explanation of mechanism of moon capturing

[Jon Steensen]

When watching documentaries, it happens quite often that some kind of scientist says that a planet might have captured one of its moon. I have never understood how that should be the case. When a moon orbits a planet, we can predict where it will be in future according to Newton's laws of gravity. The result will be that it is the same orbit as it is currently in. Likewise we can let time “run backwards” and calculate where the moon must have been in the past.  We get that it must have been in the same orbit as it is currently in.

As we do not see moons suddenly leave their orbits and fly into space, I would not expect them to suddenly fly in from space and start orbiting a planet. Either they are in orbit, always has been and always will be, or they are not in orbit and will just fly by once (maybe more if they orbit the same star as the planet). The only thing that can change an orbit is some kind of interaction with another celestrial body, e.g. colliding with an asteriode. In order for that to change the moon-to-be’s orbit so it orbits a planet, it must enter a collision of just the right mass/velocity combination at just the right time near a planet. As space I mostly empty, especially around the planets (as part of the planet definition is that they have cleared their surrounding area of debris), I find this to be a very unlikely explanation of the capturing mechanism. So how does it work?

[Strange]

1 hour ago, Jon Steensen said:

When watching documentaries, it happens quite often that some kind of scientist says that a planet might have captured one of its moon. I have never understood how that should be the case. When a moon orbits a planet, we can predict where it will be in future according to Newton's laws of gravity. The result will be that it is the same orbit as it is currently in. Likewise we can let time “run backwards” and calculate where the moon must have been in the past.  We get that it must have been in the same orbit as it is currently in.

By that argument, the fact that we see satellites in orbit around the Earth means they must have been there, rather than us launching them from Earth. 

In other words, the current stable relationship says nothing, necessarily, about the past history.

1 hour ago, Jon Steensen said:

The only thing that can change an orbit is some kind of interaction with another celestrial body, e.g. colliding with an asteriode.

It doesn't require a collision. A near miss can change the direction of an object. And a number of small interactions over many orbits can cause an object to be ejected from an apparently stable orbit.

And if that body then moves past another at the right sort of distance and speed then it may enter a new orbit. It can be fast and close, or slow and distant, so there is no "magic number" for it to happen. 

[Jon Steensen]

39 minutes ago, Strange said:

By that argument, the fact that we see satellites in orbit around the Earth means they must have been there, rather than us launching them from Earth. 

In other words, the current stable relationship says nothing, necessarily, about the past history.

It doesn't require a collision. A near miss can change the direction of an object. And a number of small interactions over many orbits can cause an object to be ejected from an apparently stable orbit.

And if that body then moves past another at the right sort of distance and speed then it may enter a new orbit. It can be fast and close, or slow and distant, so there is no "magic number" for it to happen. 

Man-made satellites were put into orbit by the thrust of one or more rocket engines, which continuously changed their orbits gradually all the way up. It was not by a collision or quick change of orbit from a close fly by or a collision. In fact you cannot put an object into orbit with a quick impulse addition at surface level, as the object would return to the same point after one roundtrip around the earth. Unlike man-made satellites, most asteroids do not have that slow thrust over long periods, which slowly change their orbits, unless they thrust something into space, or are close to a massive object at nearly the same speed, following for longer periods. I know that celestrial objects can have an in build “rocket engine” e.g. do comets boil, when they come close to the sun, and Io has geysers that erupt into space due to tidal heating.

Regarding the ejection of planets or moons from stable orbits it normally requires many interactions, so if that mechanism should work in reverse to catch objects, I would expect that it would require many interactions too. Most objects coming at a random speed from deep space I expect would most likely come in at such a speed and angle that they will only get one flyby, and get their orbit changed once e.g. by flying close to a moon and continue back into deep space. For example calculating the gravity assist manouvers to eject something from the solar system is very complicated, and it does not just happen by change that you are close enough to the planets for their gravity field to become significant. Likewise I think getting an object comming in at high speed from deep space  and getting it to slow down enough to go into orbit, by it flying close to a planets moons, is very unlikely to happen by change.

If the object got captured by interacting with something else, changing its path from a non-orbit to an orbit, I would expect to see orbits with a shape very similar to non-orbits. E.i. orbits that are far from circular, but instead reaches far into space where the object captured originally came from, and where it got its orbit influenced. I.e. If the orbit was changed by a collision in deep space, the conservation of energy dictates that orbiting object reaches the same point after one orbit. I know that many orbits exist, and non of them are especially magical, but I think it is much easier to achive a non-orbit, than for the speed distance relation to be such that an orbit is achived.

 

[Strange]

4 minutes ago, Jon Steensen said:

Man-made satellites were put into orbit by the thrust of one or more rocket engines, which continuously changed their orbits gradually all the way up. It was not by a collision or quick change of orbit from a close fly by or a collision.

That wasn't the point. But never mind.

[Sensei]

3 hours ago, Jon Steensen said:

The only thing that can change an orbit is some kind of interaction with another celestrial body, e.g. colliding with an asteriode.

Collision is not the only option. One (especially massive) object can change (otherwise stable) path of other cosmic object, because of close enough flight. To calculate force of attraction of gravity we use inverse square law. The closer flight to object, the stronger attraction of that object. Calculation of overall force between three and more objects is very complicated task. It's called three-body-problem. https://en.wikipedia.org/wiki/Three-body_problem

You can see it playing one of many games simulating stars, planets and moons.

e.g.

https://dan-ball.jp/en/javagame/planet/

http://www.testtubegames.com/gravity.html

(or search the net for "planet simulator online", "star simulator online" etc. keywords)

 

ps. There is no stable orbits. There are only metastable for billions or more years. Close enough flight of high velocity star (e.g. ejected from binary star system by black hole) can easily perturb orbits of entire solar system.

 

[Sensei]

This is also very cool simulator:

https://hermann.is/gravity/

It has couple nice features, not present in other simulators:

- cosmic objects grow when they collide each other.

- change of focus (reference frame), so we can see from perspective of star, planet or moon.

 

 

[Janus]

4 hours ago, Jon Steensen said:

 

Regarding the ejection of planets or moons from stable orbits it normally requires many interactions, so if that mechanism should work in reverse to catch objects, I would expect that it would require many interactions too. Most objects coming at a random speed from deep space I expect would most likely come in at such a speed and angle that they will only get one flyby, and get their orbit changed once e.g. by flying close to a moon and continue back into deep space. For example calculating the gravity assist manouvers to eject something from the solar system is very complicated, and it does not just happen by change that you are close enough to the planets for their gravity field to become significant. Likewise I think getting an object comming in at high speed from deep space  and getting it to slow down enough to go into orbit, by it flying close to a planets moons, is very unlikely to happen by change.

If the object got captured by interacting with something else, changing its path from a non-orbit to an orbit, I would expect to see orbits with a shape very similar to non-orbits. E.i. orbits that are far from circular, but instead reaches far into space where the object captured originally came from, and where it got its orbit influenced. I.e. If the orbit was changed by a collision in deep space, the conservation of energy dictates that orbiting object reaches the same point after one orbit. I know that many orbits exist, and non of them are especially magical, but I think it is much easier to achive a non-orbit, than for the speed distance relation to be such that an orbit is achived.

 

As long as the object is making its approach at a speed not much above escape velocity, a single gravitational interaction with anther body could be enough to cause a capture.   Calculating a gravity boost to kick something out of the solar system is actually quite simple, if that's all your after.     The gravity assists NASA uses aren't because they are not just interested in boosting the probe's speed, but also giving it just the right speed and direction in order to  make a timed rendezvous  with another planetary body.  It's like the difference between hitting a baseball right so that it leaves the ball park compared to hitting it just right so that it flies through the open window of a passing car and lands on the seat.

And while a captured satellite will initially have a highly eccentric orbit, tidal friction between it and the planet will have a tendency to circularize the orbit over time.  One a couple of good indicators that the satellite is a captured one is its eccentricity.  For example, the  outer Jovian moons  Carme, Pasiphae and Sinope have eccentricities of 0.21, 0.38. and 0.28. Another good indicator is that the orbits are retrograde (opposite to that of the planet's rotation).  These three moon also share that characteristic.   It is easier for a planet to hold a satellite in a retrograde orbit over a direct orbit because tidal acceleration (the effect that is causing our moon to slowly recede) works in the opposite manner for retrograde satellite than it does for direct orbiting one*, which tends to draw the satellite in towards the planet.

 

* this drawing in effect also occurs if the period of a direct orbiting satellite is shorter than the rotational period of the planet.  Phobos is a case in point. It orbit overtakes Mars rotation, and as a result is being drawn in towards Mars.

[Airbrush]

As I recall, our moon could not have been captured because the mass of our moon is too great for that.  Neptune, however, has a moon Triton that is in a retrograde orbit which means it was probably captured because the mass of Triton is not too great for that (compared to Neptune's mass).

[dimreepr]

55 minutes ago, Airbrush said:

As I recall, our moon could not have been captured because the mass of our moon is too great for that.

Yet here we are...

[zapatos]

1 hour ago, dimreepr said:

Yet here we are...

Not all moons are captured.

[dimreepr]

1 minute ago, zapatos said:

Not all moons are captured.

Yet here we are...

Sorry, I couldn't resist. 

 

5 minutes ago, zapatos said:

Not all moons are captured.

Please explain how our moon isn't.

[zapatos]

16 minutes ago, dimreepr said:

Yet here we are...

Sorry, I couldn't resist. 

 

Please explain how our moon isn't.

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

[Janus]

12 minutes ago, dimreepr said:

Yet here we are...

Sorry, I couldn't resist. 

 

Please explain how our moon isn't.

Basically there are three models for possible Moon formation.

1. It condensed from the same material and at the same time as the Earth.

2. It formed at some other point of the Solar system and was later captured into orbit around the Earth.

3. It formed from material cast off by the Earth by an impact of a large body some time after the Earth was formed.

The first two have problems in that neither is consistent with the present Moon's orbit, composition, and other properties the Moon exhibits.

The giant impactor Theory is presently the favorite because it is consistent with the known factors.

[dimreepr]

1 minute ago, Janus said:

Basically there are three models for possible Moon formation.

1. It condensed from the same material and at the same time as the Earth.

2. It formed at some other point of the Solar system and was later captured into orbit around the Earth.

3. It formed from material cast off by the Earth by an impact of a large body some time after the Earth was formed.

The first two have problems in that neither is consistent with the present Moon's orbit, composition, and other properties the Moon exhibits.

The giant impactor Theory is presently the favorite because it is consistent with the known factors.

The mechanism of capture may vary, but that our moon is captured seems obvious.

[zapatos]

5 minutes ago, dimreepr said:

The mechanism of capture may vary, but that our moon is captured seems obvious.

Those are not three examples of "capture". "Capture" has a specific meaning in astronomy.

[dimreepr]

4 minutes ago, zapatos said:

Those are not three examples of "capture". 

I didn't say they were.

5 minutes ago, zapatos said:

"Capture" has a specific meaning in astronomy.

Please explain.

[zapatos]

1 minute ago, dimreepr said:

I didn't say they were.

Please explain.

[Janus]

11 minutes ago, dimreepr said:

The mechanism of capture may vary, but that our moon is captured seems obvious.

"Capture" exclusively means that the Moon was in it own independent Solar orbit before becoming becoming a Earth satellite.  Only one of the three proposed mechanisms involve that.

[dimreepr]

6 minutes ago, Janus said:

"Capture" exclusively means that the Moon was in it own independent Solar orbit before becoming becoming a Earth satellite.  Only one of the three proposed mechanisms involve that.

3

Yes, the one you favoured.

Edited May 6, 2018 by dimreepr

[Janus]

4 minutes ago, dimreepr said:

Yes, the one you favoured.

No.  The only one that involved the Moon forming in an independent orbit was option 2.

Option 3, is the presently preferred hypothesis.    In that model, the material that formed the Moon came from the Earth and was thrown in to orbit around it by the collision, where it condensed into the Moon. 

[zapatos]

9 minutes ago, dimreepr said:

Yes, the one you favoured.

No

[dimreepr]

6 minutes ago, Janus said:

No.  The only one that involved the Moon forming in an independent orbit was option 2.

Option 3, is the presently preferred hypothesis.    In that model, the material that formed the Moon came from the Earth and was thrown in to orbit around it by the collision, where it condensed into the Moon. 

OK, but at what point does that second body, after the impact, become the moon?

[zapatos]

9 minutes ago, dimreepr said:

OK, but at what point does that second body, after the impact, become the moon?

The second body did not become the moon. The debris blasted into orbit, consisting of material from the first and second body, become the moon after it coalesces in orbit.

[dimreepr]

2 minutes ago, zapatos said:

The second body did not become the moon. The debris blasted into orbit, consisting of material from the first and second body, become the moon after it coalesces in orbit.

I realise that, but a collision of these two bodies does fall within a definition of capture and one body becomes the smaller (moon).

[zapatos]

6 minutes ago, dimreepr said:

I realise that, but a collision of these two bodies does fall within a definition of capture and one body becomes the smaller (moon).

As you wish.