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.

[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.

[Agemegos]

Originally Posted by Anthony

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

No, you want lagrange points.

Even Lagrange points have substantial gravity at them. They are characterised as the "points" in the synchronous rotating frame of reference at which the gravitation is exactly right to permit an object of negligible mass to participate in a circular orbit around the barycentre of the system that has a period equal to the period of the orbits of the principal components of the system.

The equipoise, the point at which the planet's and moon's (or whatever) gravity is equal and opposite is on the line between the planet and moon rather close to the moon, whereas the barycentre is closer to the planet. And the problem with the equipoise is that it doesn't stay still (as the barycentre does), nor does any possible trajectory at the point give you a circular orbit with co-incident period. "Park" your ship at the equipoise and what happens is that the equipoise moves away following the moon in its orbit. And no possible velocity that you give your hip at the equipoise will make it keep up with the equipoise as it moves.

[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?

[Anthony]

Originally Posted by Brett

Even Lagrange points have substantial gravity at them.

Sure, but the key thing is that you can be in free fall without your path causing you to eventually get closer or further from either object.