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Inclusa
2014-Nov-10, 03:21 AM
The sad fact is: the Solar System is quite devoid of naturally habitable planets and moons; we are certain about Planet Earth, but anything beside Earth is far from habitable.
But we can still put things in scale:
1)Mars
In spite of a hostile atmosphere, low mass and gravity, it is the closest to Earth environment in the Solar System except for Earth.
2)Titan
As one of the most popular human settlement in science fiction, Titan is a little similar to Earth during Archean eon.
3)Ganymede, Callisto, Triton, Pluto and Charon, the four moons of Uranus (Titania, Oberon, Airel, Umbriel), Ceres, or anything with solid surface.
They have solid surface, but they lack major atmosphere; subterranean or protected settlements may be possible.
Let's ignore Venus and Io for the moment; their surfaces are either extremely hot or ridden with lava
Gas giants and ice giants? Some people suggest atmospheric settlements, but not YET.
Still, since the Solar System doesn't offer anything naturally habitable for humans, we may adopt a few methods:
1) Domed settlements, with protective dome over the top.
2) Subteranean settlements.

cjameshuff
2014-Nov-10, 01:24 PM
You left out Mercury. Higher surface gravity than Mars with a smaller gravity well, magnetic field providing protection from the solar wind, acceptable temperature range, polar ice deposits, ample solar power for surface use and spacecraft carrying exports out to the rest of the system...

And why ignore the asteroids? Or the small irregular satellites of the gas giants? And why assume a habitat must be buried or planted on a surface?

Inclusa
2014-Nov-11, 04:26 AM
You left out Mercury. Higher surface gravity than Mars with a smaller gravity well, magnetic field providing protection from the solar wind, acceptable temperature range, polar ice deposits, ample solar power for surface use and spacecraft carrying exports out to the rest of the system...

And why ignore the asteroids? Or the small irregular satellites of the gas giants? And why assume a habitat must be buried or planted on a surface?

Atmospheric habitat? Are we becoming Homo apus apus (just kidding) in practice?

Noclevername
2014-Nov-11, 05:33 AM
Atmospheric habitat? Are we becoming Homo apus apus (just kidding) in practice?

I think he meant orbital habitats. Once we get those right, we won't need planets at all, we can build anywhere.

Inclusa
2014-Nov-12, 03:49 AM
I think he meant orbital habitats. Once we get those right, we won't need planets at all, we can build anywhere.

This is why I talk about Homo apus apus; like Apus apus that hardly ever lands, some humans may live in orbital habitats.

eburacum45
2014-Nov-12, 10:34 PM
Actually it is reasonable to say that the closest thing to an Earth-like environment in our solar system (outside the Earth) is at the 50km level in the atmosphere of Venus. Similar temperature and pressure, but a very different composition.

Inclusa
2014-Nov-16, 03:56 AM
Actually it is reasonable to say that the closest thing to an Earth-like environment in our solar system (outside the Earth) is at the 50km level in the atmosphere of Venus. Similar temperature and pressure, but a very different composition.

We haven't mastered building Laputa, though.

Noclevername
2014-Nov-16, 08:57 AM
We haven't mastered building Laputa, though.

By the time we can reasonably build anything on Venus, we probably will have the necessary skills.

agingjb
2014-Nov-16, 01:16 PM
By the time we can reasonably build anything on Venus, we probably will have the necessary skills.

Skills developed as Earth increasingly resembles Venus.

Inclusa
2014-Nov-17, 12:59 AM
Skills developed as Earth increasingly resembles Venus.

It won't go this far for a long time, but we may experience hotter than average temperature that can render the tropical areas relatively uninhabitable.

Noclevername
2014-Nov-17, 01:17 AM
Skills developed as Earth increasingly resembles Venus.

Skills developed on Venus, as we build there.

Inclusa
2014-Nov-22, 05:19 AM
We are probably still better off with space habitats than moons and planets in the near future.

Noclevername
2014-Nov-22, 06:52 AM
We are probably still better off with space habitats than moons and planets in the near future.

True. We can adjust the "gravity" in a space hab by varying the spin.

selvaarchi
2014-Nov-22, 12:34 PM
We have lived in LEO for 13 years aboard the ISS but are no closer to be self supporting then when we started. We need at least a dedicated habitat attached to the ISS to grow crops to start the process to be self sustaining.

lpetrich
2014-Dec-05, 05:42 PM
A big problem with the more massive Solar-System objects: how do you depart from them? You need a high-thrust rocket and a big one at that. Consider what's necessary for departing from our home planet. The outer planets are worse than the Earth, Venus is as bad as the Earth, and Mercury and Mars not much better. For Moon-size and smaller, it does get easier, as is evident from the Apollo Lunar Module's specs.

The Lunar Module came in two parts, a descent stage and an ascent stage. The descent stage was left behind on the Moon with only the ascent stage departing from it. Orbital velocity for a lunar "surface satellite" is about 1.68 km/s, so the Lunar Module was designed to deliver that delta-V with both stages.

Descent stage:
Empty mass: 2.1 mt
Full mass: 10.3 mt
Propellant mass: 8.2 mt
Thrust: 45 kN
Specific impulse: 3.05 km/s
Delta V: 2.5 km/s
Propellant: Aerozine 50 + nitrogen tetroxide

Ascent stage:
Crew: 2
Cabin volume: 6.7 m^3
Habitable volume: 4.5 m^3
Atmosphere: 100% O2 at 1/3 bar
Life-support time: 48 hours, later 75 hours (some descent-stage assist?)
Empty mass: 2.15 mt
Full mass: 4.8 mt
Propellant mass (main: APS): 2.35 mt
Thrust: 16 kN
Specific impulse: 3.050 km/s
Delta V: 2.22 km/s
Propellant: Aerozine 50 + nitrogen tetroxide
Propellant mass (maneuvering: RCS): 0.3 mt
Propellant: Aerozine 50 + nitrogen tetroxide
Specific impulse: 2.850 km/s

Aerozine 50 = 50/50 mix by weight of hydrazine (N2H4) and unsymmetrical dimethylhydrazine (UDMH: N2(CH3)2H2)
Nitrogen tetroxide = N2O4
These propellants are both room-temperature liquids, though very toxic ones.


Comparison of orbital launch systems - Wikipedia (https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems) shows what's necessary to depart from the Earth into low Earth orbit and beyond.

Using the lunar-module ascent stage as a reference, I find which rockets can lift 5 metric tons into low Earth orbit. The heavier variants of the Delta II can, with a launch mass of as much as 232 mt. That's over 20 times as much as the Lunar Module's descent stage.

Sources: Apollo Lunar Module - Wikipedia (https://en.wikipedia.org/wiki/Apollo_Lunar_Module), Delta II - Wikipedia (https://en.wikipedia.org/wiki/Delta_II)

cjameshuff
2014-Dec-05, 05:53 PM
Mass drivers and various forms of slings are very practical on the moon as well. Even just providing a few hundred m/s would give a substantial assist to lunar orbit or an Earth transfer trajectory.

lpetrich
2014-Dec-05, 06:47 PM
A mass driver is essentially a coilgun, a linear-motor gun. This sort of launcher is likely practical for any airless or close-to-airless celestial body, and not just the Moon. That's nearly all of them, including all of them smaller than Titan.

To fill in, the Earth's surface-satellite speed if 7.9 km/s, about 4 times larger than the Moon's. But from Tsiolkovsky's rocket equation, that means about the fourth power of the full-to-empty ratio. For 2 to 3 for the Moon, one gets 16 to 81 for the Earth, which is about right.

I think that it's a better comparison than escape velocity, because one only needs high thrust to get into orbit, since one can then use some low-thrust engine(s) to go the rest of the way.

Here's a table of surface-satellite velocities for most of the larger objects in the Solar System:

Sun: 440 km/s, Jupiter: 42 km/s, Saturn: 25 km/s, Neptune: 17 km/s, Uranus: 15 km/s, Earth: 7.9 km/s, Venus 7.4 km/s, Mars: 3.5 km/s, Mercury: 3.0 km/s, Ganymede: 1.9 km/s, Io: 1.8 km/s, Titan: 1.8 km/s, Moon: 1.7 km/s, Callisto: 1.7 km/s, Europa: 1.4 km/s, Triton: 1.1 km/s, Pluto: 0.8 km/s, Ceres: 0.4 km/s

Surface-satellite velocity:
\displaystyle{ v_s = \sqrt{ \frac{GM}{R} } = R \sqrt{ \frac{4\pi G\rho}{3} }

So it scales as (size) * sqrt(average density)

That may be why President Obama has proposed landing on an asteroid: Obama Wants Mission to Asteroid by 2025, Mars by mid-2030ís (http://www.universetoday.com/62766/obama-wants-mission-to-asteroid-by-2025-mars-by-mid-2030s/)

Launch window
2014-Dec-28, 01:02 AM
Europa Mission Wins Big in New NASA Budget

http://news.discovery.com/space/private-spaceflight/europa-mission-wins-big-in-new-nasa-budget-141226.htm