Moons - how can their orbits occur?

[AlexL]

Moons, they exist, they orbit planets.

Wikipedia lists 3 mechanisms for their origin, but I do not understand how two of these would work:

• Formation from impact debris
  
• Capture of a passing object.
  

My current thoughts:

Formation from impact debris:

(eg Earth's moon)

To me, it implies that an object collides with the planet, and that debris 'bounces off' and collects into a single body. However, would all the individual pieces of debris not have a periapsis on, or below the parent body's surface? If so, how would they go up into a higher, stable orbit?

Capture of a passing object:

(eg Mars' moons)

As the passing object, such as an asteroid, nears the planet, where would it receive the change in velocity needed to put it into an orbit from an escape trajectory? If it were from aerobraking, then how could it recieve a boost to raise it out of the atmosphere, a collision along its velocity vector at apoapsis seems unlikely?

Explanations of these methods would be appreciated.

[Themohawkninja]

The impact theory presents on offset impact, whereby a "tail" of debris is created at first, that forms a ring due to a balance between gravity and momentum. (Imagine aerobraking, but the body moves away from your craft, so your periapsis ends up out of the atmosphere.

The passing body theory has nothing to do with aerobreaking. It has to do with the fact that the velocity of the object is too great for the object to be completely attracted into the parent body, but too slow to be at escape velocity.

[Tex_NL]

Don't forget real life orbital dynamics are way more complicated than KSP's. In KSP all bodies have uniform gravity fields and you only need to consider a single one. Real planets and moons have irregular gravity and you're always dealing with multiple bodies.

An asteroid traveling close to earth for example may have it's apogee decreased by earth and its perigee raised by the moon.

[AlexL]

On capture of a passing body, I'm guessing that this is impossible in the current KSP physics model (on which my only orbital experience comes from), with only 1 gravity source acting on an object at a time.

Edit: I need to refresh the page faster.

[john_aaron]

For theory (1): The important limiting factor here is energy, as that determines the object's height from the object it orbits. With sufficient starting energy (and launched at an angle) a rocket in KSP should find itself immediately in an orbit. I found this video especially helpful:

. As you can see, the fact that it was a glancing blow is quite significant.

For theory (2): Indeed, you have hit upon the main problem with this theory as an explanation for the origin of our moon. Proponents of this theory are forced to admit that the Earth must have had an extended atmosphere long ago, something for which they lack evidence. Incidentally, this is a decent idea of how certain Saturnian and Jovian moons were formed, based on what we know about their ancient atmospheres.

[Themohawkninja]

Proponents of this theory are forced to admit that the Earth must have had an extended atmosphere long ago, something for which they lack evidence.

Is the increased thermal output of Earth billions of years ago not sufficient to cause the gases to expand outward far enough to explain the capture theory by ways of aerobraking?

[john_aaron]

Is the increased thermal output of Earth billions of years ago not sufficient to cause the gases to expand outward far enough to explain the capture theory by ways of aerobraking?

Even if the thermal output is capable of that, there is problem with atmospheric density. If we assume the atmosphere was roughly as massive then as it is today (which is optimistic; it was probably thinner. see:http://en.wikipedia.org/wiki/Paleoatmosphere). something like the ISS with relatively little angular momentum and in relatively thick atmosphere takes months to fall 10 km (source: http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html#.Uf77bGTOsdI.

So now the atmosphere has to be stretched to the distance of the Moon. Volume goes as the cube of distance, so this stretches it out really, really thin. The moon has orders of magnitude more angular momentum than the ISS, and would be going through orders of magnitude less dense atmosphere to slow it down. Without even doing concrete calculations you can see why capture is unlikely.

Edited August 5, 2013 by john_aaron

[metaphor]

For option 2, aerobraking or an atmosphere has nothing to do with it. The reason it works is that more than one body's gravity is acting on the moon at the same time, and there are also tidal forces at play. For example (if there were no Moon), an asteroid which has barely above escape velocity comes into the Earth-Sun L1/L2, and falls towards Earth with a low periapsis. During this first orbit, the gravitational pull of the Sun/Jupiter/tidal forces from Earth can all change the asteroid's orbit a little bit so that its energy falls a little below escape, and its apoapsis is lowered into the Earth's sphere of influence. That's how the Earth could get a new moon, with its orbit eventually circularizing over millions of years from tidal forces.

Earth's moon was probably formed by the impact theory, but most of the outer moons of Jupiter and Saturn are captured moons, as well as Neptune's moon Triton. Some of Jupiter's outer moons are "temporary", their orbits lasting only a few thousand/million years before gravitational perturbations give them enough energy to escape again.

[Themohawkninja]

Even if the thermal output is capable of that, there is problem with atmospheric density. If we assume the atmosphere was roughly as massive then as it is today (which is optimistic; it was probably thinner. see:http://en.wikipedia.org/wiki/Paleoatmosphere). something like the ISS with relatively little angular momentum and in relatively thick atmosphere takes months to fall 10 km (source: http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html#.Uf77bGTOsdI.

So now the atmosphere has to be stretched to the distance of the Moon. Volume goes as the cube of distance, so this stretches it out really, really thin. The moon has orders of magnitude more angular momentum than the ISS, and would be going through orders of magnitude less dense atmosphere to slow it down. Without even doing concrete calculations you can see why capture is unlikely.

Now what about how I thought how the capture theory worked, whereby the velocity of the object is just so that the velocity is too high for the gravitational attraction of the parent body to cause an impact, but to low to be escape velocity. There seems to be too many moons in the universe to allow for aerobraking to be the only method if you ask me*.

*This has no scientific basis, but rather that aerobraking would need to be very precise in order to reduce the velocity enough but not too much, and not having the object return back into the atmosphere towards the end of it's first "orbit", whereby gravity only capture would purely depend on velocity.

[magnemoe]

Now what about how I thought how the capture theory worked, whereby the velocity of the object is just so that the velocity is too high for the gravitational attraction of the parent body to cause an impact, but to low to be escape velocity. There seems to be too many moons in the universe to allow for aerobraking to be the only method if you ask me*.

*This has no scientific basis, but rather that aerobraking would need to be very precise in order to reduce the velocity enough but not too much, and not having the object return back into the atmosphere towards the end of it's first "orbit", whereby gravity only capture would purely depend on velocity.

Yes you has to hit an 30 kilometer thick area to far in an you crash, to far out and you miss, note that even puny moons like mars ones are to large to aerobrake.

The bottom part will hit the ground. Another issue is that tidal forces will break them up before hitting the atmosphere unless they are solid.

And yes your Pe has to be lifted out of the atmosphere even with gravity forces from another moon this is also unlikely.

gravity capture on the other hand can work over millions of years slowly aligning the orbits until you get an orbit, yes you would probably either crash or slingshot it away but the chances are far higher.

[metaphor]

A moon can't get close enough to a planet to go through its atmosphere without breaking up from tidal forces first, because it's going to be inside the planet's Roche limit.