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Q: The Moon - How it might crash into Earth and how it might fluxuate our gravity ( No Answer,   7 Comments )
Subject: The Moon - How it might crash into Earth and how it might fluxuate our gravity
Category: Science > Astronomy
Asked by: vikrtain-ga
List Price: $30.00
Posted: 17 Sep 2004 23:57 PDT
Expires: 17 Oct 2004 23:57 PDT
Question ID: 402804
I need to know how it might be possible for the moon to crash into the
Earth, no matter how outrageous the concept.  If an asteroid were to
hit the moon (big enough that it tears a chunk of the moon away),
could that (albeit unlikely) push the moon into a new orbit around the
Earth?  If this new orbit brought the moon alternatingly closer and
farther than normal, could it create a change in gravity and/or the
magnetic pull that would put it on a collision course?  Also, if the
moon were headed towards Earth is there a way that our gravity could
be affected to such extremes that people either feel momentarily
weightless or objects crumble?  I know this probably would never
happen, but I need a couple of senarios, no matter how extreme or
ridiculous.  Anyone who can give me some real help on this VERY
quickly will be eligible for additional research services and fees. 

Clarification of Question by vikrtain-ga on 18 Sep 2004 07:43 PDT
Although I'm looking for extreme examples, I'm only interested in
answers from people who actually have knowledge in this area and
senerios that are based in some kind of reality.  Also, I recognize
the moon is in a stable orbit, but perhaps there is something
currently unknown to science that could change that.  The rest of the
question still stands.  Thanks.

Request for Question Clarification by pafalafa-ga on 18 Sep 2004 08:47 PDT

Interesting question...a bit scary, a bit odd...but interesting just the same.

A lot of thought seems to have gone into impacts of large objects with
the earth, but I haven't seen much having to do with the moon.  Two
things did pop up however:

1)  At this site:

the author speculates that a large object could be captured into earth
orbit and if so, it would likely (see bottom of the page) smack into
the moon after only a few years of orbiting.  The consequences of this
impact are not discussed, but presumably a large enough asteroid could
throw the moon out of orbit.

2)  It's generally thought that the moon was created by the impact of
a Mars-sized object with the planet earth.  Given that such impacts
are possible (extrememly rare....but possible), then a similar impact
with the moon could certainly throw it out of orbit and crash it (or
the object itself) into the earth.

If I find any fuller details about these scenarios, I'll post it as an
answer.  But for now, I just wanted to get your feedback as to whether
these are the sorts of scenarios you're seeking.


Clarification of Question by vikrtain-ga on 19 Sep 2004 06:00 PDT
Thank you, pafalafa.  I guess what I'm looking for is this:  It seems
plausable enough that if a large enough asteroid hit the moon it could
knock it into a different orbit.  I would like to justify the idea
that if this happened, somehow the moon is no longer able to resist
Earth's gravitational pull and is slowly sucked into our planet.  AND,
as while this is happening, it somehow has an effect on OUR gravity
here on Earth.  Thank you much.
There is no answer at this time.

Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravity
From: probonopublico-ga on 18 Sep 2004 00:19 PDT
Once the hole in our Ozone Layer is big enough, there would be a real
opportunity for a Huge Hurricane (possibly Hurricane Oink?) to escape
into the stratosphere and, if this were to happen at nightime,
Hurricane Oink would automatically head for the most powerful light
that it could find ... The Moon!

This would be a completely new phenomenon for The Moon and it would
then become seriously disorientated. Its natural curiosity would then
require it to investigate the source of Hurricane Oink and to head for
Earth but if The Moon were to get too close ... Wow! It could get
sucked into Earth by the Force of Gravity.

Then, everybody on Earth had better duck.

Scary, isn't it?
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravity
From: neilzero-ga on 18 Sep 2004 04:39 PDT
In case you didn't guess, Probono is teasing you with technobable.
Most scientists agree the orbit of the moon is very stable, and it
will not crash into Earth unless a body with more mass than Earth
misses the moon by less than 1000 miles. There are many objects that
could do this, but as far as we know none will pass though our solar
system in this century. Even if one does, they are all but certain to
miss our moon by more than 1000 miles.
 In most senarios of collision, the moon is broken into small peices,
a few of which would hit Earth.
 The largest asteroid in our solar system, Ceries, is also in a very
stable orbit. If it hit the moon, changes would be measureable, but
tides would not change much. The present orbit of the moon is out of
round by about 10%. That might increase to 20%, following impact, but
a much smaller change is more probable. 20% out of round would still
be very stable.
 If our moon missed Earth by 1000 miles, the gravity on Earth's
surface 1000 miles below the moon's surface would be reduced
temporarily by about 10%. A few badly built stuctures would fall
apart. Humans woud perceive that Earth was falling away from the moon,
which would be scarry. 90% percent of Earth's surface, would be little
affected except for the moon appearing in an unusual position. The
close passage would trigger earthquakes and volcanos. The tide effect
of Earth on the moon would cause the moon to disintigrate, but the
effect would not be generally observed until after the close approach.
Please embellish, refute, or comment.  Neil
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravity
From: pugwashjw-ga on 20 Sep 2004 02:56 PDT
TechnobaBBle is scary.
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravity
From: touf-ga on 21 Sep 2004 12:31 PDT
The problem with the moon is that the gravitational force of the moon
on the moon's surface is only 1/6 that of the earth on the earth's
surface, and has no known magnetic field.  This means that even if the
moon were in a "treetop orbit" meaning it were touching the tops of
our heads, gravity here would be reduced to only 5/6 what it currently
is -- not a big enough difference to make objects weightless.  Plus,
the magnetic field would not be affected.

I recommend you look at this posting;  it is related to your topic:
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravity
From: iang-ga on 21 Sep 2004 15:32 PDT
>Plus, the magnetic field would not be affected.

Not directly, perhaps, but some disturbance of the Earth's outer core
seems likely.  That in turn would have an effect on the magnetic

Ian G.
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravi
From: astrojohn-ga on 27 Sep 2004 20:18 PDT
The correct answer here involves conservation of angular momentum.
Right now, the earth's tides are acting to slow the spin of the earth.
Since angular momentum must be conserved in the Earth Moon system, the
Moon's orbit is increasing.  This is called spin-orbit coupling.
So to get the Moon to decrease it's orbit to zero (i.e. crash into the earth),
then one only needs to spin up the earth's rotation.

Also, the Moon won't crash into the Earth.  Tidal forces will rip it
apart once it is inside the Roche lobe.  We will have a ring of lunar
material in orbit.
Subject: Re: The Moon - How it might crash into Earth and how it might fluxuate our gravi
From: qed100-ga on 10 Oct 2004 21:02 PDT
Hi vikrtain,

What we're talking about here is an orbital maneuver, a transfer from
one orbit to another. Let's say the Moon is practically in a circular
orbit. (It's not really, but close enough.) The Moon is traveling from
west to east. To drop it to a lower orbit, it must lose some of its
speed in that direction. So let's say that an asteroid slams into it
going east to west. The Moon loses some orbital speed. But   that
alone cannot deposit it into a lower circular orbit; what's been done
is to change its orbital perigee, the lowest point on its orbit with
Earth. The apogee, the highest point, stays the same. So it ends up in
a more eccentric elliptical orbit which dips down to a closer approach
to Earth.

   In order to make its orbit into a lower circular orbit, it's
necessary to impart yet another change in the Moon's speed, right as
it approaches the new perigee. Now of course, this is at least
partially what you've asked for; an orbit which takes the Moon
alternately close to and away from Earth. But it's still true what
others have called out, which is that there simply won't be any weird
gravitational behaviors, such as regional weightlessness, objects
rolling uphill, and so on. Additionally, the only massive space bodies
which will collide retrograde with the Moon are comets, since they're
the only ones with a large spectrum of inclinations to the ecliptic,
including retrograde. All asteroids travel essentially west to east.

-Mark Martin

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