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Comment by gibolt | Hacker News Reader

A key point with space elevators is that they aren't limited to Earth. The solar system has plenty of celestial bodies that could benefit (us) from having one.

It may not be as significant as the impact of an Earth-based one, but any propellant savings is a win.

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Some of those celestial bodies are even small enough that we could build the elevators with existing materials, too.

A space elevator on Mars is probably totally feasible. But of course the benefits would be much larger on Earth.

The problem is: the benefit of a space elevator is bigger the deeper your gravity well is, but it's also much harder to impossible to make one there.

But I've also been wondering: wouldn't it be possible to have a tapering space elevator? Reduce the weight by making the parts that have to carry less weight thinner?

The designs already have to taper, so you're right, but you still need supermaterials (or powered/active support structures) to make one on Earth.

Yes, it absolutely has to taper, with an exponential curve, to keep stress constant over the length of the cable. The taper ratio (in terms of cross-sectional area) for the best currently available engineering materials for an Earth space elevator would be in the tens to hundreds of millions, but a carbon nanotube cable might only require a taper ratio of around ten. (A steel cable would need a taper ratio on the order of 10^33.)

I believe A taper ratio of 10 means a 5cm rope at the bottom would be 50cm at the top.

A steel cable would be 1mm thick at the bottom and several light years* thick at the top.

*hyperbole. Maybe.

It’s a function of area so if we scale equally in both dimensions, a taper ratio of 10 means a thickness ratio of ~3.2. But yeah, that doesn’t exactly buy us much.

> *hyperbole. Maybe.

A lightyear is around 10^19mm. Given that it's by area, a square lightyear is ~10^38mm^2. The cable is about 100 times thinner than it.

The Moon being a great example.

A martian space elevator is explored in Kim Stanley Robinson's Red Mars trilogy, definitely worth a read.

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