<blockquote>
<p>My understanding is that the moon was created a long time ago when Earth was hit by a big asteroid.</p>
</blockquote>
<p>A big asteroid? If you want to call Mars a &quot;big asteroid&quot;, then yes, the giant impact hypothesis says that the Earth was hit by a big asteroid. If the giant impact hypothesis is correct, the mass of the impactor was significantly larger (by a factor of eight to ten) than the mass of the Moon. The vast majority of the impactor's mass fell back onto the proto-Earth. A very small amount may have escaped.</p>
<p>The rest of the debris, roughly a tenth of the impactor's original mass, had enough energy to orbit but not enough energy to escape. The debris cloud then circularized, and then attracted itself.</p>
<p>This might be too pretty of a picture, which leads to my next point:</p>
<blockquote>
<p>This seems extremely unlikely.</p>
</blockquote>
<p>That this may be extremely unlikely is one of the proposed solutions to the <a href="https://en.wikipedia.org/wiki/Fermi_paradox" rel="noreferrer">Fermi Paradox</a>, which asks why alien beings haven't colonized the Earth: Where are the aliens? If intelligent life requires a planet in the Goldilocks zone, a Goldilocks collision that creates a massive moon that stabilizes the planet's orientation, a Goldilocks amount of water, and a Goldilocks climate that keeps the climate relatively stable for over a billion years, then perhaps intelligent life is extremely rare. We humans may be here because our planet was one of the few winners in an intergalactic lottery in which almost every planet is a loser.</p>

CC BY-SA 4.0
2021-01-30T13:14:22.010Z
Thanks for replying! Re: goldilock zone for life, I should have used a different word, because I really meant the « just right speed to orbit without escaping ». My new understanding is that  there is a wide range of speeds that will neither crash nor escape, which means that the odds of a moon ´staying put’ are higher than I expected.
Zoltan
CC BY-SA 4.0
2021-01-31T16:17:58.903Z
@Zoltan The key thing you might not have realized is that orbital speed and distance are tied together. The average Earth-Moon distance is ~385,000 km; *to stay at that distance*, the Moon must maintain the orbital speed it has. But there's a wide range of distances the Moon *could* be orbiting at, stably, and so also a wide range of speeds.
zwol
CC BY-SA 4.0
2021-02-02T13:38:44.707Z
A simpler explanation to the Fermi Paradox could be that time and space are too big compared to our own small neighborhood and a few thousand years of written history or millions years of archaeology. Life could have started and vanished, or will start after we are no longer here.
Rsf
CC BY-SA 4.0
2021-01-30T11:14:22.580Z
2021-01-30T11:14:22.580Z
David Hammen
<p>I just want to add some numbers. The International Space Station orbits with the speed of 7.66 km/s (27,600 km/h). On the other hand, Earth's escape velocity is around 11 km/s (40,000 km/h). This means that anything with the speed in that range will orbit Earth. So it doesn't need to be some kind of a precise speed for objects to remain in Earth's orbit. Granted, the orbits of fragments from the Giant impact had different shapes, but over millions of years they started to clump together and eventually the largest object cleaned the smaller fragments because orbits of different shapes are not sustainable in the long term.</p>

CC BY-SA 4.0
2021-01-31T22:38:43.727Z
Can you clarify why you can compare the orbital speed (which is tangential, i.e. sideways) with escape velocity (which you might assume is radial, i.e. straight up)?
gidds
CC BY-SA 4.0
2021-02-01T13:37:03.510Z
@gidds Escape velocity might actually be better named escape *speed*, since the direction does not matter. If you are moving at 11km/s relative to earth *in any direction* (so long as it's not one that results in a collision with earth itself), you have sufficient kinetic energy to go as far as you want from earth (i.e. escape).
Nuclear Hoagie
CC BY-SA 4.0
2021-02-02T07:28:57.417Z
Obligatory XKCD: https://xkcd.com/1356/
I'm with Monica
CC BY-SA 4.0
2021-01-30T20:50:50.790Z
2021-01-30T20:50:50.790Z
stackzebra
<p>You might reasonably assume that the result of the collision was a big cloud of debris of various sizes.  Some bits will have come rapidly back to what was left of Earth, other bits will have gone flying out into space, and some bits stayed more or less in orbit for long enough to coalesce into the Moon.  At this scale, solids behave more or less like liquids and so both Earth and Moon formed into roughly spherical shapes.  Mars' moons, being somewhat smaller and (IIRC) newer, are rather less spherical.</p>

CC BY-SA 4.0
2021-01-31T01:51:27.740Z
2021-01-31T01:51:27.740Z
user37879
<p>A point not mentioned above is that the primordial swarm of material that would eventually form the Moon would, on average, have the same direction of orbit around the Earth as the Moon does today. In that case, we shouldn't ignore collisions between that material where the radial momenta are cancelled and leave only, or mostly, the tangential momenta (possibly with coaleced material). Similarly, collisions with a component parallel to the axis of the orbit would generally reduce momenta parallel to the axis of the orbit and lead to a disk of material (as seen for Saturn's rings). Self-gravity would then create 'lumpiness' in the disk and the dominant lump would preferentially attract material to itself and form a single body. It's not jut an orbital dynamics problem but also one of momentum exchange and aggregation statistics.</p>

CC BY-SA 4.0
2021-02-01T15:30:43.100Z
2021-06-04T12:48:16.463Z
2021-06-04T12:48:16.463Z
Dr Paul A Daniels
<p>I&quot;m not sure if I understood your question or I just didn't answer your question right. However, the earth has been bombarded by many debris floating around in the early solar system. This was due to the fact that in the belt around the sun various aggregations of material happened through mutual attraction of smaller materials, that grew. This is how all the planets formed in star systems. At a certain period in time the diverse clumped together debris where big enough to be called proto planets and the rest was still called debris or asteroids.</p>
<p>The theory you are mentioning is called the <a href="https://en.wikipedia.org/wiki/Giant-impact_hypothesis" rel="nofollow noreferrer">giant impact theory</a> or the Theia impact. Earth was not struck by an asteroid, but by another proto planet called Theia. It is assumed that this has happening a lot, also in other star systems. The fact that from this impact the earth and the moon formed in the form they currently have, is unique in our solar system. We only see it with the earth. However, impacts between proto planets is not considered to be rare in the early solar system, so the chance that the Theia impact could happen might have been small, but considering the chance of colliding proto planets was very high, the possibility was there.</p>
<p>Now back to your remark that the moon has exactly the right speed to stay in orbit with the earth. This is not true. The moon actually has a speed too high to stay in orbit and the distance between the earth and the moon is getting bigger each year. High precision measurements are made and suggest that the moon is spiraling away from the earth with a speed of about 4 centimeter per year. Of there the course of the last 4 billion years, this has thus been quite a distance and it suggests the speed is not exactly right, but a bit too high.</p>
<p>Still, a Theia-like collision between Venus and another proto planet would not have resulted in a planet moon system, due to the tidal forces from the sun. A similar thing would apply for Mars, but here Jupiter is the reason for the absence of such a pair. The outer planets are too big and would have not resulted in material being able to escape the planet's gravity.</p>
<p>So, yes, the existence of the earth moon can be seen as a unique situation, but this is not because the kind of impacts didn't happen. It is because the earth was just at the right distance from the sun and Jupiter to result in such a pair. Impacts like the Theia impact were not rare. Do, however, understand that the Theia-impact is a theory that has good acceptance, but is not undoubted. There are several questions about the validity of this theory.</p>

CC BY-SA 4.0
2021-01-30T13:18:02.327Z
Thanks for replying! I also read about the 4cm distance increase, I ´ll argue that if it is slow enough for the moon to still be around after 4bn years then it is pretty much at the ideal speed:).  My newfound understanding is that a wide range of speeds will lead to a moon that isnt going too slow or fast, so a moon is much more likely to stay in place than I thought
Zoltan
CC BY-SA 4.0
2021-01-30T13:53:12.593Z
Is it really correct to say that the moon is going faster then a certain value? The fact that the Moon goes slowly away isn't really because it has a speed exceeding that of a special orbit. It cannot even be spelled... Isn't? I would say that being the orbiting bodies not rigid than there is acceleration /transfer of momentum...... Just to say that the sentence "thus is not true. The moon actually...." seems - although I somehow understand what you mean - suggesting something untrue to OP, or just is not a very happy formulation.
Alchimista
CC BY-SA 4.0
2021-02-01T14:03:37.787Z
The additional 4cm per year is due to tidal effects.  The energy comes from slowing the Earth's rotation.  Eventually the Earth and Moon will keep the same face to each other, meaning that there will be no tides - provided nothing happens to disturb the system before then.
Rich Farmbrough
CC BY-SA 4.0
2022-12-24T19:38:23.230Z
The Moon at perigee is moving too quickly to remain in *circular* orbit *at that height*, so it rises.  Likewise, the Moon at apogee is moving too slowly to remain in *circular* orbit *at that height*, so it falls.
Anton Sherwood
CC BY-SA 4.0
2022-12-01T17:12:49.967Z
regarding "speed too high to stay in orbit[...] therefore expanding distance": I was under the (perhaps incorrect) impression that, once in orbit, one would *decelerate* to achieve a higher orbit. No?
Yorik
CC BY-SA 4.0
2021-01-30T10:56:39.410Z
2021-09-25T20:49:26.650Z
2021-09-25T20:49:26.650Z
MacUserT
<p>There isn't a &quot;Goldilocks speed&quot; for orbit.  If you put two objects in space, and give them a velocity relative to each other, then provided that velocity is less than the escape velocity (at their relative distance) the two objects will orbit each other.</p>
<p>Those orbits will be elliptical, and it is possible that the ellipse is skinny and &quot;eccentric&quot; enough for the two bodies to collide when they are closest to each other. But for an object that is several hundred thousand km from Earth, there is a quite a wide range of possible elliptical orbits.</p>
<p>So when (and if) the grand collision happened, there was a huge amount of matter that was ejected up into space.  Some probably was moving so fast that it escaped, Some certainly went into orbits that didn't have enough energy and so were small skinny ellipses and the matter fell back to Earth.  But there was a lot that ended up in some kind of elliptical orbit.  This matter was not all in the same orbit, but it started to coalesce, and form into a single ball, under its own gravity.</p>
<p>Other moons weren't formed like this, they either formed at the same time as their planets as a &quot;mini solar system&quot; (such as the four major moons of Jupiter) or they were captured from the asteroid or Kuiper belts).  Initially, the captured moons may have had rather elliptical orbits.</p>
<p>But most moons are in rather circular orbits.  Even if the moon was originally in an elliptical orbit, tidal effects will tend to make the orbit more circular.  A planet and moon system has a certain amount of angular momentum and a certain amount of energy.  The angular momentum can't change, but energy can be converted into heat and since tides dissipate some energy as heat, the orbit will tend to change to a shape that minimizes energy, for a given amount of angular momentum.  That shape is a circle.  (See <a href="https://astronomy.stackexchange.com/questions/28962/is-the-moons-orbit-circularizing-why-does-tidal-heating-circularize-orbits">Is the moon&#39;s orbit circularizing? Why does tidal heating circularize orbits?</a>)</p>
<p>So the effect of tides is to give moons the &quot;Goldilocks speed&quot; that keeps them in a circular orbit.</p>

CC BY-SA 4.0
2021-01-30T13:09:25.287Z
Thanks for replying! I think this answers my point of « having the just right speed » by saying «  there is a wide range of speeds that will neither crash nor escape, correct?
Zoltan
CC BY-SA 4.0
2021-01-30T13:15:59.383Z
Basically, that means that the odds of a moon « staying put » are higher than I thought and you dont need to generate thousands of moons to keep one.
Zoltan
CC BY-SA 4.0
2021-01-30T13:51:15.303Z
Now that I think of it, isnt it simply because billions of rocks were thrown into orbit *each* at a different speed, and those that were at the « just right » speed eventually gathered while the others either escaped or fell back? That way I dont even need a wide range of « just right » speed.
Zoltan
CC BY-SA 4.0
2021-01-30T15:07:59.123Z
Lots of stuff would have been thrown into space,  once out in space it all would start to orbit, on lots of different orbits (there are animations of this) Some of those orbits would intersect with the ground, but there is quite a range of orbits that don't.  Those rocks then joined up to make a moon. If the moon's orbit was not circular then tides would make it circular.  There is quite a range of speeds that are potenially stable. For an orbit of the moon its roughly between 200m/s and 1400 m/s  (transverse velocity at lunar distance)
James K
CC BY-SA 4.0
2021-01-31T15:21:48.840Z
@Zoltan: "those that were at the « just right » speed eventually gathered while the others either escaped or fell back" -- that wouldn't account for why the Moon is a large-ish proportion of the mass that could plausibly have been ejected by the collision. You could perhaps see the Moon's speed as the *average* of all the stuff that was in the large range of "good enough" speeds. To see for sure that there isn't a "just right" speed, you could perhaps look up the actual orbital speeds in systems with multiple satellites (such as the moons of Jupiter, or the planets in the Solar system).
Steve Jessop
CC BY-SA 4.0
2021-02-01T14:03:56.787Z
Question: Any ejecta from the Earth's surface that don't have escape velocity are on an Earth-intersecting trajectory, aren't they? They can only achieve an orbit when they collide with other objects (or interact gravitationally with large bodies -- but there was no Moon yet, so it's collision). This friction-like interaction may be responsible for  "aligning" many orbits to a certain mean trajectory.
Peter - Reinstate Monica
CC BY-SA 4.0
2021-01-30T10:46:36.517Z
2021-02-01T14:03:39.627Z
2021-02-01T14:03:39.627Z
James K
<p>My understanding is that the moon was created a long time ago when Earth was hit by a big asteroid.</p>
<p>The debris then agglomerated into the Moon, which happens to be orbiting at the exact speed required to neither crash back into the Earth, nor escape into space.</p>
<p>Having the exact correct speed seems extremely unlikely. Yet, our moon is there, and many other planets have moons.</p>
<p>Are these just the few survivors out of thousands of events that didnt have the « goldilock » speed?</p>
<p>2022 Edit:  I got my &quot;ah HA!&quot; moment where everything makes sense after playing 10 minutes of the tutorial of the &quot;Kerbal Space Program&quot; game. Highly recommended.</p>
<p><strong>Short version</strong>:  Imagine &quot;soccer ball sized&quot; earth with no atmosphere .   It is &quot;soccer ball sized&quot; because I want to make sure nothing collides with it, but it is still very heavy.  It has no atmosphere because I dont want stuff to be slown down by the atmosphere.</p>
<p>Now imagine you are a few hundred kilometers above that soccer ball and you throw a baseball in any direction.</p>
<p>The baseball witll orbit the &quot;earth&quot; and the orbit will have some oval shape.   After every orbit, the baseball will come back to the exact same point  and speed/direction relative to the earth.</p>
<p>The speed you pick doesnt matter:  it just impacts how far the baseball will travel  before coming back.  There is no &quot;perfect&quot; &quot;goldilock&quot; speed equilibrium speed you have to launch the ball at.</p>
<p>If you throw the baseball super  fast and you are relatively close to the soccer-ball earth, then two interesting points in the orbit are</p>
<ol>
<li>where you are relative to the earth (ball travelling super fast, but low altitude/gravitational energy) and,</li>
<li>the furthest point from the earth (ball travelling much slower as it had to &quot;climb&quot;, but high altitude/gravitational energy).</li>
</ol>
<p>(aka good old energy conservation)</p>

CC BY-SA 4.0
2021-01-30T14:19:42.820Z
I did upvoted one answer below. However, a short answer is that indeed there are odds, but just if you select an orbit a priori. There are certainly odds that exactly the moon that we have forms on its orbit, but many are available if you don't put a restrain.
Alchimista
CC BY-SA 4.0
2021-01-30T20:01:10.343Z
The odds are 100%, or we wouldn't be here to make the observation. [What are the minimum set of physical characteristics to define an Earth like planet?](https://worldbuilding.stackexchange.com/a/20042/799)
Mazura
CC BY-SA 4.0
2021-01-30T20:59:57.770Z
The key is that the speed is not special. If the moon had formed with a bit higher traveling speed, it would simply be orbiting a bit *farther*; if it had formed with a lower speed, it would be orbiting closer. There is a huge range of speeds that would have resulted in some moon, at some distance and some speed.
Euro Micelli
CC BY-SA 4.0
2021-01-30T21:15:55.937Z
There exists one Goldilocks coincidence though: the Moon *currently* has such an orbital distance and size combination, *just right* so that it has about the same visual disk size as the Sun. This makes the beautiful solar eclipses we experience possible. This is temporary; the Moon slowly gains orbital speed (and distance) through a tidal interaction with the Earth and in less than a million years it will be too far and visually small to fully block the Sun’s disk, and after that there will never be another total solar eclipse. This is definitely just a lucky coincidence.
Euro Micelli
CC BY-SA 4.0
2021-01-30T22:43:22.880Z
Isnt it the opposite? Faster when closer and slower when further away? If you want to orbit 1000 from the surface of the earth ya gotta go*fast*
Zoltan
CC BY-SA 4.0
2021-01-31T04:58:54.973Z
@Zoltan Orbits farther away have lower orbital speeds, but they have more orbital energy.  So initial speed has to be higher to inject an object into a higher orbit than into a lower orbit.
Connor Garcia
CC BY-SA 4.0
2021-01-31T11:17:44.700Z
For two bodies, it should be possible to work out the velocity (speed and direction) range that would be required for collision (orbit trajectory with low enough periapsis), for two body gravitation, as well as escape velocity. There may not be a closed form solution, but you could find the ranges to any desired accuracy.
marshal craft
CC BY-SA 4.0
2021-01-31T11:22:11.383Z
https://youtu.be/ZENSCasmBzg This video is extremely informative, with simulations of the collision
Gimelist
CC BY-SA 4.0
2021-01-31T17:36:17.737Z
@ConnorGarcia  OOOh that's the bit I hadnt thought about.  If you are too fast then you will indeed move further away to an orbit where the "equilibrium" speed is lower.  My first thought was "then you will keep moving further away", but I had forgot about the part where moving up also means slowing down..
Zoltan
CC BY-SA 4.0
2021-01-31T18:17:36.287Z
@Zoltan I think you have the right idea now, orbital mechanics can be incredibly non-intuitive.
Connor Garcia
CC BY-SA 4.0
2021-02-01T08:20:08.723Z
@EuroMicelli One could make an argument that it is not a coincidence that human conciousness developped at exactly the time when sun and moon are able to produce the special eye-looking solar eclipse. It is indeed, as you pointed out, a special time in Earth's life. In fact I think we cannot be 100% certain that anything is a coincidence.
Manuki
CC BY-SA 4.0
2021-02-01T12:53:48.390Z
Is the orbital eccentricity of the Moon really that low compared to that of other planets and moons of the solar system? It seems to me like this is not an amazing coincidence but rather a side-effect from how star systems form.
Philipp
CC BY-SA 4.0
2021-02-01T16:09:34.780Z
@Manuki yes, it is conceivable. I remember Isaac Asimov speculated about that in one of his earlier F&SF columns (I believe “The Triumph of the Moon”). The thing is (as far as I know) we don’t have any evidence or serious theory for a mechanism linking the two; so far, this idea remains just a curious possibility.
Euro Micelli
CC BY-SA 4.0
2021-02-02T07:52:53.130Z
There's also the obvious survivorship bias. Planets with moons that are prone to flying off into space or crashing back to the surface tend to not have moons for very long.
Shmuel Newmark
CC BY-SA 4.0
2022-10-23T00:56:18.150Z
I'm upvoting your question because you had a misconception, yet you asked the question in such a way that invited a reasonable explanation.  You stayed humble (you hedged with words like "My understanding" and "seems"), and you explained the part that you thought justified your (mis)understanding.
Wyck
CC BY-SA 4.0
2024-07-19T18:31:55.440Z
Regarding the edit; probably want to go with "tens of thousands of kilometers above" instead of "a few meters above" a soccer-ball-sized earth-mass object. To get a baseball to _not_ impact a soccer-ball sized earth at a distance of a few meters, you'd have to give it a velocity of a few thousand kilometers per second, which is likely out of bounds for most non-Kryptonian arms.
notovny
CC BY-SA 4.0
2024-07-19T19:19:21.347Z
haha, good point, made it a bigger distance
Zoltan
<orbit><the-moon>
The moon has just the right speed not to crash on the Earth or escape into space. What are the odds?
Zoltan
# The moon has just the right speed not to crash on the Earth or escape into space. What are the odds?

(Asked by: username_0 on 2021-01-30)

My understanding is that the moon was created a long time ago when Earth was hit by a big asteroid.

The debris then agglomerated into the Moon, which happens to be orbiting at the exact speed required to neither crash back into the Earth, nor escape into space.

Having the exact correct speed seems extremely unlikely. Yet, our moon is there, and many other planets have moons.

Are these just the few survivors out of thousands of events that didnt have the « goldilock » speed?

2022 Edit: I got my "ah HA!" moment where everything makes sense after playing 10 minutes of the tutorial of the "Kerbal Space Program" game. Highly recommended.

**Short version**: Imagine "soccer ball sized" earth with no atmosphere . It is "soccer ball sized" because I want to make sure nothing collides with it, but it is still very heavy. It has no atmosphere because I dont want stuff to be slown down by the atmosphere.

Now imagine you are a few hundred kilometers above that soccer ball and you throw a baseball in any direction.

The baseball witll orbit the "earth" and the orbit will have some oval shape. After every orbit, the baseball will come back to the exact same point and speed/direction relative to the earth.

The speed you pick doesnt matter: it just impacts how far the baseball will travel before coming back. There is no "perfect" "goldilock" speed equilibrium speed you have to launch the ball at.

If you throw the baseball super fast and you are relatively close to the soccer-ball earth, then two interesting points in the orbit are

1. where you are relative to the earth (ball travelling super fast, but low altitude/gravitational energy) and,
2. the furthest point from the earth (ball travelling much slower as it had to "climb", but high altitude/gravitational energy).

(aka good old energy conservation)

**Tags:** orbit, the-moon

> **Comments:**
> **username_1**: I did upvoted one answer below. However, a short answer is that indeed there are odds, but just if you select an orbit a priori. There are certainly odds that exactly the moon that we have forms on its orbit, but many are available if you don't put a restrain.  
> **username_2**: The odds are 100%, or we wouldn't be here to make the observation. [What are the minimum set of physical characteristics to define an Earth like planet?](https://worldbuilding.stackexchange.com/a/20042/799)  
> **username_3**: The key is that the speed is not special. If the moon had formed with a bit higher traveling speed, it would simply be orbiting a bit *farther*; if it had formed with a lower speed, it would be orbiting closer. There is a huge range of speeds that would have resulted in some moon, at some distance and some speed.  
> **username_3**: There exists one Goldilocks coincidence though: the Moon *currently* has such an orbital distance and size combination, *just right* so that it has about the same visual disk size as the Sun. This makes the beautiful solar eclipses we experience possible. This is temporary; the Moon slowly gains orbital speed (and distance) through a tidal interaction with the Earth and in less than a million years it will be too far and visually small to fully block the Sun’s disk, and after that there will never be another total solar eclipse. This is definitely just a lucky coincidence.  
> **username_0**: Isnt it the opposite? Faster when closer and slower when further away? If you want to orbit 1000 from the surface of the earth ya gotta go*fast*  
> **username_4**: @username_0 Orbits farther away have lower orbital speeds, but they have more orbital energy.  So initial speed has to be higher to inject an object into a higher orbit than into a lower orbit.  
> **username_5**: For two bodies, it should be possible to work out the velocity (speed and direction) range that would be required for collision (orbit trajectory with low enough periapsis), for two body gravitation, as well as escape velocity. There may not be a closed form solution, but you could find the ranges to any desired accuracy.  
> **username_6**: https://youtu.be/ZENSCasmBzg This video is extremely informative, with simulations of the collision  
> **username_0**: @username_4  OOOh that's the bit I hadnt thought about.  If you are too fast then you will indeed move further away to an orbit where the "equilibrium" speed is lower.  My first thought was "then you will keep moving further away", but I had forgot about the part where moving up also means slowing down..  
> **username_4**: @username_0 I think you have the right idea now, orbital mechanics can be incredibly non-intuitive.  
> **username_7**: @username_3 One could make an argument that it is not a coincidence that human conciousness developped at exactly the time when sun and moon are able to produce the special eye-looking solar eclipse. It is indeed, as you pointed out, a special time in Earth's life. In fact I think we cannot be 100% certain that anything is a coincidence.  
> **username_8**: Is the orbital eccentricity of the Moon really that low compared to that of other planets and moons of the solar system? It seems to me like this is not an amazing coincidence but rather a side-effect from how star systems form.  
> **username_3**: @username_7 yes, it is conceivable. I remember Isaac Asimov speculated about that in one of his earlier F&SF columns (I believe “The Triumph of the Moon”). The thing is (as far as I know) we don’t have any evidence or serious theory for a mechanism linking the two; so far, this idea remains just a curious possibility.  
> **username_9**: There's also the obvious survivorship bias. Planets with moons that are prone to flying off into space or crashing back to the surface tend to not have moons for very long.  
> **username_10**: I'm upvoting your question because you had a misconception, yet you asked the question in such a way that invited a reasonable explanation.  You stayed humble (you hedged with words like "My understanding" and "seems"), and you explained the part that you thought justified your (mis)understanding.  
> **username_11**: Regarding the edit; probably want to go with "tens of thousands of kilometers above" instead of "a few meters above" a soccer-ball-sized earth-mass object. To get a baseball to _not_ impact a soccer-ball sized earth at a distance of a few meters, you'd have to give it a velocity of a few thousand kilometers per second, which is likely out of bounds for most non-Kryptonian arms.  
> **username_0**: haha, good point, made it a bigger distance  

---

## 6 answers

### Answer by username_25 on 2021-01-30. Score: 81 | Accepted answer

There isn't a "Goldilocks speed" for orbit. If you put two objects in space, and give them a velocity relative to each other, then provided that velocity is less than the escape velocity (at their relative distance) the two objects will orbit each other.

Those orbits will be elliptical, and it is possible that the ellipse is skinny and "eccentric" enough for the two bodies to collide when they are closest to each other. But for an object that is several hundred thousand km from Earth, there is a quite a wide range of possible elliptical orbits.

So when (and if) the grand collision happened, there was a huge amount of matter that was ejected up into space. Some probably was moving so fast that it escaped, Some certainly went into orbits that didn't have enough energy and so were small skinny ellipses and the matter fell back to Earth. But there was a lot that ended up in some kind of elliptical orbit. This matter was not all in the same orbit, but it started to coalesce, and form into a single ball, under its own gravity.

Other moons weren't formed like this, they either formed at the same time as their planets as a "mini solar system" (such as the four major moons of Jupiter) or they were captured from the asteroid or Kuiper belts). Initially, the captured moons may have had rather elliptical orbits.

But most moons are in rather circular orbits. Even if the moon was originally in an elliptical orbit, tidal effects will tend to make the orbit more circular. A planet and moon system has a certain amount of angular momentum and a certain amount of energy. The angular momentum can't change, but energy can be converted into heat and since tides dissipate some energy as heat, the orbit will tend to change to a shape that minimizes energy, for a given amount of angular momentum. That shape is a circle. (See [Is the moon's orbit circularizing? Why does tidal heating circularize orbits?](https://astronomy.stackexchange.com/questions/28962/is-the-moons-orbit-circularizing-why-does-tidal-heating-circularize-orbits))

So the effect of tides is to give moons the "Goldilocks speed" that keeps them in a circular orbit.

> **Comments:**
> **username_0**: Thanks for replying! I think this answers my point of « having the just right speed » by saying «  there is a wide range of speeds that will neither crash nor escape, correct?  
> **username_0**: Basically, that means that the odds of a moon « staying put » are higher than I thought and you dont need to generate thousands of moons to keep one.  
> **username_0**: Now that I think of it, isnt it simply because billions of rocks were thrown into orbit *each* at a different speed, and those that were at the « just right » speed eventually gathered while the others either escaped or fell back? That way I dont even need a wide range of « just right » speed.  
> **username_25**: Lots of stuff would have been thrown into space,  once out in space it all would start to orbit, on lots of different orbits (there are animations of this) Some of those orbits would intersect with the ground, but there is quite a range of orbits that don't.  Those rocks then joined up to make a moon. If the moon's orbit was not circular then tides would make it circular.  There is quite a range of speeds that are potenially stable. For an orbit of the moon its roughly between 200m/s and 1400 m/s  (transverse velocity at lunar distance)  
> **username_26**: @username_0: "those that were at the « just right » speed eventually gathered while the others either escaped or fell back" -- that wouldn't account for why the Moon is a large-ish proportion of the mass that could plausibly have been ejected by the collision. You could perhaps see the Moon's speed as the *average* of all the stuff that was in the large range of "good enough" speeds. To see for sure that there isn't a "just right" speed, you could perhaps look up the actual orbital speeds in systems with multiple satellites (such as the moons of Jupiter, or the planets in the Solar system).  
> **username_27**: Question: Any ejecta from the Earth's surface that don't have escape velocity are on an Earth-intersecting trajectory, aren't they? They can only achieve an orbit when they collide with other objects (or interact gravitationally with large bodies -- but there was no Moon yet, so it's collision). This friction-like interaction may be responsible for  "aligning" many orbits to a certain mean trajectory.  

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### Answer by username_12 on 2021-01-30. Score: 24

> My understanding is that the moon was created a long time ago when Earth was hit by a big asteroid.

A big asteroid? If you want to call Mars a "big asteroid", then yes, the giant impact hypothesis says that the Earth was hit by a big asteroid. If the giant impact hypothesis is correct, the mass of the impactor was significantly larger (by a factor of eight to ten) than the mass of the Moon. The vast majority of the impactor's mass fell back onto the proto-Earth. A very small amount may have escaped.

The rest of the debris, roughly a tenth of the impactor's original mass, had enough energy to orbit but not enough energy to escape. The debris cloud then circularized, and then attracted itself.

This might be too pretty of a picture, which leads to my next point:

> This seems extremely unlikely.

That this may be extremely unlikely is one of the proposed solutions to the [Fermi Paradox](https://en.wikipedia.org/wiki/Fermi_paradox), which asks why alien beings haven't colonized the Earth: Where are the aliens? If intelligent life requires a planet in the Goldilocks zone, a Goldilocks collision that creates a massive moon that stabilizes the planet's orientation, a Goldilocks amount of water, and a Goldilocks climate that keeps the climate relatively stable for over a billion years, then perhaps intelligent life is extremely rare. We humans may be here because our planet was one of the few winners in an intergalactic lottery in which almost every planet is a loser.

> **Comments:**
> **username_0**: Thanks for replying! Re: goldilock zone for life, I should have used a different word, because I really meant the « just right speed to orbit without escaping ». My new understanding is that  there is a wide range of speeds that will neither crash nor escape, which means that the odds of a moon ´staying put’ are higher than I expected.  
> **username_13**: @username_0 The key thing you might not have realized is that orbital speed and distance are tied together. The average Earth-Moon distance is ~385,000 km; *to stay at that distance*, the Moon must maintain the orbital speed it has. But there's a wide range of distances the Moon *could* be orbiting at, stably, and so also a wide range of speeds.  
> **username_14**: A simpler explanation to the Fermi Paradox could be that time and space are too big compared to our own small neighborhood and a few thousand years of written history or millions years of archaeology. Life could have started and vanished, or will start after we are no longer here.  

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### Answer by username_15 on 2021-01-30. Score: 9

I just want to add some numbers. The International Space Station orbits with the speed of 7.66 km/s (27,600 km/h). On the other hand, Earth's escape velocity is around 11 km/s (40,000 km/h). This means that anything with the speed in that range will orbit Earth. So it doesn't need to be some kind of a precise speed for objects to remain in Earth's orbit. Granted, the orbits of fragments from the Giant impact had different shapes, but over millions of years they started to clump together and eventually the largest object cleaned the smaller fragments because orbits of different shapes are not sustainable in the long term.

> **Comments:**
> **username_16**: Can you clarify why you can compare the orbital speed (which is tangential, i.e. sideways) with escape velocity (which you might assume is radial, i.e. straight up)?  
> **username_17**: @username_16 Escape velocity might actually be better named escape *speed*, since the direction does not matter. If you are moving at 11km/s relative to earth *in any direction* (so long as it's not one that results in a collision with earth itself), you have sufficient kinetic energy to go as far as you want from earth (i.e. escape).  
> **username_18**: Obligatory XKCD: https://xkcd.com/1356/  

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### Answer by username_19 on 2021-01-31. Score: 7

You might reasonably assume that the result of the collision was a big cloud of debris of various sizes. Some bits will have come rapidly back to what was left of Earth, other bits will have gone flying out into space, and some bits stayed more or less in orbit for long enough to coalesce into the Moon. At this scale, solids behave more or less like liquids and so both Earth and Moon formed into roughly spherical shapes. Mars' moons, being somewhat smaller and (IIRC) newer, are rather less spherical.


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### Answer by username_20 on 2021-02-01. Score: 7

A point not mentioned above is that the primordial swarm of material that would eventually form the Moon would, on average, have the same direction of orbit around the Earth as the Moon does today. In that case, we shouldn't ignore collisions between that material where the radial momenta are cancelled and leave only, or mostly, the tangential momenta (possibly with coaleced material). Similarly, collisions with a component parallel to the axis of the orbit would generally reduce momenta parallel to the axis of the orbit and lead to a disk of material (as seen for Saturn's rings). Self-gravity would then create 'lumpiness' in the disk and the dominant lump would preferentially attract material to itself and form a single body. It's not jut an orbital dynamics problem but also one of momentum exchange and aggregation statistics.


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### Answer by username_21 on 2021-01-30. Score: 6

I"m not sure if I understood your question or I just didn't answer your question right. However, the earth has been bombarded by many debris floating around in the early solar system. This was due to the fact that in the belt around the sun various aggregations of material happened through mutual attraction of smaller materials, that grew. This is how all the planets formed in star systems. At a certain period in time the diverse clumped together debris where big enough to be called proto planets and the rest was still called debris or asteroids.

The theory you are mentioning is called the [giant impact theory](https://en.wikipedia.org/wiki/Giant-impact_hypothesis) or the Theia impact. Earth was not struck by an asteroid, but by another proto planet called Theia. It is assumed that this has happening a lot, also in other star systems. The fact that from this impact the earth and the moon formed in the form they currently have, is unique in our solar system. We only see it with the earth. However, impacts between proto planets is not considered to be rare in the early solar system, so the chance that the Theia impact could happen might have been small, but considering the chance of colliding proto planets was very high, the possibility was there.

Now back to your remark that the moon has exactly the right speed to stay in orbit with the earth. This is not true. The moon actually has a speed too high to stay in orbit and the distance between the earth and the moon is getting bigger each year. High precision measurements are made and suggest that the moon is spiraling away from the earth with a speed of about 4 centimeter per year. Of there the course of the last 4 billion years, this has thus been quite a distance and it suggests the speed is not exactly right, but a bit too high.

Still, a Theia-like collision between Venus and another proto planet would not have resulted in a planet moon system, due to the tidal forces from the sun. A similar thing would apply for Mars, but here Jupiter is the reason for the absence of such a pair. The outer planets are too big and would have not resulted in material being able to escape the planet's gravity.

So, yes, the existence of the earth moon can be seen as a unique situation, but this is not because the kind of impacts didn't happen. It is because the earth was just at the right distance from the sun and Jupiter to result in such a pair. Impacts like the Theia impact were not rare. Do, however, understand that the Theia-impact is a theory that has good acceptance, but is not undoubted. There are several questions about the validity of this theory.

> **Comments:**
> **username_0**: Thanks for replying! I also read about the 4cm distance increase, I ´ll argue that if it is slow enough for the moon to still be around after 4bn years then it is pretty much at the ideal speed:).  My newfound understanding is that a wide range of speeds will lead to a moon that isnt going too slow or fast, so a moon is much more likely to stay in place than I thought  
> **username_1**: Is it really correct to say that the moon is going faster then a certain value? The fact that the Moon goes slowly away isn't really because it has a speed exceeding that of a special orbit. It cannot even be spelled... Isn't? I would say that being the orbiting bodies not rigid than there is acceleration /transfer of momentum...... Just to say that the sentence "thus is not true. The moon actually...." seems - although I somehow understand what you mean - suggesting something untrue to OP, or just is not a very happy formulation.  
> **username_22**: The additional 4cm per year is due to tidal effects.  The energy comes from slowing the Earth's rotation.  Eventually the Earth and Moon will keep the same face to each other, meaning that there will be no tides - provided nothing happens to disturb the system before then.  
> **username_24**: regarding "speed too high to stay in orbit[...] therefore expanding distance": I was under the (perhaps incorrect) impression that, once in orbit, one would *decelerate* to achieve a higher orbit. No?  
> **username_23**: The Moon at perigee is moving too quickly to remain in *circular* orbit *at that height*, so it rises.  Likewise, the Moon at apogee is moving too slowly to remain in *circular* orbit *at that height*, so it falls.  

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