One-Planet-Two-Moons Orbital Mechanics Question

[Kanastrous]

For purpose of a particular story I would like to describe circumstances wherein we begin with an Earth-sized planet with two moons, one larger than Luna and the other smaller.

The larger moon is struck by another body and its orbit changes.* This perturbs the orbit of the smaller moon, as well.

And what I want to follow is a period of twenty-five years during which their orbits continue to change until settling into regularity.

Probably based upon the description alone, it ought to be clear that I'm effectively uneducated where the mechanics are concerned and so would appreciate a review of the notion by anyone who understands them well.


* ...and everything is laid waste. I'm sorry, I had to get that out of my system.

[Simon_Jester]

I assume you've decided that the perturbations are "destined" to regularize so that both moons stay in orbit, and do not collide or crash into the planet or have one eject the other from the planetary system. All those are possible outcomes, but you've already picked one for plot reasons, so cool.

I'd be honestly surprised if the perturbations damped down in twenty-five years. Basically, for the orbits to regularize, you need all the side effects of close approaches to cancel each other out. This requires many such encounters. If the moons orbit the planet about once a month (or more, for further out orbits), then 25 years is only 300 orbits- enough time for several close encounters, but not enough to "level out" the effects over the statistical long haul.

This is, however, a gut feeling.

[Napoleon the Clown]

How much of a perturbation are we talking here? Does the perigee get pulled in closer? Kicked further out? Is the orbital inclination changed? How far apart do you plan for the moons to be before the orbit of the bigger one gets altered?


For a moon bigger than our own to have its orbit changed by terribly much, it would take an immense amount of force striking it. Enough that the debris that would no doubt come falling down onto the planet would be... catastrophic.

My alternate proposal for altering the orbit of the larger moon: A stupid huge body swung past and dumped an assload of energy in through the same mechanics as an orbital assist like you'd see in a space probe. The Big Thing's gravity gives the moon a big ol' yank, either increasing its orbital velocity or decreasing it, depending on relative directions of travel. Depending on where the smaller moon is at the time, it might get kicked out off from the planet entirely, or sent into a low enough perigee that Problems develop.

Obviously, they'd need to end up in orbits that have an approximate resonance that would eventually tug into a proper resonance. Honestly, if you want a good answer as to what would happen you need to give a starting point as to where the orbits were to begin with, and then where they were after The Event. Most likely you'd end with a 2:3 resonance, or thereabouts. Outer moon goes around twice in the time it takes the inner moon to complete one orbit.

If perturbations were relatively mild it may be possible for things to "settle" after a quarter of a century, but then that would also make it so the average person wouldn't even be aware something was getting a nice shake-up. The effects on the tides could be fun, though. If the inner moon were the lower mass moon, and relative masses were correct, you could end up having periods of extreme tidal effects, where the planet is getting tugged on good and hard from opposite sides. This could cause potential volcanic activity, and certainly increase the risk of living near large bodies of water.

Alternately, if the two moons end up getting really close to each other you could see the smaller moon start getting torn apart. If it drops low enough in orbit of Not Quite Earth it will be within the Roche limit. Think 21 times closer to the Earth than our moon currently is. This would end up fracturing it, and the planet would gain a nice, happy ring around it. Some of the crumbled remains could end up in orbit of the bigger moon, too. The bigger moon could potentially speed this process up some, even. Still, more than 25 years I'm guessing. If the smaller moon is still massive enough to be roughly spherical, you'd definitely see effects on the tides there, and even alteration of how fast Not Earth spins. You could have some... interesting effects from a moon slowly getting torn apart.

But yeah... Without more info, there's only vague information to give back.

[LaCroix]

Another interesting scenario could be if the moons were synchronized in a way that they always keep their position relative to each other, but at different orbits. (E.g. no encounters)

If some close encounter now speeds up the outer moon a bit(a couple of hours or days), you get a *relatively* short period of time where that moon catches up to the other moon, slowing that one down while getting faster, overtakes, gets slowed down while the other speeds up, etc. This would go on for a good number of encounters until they have a new stable orbit. But that visitor swings by a couple more times, always changing that balance a tiny bit.

This would cause havok on the planet with tides being extremely erratic in frequency and not yet predictable ultra-low and spring tides, etc., until people get a model for that together, create flood barriers, etc. And then the visitor changes that, again. So they would have to take some time to adapt to the new normal, which means they might spend the 25 years to repeatedly adapt until the visitor is finally gone. The stabilization of the moons will still take a lot of time, but for now, everything is predictable, again.

Or you could have the planet capture the visitor as a new moon, maybe with a strong inclination. A third moon at an 90° rotated orbit relative to the other moons would definitely make things higly interesting for the planet.

[Kanastrous]

The change in the satellites' orbits is substantial enough that earthquakes and vulcanism become noticeably more frequent and intense, and tides rise sufficiently higher that low-lying coastal towns have to be abandoned.

For plot purposes, the idea is that there is a dormant-for-millenia article of technology whose functioning in part makes use of gravitational force, and that the alteration of gravitational fields created by the change in the moons' orbit "wakes it up" for about 25 years...but their eventual stabilized orbit doesn't exert the same influences, and the device goes dormant again, for good.

[Kanastrous]

LaCroix - I dig the captured-third-moon concept but I have a very nice opening already written wherein someone watching the larger moon witnesses the asteroid impact (and, belonging to what's basically a late-medieval-level-of-technology society, pretty well loses his shit).

[Kanastrous]

Really what I had originally in mind (and what one character describes) is a single very large moon whose orbital radius contracts, then expands again in a sort of slingshot effect, but even as I typed it I pretty well figured that wasn't anywhere near realistic-sounding. Once it starts spiraling in, I figure it's either going to disintegrate or collide with its primary.

[LaCroix]

How about this:

You could have your moon have a glancing impact with a large stellar object. That would still make a very impressive light show, but wouldn't necessarily shatter or de-stabilize the moon's orbit too much, so the moon stays mostly in play, but gains an asteroid ring (at least temporarily) from the debris thrown out.

You could use the impactor as plot device to trigger the ancient tech. The impact slowed it down and deflected it enough to be *barely* caught by the planet (maybe in an odd angle and a very elliptical orbit - that minimizes encounters), and as it passes the orbit of the moon it had impacted earlier, each encounter with planetary gravity along with the (much closer) moon's gravity gives it a boost to lift its orbit until it settles on a very distant orbit.

Thus, it is at first very close, and manages to wake the machine, but that gravitational influence gets rapidly weaker as it hurls towards apogee. It's regular passes at perigee keep the machine awake (A kick in the ass, and the machine keeps running long enough for it to return, might even make the machine worse during times of proximity.). As the orbit circularizes by the accelleration and decelleration during the close passes at perigee, the apogee should get lower and perigee should rise. Making the encounters more frequent, reduce the distance at apogee, but the increase the distance to the planet at perigee. At one point, it simply doesn't come close enough to rouse the machine, anymore, so it goes to sleep, again. It could be a pretty small impactor, and it would most likey glow angryly long after its impact with the bigger moon, just like the scar on the moon would glow for a short while.

So theat medieval guy would have a lightshow to see, their moon suddenly has a scar and spins, has rings, and there is a new angry small moon on the sky that has a strange timing to its visits, unlike the moons that come and go in regular intervals.

This could even be used in a way that each time it draws close, *things* happen (or get much worse), so that the moon becomes a bad omen.

(sorry, you gave my inner writer a kick in the butt and now he's running amok in my head... )

[Simon_Jester]

The change in the satellites' orbits is substantial enough that earthquakes and vulcanism become noticeably more frequent and intense, and tides rise sufficiently higher that low-lying coastal towns have to be abandoned.

For plot purposes, the idea is that there is a dormant-for-millenia article of technology whose functioning in part makes use of gravitational force, and that the alteration of gravitational fields created by the change in the moons' orbit "wakes it up" for about 25 years...but their eventual stabilized orbit doesn't exert the same influences, and the device goes dormant again, for good.

If the ancient device is itself influencing the moons, then it might explain HOW the orbits circularize in twenty-five years- because ancient tractor beams are pushing them subtly into place, although having to exert enough force to make a planet creak in order to do so.

[Kanastrous]

That's good...

[Kanastrous]

LaCroix - also good.

Man, this site is handy.