Message-ID: <kaCcIDK00VcJA58E4U@andrew.cmu.edu>
Reply-To: space+@Andrew.CMU.EDU
From: space-request+@Andrew.CMU.EDU
To: space+@Andrew.CMU.EDU
Date: Sun, 29 Apr 90 02:09:21 -0400 (EDT)
Subject: SPACE Digest V11 #332

SPACE Digest                                     Volume 11 : Issue 332

Today's Topics:
     Re: PegBlimp (was Re: Pegasus launch from Valkyrie  (or ...)
		     Re: Fermi Paradox (Long !!!)
----------------------------------------------------------------------

Date: 29 Apr 90 01:05:43 GMT
From: ox.com!itivax!b-tech!kitenet!russ@CS.YALE.EDU  (Russ Cage)
Subject: Re: PegBlimp (was Re: Pegasus launch from Valkyrie  (or ...)

In article <15712@phoenix.Princeton.EDU> michael@astro.Princeton.EDU (Michael Woodhams) writes:
>In article <1990Apr22.184617.4836@kitenet.ann-arbor.mi.us> russ@m-net.ann-arbor.mi.us (Russ Cage) writes:
>>Note that a 9% advantage in launch speed gives a *90%* increase
>>in payload.  This translates to a big cut in $/kg to orbit.
>I can't see any 9% advantages in launch speed, and certainly not any
>90% increases in payload for small change in velocity. Almost doubling
>velocity increases the payload by 21% (from 250 m/s, 470 m/s figures.)

The 9% (720 m/sec) was relative to the 8000 m/sec total delta-V.
Next time you have a question about my numbers, please send e-mail.

>>LTA craft have other problems:  their lifting gas expands with
>>altitude, which gives them a service ceiling usually < 15,000 ft.
>
>What ever gave you this idea? Balloons go higher than anything but rockets.

Now you are talking balloons, not blimps or dirigibles.  The sort of
balloon used for high-altitude research is a very different case:

1.)	A balloon doesn't have any guidance capability.  It might
	drift with the rocket and payload over areas you'd
	rather not have it go.  This is a range-safety problem.
2.)	These research balloons don't carry a crew.  Being able
	to linger at altitude to fix things is no help if there
	is nobody there to work on it.
3.)	If the launch doesn't come off, there is no nice way
	to recover the rocket and payload to try again; landing
	under parachutes tends to break things.

Neither blimps nor dirigibles have these problems, but they are
quite inferior to aerodynes as air-breathing launchers, even
off-the-shelf subsonic ones.

>Launching Pegasus at twice the velocity of the B52 is very hard and
>expensive, but launching from three times the height and zero velocity
>from a balloon should be simple. How does payload vary with launch
>altitude?

Time for me to question YOUR numbers:  The B-52 and many other
aircraft can fly at 50,000 ft plus.  You mentioned a figure of
3 times that (150,000 ft) as an altitude reachable by balloons.
How many research balloons have been that high, and how much
did they carry?  Anything even *close* to Pegasus size?
According to my CRC, the air density there is about 1 g/m^3;
supporting a 20,000 kg load would require a balloon with a
volume of at least 20 million meters^3, or 700 million ft^3!
That's a sphere 1100 feet in diameter!  Are you for real?

Payload as a function of launch altitude... good question.  In
the simplest case, it is merely an energy input.  A 50,000 ft
altitude advantage is equivalent in energy to a 1200 MPH boost.
However, Pegasus complicates matters by using aerodynamic lift
instead of rocket thrust to fight gravity in the first seconds.
A rockoon would have to use thrust alone, which is much less
efficient.  Back-of-the-envelope engineering at my modest skill
level can't answer the question of which has the advantage.
-- 
			  Oversimplification doesn't solve problems, it just
(313) 662-4147		  changes them into less tractable problems.
Russ Cage, Robust Software Inc.            russ@m-net.ann-arbor.mi.us

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

Date: 29 Apr 90 05:13:59 GMT
From: att!cbnewsd!jfb200@ucbvax.Berkeley.EDU  (joseph.f.baugher)
Subject: Re: Fermi Paradox (Long !!!)


	There has been so much discussion about the Fermi Paradox on the
net that I thought that I'd stick in my $0.02 worth.  This is gonna be
long, so unless you have a strong dispositon, better bail out now.

I'll start with a description of the Drake equation.  This formula (in
conjunction with a whole lot of assumptions and rough guesses) gives
an estimate of the total number of technological civilizations currently
resident in the Galaxy.  A technological civilization is here defined as
one which is capable of making its presence known to other inhabitants 
of the Galaxy, either by accident or by design.

The derivation starts with an idea taken straight from elementary
chemical reaction kinetic theory.   Let R be the rate at which new stars
appear in the Galaxy and let fciv be the fraction of those stars which
ultimately give rise to a technological civilization.  So the rate at which
new technological civilizations appear is given by

			R*fciv					(1)

As new civilizations appear, some of the older ones will eventually die out.
Let Tav be the average lifetime of technological civilizations.  It can be
interpreted at the average time that a technological civilization retains
the capability of making its existence known at interstellar distances.
The rate at which technological civilizations expire is then given by

				Nciv/Tav			(2)

where Nciv is the number of civilizations currently in the Galaxy.  The net
rate at which civilizations appear is given by the difference between
equations 1 and 2:

		dNciv/dt = R*fciv - Nciv/Tav			(3)


The assumption is now made that the Galaxy is currently in a "steady state",
that is, the number of technological civilizations has now leveled off and has
reached a constant value.  This is an expression of the "principle of
mediocrity", which says that there is nothing all that special about the Earth
and nothing unusual about this particular instant in time.  Humanity is 
average and typical rather than extraordinary or special.  The "mediocrity
principle" can be said to be the central idea of the modern scientific 
revolution, in which first the Earth, then life, and finally humans themselves
have been dethrowned from their privileged positions as the center of
God's concern and attention and removed from any preferred location in either
space or time.

Under this assumption, we can set dNciv/dt equal to zero in Equation 3, 
obtaining

				Nciv = R*fciv*Tav		(4)

This is the formula that is used as the starting point for most discussions
of extraterrestrial life and contact with extraterrestrial civilizations.
It is generally attributed to Frank Drake, the radio astronomer who carried
out the first serious search for intelligent extraterrestrial signals.

Equation 4 depends on a lot of things we know very little about.  The factor
R is the easiest to estimate.  It is the rate of creation of new stars.  The
rate of new star production averaged over the lifetime of the Galaxy is
of the order of 5 new stars per year.   R must have been somewhat greater
than this ten billion years ago when the Galaxy was young.  R must be somewhat
less than this now.  However, the use of the average for R should not 
introduce too many severe errors, especially in light of the uncertainties
in the other factors in Eqauation 4.

The term fciv is usually decomposed into several factors, each of which
is separately estimated.  Several different decompositon schemes have been
used.  Here's mine:

   fciv = 
	Probability that a star is of proper spectral type *
	 Probability that a star has planets*
	  Probability that there is a planet at right distance from star *
	   Probability that planet has correct mass *
	    Probability that planet does not have too steep axial inclination*
	     Probability that planet is not spinning too fast or too slow*
	      Probability that life appears on a suitable planet*
	       Probability that life ultimately produces intelligence*
		Probability that intelligence ultimately produces technology

If any one of these factors is small, fciv will be small.  

I took the Solar System as being typical and came up with the following
numbers for the various factors:

	fciv =
			0.25 *
			  1.0 *
			    0.07 *
	                      0.5 *
				0.9 *
				  0.8 *
				    1.0 *
				      1.0 *
					0.5
                                              = 0.003

The dominant factor is the low probability of a planet being at just
the right distance from the star.  Too close and it develops a thick, hot
atmosphere of carbon dioxide and water vapor, just like Venus.  Too far
away and all the water freezes to the surface and the atmosphere 
evaporates away into space, just like Mars.  If you have a quarrel with
my estimate for any one of these factors, I invite you to come up with a
different number and give a justification for it.

Now for the estimate.  The current number of technological civilizations in the
Galaxy should be

		Nciv = R*fciv*Tav = 5 * 0.003 * Tav		(4)
			     
The number of civilizations depends on the average lifetime of a civilization.
Since we know of no civilization besides our own, we have absolutely no idea
of how large Tav is.  It could be as short as only a few decades, or it could 
conceivably be as long as the lifetime of the Galaxy itself.  I'll plug in a
few numbers:

		Tav = 70 years		Nciv = 1
		Tav = 100,000 years	Nciv = 1500
		Tav = 1 billion years   Nciv = 15 million

We can anticipate large numbers of technological civilizations only if 
average lifetimes are long.  

Herein lies the Fermi paradox.  If life and civilization are to be ubiquitous
throughout the Galaxy, then lifetimes must be long.  However, this is a       
problem, since a long-lived civilization will have adequate time to establish
a technology that is sufficiently advanced to make its existence manifestly
apparent to us on Earth.  The night skies should scream technology and
intelligence to us.  But there is no apparent sign that there is anything
out there which was produced by forces other than completely natural ones.

Another problem is that a long-lived technological civilization would have
the means and perhaps the inclination to establish outposts and colonies
on planets around other stars.  A sufficiently-ambitious civilization bent
on expanding its sphere of influence could colonize the entire Galaxy 
in a time as short as 50 million years, a mere instant in the lifetime of
the Galaxy.  If life and civilization have truly been around in the Galaxy
for a long time, changes are that the Earth would have been colonized by 
aliens a long time ago.  Extraterrestrials rather than human beings should
currently be living on the Earth.  But there is no evidence that 
extraterrestrials have ever visited the Earth.  Where, then, are they?

We have a basic problem here.  We have a theory inconsistent with
observation.  We predict a reasonable probability for the presence of
extraterrestrial technology, and yet we see no evidence of its existence.
What's wrong?  Let's go back and look again at the derivation of the Drake
equation.  Did we make some error in the derivation, or did we perhaps make an
invalid assumption? 

One possibility is that the reason that the Earth has never been visited
or colonized is that intelligent extraterrestrials do not exist.  One or
more of the factors in the Drake equation is sufficiently small to make
the origin of life and intelligence so improbable that it has happened only
once in the entire history of the Galaxy.  However, it is hard to believe
that our science is so far off the mark as to completely misunderstand the
origin of life and intelligence on Earth and to introduce so large an
error in our estimate of the probability of its origin elsewhere.  But
perhaps humility in the face of the vastness of the Universe is appropriate.  
 
Another possibility is that the average lifetime of a technological 
civilization is only a few decades.  Civilizations invariably destroy themselves
before they have a chance to produce a starfaring society capable of
colonizing other planets.  They blow themselves to bits, crowd themselves
to death, exhaust their resources, poison their environment, or simply
lose their will before they have a chance to expand to other stars.  A similar
fate awaits us.  The adoption of a technogical lifestyle is a death trap; it 
inevitably leads a society to destroy itself.  However, it is difficult to
believe that ALL societies will inevitably destroy themselves in such a 
manner.  All that would be required for the complete colonization of the
Galaxy is for just ONE civilization to have escaped self-destruction in
the distant past.  Consequently, I feel that the self-destruction hypothesis
is an unlikely explanation of the Fermi paradox.

A third possiblity is that advanced technological civilizations all
voluntarily choose not to expand outward into the Galaxy.  Perhaps mature 
technological societies lose their expansionist urges; those societies
which manage to survive technological adolescence do so only by removing
from their psyches those ancient urges which would otherwise have led them
to self-destruction.  They are content to remain forever within their
own solar systems, concentrating they attention on personal development
or philosophical contemplation or other such activities which do not
require access to more resources or more space.  However, it is unrealistic
to expect that EACH and EVERY civilization in the Galaxy would behave in
this way.  In particular, I don't think that we human beings would as an
entire species ultimately and irreversibly turn inward upon ourselves, 
abandoning forever the exploration of the Cosmos that we began so
confidently such a few short years before.  I do not think that we can
resolve the Fermi paradox by proposing that ALL other civilizations in the
Galaxy invariably behave in a manner completely opposite to the manner in
which we humans would behave!

Perhaps extraterrestrials have never come to the Earth because our planet
has nothing that they want.  The conquest of another world is an
extremely difficult undertaking and is likely to require thousands or
millions of years of effort, even for the most advanced technology.  The
terrestrial biosphere is likely to be highly toxic to extraterrestrial
life, and the Earth may be no more tractible for alien habitation than
the lifeless worlds Venus and Mars or even the moons of Jupiter or Saturn.
They would have to destroy every living thing on Earth before they could
safely walk unprotected on the surface.  For this reason, colonizing
extraterrestrials may choose to colonize other solar systems by building
O'Neill-type space colonies out of materials mined from asteroid belts.
Such colonies could be assembled in only a few years, whereas it could
take millions of years to tame a planet such as the Earth.  

Sufficiently advanced civilizations may even outgrow their need for a star
about which to make their homes.  They may have become so accustomed to
interstellar migration that they prefer to drift aimlessly from star to
star, pausing only long enough to replenish their fuel supplies and to 
pick up raw materials.  Perhaps the scars of ancient extraterrestrial
mining expeditions await discovery somewhere in the Solar System.  Perhaps
the odd features on the Uranian moon Miranda are evidence of extraterrestrial
visitation.  
 
The Drake equation assumes that the Galaxy is currently in a "steady state",
as least as far as the evolution of life and intelligence is concerned.
But we know that the Galaxy is a dynamic, evolving, ever-changing place. 
Fifteen billion years ago, the Galaxy was little more than a dense ball of
gas.  Then the first generation of metal-poor stars formed.  These exploded,
enriching the interstellar medium in heavy metals.  These metals were 
incorporated into subsequent generations of stars, creating the raw 
materials which later were to lead to the advent of life and technology. 
There must have been a time when there were few or no technological societies
in the Galaxy.  Perhaps the current era is such a time, and Humanity is 
the first, or among the first, intelligent species to have appeared in the 
Galaxy.  SOMEONE has to have been the first.  Why not us?  Several other
emerging technological species may be only a few thousand years more advanced
than we, and the spherical wavefronts of their expanding civilizations may
currently be advancing through space towards Earth but none has yet reached
our planet.  Alternatively, we may be destined to encounter the vanguard of
another civilization as we ourselves advance into the Galaxy.

The "zoo hypothesis" is another possible explanation.  There is some sort of
moral imperative that prevents extraterrestrials from colonizing or
interfering with any world upon which indigent life already exists.  The  
Earth has been set aside as sort of a wildlife preserve or scientific
observation station, to be guarded and studied but not to be touched.
Extraterrestrials may be watching and studying us at this very moment, but
we do not see them because they do not want to be seen.  They are acting as
cosmic "game wardens", protecting us against poachers or other such
unscrupulous creatures who might be tempted to take unfair advantage of
an immature society such as ours.  Alternatively, we may have been deemed
a species dangerous to the rest of the Galaxy and placed under a strict
quarantine.  If so, we will not be permitted to roam very far.
The problem with the "zoo hypothesis" as an explanation of the Fermi
paradox is that it is by definition unfalsifiable.  There is no way to
prove that it is wrong. 

Could it be possible that the basic underlying assumption of the Drake
equation is wrong?  Could it be that the "principle of mediocrity" is
ultimately invalid?  Are we humans special after all?  Is the Universe
really there only to allow us to exist and thrive?  There is a provacative
book that came out a few years ago, written by John Barrow and Frank Tipler.
It was entitled "The Anthropic Cosmoligical Principle", and it argued that
the origin of life and intelligence required a fine-tuning of the laws of
physics and an adjustement of values of the fundamental constants that just
could not be concidental.  The Universe must be the way it is because we
are here, not the other way around.  I find this an attractive
hypothesis.  However, I part company with Frank Tipler's proposal that
the Fermi Paradox is such a strong indication that intelligent
extraterrestrials do not exist that we need no longer search for them.
This is akin to the ancient Pythagorean or Platonic idea that pure theory
is sufficient to understand the Universe and that experimentation is a
waste of time. 	There is always the chance that our knowledge is incomplete
and that our theories are imperfect.  We must always be alert to the     
possibility that we may have overlooked something important or have made
an invalid assumption.  We must not cease searching for intelligence
among the stars.


Joe Baugher				**************************************
AT&T Bell Laboratories			*  "May as well be frank, monsieur.  *
200 Park Plaza				*  It would take a miracle to get    *
Naperville, Illinois 60566-7050		*  you out of Casablanca.            *
(708) 713 4548				**************************************
ihlpm!jfb

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End of SPACE Digest V11 #332
*******************