The moon on the dark side

[peacekeeper]

This is my very first post on these forums <img src="/images/icons/smile.gif" /><br /><br />The background story for this question is that I am working on a conuniverse. The story takes place about 10k years into the future. Now to the issue at hand:<br /><br />I was thinking of having a gas planet orbiting very close to its star. This planet should have a moon with an orbit that exactly corresponds in time with the orbit of the planet itself. This would make the moon always stay at the dark side of the planet.<br /><br />Now, what would the climate be like on this moon? I <i>think</i> that the planet would shield it from most of the heat from the sun, but still let enough heat through for the moon to be habitable. But I am no expert in this area, so I wouldn't know for sure if this is actually a possible case. So I ask you: Is it possible?

 

[kmarinas86]

<font color="yellow">Now, what would the climate be like on this moon? I think that the planet would shield it from most of the heat from the sun, but still let enough heat through for the moon to be habitable. But I am no expert in this area, so I wouldn't know for sure if this is actually a possible case. So I ask you: Is it possible?</font><br /><br />Light coming from around the edge of a gas planet can only diffract so much. A partial "lunar" eclipse might be enough to sustain life on such a planet given that the atmosphere of the gas planet filters out bad light and transmitts alot of good light. <br /><br />I don't know if the orbit would work though. Being so close the star, the orbit must be very precise. It would take a lot of luck to get the planet into the right orbit.

 

[Leovinus]

I don't think that such orbital mechanics are even possible. I think that at different radii, orbiting bodies have different orbital periods. The only exception to that I think is a LaGrange point, but I don't think you have one on the opposite side of a planet from a star like you mention. <div class="Discussion_UserSignature"> </div>

 

[heyo]

I believe his problem could be solved by placing the moon at the sun-planet L2 point of his system. (Lagrange point 2)<br /><br />That would have it orbiting the star, just outside the orbit of the planet perfectly alligned with it.<br /><br />This orbit, in real life, would not be stable for very long though. Example: we have our SOHO satellite at Sun-Earth L1 (same deal but just INSIDE the orbit of the Earth to watch the sun constantly), and in that case, SOHO requires constant station-keeping to keep it where it should be.<br /><br />Perhaps you will put a sufficiently advanced species on your moon that will be able to make small adjustments to it's orbit?<br /><br />Or you could put the moon at either L4 or L5 of your system, but then the moon would be in direct sunlight which I think you are trying to avoid. However, L4 and L5 have the advantage of being stable, and even if the moon is pushed out of place, it will fall back into place or will orbit in a kidney-shaped orbit around the Lagrange point.<br /><br />Looking up "Lagrange points" or "Libration points" might help.<br /><br />Hope this helps.<br /><br />Heyo

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>I don't think you have one on the opposite side of a planet from a star like you mention.<p><hr /></p></p></blockquote><br /><br />Actually, yes, you can. It would be the star-planet L2 point. The WMAP spacecraft is currently residing at the Sun-Earth L2 point. It goes around the Sun in exactly the same amount of time the Earth does, although it is further away. The WMAP website has a nice overview of Lagrange points.<br /><br />Trouble is, this point (like L1 and L3) is unstable. The Sun-Earth L1 and L2 points are unstable on a period of about 28 days, if memory serves, which means that spacecraft actually have to go into "halo orbits" around the points and have to tweak their orbits regularily with thurster firings. (One notable exception is the Genesis probe, whose spacecraft bus has been renamed Exodus as it heads out towards L2 for an extended mission. Genesis' trajectory was carefully designed so that when it naturally fell away from L1, it would return to Earth automatically, taking advantage of L1's instability.) So although there IS a point where you can stay always in the shadow of a huge gas giant (theoretically), a moon would not stay there for any significant period of time. Theoretically, a space station could sit there, if it fired its thrusters from time to time to push itself back onto the L2 point. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[Leovinus]

The last time I saw a moon capable of adjusting it's orbit, it was being blown up by Luke Skywalker. <div class="Discussion_UserSignature"> </div>

 

[peacekeeper]

That does indeed pose a problem. It seems the L2 point would be my best shot, if I could only get it close enough to avoid sunlight. That's the most important issue, you see. Is there any way to make this possible? Perhaps by having a very dim sun? Or by having a very small star and at the same time a very heavy gas giant?<br /><br />Regarding the unstability of the L2 point, I was planning on having the moon settled by humans, so making small adjustments to its orbit from time to time should be no problem at all.<br /><br />And what about the climate? Would it be possible to have a temperature of somewhere between -50 to 50 degrees Celsius?<br /><br />Thank you all for your answers, especially Calli, Heyo and Steve.

 

[heyo]

<i>Regarding the unstability of this point, I was planning on having the moon settled by humans, so making small adjustments to its orbit from time to time should be no problem at all. </i><br /><br />If that's the case, then L2 is probably your best bet. The adjustment would be not too huge, and not very often, but the technology available would still have to be far ahead of our own.<br /><br />Heyo

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>Those orbits are stable, but they are not total eclipses. <p><hr /></p></p></blockquote><br /><br />Yep. At best, they'd be annular eclipses, though really it might be more accurate to call them occultation events. The sun would dim considerably, but it would not be totally dark.<br /><br />Some interesting points:<br /><br />Lagrange points can only exist if the mass ratio between the two large bodies is greater than 24.96. Therefore, the star has to be a fair bit bigger than the planet.<br /><br />A Lagrange point can only be exploited by an object with mass which is negligible compared to the two large bodies. (In this sort of situation, a fairly big space station still satisfies the condition.)<br /><br />The Sun-Earth L2 lies 1.5 million km away from Earth. (And L1 is the same distance away, but in the opposite direction.) The Earth's shadow is about 1,356,000 km, so there cannot be a total solar eclipse at L2 due to the Earth's presence. However, it's close enough that the Earth is going to cover a heck of a lot of the Sun. I will attempt to find out how much.<br /><br />I looked it up and L1 and L2 are unstable on a period of 23 days. I misremembered. Sorry about that. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[peacekeeper]

<blockquote><font class="small">In reply to:</font><hr /><p>The Sun-Earth L2 lies 1.5 million km away from Earth. (And L1 is the same distance away, but in the opposite direction.) The Earth's shadow is about 1,356,000 km, so there cannot be a total solar eclipse at L2 due to the Earth's presence. However, it's close enough that the Earth is going to cover a heck of a lot of the Sun. I will attempt to find out how much.<p><hr /></p></p></blockquote><br /><br />But since the planet in question is much larger than the earth, and also much closer to the sun, wouldn't that make the L2 point come closer as well? And if it got close enough, that should make a total eclipse possible, right?

 

[Leovinus]

Does the presence of the Moon interfere with Earth's L2? That would not be an issue with the subject of this thread, but I'm curious. Is the L2 point a distance from the Earth/Moon center of gravity? <div class="Discussion_UserSignature"> </div>

 

[vogon13]

Is it ok if I rephrase question as follows?:<br /><br />Imagine a Jupiter sized planet orbiting close to a star and the planet orbits the star in, let's say 3 days. Around this Jupiter type planet, very close to its' star, you have asked for a satellite of this planet in a 3 day orbit phased to stay over dark side of planet.<br /><br />If we are on the same page so far...<br /><br />Is a three day orbital period around this planet possible? (assume planet in isolation) Yes<br />Does putting planet in 3day orbit around star screw up stability of previously described 3 day orbital period satellite? Suspect mass ratio of planet to star is important in calculation but since I have mentioned calculation.....<br /><br /><br />Paging Dr. Spacester, paging Dr. Spacester.... <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[Leovinus]

I'm not sure the 3-day orbit is possible for the moon around the planet. The moon might have to be inside the planet to orbit that fast. <div class="Discussion_UserSignature"> </div>

 

[peacekeeper]

<blockquote><font class="small">In reply to:</font><hr /><p>A Lagrange point can only be exploited by an object with mass which is negligible compared to the two large bodies. (In this sort of situation, a fairly big space station still satisfies the condition.)<p><hr /></p></p></blockquote><br />So would a moon be too heavy? Then perhaps I will have to go with a "normal" orbit after all? But someone said that wouldn't work either. I didn't understand why it wouldn't work though, so I would appreciate if someone who knows about it could explain it to me again.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Imagine a Jupiter sized planet orbiting close to a star and the planet orbits the star in, let's say 3 days. Around this Jupiter type planet, very close to its' star, you have asked for a satellite of this planet in a 3 day orbit phased to stay over dark side of planet.<p><hr /></p></p></blockquote><br />Yes, that's exactly what I meant. Sure, it doesn't necessarily have to be a 3-day orbit. It can just as well be one of ten days, or twenty. The main thing is that the planet is close enough to the star (and far enough from it) for the moon to be habitable without any great domes or anything like that to shield it from immense heat, cold, radiation etc.

 

[CalliArcale]

A moon can be considered negligible mass for these purposes, believe it or not. It's all a question of scale. There are fairly large asteroids that orbit at the Sun-Jupiter L4 and L5 points (called the Trojans) and the Sun-Mars L4 point. And some of Saturn's moons are trojan companions of other, larger Saturnian moons. It's just that if the third body gets significantly large, its own gravity begins to affect the other two bodies. I don't know what the cutoff is, though. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[peacekeeper]

So it is possible after all! That's great! Now I just have to figure out a way to get the L2 point close enough to the planet for there to be a constant eclipse. Can anyone who knows the math tell me how I can make this possible?<br /><br />Maybe even a dimmer star could do the trick? That way no Lagrange calculations would have to be made. But if a star is dimmer, wouldn't that make it colder as well? If the star <i>would</i> in fact become colder, that could mean it's not able to provide enough heat to the moon, right? If that is actually the case, the dimmer star scenario is no longer an option. And <i>that</i> would mean I have to calculate the Lagrange points after all...

 

[Leovinus]

To make the star colder, simply move your planet farther away. Look at the temp of Mars compared to Earth, for example. <div class="Discussion_UserSignature"> </div>

 

[peacekeeper]

Well, I know that! The question was, would the star automatically become colder if I made it dimmer? And would a dimmer star make the total eclipse possible? Either way though, the optimal scenario would be to figure out a way for the moon to be in constant darkness, even though the star is of a more "normal" brightness.<br /><br />Now, I really would need someone to tell be how to do the maths here. It's really hard to find how-to guides for these things on the internet.<br /><br />What I need is a way to calculate how to get the L2 point close enough to the planet for a total eclipse to occur - if such a thing is possible at all - by changing the masses of the star and the planet, as well as the sizes of the planet and the moon. I also need to calculate how much of the heat from the star is absorbed by the planet, and hence how warm the moon will be. In addition, I need to know how the brightness of the star is related to its temperature, and whether or not the temperature difference caused by the dimming of the star will be significant enough to affect the temperature on the moon.<br /><br />I realize that answers all these questions is pretty much to ask for, but I would really appreciate it if you could help me out here, one way or the other. Thanks again.

 

[heyo]

Some of the experts might want to confirm this with some calculations, but I think that the planets distance to the star is proportional to the L2 distance from the planet.<br /><br />Increase the distance from the star that the planet orbits, and that should increase the distance that the L2 point is out from the planet.<br /><br />This is an educated logical guess, but I could be wrong so you might want to get a second opinion.<br /><br />Heyo

 

[Leovinus]

Brightness of the star has nothing to do with whether or not an eclipse is total; that is a simple matter of geometry. <div class="Discussion_UserSignature"> </div>

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>Brightness of the star has nothing to do with whether or not an eclipse is total; that is a simple matter of geometry.<p><hr /></p></p></blockquote><br /><br />True, but it just made me think of something else.<br /><br />The determining factor is the apparent angular diameter of the star versus the apparent angular diameter of the planet. If the planet is the same size or bigger (as viewed from the moon), it will eclipse the star. This is dependent on the actual size of the planet, the actual size of the star, the distance between them, and the distance to the moon.<br /><br />What if the star were very dense, perhaps a star very late in its life such as a white dwarf? It will not be very wide, and it may be possible for the planet to ecplise it, or at least for there to be a region of totality on the moon at L2.<br /><br />If there is a region of totality, say, a round spot of shadow on the nearside of the moon, you might get very interesting weather. If the moon rotates synchronously, and thus always faces the same side towards its parent and the star, this will be a fixed point on the moon's surface. If not, then there would be a belt around the moon's middle which would see the shadow move across it over the course of a day. (If the moon's axis is inclined, the band of shadow would move over the course of a year.) We know that the Moon's shadow produces mild weather effects as it moves across the face of the Earth, so I think we could expect the same to happen here. It would be cooler in the shadow, and winds would move ahead of it and behind it.<br /><br />Another thought I had: what if the planet merely reduces the star's brightness sufficiently to render the planet hospitable? There would be no spot of totality on the moon, but perhaps the planet would dim the star just enough. The Earth doesn't eclipse the Sun at L2 but I believe solar radiation should be diminished there because of the Earth's shielding effec <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[thalion]

<<Some of the experts might want to confirm this with some calculations, but I think that the planets distance to the star is proportional to the L2 distance from the planet. >><br /><br />I'm pretty sure that's correct; the L1 and L2 points are really just selected points on a planet's Hill Sphere, which for two objects of constant mass grows larger for the secondary with distance. The formula for the Hill Radius is:<br /><br />a * (m/3(M+m)^1/3 , where<br /><br />a = semimajor axis<br />m = mass of lesser body<br />M = mass of larger body<br /><br />In the case of a planet which is much smaller than its primary, you can omit the <i>m</i> term in the denominator.

 

[peacekeeper]

Very interesting idea you've got there with the shadow area, Calli. However, I would prefer the moon to be in complete darkness. Which brings me to your other suggestion. Making the star a white dwarf is an excelent idea! I made a seach on google, and it seems that white dwarves usually have a diameter of no more than 10,000 km. Since the planet will have a diameter over 10 times as wide, a moon at the L2 point would most probably be shrouded in darkness, don't you think?<br /><br />Though if I could get a "normal" star to do the trick, that would probably be even better. I want the gas giant to be a beautiful sight when viewed from the light side (unlike the gas planets of Sol, which look very pale and boring), and I am not sure the dim light of a white dwarf could provide that wonderful effect. But of course, I could be mistaken. The light of the white dwaft might not even be very dim when seen up close. Anyone care to tell me how it really is?<br /><br />Should I decide to go with a sun like star, Tigerbiten's calculations have made me confident that doing so would work just fine. Regarding the fact that the L2 point would only be stable for about 1/10th of the orbit (which depending on the distance to the star in this case could be as short as half a day), I am sure that could be fixed with a bit of advanced technology.<br /><br />I have also been pondering the idea of having a red dwarf. That would inevitably cause a few interesting visual effects, especially if the planet has rings. But again, my limited knowledge about this is working against me. I know that red dwarves are much colder than most other stars. The question then is, are they hot enough to provide sufficient heat to the moon?<br /><br />Thalion, I thank you for your calculations as well. And I realise that the Lagrange points change with the distance. However, since I am free to decide the masses of the planet and the star as well as the distance between them, I have a few more things to play with when d

 

[peacekeeper]

That's excelent news! Thank you very much!<br /><br />And by the way, what is this Roche limit you are talking about? I know I have heard of it before, but I can't quite remember what it means.

 

[bobvanx]

>>are they hot enough to provide sufficient heat to the moon?<br /><br />If your moon is in permanent eclipse, it's going to be very very cold if it has to rely on the star's warmth.<br /><br />Vacuum is nearly perfect insulator; in it, heat travels only by radiation. Refraction of the star's light through the gas giant's atmosphere could bring some small measure of infrared radiation to your moon, but just like the permanently shadowed craters of our moon are at cryogenic temperatures, so too would be the case for your moon. <br />Unless...<br /><br />your gas giant is radiating strongly in the infrared. <br /><br />While unlikely for a planet of an old white dwarf, a large gas giant is radiating considerable infrared left over from its birth and continued gravitational contraction. You could even postulate bizarre processes and boost the heat more. Neptune gives off heat as it makes a rain of diamonds, for example.<br /><br />Your moon would also experience significant tidal heating as it wobbled around in a halo orbit, so all your buildings will need to be very quake resistant.<br /><br /> />>(unlike the gas planets of Sol, which look very pale and boring)<br /><br />!!? Dude, are you an alien!?<br /><br />I think Jupiter, with its orange clouds and great big eye-spot storm and Saturn with its butterscotch and blue orb and rings are truly stunning sights! Well, to each eye beauty is unique. How many eyes does your species have? We only have two, so perhaps that's why I think our gas giants are beautiful.<br /><br />Unfortunately, a gas giant that is radiating strongly in the infrared is likely to be even more bland than our gas giants, since the greater heat would lead to greater mixing of its layers. More powerful winds and storms, but the colors would be more bland.<br /><br /> />>, and I am not sure the dim light of a white dwarf<br /><br />A white dwarf has a beautiful, blue-white actinic arc-lamp quality to its light.<br /><br /><br />Final thought: a large orb, hanging