Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v08/v8_312.txt

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Date: Wed, 3 Aug 88 01:04:44 PDT
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Subject: SPACE Digest V8 #312

SPACE Digest                                      Volume 8 : Issue 312

Today's Topics:
           Time skew -- does it hurt SETI?
               Re: Solar Sails
            Re: Shuttle-C details
            Re: Shuttle-C details
           Re: space news from June 6 AW&ST
           Re: space news from May 30 AW&ST
       Doppler lidar, ESA publications(was: NASA news - Seasat)
               We will bury you
         Re: Time skew -- does it hurt SETI?
               Re: Solar Sails
----------------------------------------------------------------------

Date: 26 Jul 88 00:25:55 GMT
From: amanda![e-mail]  (Martin Sodos)
Subject: Time skew -- does it hurt SETI?

I'd like to put forward an idea for public consideration/debate.
Forgive me if its already been addressed, but I have been exceedingly
curious about it for some time.  Consider the following: Carl Sagan
(sp?) in his cosmic calendar shows that all of recent recorded history
represents only the last few seconds before midnight, a mere
instantaneous flicker compared to the events that have preceeded it
during the cosmic 'year'. Consider further that science has in recent
years taken on an exponential learning curve. For example, electricity
was only 'discovered' (as in defined and put to useful purpose as
opposed to merely observed in nature) within the last few hundred
years, radio within the last hundred, relativity, nuclear power and
space travel within this century. The true nature of superconductivity,
perhaps yet to come.  Practical human spacetravel outside the boundarys
of even our own solar system still only an unrealized dream.

Now, consider that even if there exists a race somewhere relatively
closeby in the universe so that radio is a practical means of
communication, and if this race has developed in a very similar manner
to our own, if they are a mere 1% behind us on the cosmic calendar they
will not even yet have evolved on their planet. For example, if the age
of our Earth is 4 billion years, 1% of 4 billion is 40 million. longer
than man has existed on this planet(by most estimates). So, let's be
generous. Let's say there is some operative factor of parallel
evolution which would align the development of the two cultures (theirs
and ours) to within .0001%. Even to this, I dare say ridiculous, level
of parallelism, they are only within +/- 4000 years of us. If they are
behind us they are somewhat before the time of their equivalent of the
Greeks, and there is no hope of communication. If they are ahead of us,
they are probably so advanced as to be almost unrecognizable.  Bear in
mind what has happened scientifically in only the last 100 years.
However, for those who would argue this, I contend that the point is
moot by the very absurdity of the tolerance specified.

Ergo, I put forward for your consideration that even if life such as
ours is fairly common on the universe, that the time alignment problem
would make it extremely unlikely that we would/will ever encounter it.

And of course, that further assumes that the culture in question (the
one relatively aligned to ours), is also the one which is reasonably
nearby.

I contend that if you add the 'time skew' factor to those formulae
which portend to predict the probability of life like our own elsewhere
in the universe you get a liklihood near zero that we will ever
encounter it.

What say you all?


sun!amanda!msodos

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

Date: 23 Jul 88 09:22:38 GMT
From: unisoft!gethen![e-mail]  (Alan Bostick)
Subject: Re: Solar Sails

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.

Consider a mirror initially in free fall.  Let us work in the reference
frame where the mirror is at rest.  The mirror has a mass of m, and in
this frame it has a kinetic energy of zero.  A photon is traveling to
the right with momentum p and energy E = c*p, where c is the speed of
light.  The photon bounces off of the mirror and is reflected in
accordance with the laws of reflection.  Now it is traveling to the left
with momentum -p' and energy E' = c*p'.  The momentum of the photon has
changed, so in order for momentum to be conserved the momentum of the
mirror must change as well.  The mirror's momentum is now p" and its
kinetic energy is now E" = (p"**2)/(2*m).  The total final momentum must
equal the total initial momentum:  p" - p' = p; and the total final
energy must equal the total initial energy: E' + E" = E.  If we take our
energy equation and make the appropriate substitutions, we have:

    c*p' + (p"**2)/(2*m) = c*p

Now make the substitution p' = p" - p (a rearrangement of the momentum
equation), and solve for p".  We then have

    p" = m*c*((1 + (4*p)/(m*c))**0.5 - 1) .

When p << mc (and we are talking about a single photon here, so it
almost certainly does,) this is closely approximated by

    p" = 2*p - 0.5*((p/(m*c))**2) .

This is the momentum of the mirror after the reflection.  We substitute
this into our momentum equation to find that

    p' = p - 0.5*((p/(m*c))**2) .

We see that the photon has a little less momentum, and hence a little
less energy than before.  This energy lost by the photon has gone into
kinetic energy of the now moving mirror.  Energy is conserved, momentum
is conserved, and the mirror is now moving, due to the kick of the
light.  No absorption is necessary.

Next time, think for a minute before shooting off your mouth.  This is
freshman physics.  This is freshman physics for liberal arts majors.

                    Alan Bostick
                    ucbvax!unisoft!gethen!abostick

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

Date: 25 Jul 88 10:55:28 GMT
From: b.gp.cs.cmu.edu![e-mail]
Subject: Re: Shuttle-C details

In article <[e-mail]>, [e-mail] (Steve Willner P-316 x57123) writes:
}[Shuttle-C] Development cost would be up to $1.5 billion. Current definition
}studies are to run through 1989.

$1.5 billion to develop a launcher from mostly off-the-shelf parts!?!?!?!
Multi-year definition studies?!?!?!  Sounds like more gold-plating to me....
--
UUCP: {ucbvax,harvard}!cs.cmu.edu!ralf -=-=-=- Voice: [phone] (school)
ARPA: [e-mail]  BIT: ralf%cs.cmu.edu@CMUCCVMA  FIDO: Ralf Brown 1:129/31
Disclaimer? I     |Ducharm's Axiom:  If you view your problem closely enough
claimed something?|   you will recognize yourself as part of the problem.

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

Date: 26 Jul 88 13:34:44 GMT
From: [e-mail]  (Paul F. Dietz)
Subject: Re: Shuttle-C details

The interesting thing about the Shuttle-C, as proposed, is that you
can launch it only once for every 6.7 shuttle flights (for the two engine
version) or every 10 flights (for the 3 engine version).  So, after the
current stock of used engines is exhausted, shuttle-C will have the
capability of lifting maybe 1/3 the mass the shuttle fleet can.  Excuse
me if I don't cheer.

    Paul F. Dietz
    [e-mail]

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

Date: 25 Jul 88 22:01:07 GMT
From: thumper![e-mail]  (Phil R. Karn)
Subject: Re: space news from June 6 AW&ST

> ... So far the station has survived, at the
> expense of major cuts to CSTI, Pathfinder, NASA expendables, and the
> Commercially Developed Space Facility.  [Those are lousy places for cuts.]

*Now* you see why the Space Station isn't such a good idea. If it
competed solely with, say, SDI for funding I'd be 110% for it. But it
instead competes, rightly or wrongly, mostly with other NASA projects,
and with the possible exception of the Shuttle they are all far more
cost-effective than the Space Station.

> Soviet Academy of Sciences and the Federation of American Scientists
> jointly propose a ban on nuclear reactors in Earth orbit.  This would
> hamper SDI and shut down the Soviet nuclear radarsats.

Agreed. We really need some enlightened self-interest on both sides. An
excellent start would be the following:

1. Ban the use of nuclear power sources in low earth orbit, "low" being
defined as anything with a lifetime less than a thousand years or so.
Deep space missions would be fine.

2. Ban any mission that involves the explosion of a warhead in orbit or
the deliberate collision of objects unless the expected lifetime of the
resulting fragments is less than a year.

These two provisions would have the following highly beneficial effects:

1. The seemingly regular series of Soviet nuclear-powered radarsats
re-entering the atmosphere would stop.

2. The increasing pollution of orbital space by ASAT and SDI tests would
stop.

3. Taken together, the two provisions would effectively prohibit many,
if not most, nuclear powered SDI tests.

We urgently needed the Limited Test Ban Treaty in 1963, and we urgently
need a treaty like this now.

Phil

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

Date: 25 Jul 88 22:50:12 GMT
From: sdcc6!calmasd![e-mail]  (John Pantone)
Subject: Re: space news from May 30 AW&ST

Henry Spencer writes:
   ...much valuable info deleted...
   Reagan will propose that the two countries study cooperation
   in solar-system exploration.  He will refrain from endorsing missions
   involving extensive hardware cooperation (e.g. Apollo-Soyuz), major
   manned projects, or specific unmanned Mars missions.  [Has it occurred
   to any of the brain-damaged bozos who put together this wonderful list
   of non-promises that maybe the Soviets are tired of studying the notion
   endlessly and would like to *do* something?!?]

Henry - I'm not really arguing about the "brain-damage" nor even the
"bozo" nor even the "non-promises".  I am, however, not too thrilled
to see an editorial comment which seems to imply that just because the
Soviets may (or may not) be getting tired of our "studies" that we
should fall through our a__holes trying to please them, or get
something done to satisfy or placate them.

These are the same people that in the Sputnik era (not that long ago
to me, anyway) promised (not threatened, mind you, *promised*) to bury
us.  A couple of "kind words" and a leader in decent tailoring (for a
change) hardly eradicates decades of threatening behavior.  I
personally think that cooperation is far more desirable than continued
competition - but not without a great deal of deliberation - and due
consideration.  If the Soviets have to stew in their juices a while -
so be it - tough luck. Gorbachev represents, PERHAPS, a new era -
we'll just have to wait and see.
--
These opinions are solely mine and in no way reflect those of my employer.
John M. Pantone @ GE/Calma R&D, 9805 Scranton Rd., San Diego, CA 92121
...{ucbvax|decvax}!sdcsvax!calmasd!jnp   [e-mail]   GEnie: J.PANTONE

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

Date:     Tue, 26 Jul 88 14:00 CDT
From: Bill Higgins-- Beam Jockey <[e-mail]>
Subject:  Doppler lidar, ESA publications(was: NASA news - Seasat)
Original_To:  SPACE

All the discussion of Seasat, oceanography, and radar reminds me of an article
(going slightly off on a tangent) in the current *ESA Journal* (Volume 11
Number 4/volume 12 Number 1 [hey, that's what it says on the contents page.
Must be a Spectacular Double Issue or something.], pages 19-36: "Spaceborne
Doppler Wind Lidars," by G. Salvetti of ESTEC.

The object is to shine a laser down, observe scattered light off dust particles
and aerosols, and measure the Doppler shift.  Do this from two angles (two
spots in the satellite's orbit) for the same location on the surface, and you
have enough velocity information to get the two-dimensional wind pattern. Time
delay of the laser pulse gives you depth. This could be a great boon to the
weather guys, especially in the Southern Hemisphere where surface measurements
are sparse.

Problems:  Laser needs lots of power, but beam density is limited by eye-safety
requirements. Space-qualified lasers and detectors are needed. Data rates are
very high (you probably need on-board processing).  You have to take Doppler
shift due to spacecraft motion and Doppler broadening due to molecular motion
and wind shear out of a very feeble signal in order to get the numbers you
want.  Coherent detection of the signal would be nice and sensitive, but may be
too tricky to achieve in a flyable system.  Still under development-- but a
rather nice technique, don't you think?

I might as well reveal a secret I've been sitting on for quite a while: The
European Space Agency's magazines are FREE.  *ESA Journal* is for technical
papers, *ESA Bulletin* is more general-- if still pretty technical-- and
includes regular status reports on every ESA program, as well as articles. Both
are quarterly.  Write to:

ESA Publications Division
ESTEC
2200 AG Noordwijk
The Netherlands

And by all means, get your library to subscribe!!

The ESA Publications Division can also give you information on buying copies of
ESA's technical reports and books, which flow in great quantity from the
European space program.  And there are a couple of other free publications.


                         ______meson      Bill Higgins
                      _-~
        ____________-~______neutrino      Fermi National Accelerator Laboratory
      -   -         ~-_
    /       \          ~----- proton      Bitnet: [e-mail]
    |       |
    \       /                             SPAN/Hepnet/Physnet: 43011::HIGGINS
      -   -
        ~

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

Date: 26 Jul 88 19:05:09 GMT
From: agate!garnet![e-mail]  (Obnoxious Math Grad Student)
Subject: We will bury you

>These are the same people that in the Sputnik era (not that long ago to
>me, anyway) promised (not threatened, mind you, *promised*) to bury us.

Like or dislike the Soviets as you think proper, but don't keep citing
this famous line incorrectly.  "We will bury you" is Russian idiom that
meant nothing more than the trivial assertion "we will live longer than
you".  "Bury" here has the sense of "be in attendance at your funeral",
and not the "go out of our way to put you six feet under" interpretation.

And in case you think the sentence is possibly ambiguous, it most certain-
ly wasn't so in context.  Funny how Krushchev's full speech with this line
is so rarely printed--it was just simple Marxism about how the capitalist
system is going to self-destruct, the socialist system is going to endure,
and all that rot.

ucbvax!garnet!weemba    Matthew P Wiener/Brahms Gang/Berkeley CA 94720

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

Date: 26 Jul 88 13:57:28 GMT
From: rochester!rocksanne!entire![e-mail]  (Edward L. Taychert)
Subject: Re: Time skew -- does it hurt SETI?

My astrophysics text discusses extraterrestrial communications based
most likely on radio. Different probabilites of contact are plotted
against the population density of the universe on one axis and
the lifetime of a sufficiently advanced culture on the other.
Most depressing was the notion that we advance sufficiently to
call out, then kill ourselves off. The reply falls on a dead world :-(

Even optimistically, communications (a message and reply) are likely
to take tens of thousands of years. We may hear a beacon in the night,
or we may be heard on a distant world, but the odds of a dialog
don't look good.
--

____________________________________________________________________________

Ed Taychert                Phone: USA [phone]
Entire Inc.                UUCP: rochester!rocksanne!entire!elt
445 E. Commercial Street
East Rochester, N.Y. 14445
_____________________________________________________________________________

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

Date: 26 Jul 88 18:45:53 GMT
From: grasp.cis.upenn.edu![e-mail]  (Nathan Ulrich)
Subject: Re: Solar Sails

In article <[e-mail]> [e-mail] (Alan Bostick) writes:
>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.
>
>Next time, think for a minute before shooting off your mouth.  This is
>freshman physics.  This is freshman physics for liberal arts majors.
>
I think your reply is rather pompous and insulting.  I did not read the
original posting, but it appeared to be asking a good question, which you
did not answer.  Why isn't absorption most efficient for a photon sail?

Relativistic physics has some interesting quirks; one of the them is the
characterization of photons.  They are generally considered to have momentum
but not mass, and always travel at the speed of light (c).  It has been
experimentally determined that photons can exert pressure, and the momentum
of photons at various wavelengths has been measured.  However, the theory
of relativity does not allow for objects with mass to travel at the speed
of light, therefore photons are massless.

We do know that E = hn, where E is the energy of the photon, h is Planck's
constant, and n is the frequency of the photon.  In a collision with a mirror,
a photon will transfer some of its momentum and its energy to the mirror.
The new energy E' = hn' is less than the original energy E, therefore the
new frequency n' is less than the original frequency n.  The result?  Light
reflected from a mirror changes color.

If we want to build the most efficient sail possible, then we need to do two
things:  maximize the kinetic energy gain per photon collision, and maximize
the number of photons hitting the sail.  We can handle the second by making
the biggest sail possible.  The first is a little more interesting.  It would
seem intuitively that the best sail material would absorb all the photons
hitting it, because then all the energy of the photons would be transferred to
the sail.  Unfortunately, when the speed of a photon falls below the speed of
light--like when it is absorbed by the sail--it is no longer a photon, and its
energy is transferred to some other form:  electricity via photoelectric cells,
heat via black clothing, etc.  It turns out that the energy of a photon will
not be converted into the kinetic energy of the sail, but into one of these
other forms.  The way to gain kinetic energy from photons is to reflect them
as efficiently as possible--the energy transfer results in an increase in the
kinetic energy of the sail, rather than to heating it up.

In practice, you need a very big, very light, very reflective sail.

Nathan Ulrich
[e-mail]

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

End of SPACE Digest V8 #312
*******************