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HOW TO GET AND
HOLD ON TO A MOON
C
ounting out from the Sun, Earth is the
rst planet with a satellite, or moon.
Mercury, closest planet to the Sun, doesn’’t
have a moon, nor does Venus. Mars, the
fourth planet out, has two moons, but they
are probably just a couple of big old rocks
that ended up in Mars’’ orbit after they were
fully formed.
It is suspected that Earth didn’’t have a moon
at rst, but acquired one early in its history as
a result of a gigantic planetary collision.
Visualize the event as it may have happened
about 4.5 billion years ago.
MAKING THE MOON
Earth was pretty much formed as a planet.
Most of the dust and gas in the region had
been pulled in, and the proto-Earth was
revolving around the Sun more or less in the
orbit it travels today. However, these were
the early days of the Solar System. There
were a lot of large chunks of matter ying
around in unstable orbits. Some of the
chunks were huge——the size of small planets.
Planetary scientists now think that one of
these large planetesimals, perhaps the size of
Mars, was traveling around the Sun in an
exaggerated elliptical orbit. It’’s not known
why it had such a peculiar orbit——perhaps it
was pulled by the gravitational inuence of
a large planet, or perhaps there were lots of
such strange objects early in the Solar
System’’s history. Anyway, it ended up
heading for Earth.
Had you been on Earth to witness the event,
the incoming object would have rst
appeared as a dot in the heavens. Over a
period of days and weeks, it grew bigger
and bigger until it completely lled the eld
of view above Earth. Then it struck.
Because the colliding object was so large,
the impact itself seemed to happen in slow
motion, lasting several minutes, even
though the planetesimal was traveling at
perhaps 40,000 km/h.
What chaos must have followed the crash!
The incoming object was destroyed on
impact, reduced to vapor, dust, and chunks.
Large surviving parts were driven deep into
the interior of Earth. A signicant portion of
Earth was destroyed as well. The energy
that resulted from the crash produced an
explosion of unimaginable magnitude.
The force of the impact threw a tremendous
quantity of matter into motion——at least 20
billion cubic kilometers of matter. One
portion of the matter, the pieces traveling at
67
542-1460_Force_RB_Pgs_1-76.indd 67
5/7/08 11:46:59 AM
the highest speeds, ew out into space, never
to be seen again. Another portion of the matter ew up into the air and then was pulled
back to Earth by gravity. Some of the falling
matter fell back almost immediately as huge
rocks, some a little later as granules of various
sizes, and some months or even years later in
the form of dust and chemicals held aloft in
the atmosphere.
A third and signicant portion of the debris
didn’’t y off into space, and it didn’’t return to
Earth. It began orbiting Earth in a disk, like
the rings of Saturn. The ring was probably
about two Earth diameters from the surface of
Earth.
Over the next millions of years, the force due
to gravitation started bringing the pieces of
matter together. Tiny grains formed larger
and larger chunks, which eventually all
pulled together to form the Moon.
the mass, the stronger the force due to
gravitation.
The gravitational force plays two major
roles in planetary system formation.
Gravitational force pulls bits of matter
together to form massive objects. If a little
bit of matter is pulled together, the shape
of the object might be irregular because the
gravity will not be strong. Asteroids are
funny shapes because of their low mass. If a
lot of matter accumulates, however, the
mass will be pulled into a sphere. The other
role of gravitation is to hold planets and
satellites in orbit. The straight-line paths of
planets are deected into circular paths by
the constant application of a force. That
force is the universal force of gravitation.
GRAVITY VERSUS VELOCITY
GRAVITY AND ITS EFFECTS
When the planetesimal hit Earth, matter
ew everywhere. Pieces of matter launched
with high velocity escaped Earth’’s
gravitational attraction and began a phase
of existence as loose space debris. Matter
that didn’’t achieve escape velocity
continued to be inuenced by Earth’’s
gravity. It’’s just that simple.
Gravity is one of the four known forces
in the universe. Gravity, along with
electromagnetism and two forces at work in
the nucleus of atoms, makes everything in the
world behave in ways we understand. It is
the force due to gravitation that causes two
masses to attract each other. Gravitational
force exerted by a small mass, like a marble
or an apple, is so small that we can’’t detect it.
But the force exerted by a large mass, like a
planet or a star, is tremendous. The larger
Before we think more about the matter that
didn’’t y into space, consider another piece
of information about the behavior of mater.
Isaac Newton gured out that an object in
motion will travel in a straight line forever
unless it is acted on by a force that changes
its direction. In other words, things don’’t
travel in curves, circles, spirals, zigzags, or
any other nonstraight paths unless
something acts on them to change their
motion.
We had a moon where previously there was
none, and it must have been a sight hanging
up there maybe 30,000 km above Earth, rather
than the 385,000-km distance we see today.
68
542-1460_Force_RB_Pgs_1-76.indd 68
5/7/08 11:26:35 AM
F
gra
Imagine taking a yo-yo by the end of its
string and swinging it around over
your head in a nice circle. If you
let go of the string, what happens?
The yo-yo stops going in a circle
and ies off in a straight line. As
long as you keep applying a force
(pulling on the string) to change the
direction of the yo-yo, it continues to
orbit your st.
y
vit
F gra
Earth
“Natural” path of Moon
vit
y
Back to the impact debris. Some of the matter
fell back to Earth in the usual way. But some
of the matter that ew out in a straight line
had its path altered by the force due to Earth’’s
gravity. Take a rock the size of a trash can
as an example. It ew off in a straight line
like that shown on the illustration labeled
““Natural”” path of Moon. If there were no
gravity, the rock would keep going off into
space. However, the force of gravity pulls the
rock toward Earth. The pull didn’’t bring the
rock to Earth’’s surface, but it did change the
direction of the rock’’s travel. Remember, an
object travels in a straight line until acted on
by a force. The force that changed the rock’’s
direction of travel in this case was due to
Earth’’s gravity. The continuously altered path
of the rock brought it into orbit.
Fgravity
Force due to gravitation is the ““string””
pulling on the Moon to keep it in a circular
path. Similarly, gravitation is the force
keeping Earth in a circular orbit around the
Sun. In fact, everything that is going around
something else in the Solar System is doing so
because of gravitational force. Gravity rules!
69
542-1460_Force_RB_Pgs_1-76.indd 69
5/21/08 3:35:26 PM
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