Capturing planetary bodies

[Hannu]

tidal forces will make an orbit more circular over time, they wouldn't be involved in a capture much though.

Have you never had an asteroid self capture in KSP?

No, but if I remember correct, I have had spacecraft captured aroind Jool's moons. However, it is due to inaccuracy of model. It would be impossible in realistic newtonian system.

Triton may have been part of a binary system (like Pluto-Charon), and then its possible for one of the members to be captured while the other escapes, but we'd never see such a situation in KSP.

As far as I know, captured moons are parts of binary objects. There was much more bodies couple of billions of years ago and binary objects were common. When binary asteroid fly by a planet it is possible that one component gives energy to another and get captured on orbit. Other body leaves the planet.

There are also other possibilities of many body interactions. For example collision. There are groups of small asteroid like moons aroun Jupiter. Their origin may be collision of two larger bodies near Jupiter. Larger moon of the planet can also take excess energy of body flying by.

[KerikBalm]

No, but if I remember correct, I have had spacecraft captured aroind Jool's moons. However, it is due to inaccuracy of model. It would be impossible in realistic newtonian system.

No it would not. Gravity assists work in KSP, as in a realistic newtonian system.

Approach the Mun from behind, get flung off to escape velocity, approach it from the front (as in a free return trajectory), and lose energy (in a realistic system, the Mun would gain energy... but its on rails, and the amount would be miniscule anyway). Just as you can use it to get a free return trajectory, interaction with a moon can cause a capture around a planet. It may later be ejected by the same moon, crash into the moon, rash into the planet... but if the parameters are right, tidal forces, etc can circularize the orbit and make it stable before it encounters the moon again.

[GreeningGalaxy]

This sounds a little familiar... when I was in high school, we studied the solar system a bit, and we were told that Mars's moons were captured asteroids. I joked that they used aerocapture and a short burn at apoapsis to do that, but no one understood what I was talking about, of course.

In a perfect Newtonian system, captures like that might be impossible (from the frame of reference of the capturing body this seems easily the case, since something that drops into its gravity well will inevitably find its way back to the starting altitude with equal speed as before, just going the other way). However, in the real universe, we have tidal forces that drag on eccentric orbits at their periapses, and it is possible at the very least for an object to get a very slow flyby by orbiting the sun (or whatever parent body) slightly above the body it will encounter, and simply waiting to be caught up to. If this happens slowly enough, the low speed of the flyby could probably make it take long enough for the tidal forces to drag the orbit's eccentricity down below 1 and capture the incident body into orbit.

[Hannu]

No it would not. Gravity assists work in KSP, as in a realistic newtonian system.

This is not true. Patched conic approximation produces large errors. For example very slow transition over Kerbin's SOI changes velocity about 200 m/s.

In most practical cases slingshot maneuvers work qualitatively quite correct way. However, Jool's large moons have very small SOIs and errors are also qualitatively very severe. But this kind of capture is impossible in patched conics model. It ignores interactions with other moons, which in some very special situation could cause a capture. In patched conic model what comes in SOI goes out of the SOI if it does not change velocity in SOI. Orbit is an ellipse with apoapsis outside of the SOI or a hyperbola. My experience was due to error in SOI crossing detection. It was many versions ago and maybe it is fixed after that.

but if the parameters are right, tidal forces, etc can circularize the orbit and make it stable before it encounters the moon again.

KSP does not take tidal forces into account. Things are handled as point masses.

[Deadpangod3]

The Moon gains momentum while Earth's rotation loses it. So, when they relatively stop moving, no momentum exchange will happen, but the varying gravity will cause the Moon to lose momentum, and beyond that I don't know. This is all very far into the future. And Earth is slowing down by about 15 microseconds a year..

I'm having a bit of trouble finding a source. i found one a while back explaining that the momentum exchange will eventually stop, resulting in the Moon reaching the Roche Limit in the far future, but I can't seem to find it. I'll have to get back to you...

EDIT:

Okay...

It seems that the Earth-Moon system will be tidally locked to each other, and then momentum will be transferred between it and the Sun. This causes the Moon to lose momentum and hit the RL. But it's about 50 billion years in the future, which is well beyond the Sun's giant phase...

asuming the sun didnt go kapoof in a few billion years, eventually the moon would reach the height where through rising and slowing earths rotation, it both will be tidally locked, and it would stay there, also, tides are caused by the moon, they dont keep the moon in orbit

but the sun will be long gone by then, if not earth in general.

[Newt]

Yeah. So much can happen in fifty billion years. Like the entire history of the Universe after the Big Bang, several times over.

As for capturing objects, I think part of the issue is that KSP has rather few asteroids, whereas the Solar System has many, many, many asteroids and other objects. It does not have to be a simple thing for a rock to land in Mars orbit, if there are enough objects that pass close enough, one may eventually have just the right circumstances to become a lasting satellite.

[Bryce Ring]

Have you guys/gals ever played Universe Sandbox ?

[KerikBalm]

In a perfect Newtonian system, captures like that might be impossible (from the frame of reference of the capturing body this seems easily the case, since something that drops into its gravity well will inevitably find its way back to the starting altitude with equal speed as before, just going the other way). However, in the real universe...

Again, you forget the presence of a 2nd body orbiting the first. Earth can capture things in ways that venus cannot because of our moon.

This is not true. Patched conic approximation produces large errors. For example very slow transition over Kerbin's SOI changes velocity about 200 m/s.

Yes, there are situations where there are large errors.... but you misread and didn't understand my post.

Gravity assists work in real life.

Gravity assists work in KSP.

In both cases the body's motion + gravity can add or subtract from the KE of the object (which you want to capture).

In KSP, the edge of the SOI is sharply defined.

In real life, we can mathematically describe a "sphere of influence", and most of the effect will take place within that sphere, but its not like the force of gravity becomes zero when crossing the threshold.

Still the basic principles are the same.

this kind of capture is impossible in patched conics model.

No its not, and it happens often in KSP on its own.

It ignores interactions with other moons, which in some very special situation could cause a capture. In patched conic model what comes in SOI goes out of the SOI if it does not change velocity in SOI. Orbit is an ellipse with apoapsis outside of the SOI or a hyperbola. My experience was due to error in SOI crossing detection. It was many versions ago and maybe it is fixed after that.

You seem to be confusing the conics not displaying correctly with captures.

"what comes in SOI goes out of the SOI if it does not change velocity in SOI."

Yes, and velocity changes can happen when you enter and exit an SOI. Very similar to real life... if you are in front of the moon, and its going 200 m/s faster than you, then after falling towards it, passing PE, and going towards escape again... you'll now be going 200 m/s faster than then moon instead of slower than it. This basically works the same way in KSP and real life, just with sharp transitions at the edges (though the majority of the effect has already taken place, so the outcome is very similar, except when the crossing happens very slowly.)

When one enters the SOI of a planet, you'll be on an escape trajectory right away again (assuming you don't intersect the planet)

If you happen to encounter one of the planets moons on the way, your velocity relative to the planet can change significantly, and the result will be a capture with an orbit intersecting the moon's orbit -> thus eventually it will encounter the moon again.

KSP does not take tidal forces into account. Things are handled as point masses.

I know, I was talking about what would follow in real life.

[Hannu]

but you misread and didn't understand my post.

Oh, I see. You thought that you use moon which is already in planet's SOI to lose energy and I though a situation of single or binary object without interaction with Moon.

[Rosco P. Coltrane]

This just happened to me and reminded me of this post... Self circularizing capture? Hell yeah, right? Well, no... KSP's predictions were WAY off. Didn't take a screenshot, but I ended up leaving the Duna system for good.