Real-life orbital questions

[zarakon]

Everything I know about orbital mechanics, I learned from KSP

So that has led me to a couple questions..

1. How does an asteroid get "captured" by a planet to become a moon? It seems impossible to enter orbit around a planet without a powered deceleration.

2. The giant impact theory for the formation of Earth's moon says that some large object crashed into the Earth, and the debris sat in orbit until it eventually all coalesced to form the Moon. But how did that debris end up in orbit? It seems that with only an initial velocity from the surface, each piece of debris would either escape the system or fall back to Earth.

[nhnifong]

As for the captured asteroids, there are other forces slowing them down, like tidal forces, and interaction with magnetic fields.

As for the parts of the moon, they are not just gravitationally attracted to the earth, they are gravitationally attracted to each other.

This is just my guess, I hope someone will have a better answer. This is a great question by the way you have the mind of a scientist.

[A Fat Pokemon]

Well, I'd say that with the asteroid, it might have entered the system, and just got captured by the planet, didn't have to be going fast at all.

With the moon, well, if something hit the earth, and debris came off, the Earth's orbit would have been altered maybe, which allowed for the debris to have an orbit that wouldn't bring it back, crashing into Earth. Also, not just the Earth's gravity would be acting upon the debris, the sun could have helped with the creation of the orbit.

This is just what I think, as a lot of my knowledge also comes from KSP but I also have a bit of background knowledge on it

[Bluejayek]

In relation to the moon question:

Material that has been ejected in an impact of that nature would be very hot. I think it is reasonable to assume it would be molten, with some portion of it possibly vaporizing. This could cause geysers of vapor off small chunks of it that could act somewhat like rocket engines to give it the little boost into orbit. Further, most of it DID fall back down to earth; the moon is a lot smaller!

Another point is, it may be more accurate to think of this not as one object that has a certain amount of matter ejected to form another, but one object that is shattered, and then coalesced to form two new objects of different masses. That way, the asteroid hits, breaks apart the earth, the bits float around in orbit around the sun and slowly are attracted together to form the earth and the moon.

[illectro]

Asteroid capture is typically a 3 body process, the planets that have captured asteroid moons also have other moons which have acted during the capture process.

This of course makes Gilly's classification as a captured asteroid questionable, but it's entirely possible that there was a third body which has since been lost back into a heliocentric orbit.

[illectro]

As for the moon, the giant impact hypothesis has some material being vapourised into a gas phase, this gas cloud induces drag on the particles that are ejected and circularised their orbits before they fall back. Essentially you end up with planar debris cloud, some of which falls back, some of which gets driven away by the solar radiation, and some which collects back to form the moon. Since the light elements get blown away faster this would explain why the moon has such a low volatile content compared to Earth/Mars/Venus

[maltesh]

An incoming object can gravitationally interact on a close pass with a satellite object already in orbit of the planet, and exchange momentum as it swings past the satellite. If enough momentum is lost, the incoming object can wind up captured, and if enough momentum is delivered to the satellite, the satellite may wind up being ejected.

There are also weird paths to capture that result from the actual gravitational gradients in a multiple-body system that Kerbal Space Program does not simulate.

When considering the Giant Impact Hypothesis, you have a /lot/ of mass being splashed around and interacting with itself gravitationally.

[zarakon]

Asteroid capture is typically a 3 body process, the planets that have captured asteroid moons also have other moons which have acted during the capture process.

This of course makes Gilly's classification as a captured asteroid questionable, but it's entirely possible that there was a third body which has since been lost back into a heliocentric orbit.

Ah, gravity assist from a moon, that would make sense.

[PakledHostage]

There's also the case where a binary asteroid pair gets close enough to a larger celestial body that one of the two gets captured by the celestial body while the other escapes. This is the same mechanism that is believed to produce hypervelocity planets and stars. This mechanism could also explain how Gilly's moon was captured.

Edited October 17, 2012 by PakledHostage

[Vanamonde]

What excellent questions! To rephrase Zarakon's point, out of all the possible velocities two objects might have relative to each other, what are the odds that the conditions are right to result in a closed orbital path? The odds that this would result by chance are indeed pretty low, which is why in the game, it only results from intentional manuevering. But as the other posters have already noted, random trajectory isn't the only factor at work.

Bluejayek makes the interesting point that the Earth-Moon system may have been one object that became two, but is still one system. It's not coincidence that they ended up at appropriate velocities to orbit each other, because that same mass was ALL at the same velocity (except for rotation around its axis) before the impactor came along and made a mess out of things.

As for Gilly, perhaps it's the rocky core of a captured comet, the rest of which evaporated, with the outgassing serving as reaction mass that was ejected in the process of arriving at its current orbit.

[Yeomans]

In response to the moon question, it's also important to note that if this collision took place, it would have happened during a time when the Earth was still mostly molten. Molten rock acts in a much different manner than non-molten rock, and if you can imagine it, the collision probably yielded a large mass of molten rock into space, but enough for the mass to bind together under the force of gravity. In this early time of the planet's development, it wasn't as dense, and so it's sphere of influence, albeit would have been weaker, but more widespread. An unstable mass such as the soon-to-be moon could have been easily captured if it was ejected from the planet at a relatively slow velocity, but not too slow as to cause it to rebound. You have to also remember the moon bears a significant pull on the Earth, and so being completely ejected from the earth's SOI isn't as easy as it sounds.

[illectro]

what are the odds that the conditions are right to result in a closed orbital path?

Very small for each encounter, but over the billions of years it takes a planetary system to form you'll have lots of chances.

As for Gilly, perhaps it's the rocky core of a captured comet, the rest of which evaporated, with the outgassing serving as reaction mass that was ejected in the process of arriving at its current orbit.

Outgassing from a comet would not alter a body's orbit sufficiently quickly to result in a capture around a planet, ok there's a finite chance, but the window for this to happen millions of times smaller than the window for a 3 body capture.

[Vanamonde]

Very small for each encounter

Right, which I think was part of the original poster's confusion; he was comparing single spaceship-to-world encounters he's coordinating in KSP with the results of real-universe situations in which many bodies are flying around, but only a few are captured as moons.

Outgassing from a comet would not alter a body's orbit sufficiently

My suggestion was somewhat whimsical, in the same spirit that allows Minmus to be frozen while Kerbin is not. Sorry I wasn't clear about my tone.

[UNSC]

I just had another one of those "am I playing a game or taking a class?" moments.

[Elmach]

Since we are going with whimsical ideas, how about this one:

Bop and Gilly were once a binary asteroid (Is this the right term?) between Duna and Jool. Their resonance with Jool ejected them into the inner planetary system, sending them into an encounter with Eve.

The interaction between Kerbol, Eve, Bop, and Gilly caused Gilly to be captured by Eve, but Bop to be ejected into an orbit crossing Jool's. (Is this possible?)

Bop encounters Jool, and gets captured, with the help of Laythe, Vall, and Tylo. The stable resonance between Laythe, Vall, and Tylo leads to their orbits not changing much, but Bop's low mass allows relatively little force to capture it. (Not little with respect to spacecraft, moreso the mass of Laythe, Vall, and Tylo.)

Is this plausible?

(A minor note: Tylo should be moved a quarter of an orbit ahead, so the Laplace resonance is stable, not unstable.)

[Jesrad]

I just had another one of those "am I playing a game or taking a class?" moments.

Fun in games comes from learning new tricks and applicable theories, according to Raph Koster. I think that is why KSP is so much fun

[geb]

Another way that you can have an asteroid become captured is if it breaks apart while passing by a planet. If you've got a spinning asteroid that splits into two parts, one part could be flung back and captured, while the other part is flung forward and continues on escape trajectory.

Some asteroids are really loose collections of rubble, held together only by weak gravity, and could break apart really easily from a slight nudge like you might get from tidal interaction with a planet.

Other asteroids are a bit more solid and spin quite fast so that only material strength keeps them held together. If something like that got hit by a smaller rock, it could fly apart too.

[Dweller_Benthos]

Best explanation of the moon impact I could find quick.

Of course, all theory, but pretty cool anyway.

[pwnedbyscope]

another video to go with dwellers one

[Jason Patterson]

Very small for each encounter, but over the billions of years it takes a planetary system to form you'll have lots of chances.

This was my thought as well. Take millions of asteroids and billions of years. Shake well.

[deviouswatch]

From what I understand, as long as an asteroid enters a body's gravitational field with less velocity than the escape velocity required to remain on their current trajectory, the asteroid is at least largely affected by the gravity of the planet. Usually this will involve ridiculous parabolic orbits or escape trajectories unless the moon and earth both interact to bring the velocity relative to Earth slower and then we may see a respectable elliptical trajectory.