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Comment by vardump | Hacker News Reader

> Starship should bring those costs below $1000.

It might even bring the costs below $100/kg.

It might, but it's also at a scale where people can dust off the old plans for orbital rings and ask if this time the economics work out.

(My guess is the economics are fine, but the politics would kill it on earth, so the moon or mars will get one, but that's just an interested amateur opinion).

Once you put something big in the LEO, you'll have to be able to boost its orbit indefinitely, because the orbit will otherwise slowly decay.

That depends on how high the orbit is. If it is high enough, the decay will take centuries.

IIRC, above something like geostationary they tend to decay upwards? Though the old orbital ring white paper wasn't suggesting anything like that, this was an alternative to needing to go so high in the first place.

(I may be misremembering or getting confused with a thing specific to tidal locking?)

I wonder if they could have an orbit high enough to move away from earth with some kind of drag cables dangling from them into low orbit to counter the outward movement. Would that work?

You mean something like a skyhook?

Yes but no hook or anchor needed I'd assume. The cable alone in lower orbit would weigh enough to be pulled by gravity.

These orbits have vastly different speeds though. Consider a high geosynchronous orbit vs. something like ISS which goes around the Earth in an hour or two.

but you are towing the cable and you'd only get so far down for the counteraction of force from gravity to pull on the cable. Would speed or friction on a cable be a problem.

That is indeed what you get with tidal forces - bodies closer than geostationary orbit lose angular momentum and decay inward, bodies further out steal angular momentum from Earth and move outward.

I suppose the same effect is there with satellites much smaller than the moon, but it would be tiny.

The L in LEO means we're talking about orbits that are not very big.

By definition, LEO extends up to 2,000 km. At that altitude orbital lifetimes can exceed 10,000 years.

https://commons.wikimedia.org/wiki/File:Orbital_Debris_Lifet...

Contiguous rings filling an entire orbit don't have much air resistance, and what little they do have is small in comparison to their momentum.

But yes, eventually things decay.

If you have the infrastructure to build an orbital ring, you have the infrastructure to keep it supplied too

There is some SciFi story about civilization collapsing and the survivors worrying that the sky will fall on them hidden in there.

Gotta make the individual pieces small enough to mostly burn up completely on rentry.

Term of art for this is "design for demise", i.e. make everything of pieces small enough and materials ablative enough that there's less than 1 in 10k chance that any debris will survive to the surface.

https://www.esa.int/Enabling_Support/Space_Engineering_Techn...

Planned obsolescence but in a good way.

Already exists, Chasm City by Alastair Reynolds. They absolutely trash orbit by planning very badly and over-packing habitats.

Presuming it’s in LEO. When and if we ever get around to building these things they probably won’t be in LEO at least not for long. Some of them might not even be constructed from materials launched from Earth.

How does the orbit or an orbital ring decay?

There are relatively stable orbits over the equator.

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