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.cb INTERSTELLAR TRAVEL WITH 20TH CENTURY TECHNOLOGY


	There are about a hundred billion stars in our galaxy (about 25
stars for each person on the earth today), and it would be wonderful if we
could travel to them in a few months or years.  Unfortunately, the galaxy
is a 100,000 light years across (30,000 light years from here to the
galactic core), and our present science tells us that we can't travel
faster than the speed of light.  Present science may be wrong, but the
special theory of relativity has survived more experimental tests than
almost any other part of physics.

	For once, let's be conservative and ask what interstellar travel
is surely possible rather than what we might do if Nature is more
co-operative than we can be sure of.  We shall assume only present science
and not much beyond present technology.  We will have to accept
multi-generation journeys, consider how they can be done, and why people
might want to.

	Consider a rocket that uses a nuclear reactor to generate
electricity and uses the electricity to propel charged particles out the
back - the reaction propelling the rocket forward.  The reactors now used
in nuclear submarines would even work; they produce the most power for a
given mass of reactor of any existing reactor, but a new design could do
much better.  Rockets that expel charged particles have been built, one is
being tested now in orbit, and NASA's hopes for a 1986 %2rendez-vous%1
with Halley's comet were based on an electric rocket (solar powered).

	Optimizing a rocket system that generates power and uses that
power to expel a "working fluid" is more complicated than optimizing a
chemical rocket.  In a chemical rocket, the %2figure of merit%1 (the
number that tells how good the system is) is the velocity of the exhaust -
the higher the better.
The %2figure of merit%1 of a %2power-limited%1 rocket is the %2specific power%1.
This number is obtained by dividing the power usable for expelling
working fluid by the
mass of the rocket system minus fuel and working fluid.  The
exhaust velocity is at the disposal of the designer and can be
chosen to optimize performance.  The main limitation
on power is the need to dissipate waste heat by
radiation; dissipating a lot of heat requires either a high
temperature which reduces efficiency or a big radiator which weighs
a lot and has to be accelerated along with the payload.  Too high an
exhaust velocity produces a low thrust for a given power and this makes
the journey take long.
Too low an exhaust velocity requires that a large mass of working
fluid be accelerated.

	Between the extremes there is an optimum exhaust velocity,
and it turns out that this velocity should vary during the
journey.  Consider a journey through a distance %2s%1, starting from zero
velocity and ending at zero velocity.  Assume that the propulsion system
has a %2specific power p%1, i.e. %2p%1 is the the power output of the
system divided by its mass.  Finally, let the mass ratio be %2k%1, i.e.
%2k%1 kilograms start the journey for every kilogram that arrives.  The
time %2T%1 of the journey is then given by the formula

!!a1:	%2T = 1.8s%52/3%2p%5-1/3%2(log k)%5-1/3%1.

	We will spare the reader the derivation of formula ({eq a1}).  (It
uses the calculus of variations, and has the interesting consequence that
the acceleration varies linearly with time becoming zero at the midpoint
of the journey.  The part involving %2s%1 and %2p%1 can be derived by an
easy method that physicists call dimensional analysis).  However, the
formula has consequences that should interest even the mathematically
handicapped reader.

	First consider the %2s%52/3%1  part.  The time required for the
journey is less than proportional to the time.  This is because
on a longer journey we can use our power source
longer, and therefore can use a higher average exhaust velocity and
achieve a higher average velocity during the journey.  Thus
reaching stars 80 light years away takes only four times
as long as reaching stars 10 light years away - not eight times
as long.  Since we can expect 512 times as many stars in that sphere,
if any stars are visitable, many will be visitable.

	The %2p%5-1/3%1 tells us that the rocket system must be improved
by a factor of a 1000 in order to reduce the travel times by a factor of
10.  Conversely, you don't lose much by a bulkier system with a low
specific power.  Perhaps it also implies that very advanced civilizations
will only be able to travel a little farther than backward civilizations.
The %2(log k)%5-1/3%1 term tells us that starting with a very large mass
doesn't help a lot either.

	Table 1 gives some travel times for different
assumptions about the distance, the specific power, and the mass-ratio.


table 1


	I don't claim this is the only possible method of interstellar
travel or even the best, %3but it is one we can be sure will work%1.
Therefore, our speculations about the occupation of the universe by humans
or other intelligences should be based on having at least this good a
method of interstellar travel.  I find it annoying that many scientists
interested in interstellar
communication like to assume that interstellar travel is impossible.
(They justify the assumption in two ways: pointing out that we haven't
been visited and building up straw man impractical interstellar
travel systems and pointing out that they are impractical).

	Suppose that this is the best we can do technologically.
When can we expect what interstellar travel?


.cb SCENARIOS FOR LAUNCHING INTERSTELLAR TRAVEL

	There are several cases.

	1. Suppose that humanity remains relatively peaceful, solves
its energy and resource problems well enough so that life is no
worse than today, and retains its enthusiasm for technology.
Suppose also that during the next thousand years
we do not become aware of any other intelligence in the universe.
In that case we can predict the time of arrival
at the nearer stars more accurately than we can predict the time
of departure.  Almost certainly interstellar expeditions will arrive
at the nearer stars before the year 3000 and most likely before 2500.

	Clearly it is pointless to start a thousand year expedition at a
time when waiting twenty years to start will reduce the journey to 900 years.
If technology based on present science improves at its present rate,
this will be true for one or two hundred years, e.g. an expedition
starting in the year 2100 will arrive before an expedition starting
in the year 2000.

	Most likely, before the first expedition, much more will be
known about the prospects of planets and asteroids around the
potential targets.  Large space-based telescopes can answer many of these
questions and are much cheaper than even unmanned probes.
A fly-by mission that accelerates all the way takes half the time
of a mission that must stop, but it must collect whatever information
it will radio back in a short time while travelling at high velocity.

	2. There is a danger that the anti-technology intellectual disease
will prevail and that humanity will lose its enthusiasm for science and
technology, abandon space travel and nuclear energy, and enter a new dark
ages in which the necessary science will be forbidden.  Fortunately, this
can't happen if even one country holds out, and the others won't or can't
conquer it.  Moreover, if anti-technology doesn't conquer the U.S.  in the
next 50 years, it will be too late, because by then there will colonies in
interplanetary space whose inhabitants will be self-selected for 
adventurousness and liking technology. Such colonists will be tempted to
declare independence anyway and will be able to launch an interstellar
expedition if they have to escape.

	3. In fact, if interstellar travel continues to require
multi-generation expeditions, escape is one of the more probable
motivations for them.  One possibility is that a colony in interplanetary
space already runs afoul of a would-be interplanetary government or
governments.  Perhaps they keep slaves, perhaps they practice free
enterprise, perhaps they won't acknowledge the true religion, perhaps
their communes are found repellent by the media or the majority or the
emperor, or perhaps they are some kind of technological hippies,
or perhaps they are the remnants of the Red Brigades or the American
Nazi Party.  In short, whether you or I would regard them as heroic
seekers or criminals fleeing just punishment, they may want to
escape.

	Whatever the reason for it, escape won't be very difficult
for a group living in space already.  If they have nuclear energy
and can make a closed eco-system, they can reach the star of their
choice.  They face two problems: their society must sustain a
multi-generation journey, and the new solar system must not be
occupied by someone willing and able to prevent their settlement.
The more kooky societies will be transformed by a multi-generation
journey into something more "average".

	Residents of Earth will find escape more difficult, and unless
they are very rich, they will only be able to send representatives
to re-establish their way of life somewhere else.  The losing side
in a war may do this; one can even imagine interstellar "boat
people".


.cb THE MULTI-GENERATION EXPEDITION

	Table 1 shows that most journeys will be multi-generation
unless a way of living longer is found.  Living longer is difficult, but
unlike exceeding the velocity of light, there is nothing in current
science that says it is impossible.

	Since the 1940s, the multi-generation expedition has been
a common science fiction theme.  Usually the writer gave the expedition
sociological difficulties so that it didn't preserve its purpose.

	It seems to me that most multi-generation expeditions
will make it provided they are not asked to preserve some goal
of the people that sent them that has no relation to comfortable
survival in a space environment.  They shouuld be no worse off
than isolated Eskimo villages that preserved and developed a
rather complex technology with only an oral tradition.
The highest imaginable exhaust velocity is the
velocity of light, and some years ago many engineers believed that this
would be the best rocket, because it would give the highest ultimate
velocity of the rocket for a given expenditure of mass.  Pessimistically
inclined scientists then pointed out that the a photon rocket must expend
enormous power in order to provide a moderate thrust.  Indeed
a photon rocket with 100 tons thrust 
 and argued that
interstellar travel is impossible.

 For some reason, most of the
discussion of interstellar communication assumes that interstellar travel
is impossible.
notes:

1. can a multi-generation expedition succeed

2. do we need an earth-like planet
	What?  Go back in the well!

3. Will civilization fission into pro- and anti-technology?
	The bureaucratic ethic.

4. 
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.begin verbatim
John McCarthy
Artificial Intelligence Laboratory
Computer Science Department
Stanford University
Stanford, California 94305

ARPANET: MCCARTHY@SU-AI
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