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Old 10-05-2010, 01:46 AM   #31
gjc8
 
Join Date: Oct 2005
Default Re: [Space] Gliese 581

Quote:
Originally Posted by lwcamp View Post
Thermodynamics does impose an ultimate limit on what fraction of the energy can be harvested from the sunlight, because any physical process can be driven in reverse, and the absorption mechanisms can be thermally activated to cause emission. The ultimate efficiency depends on the temperature of the star's surface and the temperature at which your photopigment operates. For life on earth, operating at about 300 K and harvesting light from a 6000 K surface, the maximum theoretical efficiency is (6000 K - 300 K)/(6000 K) = 95%. You can see that life on earth has a long way to go before the physical limit is reached. For life around a red dwarf sun, with a temperature of 3000 K, the maximum efficiency drops to (3000 K - 300 K)/(3000 K) = 90%. Again, this limit is probably well above the efficiency of any actual photopigments that would have evolved.
Ahh, right. But as far as I can tell, photosynthesis is fairly far from this theoretical limit even at the peak limits. If I recall, efficiency is closer to 30% for energy absorbed by the chlorophyll. Now, I expect you'll see some loss of efficiency (just as you do for heat engines operating with lower temperature differences), but I don't think that's likely to be a problem. The difference is small, compared to other inefficiencies.
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Old 10-05-2010, 02:42 AM   #32
Sunrunners_Fire
 
Join Date: Mar 2010
Default Re: [Space] Gliese 581

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Originally Posted by gjc8 View Post
Ahh, right. But as far as I can tell, photosynthesis is fairly far from this theoretical limit even at the peak limits. If I recall, efficiency is closer to 30% for energy absorbed by the chlorophyll. Now, I expect you'll see some loss of efficiency (just as you do for heat engines operating with lower temperature differences), but I don't think that's likely to be a problem. The difference is small, compared to other inefficiencies.
Per Wikipedia (yes, I know) (http://en.wikipedia.org/wiki/Photosynthetic_efficiency), terrestrial photosynthesis is between 3 and 6 percent. Plants are very inefficient. :)
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Old 10-05-2010, 06:45 AM   #33
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Space] Gliese 581

Quote:
Originally Posted by lwcamp View Post
There are possible abiological methods of producing oxygen
That's why I said "overwhelming" rather than "absolute". I was trying to allow for the theoretically possible even if it's probably rather unlikely.
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Old 10-05-2010, 04:21 PM   #34
doctorevilbrain
 
Join Date: May 2009
Default Re: [Space] Gliese 581

Sorry, Dataweaver, I should have said links.
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Old 10-05-2010, 04:41 PM   #35
Mgellis
 
Join Date: Nov 2004
Location: Flushing, Michigan
Default Re: [Space] Gliese 581

Just for fun, I ran some of the numbers for a possible version of Gliese 581g, using the rules from GURPS Space. Interesting...


class of star M4 V
mean distance 0.15 a.u.
perihelion 0.14 a.u.
aphelion 0.15 a.u.
axial tilt 5°
annual period 0.102 years
tidelocked
local day infinite

large garden planet
diameter 1.6 x Earth's
19843 km
density 0.90 x Earth's
5.0 g/cm^3
surface gravity 1.4 g.
14 m/s^2
escape velocity 17 km/s
vulcanism moderate
tectonics moderate
climate cool
temperature
average 7 C
dayside 21 C
nightside -7 C
illumination 11 % Earth's
oceans 80%
composition water
tidal range 0.00 m
atmosphere
main gases He, N2, O2
traces &c.
class breathable
pressure 4.9 bar (very dense)
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Old 10-05-2010, 04:47 PM   #36
gjc8
 
Join Date: Oct 2005
Default Re: [Space] Gliese 581

Quote:
Originally Posted by Sunrunners_Fire View Post
Per Wikipedia (yes, I know) (http://en.wikipedia.org/wiki/Photosynthetic_efficiency), terrestrial photosynthesis is between 3 and 6 percent. Plants are very inefficient. :)
3-6% is relative to all light received. Photosynthesis pigments don't absorb all wavelengths of light (and definitely don't absorb 100% of of all wavelength of light), and don't use all wavelengths of light with equal efficiency. Also, not all light hitting a leaf (or equivalent structure) reaches the chloroplasts. The 30% figure was the ideal case (optimum wavelength, striking the pigment), which the maximum thermodynamic efficiency applies to.

I actually think the problem of incomplete absorption is a larger problem for red star photosynthesis than the thermodynamic limit. If you look at the blackbody spectrum for ~6000K (a G-class star, like Sol), it's sharply peaked. At ~3500 K (like a M-class Gliese 581), the distribution is much broader. Since photosynthesis pigments generally only absorb over a limited range of wavelengths, plants would have to use a larger set of pigments, evolve broader pigments (unlikely, as terrestrial plants tend to use multiple pigments instead of broader ones), or miss more of the light.
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Old 10-06-2010, 04:48 PM   #37
doctorevilbrain
 
Join Date: May 2009
Default Re: [Space] Gliese 581

Dataweaver, could you please post those links again? Thanks.
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Old 03-17-2014, 12:34 PM   #38
strange7person
 
Join Date: Mar 2014
Default Re: [Space] Gliese 581

Let's say Earth has late TL 10 orbital economic activity in the $10^12 per month range, with limited superscience, vast reserves of antimatter which need to be disposed of in some appropriately symbolic way as part of disarmament in the aftermath of a cold war, a similar surplus of frightfully competent people who find themselves unable to adapt to peacetime, and public sentiment backing the idea that humanity faces certain existential risks so long as it's confined to a single star system. So, there's a push to colonize Gliese 518, even if doing so makes little or no sense economically.

Here's the initial concept for the colony ship (or rather, one of the human-habitable ships sent out as part of a larger colony project):
front hull: [1] nanocomposite armor,
[2!] reconfigurable system: ramscoop/magsail,
[3] reconfigurable system": open space/science array/solar panels,
[4-6] hangar bays,
[core] control room
center hull: [1] nanocomposite armor,
[2] de-rated fusion reactor,
[3-6 habitats (500 cabins, 10 establishments, 100 automed sickbays, 100 Science! labs 400 tons steerage cargo),
[core!] robofac,
rear hull: [1] nanocomposite armor,
[2-5 fuel tanks (3000 tons antimatter-boosted hydrogen),
[6] reconfigurable system: antimatter plasma rocket/fusion torch
features: unstreamlined, lacks automation

All the reconfigurable systems are TL 11 prototypes, but even with all those multipliers the actual hull & machinery of the ship cost less than half as much as the antimatter fuel. Science array and solar panel both attempt to catch infalling photons and convert them to electricity; the former takes measurements, while the latter simply channels the power. The same equipment transforms into an open space for farming and claustrophobia relief by delaminating and inflating various membranes.
The engines both mostly take individual high-energy particles in one end and spit hydrogen plasma out the other.
The magsail and ramscoop are both mostly enormous superconducting hoops.

mission profile: Load up with five hundred to a thousand troublesome young heroes and disgruntled veterans, hundreds of tons of personal belongings and lifebanks, and load the hangar bay full of whatever pre-packaged parts seem likely to come in handy. 17000 hours accelerating with the magsail to just under Sol escape velocity.
6552 hours accelerating with antimatter plasma rocket to 1815 mps.
? hours ramscoop-fed fusion torch up to relativistic speed and back down.
81900 hours decelerating from minimum ramscoop velocity to orbital insertion.
Overall travel time depends on how close to C you're willing to push. No less than 35 years, likely 60 or more if front armor ablation is a concern. In any case, all the antimatter is expended before relativistic velocities are attained, as a safety measure.

Please poke holes in this plan.
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Old 03-17-2014, 03:26 PM   #39
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Space] Gliese 581

Quote:
Originally Posted by strange7person View Post
Please poke holes in this plan.
You start out with the basic issue with ramscoops.

Earth is about 500 ly for the pulsar Geminga. That is the remnant of a supernova from c. 30,000 years ago. The interstellar medium for 600 ly around Geminga is still thin (and hot) from the expansion of the shockwave when Geminga went "boom".

Realistic ramscoops might be borderline efficient in areas with "average" density but really won't work well at all in the local bubble. Even with denser material to work with they appear to be speed-limited. They tend to make better brakes than accelerators.

If you only want "pseudo-hard" science you can ignore all this and use the "cinematic" ramscoops form Spaceships.

If you do want to change from ramscoops just pick whatever hard sioccne drive is left with the best Delta-V figures and be patient, either freezing the crew (another probably cinematic option) or making this a generation ship.

An alternative to all this cinematic stuff is to go with a low-perfoming option from flat-out superscience like a low acceleration reactionless drive or maybe some species of total conversion drive.
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Old 03-17-2014, 03:41 PM   #40
johndallman
Night Watchman
 
Join Date: Oct 2010
Location: Cambridge, UK
Default Re: [Space] Gliese 581

Quote:
Originally Posted by strange7person View Post
... surplus of frightfully competent people who find themselves unable to adapt to peacetime ... Load up with five hundred to a thousand troublesome young heroes and disgruntled veterans No less than 35 years, likely 60 or more
There appears to be a basic social problem. These people are going to go stir-crazy in this limited space for 40 years, and will end up killing each other. Likely in some ritualised-duelling kind of a way, or something like that, but still killing each other. They don't have the space to raise families, and when they arrive, the survivors will no longer be young and dynamic, but middle-aged by TL10 standards.
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