So, we sufficiently anchor a chain into the Earth and manage to bring the chain into orbit – what would happen?
Could the chain be long enough to maintain orbit? Or…
…would it not matter, since so much of the chain’s mass was closer to Earth, resulting in the chain losing orbit? Could we get to a point where the chain is so long it stays in a balance, so that the outermost part is pulling away from Earth and the lower part is pulling in towards Earth?
Would the chain just fall to the ground, or would it begin to wrap itself around the world like twine 'round a spinning ball?
What effect would Earth’s rotation have on it? Would the chain be deformed into an arch, stay straight, etc?
The effects of the air drag would bring it down at some point. Something has to counter the air drag, like a big massive station in space that can apply thrust. Doing the experiment on an airless planet makes things much simpler.
I wonder if a station could deploy a solar sail at the right time every orbit to counter falling to earth, and forgo the need for chemical thrusters. Turn the sail panels on end for a low profile to the sun when heading towards it.
Problems like this are usually answered starting with the words ‘All you have to do is just…’, as if it’s a walk in the park.
In this case, all you have to do is just construct the thing with a centre of mass sufficiently far up that the centrifugal (or centripetal, maybe) from the Earth’s rotation force pulls it taut.
Unfortunately, it’s unlikely than we’ll be able to build a space elevator in the foreseeable future, despite proponents’ thinking otherwise. There are a lot of basic physics issues which we really don’t have a good way around right now. Short version: materials science is probably the easiest problem we can look at.
Centrifugal is the outward force exerted by an object changing directions (i.e. changing its vector direction, on some radius). Centripetal force is a force requirement that keeps the object on said arc.
But I think in the case you describe, the chain would wrap around the earth. You can’t really keep it taut from the center, except by making the chain effectively shorter.
Would you care to elaborate? I won’t deny that it’s one heck of an engineering problem (anything tens of thousands of miles long is going to be one heck of an engineering problem), but given the nanofiber cable material, I don’t know of any other fundamental difficulties.
I can think of a few major issues (resonance in the presumably taut cable, securely anchoring to ground, avoiding impact on the cable, creating a conduction path from the upper atmosphere to ground, et cetera) but these are all engineering problems not “basic physics issues”. The physics of a beanstalk are pretty straightforward, and the essential showstopper is being able to produce a material with sufficent tensile strength for the tether. The construction if it, on the other hand, is a ferrociously difficult problem even once we have the material science issues solved. I doubt we’ll even see a serious attempt to construct one in the next fifty years. At a minimum, you’d need a reliable and cost-effective way to reach orbit, and to move or build an anchoring satellite into the correct orbit, and right now that’s not even on the table.
The proposals I’ve seen call for an initially very thin cable, the material for which could be launched on a small number of conventional rockets (I believe the estimate was five Deltas), which would be gradually thickened by progressively larger climbers laying down additions to the cable (each climber could carry enough cable material to thicken it by a few percent of the thickness at the time). As these progressively larger climbers went up, the cable thickness would grow exponentially (albeit with a fairly long timescale) until it was finished.
Oh c’mon, guys, it’s easy. All you do is calculate the stresses on your quantum computers and then use the power of laser fusion plants to put it up. It’s only 25 years away, guaranteed.
Air drag? For the most part, the atmosphere rotates with the earth and the cable would also. It’s not like the atmosphere is stationary and the earth rotates inside it, returning to the same spot once every 24 hours. Mostly the same forces that act on a flying plane would be present – just local weather and jet streams.
The article says the strongest known steel has a tensile strength of around 5.5 GPa. Somebody earlier mentioned carbon nanotubes:
So you’re thinking, Aha! It’s just a matter of time! But keep reading:
So basically, the material strong enough to make such a cable is theoretical. The technology to make a cable of any length from the theoretical material doesn’t yet exist. I think we’re in for a LONG wait on this one.
Yep, local weather and jet streams won’t impart any force on the cable.
Ever flown a kite? Now imagine a “kite” let’s say, one foot thick and a thousand miles long. That’s going to take alot of force to anchor. Granted, not all the forces are going to be in the same direction, but they’ll still add vectorally to make some pretty large anchors on earth required, and a VERY strong cable.
Yes, I know the coefficient of drag is going to be alot less for a cylindrical cable than for a kite, but there’s still exponentially more surface area involved.
I think the top down construction plan is simpler, although I don’t know about the energy budget for such a thing.
You start with a construction shack at a geostationary orbital position. Then you string out your balanced pair of cables one up, one down. The downward end is much less massive per meter of length, and can be very much longer, while the upper cable does not need length, only mass necessary to keep the center of mass at geostationary orbit height.
Still, you have to keep the weight budget in mind, even after you begin using the thing. If you load up the station with cargo from Earth, you have to move the counterbalance portion inward, to keep the center of mass geostationary. When you bring stuff down, you have to move the mass outward. This gives you hard limits on cargo and personnel transfer. You can download mass at the same rate that you upload, of course, but the chance that your logistics will keep that balanced are one of those things where theory needs to leave room for real world variability.
The level of tensile strength needed to support the weight of a twenty two thousand mile cable makes the tug of hurricane winds in the first seven or eight miles more or less a “spare change” magnitude consideration. In fact, the ground station doesn’t need to be all that strong, since the major force on it is maintained as close to zero as is practical. A few hundred tons or so should be enough. More than that, and it puts unreasonable extra tension on the cable.
