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ozsmurf
2004-Nov-12, 01:33 AM
One of the great images in sci fi is when mutiple planets share the same orbit. Not orbiting each other like the Earth- Moon system, but orbiting a common point like beads on a string. This has raised a number of questions in my mind.

1. Is this possible to be naturally occurring? As I understand it planet formation effectively sweep the orbit path (is this the correct term?).

2. Would these shared orbits be stable? I imagine there would be all sorts of dynamics occurring. If yes is there an upper number on the number of items in this orbit.

3. Does orbit speed change in an elliptical orbit, like a slingshot effect or does it stay constant all the time? In effect would the planets bunch up at the short “corners” and string out on the longer “corners”?

4. Would the planets gravities interact to cause a tide-locking effect?

I know that several planets have multiple moons but I don't know if the moons share the same track or not.

Wow, that was a long one. Any guidance would be good cause I like the imagery created but I always thought hmmm.

Nowhere Man
2004-Nov-12, 02:04 AM
One possible answer is to have co-orbital planets. Co-orbital moons have been sighted orbiting Saturn. One speculation is that they were part of a larger moon that was broken.

Basically, two bodies share nearly the same orbit. One is a bit closer to the primary (call it A), the other a bit farther out (call it B). A, being in a smaller orbit, moves faster and eventually undertakes B. At that point, their mutual gravity causes them to swap orbits. Then B, now in the closer orbit, pulls away from A, until it undertakes A, and they swap again.

For a short time, they are near each other, and pretty far apart for the rest of the time. Google up co-orbital moons for more info.

Fred

Ilya
2004-Nov-12, 03:10 AM
Two planets orbiting a star in the same orbit 60 degrees apart are stable. Even three are stable, as long as the one in the middle is much more massive than the other two. It's called Trojan orbit, and many asteroids in the Solar System thus share orbits with planets.

Three or more planets orbiting at equal angles apart (120 for three, 90 for four, etc.) is also stable, - it's called Kemplerer Rosette, - but there is no known way for such arrangement to come together. Unlike Trojan orbit, all members of a Kemplerer Rosette must have equal mass.


One possible answer is to have co-orbital planets. Co-orbital moons have been sighted orbiting Saturn. One speculation is that they were part of a larger moon that was broken.

Basically, two bodies share nearly the same orbit. One is a bit closer to the primary (call it A), the other a bit farther out (call it B). A, being in a smaller orbit, moves faster and eventually undertakes B. At that point, their mutual gravity causes them to swap orbits. Then B, now in the closer orbit, pulls away from A, until it undertakes A, and they swap again.

For a short time, they are near each other, and pretty far apart for the rest of the time. Google up co-orbital moons for more info.


If you plot the motion of such moons in a coordinate system where their center of mass is at rest, you will see that they simply orbit each other in mundane elliptical orbits. It's just as they (and their center of mass) circle Saturn, these ellipses are stretched to the point of being completely unrecognizable.

For that matter, Earth and Moon are doing same thing around the Sun. Viewed as orbiting the Sun, Moon never moves backward, and just falls behind and ahead of Earth at regular intervals. In fact, the path Moon traces around Sun is never concave, always convex.

ozsmurf
2004-Nov-12, 05:40 AM
Some food for thought there. Does this apply for satellites in LEO as well? Can you only have a max of four on the same orbit track or does it not matter since they are so small so their relative gravities don't matter so much, or is that why they have manueuvering rockets for fine adjustments? (Guess who was one of those ... why ... kids :P )

I hate it when I get one of these thoughts going as they tend to spiral into lots of questions that I have no way of answering with my current knowledge #-o

eburacum45
2004-Nov-12, 10:05 AM
This page discusses the stability of moons and planets in multiple orbits; I am not sure the simulation is entirely reliable, but it does seem to show that some configurations are unstable, and some are stable at least in the medium term.
http://burtleburtle.net/bob/physics/kempler.html

I don't kow hnow stable the fictional Ecotopia Lab ring in Orion's Arm is;
I suspect not very stable at all, and it must need a lot of active station keeping. Here it is in Celestia;
http://www.orionsarm.com/worlds/Ecotopia-theLab.jpg
there are 312 in total, but I have only made 43 so far.

Weird Dave
2004-Nov-12, 03:34 PM
One of the great images in sci fi is when mutiple planets share the same orbit. Not orbiting each other like the Earth- Moon system, but orbiting a common point like beads on a string. This has raised a number of questions in my mind.

1. Is this possible to be naturally occurring? As I understand it planet formation effectively sweep the orbit path (is this the correct term?).

I'd be surprised if it was absolutely totally impossible, but it is probably quite rare otherwise we'd see moons or planets like this in our solar system.


2. Would these shared orbits be stable? I imagine there would be all sorts of dynamics occurring. If yes is there an upper number on the number of items in this orbit.

3. Does orbit speed change in an elliptical orbit, like a slingshot effect or does it stay constant all the time? In effect would the planets bunch up at the short “corners” and string out on the longer “corners”?

Elliptical orbits have variable speed. Look up Kepler's Laws for the details, but simply put, the planet moves slowest furthest from the Sun, and fastest when it is closest to the Sun. This difference can be large for a very elongated orbit like a comet's, but most planets have nearly circular orbits so the effect would be small.


4. Would the planets gravities interact to cause a tide-locking effect?

Yes, but it might be infinitessimally slow. Depends on their compositions, sizes, separations etc. I would expect the effects to be much smaller than the effect of the Sun on Earth's tides.


I know that several planets have multiple moons but I don't know if the moons share the same track or not.

Wow, that was a long one. Any guidance would be good cause I like the imagery created but I always thought hmmm.

All the large moons that I know about are in different orbits, just like the planets round the sun. Effectively, Saturn's rings are made up of very many tiny moons. Many of these share very similar orbits. Note that the probability of two bodies randomly sharing exactly the same orbit is zero. It will only happen approximately, although this may be good enough to last the lifetime of the star they orbit. It could be stable forever if there is an interaction that "locks" them together like Trojan asteroids.

The asteroid Cruithne almost shares Earth's orbit. I'll add a link later. I presume that this arrangement isn't long-term stable.

eburacum45
2004-Nov-12, 04:34 PM
This page discusses the stability of moons and planets in multiple orbits; I am not sure the simulation is entirely reliable, but it does seem to show that some configurations are unstable, and some are stable at least in the medium term.
http://burtleburtle.net/bob/physics/kempler.html

I don't know how stable the fictional Ecotopia Lab ring in Orion's Arm is;
I suspect not very stable at all, and it must need a lot of active station keeping. Here it is in Celestia;
http://www.orionsarm.com/worlds/Ecotopia-theLab.jpg
there are 312 in total, but I have only made 43 so far.

Weird Dave
2004-Nov-12, 04:48 PM
This page discusses the stability of moons and planets in multiple orbits; I am not sure the simulation is entirely reliable, but it does seem to show that some configurations are unstable, and some are stable at least in the medium term.
http://burtleburtle.net/bob/physics/kempler.html

I don't know how stable the fictional Ecotopia Lab ring in Orion's Arm is;
I suspect not very stable at all, and it must need a lot of active station keeping. Here it is in Celestia;
http://www.orionsarm.com/worlds/Ecotopia-theLab.jpg
there are 312 in total, but I have only made 43 so far.

The Cruithne link in your first example didn't work (your link is good, though). The page is, in fact, here (http://burtleburtle.net/bob/physics/cruithne.html). Also, I was thinking of this (http://www.astro.uwo.ca/~wiegert/3753/3753.html), which has mpegs rather than java.

DreadCthulhu
2004-Nov-12, 04:49 PM
Three or more planets orbiting at equal angles apart (120 for three, 90 for four, etc.) is also stable, - it's called Kemplerer Rosette, - but there is no known way for such arrangement to come together. Unlike Trojan orbit, all members of a Kemplerer Rosette must have equal mass.



All you need for a Kemplerer Rosette are some sufficently motivated puppeteers.

Rich
2004-Nov-12, 05:53 PM
All you need for a Kemplerer Rosette are some sufficently motivated puppeteers.

Hey! I was going to say that!

Avatar28
2004-Nov-12, 11:05 PM
Maybe I'm off, but I got the impression that the OP was asking about having two planets of roughly equal mass orbiting a common barycenter while also orbiting their star. So, say, you had two planets the size of the earth orbiting around a common center at a distance of a quarter or half million miles or some such.

NM, I'm an idiot. I just reread the OP and it's exactly NOT what I'm talking about.

ozsmurf
2004-Nov-14, 11:09 PM
Avatar 28 wrote

Maybe I'm off, but I got the impression that the OP was asking about having two planets of roughly equal mass orbiting a common barycenter while also orbiting their star. So, say, you had two planets the size of the earth orbiting around a common center at a distance of a quarter or half million miles or some such.

In the "Nights Dawn" trilogy they mention a string of planets around a star, a number isn't given but the impression given is of more than 100. All of these planets were of roughly equal mass and had roughly Earth like conditions.

Van Rijn
2004-Nov-16, 02:23 AM
In the "Nights Dawn" trilogy they mention a string of planets around a star, a number isn't given but the impression given is of more than 100. All of these planets were of roughly equal mass and had roughly Earth like conditions.

That's just a wee bit on the high side :) ... that would have to be artificially set up and maintained by arbitrarily advanced technology.

Doe, John
2004-Nov-16, 02:59 AM
In the "Nights Dawn" trilogy they mention a string of planets around a star, a number isn't given but the impression given is of more than 100. All of these planets were of roughly equal mass and had roughly Earth like conditions.

That's just a wee bit on the high side :) ... that would have to be artificially set up and maintained by arbitrarily advanced technology.iirc that was exactly the scenario in that particular story. They also had devices called "providers" that could recreate any physical object they had a template for. They assigned one of them (with appropriate safeguards) to provide for a pre-pubescent human girl they had kidnapped.

eburacum45
2004-Nov-16, 08:03 PM
Quite. I am interested in how arbitarily advanced this technology would have to be; the 100+ planets of the Kiint world necklace and the three hundred worlds of the Ecotopia Lab in Orion's Arm (a system thought up by Todd Drashner) are both products of very high technology.

I imagine that they both owe a lot to the circlets of artificial worlds in Olaf Stapledon's Starmaker, but if such situations takes massive amounts of energy to keep stable then that makes both systems more difficult to maintain.

Van Rijn
2004-Nov-16, 10:28 PM
The primary issue would be finding - or creating - all those planets. Perhaps you could find a star with that many appropriate worlds, but it seems unlikely. Or we might imagine using self replicating machines to create a "star lifting" system to remove mass from a star or gas giants (with self replicating machines and a reasonable replication time, hardware can be scaled up to whatever level needed quickly). Most of the mass wouldn't be the preferred elements, so it might require massive transmutation. In principle, that might be possible in arrays of huge fusion reactors (scale up a magnetic confinement system large enough, and you can probably fuse up to iron). The waste heat would be enormous if you wanted to transmute a hundred earth masses within a few thousand years - on the order of the sun's energy output, continuously.

Then you have to coalesce the planets in position. You'll have to do that very carefully to keep the temperature down, unless you can wait a very long time for them to cool. You'll need to add oceans and atmosphere, but if you can do the rest, that's no problem.

Actually keeping them in place may not be that difficult, or energy intensive, but would require ongoing attention. Here's a method to possibly move Mars by working with perterbations:

http://www-formal.stanford.edu/jmc/future/mars/mars.html

You could use a similar idea to stabilize planets that are in somewhat unstable orbits.

It would be far more mass efficient to build large O'Neill colony type structures. There's also the possibility of building mini ringworlds with a surface area similar to earth. (You use a non-spinning "support ring" with two counter rotating "spin rings" inside, floating magnetically on the support ring. This would solve the structural problems for large spinning rings.)

eburacum45
2004-Nov-17, 10:05 AM
(You use a non-spinning "support ring" with two counter rotating "spin rings" inside, floating magnetically on the support ring. This would solve the structural problems for large spinning rings.)

That sounds like my own design for a ringworld, http://www.orionsarm.com/worlds/Ouaddai.html
which uses counterotating rings balanced against the gravity of the central star.

Yes, there are plenty of sensible O'Neill colonies in our imaginary OA civilisation, but starlifting and transmutation are relatively commonplace, and some systems just like to push the envelope.