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My kerbin have a third moon !


Elowiny
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Hello ! I don't know if it happened to you, but my Kerbin have a third "Moon". An asteroid put itself in Kerbin orbit without needing redirection !

Do it happened to you ? Is it rare ?

However, this asteroid quitted Kerbin orbit a bit after, so it was a temporary moon :(

Edited by Elowiny
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yeah this does happen from time to time. I don't think its possible for one to fall into a stable orbit on its own though(?), so like you said, they're temporary.

I guess if one happened to put itself on an course to aerobrake it could happen, but you'd still have to get a craft up to it and be focused on it as it went through the atmo.

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yeah this does happen from time to time. I don't think its possible for one to fall into a stable orbit on its own though(?), so like you said, they're temporary.

I guess if one happened to put itself on an course to aerobrake it could happen, but you'd still have to get a craft up to it and be focused on it as it went through the atmo.

In reality, it is possible for an asteroid or moonlet to be captured and end up in a stable orbit, but it's extremely rare. As far as I know, the only way for it to happen is for two small objects orbiting each other, they enter the sphere of influence of the larger body. After the encounter, one of the two smaller objects has been "sped up" on its hyperbolic trajectory, and the other slowed down. It's possible for the slowed down one to actually find itself on a closed high eccentricity orbit that way. Over a few hundred million years, this orbit will tend to circularise due to tidal forces and whatnot. That's thought to be how Triton was captured by Neptune.

Needless to say, this does not happen in KSP, because the asteroids have no gravity of their own. And even if I did, I don't think the code allows for spheres of influence to change, which would be necessary for such an event to occur.

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Yeah, this happens sometimes. Including in real life! For example, there was an asteroid a while back that was captured in earth orbit by the help of the moon. After several months, though, it eventually left earth's SOI. Pretty cool though, eh?

I've had some close natural captures myself, but nothing has actually put itself in a stable orbit.

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Hello ! I don't know if it happened to you, but my Kerbin have a third "Moon". An asteroid put itself in Kerbin orbit without needing redirection !

Do it happened to you ? Is it rare ?

However, this asteroid quitted Kerbin orbit a bit after, so it was a temporary moon :(

Things like this even happen in the Real Solar System.

There was once this S-IVB stage of Apollo 12 that flew past the Moon and into interplanetary space. Years after that it came back and re-encountered Earth's system and stayed there for a while until it left again.

J002e3f_orbit.gif

Sauce: http://en.wikipedia.org/wiki/J002E3

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It is pretty rare in game, and was probably either captured by the mun (or less likely, Minmus), or entered orbit due to slight physics errors from rounding, time acceleration, etc.

If it was captured by the Mun, it's orbit would be Mun crossing, meaning that eventually (whether it takes 4 orbits or 40billion orbits) it would enter the Mun's SOI again. And after a few encounters (again, it might take hundreds of encounters, or just one) it would be sent back into solar orbit, or be set on a collision course with Kerbin/the Mun.

Edited by sporkafife
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I wish I understood this - once a satellite has been captured by a body and made a couple of orbits, how does that satellite then gain enough kinetic energy to reach escape velocity without input from some external energy source? With J002E3 is the additional energy just provided by crossing the moon's orbit?

Cheers,

Adam

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Aerobrake isnt necessary, a gravity assist from a moon is enough (as our apollo stage shows), but if the moon can capture it, it can also throw it back out

That.

Aerobraking is impossible anyway, since aerodynamics aren't considered for anything that you're not currently within 2.5km of.

Anything that appears to have entered a stable Kerbin orbit will have done so because it was captured by the Mun or Minmus (although unlikely, because it's so small). That object will have an orbit which crosses the orbit of either moon at some point, and so eventually it will either be 'assisted' back into interplanetary space by a moon or crash into the same moon.

In other words, you'll have a 3rd moon for a while, but it won't last forever. Unless time warp messed with physics at the right time, that is.

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I wish I understood this - once a satellite has been captured by a body and made a couple of orbits, how does that satellite then gain enough kinetic energy to reach escape velocity without input from some external energy source? With J002E3 is the additional energy just provided by crossing the moon's orbit?

Cheers,

Adam

You might find this article interesting then:

http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html

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You know when you're spending too much time on the Internet when you can't stop reading "J00 2, E3"

Anyway, one possibility that has not been discussed yet is, that it came into an orbit that was just slow enough to still be elliptic, yet fast enough to have its Apokee a teeny tiny bit outside of Kerbins SOI. In map viev, that would be shown as "stable" (i.e. a closed ellipse) but the game would eject it from Kerbin at, or shortly before, the next Apokee

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You know when you're spending too much time on the Internet when you can't stop reading "J00 2, E3"

Anyway, one possibility that has not been discussed yet is, that it came into an orbit that was just slow enough to still be elliptic, yet fast enough to have its Apokee a teeny tiny bit outside of Kerbins SOI. In map viev, that would be shown as "stable" (i.e. a closed ellipse) but the game would eject it from Kerbin at, or shortly before, the next Apokee

I've had them already captured. No aerobraking required. Monitored them from the Space Center and they stayed in orbit for years.

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I've seen the "asteroid captured by the Mun's gravity" at least once.

But to the picture above me: isn't the issue there that elliptic orbits with apoapsis outside the sphere of influence (yes, those exist) don't show the SoI exit point? (Or maybe it's that that point only shows up when you're inside the sphere of influence?)

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I think it likely happens in KSP due to errors introduced during SOI changes if you're at a high time warp. Asteroids generated by KSP usually only have a few m/s beyond escape velocity (and sometimes, it's pretty clear that they're in a non-escape orbit, and only escape because the AP lies beyond Kerbin's SOI). If you time warp really quickly, it's quite possible that the velocity and entry point are shifted though that it just gets captured.

A Munar gravity brake is also possible, but that can only happen if the asteroid's orbit intersects the Mun's.

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Ah - OK. So the moon adds to the velocity and as J002E3 exits the moon's SOI on the opposite side to the earth, that's enough energy to escape. I guess if it exited the moon's SOI towards the Earth, it would still remain in orbit. Got it (I think)!

Yes, but since the orbit still crosses that off the moon, it will eventually encounter the moon again, and get another gravity assist. And then it can again get kicked out, or kicked in even further and crash into the planet. Or just get some weird ass new orbit and get another chance at getting kicked out

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In reality, it is possible for an asteroid or moonlet to be captured and end up in a stable orbit, but it's extremely rare. As far as I know, the only way for it to happen is for two small objects orbiting each other, they enter the sphere of influence of the larger body. After the encounter, one of the two smaller objects has been "sped up" on its hyperbolic trajectory, and the other slowed down. It's possible for the slowed down one to actually find itself on a closed high eccentricity orbit that way. Over a few hundred million years, this orbit will tend to circularise due to tidal forces and whatnot. That's thought to be how Triton was captured by Neptune.

The question is what timespan is needed that it can be called a "stable orbit" ? In the end no orbit is stable, they all drift slowly away from each other, affect each other and so on (just look at the whole Jupiter system, his moons are playing a very interesting gravity game to each other, doing weird things like loosing constantly mass to the ring systems)...

And as a funny sidenote, Earth "currently" has a second moon, since at least 775 years and for around the next 165 years "2014 OL339" - it's officially a quasi-satellite, but hard to say after how many years in an orbit it can be called stable ;)

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I wish I understood this - once a satellite has been captured by a body and made a couple of orbits, how does that satellite then gain enough kinetic energy to reach escape velocity without input from some external energy source? With J002E3 is the additional energy just provided by crossing the moon's orbit?

Cheers,

Adam

There is a misconception that there is no gravity in space...

Gravity is everywhere in space... Everything is pulling on everything else all the time... only to lesser or greater extents depending on proximity. Everything is also moving...

If there was no gravity in space then you couldn't have an orbit. Orbits are only possible because you are being pulled on by the gravity of the object you are near.

If you cross behind an object in it's orbital path then you will be pulled toward it... Adding velocity to you and changing your direction a little... but since it is moving away from you and your direction of travel is in approximately the same direction as it then you have a net gain of velocity.

If you cross in front of the object... you get pulled toward it and the direction of your travel changes a little to be more retro grade... thus reducing your velocity relative to the object the both of you are orbiting. Such as Kerbin. This could effectively slow down an object into a closed orbit... but it is highly likely to encounter the Mun again and more adjustments made.

The asteroid in question is being acted upon by an outside force "gravity" just as effectively as if you had strapped a rocket onto it.

Here's a quick and dirty image I whipped up to hopefully help.

bLIxsVE.png

Edited by FITorion
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