[music playing] NARRATOR: It's so close. For thousands of years man has
found comfort in its presence. It's been a beacon for
nocturnal travelers, a timekeeper for farmers, and
a location finder for sailors at sea. For some cultures,
it's even been a God. It's the only cosmic body
ever visited by human beings. And today NASA is planning
a permanent outpost there. But how did it get there
in the first place? How did the Moon come to be? The answer is more
astounding and spectacular than most residents of
Earth have ever imagined. At last count, over 150 moons
populate our solar system. Neptune claims 13 of them. Saturn has 48. And Jupiter hosts
an astounding 62. [music playing] Earth, on the other
hand, has just one. But it's a special one. Our Moon, Luna, as
the Romans named it, is remarkable in its size. It is by no means the largest
Moon in the Solar System. Several others are bigger. One of Saturn's Moon, Titan,
for instance, is twice the size. But our Moon is the largest in
relation to its host planet. WILLIAM HARTMANN: It's a
quarter the size of the Earth. It's really big
compared to the Earth. If you look through a telescope
at the Earth from a distance, you'd see the Earth and
this other big thing. If you look at Jupiter
or any other planet, you've got the big planet
and the little tiny moons right next to it. So our Moon is so much bigger. And it's the only one
of the now eight planets that has that situation. NARRATOR: The relative
sizes of the two bodies are close enough that
some astronomers go so far as to refer to the Earth Moon
system as a double planet. [music playing] The mean distance from
the Earth to the Moon is 234,000 miles, a three-day
flight through space. Luna's diameter is roughly one
quarter the size of Earth's measuring 2,160 miles. A single day on the Moon is the
equivalent of 27.3 Earth days. This is because one side of
the Moon permanently faces us. Luna is phase locked
with our planet. So the Moon must fully
orbit the Earth in order to make one complete rotation on
its axis, sort of like children in a game of ring
around the rosy. They always face inward as
they hold hands and move in a circle. [music playing] MAN 1: [inaudible]
all engines running. Liftoff-- we have a liftoff. NARRATOR: Yet, as closely
related as the Moon is to our Earth, it would only
take a brief visit to Luna to make it quite clear that it
is, in fact, a very different world from our planet-- [liftoff] --and an exceedingly
dangerous place. MAN 2: Houston,
Tranquility Base here. The Eagle has landed. There's no air, of course
so you need your space suit. NARRATOR: Indeed, the Moon
has absolutely no atmosphere at all. So there is nothing
to carry sound waves. [music playing] If you were standing on
the Moon with a friend and tried to
converse, the person wouldn't be able to hear
you, except by radio. MAN 3: (SINGING) I was
going on the Moon one day-- BOTH: In the merry,
merry month of December. MAN 4: No, May. NARRATOR: No
atmosphere also means that there are no air molecules
to scatter light from the sun. So the sky is always black. And the landscape does little
to help brighten the scene. WILLIAM HARTMANN:
Kind of monochromatic, you know-- not much color there. The rocks are mostly
gray, brown colors. This may be a little warmer
tone in one direction-- maybe toward the sun and cooler
tones, grayer tones looking in other directions,
but not so colorful. NARRATOR: Extreme
temperatures would also add to the unpleasantness
of a visit. The swings between hot and cold
are brutal, from 270 degrees above 0 at midday to 240
degrees below 0 at night. Even the Moon's low gravity,
just one sixth of Earth's, could present a hazard
to a tourist, a reality the Apollo astronauts kept
at the tops of their minds during their many moonwalks. EVERETT GIBSON:
But they realized that beyond the thickness of
their visor in their spacesuit was death. Should they do something
and fall into a rock and lose their footing and
fall and hit an outcrop, this visor could be cracked,
exposed to the lunar vacuum, we would have had a serious
situation on our hands. [music playing] NARRATOR: But while a space suit
could protect lunar tourists against the vacuum of
space, a lack of oxygen, temperature extremes, and
lethal solar radiation, a potential hazard
a space suit would do little to protect against is
high velocity micrometeorites. They pummel the lunar
surface frequently. Tiny meteorites, the little--
the little ones that burn up in our atmosphere and
make shooting stars-- those-- there's no
atmosphere on the Moon. They're coming right
down, hitting surface. NARRATOR: They pulverize
the lunar surface, generating a dusty blanket of
gravel-like material called regolith. Dr. Amanda Hendrix studies the
Moon at NASA's Jet Propulsion Laboratory. AMANDA HENDRIX:
This plant here is a kind of a regolith factory. It's ground-up
rock is what it is. The time scales at this
gravel mill are a lot faster. Here, it's made in
less than an hour. But on the Moon, it is the
result of more than 4 billion years of bombardment by
meteoroids and micrometeoroids raining in on the surface
and breaking up the rock. NARRATOR: Like the product
of this gravel mill, lunar regolith has formed
in a variety of grain sizes, from large rocks to find dust. AMANDA HENDRIX: This very
fine dust is familiar to us from Apollo images that were
seen of astronauts' footprints in the dust. It's very fine and
so fine, in fact, that the astronauts
had problems with it, sticking to their spacesuits,
getting into the equipment. And it can end up causing
a bit of a problem. [music playing] Really big meteorites have
also hit the Moon in the past. In fact, these
massive impacts are responsible for the
dark circular regions on the lunar surface-- the shapes that
to many observers seem to be arranged like
the eyes, nose, and mouth of a human face. They make up the illusion
of the man on the Moon. [collision] They blasted huge basins
in the Moon's surface. Some of them are
700 miles across. Dark lava eventually burst
through at the impact points and flooded the basins. Today we call these dark regions
maria, the Latin word for seas. This dates back to the 17th
century when Renaissance era observers looked up at the Moon. They speculated that the
dark areas might be oceans. And astronomers at the
time gave the many dark spots, or seas, whimsical names. DANA MACKENZIE: The seas are all
named after effects that were once attributed to the Moon. So you have the Sea of
Storms and the Sea of Crises and the Sea of
Tranquility and so forth. NARRATOR: Some of the
younger impact basins have had less time to erode. So their features are
still relatively crisp. One in particular is called Mare
Orientale, or the Eastern Sea. It's some 600 miles across. And the impact that
created it was so massive and direct the result looks
very much like the bullseye on a target. WILLIAM HARTMANN: This orientale
scar is kind of like a bullet shot in the glass, where you see
all the sort of rings around it and fractures going out. People have often said if that
had been on the side facing directly toward
the Earth, we might have had a whole different
mythology about the Moon, because it would have looked
like a big eye staring at us. [music playing] NARRATOR: Three concentric
rings of mountain ranges surround Mare Orientale. And some of the peaks rise
to several thousand feet. They're all effects
of the monster impact. When we see mountain
ranges on the Earth, they're mostly caused, first,
by the continents moving around, crashing into each other
very slowly, buckling up, and creating mountain ranges,
which then erode into all the spectacular
shapes that we see-- the Matterhorn and so forth-- from water erosion. NARRATOR: On the Moon, there are
no continental plate tectonics. The surface is static. WILLIAM HARTMANN: Yet, you still
have mountains on the Moon. And the reason is that
those big impact basins [collision] --that's a big explosion,
excavates a pit, throws material
up on the outside. So you have these rings of
mountains, arcs of mountains that surround the impact sites. But they're impact features. You know, they're caused
by the external forces, not the internal forces. NARRATOR: Another
notable impact crater, much smaller than
Mare Orientale, but every bit as
spectacular is Tycho. Named after a prominent 17th
century Danish astronomer, this impact feature is located
in the southwestern quadrant of the Moon's nearside. WILLIAM HARTMANN: It's
in the bright area. You can almost see it
with your naked eye. And when that
explosion happened, as with many other
craters, it shot out jets of bright, powdery dust. So there are these rays
that go out from it. It's very striking. NARRATOR: The rays of
dusty, ejected material extend some 900 miles from
the Tycho impact site. A mountain of debris also rises
from the center of the crater. This is a result of recoil
from the asteroid strike. Some of the highly compressed
material at the center of the impact site springs
outward once the impacting body either ricochets
or disintegrates. But long before any of the
Moon's specific features could be discerned through
sophisticated telescopes and space travel, careful
observation of lunar behavior and appearance was still vitally
important to Earth dwellers. [water rushing] For 15,000 years at least,
man revered the Moon as a source of light,
as a navigational guide, as a reference in agricultural
pursuits, and most of all-- [bell tolling] --as a convenient timekeeper. In the days before
our modern systems, timekeeping was no simple task. Early timekeepers
had two choices. They could monitor
the sun or the Moon. If you think about trying
to keep track of dates, if you use a solar calendar
like we do nowadays, there are 365 days in a year. And that's an awful lot
of days to keep track of. And it's not something
that the ordinary person can do very well. Compare that with
the lunar calendar. Everybody can tell when the full
Moon is, when the new Moon is. You don't see the
Moon at new Moon. You see it that's big
and round at full Moon. So it's easy to tell. There are only 28 to 29
days in a lunar cycle. So it's easy to count them. And so most societies
actually start out with a lunar calendar. NARRATOR: Early
observers of the Moon also recognize that our
planetary neighbor has a very real physical
effect on the Earth itself. The Moon is responsible for
the rise and fall of our ocean tides. If we think of the Moon as being
this tennis ball and the Earth as being this football,
the-- the tides are caused by the Moon's
gravitational attraction. So it's obvious-- OK, so the
Moon kind of pulls the Earth's water towards it. And so this creates
a slight bulge in the direction of the Moon. What's less obvious is there's
also a bulge in the direction away from the Moon. So there is, in fact,
two high tides every day. The other high
tide, you can think of as being caused by the
Earth's centripetal force. The Earth and the Moon
are both rotating around. And that causes the water on
the far side to also move out. NARRATOR: An extreme
illustration of the difference between high and low tides
can be found along the shore of Canada's Bay of Fundy. The water level from
high tide to low tide drops an astounding 55 feet. For some forms of life on
Earth, the advance and retreat of the tides creates
useful habitats. But another of the Moon's
gravitational effects on our planet is directly
responsible for nothing less than the continued survival
of terrestrial life itself. The Moon stabilizes
Earth's climate. The gravitational
effect of the Moon keeps the degree of tilt in
the Earth's rotational axis constant. This tilt is what maintains
the repeatable cycle of seasons as the Earth orbits the sun. ROBIN CANUP: If we
didn't have the Moon or if we had a much
smaller moon, for example, then you can mathematically show
that the tilt of our North Pole would vary widely-- [music playing] --with that angle going
from, say, 0 to 90 degrees. Currently it's 23
and 1/2 degrees. And it would actually
vary chaotically. And so the Moon has played an
important role in the stability of our axis of
rotation, of our planet, and therefore, in our climate. NARRATOR: Over the
millennia, with the Moon's prominent and constant
presence in our night sky, men ultimately began to
speculate on its origin. How did it form? How did the Moon come to be? In 455 BCE, the Greek
scholar Anaxagoras theorized that the Moon
was simply a rock that was flung off by the Earth. Most of his contemporaries,
on the other hand, were convinced that the
Moon was a god or maybe a huge ball of fire. So Anaxagoras' notion did
not get much traction. Quiet speculation,
no doubt, continued. But no hard information
about the Moon came until 1609 when Italian
astronomer, Galileo Galilei, pointed one of the
first telescopes at the Moon, recognized that
he was looking at a landscape, the terrain of another world. DANA MACKENZIE: When you look
at the Moon through a telescope, it looks completely
different from the way it looks to the naked eye. Instead of looking flat the
way it does to the naked eye, it really looks round. And you can see the shadows. You can see all these craters
that the naked eye does not see. And it just immediately
looks like a world. It jumps out at you
into three dimensions. NARRATOR: Galileo made detailed
drawings of the small planet surface and established
once and for all that the Moon is a solid
world, not a god or a fireball. But the groundbreaking
astronomer never publicly speculated on the
Moon's origin primarily because his interest soon
moved to other planets. Not until 1873 did the first
science based theory regarding the origin of the
Moon publicly emerge. It sprang from the mind of
a talented French astronomer named douard Roche. DANA MACKENZIE: Roche
advocated what's called the co-accretion theory,
which says that basically Earth and the Moon grew up at the same
time out of the same materials. NARRATOR: In Roche's
day, many scientists began to believe that the
planets might have formed from hot, condensing
clouds of gas. DANA MACKENZIE: They gradually
contracted and cooled. And as it contracted, it would
separate out rings of gas. So you'd have a
ring of gas here, a ring here, and so forth. And these rings of gas
would then eventually coalesce and form the planets. Roche saw the Earth and the Moon
as a solar system in miniature. His idea was that the Earth
starts out as a ball of gas and then cools and contracts
and sheds a ring of gas that then, itself, coalesces
and forms the Moon. NARRATOR: But there were
problems with this theory. For one, our Moon has a
much lower iron content than the Earth's. If the two bodies formed
from the same materials, their basic composition
should be the same. But they are not. This and other inconsistencies
soon led fellow astronomers to quest for new ideas
to explain the existence of the Moon. In the last third
of the 19th century, advanced theories about
the origin of the Moon started to emerge. In 1873, French scientist
douard Roche proposed that the Moon simply formed alongside
the Earth out of, essentially, the same nebular cloud
of particles and gases. But this idea had a
fundamental weakness. The Moon has a much lower
iron content than the Earth's. It's much less dense. The big thing to
remember about the Moon and its composition is that
it doesn't have any iron core like the Earth does. So you look at the Earth. And there's a very large
central area, something like half the inside of the
Earth, is iron-- nickel iron. And that's metal drained down
to the center of the Earth when the Earth was hot, and it
formed at the beginning. The Moon is more
like just plain rock. [music playing] NARRATOR: Scientists initially
deduced the mass of the Moon through observation and
mathematical calculations. If the Moon had formed from the
same stuff that made the Earth, the iron content
should be similar. It was a hole in the theory
douard Roche couldn't explain. But another idea soon
followed on the heels of the co-accretion hypothesis. In 1878, George Darwin
announced his vision theory of lunar origin. This idea received
some attention in part because Darwin had a celebrated
father, Charles Darwin, author of "Origin of Species." In time, though, George
Darwin stepped out of his father's shadow and
became known as England's leading expert on tides. And through extensive analysis
of the tide-Moon relationship, George Darwin came
to the realization that the Moon is gradually
moving farther and farther away from the Earth. DANA MACKENZIE: It wasn't
proved until 95 years later when astronauts
landed on the Moon. They put little
mirrors on the Moon. And you can shine a
laser at the Moon. And the laser will bounce
off the mirror, come back. And you can actually
measure the exact distance between the Earth and the Moon. And the rate at which the
distance is increasing is 3.8 centimeters per year. NARRATOR: 3.8 centimeters
is about an inch and a half. DANA MACKENZIE: If you made
a movie, extreme timelapse, you would see that the
Moon moving away from Earth gradually-- that's
what we see nowadays. [music playing] NARRATOR: Darwin began to
consider what would happen if you reversed the process,
if you ran the movie backwards. As we move backwards in time
and the Moon moves closer, both the Moon's orbit and
the rotation of the Earth get faster and faster. And so, well, what happens is
that eventually the Moon must coalesce with the Earth. It must-- it must hit the Earth. NARRATOR: The logical
conclusion for Darwin was that a portion of the
molten, rapidly spinning Earth must have separated
from the main mass and spun off to become our Moon. He immediately began work
on mathematical calculations to reverse the
trajectory of the Moon all the way back to the Earth. DANA MACKENZIE: Frustratingly,
he reaches a point where it gets almost to Earth. And then he couldn't
work it out any farther. The mathematics doesn't
let you go any farther. What you get to is a point
where the Moon is whipping around the Earth at a rate
of five revolutions a day or six revolutions a day. So it's just zzzzzz, zooming
around the Earth, OK? And it's-- it's about 5,000
miles away from the Earth. NARRATOR: Still, the
mathematics did not allow Darwin to bring the two
cosmic bodies into contact. The fission theory was
debated for decades. But scientists
eventually concluded that the relative movements
of the Earth and Moon could not have resulted from it. The Earth would have been
spinning too fast to account for its present rotation rate. The quest for an explanation
of lunar creation continued. And a new theory would soon
originate with an American. In 1909, Thomas
Jefferson Jackson See was a US Naval captain, based at
Mare Island near San Francisco. His official job was to keep the
standard time for the US West Coast. But See was also a
gifted scientist. As a young man, he had
trained as an astronomer. DANA MACKENZIE: He was
one of the first Americans to actually get a
doctorate in astronomy. He went to Germany and
got a PhD in astronomy, which was, you know, almost
unheard of back then. The US was still scientifically
a total backwater in the 1800s. NARRATOR: See's duty
station, Mare Island, had an observatory. And the captain's job allowed
him plenty of time to theorize. He had spent time analyzing
both the co-accretion and the fission
hypotheses regarding the origin of the Moon. And he wasn't convinced. Gradually, Thomas See developed
a completely different idea. It came to be called
the capture theory. Essentially, See theorized that
the Moon had actually formed in a different part
of the solar system from the Earth, that it
orbited the sun, just like the other planets,
but that at some point, it had moved too
close to Earth and was captured by Earth's gravity. His idea was that
there is something that he called the resisting
medium in outer space, which we know is not there now. NARRATOR: See never
adequately explained what this resisting medium
was supposed to be-- possibly tiny
particles of matter. Regardless, he was sure
it no longer existed. DANA MACKENZIE: His idea
is that for the Earth to capture the
Moon, the Moon would have to come in from far away. And then it would hit
this resisting medium, and slow down and then gradually
get captured into an orbit-- maybe not at once, might
take a little while. But gradually it would be
captured into Earth's orbit. It's a little bit like a bungee
jumper jumping off a bridge-- go down, come back
up not quite as far, go down and come back up. [music playing] You know, and eventually they'd
settle down at the bottom. NARRATOR: Thomas See's
capture theory would certainly explain the difference in
iron content between the Earth and the Moon. If the Moon formed
elsewhere, its composition could be very different. On the other hand, the notion
that Earth's gravity could capture and retain
such a large object was unlikely since
there is no obvious resisting medium to slow down
an object as big as the Moon. All three theories had
significant weaknesses. The origin of the Moon
remained a mystery. [music playing] On July 20, 1969, US astronauts
set foot on the lunar surface for the first time. NEIL ARMSTRONG: That's one small
step for man, one giant leap for mankind. NARRATOR: They had
landed their lunar module on the hardened lava plane
known as the Sea of Tranquility. And mission pilot Buzz
Aldrin described the view as magnificent desolation. BUZZ ALDRIN:
Beautiful, beautiful. NEIL ARMSTRONG:
Isn't that something? BUZZ ALDRIN: Magnificent
sight out here. NARRATOR: The astronauts brought
back 48 pounds of Moon rocks and dusty soil, or regolith. CREW: [inaudible]
position 1-3-3-0. [inaudible] NARRATOR: Geologists at
Johnson Space Center in Houston eagerly awaited their arrival. The excitement when
samples first came back was just electric. I mean, it was just incredible. Nobody really knew
quite what to expect. And we realized right away
that we had basaltic rocks, like one might see in Hawaii-- in fact, very similar to
rocks one would see in Hawaii. NARRATOR: The NASA
geologists were intrigued to find not
only basalts, rocks of another sort-- rocks called breccias. Breccias formed during
asteroid impacts. GARY LOFGREN: So
if you can imagine, a large impact body
hitting the Moon is going to be very chaotic
it's going to throw up debris. Debris is going to come flying
out of this, what winds up to be a crater after it's over. And you can see the many
craters on the Moon. This debris gets all jumbled
up and lands on the ground around the crater. And then it gets compacted
with the heat generated by the crater and
gets turned into rocks that look very chaotic. You see all sorts of pieces in
the rock of different sizes, different shapes, all pieces of
the rock that were in the area that the impacting
body impacted. We have just not seen
anything like this on Earth. [music playing] NARRATOR: The Moon
rocks soon began to tell a fascinating tale. First, the rocks
and soil samples contain particles indicating
that the Moon must have been covered by a deep ocean
of lava after it formed. This notion was reinforced
by the discovery that the rocks were
lacking significantly in what scientists
call volatile elements. ROBIN CANUP:
Volatile elements are those that can evaporate easily
and therefore be lost when you heat up a rock. And some examples
of volatile elements include water or
potassium, for example. And if you compare the bulk
Earth rocks to lunar rocks, you find that the lunar
rocks are extremely parched. It's as if they've been
heated, and they've lost a lot of their
volatile elements. NARRATOR: But along with
these stark contrasts, the lunar samples also
revealed at least one astonishing similarity between
the lunar surface and rocks and soil from Earth. WILLIAM HARTMANN: The isotopes
of individual elements like oxygen, in particular-- you have different
forms of oxygen-- the Moon had exactly the same
ratios of different forms as the Earth did. But all the other rocks
we knew from elsewhere in the solar system, which
are meteorites that fall out of space, all have different
oxygen isotope ratios, which tells you that the Moon
material and the Earth material are very, very similar. NARRATOR: In the end,
the lunar samples had supplied a wealth of
hard evidence regarding the geological
makeup of the Moon. But astronomers trying to
understand the Moon's origin were still left with a puzzle. For William Hartmann of the
Planetary Science Institute in Tucson, Arizona,
the information gleaned from the Moon
rocks supported some ideas he'd been working on
for nearly a decade. Now a distinguished astronomer
and painter of space images, in the early '60s, Hartmann
was a graduate student at the University of Arizona,
taking part in a project to map impact craters on the Moon,
from the enormous basins or seas to the smallest visible dimples. We realized during this
mapping in the '60s that the big basins are
actually impact features. Very large asteroids
hit the Moon and made these huge explosions. Some of those are
600 miles across. How big an object does
it take to do that? Something like 100 miles across. So we had large
objects running around in the inner solar system
as the Earth was forming. And they were
crashing into planets. NARRATOR: For Hartmann,
the notion that 100 mile wide asteroids once
impacted planets begged a couple of questions. Could planet-sized objects
have ever collided? And could that have
something to do with the formation of the Moon? By 1972, Hartman and fellow
Tucson astronomer Don Davis had created a computer
program to help them explore these ideas. The program made a rough attempt
to simulate the accretion process in the
early solar system. The astronomers wanted to see
if any other planetary bodies formed near the Earth that
could have crashed into it. WILLIAM HARTMANN:
Well, the idea was, if there was some
other body there, and it finally crashes
into the Earth and blows off all this crustal
material from the Earth, as well as from the
impactor itself, maybe the Moon could
form out of that. NARRATOR: Sure
enough, the simulation showed that a second
planet could have formed in the Earth's accretion zone,
one about the size of Mars. It wasn't the Moon,
because it had formed from the same elements
as the Earth and should, therefore, have had the same
iron core and heavy density as the Earth. The Moon, of course, does not. That was evidence
enough for Hartmann to formulate a new
fourth hypothesis. It came to be called
the giant-impact theory. [collision] In 1974, a new hypothesis
explaining the origin of Earth's Moon debuted on
the world scientific stage. Its proponents dubbed it
the giant-impact theory. The basic idea is that about
4 and 1/2 billion years ago, Earth collided with an
object roughly the size of the current planet Mars. It's a very large collision. And it started the
Earth spinning. It's what gave us our current
24-hour day, we believe. And this collision
was so massive that it launched material
into orbit around the Earth. And it's from that material
that we believe the Moon later coalesced. NARRATOR: Dr. Robin Canup
and others at the Southwest Research Institute in Boulder
have constructed a computer model to study the details
of the giant-impact scenario. ROBIN CANUP: It divides up
the Earth and this colliding rogue protoplanet into
many different particles. And then the evolution
of each particle is tracked through the
course of the impact. NARRATOR: The simulation depicts
the impact from a bird's eye view, from the top down. ROBIN CANUP: The Earth was
probably partly molten even before this impact. So initially the impactor
came in from this direction. It hit the Earth with
an off-center collision. And you can see the impactor
has been stretched out into this long arm of
material right here. NARRATOR: Scientists believe
the upper layers of the Earth became completely molten
following the impact. The collision starts
the Earth rotating. And you can also see that the
collision has substantially distorted the shape
of the Earth itself. After a couple hours, we see
that this arm of impactor material has coalesced
gravitationally into two large clumps. The inner clump of
the impactor material is actually composed
overwhelmingly of the impactor's core so
that when this inner clump recollides with the Earth,
which happens right there, the vast majority of the iron
that came in with the impactor is actually accumulated
by the Earth while this outer clump comes
in and makes a close pass by the Earth and is stripped
by the Earth's gravity into a long arm of material,
which then breaks up to form this disk. NARRATOR: The material is
thought to have coalesced to form the Moon in
less than a year, following the massive impact. There is no sign of this
impact on the Earth today, because at the time, our planet
had only developed to about 90% of its current size. The remaining 10% would
accumulate from later, much smaller impacts. Also, the Earth's own gravity
had a reshaping effect. ROBIN CANUP: Within a
day after the impact, the Earth had reassumed a
basically spherical shape. And any depression
that the impact caused would have been smoothed over. Giant-impact theory
originator Bill Hartmann presented his hypothesis at a
scientific conference in 1974. But it received little
attention for nearly a decade. Interest in the Moon
dropped off sharply at the end of the
Apollo missions. CREW: We had a beautiful,
very smooth, very quiet ride. NARRATOR: Finally, at a lunar
conference in Hawaii in 1984, 12 years after the
last Moon shot, the world's foremost
astronomers reached a consensus. The giant-impact theory was
the most acceptable explanation to date for the
origin of the Moon. DANA MACKENZIE: I think there's
still some people who say, OK, it's not really proven yet. But it's what I would
call the default theory. If you look at any textbook of
planetary science or geology, it's just the theory
that's accepted. And there are certain
small discrepancies, which have yet to be explained. But by and large, it works. NARRATOR: Even with
an agreed upon theory of how our Moon came
to be, scientists have not finished studying
our closest cosmic neighbor by any means. In fact, talk of returning
to the lunar surface with a permanent manned
outpost has recently emanated from the White
House and NASA headquarters. ASTRONAUT: Gorgeous
views out the window. CREW: [inaudible] sequence
good, everything go. NARRATOR: Such a base
could provide a place to train astronauts to
live in space long term, as well as provide a more
efficient launch point for eventual missions to Mars. But such lofty
scientific goals are far from the minds of
average residents of Earth, who occasionally peer up at
the glowing, mesmerizing lunar disk in the night sky. For us, talk of lava
oceans, regolith forming micrometeorites,
and giant impacts will never diminish the
fascination and romance of our mysterious
neighbor world, the Moon. [music playing]
