planets, moons, satellites and stuff

[chrispenycate]

If the moon was cycling through the L2 Lagrangian point it could be in shadow for weeks at a time. Although objects at the Lagrangian points are typically not thought of as moons.

Because they are not actually orbiting the planet, right.

But, apart from 'Trojan asteroids', how would you specify such bodies?

someone there who had great knowledge of astronomy who stated that it was impossible for a satellite of a planet to have a satellite of its own.

Unlikely, maybe. But I can quite easily specify a stable orbit for a 'satellite of a satellite' (stable for a few million years, at least). I wouldn't like to do it for the Jupiter system, littered with moons, but on a simple 'one star, one planet, one moon, one sub moon' system I could specify orbits and make a reasonable explanation of how it condensed like that out of the original protoplanetary disc

 

[pixallmighty]

I thought about what all of said, made some decisions and found what was important to me. I had a lot of ideas that were just cool, but weren't necessary: so I have some numbers to work with, and if I have to give up some ideas, than so be it.

-One day and one night last 28 hours.

-The diameter of the moon is between 4700 and 6300 km (smaller than what I originally said)

However, if you already have a huge planet in the sky, and that planet's rings, do you really need a moon in the sky as well?

Maybe I said it wrong, but the moon DOES NOT have moons. As Psychotick said, they wouldn't be necessary.

Plus, the planet would always be near the sun from the perspective of the "moon" and therefore you don't get the huge beautiful planet filling the night sky.

That is the point of having my story on a moon. In the end, it doesn't need to be a gas giant, but since my moon is more or less the size of Titan, I hope that something would be possible.

-For the eclipses I think a few hours are okay. If I must choose, I prefer to have fewer and shorter eclipses during a year, but to have the planet in the night sky.

-I problem I encountered was the moon's gravity, that I need to be similar to the Earth's. If I understood things correctly, that mean the moon's density, because of it's smaller size, would have to be two times the Earth's,. Would that be plausible? It wouldn't matter if the moon's gravity was inferior to the Earth's, maybe 0.9 G (?).

-Humans must be able to live on the moon.

I think it sums up all the things I really need; the rest is accessory. And as Chrispenycate said:

the readers don't need to be told any of this. The ones like me will work it out for themselves, the others don't want to know.

 

[Wormtongue]

Because they are not actually orbiting the planet, right.

But, apart from 'Trojan asteroids', how would you specify such bodies?

No, trojans do not orbit the body they are in synch with. I suppose if a planet was at a Lagrangian point it would be a trojan planet.

My world has a trojan asteroid at the L2 point. Obviously the inhabitants don't know what asteroids are, much less what the Lagrangian points are. To them it's just a star that always rises at sunset and sets at sunrise. They call it the Watchstar.

 

[Ravana]

That is the point of having my story on a moon. In the end, it doesn't need to be a gas giant, but since my moon is more or less the size of Titan, I hope that something would be possible.

The Kepler mission has discovered numerous gas giants very close in to their stars… far closer than we previosly believed possible, as chrispenycate gestured toward. Whether or not the gases escape will depend not only on proximity to the star, but also on the mass of the planet, the amount of radiation coming off the star, the extent to which the planet's magnetosphere helps deflect said radiation… probably a few other factors not coming to mind right away. Above all else, though, the process still takes place over time, so you can simply put your system at a younger age and say the gases haven't all been blown away yet.

-I problem I encountered was the moon's gravity, that I need to be similar to the Earth's. If I understood things correctly, that mean the moon's density, because of it's smaller size, would have to be two times the Earth's,. Would that be plausible? It wouldn't matter if the moon's gravity was inferior to the Earth's, maybe 0.9 G (?).

Plausible, or possible? Earth's pretty dense, as far as observed bodies go. It could be denser… if, say, the amount of lead in the planet far exceeded the amounts of iron or silicon. Also, density might be greater if the moon has fully cooled rather than having a molten core (and is thus smaller). I can't actually think of a way this would be likely to happen–and I'm pretty sure that the younger the system, the less likely it would be–but that doesn't make it physically impossible. The universe is a strange place.

Note that a hyperdense moon might also imply a greater density at the core of the planet it orbits (if they formed at the same time, and the planet got its own greater-than-normal chunk of heavier materials), making it easier for the planet to keep its gaseous envelope longer.

 

[Wormtongue]

-The diameter of the moon is between 4700 and 6300 km (smaller than what I originally said)

-For the eclipses I think a few hours are okay. If I must choose, I prefer to have fewer and shorter eclipses during a year, but to have the planet in the night sky.

-I problem I encountered was the moon's gravity, that I need to be similar to the Earth's. If I understood things correctly, that mean the moon's density, because of it's smaller size, would have to be two times the Earth's,. Would that be plausible? It wouldn't matter if the moon's gravity was inferior to the Earth's, maybe 0.9 G (?).

-Humans must be able to live on the moon.

I don't know what your motivations are behind the size but why not just make it near earth size? That's the easiest way to get the gravity and atmosphere you need for humans.

As far as the eclipses, depending on the inclination to the plane of the ecliptic the eclipses could be every day or only a few eclipses a year. If only a few times a year they would happen in clusters twice a year. The clusters would have a pattern of short, medium, long, medium, short. The number of eclipses in each cluster could be any number you choose but both clusters would be about the same and it would not vary much year-to-year.

 

[Wormtongue]

As far as the eclipses, depending on the inclination to the plane of the ecliptic the eclipses could be every day or only a few eclipses a year.

Can't edit my post.

What I meant to say is that the eclipses could be every orbit, not every day. That could be as little as a few days or up to a month or more depending on the orbit. It's not practical to have an orbit shorter than 3 days due to the extreme tidal forces and probability of being tide locked. Plus the radiation close to a gas giant is lethal.

And speaking of tides, a moon around a gas giant is going to experience some spectacular tides. Sea travel would be much more challenging. Harbors as we know them would not be possible. Think Bay of Fundy X10.

 

[Ravana]

Plus the radiation close to a gas giant is lethal.

Possibly. Depends on how the planet's magnetosphere works: how strong it is, how far it extends, etc. Compare the magnetospheres of Jupiter and Saturn, for instance. Certainly, it's better to bet this way rather than otherwise, but very little is impossible, only less probable. Regardless of these factors, the moon's atmosphere will still shield the surface to a considerable extent… though the radiation might produce some interesting effects in the upper atmosphere. Global auroras, anyone?

Note that not only will distance be a factor here, but depending on the inclination of the moon's orbit to the planet, the moon may pass in and out of the magnetosphere. Which itself could make a fascinating plot point… possibly even replace what pixallmighty had initially been hoping to achieve with eclipses.

And speaking of tides, a moon around a gas giant is going to experience some spectacular tides. Sea travel would be much more challenging. Harbors as we know them would not be possible. Think Bay of Fundy X10.

Will depend in part on the distance between the planet and moon: the differences between Io and Callisto, for example, are pretty dramatic (as are the differences of the effects from Jupiter's magnetosphere). Again, large tidal forces would be the default assumption, though.

Note that even Callisto is regarded as an "inner" moon of Jupiter: move farther out, everything drops off all the more. So ultimately distance will be an overriding factor.

 

[pixallmighty]

Note that not only will distance be a factor here, but depending on the inclination of the moon's orbit to the planet, the moon may pass in and out of the magnetosphere.

Do you have some examples of things that could happen? You mentioned auroras, but do you know what would be the impact(s) on the moon of such phenomenon?

 

[Ravana]

I don't know offhand what effects might be expected. Apart from the pretty lights, at least. So I'll speculate.

One would expect a significant increase in ionizing radiation bombarding the upper atmosphere; possibly, some of the radiation might reach the lower atmosphere, causing… whatever such radiation might cause. Dramatic electrical storms, possibly even noticeable ground discharges (that is, ones that follow along the terrain, not ground strikes), if enough of the charge reaches the moon… perhaps channeled by the moon's (weaker) magnetic field toward its poles, creating differences in static levels between poles and other regions.

The upper atmosphere might see a significant increase in the amount of ozone, which would help shield the surface but which could have follow-on effects such as greenhousing. Note that the ozone increase will be short-term, as it will break down into normal O[SUB]2[/SUB] over time; depending on how often the moon passes through the planet's magnetosphere, this may create cyclical effects–more likely if this only happens every few orbits, since otherwise, the ratios would probably remain fairly steady (it takes time for the ozone to break down). The normal ozone-oxygen cycle is caused by UV radiation, which both creates and breaks down ozone (yeah, it's strange); I'm not sure whether something similar would happen in this situation. Probably not… unless you can find a way for the magnetosphere to be concentrating UV rad from the star onto the moon. Which is probably possible.

Depending on how you set things up, passing through the planet's magnetosphere might be a respite rather than a bombardment, as solar winds are redirected by it around the moon. If the period within the magnetosphere is minimal (i.e. the moon's orbit is a distant one), this probably wouldn't be of much use as a plot point. If the period is large (due to a closer orbit, a larger magnetosphere, or both), and the moon only left its protection occasionally, then you'd see an increase in bombardment of all radiation types emitted by the star. And the transition period, as the moon passes through the highly-charged outer edges of the magnetosphere, could be potentially dramatic as well.

Check the following terms for a start on what might happen: ionizing radiation, ionosphere, stellar/solar wind; perhaps cosmic ray. (Plus magnetosphere, if you haven't already.) After that, follow whatever links seem most promising. I imagine the possible effects are pretty broad, depending on how you want to set things up. Unfortunately, you'll eventually start hitting heavy science, so if that isn't your thing, you may just want to find a few nifty potential effects, say they happen, and hope no one reading your book bothers to work through the science to find out where you cut corners. Which, honestly, is a pretty safe bet.

Besides, this is fantasy, so there may be unexplained phenomena involved.