# Why would a space elevator / structure need to be in Geosynchronous orbit?

I've read a bunch on space elevators and from what everyone is saying, it would be required that the elevator is in geosynchronous orbit. From my understanding, that is so that it would stay fixed in one position over the earth.

Also, from my understanding a space elevator would be some sort of cable that runs up into space and so people would essentially ride that cable up. Now that would also explain why it would need to be in geosynchronous orbit, since the cable would then otherwise wrap itself around the earth.

That got me thinking though, what if you were to build a super-structure from the ground up. Something similar to the image below. If you were able to create a structure strong enough to withstand the all of the elemental forces in the upper atmosphere:

• Would it be possible to have the structure sticking out into space, but not in geosynchronous orbit?
• What would be the challenges in creating a structure like this?
• Could something like this change the earths rotational period?

• The thing though is that the compression strength of materials is generally a lot smaller than their tensile strength. Add that to how even small strength differences grow exponentially, thus the ground-up design has a major drawback. – SE - stop firing the good guys Aug 19 '16 at 17:17
• Arthur C. Clarke, if I remember correctly, had such structures in his book titled 3001. There were four towers, every 90 degrees around the equator, made of diamond. They reached into space, where a ring connected the tops of all of them. All that diamond came from the core of Jupiter (or something like that)... – Steve Aug 19 '16 at 17:33
• You'll need to build your towers from unobtainium. – Loren Pechtel Aug 19 '16 at 21:44

I've read a bunch on space elevators and from what everyone is saying, it would be required that the elevator is in geosynchronous orbit.

The center of gravity of a space elevator would need to be at geosynchronous altitude, which means that a space elevator would need to be extend well beyond geosynchronous altitude.

• Would it be possible to have the structure sticking out into space, but not in geosynchronous orbit?

• What would be the challenges in creating a structure like this?

The key challenge is strength of materials. The tallest structure built to date is "only" 829.8 meters tall (the Burj Khalifa), and the very top of that structure is exceedingly narrow compared to the base. A building under construction will reach about a kilometer into the sky. It might be possible to build a structure ten times this tall, but that means the top is still well within the troposphere. Building a structure that is a few hundred kilometers high is not possible given our current understanding of strengths of materials, not even a mountain made of diamond.

Furthermore, even if it was possible to build a structure that was over a hundred kilometers high, it wouldn't be particularly useful for space exploration. There are two key obstacles regarding space exploration. The lesser of the two is climbing out of the bulk of the Earth's atmosphere. The far more significant obstacle is achieving sufficient horizontal velocity so as to be in orbit, and a one hundred kilometer tall structure will not help solve that problem.

A space elevator is a bit beyond the reach of current science and technology, but it's within an order of magnitude. A structure that reaches only partly into the sky is several orders of magnitude beyond the reach of current science and technology.

Yes, if you could build a structure like that, you'd be able to build it almost anywhere you want. A structure on the equator would be the most efficient: build anywhere else and the structure would have to be higher in order have the top of the structure be at orbital speed, so you can dock a spaceship to it. The only place you can't build such a structure is at the poles. No rotation means you never get to orbital speed.

There are significant challenges to building such a tower though. The biggest one is the structure has to be very strong in compression: the first floor has to be able to bear the weight of the structure above it. If you build in concrete, you can build a tower about 1 km high before it collapses under its own weight. Stronger materials get you a bit higher, but nowhere near the 36000 km you need. You could build a stronger structure. A pyramid is nice: the lower floors are bigger so they spread the load. But a pyramid 36000 km high has to be 36000 km across at the base. That doesn't fit on Earth. And all of the construction companies on Earth would need aeons to build it.

Due to the gigantic mass of such a tower, yes you'd likely see measurable changes in Earth's orbital period.

A cable 36000 km long, on the other hand is doable. In fact we've built cables on that order of magnitude (undersea fiberoptic cables).

• Why would that object need to be at orbital speed? Wouldn't a taller object mean a lower orbital speed? – Adjit Aug 19 '16 at 17:20
• At 36000 km, an object attached to Earth (like a tower) moves around at orbital speed. At any other altitude, there is a mismatch: at lower altitudes the tower is below orbital speed, at higher altitudes the tower is above orbital speed. So if you want to go to space without needing a rocket, 36000 km is your only option. – Hobbes Aug 19 '16 at 19:05
• Orbital speed relative to what? At lower orbits you need a greater orbital speed to keep yourself in orbit. Not sure I'm following why 36000km is the magic number. – Adjit Aug 19 '16 at 19:14
• @Adjit, because the structure is connected to the earth, it's angular frequency is fixed. Assuming it's on the equator, then at altitudes lower than 36000km, this frequency is less than that of an orbiting object. – BowlOfRed Aug 19 '16 at 19:41
• Shouldn't the orbital anchor of a space elevator be significantly above geosynchronous altitude? To keep a tether in tension, there has to be something to pull against. At GEO the available force would be zero. You'd need to put a mass above GEO so that it requires tension from the tether to keep it in place - tension you would need to both support the lower portion of the tether, the climber, and its payload. – Anthony X Aug 20 '16 at 14:21