The Speculist

An interesting idea, but I'm not sure how stable it would be. Besides, once you have a cable reaching 60,000 miles and then some, what difference does another 100 make? Wouldn't it make more sense to skip the "planetoid" and attach the cable directly to the ground?

You don't need a 60,000 mile long cable. All you need is a cable that links to a higher slower moving point. I believe these are called "sky hooks". I don't know how long they have to be, but they can be made with current known materials (eg, kevlar or carbon fiber) rather than having to develope a new material.

Actually, I'm surprised that the space elevator people aren't pushing the obvious intermediate step. Launch something suborbital and then have a cable lift it to orbit. Then something like Scaled Composites' Spaceship One (perhaps with a little more pep so it could have a faster velocity at the top when it's caught by the sky hook) could get to orbit now.

That bothers me. A sky hook should be a far more achievable project with a fair fraction of the economic benefit that you could get from the space elevator. But these people are going with the big money, possible project rather than the smaller doable project. It's sounding to me more like they're angling for those "cost plus" contracts rather than seriously pursuing incremental efforts. Well, at least they'll produce some useful results assuming those results become public domain after a time.

JW:

I think a big plus to Phil's Dark Sky/Space Elevator idea is security. Terrorists would have a much harder time attacking a station that's not attached to the ground.

Another plus - this station would not have to be near the equator. We could have a space elevator parked right over the U.S.

Um . . . guys . . .

A Beanstalk/Space Elevator/whatever is kept in position by tension in the cable. The Earth-side anchorpoint has to hold some massive force to keep it from floating off. An oil platform type setup will work because you can enclose some big tanks of water as your ballast. The force is pretty constant (varies some as payloads go up and down the elevator) so you can keep the platform at sea level with some trim tanks.

As for the suborbital-rocket-to-sky-hook concept . . . There's two kinds of sky hook in that concept, rotating and non-rotating. Either one can give a boost to a payload but trades off its own orbital momentum to do so. So it has to have thrusters to boost it back up, or have an equal mass of payload coming back down so it can steal their momentum. An Earth-to-GEO elevator doesn't have this problem because it can steal the Earth's momentum.

A non-rotating skyhook catches payloads at the bottom end and gives them an elevator ride up to the top. Slow and not a big boost, but your rocket has a constant-altitude target to aim for. Rotating skyhooks grab the payload on the low end and then release it as that end reaches peak altitude, a very quick and energetic boost. But then the rocket has a fraction of a second landing window to catch the hook as it comes by. Either way your rocket is probably going to need a powered approach to have any chance of making the hookup.

I think the Earth-anchored Beanstalk is probably a much easier project to develop and operate than a free-flying Skyhook. The payloads have no problem getting on or off and there's much less chance of losing the whole thing if your operations get in trouble.

Steven:

A space elevator has to go through the geostationary orbit position. If you don't put it on the equator it goes up at an angle and puts a lot of shear force on the anchor point.

Karl H -

I like the sky hook idea!

Karl G -

I'm the first to admit that I'm not on top of the physics that would lie behind engineering something like that. However, the idea behind making the station into the planetoid was to make it as massive as possible. (Though it is weightless, it would obviously have to have a mass several times that of the entire cable and the space station at the top.) If it is floating on top of the atmosphere the same way a ship floats on top of the water, couldn't you secure it using high altitude versions of ballast and trim tanks?

Phil -

The problem with the planetoid "floating on top of the atmosphere" is it would have to have a mass in the hundreds or thousands of tons. I suspect a balloon of that size might block enough sunlight to affect local ecologies. Even if you solve that, you have the question of where to get more ballast. An ocean platform can have a sealed tank of air below the waterline. If the topside payloads get out of balance, you open the valves, the water rushes in, close the valves again, and you've added tons of ballast. Replacing a vacuum with a near vacuum won't have nearly the impact or work as quickly.

All that can be solved. You'd probably wind up having a bunch of elevator cars on both sides of GEO that position themselves to handle ballast duty so the planetoid doesn't have to change. But that doesn't strike me as an easier engineering challenge than dealing with the ocean or bedrock.

Oh, and a clarification on my comment to Stephen (in addition to an apology for misspelling his name). An elevator doesn't have to go through the geostationary satellite belt, it has to go past that altitude in a plane parallel to the equator. So the US-based elevator would be at an angle to the vertical equal to the site's latitude. A bedrock site might handle that but it would be costlier than an equatorial site.

Karl,

I would think that the orbital station would eventual end up being just as massive if not more so than the theoretical sub-orbital station. Your 'dangerous' shadow problem would still inevitably manifest.

The station at the end of the 'beanstalk' would be a center of commerce. It could quite possibly become the first orbital city. It would be the Atlanta airport a thousand fold. There would be stores, resteraunts, hotels and quite possibly permanent living communities for the employees of all those businesses.

I like your oil-platform idea, that sounds the most promising solution I've heard. It could be positioned outside the boundaries of any equatorial nations, and might make it more easily defensible.

I am tardy in blogging about this, having read it in Discover last monht. Oh well, when I get around to my Seasoned Words post for Guerrero Knew, you'll see where I"m heading with it.

I've just posted my space elevator story at Seasoned Words under the "Guerrero Knew" category.