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Thread: Need a life-sustaining satellite for a story

  1. #1
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    Need a life-sustaining satellite for a story

    I'm currently writing up a story for a roleplaying game that takes place on a roughly Earth-sized planet. As part of the story, I want to have the moon that orbits the planet to be able to sustain life, like on the planet. What would the mass and size of this moon have to be in order to keep an atmosphere, say, like Earth? The setting is a D&D-esque fantasy world, but I want to try and keep the astronomy as close as I can.

    Thanks!

  2. #2
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    How about the same size as the home world? Or, if you don't have to have life evolving on the moon, then luna-up-to-Mars would work, but you'd have to BYOA (Bring Your Own Atmosphere)...dump comets by the gross and enjoy the blue skies while they last. A few thousand years? Would that be enough for your action to develop?

  3. #3
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    THAT would be a neat idea...a double planet system? But yeah, I'd like to have life evolve on both bodies. I figure they are about 5BYO.

    [EDIT] Hey! I'm a Bad Fellow now! Been a few months since I first joined. *has sparklers around*

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    Say about the size of Mars, that way you could say that one was really a moon, orbiting the other (not in reality but good enough). Is one supposed to be arid and the other Earth-like? Both the Earth-like? What type of animals will there be on the moon? What level of technology? Or will they magic themselves there? If you need any help with military stuff, weapons from the lowly derringer to the laser cannon, simply ask. Just call me Colt. :wink: -Colt

  5. #5
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    Both are to be Earth-like with a similiar structure and environment (so animals wouldn't be comepletely different). The setting is taking place in several time periods (I'm invoking magic to deal with Time Travel) with the "first" age so to say be around 1400. I'm taking liberties and incorperating crude steam machines, and magic is widely popular in both worlds. Trade can commence on both worlds through magical portals that were put in place a long time ago, but travelling is costly (kinda difficult to have and maintain a two-way magical teleporter spanning hundreds of thousands of kilometers). Later on as technology progresses, shuttles become more efficient to go from point A to point B, still enhanced by magic.

  6. #6
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    Could the moon be smaller but denser? If you took the Earth - Luna system but made Luna dense enough to have gravity that would hold down an atmosphere, how much more dense would Luna have to be?

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    Quote Originally Posted by Musashi
    Could the moon be smaller but denser? If you took the Earth - Luna system but made Luna dense enough to have gravity that would hold down an atmosphere, how much more dense would Luna have to be?
    Since:

    surface gravity = G * mass / radius²
    mass = density * volume = density * (4 * pi * radius³ / 3)

    surface gravity = (G * 4 * pi / 3) * density * radius

    So, we see that the surface gravity is linearly proportional to the average density. This is a very simplified model that doesn't take into concerns mascons, non-uniform densities and so forth.

    I'm not sure how high gravity needs to be to be able to hold an atmosphere; if I'm not wrong this also depends on how fast the body is spinning around its axis. No doubt there are approximative formulas to use, but let's say it needs to be half of Earth's gravity, or 3 times that of the moon.

    If gravity is to be 3 times higher for the same radius, density needs to be 3 times higher. The moon's average density is something like 3.34 tons/m³, so this would be something like 10.0 tons/m³.

    The highest observed density for any moon in the solar system is that of Jupiter's moon Adrastea, with 4.6 tons/m³. For planets, the highest number is 5.52 tons/m³ for Earth, with Mercury and Venus close seconds. (Corrections to my figures are welcome)

    So, the best bet seems to be making the planet larger, unless you want a planet made out of lead.
    (Edited - speling errors)

  8. #8
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    Hmm, so you could go in the other direction, and rather reduce the density of the "planet" in the scenario. Stick an Earth-sized Earth in orbit round a bigger, less dense object.

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    Quote Originally Posted by Iain Lambert
    Hmm, so you could go in the other direction, and rather reduce the density of the "planet" in the scenario. Stick an Earth-sized Earth in orbit round a bigger, less dense object.
    Jupiter size + planet/brown dwarf.

    Two earth size sattilites sharing an orbit

    More.

    Just two hits off a Google....

    Should lead to some cool night skys, and interesting seasons. If the main body is a brown dwarf, could be enough heat from it for life, and the sun is just there for light/ really warm summers...the possibilites are endless.

    Let us know what works out

    edit: forgot to say about they swap places every [I can't remember how often]. One is in a slightly lower fast orbit than the other, then when it catches up, the outer one is pulled down, and the inner one is boosted out (the same principle as a gravitaional sling shot). Don't know how stable it is, but in a D&D world there can be more than first meets the eye (sounds like a great quest sooner or later)

  10. #10
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    A martian sized moon would retain an atmsphere for many millions of years- look at Titan, smaller than Mars but with a dense atmosphere (but miuch colder).
    I would expect any two earth type planets that were so closely associated to have been the product of a prehistoric terraforming campaign by person or persons unknown;

    failing that, a true double planet, with both planets approximately Earth sized, might be possible... they may have been separated by a Big Splash billions of years ago, or simply coalesced together like two close orbiting binary stars.
    It would be common for such binary planets to develop in similar ways, and abiogenesis may happen on one planet, then life could transfer to the other by ejected material carrying biological spores or similar.

  11. #11
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    If you want to pick up a couple of good boks on the subject, Allen Steele's Coyote stories (or the book) is a good place to look - he describes a planet-sized moon orbiting a gas giant.

    Stephen Baxter's Origin also talks about a moon big enough to hold an atmosphere. He describes it as being larger than Mars. Rememebr CO2 is pretty dense as gases go and Mars' atmosphere is darn thin.

    Here are some ways to figure out how thick an atmosphere is. By the way, how fast a planet spins has little effect. (The centripetal forces are't big enough).

    1. Figure out surface gravity. There is an earlier post on how to do that.

    2. What is the atmosphere made of? This is simpler if you assume an eartly composition.

    3. How hot or cold is it? Colder= denser gases = easier to hold air.

    Now, asuming there is as much air (by mass) as on Earth, one could approximate it by multiplying the surface gravity in gees (fractions of that of Earth) by 14.7 and get a figure for the atmospheric pressure at the surface.

    With a little calculus, one could figure the volume of the atmosphere and then get an idea of how far up it would go, and what altitude the molecules would start escaping to space.

    This would be a gross oversimplification but it might work.

  12. #12
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    Thanks for all the help, folks! I really liked some of the ideas (like darkhunter's satellites orbiting a brown dwarf which in turn orbits a star). It's leading to some interesting ideas that are cropping up. Now that you kind folks gave me some material and formulae to work with, I'll do some math and get some estimates going. Thanks again!

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    If this universe includes magic, you don't really need to worry about the science. Some ancient wizard could simply have established an atmosphere magically, regardless of the moon's size.

    If you insist on getting the science right :P then Emspak's points are all excellent. The planet's temperature (which is closely related to its distance from the star and the star's output) has much more to do with the kind of atmosphere it can hold than the rotation rate does. That's because the main mechanism for losing an atmosphere is thermal - "hotter" molecules at the top of the atmosphere can reach escape velocity, and fly off (never to return).

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    On a planet with a lower gravity the atmosphere gets thinner more slowly with altitude; instead of having an atmosphere 50 km high you might have the same thickness of atmosphere at 100 km.
    The loss of molecules at the top of such a high column is greater than on Earth, but the total mass of the atmosphere will be quite large- you could terraform Mars, and expect the planet to keep its atmosphere for millions of years. Of course the planet is billions of years old, so has lost most of it's original atmosphere (if it ever had one)
    but you could make it livable for a while.

    Be careful about putting planets in orbit around a jupiter type planet, though- if it is Saturn sized the local temperature is probably too cold,
    if it is Jupiter sized you can get heat from Tidal warming but there is uncomfortable radiation,
    If it is an Epistellar Jovian (a hot Jupiter, near the local star) the orbit of any satellite is likely to be unstable,
    and if it is a Brown Dwarf the moons will probably be tidally locked, and have one warm side and one cold side.

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    I've read about binary stars that are so close together that they're practically touching. Set up the planet and moon the same way. They'd then share the same atmosphere. Make them tidally locked so there won't be excessive wind and tides, and so the moon won't be slowed by atmospheric friction. People could travel between them using aircraft. No space program would be necessary.

  16. #16
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    Quote Originally Posted by Chuck
    I've read about binary stars that are so close together that they're practically touching. Set up the planet and moon the same way. They'd then share the same atmosphere. Make them tidally locked so there won't be excessive wind and tides, and so the moon won't be slowed by atmospheric friction. People could travel between them using aircraft. No space program would be necessary.
    They'd have to be just outside the Roche limit to share atmosphere yet not break apart due to tidal forces. Robert Forward wrote a book about explorers who find a pair of worlds like this, where you could fly from one planet to the other. It would be an interesting place.

  17. #17
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    If they're tidally locked would tidal forces break them up? The moon would be elongated pointing toward and way from the planet but there would be no forces acting on it to break it up. Or perhaps it couldn't have gotten into that position without breaking up.

  18. #18
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    bob forward's book

    The Bob Forward book is 'Flight of the Dragonfly'. I think it was serialized in Analog as 'Rocheworld'. Forward's physics is pretty darn trustworthy. By the way, for those who haven't heard, Dr. Forward died recently.

  19. #19
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    Even if they're tidally locked (which they would have to be, given the proximity), they will still break apart if they're too close together. It has to do with the difference in gravitational force from one side of the planet to the other - if there's too much difference, it just pulls the planet apart. It is possible to have two planets close enough together that they share some atmosphere but don't break apart, but only just barely.

    How wuch a tightly coupled double planet could form is also a good question. It's not a likely thing to see happen naturally.

  20. #20
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    Ok, so let's say the planet and moon are close together and share the same atmosphere. How large would the planet and moon (or double planet) be? How close together would they be? Would an Earth-Mars sized system work out? You guys are definately getting credit for this stuff!

  21. #21
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    I've read about binary stars that are so close together that they're practically touching. Set up the planet and moon the same way. They'd then share the same atmosphere. Make them tidally locked so there won't be excessive wind and tides, and so the moon won't be slowed by atmospheric friction. People could travel between them using aircraft. No space program would be necessary.
    Boy, at that range, the gravity might be a little wacky. According to this site, http://www.rcn27.dial.pipex.com/clou...tmosphere.html, the separationg between the planets should be about 100 km for easy air travel back and forth. Compared to the Earth's radius of ~6000 km, that's pretty darn close. There would be quite a change in gravitational pull between the near and far sides of either planet. That would be pretty cool, actually, and would make for smaller escape velocity on the neighbor-facing surface. Oh, a quick back-of-the envelope calculation shows that for two earth-like planets, the gravity would be about 3.3% of a gee on the near side.

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