Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V08/V8_320.TXT

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Date: Wed, 10 Aug 88 01:04:39 PDT
From: Ted Anderson <[e-mail]>
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Subject: SPACE Digest V8 #320

SPACE Digest                                      Volume 8 : Issue 320

Today's Topics:
               Re: Solar Sails
             Re: Libertarian space policy
                 Solar flares
             20-year anniversary
                  Satellites
              Re: Lithium cells
        Re: Space Station Name chosen -- Fred
                 Re: E stamp
               RE:  Solar Sails
             Re: Libertarian space policy
               Re: 20-year anniversary
----------------------------------------------------------------------

Date: 2 Aug 88 09:47:50 GMT
From: polya![e-mail]  (Roger Crew)
Subject: Re: Solar Sails

In <[e-mail]> sedspace (Steve Abrams) writes:
> In V8 #306, Eric "TheBoo" Bazan ([e-mail]) writes:
>> My question is this: just how does the sun 'push' against the sail?
>> Is it the solar wind of charged particles (protons and electrons),
>> or the actual photonic flux, or both?
>
>     Since the wave and particle paradigms for electromagnetic radiation are
> equivalent, you can certainly think of "material" photons bouncing off a sail
> film and, thereby, transferring momentum to the sail.  However, I assume you
> want to view it from the wave perspective.
>
>  [...stuff about photons ...]

Look, if you want to view it from the wave perspective,
you're supposed to forget you ever heard the word ``photon.''

An electromagnetic wave in a vacuum consists of an electric field and
a magnetic field oscillating perpendicularly to one another, both
perpendicular to the direction of motion.  So consider what happens if
you have a single charged particle sitting off in the middle of
nowhere when an EM wave hits it:

Let's just put a positron (+ charge so that we don't get too messed up by
sign changes) in the middle of your terminal screen and have the wave
be moving into the screen.  Just to keep things simple let's have the wave
be polarized so that the electric field (E) is oscillating up and
down.

By itself, the electric field causes the positron to vibrate up and
down.  As with any simple harmonic oscillator the force will be 180
degrees out of phase with the displacement; the positron reaches its
lowest point just when the electric field is at its strongest pointing
upward (thus pushing the particle back up...) and vice versa.  Note
that the velocity of the particle is 90 degrees ahead of the dislacement.

No problem so far, but now we've also got a magnetic field (B) as well
which will be oscillating left to right.  Recall that the magnetic
field is generated by the changing electric field (Maxwell's version of
Ampere's Law) [B ~ dE/dt]
   --- and vice versa (Faraday's Law  E ~ -dB/dt), after all, this is
       how we get an electromagnetic wave in the first place ---

and will thus be 90 degrees ahead of the electric field.  After lots
of exercises with your right thumb and index finger, you'll get that
whenever the positron is moving upwards (electric field changing
downwards), the magnetic field points to the right, and whenever the
positron is moving downwards, the magnetic field is to the left.

Thus, at all times, the magnetic force on the particle q(v x B) is
pointed into the screen.  If the particle were an electron, the
magnetic force would be reversed, but we also have that it would be
vibrating in the opposite direction --- still ends up being pushed
into the screen.

To first order, this is what's happening.

But it's not the whole story.  Whenever a charged particle
accelerates, it radiates a wave of its own.  Part of this wave heads
into the screen, cancelling out a part of the original wave headed in
that direction.  The other part heads out of the screen back towards
your face.
Thus, energy has been taken out of the original wave; some of it goes
to the particle, the rest gets reflected back.  If you consider the
waves to have momentum inversely proportional to the wavelength,
it turns out that momentum is conserved as well.

Note that this is a completely classical explanation; at no point have
we drawn on anything from quantum mechanics.

--
Roger Crew                    ``Beam Wesley into the sun!''
Usenet:    {arpa gateways, decwrl, uunet, rutgers}!polya.stanford.edu!crew
Internet:  [e-mail]

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

Date: 2 Aug 88 04:51:39 GMT
From: clyde!watmath!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Libertarian space policy

In article <[e-mail]> [e-mail] (Leonard Erickson) writes:
>Note that Argentina has *already* started claiming *soveriegnty
>over it's portion of Antartica...

Claims of sovereignty over parts of Antarctica are nothing new; I don't
believe it has *ever* been the case that *all* interested parties were
willing to renounce them completely.  Just because the superpowers have
renounced them does not mean everybody has.  (In fact, this is the sort
of assumption that really irritates citizens of non-superpower nations.)
However, the big powers *have* played a key role in keeping those claims
in the realm of unimportant legal fiction.  When the US and the USSR
ignore your claims and violate them regularly, nobody else is likely
to take them seriously either.
--
MSDOS is not dead, it just     |     Henry Spencer at U of Toronto Zoology
smells that way.               | uunet!mnetor!utzoo!henry [e-mail]

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

Date: Wed,  3 Aug 88 02:17:17 EDT
From: "Keith F. Lynch" <[e-mail]>
Subject: Solar flares
To: [e-mail]
Cc: [e-mail], [e-mail]

> From: "Pat Reiff" <[e-mail]>

> ... the great flare of August 2, 1982 ... the Apollo program was
> still going on at that time ...

Are you sure you don't mean 1972?
                                ...Keith

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

Date: 3 Aug 88 01:10:02 GMT
From: tektronix!teklds!mrloog![e-mail]  (Dan Tilque;1893;92-101;)
Subject: 20-year anniversary

Next year will be the 20th anniversary of Apollo 11 and I'm sure that
everyone would like to celebrate it.  It never hurts to be prepared in
advance, so lets start to kick ideas around.

My idea is that they launch Discovery on or before July 20, 1989, even
if its faucets do leak.  Any others?


---
Dan Tilque    --    [e-mail]

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

Date: 2 Aug 88 22:45:37 GMT
From: unccvax![e-mail]  (Nitin R Kulkarni)
Subject: Satellites

Hello, fellow-netters.

I am a newcomer to this group. Can anybody in net-world give me the
following information :

    1. How many satellites has the US and the USSR launched into
       deep space ?
    2. Their probable location in our solar system as of today.
    3. The dates when they were launched.

Thanks in advance.

Nitin.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UUCP : [e-mail]
ARPA : [e-mail]

******    Is it clean in other dimensions ?   ******

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

Date: 3 Aug 88 02:14:51 GMT
From: killer!tness7!tness1!nuchat!splut![e-mail]  (Jay "you ignorant splut!" Maynard)
Subject: Re: Lithium cells

In article <880729-124705-5997@Xerox> "chaz_heritage.WGC1RX"@XEROX.COM writes:
>Some reported explosions of lithium cells are due not to short-circuiting but to
>application of AC.
>[...]
>Since in the astronautics application no high-voltage AC will be present in the
>circuit, there appears to be no risk of such violent explosions. Assessment of
>the outgas hazard is clearly in the hands of experts.

Uhm...not quite.
Main power on the space station is specified as 220 VAC at 20 kHz.

This, of course, makes power supplies efficient (since, in essence,
they're giving you the front half of a switching power supply), but can
indeed generate AC across a lithium battery in the manner described.

--
Jay Maynard, EMT-P, K5ZC...>splut!< | Never ascribe to malice that which can
uucp:       uunet!nuchat!           | adequately be explained by stupidity.
   hoptoad!academ!uhnix1!splut!jay  +----------------------------------------
{killer,bellcore}!tness1!           | Birthright Party '88: let's get spaced!

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

Date: 2 Aug 88 08:33:11 GMT
From: mcvax!enea!liuida![e-mail]  (Yngve Larsson)
Subject: Re: Space Station Name chosen -- Fred

In article <[e-mail]> [e-mail] (John Gregor) writes:
>>space station "Freedom".
>                ^^^ ^
Well, coincindence of coincidences; The strange fact is that "Fred" is the
Swedish word for "Peace". How nice of you to (sort of) honor Mir in this
way.. :-)
>
>John Gregor                                       [e-mail]


--
Yngve Larsson                               UUCP: ...mcvax!enea!liuida!yla
Dept of CIS                                       Internet: [e-mail]
Linkoping University, Sweden                          Phone: [phone]

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

Date: 1 Aug 88 19:15:32 GMT
From: trwrb!aero!venera.isi.edu![e-mail]  (Craig Milo Rogers; FAST)
Subject: Re: E stamp


    Since the center of illumination lies over Siberia, and the
orange bands in the background might have the correct elevation for
orbital platforms, perhaps the E stamp depicts SDI deployment.  What a
subtle method to subliminally influence the US voting public!  This,
too, would explain why the stamp is limited to "domestic" use;
wouldn't want too many Soviets or Europeans getting the wrong ideas
through repeated exposure to these stamps.

                    Craig Milo Rogers

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

Date: Wed, 3 Aug 88 12:20:22 CDT
From: [e-mail] (Steve Abrams)
Posted-Date: Wed, 3 Aug 88 12:20:22 CDT
Subject: RE:  Solar Sails


    In V8 #312, Alan Bostick replies to a simple question (FLAME
ON)  in his ubiquitously obnoxious and condescending manner
intended, not so much to answer the question or enlighten the
questioner, as to impress us with his virtuosity in physics.  However,
as a former professor of mine used to say, "If you can't answer a
physics question in words, *without resorting to formulae*, then you
don't understand the topic well enough. (FLAME OFF)

    Now, the original poster asked and Alan responded:

>In article <[e-mail]> [e-mail] (Steve) writes:
>>If the light bounces off the sail, how does it impart momentum.  What
>>energy of the photon is now reduced?  I think the photons must be
>>ABSORBED by the sail for this to work.
>
>Well, you're wrong.

    However, Alan (in his response) implicitly agreed that the
momentum trasnfer *does* occur when the photon *is* absorbed by
the sail film...

>The photon bounces off of the mirror and is reflected in
>accordance with the laws of reflection.

    Well, Alan, you kinda' glossed over the mechanism of
momentum transfer by the "in accordance with the laws of
reflection" phrase.  Let's look at exactly what happens when the
photon is "reflected."  In an "ideal" reflective surface, the incident
photon is *temporarily* absorbed by the electrons (at some
equilibrium energy level dictated by thermal effects and its "nearest
neighbor" electrons) in the surface of the sail (look up "classical skin
depth" and "anomalous skin effects").  An electron absorbing a
photon increases its energy level.  The photon's momentum is
transmitted, via interaction with the electron's potential "barrier," to
the metallic lattice of atoms of the sail film.  In addition, the electron
with the higher energy level is unstable and perturbations induced
by the "re-bound" of the potential barrier cause it to decay to its
original energy level thereby emitting a photon with the original
energy *but* in a direction of "least opposition" (i.e. back towards the
surface and not further into the film; this new photon's polarization
is also reversed from the original's) thereby imparting more
momentum because of the change in direction; of course, in a *real*
reflective surface, the electron decays to a higher energy level and
the re-emitted photon has less energy than the original.

    Occasionally, some "higher energy" electrons are more stable in
their new configuration and survive the "re-bound."  These photons
are the ones we call "absorbed."  Also, since skin depths for most
metals are for the same order of magnitude as the thicknesses of sail
films and thin films are close (within a couple of orders of
magnitude) in thickness to the wavelengths of the radiation incident,
some photons make it all the way through and are "transmitted."

    BOTTOM LINE:  For the ideal sail films you were discussing,
*all* incident photons *are* absorbed, transmit a "unit" of
momentum, and are re-emitted (transmitting another "unit" of
momentum).

    FLAME RE-IGNITED:   See, Alan, it can be explained without the
tedium of working through ASCII equations and in a manner that
conveys the *physics* of the topic and not just the math.  So,

>Next time, think for a minute before shooting off your mouth...

    FLAME RE-DOUSED


    I've been wondering if anyone has looked into the possibility of
using thin, conducting polymer films rather than metallic films for
sail films.  I know that polymers were once used for backing for a
metallic film and dropped to improve the thrust to mass ratio.  Can
thin films be polymerized to appropriate thicknesses?  If so, the
doping to make them electrically conducting (and reflective) can be
done rather easily.  The electrical conduction mechanisms are
different in these polymers from metals so, presumably, are the
reflections mechanisms.  With metals, you must consider specular,
diffuse, and back reflectances.  Anyone know if all of these apply to
the reflection mechanism of conducting polymers?  Are there any
different ones?


                    Ad Astra,

                    Steve Abrams

 \                                                                       /
--"You have twenty seconds to put down your slide rule...19...18...17..."--
 /                                                                       \
                                                     --  RoboDweeb

2721 Hemphill Park, Apt. C    ARPANET:  [e-mail]
Austin, TX  78705        CompuServe:  [70376,1025]
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                OR
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              Austin, TX  78713-7883  [phone]

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

Sender: "James J. Lippard" <[e-mail]>
Date:  Wed, 3 Aug 88 10:49 MST
From: "James J. Lippard" <[e-mail]>
Subject:  Re: Libertarian space policy
Reply-To: [e-mail]
To: [e-mail]
Cc: [e-mail]

>Date:     Mon, 25 Jul 88 15:05 EDT
>From: <[e-mail]> (Matt)

> I can hardly wait to see the outer space equivalent of Love Canal and
> Manville asbestos.

Ah yes, Love Canal--where the government used its power of eminent domain
to take a Hooker Chemical dump site and build a school on it despite
Hooker's warnings against it.

Jim Lippard
Lippard at BCO-MULTICS.ARPA

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

Date: 3 Aug 88 21:58:52 GMT
From: [e-mail]  (James E. Conley)
Subject: Re: 20-year anniversary

In article <[e-mail]> [e-mail] (kannan) writes:
>In article <[e-mail]> [e-mail] (Dan Tilque) writes:
><Next year will be the 20th anniversary of Apollo 11 and I'm sure that
><everyone would like to celebrate it.  It never hurts to be prepared in
><advance, so lets start to kick ideas around.
>
><My idea is that they launch Discovery on or before July 20, 1989, even
><if its faucets do leak.  Any others?

I would hope that NASA would cancel the launch if there we any unreasonable
possibility of failure.  Last thing we need to be remindered is how twenty
years ago we would put men on the moon, and now all we can do is scatter
them over the Atlantic Ocean.

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

End of SPACE Digest V8 #320
*******************