How do moons get into orbit?

worir1

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I was wondering how did the moon get into orbit. Some places when i have read say that our moon came from debris left over from a collision between earth and another planet. I done under stand how it is possible for the moon to be in orbit. Would it not have been flung out on either an escape orbit or a parabolic orbit. Would it not require some thing to slow it down to get into a stable orbit. Also how is our moons orbit so relatively circular?
 

Urwumpe

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I done under stand how it is possible for the moon to be in orbit. Would it not have been flung out on either an escape orbit or a parabolic orbit. Would it not require some thing to slow it down to get into a stable orbit.

You have to understand, that when the moon formed according to the currently best theory (yes, that means it can be slightly wrong. There are still many open questions left for improvements), Earth did no longer exist, except a small practically liquid rock remainder.

The moon is not suddenly there. It orbited around the badly battered Earth as a cloud of ejected mass, much more massive than the moon today. A large part of the debris cloud dropped back to Earth over a relative long time (a few dozen thousands of years), another part formed the moon, a small part formed possibly NEOs around Earth.

Also how is our moons orbit so relatively circular?

You have to think of the debris cloud like a subway station during rush hour. When nothing is around, you can walk from left to right in the passage way without problems, but when thousands of people try to catch their subway, you will bounce against a few people before you better move with the flow of people.

Same happens in space in such a debris cloud. Particles collide, with each collision equalizing the momentum. the velocities become quickly similar with only very small variations.

So, the moon already started in a pretty good orbit. And then, you get the tidal effects. The acceleration that makes the moon move slowly away from Earth (As result of the moon slowing down Earth rotation) also removes a large deal of eccentricity. The moons orbit is still pretty elliptic because of the effects of the sun and the other planets, but Earth also pushes the moon back to a more circular orbit, since the moons eccentric movement in an elliptic orbit relative to Earth constant rotation during the day means, that when the moon moves away, it will fall behind the tidal bulge on Earth and be accelerated stronger by Earth, when the moon moves closer to Earth, it will fall less far behind and receive less acceleration.
 

garyw

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There is a whole programme on it here:

 

worir1

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That's like the one i watched. The thing i dont understand is how did the first part of debris get into orbit?
 

garyw

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During the formation of the solar system they'd be a lot of large debris and proto-planets which generally went around the solar system like dodgems.
 

worir1

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Let me give an example. Say on orbiter you have just made a trip from mars and are now on your way back to earth. When you reach earth You will have an escape orbit and will fly away again. Normaly you would perform a retrograde burn to slow down. Now imagine that was a moon. What stood in place of the retrograde burn?
 

Codz

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Let me give an example. Say on orbiter you have just made a trip from mars and are now on your way back to earth. When you reach earth You will have an escape orbit and will fly away again. Normaly you would perform a retrograde burn to slow down. Now imagine that was a moon. What stood in place of the retrograde burn?

The Moon didn't just "arrive" all in one piece one day. According to the giant impact hypothesis, it was originally a large debris field from an enormous impact that ripped much of the Earth's surface off and completely destroyed the impactor. After some time much of the debris field eventually coalesced into what we now know as the Moon.

Here's an excerpt from the wiki page on it.

Astronomers think the collision between Earth and Theia happened at approximately 4.53 bya; about 30–50 million years after the Solar System began to form. In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s.[11] Theia's iron core would have sunk into the young Earth's core, and most of Theia's mantle accreted onto the Earth's mantle, however, a significant portion of the mantle material from both Theia and the Earth would have been ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulations of such an event suggest that some twenty percent of the original mass of Theia would have ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon.
 

Linguofreak

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Let me give an example. Say on orbiter you have just made a trip from mars and are now on your way back to earth. When you reach earth You will have an escape orbit and will fly away again. Normaly you would perform a retrograde burn to slow down. Now imagine that was a moon. What stood in place of the retrograde burn?

Lithobraking.
 

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The most important key to this theory is that the proto planet struck the Earth at a glancing angle.
That mean that not all the velocity of the proto planet was lost.
In effect the molten material was "launch" into orbit.
Some material reach orbital velocity.
The other material fall back to Earth.
In fact, according to this theory, Earth gain mass after this impact.
Over the eons, the material in orbital velocity coalesced into what we know as the Moon today, as stated by Codz.
Had that proto planet struck the Earth head one....well, I would not have the opportunity to type this message.
 

worir1

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The Moon didn't just "arrive" all in one piece one day. According to the giant impact hypothesis, it was originally a large debris field from an enormous impact that ripped much of the Earth's surface off and completely destroyed the impactor. After some time much of the debris field eventually coalesced into what we now know as the Moon.

Here's an excerpt from the wiki page on it.

Take Triton for example. We know that it must have been once an asteroid and formed else where due to its retrograde orbit. How did Triton manage to get into orbit of neptune and not just simply sling off on an scape orbit. What slowed it down to orbital velocity. If it was already coming in slow what stopped it from hitting neptune.
As for our moon i understand that the moon was formed from debris left over however how did it reach orbital velocity. From that wiki quote it says the planet hit at 4km/s which is much too slow to reach orbit.
 

garyw

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Take Triton for example. We know that it must have been once an asteroid and formed else where due to its retrograde orbit. How did Triton manage to get into orbit of neptune and not just simply sling off on an scape orbit. What slowed it down to orbital velocity. If it was already coming in slow what stopped it from hitting neptune.

What's a 'scape' orbit?

Do you know what a sling shot is? As a probe passes a planet it can steal orbital energy from the planet and travel outbound even faster.

Its' also possible to approach a world retrograde and donate orbital energy. In this case triton would have done exactly that and it would have slown it down enough to be captured.

Alternatively, it may have been several times it's current size and simply smashed through the atmosphere and don't a very impressive weight-shedding aerobrake.
 

Urwumpe

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Also, you have to understand, that the gravity field of a planet is not a constant static thing. It varies, especially at the border of the "sphere of influence".

It is perfectly possible for an asteroid in a multi-body solar system to enter the gravity well of a planet but not have enough energy left to leave it again, going from a low hyperbolic orbit (relative to the planet) to a highly eccentric orbit. And then be forced into a more circular orbit by the influence of the other moons.
 

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As for our moon i understand that the moon was formed from debris left over however how did it reach orbital velocity. From that wiki quote it says the planet hit at 4km/s which is much too slow to reach orbit.

Hitting target at 4 km/s doesn't mean that derbis cannot fly faster. It's momentum (or energy to be precise) exchange not velocity. Less massive derbis retained a lot of it which resulted in higher velocity.

As Urwumpe stated before, derbis fields tend to create disk like structures (acretion) in which momenta of particular derbis are slowly equalized (by collisions and near pass gravitational atraction). That resulted in nice disk placed outside [ame="http://en.wikipedia.org/wiki/Roche_limit"]Roche Limit[/ame] and condensed into the moon.

As for "retrograde burns": Remember that when you calculate orbital insertion in orbiter, you usually use 2-body problem, while in reality each body experiences gravitational influences of other bodies. In 3-body problem results may vary from ejection of 1 body to moving into stable orbit. 3 and more body problem is very sensitive to initial conditions and minor influence can cause great trajectory changes. In triton's case it could have been Jupiter or Saturn that pushed/pulled triton from hyperbolic to eliptic orbit.
 

worir1

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What's a 'scape' orbit?

Do you know what a sling shot is? As a probe passes a planet it can steal orbital energy from the planet and travel outbound even faster.

Its' also possible to approach a world retrograde and donate orbital energy. In this case triton would have done exactly that and it would have slown it down enough to be captured.

Alternatively, it may have been several times it's current size and simply smashed through the atmosphere and don't a very impressive weight-shedding aerobrake.

It was a typing error. I ment to say escape, Im sure you could work that out.
As for sling i simply ment a change in direction due to a gravitational pull. I apologise if i have used incorrect terminology.

As for the rings, what stops Jupiters rings from forming a moon(s)
 

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Simple answer: Jupiter.

More complex answer: Jupiter has a mind-boggling huge mass, which means that it's gravity, and hence its tidal forces, have a major effect on a huge area of space. To form a larger body, an aggregate of material must have a greater gravitational attraction to itself than the tidal forces of the parent body trying to pull it apart. See [ame="http://en.wikipedia.org/wiki/Roche_limit"]Roche Limit[/ame] for further information. In the case of the rings of the gas giants, the material therein has insufficient self-gravity to coallesce at the altitude of their orbit.
 

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As for our moon i understand that the moon was formed from debris left over however how did it reach orbital velocity. From that wiki quote it says the planet hit at 4km/s which is much too slow to reach orbit.

The simple answer to that is that the Earth was smaller before the Proto planet impacted it at that time.
So it had less mass and a smaller orbital velocity.
Added the sling shot effect and it is quite plausible for the ejected mass to reach orbital velocity.
Another thing is that orbital velocity changed with distance above the surface of the planet.
0 km: 7.89 km/s (17,650 mph)
200 to 2,000 km: circular orbit: 7.8 to 6.9 km/s (17,450 mph to 15,430 mph) respectively
elliptic orbit: 8.2 to 6.5 km/s respectively.
35,786 km: 3.1 km/s (6,935 mph)
357,000 to 399,000 km: 1.08 to 0.97 km/s (2,416 to 2,170 mph) respectively
 

Linguofreak

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From that wiki quote it says the planet hit at 4km/s which is much too slow to reach orbit.

That's an *initial* velocity (when it was still far away from Earth) of less than 4 km/s. As it fell in towards Earth it would have gained velocity, and hit Earth at at least escape velocity (exactly escape velocity, more or less, if it started out at zero speed).
 

worir1

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Thanks for sharing your great knowlagr guys. I think i understand now.
 
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