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Do orbits decay in KSP?


hunter558
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Not sure exactly how they work in reality but at some point, wouldn\'t much of the debris, especially those close to the surface of Kerbin(for example) have decaying orbits that eventually land them on the ground? Or do they constantly orbit forever? I dont know whether this takes place in KSP over extended periods of time but in real life this does happen to many of our older craft and debris. I was wondering if this will be a feature in KSP or whether debris will exist in orbit forever if you choose to leave it on.

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Orbits ONLY decay if there is atmospheric drag or engine thrust. If you want your orbit to gradually decay, orbit at 68 kilometers. That\'s just a hassel, though, as you\'ll have to keep boosting. If the ISS was totally out of the atmosphere, it would not have a decay in its orbit.

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Unless they are on rails with the current rules. When you leave a ship in orbit to fly another one, if that ship\'s orbit crosses into the atmosphere during its perigee, it will ignore the friction of the atmosphere and fly through as if it wasn\'t there. But that is with the current rules. I do believe I remember Harv mentioning some work being done on having the on-rails system recognize when a ship hits atmo. Don\'t quote me on that. It\'s not mentioned in the list for 0.16 so if it is something being worked on, it isn\'t projected for anytime soon.

That being said, I\'d like to throw some science in the room. Careful, you may get some on you.

In a perfect vacuum, a craft at orbital velocity around a solitary gravity well(like a planet) will remain at that speed indefinitely, as it has no forces to counteract or interfere with said velocity. Add a moon to that planet, and that becomes a force to interfere with the craft\'s orbit, skewing it to either go down to the planet eventually, or even get thrown out of the well and either the moon captures that craft, or it leaves the vicinity altogether.

Then there is the part where space is not a perfect vacuum. There are molecules in space all around, even between the stars and galaxies. Those all bump into the craft and do their part to slow that craft down. When you are in Low Earth Orbit, as the ISS is, then there is a much thicker amount of molecules for the station to deal with. The ISS is consistently making course corrections to remain in orbit. If it was left to its own devices, its orbit would eventually degrade due to friction from the atmosphere still present at that altitude, as well as the tugs that the Moon, the Sun, and the other planets in the solar system exert on that wee little station. I wouldn\'t think about it too much, as most of it is minuscule by itself.

There\'s all that in a nutshell on how any orbit has forces against it in reality.

Dessert?

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In KSP, all masses are point sources and so gravity fields are perfectly spherical. There\'s no gravity perturbation. That means that in the game, only air friction makes an orbit decay, and the air stops at 70km up. Above 70km, orbits are permanent.

Out in the real world, orbits can decay even around totally airless worlds. Anything rocky has lumps in its mass distribution, which make its gravity field lumpy too. That gives an uneven force on an orbit over time, which tends to make the orbit very slowly more and more elliptical until finally it clips the surface.

http://en.wikipedia.org/wiki/Mass_concentration_(astronomy)

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I would also like to add, and do correct me if I\'m wrong, but unless you are switched to the debris that are going through the atmosphere at the moment they do so, the debris would just go on with the orbit, as if no air was there. So, in my case where I leave a spent stage on a low orbit that intersects atmosphere, I have to switch to that stage for some 10 minutes after the mission is complete (or when I have a spare window within the mission), just to watch (and make) it slow down in the atmosphere and fall to the surface.

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Harv said some time ago that a friend of his passed him some closed-form approximate solutions for the decay of an orbit in presence of very faint drag. If possible, he will add these to the rails system to simulate 'well enough' the orbital decay of debris, and maybe of low Kerbin orbit crafts.

I would also like to add, and do correct me if I\'m wrong, but unless you are switched to the debris that are going through the atmosphere at the moment they do so, the debris would just go on with the orbit, as if no air was there. So [...] I have to switch to that stage for some 10 minutes [...] just to watch (and make) it slow down in the atmosphere and fall to the surface.

You are right. Since I like to keep LKO clean, for now it\'s there in my checklists: 'ensure destruction' ;)

I think that this will be half-fixed, in the beginning, by just destroying debris below some altitude, and afterwards thoroughly fixed with the decay/reentry model I was talking about.

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I am all for the micro-drag. We can always compose a sat which would have the means for correcting an orbit. What I started doing recently though, is include deorbiting solid motors for my stages, 2 angled so the whole thing rotates and stabilizes, and another 2 straight back, to aid in braking. If the periapsis is lower than the surface, when switched out of the debris in question, it disappears at an altitude of about 4km.

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i have a couple of pieces of pieces of debris that are losing about a metre of altitude every third orbit, with or without me switched to them. Good to know the skies will eventually be clear, if i wasn\'t still putting things up there

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Over very long periods of time, I believe tidal effects cause bodies to alter their rotational rate and orbital speed until they are mutually tidally locked. And by 'very long periods of time,' I mean, most orbiting pairs in the solar system haven\'t yet reached this state in ~5 billion years. Of course it\'s perturbed by the existence of other bodies, and would never be anything the game would have to take into account anyway. But it does cause orbits to 'decay' (decelerate) or accelerate, depending on the circumstances.

For example, earth\'s rotation has slowed gradually as it transfers angular momentum to the moon, whose orbital radius increases a bit over 1 inch per year in response. Moon is already tidally locked to us, and we will eventually be tidally locked to it. I believe Pluto and Charon are mutually locked like that, though I\'m not sure what effect a recently discovered 2nd Plutonian moon has on that arrangement.

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I\'ve noticed that debris will only ignore atmosphere above 23000m. If you time your stages to be jettisoned when your Pe is just below that, it will not stay in orbit. That\'s where I tend to ditch my final ascent stages (even if they have some fuel left) to avoid clutter.

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