GURPS Space - assistance

[Anthony]

Originally Posted by Whill

Which brings me to an astronomical question. Before I get back to system generation questions, what would it be like at the barycenter?

Uninteresting; you won't get star motion due to orbiting, but that's not human-visible to start with.

Originally Posted by Whill

If you could park your ship there (not moving with respect to the Thanatos-Astra system), would it just be completely weightless with no gravitational forces pulling in either direction?

No, you want lagrange points.

[Whill]

I'm really liking this forum.

Even though I don't roll for the aspects most important to me, I try to keep my choices within the ranges possible from rolling. For a Standard Garden world, the minimum hydrographical coverage is 45% (min roll with optional variation of the result by 5%). Is this worldbuilding system indicating that desert planets like Arrakis or Tatoonie (with maybe single-digit hydro) are just not possible?

What if the planet had 45% at one time, leading to plant life, oxygen, animal life and conditions suitable for human colonization, but then the oceans, seas and lakes were somehow drained into massive underground reservoirs within the planet? Step 4 on p.81 mentions worlds possibly having extensive underground water supplies but that doesn't count towards its hydro rating. Could something like that make a 5% hydro desert planet with polar seas at all reasonable?

Before the loss of the oceans, if there were already large deserts on the planet with lifeforms adapted for that environment and the change happened slow enough, I'm thinking a desert planet with a smattering of plant and animal life of its own is possible. As unoriginal as it may be in sci-fi, I really want desert planet in the Astral System!

[Anthony]

Originally Posted by Whill

I'm really liking this forum.

Even though I don't roll for the aspects most important to me, I try to keep my choices within the ranges possible from rolling. For a Standard Garden world, the minimum hydrographical coverage is 45% (min roll with optional variation of the result by 5%). Is this worldbuilding system indicating that desert planets like Arrakis or Tatoonie (with maybe single-digit hydro) are just not possible?

They probably aren't possible; the hydrological cycle is too important to keeping a planet habitable. On the other hand, you can simply ignore realism.

[Whill]

That's not what I wanted to hear, but I truly do appreciate your reply. I really am trying to make my star systems as plausible as possible, but if I absolutely have to whip out the cosmic hand and waive something into existence, I will. Perhaps a long time ago, a very advanced species reverse-terraformed the planet to degree for some unknown purpose, and solving that ancient mystery as well as setting things in motion towards a return to its more garden-like past will be plot point in the campaign. Yeah, that's the ticket.

[Whill]

In an eccentric gas giant arrangement where a gas giant formed outside of the snow line and migrated sunward before stablizing into a new orbit, shouldn't one of the outer orbits be left empty of any gas giants to represent the inner gas giant's original orbit before migration?

Or is that not necessary because a cause for the migration could be that the original gas giant orbit was not stable in the long term, which is what may have caused the close encounter that sent the eccentric gas giant inward in the first place?

[Agemegos]

Originally Posted by Whill

In an eccentric gas giant arrangement where a gas giant formed outside of the snow line and migrated sunward before stablizing into a new orbit, shouldn't one of the outer orbits be left empty of any gas giants to represent the inner gas giant's original orbit before migration?

Not necessarily. Migrating gas giants don't necessarily move alone, but dragging things in from outer orbits behind them. In our system Jupiter formed at about 3.5 AU, migrated in to 1.5 AU by interactions with the disk of planetismals, then got into a resonance with Saturn that first dragged Saturn inwards and then pulled both of them outwards. It ended up further out than it formed, but there was an era in which Jupiter had migrated inwards from the distance that it formed at and had pulled Saturn in to a distance less than that which Jupiter formed at.

[Whill]

Originally Posted by Brett

Not necessarily. Migrating gas giants don't necessarily move alone, but dragging things in from outer orbits behind them. In our system Jupiter formed at about 3.5 AU, migrated in to 1.5 AU by interactions with the disk of planetismals, then got into a resonance with Saturn that first dragged Saturn inwards and then pulled both of them outwards. It ended up further out than it formed, but there was an era in which Jupiter had migrated inwards from the distance that it formed at and had pulled Saturn in to a distance less than that which Jupiter formed at.

I hadn't read all of that early migration in and out of Jupiter and Saturn. Cool. But does this mean that the book's formula for calculating snow lines is incorrect or out-of-date? On p.106 it says R = 4.85 x square root of L. According to the chart on p.103 has the L-Min for a G2 as .68. So according to the book, Sol's snow line would be (4.85 x square root of .68), about 4 AU.

[Agemegos]

Originally Posted by Anthony

They probably aren't possible; the hydrological cycle is too important to keeping a planet habitable.

This is one of the reasons that I am intensely skeptical about the habitable tide-locked worlds that the GURPS Space 4th ed. generator produces in such profusion.

[Flyndaran]

Originally Posted by Brett

This is one of the reasons that I am intensely skeptical about the habitable tide-locked worlds that the GURPS Space 4th ed. generator produces in such profusion.

Wouldn't that just make a very different but possibly still life sustaining water cycle?

[Agemegos]

Originally Posted by Flyndaran

Wouldn't that just make a very different but possibly still life sustaining water cycle?

Possibly. If you start with a nice thick atmosphere to convect plenty of heat to the cold side, and if you start out with so much water that there is still plenty on the sunny side after the formation on the shady side of an ice cap fifteen times the size of Antarctica. I suspect that the rule for synchronously-rotating planets on Space p.125 may be generous in the matter of the effect on hydrographics. I'm pretty sure that it does not take into account that the ocean will freeze solid on the shadyside, so final hydrographics figures should be cut in half after the present adjustment.