# Thread: moon leaving earth's gravity...possible?

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## moon leaving earth's gravity...possible?

the moon is spiraling away from the earth at a rate of 38 mm per year. I heard on a tv show in the past week, I forget which one, that the moon can and will break free of Earth's gravity sometime in the future. The number 25,000 was stated, but I can't recall if that was 25,000 more miles (?) to go before the moon goes bye-bye...at a rate of 38 mm per year, or about 1.49607 inches per year. I remember doing some fast calculations on my tiny calculator and got 16,000 somethings before we are in big trouble, but don't remember if I converterted this to that properly. I am positive about the 38mm per year, however. Anyone hear anything else, recently? I realize there was a similar thread about this question in 2005, but I just heard the "new" 38mm only a week ago. Serious replies, please. Thanks, Sara.

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Welcome, sara7.

The rate of recession will slow as the Moon moves farther away. It will stop receding when it gets to be about 1.4 times its present distance, at which time its orbital period will be around 47 days, and the Earth's rotation will have slowed to match that period: Earth and Moon will always turn the same face towards each other. It's estimated it will take about 50 billion years before that happens, which is long after the Sun turns into a red giant.
The distance at which the Moon stops receding is about one third of the Earth's "Hill radius" (the region in which Earth's gravity dominates over the Sun's). That's far enough out to make the Moon susceptible to perturbations from the gravity of other planets, but not far enough to make it certain that the Moon will wander off on its own.

Grant Hutchison

3. Originally Posted by sara7
I remember doing some fast calculations on my tiny calculator and got 16,000 somethings before we are in big trouble, but don't remember if I converted this to that properly. I am positive about the 38mm per year.
Just out of curiosity, why do you think we would be in big trouble even if the moon were to escape from earth's gravity?

4. I think it is safe to say that any of us will be in big trouble in 16,000 somethings. But the Moon cannot leave the Earth due to the effect you mention (it might due to the effect grant hutchison mentions), because the Moon saps angular momentum from the Earth's spin as it moves outward, and the Earth only has enough angular momentum in its spin to move the Moon so far-- roughly the distance that grant hutchison described. From then on, it would be up to other processes to expel the Moon, if they did indeed occur. You raise a good point though-- if the Moon were made vulnerable to being stripped by other forces at that distance, it would still have a significant tidal influence, and I expect that stripping the Moon, even at 1.4 times its current distance, might cause earthquakes or some such thing-- though it would probably also happen rather slowly and perhaps we wouldn't be so affected at all. In any event, the Sun will be so much different that life on Earth would probably not exist any more (Grant was correct that the Sun would have turned into a red giant by then, but also, it would have gone beyond a red giant into a white dwarf, and a pretty cool one at that).

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Originally Posted by Jens
Just out of curiosity, why do you think we would be in big trouble even if the moon were to escape from earth's gravity?
That would leave a planet-sized thing in an Earth-crossing orbit.

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Originally Posted by grant hutchison
It's estimated it will take about 50 billion years before that happens,
50 Gyr? Wow.

7. "moon leaving earth's gravity...possible? "
Well, no, actually.
One of gravity's extraordinary properties is its ability to act at any distance. Yes, ANY distance.
The Sun's planets influence each others orbits.
The mass of the Milky Way, keeps the Sun in orbit.
The Small and Large Magellanic Clouds, and other satellite star groups orbit the Milky Way which, with the Andromeda Galaxy, orbits as part of the Local Group of galaxies.
All linked by gravity and the last about 100 Million light years across.

You never leave anything's gravity!

John

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Well yes, and no, JohnD. sara7's OP was asking about whether the Moon's 38 mm per year would is constant or variable, and how long it would take before it ceased being a Moon and became just another solar satellite, same as Venus, Mars, or any one of many asteroids in the belt.

It is currently far too closely bound to Earth for anything smaller than the near miss of a comparably-sized body to fling it from it's orbit, but in a billion years, when it's 23,000 miles further? Three billion?

Even then, the likelihood of any such body still being around is slim, so I'd hazard to say the Moon will be with us throughout it's destruction (along with the Earth's) when out Sun expands and gobbles us both.

9. Originally Posted by sara7
the moon is spiraling away from the earth at a rate of 38 mm per year. I heard on a tv show in the past week, I forget which one, that the moon can and will break free of Earth's gravity sometime in the future. The number 25,000 was stated, but I can't recall if that was 25,000 more miles (?) to go before the moon goes bye-bye...at a rate of 38 mm per year, or about 1.49607 inches per year. I remember doing some fast calculations on my tiny calculator and got 16,000 somethings before we are in big trouble, but don't remember if I converterted this to that properly. I am positive about the 38mm per year, however. Anyone hear anything else, recently? I realize there was a similar thread about this question in 2005, but I just heard the "new" 38mm only a week ago. Serious replies, please. Thanks, Sara.
As to often the TV show probably got the science terribly wrong. This is either by ignorance and accident or on purpose because it makes for a more exciting story.
As it has been stated by the time our moon had enough time to drift out to the distance where it would be tidally locked our sun would be well over 10 times its current age. Its like you worrying about how you are going to support your great great great great great great great great great grandchildren after world war 10 happens.

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The moon has a planet size thing = Earth approximately in it's orbital path every 27 days. That will increase very slowly to 47 days in about 50 Gyr. Since they are and will be orbiting each other; collision can't happen unless the orbit is perturbed by a massive body within a million kilometers or so, or some other unlikely event. There are lots of possibilities in 50 billion years. Neil

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The "25,000" figure may be the speed in miles per hour which a
body falling all the way from the Moon to the Earth has when it
hits Earth's atmosphere. That applied to the returning Apollo
spacecraft.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
The "25,000" figure may be the speed in miles per hour which a
body falling all the way from the Moon to the Earth has when it
hits Earth's atmosphere. That applied to the returning Apollo
spacecraft.
No,· the Apollo astronauts didn't somehow come to rest relative to Earth and then drop on it from the distance of the moon.

13. Originally Posted by whimsyfree
No,· the Apollo astronauts didn't somehow come to rest relative to Earth and then drop on it from the distance of the moon.
No. and it was not meant to be understood that way. Jeff was just putting things right... We are talking of the moon and its drifting away....
The moon is about 390,000 km away, thats 238,000 miles. and yes it is drifting away at about 38mm or 1.4 inches per year... This subject has been talked through at length and a search would find that thread.... but as you now know. The sun will grow and engulf the Earth and Moon before the moon gets to be tidally locked... and geostationary...
" The film record of this event is in the Libraries of Magrathea." according to Douglas Adams....

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25,000 miles per hour is 11 km.s-1, which is Earth's escape velocity, which is a good approximation of the re-entry velocity for the returning Apollo lunar missions. It's also a good approximation for the velocity of something falling to Earth from rest at the distance of the Moon (in the absence of the Moon itself), or from the L1 libration point between the Earth and Moon (which is perhaps more representative of the route taken by Apollo).

Grant Hutchison

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Asimov pointed out that we do not have a moon, we have a co-planet. I.e. the moon does not orbit the earth, but we dance around a common point located outside of the earth's surface. (Definition of a moon has the common point inside of the planet.) The force of attraction acting on the moon are first the Sun, and secondly the earth. Same holds for the earth. The sun is the controlling force. I know, just a technical point, but the watered down stuff the TV's shows feed the masses can't deal with anything too technical.
From Wikipedia: The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass. http://en.wikipedia.org/wiki/Natural_satellite That holds true for our Earth and Moon.

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Originally Posted by SparkyVA
Asimov pointed out that we do not have a moon, we have a co-planet. I.e. the moon does not orbit the earth, but we dance around a common point located outside of the earth's surface. (Definition of a moon has the common point inside of the planet.)
The centre of gravity of the Earth-Moon system (around which both bodies revolve) actually lies well inside the Earth. The Moon has about 1/81 of the Earth's mass. Moon and Earth are separated by about 384400 kilometres. The barycentre is therefore 384400/82 = ~4700 km from the centre of the Earth, while the Earth has a radius of 6378 km. Call it a thousand miles underground, and you're not far wrong.

From memory, what Asimov pointed out was that the Moon's orbit is everywhere concave towards the Sun, which is unusual for a major satellite.

Grant Hutchison

17. Originally Posted by grant hutchison
The Moon has about 1/81 of the Earth's mass. Moon and Earth are separated by about 384400 kilometres. The barycentre is therefore 384400/82 = ~4700 km from the centre of the Earth, while the Earth has a radius of 6378 km.
And note that 1.4 * 4700 = 6580 > 6378, so it sounds like perhaps 50 milion years from now our Moon really will qualify as a coplanet!
From memory, what Asimov pointed out was that the Moon's orbit is everywhere concave towards the Sun, which is unusual for a major satellite.
Interesting point.

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I made a diagram for a discussion here early last year, of the
Moon's orbit around the Sun, to show the geometry pointed out
by Isaac and Grant:

http://www.freemars.org/jeff2/MoonOrb1.png

The image is 1600 pixels wide, so you may need to maximize or
widen your image viewer window to see the whole thing.

I've seen many people suggest that the barycenter being inside or
outside the primary should be what determines whether a body is
a moon or not, but that is only a suggestion, it isn't an "official"
definition. Earth's Moon clearly orbits the Earth, as well as orbiting
the Sun.

-- Jeff, in Minneapolis

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Originally Posted by Ken G
And note that 1.4 * 4700 = 6580 > 6378, so it sounds like perhaps 50 milion years from now our Moon really will qualify as a coplanet!Interesting point.
Ken G. Yep, and I'd like to be the first to name the baby....Pluto. pete

20. Also, the barycenter of the Sun-Jupiter system lies outside of the sun, but I don't think anybody has suggested that Jupiter shouldn't be considered a planet (satellite) of the sun. So it has to be a little bit more complicated than that. Also, with regard to the issue in general, I should point out that "moon" is a word coined by humans to describe a category that we perceive. But nature doesn't have to follow our categories.

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