From what I’ve read so far, a space elevator’s counterweight must extend beyond geostationary orbit in order to keep tension on the tether. If you’re higher than GEO, your orbital period will be longer than a day, so how is the counterweight kept stationary when it’s higher than GEO?
In: Physics
The counterweight isn’t in orbit – it is tethered to the Earth.
It would stay there the same way the rock on the end of a string stays extended when you whirl it around your head
The counterweight’s not moving at orbital speed – if it was it wouldn’t be a very good counterweight because it would be weightless. It’s moving faster than orbital speed, and so its (non-orbital)period is one day.
You want the center of gravity of the whole space elevator to be at geosynchronous orbit. So you need a counterweight at a higher orbit to balance out the cable below the CoG.
In KSR’s Red Mars, they called for a Martian Space Elevator of approx 10 meters diameter of diamond/carbon construction, from boring out a captured carbon rich asteroid. Wild!
Different way to think about it: attaching the tether to the ground *forces* the entire thing to turn geosynchronously.
Now, think of the tether as made of three pieces – one of them is at perfect geosynchronous altitude, one is lower, one is higher.
For the piece at the perfect altitude, Earth’s gravity is precisely enough to maintain the 24-hour orbit. For the piece that’s lower, Earth’s gravity is more than enough so that piece wants to move radially inward. For the piece that’s higher, Earth’s gravity is less than required to maintain the orbit so that piece wants to move radially outward. (Think of it as centrifugal force being weaker for the inner piece, equal to gravity for the middle piece and stronger for the outer piece, if you’re comfortable with that.)
So the low piece wants to drag the elevator to the ground, the middle piece (which is miniscule) is neutral, and the outer piece wants to pull the elevator out into space. As long as you have more mass on the outer piece, the net force will be outward and the tether will stay taut.
It’s just a very tall skyscraper. That’s literally all it is. A very tall building, so it will stay in line because it’s attached to the Earth like any tall building is. Does the Burj Khalifa doesn’t need to do anything special besides not collapse under it owns weight. A space elevator is exactly the same.
Just a warning, there’s a lot of *almost* correct but wrong information in this thread, especially in the comments. Orbital mechanics are tricky.
An orbit is stable because the force that pulls you towards earth and the speed that makes you go sideways are ”in balance” with the position. Chance one of them and the others must chance. The counterweight is under tension by the space elevator, so there is an additional force pulling on it. This means that even though it is in a position that would normally be beyond geostationary orbit, the extra force makes it act like it’s orbiting a bigger planet, and thus must go faster.
Space elevator is not a practical concept. It only works if you ignore a bunch of things that make it not work.
For an elevator off of an earth sized mass, rotating at earth speeds, you need an impossibly tall structure to reach a position in space that is at the point of a geosynchronous orbit. This structure can be counter balanced by making it twice as long.
We don’t have any materials that are capable of making the structure feasible.
One problem they usually ignore about space elevators is that our planet has an atmosphere, which will flow around the base of the elevator, putting forces on it, and the resulting drag needs to be compensated for.
Even less often mentioned is that the elevator will tend to wick atmosphere up, resulting in the gradual escape of air molecules which are then lost to space, and eventually the planet loses its atmosphere.
And another problem with the concept is that the elevator will slow the spin of the planet, as rotational velocity will be bled off as mass ascends the elevator. The more mass you send up, the further out, the more you slow down the planet’s rotation and lengthen the day. Put enough mass up there and the earth’s center of gravity moves and the whole planet starts to wobble. The effects of this will be tiny and slow, but over a long time scale are not negligible, and would be ruinous to life on the planet.
The space elevator concept might work better on smaller planets without any atmosphere, but then on such a body you probably don’t need an elevator since conventional lift rockets will also have an easier job to do and so will work better than on earth, making the advantages of the elevator less.
the idea with a space elevator is that you counter weight with an object ABOVE geostationary orbit. as you get higher the orbital speed drops AND orbital duration increases. so an object at geostationary orbit is travelling at 3.3km/sec. an object further out but keeping station with it would have to travel a bit faster, but for the sake of simpolicity lets say its the same. geostatrionaary orbit is 35000km give or take. if we put a weight at 50,000km its orbital velocity to be in a circular orbit is 2.8km/s, but it has to be moving faster than 3.3km/s to keep up with the earths surface. this means it wants to fly out into a higher (excentric) orbit but it cant because its tied to the earth. this then applies the tension to keep the tether upright. but it is worth considering the energy for something getting to geostationary orbit has to come from somewhere. there is potential energy from hauling yourself up there but also kinetic energy because you are getting faster to maintain the same orbital period at greater radii. so the counterwieight must have some form of boosting or retrograde engines depending on if more mass is going up or coming down. (coming down the payload would try and overtake the cable as its going 3km/s but the ground is only going 0.4km/sec
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