Disposal of spent stages in interplanetary and interstellar space

[Kerbocracy]

Hi all, I have a garbage disposal question.

I've been disposing of my spent stages in interplanetary and interstellar space. For example, on my way to the Mun, I'll plan a trajectory with my transfer burn that goes through the Mun's influence (behind it) and out the other side into a Kerbin Escape trajectory. Once I've achieved that trajectory, I dump my transfer stages and then my lander or probe does burn at periapsis to get into orbit. The other stages etc just sail on into interplanetary space. If I give them enough energy, apparently they escape Kerbol as well. I like this process because while there is still litter, it is over such a huge space that I'm guaranteed never to see it again. In doing this I have noticed some quirks that I wanted to mention.

1) Let's say I separate from my transfer stage prior to my Mun encounter. My transfer stages are drifting very slowly away from my lander. Upon Mun encounter, they suddenly accelerate away from my lander (to about 10km within 10 seconds or so). Why does this happen? It's as if my trajectory has changed upon Mun encounter and they have continued on my previous trajectory. If so, why does my trajectory suddenly change upon Mun encounter...?

2) A related question is: if I wait until I make my Mun encounter to jettison the transfer stages, they continue past periapsis and to Mun escape on the other side. Meanwhile I've burned into orbit at Pe. Do those spent stages then achieve Kerbin escape as well following the Mun escape? The reason I ask is that under 1), when I left those stages, I still had the trajectory plot showing Mun encounter, Mun Escape, and Kerbin escape, indicating [to me] that the spent stages would eventually achieve Kerbin escape. When I jettison them in the Mun's influence, they only have 'Mun Escape' listed, and I go on faith that they will also achieve Kerbin Escape. However, because I suspect a sudden trajectory change upon entering Munar influence (described in 1), I no longer know if the jettisoned stages will escape Kerbin. In testing, I've found that they do. But I've also found some jettisoned stages that I thought would have Kerbin escape doing a very wide Apoapsis orbit of Kerbin...

3) In examining my spent stages that have managed to escape Kerbin orbit, I find them listed as 'Orbiting kerbol' or 'on an escape trajectory from Kerbol'. The orbiting Kerbol ones make sense, but the escape trajectory ones seem to have defined Kerbol orbits. So I'm not sure why they are listed as escaping; their orbits are defined (and fall inside of Eeloo's orbit).

4) A related question: Are unmanned debris items' orbits updated by encounters? For example, for my debris that is orbiting Kerbol, if a piece happens to encounter Duna's influence when I'm not looking, will it get captured or have it's orbit altered? This relates to my question in 1) where I was wondering if I jettison debris just prior to an encounter, does that debris still have the encounter. I suspect based on my observation that the answer is 'no'.

Thanks in advance for any feedback.

[Eric S]

I suspect what you're seeing is that trajectories crossing an SoI can go wonky if done at high warp. Since you're not paying attention to the debris, it's quite likely that you're warping when the debris makes the SoI transition.

Debris definitely follows ballistic trajectories including gravity of SoI, etc. It doesn't encounter atmospheric resistance, though it will get deleted if it goes far enough into an atmosphere (the limit for Kerbin is somewhere around 25km).

[Kerbocracy]

I suspect what you're seeing is that trajectories crossing an SoI can go wonky if done at high warp. Since you're not paying attention to the debris, it's quite likely that you're warping when the debris makes the SoI transition.

Debris definitely follows ballistic trajectories including gravity of SoI, etc. It doesn't encounter atmospheric resistance, though it will get deleted if it goes far enough into an atmosphere (the limit for Kerbin is somewhere around 25km).

Perhaps you're right - I might have been at > 1x time accel. when I crossed the SoI boundary. Next time I dump my transfer stage I'll make sure to cross the SoI boundary in 1x.

But the more basic question remains: will the orbits of a piece of debris be updated by an encounter with a celestial body? In the case of my debris that is orbiting Kerbol, if it happens to encounter Eve, say, when I'm not watching it, does it's trajectory get updated by the encounter or does it continue on 'rails' as if it never had the encounter?

[Taki117]

Near as I can tell, if it doesn't encounter another bodies SOI on the flight path it never will. (I had a probe orbiting Kerbol for somewhere on the order of 238 in game days, never once encountered another planet) Planetary transfers have to be precisely timed and executed so that you end up in the SOI.

[Kerbocracy]

Near as I can tell, if it doesn't encounter another bodies SOI on the flight path it never will. (I had a probe orbiting Kerbol for somewhere on the order of 238 in game days, never once encountered another planet) Planetary transfers have to be precisely timed and executed so that you end up in the SOI.

Spurious encounters happen often close to Kerbin though. I've had spent stages that are on a wide Apoapsis around kerbin and had them cross the Mun or Minmus's path. When this happened I was excited because it meant that the object's orbit would change and just _maybe_ I wouldn't have to track it down and de-orbit it myself. In this case I focused on the debris object (flew it) and saw it through the encounter, which indeed slowed it down and crashed it into Kerbin.

The question remains - if I had not 'flown' the debris, and had left it unattended, would it have had the encounter with the Mun and thus had its trajectory changed?

[toomuchbrew]

I had a stranded capsule in a long elliptic orbit between Kernin and the Mun. After a bunch of game hours, probably a couple in game years, the capsule must have had an encounter with the Mun or somehow entered its soi, it got ejected from The Kerbin soi and now orbits Kerbol. No rescue for them...

[Deadpangod3]

When I saw the "in interplanetary space" I was like

You monster...

I think debris inker in orbit is bad enough, but in interplanetary space is like polluting a solar system...

But I see the advantages in it

[Kasuha]

1/ when getting to periapsis, your speed increases and so does increase your relative speed towards parts you dumped.

2/ If the trajectory was showing Kerbin escape, then they will most probably escape. You can visualise their paths by moving your mouse at the upper edge of the screen and right-clicking on the debris icon.

3/ That's just a harmless game bug.

4/ Yes, if debris enters SOI of a body on its way it may get slingshotted to higher or lower orbit depending on what direction it entered and exited. There's even chance they will crash into the body.

I never have problems with debris. I make reasonable effort to crash it into planets, but if I fail, my tracking station has powerful lasers which can obliterate any floating debris with pinpoint accuracy.

[magnemoe]

Stuff dropped so it can intercept Mun SOI usually end up in solar orbit, this is more likely than them hitting Mun or Kerbin.

Now because of an bug they might end up in retrograde orbit around the sun, I manage to send a probe on retrograde orbit to Eve as I did not follow it then it escaped Kerbin SOI.

Now if you look at items on an Kerbin escape trajectory in the map view it tend to show that the item has an orbit around Kerbin, switching to the object and the real trajectory come up.

[Eric S]

Perhaps you're right - I might have been at > 1x time accel. when I crossed the SoI boundary. Next time I dump my transfer stage I'll make sure to cross the SoI boundary in 1x.

Also note that they left the Mun's SoI when you were off doing something else, so it's even more likely that you had warp going at that point.

But the more basic question remains: will the orbits of a piece of debris be updated by an encounter with a celestial body?

I've never seen a case where that hasn't happened, but I haven't exactly gone digging to make sure that it never happened. I have seen it happen, but most of the time I'm not paying any attention to it.

[Kerbocracy]

I've never seen a case where that hasn't happened, but I haven't exactly gone digging to make sure that it never happened. I have seen it happen, but most of the time I'm not paying any attention to it.

The reason I wonder about it is because when not focused on an item and if it's > 2.5km away the game stops updating the object's trajectory based on the physics model. This is when when you detach from stages in the atmosphere, once you separate by 2.5 km, those parts continue on rails and you continue to slow down because the game stops applying the air resistance to those parts. They only disappear from their 'rails' if their track takes them below 25000m in altitude. So, if you put debris on a decaying orbit (say Pe = 40km), the object's orbit will only decay while you are 'flying' it. Otherwise it continues on rails and ignores the atmosphere. By this logic, if unattended encounters with celestial bodies rely on the physics model, then I guess they won't happen. This would be consistent with the developer's choice to update the physics of only the object being 'flown'.

But encounters are different than drag and might not take the physics model into account (thought they might take the object's mass into account, mind you the mass of our orbiting objects is negligble compared to the celestial bodies).

Another related question is if orbital collisions can happen with unattended objects. For example, can your unattended space station orbiting Kerbin collide with space debris while you are doing an EVA on Duna? I think not.

But we haven't had a definitive answer to these questions; everybody's got some kind of semi-related opinion. It would be nice to have a Dev chime in on this. I suppose I could go do the experimentation myself though:)

[Kasuha]

Another related question is if orbital collisions can happen with unattended objects. For example, can your unattended space station orbiting Kerbin collide with space debris while you are doing an EVA on Duna? I think not.

But we haven't had a definitive answer to these questions; everybody's got some kind of semi-related opinion. It would be nice to have a Dev chime in on this. I suppose I could go do the experimentation myself though:)

No. Definite answer, I just tried it - took my space station, undocked a part, then used RCS to send it on collision course, switched to another station, ran the time for a while and then switched back. The separated part was on the other side with no damage on either. I just gotta try what happens when I return while they are overlapping each other. I guess explosions will be the answer.

Edit: yeps, guessed right.

Edited October 25, 2013 by Kasuha

[Kerbocracy]

Ok thanks for that. Collisions clearly depend on the physics of the crafts being modeled, so if we're focused elswhere or outside of 2.5km, we have to live with a no-clipping reality.

So the question then (still) is - do unattended objects have their trajectory updated by encounter with a gravity well...

If so, that's really cool: it means that my debris in interplanetary space might eventually do some unpredictable stuff, or might dispose of itself:)

If not, that's less cool. But more functionally, in the context of my original question, it means that if I want to dispose of my transfer tanks in interplanetary space using a gravity assist (as in my Mun approach), I need to detach from the transfer stage after the encounter with the gravity well in question; detaching before will result (likely, if detached debris > 2.5km away) result in the transfer stage not being gravity assisted and remaining in a kerbin orbit. I suspect that this is the case...

[ScottyDoesKnow]

The reason I wonder about it is because when not focused on an item and if it's > 2.5km away the game stops updating the object's trajectory based on the physics model. This is when when you detach from stages in the atmosphere, once you separate by 2.5 km, those parts continue on rails and you continue to slow down because the game stops applying the air resistance to those parts. They only disappear from their 'rails' if their track takes them below 25000m in altitude. So, if you put debris on a decaying orbit (say Pe = 40km), the object's orbit will only decay while you are 'flying' it. Otherwise it continues on rails and ignores the atmosphere. By this logic, if unattended encounters with celestial bodies rely on the physics model, then I guess they won't happen. This would be consistent with the developer's choice to update the physics of only the object being 'flown'.

But encounters are different than drag and might not take the physics model into account (thought they might take the object's mass into account, mind you the mass of our orbiting objects is negligble compared to the celestial bodies).

Another related question is if orbital collisions can happen with unattended objects. For example, can your unattended space station orbiting Kerbin collide with space debris while you are doing an EVA on Duna? I think not.

But we haven't had a definitive answer to these questions; everybody's got some kind of semi-related opinion. It would be nice to have a Dev chime in on this. I suppose I could go do the experimentation myself though:)

Objects outside of that range ignore certain things like drag in atmosphere (though if they get below a certain point they disappear, above that point they won't be slowed like they should) and collisions with other objects. They will still calculate gravity interactions with other planets and moons though. I've definitely left ships in jools orbit only to come back and have them in the suns orbit or leaving the solar system. The only thing I don't know is if debris is different and this isn't calculated for it, but I would assume debris is treated the same way.

[Kerbocracy]

I've definitely left ships in jools orbit only to come back and have them in the suns orbit or leaving the solar system.

Yes! That's cool. I noticed that when you enter the Jool gravity well you get so many encounters with its moons. It feels like you would have to think carefully to produce a stable orbit without any spurious encounters. I love the concept that that could happen though. I'm going to run a couple experiments on this when I get home...

[Mesons]

If so, why does my trajectory suddenly change upon Mun encounter...?

Three-body physics. It's hard, so we don't do it. The approximation that Squad uses is to use only the gravity of the host of your sphere of influence. When you change SOIs, you are suddenly ignoring the gravity of your previous body and now subject to the new gravity, so though your velocity remains the same in crossing SOIs, the force changing your velocity differs. Depending on the angle at which you enter the SOI, this can mean that your separation from your disposed stages can change in non-intuitive ways.

[Bengsch85]

I can confirm that debris, although on rails, will be influenced by SoI changes. The drive unit for my Jool mapping probes didn't have a probe body on it so after the last probe left for Pol it was listed as debris. Its orbit has changed numerous times as it passes near the inner moons. (I know because I was curious if it would change so I've been watching it occasionally.)

[Kasuha]

Unattended objects "on rails" generally follow SOI changes and can be destroyed if they collide with a planet, but it depends on time warp factor used. Even though these objects are "on rails" and completely predictable, the game still for whatever reason evaluates them in steps and if the step is large enough, the object may miss a SOI (or, more often, the warp factor may significantly affect the shape of the trajectory after SOI change) or skip through a planet.

[Kerbocracy]

Three-body physics. It's hard, so we don't do it. The approximation that Squad uses is to use only the gravity of the host of your sphere of influence. When you change SOIs, you are suddenly ignoring the gravity of your previous body and now subject to the new gravity, so though your velocity remains the same in crossing SOIs, the force changing your velocity differs. Depending on the angle at which you enter the SOI, this can mean that your separation from your disposed stages can change in non-intuitive ways.

Ok makes sense. Though - if I separate from my stages _right_ before crossing into the new SOI, shouldn't both me and my debris be subject to the same changes upon crossing into the new SOI? Therefore both of our trajectories might change in similar non-intuitive ways?