Orbital elevators and long, long days
 

My algorithm for determining what planets in my SF setting ought to have tower facilities (a.k.a. orbital elevators, a.k.a. beanstalks) takes into account economic volume and tech level, but not the cost of building the elevator. Now that I am using (modified) GURPS Space to generate more realistic star systems I am getting quite a lot with much, much longer days. And that means longer, more massive, more expensive elevators.

So what's the story? In the case of a planet that is tide-locked to one of its moons the geostationary orbit is at the moon's altitude, which means that the options are to build an elevator to the moon (allowing somehow for variations in its length and orientation if the moon's orbit is at all eccentric) or to build an elevator to L1, L3, L4, or L5. (In the case of a tide-locked habitable moon L1, L2, L4, and L5 are available.) In the case of a planet the solar L1, L2, L4, and L5 points are the only possible places for the centre of mass of an orbital elevator.

Intuition suggests to me that elevators to moons or to the Lagrange points, and especially to solar Lagrange points would be prohibitively expensive, and possibly impractical, perhaps even impossible at TL10. But intuition is notoriously unreliable. Has anyone seen studies?

Re: Orbital elevators and long, long days
 

My understanding is that under current science and on Earth, a beanstalk would have to be made of unobtanium. Specifically, it needs to be both stronger and lighter than carbon nanotubes, even disregarding the current production problems with attempts to grow nanotubes to any reasonable size. Apparently the distance is such that the weight of the cable alone is prohibitive.

Re: Orbital elevators and long, long days
 

From what I know, this isn't quite true. It needs to be stronger than current-production carbon nanotubes, but the requirements are well within the theoretical capabilities of them.

Re: Orbital elevators and long, long days
 

Pinwheels can be built in highly elliptical orbits, but otherwise have similar engineering issues and solutions to beanstalks. Maybe they would be the preferred lift solution on your tide locked planets?

Re: Orbital elevators and long, long days
 

Are those the things I call "rotovators", or something else? Rotovators are a very likely alternative to elevators.

Re: Orbital elevators and long, long days
 

Beanstalks just won't work someplaces. Simple as that.

You may also be exaggerating the habitability of planets with extremely long days. It may depend on what you're calling "much, much longer" but there probably are day lengths that will result in too much temperature swing to be viable.

I don't remember my exact criteria, but back when I was trying to determine how common Earth-like planets would be according to the assumptions of Gurps Space I simply kicked planets with excessive day length out of the results.

Too long and you might not even get a really Earth-like atmosphere. Plants have to subsist on stored energy during the night cycle and algae-analogies are very likely to come before macroflora. A cycle that had many, many hours of light followed by many, many hours of darkness might be too much of a stretch for early microflora.

Re: Orbital elevators and long, long days
 

I am inclined to think so (following Stephen Dole), but GURPS Space does not calculate daily temperature variation, and does not take any account of day length when calculating Habitability.

I have been doing a write-up of a planet that the starsystem generation sequence gave me as Affinity 8. Its days are 317.5 hours long. Which has no effect on the Habitability calculation--and neither does the temperature range induced by the absurd orbital eccentricity, nor the obliquity of 37°.

Re: Orbital elevators and long, long days
 

When I was doing the Omicron Polypi design that eventually turned into a Pyramid article, I came up with a world in a 3:2 resonance. Not having rules for determining different average temperatures for midday and midnight, I used the multipliers for tide-locked worlds as an upper bound. This didn't result in an uninhabitable world. With a thinner atmosphere, it might have.

I think it's probably safe to say that the temperature extremes for a 3:2 resonance would not be greater than those for a tide-locked world.

See p. 125 of Space for all this.

Bill Stoddard

Re: Orbital elevators and long, long days
 

Since early microflora aren't unlikely to be subsisting on the abundant geothermal energy of a young world, possibly not so much of an issue. Also of course worlds aren't limited to native life.

Re: Orbital elevators and long, long days
 

Huh? Thermal vent lifeforms? What does that have to do with photosynthesis? Extremophiles would be microfauna.

You've got to have photosynthesis or there will be no atmospheric oxygen. That's what I was getting at.