Search for Extraterrestrial Intelligence
Roadblocks to Intelligence
In a pursuit of an objective look at the probability of alien visits to our
planet, as well as a look at the probability of life existing elsewhere in the
Universe, one must first begin without any a priori opinions. First, one must
assume that there is no Creator, or if there is, that the Big Bang and natural
selection plus random events led to intelligent life originating on earth. By
no means am I attempting to discredit any particular religion, but for this
argument to effectively work, we must move away from anything which might require
a step of faith which leaps beyond logic, and is untestable. You are welcome
to hold to any personal faith in a Creator that you might have. This section
of the course looks at the extreme difficulty in accounting for the presence
of intelligent life ANYWHERE in the Universe, much less here and in lots of
other places. Looking at things from a strict mathematical probability standpoint
is the goal, and then you can draw your own conclusions later relative to your
The following pieces of information have been pieced together from articles
in "Sky & Telescope Magazine," "Astronomy Magazine,"
and other resource books. The work on this document is still in progress, but
here is the information which attempts to demonstrate how difficult it is for
circumstances to be sufficiently correct for life to not just evolve, but to
get to the present level of high and conscious intelligence. Most students come
into an Astronomy course with apriori assumptions, well-grounded by years of
inundation with television, theater, and literature dealing with extraterrestrials
and UFO's. Please keep an open mind when working through this page, and consider
what all must happen simultaneously for us to be here.
Major Fact About the Materials in Life
Scientists agree, but fiction writers do not when it comes to discussing the
material upon which life will be based. Life will be based on elements that
naturally occur in nature. Stars are formed from hydrogen and helium. In their
cores, nuclear fusion creates other elements, and when the star dies, many of
the elements are released into space. Some of the most common stellar elements
are Oxygen and Carbon. Oxygen is the one of the most electronegative elements
on the Periodic Table (see both charts below - top is periodic table, and bottom
holds the electronegativity values).
Due to the properties and abundance of elements scattered by stars, oxygen
readily binds hydrogen and makes water. Water is everywhere in the Universe.
What is so amazing is the properties that are unique to it ... 1)cohesion, adhesion
means it sticks to itself and to other things, 2)high heat of vaporization means
it takes a lot of energy to changes from liquid to gas, and thus prevents the
oceans from boiling away everyday, 3)it floats on itself when it freezes ...
and water is the ONLY solid that floats on its own liquid state. Imagine the
disaster is winter lakes and oceans froze from the bottom up, 4) Water dissolves
ionic compounds unlike any other liquid. Water
is common throughout space. In fact, there are vast nebulous clouds, light
years in diameter, that are made of frozen water.
Beyond the value of water, carbon is a common element in space, and carbon
has the property of being able to form chains and build structures. No other
element has the properties of carbon and can do what it does.
Because carbon, oxygen, and hydrogen form such wonderful molecules, it is
expected that life will be based on carbon-oxygen compounds. It certainly might
look like us, with it will be made of the same stuff. Indeed, deep
space nebulae contain amino acids that are the product of chemisty in cold,
clouds of dust and gas. If amino acids, hydrocarbons, and water are the compounds
that are made in space, then we conclude that any lifeforms will be based on
these molecules, and thus the properties of the host star and planet must be
agreeable with biomolecules.
1) The host star must be
of the right variety to provide stable temperatures for billions of years. The
following bullet points demonstrate why most stars in space do not qualify as
candidates for life-support. The HR Diagram is given below for your reference.
a) It cannot belong to a binary system (eliminates 2/3 of stars) since planets
with circular orbits cannot readily form there due to tremendous tidal forces
from the double star system, and if planets do form and then orbit the system,
the radiation flux would be overwhelmingly varied. At times, the planet would
receive the energy from 2 stars, and then from only 1 star as it orbits the
binary. It is estimated that 70% of the stars in space are binary systems or
composed of even more that two stars. Below is a photograph of Alberio, the
head of Cygnus the swan, and a blue-orange binary system. An additional problem
with binary systems is that the mass is not distributed equally between the
two stars. Therefore, one star will evolved faster and pass through advanced
life stages sooner than the other, and when this occurs, planets around the
companion are threatened by the death events of the larger star.
b) It must be the right size
smaller stars require the planet to orbit
closer to get in the habitable zone (the orbital area where conditions are suitable
for life), but this would create greater tidal forces, slowing the rotation
like that of Mercury, which results in one side of the planet always facing
the star, and the other side in darkness. The temperature extremes would make
evolution of advanced life impossible
c) If the star were too large it would burn out before life could
get a foothold, since upper level mass stars burn and die within millions of
years instead of billions, and thus get rid of their planets in their deaths
before life could get a start on the planet.
d) The spectral class must be near G because these stars produce
predominant energy in the visible spectrum which is far less harmful on the
types of biomolecules which the universe would make randomly. Hot O, B, and
A stars shed so much radiation that biomolecules are destroyed.
e) The star must come from a previous Supernova event or else
the heavier elements upon which life depends cannot be on the planet. Population
I stars are composed exclusively of hydrogen and helium gases, and any planets
around them are made of the same two elements. Only stars that begin their lives
in excess of 10 solar masses might generate enough internal gravitational pressure
due to mass. This permits the entire sequence of nucleosynthesis up to iron,
triggers a core collapse, and detonates. In the extreme energy supernova explosion
event the rest of the elements in the periodic table are created and flung out
into space. From this explosive event might come a future collapse of the debris
cloud into a Population II star with sufficient number of elements for the evolution
of life. Since only 5% of stars in space explode as Supernovae, this makes such
an event rare. Sure, there are plenty of Population II stars out there, but
they are still far less common that the Population I class. The Veil Nebula,
pictured below, is the result of a supernova event a long time ago.
2) The host galaxy must be a spiral. Elliptical galaxies (by far the most
common form and seen below left) appear to be populated with Population I stars
which are hydrogen/helium mixes. These large galaxies lack the starbursting
pattern necessary for repeated regeneration of Population II stars from parental
material. Imagine entire galaxies that are not candidates for life-forming star
systems. Most of the galaxies in the Universe are eliptical (lower left), and
thus not of the right type for life because they do not recycle their stars
as spiral galaxies do (lower right).
a) Spiral galaxies (above right and below) have been shown to be composed of
more material than the visible matter can account for. Large lobes of dark matter
called density lobes, orbit within the spiral galaxies, and at rates slower
than the visible arms. As the spiral arms pass through these lobes, the dark
matter mass interacts with the visible matter, pushing stars, igniting stars
from nebulae, and disrupting cometary clouds thought to exist in the distant
gravitational fields of stars. Repeated passage through density lobes poses
repeated threats to life-hosting planets due to shuffling of cometary bodies.
The comets are pushed toward the Sun, and if the Earth is in the way, it is
struck and much of life is probably extincted. The galaxy below has density
lobes and spiral arms where star bursting is occurring.
b) The spiral galaxy has a habitable zone that is quite narrow. Stars in the
galactic core are Population I stars, and additionally most spirals have a massive
black hole which generates harmful radiation beams. The inner arm stars are
sufficiently close to the core that they experience excessive gravitational
interactions with density lobes of dark matter throughout their orbits that
planetary systems are disrupted. The outer arm stars might not experience enough
gravitation interaction with density lobes of dark matter to regenerate stars
from supernova debris.
tenth of stars may support life
19:00 01 January 04
NewScientist.com news service
One tenth of the stars in our galaxy might provide the right conditions to support
complex life, according to a new analysis by Australian researchers. And most
of these stars are on average one billion years older than the Sun, allowing
much more time, in theory, for any life to evolve.
The concept of a "galactic habitable zone" (GHZ) for
the Milky Way was first proposed in 2001. Now Charles Lineweaver of the University
of New South Wales and colleagues have defined a life-friendly GHZ using a detailed
model of the evolution of the Milky Way to map the distribution in space and
time of four major factors thought essential for complex life.
"We're looking at what we think are the most robust and
conservative pre-requisites for life - but they are very, very basic,"
The researchers conclude that a ring-shaped habitable zone emerged
about eight billion years ago, roughly 25,000 light years from the core of the
Milky Way. The zone has expanded slowly and includes stars born up to about
four billion years ago. It encompasses close to ten per cent of all stars ever
born in this galaxy.
But other researchers say that too little is known about the
prerequisites for life for this kind of mapping to have a great deal of meaning.
"We hardly understand the origin of life, let alone the
evolution of complex life. Until we do, it is extraordinarily difficult to talk
about habitable zones," Mario Livio of the Space Telescope Science Institute
in Maryland, US, told Science.
Lineweaver stresses that his team is not arguing that complex extra-terrestrial
life is probable, or even exists, within their GHZ. "What we're saying
is that this is the region that has the most potential for the formation of
complex life," he says.
The first factor the team considered in mapping the GHZ was the
presence of host stars for a solar system. The second was the presence of sufficient
heavy elements to form terrestrial planets. The third was a sufficiently safe
distance from exploding supernovae. And the fourth was enough time for biological
evolution. The team set this figure at a minimum of four billion years, since
this was the amount of time it took for complex life to emerge on Earth.
Future work will test the importance of some of these factors,
the team adds. Only about 100 extra-solar planets have been spotted to date,
and these are all Jupiter-like gas giants. But the launch of NASA's Terrestrial
Planet Finder telescope in about 2013 will mark the start of a major search
for nearby planets that could harbour life.
Journal reference: Science (vol 303, p59)
c) The host galaxy cannot have gamma
bursts anywhere near, nor once life really gets a foothold. Gamma ray bursts
(seen in artistic impression below) are among the most extremely deadly events
in the universe, and they happen frequently on a cosmic time scale. These events
sterilize large regions of the host galaxy. If gamma ray bursts are as common
as currently revealed, it seems difficult to believe that any planet could have
highly evolved life forms over billions of years of time without being sterilized
by Robert Matthews at New Scientist
London - January 21, 1999 - Gamma-Ray bursts -- incredibly powerful explosions
that may be caused by collisions between collapsed stars -- could solve one
of the oldest riddles about extraterrestrial civilisations: why haven't they
reached Earth already? After studying the effects of gamma-ray bursts on life,
an astrophysicist has concluded that aliens may have just started to explore
Enthusiasts for the existence of extraterrestrials have long been haunted by
a simple question supposedly posed by the Nobel prizewinning physicist Enrico
Fermi around 1950. Fermi pointed out that the Galaxy is about 100 000 light
years across. So even if a spacefaring race could explore the Galaxy at only
a thousandth of the speed of light, it would take them just 100 million years
to spread across the entire Galaxy. This is far less than the Galaxy's age of
about 10 billion years.
So if ETs exist in the Milky Way, where are they? Maybe they don't share the
human urge to explore. Or perhaps there's another reason, says James Annis,
an astrophysicist at Fermilab near Chicago. He thinks cataclysmic gamma-ray
bursts often sterilise galaxies, wiping out life forms before they have evolved
sufficiently to leave their planet (Journal of the British Interplanetary Society,
vol 52, p 19). GRBs are thought to be the most powerful explosions in the universe,
releasing as much energy as a supernova in seconds. Many scientists think the
bursts occur when the remnants of dead stars such as neutron stars or black
Annis points out that each GRB unleashes devastating amounts of radiation.
"If one went off in the Galactic centre, we here two-thirds of the way
out on the Galactic disc would be exposed over a few seconds to a wave of powerful
gamma rays." He believes this would be lethal to life on land.
The rate of GRBs is about one burst per galaxy every few hundred million years.
But Annis says theories of GRBs suggest the rate was much higher in the past,
with galaxies suffering one strike every few million years -- far shorter than
any plausible time scale for the emergence of intelligent life capable of space
travel. That, says Annis, may be the answer to Fermi's question. "They
just haven't had enough time to get here yet," he says. "The GRB model
essentially resets the available time for the rise of intelligent life to zero
each time a burst occurs."
Paul Davies, a visiting physicist at Imperial College, London, says the basic
idea for resolving the paradox makes sense. "Any Galaxy-wide sterilising
event would do," he says. However, he adds that GRBs may be too brief:
"If the drama is all over in seconds, you only zap half a planet. The planet's
mass shields the shadowed side." Annis counters that GRBs are likely to
have many indirect effects, such as wrecking ozone layers that protect planets
from deadly levels of ultraviolet radiation.
Annis also highlights an intriguing implication of the theory: the current
rate of GRBs allows intelligent life to evolve for a few hundred million years
before being zapped, possibly giving it enough time to reach the spacefaring
stage. "It may be that intelligent life has recently sprouted up at many
places in the Galaxy and that at least a few groups are busily engaged in spreading."
2) The star must begin its life surrounded by a protoplanetary disk (1/2 of
single stars do) and the disk must be small. Planets with medium or massive
disks are doomed to spiral their way into the host stars.
3) The system must be devoid of large planets with elliptical orbits (which
are being discovered currently), that would eject or destroy any smaller planets.
So, we cannot have Jupiters orbiting the system in an elliptical path, and yet
we need Jupiters for other reasons.
4) Large planets with circular orbits are needed --- at the right distance
--- to sweep the way clear of those pesky asteroids or comets what would otherwise
strike inner planets with regularity, causing mass extinctions every 100 thousand
years instead of every 100 million.
5) Earth has to be in a perfect place within its circular orbit inside of
the habitable zone. If it were 5% closer, during the early ages, a runaway greenhouse
effect would have boiled all the oceans away. If it were 1% farther, there would
have been runaway glaciation 2 billion years ago, with a mean global temperature
of -50oF and permanently frozen oceans.
Life Zones and Suitable
Stars for E.T.
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This material (including images) is copyrighted!. See Nick's
copyright notice for fair use practices. I received permission to place
it into this course.
The life zone, or habitable zone, is the distance from the star where the temperature
is between the freezing point (0° C) and boiling point (100° C) of water.
If you consider a planet with the same reflectivity (clouds and surface material)
as the Earth, reradiates the solar energy it absorbed as efficiently as the
Earth does, and rotates as quickly as the Earth does, then the life zone for
the Sun (a G2 main sequence star) is between approximately 0.63 and 1.15 A.U.
Calculations that include the effects of the greenhouse effect and whether or
not there is a runaway process and ultraviolet dissociation of water like what
happened on Venus shift the Sun's life zone outward so that the Earth is nearer
the inside edge of the life zone. Climate research is still at the beginning
stages of development, so the life zone boundaries are a bit uncertain.
The life zone of a hotter main sequence star will be farther
out and wider because of the hotter star's greater luminosity. Using the same
line of reasoning, the life zone of a cooler main sequence star will be closer
to the star and narrower. You can use the inverse square law of light brightness
to determine the extent of the lifez ones for different luminosity stars. The
boundary distance is
star boundary = Sun boundary × Sqrt[(star luminosity)/(Sun luminosity].
For example, if the Sun's life zone boundaries are 0.9 and 1.5 A.U, the inner
and outer bounds of the life zone for a star like Vega (an A0-type main sequence
star with (Vega luminosity/Sun luminosity = 53) are 6.6 to 10.9 A.U., respectively.
For a cool star like Kapteyn's Star (a M0 main sequence star with Kapteyn's
star luminosity/Sun luminosity = 0.004), the life zone stretches from only 0.056
to 0.095 A.U.
Despite the fact that hotter, more massive stars have wider life zones, astronomers
are focusing their search on main sequence stars with masses of 0.5 to 1.4 solar
masses. Why are these types of stars more likely to have intelligent life evolve
on planets around them? Let's assume that it takes 3 billion years for intelligence
to evolve on a planet. You will need to include main sequence lifetime and the
distance and width of the star's life zone in our considerations.
First consider the lifetime of a star. The star must last at least 3 billion
years! Use lifetime = (mass/luminosity) × 10 billion years = 1/M3 ×
10 billion years if the star's mass is in units of solar masses. The most massive
star's (1.4 solar masses) lifetime = 3.6 billion years (a 1.5-solar mass star
with a lifetime = 3.0 billion years would just barely work too).
The less massive stars have longer lifetimes but the life zones get narrower
and closer to the star as you consider less and less massive stars. At the outer
boundary of the life zone the temperature is 0° C for all of the stars and
the inner boundary is at 100° C for all of the stars. You can use the observed
mass-luminosity relation L = M4 in the life zone boundary relation given above
to put everything in terms of just the mass. Substituting M4 for the luminosity
L, the 1.4-solar mass star's life zone is between 1.76 A.U. and 2.94 A.U. from
the star (plenty wide enough). The 0.5-solar mass star's life zone is only 0.23
A.U. to 0.38 A.U. from the star. Planets too close to the star will get their
rotations tidally locked so one side of planet always faces the star (this is
what has happened to the Moon's spin as it orbits the Earth, for example). This
actually happens for 0.7-solar mass stars, but if the planet has a massive moon
close by, then the tidal locking will happen between the planet and moon. This
lowers the least massive star limit to around 0.5 solar masses.
Any life forms will need to use some of the elements heavier than helium (e.g.,
carbon, nitrogen, oxygen, phosphorus, sulfur, chromium, iron, and nickel) for
biochemical reactions. This means that the gas cloud which forms the star and
its planets will have to be enriched with these heavy elements from previous
generations of stars. If the star has a metal-rich spectrum, then any planets
forming around it will be enriched as well. This narrows the stars to the ones
of Population I---in the disk of the Galaxy.
6) The habitable planet must be large enough to hold an atmosphere and small
enough so that its gravity doesn't crush everything on its surface. If its mass
is too small, it cannot hold water vapor; too large and it will hold on to hydrogen,
methane, and ammonia, like Jupiter. In fact, unless a planet has a mass at least
0.85 of the Earth's but no larger than 1.33 of the Earth's mass, temperature
variations would make the planet uninhabitable within 2 billion years. Mars
(below and left) is too light in mass to hold water, Uranus (below and left)
is too massive to release hydrogen, and Venus (below and center) is the right
mass but has a rotational spin that makes the surface unbearably hot.
7) The planet must be a member of a double-planet system, like ours. The moon
acts as a stabilizing anchor for the Earth from undue attraction by Jupiter
that would otherwise cause Earth to tilt too far on its axis, which would lead
either to a runaway greenhouse effect or to a permanent ice age. The odds of
the collision which formed the moon yielding enough mass is 1/1,000,000.
8) Our sun was 30% dimmer when the Earth was born, but as the sun's heat increased,
the Earth's surface managed to stay within a 25-degrees-centigrade window while
the warming greenhouse gases disappeared (see Sky
& Telescope article). Plant life appeared at just the right time to
reduce the accumulating carbon dioxide. How Earth did not experience a runaway
greenhouse effect or permanent ice age is difficult to conceive.
9) In order to cause precipitation and to keep from freezing, a habitable
planet must have some mechanism to keep carbon dioxide from disappearing. The
presence of liquid water begins a chain reaction that always depletes the atmosphere
of carbon dioxide. The atmospheric carbon dioxide combines with water to make
carbonate that precipitates as a solid and sinks into the ocean or falls onto
the ground. Slowly, the Earth would deplete its entire store of carbon dioxide
gas. However, Earth recycles carbon dioxide. On Earth, parts of the surface
always sink to depths where carbonate decomposes to create carbon dioxide and
then cycles to the surface via volcanic activity, replenishing the atmosphere.
We observe no other planet with plate tectonic activity like Earth. Why should
we assume that all habitable planets in the universe would be so?
10) Plate tectonics plays a crucial role in maintaining a balance between
continental and oceanic crust rocks. Our Earth has just the right low-density
continental rock composed of granitic rock, causing it to maintain itself on
average just 125 meters above sea level, while the oceanic crust, composed of
basalt, gives us an ocean floor that averages about 2.5 miles below sea level.
Without a continuous recycling of materials through nonstop plate tectonic activity,
granitic magmas would not give us our high-standing land masses for very long,
and we would soon have a water world. No other planet has been able to generate
the sizable patches of stable continental crust as we see here.
11) A habitable planet must be inhabited. Contrary to what is commonly taught,
microbiologists still do not have a clue as to how the first living thing formed
from inorganic matter. A combination lock may contain 20 bits of information,
representing about a million possible combinations. "It would seem impossible
for the prebiotic Earth to have generated more than about 200 bits of information,
an amount far short of the 6 million bits in E. coli by a factor of 30,000.
For E. coli to just happen into existence from random chance would require 10^1,800,000
possible combinations and only would be correct. The simplest known organism
which is capable of independent existence includes about 100 different genes.
For each of 100 different genes to be formed simultaneously (in 10 billion years)
the probability is 10^-3,000. For them to be formed at the same time, and in
close proximity, the probability is much lower. The most currently accepted
theory for the origin of life is that biomolecules were concentrated in water,
trapped in fissures of the Earth's crust. Geothermal heat provided energy for
these molecules to collide and begin associations, creating proteins from amino
acids, RNA from free nucleotides, and micelle membranes from free phospholipids.
The membranes surrounded RNA that was self-replicating with the help of randomnly
made proteins to do the process, energized by ATP. And, this life form existed
almost immediately after the period of meteor bombardment ended, 3.8 billion
12) During the Earth's entire 4.6 billion year history, only one species out
of an estimated 50 billion developed here, have evolved to high intelligence.
If intelligence has such a high value, why has it not developed more frequently?
Robert Naeye in Astronomy, July 1996 concludes, "On Earth, a long sequence
of improbable events transpired in just the right way to bring forth our existence,
as if we had won a million-dollar lottery a million times in a row. Contrary
to the prevailing belief, maybe we are special
it seems prudent to conclude
that we are alone in a vast cosmic ocean, that in one sense, we ourselves are
special in that we go against the Copernican grain."
To date SETI has found nothing, infrared satellites have found no signatures
of life, and it appears we are alone. Enrico Fermi figures that there are billions
of stars in our galaxy alone which are older than our sun. If life routinely
develops, then there should be many civilizations in our galaxy that have had
billions of years to advance in their space travel abilities before us. Even
if their travel speeds never exceed those of our own age by much, any civilization
with a desire to colonize should be able to settle the entire galaxy within
about 5 million years. Assuming 10 billion years for the age of our galaxy,
this means that there have been at least 2,000 chances for such a cycle to occur
for each early civilization. Where are they?
Science has supplied us with ample evidence that our universe and time itself
had a beginning. Now
explain where it all came from
Mass and energy can be converted into each other, but neither mass nor energy
can appear from nothing
1st Law of Thermodynamics
If the universe is becoming more disordered, where did the initial order come
2nd Law of Thermodynamics
More odd accidents of nature
1) The resonance of carbon is so perfect, that just a bit smaller, and it
would not form, just a bit greater and it would destroy itself. Without it,
life cannot exist.
2) The proton is 1,836 times heavier than the electron. We know of no reason.
If it were ANY different, molecules would not form
no chemistry, no physics,
3) If the relationship between the four forces of nature were ANY different,
we would either have no atoms, or we'd have atoms but no stars of planets. If
electrical forces were greater, no element heavier than hydrogen would form.
If electrical repulsion were too weak, then protons would combine too easily,
and stars would blow up in short time. If the strong force were too weak, multiproton
nuclei would not hold together. Stronger forces would have caused the entire
universe to fuse all of its hydrogen into helium. Without hydrogen, stars would
4) The odds that all of the functional proteins necessary for life forming
in one place by random events is one chance in 10^40,000. Since there are only
about 10^80 subatomic particles in the entire visible universe, the physicists
conclude that this was an "outrageously small probability that could not
be faced even if the whole universe consisted of organic soup."
5) There is a very tight correlation between the relative strength of gravitational
forces and electromagnetic radiation in stars. Outside of an exceedingly narrow
range and all stars become either Red Giants or Blue Dwarfs
impossible. If the strength of such gravitational force were altered by one
part in 10^40, we'd have a world of all red dwarfs or blue giants.
6) Given the random distribution of gravitating matter, it is far more probable
that the early universe should have quickly collapsed into a black hole. The
present arrangement of matter and energy, with matter spread thinly at relatively
low density, in the form of stars and gas clouds would, apparently, only result
from a very special set of initial conditions. Roger Penrose estimates a figure
7) If our universe had been expanding at a rate that was slower by a factor
of one part in one million, then the expansion would have stopped when it was
only 30,000 years old. Too rapid expansion, and no stars or galaxies would have
8) Why didn't all of the matter and antimatter annihilate soon after the beginning?
Apparently, only one part in 10^10 survived to form light atomic nuclei three
minutes into the universe's age, and then after a million years form atoms.
ask why we are here at all. Why we have not been destroyed in all
of our galactic orbits. The location of the continents. The fast spin of this
planet. The correct axial tilt. The right distance. On and on and on.
Arroway is wrong in the movie "Contact." It was not a waste of space,
but necessary so that in one place, life could develop, and even then, it never
Any yet, people continue to believe that aliens exist. They continue to believe
that aliens have visited our planet, abducted humans, and manipulated human
genetics and influenced our greatest leaders throughout history. Perhaps we
need to look more closely at alien encounters and learn some truths. Please
move forward to Roswell ... the town in New
Mexico where a UFO crashed in 1947 and set off a huge worldwide surge of interest.
Return to SETI introduction or the Syllabus.
DON'T FORGET TO DO THE SETI
ASSIGNMENT! when you are finished with this unit.
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