Serenity for d20?

[Johnny Angel]

Teraforming (increasing a moons gravity to hold in atmosphere)

Seems like this would take an awful lot of energy.

 

[Agemegos]

Seems like this would take an awful lot of energy.

It would. Gibberingly large quantities. Comparable to the gravitational binding energy of a habitable planet.

And even when you had done it, you would still need to deliver or synthesise the atmosphere.

 

[Agemegos]

How much gravity does it take to be able to hold an atmosphere?

The trick actually requires a certain escape velocity, and the escape velocity of a sphere is equal to its surface gravity times its radius. So moons (which are presumably small) need higher surface gravity than planets (which are presumably larger).

For a world to be habitable (and to stay terraformed after it has been terraformed) the escape velocity has to be low enough that excess hydrogen and helium (with molecular weights of 2 amu and 4 amu respectively) will boil off by the process called (if memory serves) Jeans Escape, but that water vapour (with a molecule weight of 18) will not. This depends on the mean velocity of the molecules in a gas at the temperature of the atmosphere being well less than escape velocity, and molecular velocity is inversely proportional to molecular weight (the temperature dependency is fixed by the narrow range of comfortable temperatures. Anyway, the margin between the molecular weights of helium and water vapour is more than a factor of fourt, which gives a comfortably wide margin. Judging by the fact that helium is rare in Earth's atmosphere (but abundant in the primordial nebula) we can say with fair confidence that if a planet were the same size as Earth by 1/4 the gravity (requiring 1/4 the density) all its water would undergo Jeans escape, and that if it were the same surface gravity as Earth but 1/4 the size (requiring four times the density) the same would happen. Given that any habitable planet is likely to be about as dense as Earth (maybe a little less dense, like almost all the other objects in the Solar System), the lower bound is somewhere above a surface gravity of 1/2 g.

An object with an equable surface temperature that has less than 1/2 the surface gravity is Earth is going to lose its all its water to Jeans escape in a time that is short compared to the age of the Earth. So it is overwhelmingly likely that any such object discovered is going to be desiccated and deficient in hydrogen. But Jeans Escape is a slow process by human standards, and it is conceivable that such an object much be terraformed by hauling in huge quantities of ice from out where temperatures are lower (and then introducing algae to make oxygen out of the ice and waiting a century or so). If this were done, it might retain its artificial atmosphere for a time long compared with human lives.

So we might see a terrafomed world with less than 1/2 of Earth's surface gravity.

 

[Agemegos]

the escape velocity of a sphere is equal to its surface gravity times its radius.

Oops. Escape velocity is actually proportional to the square root of the product of surface gravity and radius. Surface gravity is proportional to density times radius. Which is to say that escape velocity is proportional to radius times the square root of density. A habitable planet or moon will be about as dense as Earth or a little less.

The velocity of a molecule in a gas is proportional to the square root of its molecular weight. Water has a molecular weight at little over four times that of helium. So the fact that helium has almost completely escaped from Earth and that water has not establishes that a planet or moon requires an escape velocity at least about half that of Earth. That means a radius about half that of Earth (or more, if density is lower). And that means a surface gravity at least about half that of Earth.

So I ended up at the right place because my two errors cancelled out. For a naturally habitable world in a mature solar system there is a lower bound on surface gravity somewhere above half a gee. But if an ocean's worth of water is delivered to a planet or moon with lighter gravity if can hang on to it for a long time by human standards.

 

[Rhun]

It would. Gibberingly large quantities. Comparable to the gravitational binding energy of a habitable planet.

And even when you had done it, you would still need to deliver or synthesise the atmosphere.

Regardless of whether it is plausible or not, it is canon from the setting. In the pilot episode, Zoe mentions that terraforming includes setting the gravity as close to Earth-That-Was as possible. You may not like it, but if you are going to play Firefly it is part of the setting.

 

[Felon]

Regardless of whether it is plausible or not, it is canon from the setting. In the pilot episode, Zoe mentions that terraforming includes setting the gravity as close to Earth-That-Was as possible. You may not like it, but if you are going to play Firefly it is part of the setting.

But note that the lack of FTL propulsion systems and the one-big-solar-system thng aren't covered in the series. They were contrived after-the-fact.

 

[MavrickWeirdo]

But note that the lack of FTL propulsion systems and the one-big-solar-system thng aren't covered in the series. They were contrived after-the-fact.

Was FTL Explicitly shown in the series? It is possible that it did not exist from the begining, but never made it into the dialogue. It is also possible that someone decided "this only makes sense if..." after the fact.

The original question of the thread was asking for info on the "Serenity Universe", according to reliable sources FTL does not exist in the "Serenity Universe".

 

[Felon]

Was FTL Explicitly shown in the series? It is possible that it did not exist from the begining, but never made it into the dialogue. It is also possible that someone decided "this only makes sense if..." after the fact.

Well, how do you explicitly "show" FTL? Propulsion methods are not discussed one way or the other, which is most likely intentional, just as money and fuel are never referred to by any specific terms. Like I said before, he wanted fans to get the impression that Serenity was powered by some kind of gas-powered turbine engine, despite the outright impossibility of it all.

 

[Ghoti]

Back to the issue of Serenity being based in a single star system.

This is being debated on the Serenity forum.

It seems that some think it is one star system while others think it is a stellar cluster with 5 or so stars.

 

[KaosDevice]

I'm suprised the one solar system thing is being debated. The movie makes it pretty clear it is just one solar system, just as Mal makes it clear that there is less and less room to maneuver on the edge as time goes by and the strength of the Alliance grows. I think this whole 'vanishing frontier' theme is a pretty big part of the backdrop of Serenity.