Capturing planetary bodies

[Dominatus]

I've never been able to grasp how this might be done. In ksp, I always have to burn retrograde in order to ensure an or it around another body, otherwise I maintain my velocity and am .... out of the gravity well. In the real world, planets are moving far faster than your craft when they enter another planets gravity well. So how is it these planets are captured and go into orbit? Why don't they jus get a boost in velocity form the other body and continue on their way?

[Bryce Ring]

One thought, aero breaking with atmosphere or collision with surface at very low sub surface pe . but this would result in a very elliptical orbit for the aero breaking ? maybe over a very long time it would settle. You have captured my attention, Posting to sub

I imagine there have been many that have resulted in a Velocity change resulting in Planet "Moo Moo" being BBQ'd by the sun or Planet "Marathon" being sent on a mission to somewhere not here.

[Frida Space]

Yes, according to the current capture hypothesis for the two Martian moons, their orbits have been circularized and brought to an inclination of almost 0 degrees thanks to a combination of atmospheric drag + tidal forces. These latter ones in particular would have been enhanced by a binary asteroid. The only problem is time.

[Dominatus]

That can explain asteroids that hit the planets atmosphere but explain Titan. It is thought to be a captured planet, possibly even an exoplanet. How could it have aerobraked without being torn apart from tidal forces?

[Frida Space]

That can explain asteroids that hit the planets atmosphere but explain Titan. It is thought to be a captured planet, possibly even an exoplanet. How could it have aerobraked without being torn apart from tidal forces?

The most widely accepted theory on the formation of Titan is that it resulted from a series of giant impacts against Saturn and a series of Galileian-like satellites. These would explain its high orbital eccentricity and its Oort Cloud-like atmospheric nitrogen.

[worir4]

I was about to mame a thread on this also!

I always wondered how did the Moon get captured? If it was the planet collision theory then why is the orbit so circular?

[Moar Boosters]

I imagine capture would be closer to a docking maneuver than the orbital burn at the end of a hohmann transfer. Remember real planets have exospheres which extend out much further than the atmosphere which could provide gentle aerobraking effects with enough time. It might require the smaller body to enter a cyclical orbit with the larger one, taking many passes for it to eventually be pulled into the same orbit and ultimately captured.

[Dominatus]

Triton, then? If I remember correctly is is also thought to be a captured dwarf planet.

[Newt]

Triton, then? If I remember correctly is is also thought to be a captured dwarf planet.

So it is. I read a paper about how its orbit may have come to be as it is, that paper was speculating about a puffy stage in Neptune's existence, it was possible but fairly difficult to really justify with the evidence they had. There also have been hypothesis that connect Pluto as an old moon of Neptune, and something threw off Triton's orbit, while launching Pluto far away. As far as I know, that is not the current belief, it was from before we realized that there were more trans neptunian objects.

Triton's orbit is weird, though. It is bizarre in how circular it is, e=0.000016 (Luna's e=0.0549), but retrograde, and fairly inclined.

[Bill Phil]

I was about to mame a thread on this also!

I always wondered how did the Moon get captured? If it was the planet collision theory then why is the orbit so circular?

The tidal forces between Earth and Moon pull it away, also circularizing its orbit. Eventually the Moon will die when it starts to fall back and hit the Roche Limit.

[KerikBalm]

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?

I've had some asteroids self capture from interactions with Mun or even minmus - of course, eventually they cross the SOI of one of the moons again, so its not a stable orbit - but IRL that could change through tidal forces and lead to a stable orbit.

Titan seems to be a normally formed moon.

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.

Then it may have interacted with the previous moons of Nepture (which were likely destroyed by Triton's arrival)

I doubt aerobraking would ever lead to capture of a "large" (relatively speaking here, ie large enough to be spherical) body - but residual atmospheric drag will shape the orbit's evolution after capture.

[FenrirWolf]

The current theory is that the Moon wasn't captured, but instead it was the result of a collision between Earth and a Mars-sized body.

[worir4]

The current theory is that the Moon wasn't captured, but instead it was the result of a collision between Earth and a Mars-sized body.

This is what I don't understand.

wouldn't the debris just

1. Be flinged out at escape velocity

2. Crash into Earth

3. Go into a highly eccentric orbit probably later to crash into Earth.

How would it's periapse raise before it crashed back into Earth?

[Bill Phil]

This is what I don't understand.

wouldn't the debris just

1. Be flinged out at escape velocity

2. Crash into Earth

3. Go into a highly eccentric orbit probably later to crash into Earth.

How would it's periapse raise before it crashed back into Earth?

The Moon accreted into a sphere, then tidal forces, magnified by a large factor back then compared to now because it's much closer, brought it into a circular orbit.

[Newt]

Much stuff would have crashed back into Earth, much would have flung far away, and the rest would likely have been in weird eccentric inclined orbits. But it would bump into other material, and average ultimately to a less weird orbit, and, with the help of tidal forces, go into the orbit it is in now.

[Dominatus]

Yes, I actually have another thread (way less attention than it deserved IMHO) that discussed a theory that Mercury may have been the supposed Impactor "Theia"... But the accretion around earth after such an impact happened likely within a century, resulting in two moons forming. Some was flung out into space, a lot probably crashed back into earth, but there was more than enough left behind in a ring-like formation around earth to form the moon. That sort of formation is fairly well explained. But I still am unsure as to how planet-sized bodies, many of which are gas giants, could be captured by another star. There are some large gas giants that were most likely captured by their current parent star. I can't seem to fathom how that might happen, either. Gravitational bumps and the gravity wells of other planets slowing it's velocity somehow, enough for tidal forces to capture it? Or massive impacts with asteroids/ other planets that slowed it's velocity, similar to how Neptune slowed down after the resonance between Jupiter and Saturn dragged it off it's orbit, and the asteroid belt helped to re-circularize it's orbit? (That info is from a documentary I watched 2-3 years ago so it may not be up to date)

[VirtualCLD]

The tidal forces between Earth and Moon pull it away, also circularizing its orbit. Eventually the Moon will die when it starts to fall back and hit the Roche Limit.

I'm fairly certain the moon is slowly getting further away and will eventually escape earth orbit.

[Newt]

I'm fairly certain the moon is slowly getting further away and will eventually escape earth orbit.

Yeah.

Some objects fall in (like Triton), but the Moon is slowly receding from the Earth.

[Bill Phil]

The Moon will actually start to fall back towards Earth eventually. In a LONG time, though. Earth has to face the Moon in a tidally locked position before the Moon begins to get pulled back.

[Newt]

Oh, you are talking about a really huge time scale, then. Are you certain that the Moon would not escape, first? How far out will it have to orbit before that happens?

[Dominatus]

The moon will not escape unless it suffers an impact that gives it enough velocity to do so. It will retreat to the Lagrange point (think that's what it's called) at it's current rate, with both the earths rotation and the moons slowing down. At this point the moon is tidally locked to the earth, and begins gradually falling in... At least that's how I understand it.

[Newt]

The Lagrange points are defined by the Earth's gravity and the Moon's gravity, or generally by the gravity of objects in a two body system. As the Moon moves further out, so do the L points. At present the Moon is tidally locked to the Earth, though the Earth is not tidally locked back.

The L points

The L points are areas, where, the gravity of the Moon and earth balance to some degree. For example, in L1, the gravity of the Moon effectively lessens the gravity of the Earth, such that a body in L1 will orbit the Earth at the same rate as the Moon does. In L2, the Moons gravity compounds with the Earth's, making the object need to orbit faster, matching the orbital period of the Moon.

If the Moon is slowly receding, should it not escape? If the only two objects in the Universe were we two, I concede that it would be unlikely, but from my guess, it would probably move out of the Earth's domain prior to its falling back to Earth in the distant future. Do you have any references?

(I do not mean to sound defensive or argumentative, and if I do, it is only because I am trying to understand what you are describing, and do not understand certain aspects)

Edited January 19, 2015 by Newt
Error with photo needed repair

[Bill Phil]

The Moon's moving outwards is caused by the tides, or essentially the Earth's water sloshing around. Eventually, when Earth becomes tidally locked to the Moon, the tides will be at a constant, and then the Moon will fall back into the Earth. Hitting the Roche Limit, and breaking apart.

[Newt]

Okay. Do you know when that will happen? My very (very) cursory investigation suggests that the Moon moves away currently about 3.78cm/year, and slows the Earth down about 1hr/rotation/9450km receding. In order to stay tidally locked, the Moon will slow down as it moves outward, and so the Earth will have to slow down more still to become tidally locked to the Moon. If when the Moon's orbit hits 1.5 gigameter (about the Earth's Hill sphere. Not a great thing to go by but I am in a rush), it is no longer in orbit, and when the Earth's rotation equals the Moon's orbital period, the recession stops, and all those numbers for changes are constant (which they are not), it looks to me like the Moon should be gone by the time our day is 130 hours, or so.

I would check this more carefully, but have not the time at the moment. Do you have any references as support that the Moon will fall back down, or is it just a (logical) forecast?

[Bill Phil]

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...

Edited January 19, 2015 by Bill Phil