Questions of a weird mind

[Elemental]

So more than 40 times the size of a full moon.
OK, interesting.
The reason I asked is because I had an idea for a graphic novel involving two races in a double planet system launching the equivalent of the Apollo-Soyuz Test Project, told from the point of view of one of the astronauts, maybe even twice, once from a crewmember on each spacecraft.

Sounds interesting.
However, I think at that distance the two would be tidally locked, which would cause problems.
But you can ignore all that. Science should not get in the way of a story, only enhance it.

[Ravens_cry]

Sounds interesting.
However, I think at that distance the two would be tidally locked, which would cause problems.
But you can ignore all that. Science should not get in the way of a story, only enhance it.

Like what? Could they spin around a central point, like some binary stars?

[Elemental]

Like what? Could they spin around a central point, like some binary stars?

I would think that they would spin around a central point because the centre of mass of a system of two equally sized objects would be between them somewhere.
However, the problems I was thinking of would be concerning ocean and land tides. Tides caused by the Moon are already significant, tides caused by an object with the mass of the Earth would be substantially greater.
Edit: Yes, the land tides would be significant, resulting in earthquakes.

[Ravens_cry]

I would think that they would spin around a central point because the centre of mass of a system of two equally sized objects would be between them somewhere.
However, the problems I was thinking of would be concerning ocean tides (possibly land tides, but I'm not certain). Tides caused by the Moon are already significant, tides caused by an object with the mass of the Earth would be substantially greater.

Not to mention closer. It could be background stuff, alluded to but normal for them so not exactly mentioned much.

[Douglas]

Physics question: is it possible for a planet to have a natural geostationary satellite, as in, a moon? Could that moon also be tidal locked?

Because I had an idea for a weird SciFi story, basically.

That's pretty much the definition of tidal locking, I believe. All planet/satellite systems will shift towards that configuration over time, it's just a matter of how quickly and how far off the initial state was.

I would think that they would spin around a central point because the centre of mass of a system of two equally sized objects would be between them somewhere.
However, the problems I was thinking of would be concerning ocean and land tides. Tides caused by the Moon are already significant, tides caused by an object with the mass of the Earth would be substantially greater.
Edit: Yes, the land tides would be significant, resulting in earthquakes.

Ah, but we're talking about two objects that are tidally locked with each other - which means that there aren't any tides.

Tides are the mechanism that causes tidal locking, and they are caused by two objects orbiting each other without being tidally locked. If tidal locking has already been achieved, then there are no more tides.

[Elemental]

Ah, but we're talking about two objects that are tidally locked with each other - which means that there aren't any tides.

Tides are the mechanism that causes tidal locking, and they are caused by two objects orbiting each other without being tidally locked. If tidal locking has already been achieved, then there are no more tides.

Of course, you're right about that. How could I forget that?
Anyway... I'm pretty sure there'd be some significant seismic effects on a pair of geologically active planets that are tidally locked to one another.

[Ravens_cry]

Perhaps the side that points toward the other planet would bulge somewhat, but by how much?

[Elemental]

Perhaps the side that points toward the other planet would bulge somewhat, but by how much?

That question is up to the people who know how to work that out.
However, the seismic effects I was referring to would likely take the form of increased volcanic activity and a higher number of earthquakes.

[AtomicKitKat]

That question is up to the people who know how to work that out.
However, the seismic effects I was referring to would likely take the form of increased volcanic activity and a higher number of earthquakes.

That is just the pre-Apocalypse. As time goes on, the planets will eventually become smooshed into one another, and a new, bigger planet, with maybe 1.5 times the mass(the remaining 0.5 got blown off into space on impact, before the combined gravity trapped the rest in orbit, and subsequently reabsorbs them) of either of the originals will be formed.

All of the above of course, assumes Earth-like planetary composition. If the planet were a solid chunk of *insert metal here*, there'd be more gravity(increased mass/density being the cause), but far fewer seismic effects upon its surface. If the planet was light to begin with, then more of its surface would be jettisoned on impact.

[Yora]

Are we speaking of two earth-size planets, with earth-mass, at the distance of pluto and charon, which are both tidally locked to each other?

In a perfect configuration, there would be no tidal forces at all. Both planets would have entered a gravitational equilibrium and the gravitational effect at each point of both planets would always be the same. So you don't have the squeezing effect that is found on the moons of Jupiter and Saturn. These moons do not orbit their planets in a perfect circle.
But of course, you wouldn't find a perfect circle anywhere and there will always be minor irregularities in the gravitational effects, so a bit of squeezing on both objects will always appear. How great that effect would be would be whatever you chose.
However, given that Ploto and Charon are both a small fraction of the mass of Earth, that distance between two earth-size planets would probably not work.

[QUOTE=factotum;13187236]It would be a fairly unlikely astronomical coincidence, of course, but then, so is our Moon being so close in size to the Sun (as seen from the Earth's surface) that during an eclipse it can exactly block out the main part of it and leave the solar corona visible.QUOTE]
Pure coincidence, since the distance of the moon increases over time. Some million years ago, the corona would not have been visible. Some million years in the future, it will not be able to cover the sun.

[Ravens_cry]

Not Pluto Charon distances, a bit further actually, at geosyncranous orbit, though it can be farther if I make the day longer.

[Yora]

Now I see. For doube-tidal lock, you probably would need a geosynchronous orbit.

However, that applies only if one body is significantly more massive than the other. With two bodies of similar size, they would be orbiting a shared center of gravity, neither would be orbiting the other.

Let's aproach this whole thing from the other side: What are the parameters that you want to be included in the final result?
I believe it is to have a large planet visible in the sky at a fixed position, for people on either planet?

[factotum]

I wonder how long it would take for two Earth-sized bodies to become tidally locked to each other when orbiting at, say, Moon distances? Bear in mind here that your geostationary orbit distance doesn't have to be the same 22k miles (or thereabouts) that it is for Earth--if the "prime" planet spins slower then geostationary orbit is higher. If you had two Earth-sized planets 250k miles apart that were tidally locked then the day would be an Earthly month long, mind you, which might cause problems.

Actually, there's another issue I can see as well. If there was a massive body in geosynchronous orbit, you effectively lose most of your ability to put communications satellites in that orbit; only the Lagrange points of the system would be stable points, anywhere else would require so much station-keeping thrust due to the disruption caused by the moon that it would be impractical to put anything up there. Imagine how our modern world would be without communications satellites!

[Elemental]

Actually, there's another issue I can see as well. If there was a massive body in geosynchronous orbit, you effectively lose most of your ability to put communications satellites in that orbit; only the Lagrange points of the system would be stable points, anywhere else would require so much station-keeping thrust due to the disruption caused by the moon that it would be impractical to put anything up there. Imagine how our modern world would be without communications satellites!

Oh no! We would have to make do with a land based system!
Seriously, we'd be fine. If we never used them in the first place, we'd never become reliant on them.

[Ravens_cry]

Now I see. For doube-tidal lock, you probably would need a geosynchronous orbit.

However, that applies only if one body is significantly more massive than the other. With two bodies of similar size, they would be orbiting a shared center of gravity, neither would be orbiting the other.

Let's aproach this whole thing from the other side: What are the parameters that you want to be included in the final result?
I believe it is to have a large planet visible in the sky at a fixed position, for people on either planet?

Basically the opening splash screen is a great curve of one planet, with the other hanging overhead also only partially visible with one spacecraft shutting down the third stage booster (or something like the Agena Target Vehicle) after orbital insertion with the sun to one side, probably the left.
No stars.

[Elemental]

Basically the opening splash screen is a great curve of one planet, with the other hanging overhead also only partially visible with one spacecraft shutting down the third stage booster (or something like the Agena Target Vehicle) after orbital insertion with the sun to one side, probably the left.
No stars.

Of course there wouldn't be any stars. You've got the sun in the sky.
You can't see stars during the day time on the Moon where there's very little atmosphere, so you're unlikely to in space.

Also, there would be tides, but they'd be solar tides, and thus not as high.

[Ravens_cry]

Of course there wouldn't be any stars. You've got the sun in the sky.
You can't see stars during the day time on the Moon where there's very little atmosphere, so you're unlikely to in space.

Also, there would be tides, but they'd be solar tides, and thus not as high.

Exactly. I'm also working on some space vehicle designs that are hybrids of various flown, and unflown, capsules, like the Apollo D-2 proposal.

[Brother Oni]

In fantasy and sci-fi settings, is there a term for a sentient species that eats other sentient species?

I hesitate to use the term cannibal since that generally implies eating your own species.

An example would be the Pak'ma'ra from Babylon 5, but all available sources I can find state that they're carrion eaters and distain fresh meat, to the point that such foodstuffs can cause intense nausea and involuntary projectile vomiting.

I was thinking some cobbled together term (xenovore or something), but I can't think of a more suitable generic term for a sentient being.

[Elemental]

Hmm... That's a tricky one.
All I know is that there is no word in the English language. Unfortunately, it can be difficult to come up with a suitable word for these kinds of things.
All I can think of is that xenovore is a terrible idea because it involves eating things that are classified as foreign, strange or unknown. Not really what you're going for.

[Brother Oni]

Hmm... That's a tricky one.
All I know is that there is no word in the English language.

I believe that most of these terms have Greek/Latin roots anyway (carnivore, omnivore, herbivore, with the various subdivisions like piscivore, insectivore, etc), so wasn't really expecting to find something in English.

All I can think of is that xenovore is a terrible idea because it involves eating things that are classified as foreign, strange or unknown. Not really what you're going for.

Yeah, I wasn't really expecting that one to work - was thinking of the 40K term Xenos (meaning any non-human), which probably wasn't a good idea to begin with.
I know the Aliens from the Alien franchise are sometimes referred to as xenomorphs and the Colonial Marines generally refer to such things as 'bugs' in a derogatory manner, indicating that they encounter (and exterminate) such extraterrestial critters on a fairly common basis.

Hmmm, maybe something involving 'extraterrestrial'?

[Elemental]

Hmmm, maybe something involving 'extraterrestrial'?

Extraterrarian?
It's similar to vegetarian.

[Brother Oni]

Extraterrarian?
It's similar to vegetarian.

Hmmm, true. The problem is that it sounds very similar to a Terry Pratchet joke about werewolves and humanitarians, so was trying to come up with something a bit more grandiose.

The more I think about it, extraterrarian sounds fine for colloquial/dergoatory term for a sci-fi setting and I can probably find something in-universe blasphemous for a fantasy one.

[Matthias2207]

Carnivore is Latin, Xenos is Greek. So xenovore would be some weird crossover, we don't want that. Extraterrestrial isn't a good choice either, because that would mean eating things from other planets than Earth. We want sentient beings eating other sentient beings, so we need a word with 'intelligence', 'thinking', 'conscious' or something along those lines. Sapivore? From Homo Sapiens, the thinking man?

[Elemental]

I was thinking of something along those lines, I just couldn't think of a good way to say it.
But, wouldn't it mean something along the lines of "thought-eater"?

[Matthias2207]

... Yes... Probably.
Meat-eater, plant-eater, all-eater, thought-eater.
A little awkward, but with a little explanation, most would understand what you meant by it.
Sentivore?
Animus is consciousness, so animovore?
But why would someone want to specifically eat meat from sentient beings anyway?

[Elemental]

Either religion, culture or superstition.
Religion as a form of ritual sacrifice; culture as a way of proving yourself; and superstition because you believe that you gain the strength of the consumed individual.

Other reasons are possible.

[Brother Oni]

Either religion, culture or superstition.
Religion as a form of ritual sacrifice; culture as a way of proving yourself; and superstition because you believe that you gain the strength of the consumed individual.

Other reasons are possible.

In my case, I was thinking about a race that essentially had the W40K Space Marine omophagea implant, and how it would affect their development, culture and general perception by the larger universe.

Given how their closest equivalent in B5 are viewed, the general consensus is 'not very well'.

Thinking about it some more, aren't mindflayers from D&D pretty much the same, or do they subsist on other foodstuffs?

[Yora]

I'd like to ask my own question about planetary systems:

What would the day-night cycle on moons like those of Jupiter and Saturn be like? Assuming tidal locking for simplicity.
On the one hand you would have strong planetshine during the "night" but a massive solar eclipse every "day". And how long would a full cycle last?

[factotum]

If the moon is tidally locked then its day/night cycle would be the same as its orbital period--those are listed on Wikipedia, so for Jupiter that would range from a little over seven hours (Metis) to nearly three Earth years (S/2003 J 2--catchy name there ). A daily solar eclipse wouldn't necessarily happen, though, because most of the moons' orbits are not exactly aligned with the ecliptic (e.g. the moon would pass "above" or "below" Jupiter relative to the Sun). In addition, for a tidally locked moon, there would be many places on the surface where you would spend the entire day with Jupiter on the opposite side of the moon from you, so those places would not see a solar eclipse in the way you're thinking. Of course, that would vary according to the time of year, so some parts would get eclipses in summer, some in winter.

[Elemental]

Clearly, Yora desires real estate on the planet side of the moon.
Wise decision as the views would be spectacular.

And yes... There'd be a lot of planetshine.