Orbital mechanics

RHJunior · Post by **RHJunior** » Sun Jul 30, 2006 4:46 am

After much tinkerage, I've concluded the given orbital design for Aerith's two moons can be made stable.

The moons, abhall and caashey, are 120,000 miles and 240,000 miles distant from Aerith, respectively. The inner moon orbits approximately once every 10 days, the outer, approximately once every 30 days. Abhall is 1042 miles in diameter. Caashey is 4320 miles in diameter.... meaning abhall looks as large as our moon from the surface, and Caashey looks twice as large.

This orbit and planetary setup IS STABLE, given certain prerequisites.

1)The combined mass of the two moons is approximately equal to or slightly greater than Earth's own moon. (about 7.3X10^22 kg. The system remained stable even at 9X10^22 kg.) this prerequisite isnt necessary for orbital stability--- but without it there would be insufficient gravitational pull to cause lunar tides, which are vital to atmospheric cycles, tides, and many other features of ecosystems and the biosphere.)

2)Abhall is at least equal to, or greater than, Caashey in mass. The larger the difference the more stable the orbit. (for comparison the mean density of the moon is 3.35 grams per cubic centimeter.)

Speculations about the composition and structure of both moons may now run riot. But you can assume safely that Abhall has a greater influence on the tides, on average, than caashey.

Rokas · Post by **Rokas** » Sun Jul 30, 2006 5:29 am

Not to mention, those tides will be a bit more numerous and varied than tides here on Earth, which are caused by a Sun-Terra-Luna interaction. Now, you add a fourth body to the mix, with a different orbital period, and you'll get it so that sailing will probably be a bit trickier, not to mention, the possibility of inceased geological activity due to additional and changing stresses coming from two different sources.

MikeVanPelt · Post by **MikeVanPelt** » Mon Jul 31, 2006 12:56 am

I don't see any problem with that at all... the only thing that was giving me trouble was, someone (not you) said that both moons were always in the sky at all times. That, I don't see how it can be arranged.

I believe it was a statement that if the Rac Conans saw only one moon in the sky, they would find that Really Weird... They wouldn't immediately notice anything weird if the moons are in normal orbits like you describe; sometimes there'd be one, sometimes none in the sky.

Of course, Luna probably radically different from either of their moons, which probably would weird them out.

Tom Mazanec · Post by **Tom Mazanec** » Mon Jul 31, 2006 8:40 am

Unless the densities are way less for the larger APPEARING moon, it will have the greater effect on tides. The sun looks the same size as the moon (giving us great eclipses, btw) but is less dense, so has less effect on tides (but still "sorts" them into spring or neap tides, sepending on the moon phase).

RHJunior · Post by **RHJunior** » Mon Jul 31, 2006 9:41 am

Oh yes indeed. Caashey has a far lower average density than Abhall.
Far, FAR less.

PeCULiarly so......

Speculations about its composition may now begin in earnest.

Astral · Post by **Astral** » Mon Jul 31, 2006 11:41 am

It's a space probe sent by the BataCentarians!

... Ahem... excuse me...

Well, from what little I know that dose seem to make sence. It is thought that most large satellites are just debris left over from the beginning of the solar system, miniature planetoids that got caught in the gravitational pull of the mother planet but were sucked into orbit rather then ending up crashing into and simply being assimilated by the still liquid surface. Earth's moon is slightly different though, as it is composed of almost exactly the same material as the earth. Current theory suggests that a planetoid nearly as large as the infant earth collided with the planet and caused a kind of 'droplet in a pool of water' effect (If you look closely at a rain drop falling into a pool of water you'll sometimes notice another drop of water bouncing back out a few millimeters from the surface of the pool).
These factors combined, I would think that the smaller, denser moon was formed by the collision effect, and the larger, lighter moon was simply caught in the gravitational pull.... then again, I know only what I've seen in documentaries and various books.

Nick012000 · Post by **Nick012000** » Mon Jul 31, 2006 6:05 pm

The lighter one is primarly made of liquid, gas, or a combination of the two.

BrockthePaine · Post by **BrockthePaine** » Mon Jul 31, 2006 6:21 pm

It'd need to be liquid, I think... gases just don't collect in Moon-sized bodies unless they have a pre-existing body to keep them in. Gotta be a liquid for its size, I'd say.

Maybe ice, covering a deep ocean of water or methane, with a solid core? It's albedo would be seriously tweaked if it was ice, though. Nice and SHINY.

The JAM · Post by **The JAM** » Mon Jul 31, 2006 7:29 pm

That's no moon....
It can't be ice, otherwise it would behave like a comet. Can't be rock or iron...

Nick012000 · Post by **Nick012000** » Mon Jul 31, 2006 9:24 pm

Maybe there's another liquid mixed into the water to raise the boiling point?

Calbeck · Post by **Calbeck** » Mon Jul 31, 2006 10:43 pm

You know what I like most about this comic strip? That the creator has enough concern about the details to actually WORK OUT THE MATH FOR A DOUBLE-LUNAR ORBITAL SYSTEM.

Now that's frickin' commitment, man.

RHJunior · Post by **RHJunior** » Tue Aug 01, 2006 6:53 am

either that or he's anal retentive....

Anthony Lion · Post by **Anthony Lion** » Tue Aug 01, 2006 7:20 am

Well...

You said it, not us...

Where did you pick up the maths for all that?
(Just asking in case I ever decided to do a bit of world-building of my own)

Tom Mazanec · Post by **Tom Mazanec** » Tue Aug 01, 2006 9:11 am

I thought it might be filled with cave like cavities. Some of the asteroids we have probed seem to be like that. But the core would tend to collapse them for an object that big. Best guess I can come up with. Still, I suppose it is possible. I gave up on trying to say what is impossible in astronomy when they found those pulsar planets. I remember having a good laugh with an astronomy professor at the idea of looking for planets around a pulsar. And he thought I was nuts for suggesting that an asteroid could have a moon (and I was thinking about Ceres, not a pipsqueek like Gaspra). AFAIK theorists are still scratching their heads over that one...how do you get a planet, much less three or four, orbiting what is essentially a supernova site?

Kerry Skydancer · Post by **Kerry Skydancer** » Tue Aug 01, 2006 12:58 pm

If Caashey has that low a density -- gas wouldn't hold together at that size. It's gotta be a structure. Hollow space is the only way it would work. And the -surface- has to be solid, since the drawings seem to show consistent features. Possibly the ship that brought them here? And they lost their technology somehow?

Maybe we -will- get to see Quentyn in Space...

LoneWolf23k · Post by **LoneWolf23k** » Tue Aug 01, 2006 3:27 pm

Maybe it's actually a Vok mega-artifact, designed to turn into a planet-killer if enough Vok experiments on the planet are disrupted by outside interferences..

(Shameless Beastwars reference)

Earl McClaw · Post by **Earl McClaw** » Tue Aug 01, 2006 8:24 pm

You have to be careful about specifying the chemical composition. There's reasonable evidence that distance from the Sun is a factor (along with planet size/gravity and maybe others) in determining what substances will remain solid / liquid long enough to collect.

I'll speculate that Caashey is hollow to some extent. How hollow and why, I'll leave to those who want to work out the math.

EdBecerra · Post by **EdBecerra** » Wed Aug 02, 2006 3:18 am

Tom Mazanec wrote:AFAIK theorists are still scratching their heads over that one...how do you get a planet, much less three or four, orbiting what is essentially a supernova site?

I've always wondered about their wondering. Seems that it might be as simple as this - the planet wasn't always a planet. A brown dwarf with an impacted core might - JUST might - be tough enough to have that core remain intact after Mr. Super Nova goes *bang*. IF it were far enough away and tough enough.

I'm reminded of a game world used in Battletech, and in one of the novels... it's a world that USED to have a life-sphere, but it was ripped away in a cosmic accident. Now what life is left is at the bottoms of what were the abyssal depths of the now dry ocean beds, while the continental landmasses are, for all intents and purposes, so "high" above the little atmosphere that remains, they're airless.

Take that a step further... a planet that ALMOST ignited into a star, but wasn't able to. It's near another star that goes *bang*. The gasses are stripped away, but a small crushed core remains.

Be interesting to see someone calculate the physics on that, see if it's possible.

Tom Mazanec · Post by **Tom Mazanec** » Wed Aug 02, 2006 7:18 am

Pulsars are supernova remnants. Supernovae eject enough of their mass that any compnion star will fly off its orbit in a tangent (such stars have been discovered moving rapidly away from a star cluster). If by some miracle you get one to stay in orbit, how do you get three or four to do so in a nice solar system? Using Sherlock Holmses' principle, since this is impossible, theorists decided that they must have formed after the explosion. But the ejected mass moves way faster than escape velocity, and as well it "snowplows" the interstellar medium ahead of it. So how did a preplanetary disk form?

Aurrin · Post by **Aurrin** » Wed Aug 02, 2006 8:59 am

I'm curious as to how you figured this out. Did you use a program of some kind to check, or are you basing it off of forumlas, or what?