How are moons captured?

[Rdivine]

I've always wondered... How are moons captured? Say, Triton. Triton is in a retrograde orbit relative to neptune, so it suggests that it may be captured.

 

However, how does the moon even end up permanently inside the SOI of Neptune? When it encounters Neptune's SOI, it is in an escape trajectory. Multiple encounters with Neptune later, it still would be in an escape trajectory, unless the moon aerocaptured itself .

Now we assume that Neptune captured Triton by swinging a moon into space, but how does it's orbit get circular? Wouldn't an encounter with another moon set it's trajectory to be very hyperbolic? Or is it just a mere coincidence?

[K^2]

Capture always requires a three-body interaction. There are two typical scenarios. Either an object enters a system and gets a gravity assist from one of the bodies already orbiting in that system, or a pair of bodies enter the system, one of them gets ejected, and the other captured. Either way, a lone planet cannot capture a moon. It must get help from another object.

[legoclone09]

Actually it only needs two bodies (I think). I sent something to the Mun once and om the flyby it was captured, no input needed. This was a long time ago so not sure.

[K^2]

1 minute ago, legoclone09 said:

Actually it only needs two bodies (I think). I sent something to the Mun once and om the flyby it was captured, no input needed. This was a long time ago so not sure.

That's due to numerical errors in KSP. It's a well known "bug", if you will. Real physics doesn't work that way. While it's possible for a very tiny momentum transfer to take place in a two-body encounter, in practice, it never leads to a lasting capture, since the tiniest perturbation from an outside source will knock the would-be-moon loose again. So you need a three body interaction to actually result in a capture.

[legoclone09]

9 minutes ago, K^2 said:

That's due to numerical errors in KSP. It's a well known "bug", if you will. Real physics doesn't work that way. While it's possible for a very tiny momentum transfer to take place in a two-body encounter, in practice, it never leads to a lasting capture, since the tiniest perturbation from an outside source will knock the would-be-moon loose again. So you need a three body interaction to actually result in a capture.

Ok, thanks, That makes sense and explains it.

[Rdivine]

3 hours ago, K^2 said:

That's due to numerical errors in KSP. It's a well known "bug", if you will. Real physics doesn't work that way. While it's possible for a very tiny momentum transfer to take place in a two-body encounter, in practice, it never leads to a lasting capture, since the tiniest perturbation from an outside source will knock the would-be-moon loose again. So you need a three body interaction to actually result in a capture.

But how does the orbit get circular?

[KerikBalm]

I don't fully understand it, but I think it has to do with tidal interactions, or mass not being perfectly evenly distributed in both bodies... but I too would like a more detailed explanation

[K^2]

52 minutes ago, Rdivine said:

But how does the orbit get circular?

Unless there is a system of moons there, it doesn't. Circular orbits are a result of many, many interactions between multiple objects orbiting the same parent body.

[KerikBalm]

then how did our moon's orbit get so circular?

[Gaarst]

1 minute ago, KerikBalm said:

then how did our moon's orbit get so circular?

The Moon wasn't captured: it came from the debris of a collision between Earth and a smaller planet. (According to the most popular hypothesis)

Edited January 22, 2016 by Gaarst

[magnemoe]

8 hours ago, K^2 said:

That's due to numerical errors in KSP. It's a well known "bug", if you will. Real physics doesn't work that way. While it's possible for a very tiny momentum transfer to take place in a two-body encounter, in practice, it never leads to a lasting capture, since the tiniest perturbation from an outside source will knock the would-be-moon loose again. So you need a three body interaction to actually result in a capture.

An asteroid can be captured in KSP if it enter Mun SOI and this changes the orbit, typicaly raising Pe who will lower Ap so its inside kerbin SOI, This is unstable as the asteroid orbit  cross Mun SOI and later passes might kick it out. 
 

If you look at trajectories in the tracking center it might show wrong trajectories, it will commonly see objects on an low speed escape trajectory as in orbit.
This is common if you do an fast run to Minmus, using 50-100 m/s extra to save time. 

[magnemoe]

4 hours ago, K^2 said:

Unless there is a system of moons there, it doesn't. Circular orbits are a result of many, many interactions between multiple objects orbiting the same parent body.

Tides will circulate an orbit, this is linked to having the same side facing the parent who only work with an circular orbit. 
mars has two small asteroid moons, earth has only the Moon, Moon gravity makes it hard for other captured bodies to get captured in an stable orbit. 

 

[Just Jim]

I actually had something similar happen in my career game with a huge asteroid.... it's what my fan fiction story is (more or less) about. 

What happened is a huge class-E asteroid came into Kerbin SOI and made a near, but not completely circular, orbit.  However, near the end of it's trajectory, it swung into Minmus's SOI just enough to add a couple degrees more and completely circularize the orbit.  Without Minmus being in the right place at the right time, it would have just kept going.  Later I moved it in more so it wouldn't intersect with Minmus again in the future and cause problems.

Granted, this was the game, but I have to think something similar would have to happen for a planet to capture and keep a moon or other large body.

[rudi1291]

Isn´t every encounter technically an 3-body (or better: n-body) encounter? Gravity doesn´t stop somewhere, as far as i know, so there are multiple bodies acting on a single one at every point in time. Most of that might be to weak to actually have an effect... but maybe the sun might have a noticable effect?

[A35K]

1 hour ago, Gaarst said:

The Moon wasn't captured: it came from the debris of a collision between Earth and a smaller planet. (According to the most popular hypothesis)

One thing I don't get about this theory: How does a collision put objects in a near circular orbit? Shouldn't the debris have been blasted into a ballistic arc? What caused it to 'accelerate' at apogee to make a circular orbit?

[wumpus]

10 hours ago, K^2 said:

That's due to numerical errors in KSP. It's a well known "bug", if you will. Real physics doesn't work that way. While it's possible for a very tiny momentum transfer to take place in a two-body encounter, in practice, it never leads to a lasting capture, since the tiniest perturbation from an outside source will knock the would-be-moon loose again. So you need a three body interaction to actually result in a capture.

Note that this ignores aerobraking.  A planet can capture a moon on its own if it dips into the atmosphere (assuming it has one).  I'd expect that real n-body physics is more likely involved in most captures (I'm curious to know if asteroids with moons involved capture or break up).

[Gaarst]

1 minute ago, A35K said:

One thing I don't get about this theory: How does a collision put objects in a near circular orbit? Shouldn't the debris have been blasted into a ballistic arc? What caused it to 'accelerate' at apogee to make a circular orbit?

Interaction between the objects (collisions and gravitational attraction) cause transfers of momentum towards the exterior of the system and end up by circularising and homogenise the matter ejected in orbit, before gravity shapes it into a moon.

Right after the impact, the energy and matter density are dense enough for it to be modeled like a fluid, hence non-elastic collisions. A great majority of the matter ejected by the impact falls back on Earth, but these collisions are enough change the trajectory of a small fraction of the debris so that they are in orbit, highly elliptical at the beginning, and slowly circularising with time.

This isn't really easy to explain, and I'm not that cultivated about the topic do it properly. If you're interested, Wikipedia will probably explain the thing better than I do.

[Wjolcz]

16 minutes ago, A35K said:

One thing I don't get about this theory: How does a collision put objects in a near circular orbit? Shouldn't the debris have been blasted into a ballistic arc? What caused it to 'accelerate' at apogee to make a circular orbit?

I saw a few times a computer animation of the event and it clearly showed it's possible.

[Gaarst]

3 minutes ago, wumpus said:

Note that this ignores aerobraking.  A planet can capture a moon on its own if it dips into the atmosphere (assuming it has one).  I'd expect that real n-body physics is more likely involved in most captures (I'm curious to know if asteroids with moons involved capture or break up).

In real life, a moon can't be aerocaptured by a planet for several reasons:

• The moon is going to be inside the planet's Roche limit and will disaggregate due to tidal forces
• If a large enough moon somehow manages to enter the atmosphere, you have to keep in mind that the ground is really close, so it will very probably impact it (an asteroid will impact the planet too, or disintegrate upon atmospheric entry).

Edited January 22, 2016 by Gaarst

[Findthepin1]

So what was Neptune-Triton's third massive body? What happened to it?

[Gaarst]

2 minutes ago, Findthepin1 said:

So what was Neptune-Triton's third massive body? What happened to it?

An ancient moon that was disrupted since then, gravitational influence of other planets, or, as K^2 mentioned it, Triton was part of a 2-body system, and its peer got ejected by Neptune while Triton was captured. AFAIK, I doubt we will ever find out the cause of the formation/capture of every moon of the Solar System (especially considering the fact that we are not 100% sure how the Moon was created).

[wumpus]

3 hours ago, Gaarst said:

In real life, a moon can't be aerocaptured by a planet for several reasons:

• The moon is going to be inside the planet's Roche limit and will disaggregate due to tidal forces
• If a large enough moon somehow manages to enter the atmosphere, you have to keep in mind that the ground is really close, so it will very probably impact it (an asteroid will impact the planet too, or disintegrate upon atmospheric entry).

The moon (and to a lesser extent the Earth) show some pretty big craters.  Presumably plenty of asteroids *did* hit.  If one of them grazed the atmosphere *just* right, it could presumably be captured.  Note that aerobraking and location of deceleration only define apogee, and the remaining velocity will determine perigee.  It hardly mandates being inside of the Roche limit.

That being said, my guess is that this would work for Phobos and Deimos, with the moon (and maybe some others) generated from the original planets and the rest captured via 3+ body gravitation (and note that Phobos and Deimos have iffy futures).  I guess a better question is "how many moons are stuck in Saturn-Titan Lagrange points"?  That appears to be a natural spot for gravity-captured moons to wind up.  Jupiter's big moons are tied together, making such things complicated.  I'd also have to wonder what should be in Earth's.  It might not have the size of Saturn, nor a neighbor like Jupiter, but wouldn't it accumulate some stuff in the (stable) Lagrange points?

[Bill Phil]

Gravity isn't uniform above the planet, and this gets compounded by the other planets, changing up gravitational forces. An asteroid might enter the SOI and be above escape velocity, but perturbations might change the escape velocity slightly, and of course, pull on the object. Perhaps pulling it into orbit?

[Exploro]

This might give you the answer that you seek Rdivine (see the bottom paragraph for that section). In summary two possibilities for how Triton (and perhaps some of the other captured moons) wound up around Neptune. Triton likely collided with another body already in orbit around Neptune and the collision caused Triton to enter into orbit around Neptune. Alternatively at one point in time Triton had a binary partner that; when the two came into proximity of Neptune; the gravitational interactions tossed the partner body back out into the Solar System and robbed sufficient energy from Triton for Neptune's gravity to capture it.

[Rdivine]

1 hour ago, Exploro said:

This might give you the answer that you seek Rdivine (see the bottom paragraph for that section). In summary two possibilities for how Triton (and perhaps some of the other captured moons) wound up around Neptune. Triton likely collided with another body already in orbit around Neptune and the collision caused Triton to enter into orbit around Neptune. Alternatively at one point in time Triton had a binary partner that; when the two came into proximity of Neptune; the gravitational interactions tossed the partner body back out into the Solar System and robbed sufficient energy from Triton for Neptune's gravity to capture it.

Indeed, i how have a better understanding.. Thanks!