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Interesting Orbits Throughout the Solar System
OK, how about Venus?

[Image: pvo_uv_790226.jpg]
Venus cloud cities are an intriguing possibility, but of course here we're talking about orbital cities around Venus.  Some quick stats:

  • solar intensity: 1.9 times that at Earth
  • gravity: 90% of Earth's
  • atmosphere: mostly CO2, plus 3% nitrogen and small amounts of other stuff
  • magnetic field: essentially none (except for a small induced field)
I'm not bothering to look up what the rocks are made of, because they're not only at the bottom of a deep gravity well, but also at the bottom of a thick, soupy atmosphere at over 800°F all the time (unlike Mercury, which cools off at night).  I consider them inaccessible, though who knows, people are clever — we'll probably find a way someday.  But let's assume not for now.

The atmosphere, however, could be skimmed off by a craft in an elliptical orbit, or perhaps even harvested by something dangling from a craft (or colony!) in low Venus orbit.  That would give you carbon, oxygen, and nitrogen — very handy stuff for building, breathing, and growing.

The lack of a magnetosphere means that even in low Venusian orbit, a colony would need as much shielding as one in free space.  (Though there is still that question of what benefits you get from being closer to the Sun.)

Since Venus has no moons, it doesn't have as many interesting orbits as Earth.  Basically it looks like a choice between low and high Venus orbit.  The low orbit would have cheaper access to the atmosphere, though probably higher orbital maintenance cost (that is, what you have to spend to reboost as the atmosphere slowly drags you down).  A high orbit would have the opposite.

But if anybody can think of anything I've overlooked here, speak now, or hold your peace until you think of it later!

Joe Strout
Lead Developer, High Frontier

Ah, Venus. I see you mentioned cloud cities. I think the advantages of that are superior to an orbital one. Really, the main thing an orbital city might have is easy access to the solar system and not deep in the gravity well of the planet. Pretty good solar energy too, I suppose, but not much else going for it.

But a floating cloud city has:

- gravity almost like earths, no adaptation needed for Earth people moving there

- atmosphere gives some radiation protection

- atmosphere mining gives direct access to oxygen, carbon (crack CO2), maybe even enough water?

- pressure could also be equal to internal living quarters, meaning a breech won't be deadly...slow leaks are easy to deal with.

OK, you have to deal with powerful acidic clouds perhaps...I do not know how high up they typically are.

I think if you were going to invest in a Venusian orbital city, it probably makes more sense to just put it in Earth orbit Wink
OK, next up: Mars.

Mars is 227 million km from the Sun, or about 1.5 AU.  So, solar power there (588.6 W/m^2) is only 43% of what we get near Earth.

But it's a somewhat more interesting system than Venus or Mercury because, of course, it has two moons.  However, both of these are quite small — much too small for a colony to orbit.  However, there are quasi-synchronous orbits around Mars which stay within a few km of Phobos, and I suspect (though have not found) that there are similar orbits for Deimos.  Those orbits would be interesting because of the ready supply of materials that Phobos and Deimos could provide.

[Image: 300px-Phobos_colour_2008.jpg]   [Image: 294px-Deimos-MRO.jpg]
It's not known exactly what these moons are made of; both are quite porous, similar to C-type asteroids, and it is speculated that Phobos may contain significant amounts of water.

So, from innermost to outermost, interesting orbits around Mars would be:

  • Low Mars Orbit
  • Phobos Quasi-Synchronous Orbit
  • Mars Synchronous Orbit
  • Deimos Quasi-Synchronous Orbit
Note that I didn't include a "high Mars orbit" — I don't at the moment see any advantage in orbiting at some arbitrary distance that's far from Mars, but without easy access to Deimos.  (But jump in if I've overlooked something!)

As for the Mars system in general, it's got no magnetic field, so full shielding would be needed (possibly even more than in cislunar space, since we're further from the Sun).  And Mars itself is a bit of a thorny source of materials, since its atmosphere is, as they say, "too thin to be useful, but too thick to ignore."

But on the other hand, it's clear that Mars will be a huge tourist destination, and it's in an important location halfway to the asteroid belt and outer planets.  For both reasons, a port city in orbit around it is sure to see plenty of economic activity — especially if (as seems likely) it turns out that you can't safely raise children in 1/3 G.

Joe Strout
Lead Developer, High Frontier

(09-21-2015, 10:43 AM)JoeP Wrote: - pressure could also be equal to internal living quarters, meaning a breech won't be deadly...slow leaks are easy to deal with.

I'm not so sure. Atmospheric pressure on Venus is 92 times that of Earth! I'm not entirely sure, but I doubt humans could deal with that. At such pressures, just the act of inhaling and exhaling would be a Herculian feat. If anything, Venus would be more challenging than space in that regard. In space, the pressure differential is just 1 atmosphere. On Venus it would be a whopping 91 atmospheres, so the habitat's walls would have to be extremely strong.
Feel free to correct me if I'm wrong.

EDIT: Ok, at the altitude at which these floating cities would hang out, the pressure would probably be a lot lower.

EDIT #2: Yep. Just read on Wikipedia that at an altitude of 50 km, the pressure would be exactly 1 bar. So you're absolutely right. Smile
Other orbits of interest around Mars:
- Communication satellites in a horseshoe orbit with Phobos.
- Transfer orbits to and from tethers anchored to Phobos.
@hanelyp, those are interesting ideas.  In game we might lump them both under "Phobos orbit," but in reality they would be distinct.

OK, moving on out... the main asteroid belt is an obvious win.  Ceres and Vesta are obvious targets, but really, any largish hunk of rock will do (and there are millions of those). But there's not a lot more to say about them; in each case you'd be in a coincident orbit around the sun, making good use of the resources it provides.  The main belt is wide, but its center is about 400 million km from the sun, for an irradiance of about 180 W/m^2, or 13% of what we get here.  Starting to get dark out there.

[Image: asteroid-243-Ida.jpg]

Up next: Jupiter!

Joe Strout
Lead Developer, High Frontier

(09-24-2015, 11:41 AM)JoeStrout Wrote: Ceres and Vesta are obvious targets, but really, any largish hunk of rock will do (and there are millions of those).

...though as it happens, I just stumbled upon this essay about Ceres as the main base and hub for future asteroid belt mining.

[Image: Ceres_Orbit.svg.png]

Joe Strout
Lead Developer, High Frontier


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