Auto Capture - How does it work?

Spike Spiegel

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I understand how to achieve a circular orbit with a spacecraft, thanks to Orbiter. What I don't understand is how natural objects do it. I saw a post in the forum asking the same question, but it was off-topic and the answer was basically "tidal interactions over a long time". (I'm paraphrasing a bit.)

I'm trying to understand the process in more detail. My assumption is that an object would have to come in at just the right speed (say, slower than escape velocity for the larger body) and just the right angle. Initially, my questions are:

1. What's the "right" speed?
2. What's the "right" angle?
3. How does the orbit become circular (or low eccentricity)?
 

Mantis

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I'm not an expert but based on my understanding, 1 and 2 depend entirely on the capturing body. With regards to number 3, it usually doesn't - "moons" that are actually captured asteroids usually have much more eccentric orbits than naturally formed moons. In fact it's not unusual for such objects to eventually collide with the capturing body. Take the moons of Mars which are thought to be captured asteroids. Phobos is in retrograde orbit (west to east) and tidal forces are slowing decreasing the radius of it's orbit and stressing it to the point that it will eventually break up and the fragments will impact the surface.
 

n72.75

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If you think about it: any object entering a circular or elliptical orbit from a hyperbolic trajectory would have to louse energy. This could happen by collision or by entering the Roche limit.

So to answer your question about the "right" angle and velocity. The only "right" ones are ones along a circular or elliptical trajectory.
 

tblaxland

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My assumption is that an object would have to come in at just the right speed (say, slower than escape velocity for the larger body)
That is impossible - if the object is not already in orbit around the planet, the lowest speed it can approach at is the escape velocity.

Ultimately, there needs to be some way to reduce the orbital momentum of the object with respect to the planet. This could be:
  • a collision with the planet or one of its existing moons (or a specialised version of a collision - aerobraking)
  • momentum exchange with existing moons through gravitational forces
  • ???
 

jedidia

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That is impossible - if the object is not already in orbit around the planet, the lowest speed it can approach at is the escape velocity.

Well, the "Slingshot-principle" might provide an option for this scenario: a small body with almost the same orbit as a larger one gets overtaken, and draged along enough to raise it's velocity below escape velocity ("raise below"?? just got to love orbital mechanics... :lol:). But I guess this would be a V-E-R-Y specialized case that could be described as "almost impossible". For one, if the Orbits are similiar enough, the two bodies might never meet...

And for the orbits to be similiar enough, they probably would have had to collide one or two times already to match orbits...
 

Spike Spiegel

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I've found this article about a comet that was temporarily captured by Jupiter:

http://www.astronomy.com/asy/default.aspx?c=a&id=8616

And apparently there are a few others known. The problem is that it's temporary... this comet left orbit again.

So, if an object can't approach a planet at a speed lower than the planet's escape velocity, then it has to lose energy, like n72.75 said. If it does this by aerobraking, I'd expect the captured object to collide with the parent object relatively quickly, since it has no way of raising its periapsis after achieving orbit.

In the case of a moon like Phobos or Deimos, a collision could have been responsible... but it would have to be a small one. I can't imagine a tiny object like that surviving much of an impact.

This process still puzzles me. If Phobos and Deimos are both captured asteroids, there must have been some very rare circumstances that resulted in their near-circular, near-zero inclination orbits. Do objects tend to stabilize into equatorial, circular orbits (and how)?
 

Grover

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i was doing this very topic in physics recently:

basically, anything can have the right orbital properties, and these can be changed by other orbiting bodies or other things. most bodies in the solar system (all as far as i know) were created at roughly the same time at the formation of our solar system, in the order: mercury, venus, earth, mars, [neptune, uranus], jupiter, saturn [and unknown gas giant, which was knocked out of orbit somehow and crashed into the sun, all theory, but still interesting :D]

as you should notice, uranus and neptune are in the wrong place, and back to front. this is because an alignment between jupiter and saturn caused them to accelerate away from the sun due to the high gravitational force that they exerted. this caused them to move to the other side of the two gas giants and invert their positions. they then orbited into an asteroid belt shich slowed them down to their current orbits (and the meteor shower that they generated covered our mon with craters and brought water to earth!).

this is a good example of orbits changing, and natural capture. as far as i know, there is no automatic balancing system to bring them into perfect orbits (in fact, there is no perfect orbit in our solar system), but rather chance. those that were formed at the birth of out own system are an exception; due to the slow formation, and the was that the gasses and dust formed into rock, they were already in perfect orbit before they became the planetary bodies that we see today, the leftover material from the sun's birth orbiting, was attracred together into planets, and their proximity to the sun determined the planet type (eg gas giant, solid structure and so on)

this is currently the most sensible theory i can find, ive 'expanded' a little (sorry about that) but to e entirely honest, nobody can truly answer your questions, because we have never found the right answer

its a case of which theory you believe

happy orbiting!
-=Grover=-
 

jguillaumes

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Phobos orbit is NOT retrograde. I guess you are a little bit confused by the fact that the OBSERVED trajectory of Phobos from Mars surface is west to east instead of the usual east-to-west because the orbital period of Phobos is shorter than the Mars rotation.
 

Urwumpe

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You can also achieve initial capture at the weak stability boundary. The border of the gravity well of a planet is not static, it changes over time, can locally expand or shrink. An asteroid close to this region with only tiny velocity difference to the planet could slip into the gravity well of the planet in hyperbolic orbit and be forced to leave the gravity well at a point with higher potential, making it loose energy and slow down to be in a huge nearly circular orbit around the planet.

Another such "wave" in the gravity field could make the same asteroid drop closer to the planet - multi-body dynamics are great for people who like fractals.
 

Hielor

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basically, anything can have the right orbital properties, and these can be changed by other orbiting bodies or other things. most bodies in the solar system (all as far as i know) were created at roughly the same time at the formation of our solar system, in the order: mercury, venus, earth, mars, [neptune, uranus], jupiter, saturn [and unknown gas giant, which was knocked out of orbit somehow and crashed into the sun, all theory, but still interesting :D]

as you should notice, uranus and neptune are in the wrong place, and back to front. this is because an alignment between jupiter and saturn caused them to accelerate away from the sun due to the high gravitational force that they exerted. this caused them to move to the other side of the two gas giants and invert their positions. they then orbited into an asteroid belt shich slowed them down to their current orbits (and the meteor shower that they generated covered our mon with craters and brought water to earth!).

this is a good example of orbits changing, and natural capture. as far as i know, there is no automatic balancing system to bring them into perfect orbits (in fact, there is no perfect orbit in our solar system), but rather chance. those that were formed at the birth of out own system are an exception; due to the slow formation, and the was that the gasses and dust formed into rock, they were already in perfect orbit before they became the planetary bodies that we see today, the leftover material from the sun's birth orbiting, was attracred together into planets, and their proximity to the sun determined the planet type (eg gas giant, solid structure and so on)

this is currently the most sensible theory i can find, ive 'expanded' a little (sorry about that) but to e entirely honest, nobody can truly answer your questions, because we have never found the right answer

its a case of which theory you believe
Uh... :facepalm:

I think Occam's Razor might have something to say about this one.
 

Urwumpe

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Uh... :facepalm:

I think Occam's Razor might have something to say about this one.

And the Nice model.

[ame="http://en.wikipedia.org/wiki/Nice_model"]Nice model - Wikipedia, the free encyclopedia[/ame]

Grovers theory sounds like it was inspired by the data-free predictions of Zecharia Sitchin, like in his book "The twelfth planet"..
 
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Urwumpe

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I can accept uranus and neptune swapping places at some point.

I have a hard time accepting both of them moving out past Jupiter and Saturn.

Just like it is very hard accepting the existence of a gas giant that disappeared without a trace. ;)
 

Spike Spiegel

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Thanks for all the responses so far.

The theories do seem to share a few similarities, but the one Grover outlines has some details that appear to be more unlikely.

About this Nice model; if the orbits of Uranus and Neptune became more eccentric to the point where they migrated further out into the planetesimal disk, then wouldn't the interactions with those planetesimals lower the planet's periapsis? Shouldn't that result in greater eccentricity?
 

Urwumpe

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About this Nice model; if the orbits of Uranus and Neptune became more eccentric to the point where they migrated further out into the planetesimal disk, then wouldn't the interactions with those planetesimals lower the planet's periapsis? Shouldn't that result in greater eccentricity?

These planetesimals are in total something like 1% of the mass of the solar system, the interaction with them is tiny to negligible.
 

tblaxland

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Well, the "Slingshot-principle" might provide an option for this scenario: a small body with almost the same orbit as a larger one gets overtaken, and draged along enough to raise it's velocity below escape velocity ("raise below"?? just got to love orbital mechanics... :lol:).
In a slingshot, the relative angular momentum of the two bodies is unchanged, so one body approaching another will not enter orbit around it without some other momentum exchange occurring. Such a momentum change may occur with another moon in the system or with other planets in the system, this is what happens at the weak stability boundary that Urwumpe mentioned.

In the case of a moon like Phobos or Deimos, a collision could have been responsible... but it would have to be a small one. I can't imagine a tiny object like that surviving much of an impact.
Or it could have been a large collision with Mars itself, the moons being the accretion of the debris.

This process still puzzles me. If Phobos and Deimos are both captured asteroids, there must have been some very rare circumstances that resulted in their near-circular, near-zero inclination orbits.
Indeed, it is still the subject of much speculation and study amongst better minds than mine.

Do objects tend to stabilize into equatorial, circular orbits (and how)?
Tidal forces can circularise an orbit. For example, our Moon's tidal effect on the Earth slows down the rotation of the Earth. The momentum lost from Earth's rotation is added to the orbital angular momentum of the Moon and the Moon moves a little further from Earth. Since the tidal affects are greater at periapsis than at apoapsis (because the gradient of the gravitational field is higher closer to the planet), the periapsis tends to rise faster than the apoapsis and the orbit becomes circularised. It takes a long time though - the Moon's semi-major axis increases at about only 38 mm per year.
 

Keatah

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You mean to say jupiter and saturn were the outermost planets? And they moved uranus and neptune to positions 7 & 8? kewl!
 
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