sun, and watch an eclipse for
several minutes.

Theory:
In a "camera obscura", the rays of light from different parts of the sun pass
through the tiny aperture (at the far end of the tube), and since they come
in at different angles, they hit the screen at different spots. In this way
an image is formed on the screen. Since the light from the highest part of
the sun is projected onto the lowest part of the image and vice-versa, the
image will be inverted (upside-down). The purpose of the box is to make the
area around the screen dark, so you can see a much dimmer image than would
be possible in broad daylight. The purpose of the tube is to increase the
length of the path from the aperture to the screen to produce a larger image,
without adding much to the weight of the box. (If you want, you can use
just the box and the aperture without the tube, but good long boxes are
hard to find and difficult to hold for long periods.)

If you do not see an image, there are three possibilities:
 - The aperture is too small, so the image is too dim (try a larger aperture).
 - The tube is not pointed directly at the sun (realign the tube).
 - You stuck your head in too far, and the image is projected on the back of
   your head (pull your head a little further out, or use a higher box).

Design principles:
 - The diameter of the aperture determines the smallest "pixel size", and
   therefore the resolution of the image, as well as the brightness. A small
   aperture gives a very detailed, but dim image. A large aperture gives a
   bright but fuzzy image. A very large aperture gives a bright fuzzy blob,
   and you could possibly hurt your eyes looking at it in the dim light (it's
   no brighter than the outside sunlight, but your eyes are dark-adapted).

 - A round aperture gives the best image, since it has the highest "area
   to maximum dimension" ratio. Any other shape blurs the image more than
   necessary in the direction of elongation of the aperture.

 - The length of the path from the aperture to the screen affects image size,
   resolution, and brightness. A short path gives a small, bright, fuzzy
   image (which is what you would get using two pieces of cardboard - just
   enough to see the crescent shape of the sun). A long path gives a bigger,
   more detailed, but dimmer image. With a longer path length, you can use
   a larger aperture to get a bigger image with the same brightness and
   resolution, which makes it easier to see details. I've been able to
   see brightly-lit clouds near the sun. I suppose in principle the device
   could could be improved to show sunspots. Some solar observatories do this.

 - If you use a fairly wide tube, alignment is easier, since it does not have
   to be exact. A longer tube gives a bigger image, but is harder to align.

 - It is possible to get a bigger/brighter image by replacing the aperture
   with an arrangement of lenses. I've never tried this. It is also possible
   when trying this to get a very small, bright image that will hurt your eyes
   and burn a hole through the screen. If you try this, be careful!

 - A further refinement in the design is possible, which will make alignment
   easier. Make a tiny hole in the side of the box (just large enough to
   cast a spot of light that you can see) on the side of the box where the
   tube is attached, somewhere to the side of the tube. Get an assistant to
   line the tube up with the sun, stick your head in the box, and observe where
   the new spot of light hits the side of the box where the screen is attached.
   Mark this spot in a clearly visible manner (you may have to temporarily
   open the flaps of the box to do this). Now you can align the device
   yourself, by sticking your head in the box and moving the assembly around
   until the new spot of light strikes the mark you made. If you make the
   sighting hole too large, the spot of light it produces (which is itself
   an image of the sun) will wash out the larger image you're trying to
   view from the tube. If you make the sighting hole too small, its
   image will be too dim, making it hard to align the tube.
   Here is a view from the tube end of the box:

                 +---------------------------+
                 |    o       ___            |
                 | sighting  /   \ tube      |
                 | hole      \___/           |
                 |                           |
                 |                           |
                 |                           |
                 |           . . .  your     |
                 |          .     . head     |
                 |          .     . (in box) |
                 +---------------------------+

 - Another possible refinement is to install some sort of viewer in the
   back of the box, pointed at the screen inside, with covers to shield
   your eyes from the daylight. If this is done, the large hole in the
   bottom of the box can be eliminated.

I like this design, because it can be easily, quickly, and cheaply built
from readily-available materials, and it actually works very well. You
can use it to view solar eclipses, or any time to look at brightly-lit
distant objects, and you can throw it away when you get tired of it.
Just make sure to tell your neighbors what you're doing (or better yet
write "Solar Eclipse Viewer" on the side), so they won't have you hauled
off to the asylum. :-) During the last partial eclipse, I had about twenty
people looking through my viewer. Most of them were favorably impressed.

Disclaimer: Never look at the sun. Also, never perform any solar observation
unless you know what you're doing. Make sure you're in a comfortable position,
and that you have plenty of air. It's probably safest to sit down while using
this viewer. Don't blame me if you get a crick in your neck or hurt your eyes
or suffocate or trip over the dog. Also don't blame me if you paint the inside
of the box (except the screen) black and the paint rubs off on your head or
you breathe the fumes and get sick. Also don't do anything dangerous. I built
the viewer as described, and it worked very nicely, with no harmful effects.

(my part) Copyright (C) 1990 by John Roberts. Unlimited distribution
permitted provided my name is included.    January 12, 1990

------------------------------

End of SPACE Digest V10 #421
*******************