A space elevator, as a whole, is in a stable orbit, but not every part of it is. The center of mass is at 22,200 miles high (or close to it). And because there’s 22,200 miles of cable below that point, there needs to be a hell of a lot of mass above that point, too. If you were in an elevator climbing up this cable, at 22,200 miles you’d be weightless. Your height and speed would be just right to stay in orbit, so it doesn’t matter if the elevator cab is around you or not. But below that point you’d be going slower than a satellite, so you’d still feel some sense of gravity inside the elevator. Above that, you’d be going faster than a satellite and you’d be pressed, slightly, against the ceiling of the elevator.
I’m not sure what you mean by “stable orbit” but that’s not correct by most definitions, and you certainly would not be weightless at the top of a space elevator if by “top” you mean the maximum extent of the cable, where the counterweight is. The counterweight is in an artificially constrained orbit, orbiting with a geosychronous period but at a higher altitude where its velocity is higher than it should be for a stable orbit. In effect everything at that speed and altitude has negative weight; if the counterweight were released, it would climb to a higher orbit, just as a satellite would do if it got a rocket boost. That’s what gives the cable its tension. It would keep climbing until the earth starting pulling it back, and that point would become the apogee of a new stable orbit.
The only point on a space elevator where you would be weightless would be at the geosynchronous point. You would get lighter and lighter until you weighed zero at that point. Climbing further, toward the center of mass and the counterweight, your weight would become increasingly negative, pointing away from the earth. If you jumped out into space you would of course immediately become weightless since you would be on a ballistic trajectory, but you’d end up rising higher and then settling into a stable higher orbit.
So if I understand correctly, a space elevator would be of little use getting to low earth orbit because at that altitude, whatever you are lifting is not moving fast enough to maintain orbit.
Plus, if my quick calculations are correct, if you went past about 11km above the geosynchronous point and let go you would be moving faster than escape velocity - you would go not just a higher orbit but out of the earth’s gravitational field completely. I believe Clarke made this point also in Fountains of Paradise - the space elevator could also be used as a slingshot to launch interstellar payloads.
With a space elevator, getting to LEO would be cheaper than it is now, but it’d still be awkward. But that’s OK, because there’s very little value in LEO in its own right. The only reason we launch so much stuff to LEO right now is because it’s the cheapest orbit (with our current technology). A space elevator would make GEO orbits and highly-eccentric orbits much cheaper, so most of what we launch to LEO now would just be launched to some other orbit instead.
EDIT: And yes, you could do interstellar launches from a space elevator if you wanted. But you’d still have to solve all of the other problems with an interstellar mission, like providing power for thousands of years. Much more practical would be interplanetary missions.
Except that (as Chronos already said) getting up to an orbital altitude and with a good fraction of the needed orbital speed (even if insufficient) already saves you a very, very high percentage of the cost of trying to lift it from the ground in a rocket at standstill! Plus, if you go higher than you need to be, you have free energy to play with, though I confess I don’t have a sufficient intuitive sense of the orbital mechanics to know how you would effectively use that – I believe you would inevitably need some amount of rocket thrust to maneuver into the desired orbit. The international consortium working on this proposes a “LEO gate” for low earth orbits that would release the payload at around 24,000 km, IIRC, as opposed to the GEO (geosynchronous point) at 35,786 km. They also propose a “lunar gate” and a “Mars gate” at higher altitudes above GEO.
That intuitively seems far too little. The Wikipedia article on space elevators states that escape velocity is reached at 53,100 km, as compared to GEO at 35,786.
The Wikipedia article says, “An object attached to a space elevator at a radius of approximately 53,100 km would be at escape velocity when released.” Radius, not altitude. The radius of a geosynchronous orbit is about 42,000 km.
OK, thanks, but what caught my attention was that you wrote “11km above the geosynchronous point”, which seemed wrong. I wasn’t trying to be snarky, it actually didn’t occur to me that this was probably a typo and that you probably meant 11K km! If so, then apologies and all is well – you were indeed in the ballbark.
But on the surface of the earth, you are not at a standstill. At the equator you are moving east at over 1000mph. 300 miles up you are only going very slightly faster. 1100mph or so if my math is correct.
It is a great help to be above the atmosphere of course, but speed seems still to be an issue.
You are probably right that something may be gained by going higher (and therefore faster) and changing orbit later on.
I’m certainly not an expert on orbital mechanics, but I think that’s the general idea. Tremendous cost and energy is saved by gaining altitude, because a rocket has to lift not only the payload, but also itself and all its fuel, so it all snowballs exponentially. Plus, you’re not talking about hundreds of miles in altitude but potentially thousands – as mentioned, the LEO gate proposed by the elevator consortium to the best of my knowledge is at 24,000 km, about two-thirds of the way to GEO – which gives the payload lots of orbital speed at release. I would imagine relatively minimal thrusters could put it into a nice circular LEO.
This is not correct. It takes much more energy to achieve orbital speed than it does to achieve orbital height.
XKCD has an article about this. https://what-if.xkcd.com/58/
Ummm… Perhaps this is too simple of a view. But thinking about the OP’s question. - Wouldn’t the anchor in space have to have ‘negative’ gravity to hold up the climbing ribbon/cable to keep it taught? Not even considering if the payload climbs or is pushed up with lasers.
So, if you ‘stepped off’, you would fly into space away from earth.
Well, getting above the atmosphere does still save you something. Still, I imagine that if, for some reason, you really must have LEO, then the most efficient way to get there is probably to lift to a higher height and release, to go into an orbit with its perigee at LEO height. Then, when you’re down there, you do a rocket burn to circularize. Or maybe a combination of rocket burns and aerobraking. I haven’t done the calculations on how high you’d need to go for your original release, but the 24 Mm cited by Wikipedia seems plausible.
EDIT: enipla, the top point must be at least some amount above GEO in order to balance it, so if you went to the top and let go, you would certainly “fall” upwards, at least initially. But depending on the design of the elevator, you might end up still in an orbit around the Earth and come back down to that same height again a little over a day later (and then up again and so on), or you might end up escaping completely. How high up the end needs to be depends on how massive the counterweight is, and with a really big counterweight, it might be only a little above GEO height. Still, it’d be really nice to be able to launch things on escape trajectories for free, so in practice, I expect that any space elevator would be at least tall enough for that.
That’s quite true, and worth pointing out. My comment was badly worded. Satellite launches tend to go fairly straight up only to clear the worst of the atmosphere, and then they heel over and go for speed. The thing is, an enormous proportion of the fuel is consumed in those early seconds and minutes, which is why experimental airlifted spacecraft (“air launch to orbit”) have had some success, even though the predominant advantage offered by air launch is high altitude and thin atmosphere rather than any substantial velocity. But certainly your statement about where most of the energy goes is correct. And the beauty of space elevators is that enormous altitude and huge orbital velocity go hand in hand.
