The bowling ball would curve and hit the flintstones vitamin. Being geostationaary at 1000 miles up means you do not have enough forwards velocity to miss the earth no matter how small it is. It would take far more than a small nudge to get it to miss, something more like an extended rocket burn. Keep in mind that a flintstone sized earth would warp space more than a black hole does, since the Schwarzchild radius of the earth is a few centimeters IIRC.
Apparently, NASA and others (including some Dopers). People once said (and sadly still do) than man would never walk on the Moon. He did. So do you have some hard data on such things that NASA should know about?
That should be “Apparently, NASA and others (including some Dopers) think it will work.” :smack:
You should note my disclaimer about ignoring logistics and materials problems. I don’t expect us to have an vast enough supply of unobtainium for the ladders and sufficient launch vehicles and fuel to get it up there.
As for the ham radio towers to balance it out we’ll go to Radio Shack. They make really good ones. Nnng-glaven!
Umm, the base of the elevator cable would not be ‘mountain sized’ at all, in fact it could be extremely slender. As far as I know the nature of a space elevator would require that the cable taper from the middle to either terminus, the thickest part being the center of gravity. The cable would look like an extremely elongated diamond shape in profile. The reason it has to be so thick in the middle is to take the strain of half the cable’s weight wanting to proceed away from the earth into orbit, and the other half wanting to fall towards earth into the gravity well. To answer your original question, one hour before should see a tiny thread like cable oscillating down from the sky before being stabilized and anchored… once anchored the cable would be built up to a larger size in layers until strong enough to support a cargo. I think that’s the current idea anyway.
The thether would be built from the geostationary point outwards. It doesn’t really matter if you reel it out form the center or have robots ferrying new segments to both ends. So long as the senter of mass remains at the geostationary point, the tether will remain stable. Tidal forces will help you, not hurt - they’re what ensure that the tether remains properly aligned. Wind pushing the lower end of the tether around before it touches the ground might be a problem, but not an unsolvable one. You’d probably want to make that final period of cable extension go as quicky as possible, or have the tether end link up with a high-flying aircraft or baloon to steer it down.
Interestingly enough, a properly designed tether will have zero or near zero tension on it at the attachment point to earth. It just needs to be held in place to keep from drifting.
The only real obstacles to building a tether is building the super-strong material, and clearing all the junk out of low earth orbit before you put it up. The former we’re nearly able to do now. The latter might be an impossible barrier.
Well it wouldn’t be impossible to remove all debris from low earth orbit, just extraordinarily difficult. Though if you have the ability to built a space elevator, it would be doable.
One way I can think of right now is to release a large amount of a gas into low earth orbit. It wouldn’t have to be too thick, or last too long, just enough to create enough friction to send everything in low earth orbit spiraling down to earth. The gas could be harvested from a comet or ten. And if you don’t have the technology to destroy a few comets in low earth orbit, you don’t have the technology to built a space elevator anyway.
The biggest obstacle to building a space elevator in my opinion is political. Much of the world is still ruled by babies in big bodies, who cannot be trusted with the kind of resonsibility that can move bits of the solar system around at whim. The technological hurdles will eventually become easy, the physics certainly isn’t hard.
What happens though when load is placed on the newly completed tether for the first time? Does one little tug cause the whole thing to dip towards the Earth in an ever-accellerating crash? In other words, is the cable naturally stable or unstable? How is the tether effected by lunar and solar tides? And just how severe a problem is ocillation anyway?
The elevator could be held stable with rockets on the anchor asteroid. Also cars going up the cable would most likely be counterbalanced by the weight of cars coming down the cable. In the case of a weight imbalance, the rockets would be used to keep the orbit at the correct hight. They would also be used to counteract changing solar wind conditions and tidal effects. It would need active guidance to remain stable, and that would neccesarily be part of its design.
I think those designs having a countercable extending up from the anchor asteroid be subject to severe oscilations, since the upper end would be unanchored. It would basically be like a whip in the wind. This is easily solved by not having an upper extension and using the anchor asteroid as the counterweight. Its high mass combined with periodic rocket adjustments would make it stable.
cheesy diagram follows:
Earth…Center of mass…asteroid
(•)---------------------------------------·•
Another thought just occured to me: You could also use solar sails to stabilize the asteroid. Sunlight is cheaper than rocket fuel, and if the sun goes out we wouldn’t really care anymore about the cable. Changing their pitch would produce small constant amounts of thrust in whatever direction was needed.
Once the tether is connected, you will probably want to add mass to the far end, placing the connection between the tether and the base station under tension. This will help stabilize the tether against being pulled down by lifting loads. With enough anchor weight on the end, the tether should be stable even when you’re lifting cargoes from the ground. It’s also assumed that an orbital tether would be used to lower a fair amout of mass as well, from asteroid mining or He3 extraction plants, so the overall updard and downward forces should balance.
Oscillation may become a significant problem. Some sort of active dampening system may be required. In Arthur C Clark’s book about a tether, they worked out a way to dampen the oscillation of the tether by carefully timing the rising and falling cargo cars, using them to apply forces just as required.
In the real world, the tether wouldn’t be whisker-thin at the bottom. Not because it needs the tension, but because it has to be thick enough to hang structures on, attach the cars that ride up and down, and survive various natural and man-made disasters.
It would probably be built the way cable spans on bridges are - originally, a very, very fine thread would make its way down. The way the taper ratio works, you save huge amounts of mass by making the bottom even a tiny bit thinner. Since the original mass has to be boosted into orbit, this is important.
So you might get a thread coming down that, by the time it gets to the earth is the size of a human hair, or even smaller. It’s thick enough, though, that it can carry more tension than its own weight.
So what you do is extrude the thread towards the earth, while moving a heavy mass the other direction to maintain equilibrium. When the thread gets to within a mile or so of the ground, I imagine it would be met with a ground-based attachment cable that is slack. Attach the two together, then slowly wind the orbital mass out a bit until the slack is taken up. Then reel the whole thing down to the ground and anchor it.
Now that your very tiny thread is anchored, you start a reeling machine that takes additional threads up the cable. So the original thread has to be strong enough to handle the weight. You keep reeling up these threads until the whole cable is immensely strong.
I think the biggest two problems with the cable are terrorism and space junk. This thing would be the world’s #1 target. You’d have to establish a very large no-fly zone around it - hundreds of miles. You’d have to protect it against airliner strikes, missiles, and bombs. In the future, you’d have to protect it against missiles outside the atmosphere or killer satellites.
But the engineering sounds perfectly feasible to me, and well within our capabilities assuming we can come up with a way to spin nanotubes into long fibers.
Speaking of nanotubes - I know they are immensely strong in tension, but how are they for strength in shear? If I hit this tether with a jet, will the tether snap, or will the jet get sliced apart?
Engineering with nanotube fibers will be extrmely interesting. Assuming we can build this stuff in quantity and work with it, you’re going to see a revolution in many, many industries. Architecture, aviation, cars, you name it. Look at what carbon fiber did for everything from airplane construction to skis. Nanotube fibers would be much, much stronger. With material like that, we can build suspension bridges across huge expanses of ocean. We can build gigantic skyscrapers, and incredibly strong, lightweight cars. And like other materials revolutions, we’ll no doubt build a whole bunch new things that no one’s even thought of yet.
Using one of the strongest commerically available carbon fibers to make a composite with an epoxy matrix, a space elevator could theoretically be made if diameter changes with altitude. It would require that the diameter of the shaft be about 62,000 times bigger at its widest than at the ground. If 1 millimeter diameter at ground, this would need to eventually grow to be a maximum of 62 meters in diameter. A constant diameter would not work with this material, but would with nanotube.
Why are so many people are saying that the center of mass of the elevator would be at geosynchronous altitude? The space elevator does not have neglible size compared to the earth’s radius, so it is poor assumption to treat it as if it is in a uniform gravitational field.
Is that the taper ratio for carbon nanotubes?
Some people in this thread really need to read up on space elevator engineering. I have never seen a technical article anywhere about it that claimed that the Earth end would be anything but huge.
Again, it is up to people making extreme claims to provide the evidence. What is the state of the elevator 1 hour before linkup??? (Since I know it can’t be done, I have to idea how to argue against a fictional hypothesis.)
1 hour before linkup the end of the tether is descending towards the ground, and the counterweight is being reeled out from the other end.
You have been given a lot of answers on how a space elevator would be built. you are the one making the claim it can’t be, you are the one who needs to back up your claim rather than simply stating “I know it can’t be done”.
As I said above, the ‘finished’ elevator will have to have a pretty massive bottom end for sure, but not because it needs to hold its own weight. It needs a massive end because the whole thing needs to be under significant tension in order to handle the loads that are applied to it by the cars, the environment, etc.
No question about it - the anchor point on the ground will be huge. The cable itself will be large.
Instead of making us guess about what the state of it will be 1 hour before connection, why don’t you just tell us? Tell us what the problem is.
And if we’re going to appeal to authority, NASA doesn’t think it’s impossible. I highly recommend the 34-page PDF file on that site. It’s a summary from a conference NASA sponsored on space elevator development.
The conclusion of the panel:
In fact, money is being spent on it right now. From Space.com:
Highlift Systems is actively designing a commercial space elevator, which they say could be built and deployed in 15 years, and would be capable of lifting 5-ton payloads.
Interestingly, NASA’s conference points out that we can cut down on the mass of the tether significantly by connecting it to a REALLY tall tower. As in, a tower maybe 3,000 km high. As NASA points out, given a material of equal strength, you can build a tower into orbit just as easily as you can build a tether coming down. The big difference is that the tower would have to be stabilized, and that it’s easier to make materials that are strong in tension instead of compression.
So, you build a tether out of a combination of the two. You build a tower up, a tether down, and connect them.
Except that we don’t know of any material which could be used to construct a practical 3000 km tower, but we do know of a material which could be used to construct the cable. Realistically, I think we’re probably just looking at an oil rig-type sea platform at the bottom, with little or no compressively-strong tower.
One other thing about a space elevator is that it may be difficult to build one, but once you build one, it becomes very easy to build more. So you build a few dozen of them, and if terrorism or natural disasters destroy a few, you can easily replace them. You would, of course, try to prevent loss by either, but “it won’t last forever” is a pretty poor reason to not build something.
If the break is between the centre of gravity and the ground, the top part will go into orbit… it might even become a torist attraction.
Do please let us know what the impossible stge in space elevator construction is, ftg; there are one or two lttle hi-tech fixes that might get round it…
just like there might be ways of putting an elevator above a slow - rotating body like the moon.
To expand on this comment, I saw a presentation by Michael Laine of HighLift recently. One thing he mentioned was seeing Dr. Edwards (the guy who’s been doing technical work on space elevators for the last 3 years under a NASA grant) give a talk to a very skeptical technical audience, and at the end of the talk Edwards had convinced everyone it was possible. Edwards has spent a lot of time (3 man-years) working on tons of nitty-gritty details, and HighLift has convinced people that there are reasonable answers to any questions people raise.
Also FYI, they’re looking to tether the base at an oil-rig type of floating platform in the Pacific hundreds of miles west of South America (I forgot where, exactly). The region is on the equator, and in an area that has almost no lightning strikes or other major storms. And another tidbit I learned from the talk: apparently it’s “better” (for a reason I forget) to extend the tether out way past GEO (with some counterweights, such as some of the climbers) rather than using a big asteroid much closer to GEO. I think it helped with the tether requirements.
ftg, I’d recommend you go read the reports at the HighLift Systems website (see above for URL) before saying “it can’t be done” because they’ve dealt with issues (such as the one you keep harping on of how to connect the tether to the ground). Oh, and while the bottom of the tether might be thick/heavy, it’s thinner than the middle (the part at GEO) – I hope you’re not claiming the opposite? But if they can use nanotubes, the diameter of the first tether (which would be used to loft more tethers) was very small - on the order of a few (or tens?) centimeters, I think…
Cobalt