A long trip to the Space Elevator

The concept of a Space Elevator was first identified by Konstantin Tsiolkovsky in 18951. His idea was of a building so tall that it would reach orbit.

Unfortunately this is not practical- all building materials are so heavy that they would buckle under the load, or if an exponential taper to the building was used to combat this, the base would be hundreds of miles wide. The maximum practical height is probably well under 10 miles, and the highest current building is only about 500 yards or so (see the CN tower for an exponentially tapered building like this). Orbit is at an altitude of ~120 miles, and geosynchronous orbit is far higher. All buildings built up from the ground (compressive structures) simply are not practical.

Instead, the current modern concept is of a space elevator that consists of a long thin cable- hanging down under tension from orbit. Weight-for-weight cables under tensile loading are fantastically strong- that's why suspension bridges are so much bigger and better than arched bridges.

Arthur C. Clarke wrote a work of fiction called Foundations of Paradise about a Space Elevator that used this concept. He is widely quoted as saying that he thought it would be built 'about 50 years after everyone stops laughing'.

Well, people have stopped laughing. Recently a researcher Brad Edwards published a paper under the funding from NIAC describing his investigation into the Space Elevator. His conclusions are that it may be much, much closer than 50 years away right now, and this node mainly describes his ideas.

What keeps it up?

A common misconception about a space elevator is that it would be in orbit. It cannot be in orbit- it's attached to the ground, and the center of mass is too far from the earth for it to be in orbit.

In fact, it works just like a stone being swung around your hand on a piece of string. The gravity pulls the tether towards the Earth, of course, but the rotation (actually momentum) is stronger and keeps the cable stretched out in a straight line.

What can we make the tether out of?

This is the really hard bit. The tether must be light, as it has to hold its own weight, and really strong- because it must be more than 38000km long! The only materials that might make this distance (and not need to be many kilometers thick) are:

Carbon fiber whiskers are too short. Diamond is incredibly expensive and difficult to build with and brittle. Metallic hydrogen only exists at awesome pressures- building a tether using it would be at best, unbelieveably difficult. Carbon nanotubes are more or less strong enough; and long enough to do this; there currently exist samples more than 3cm long with a strength of 73 GPa or more. The theoretical strength is over 110 GPa. A target strength for this application is 72.5 GPa, so in principle we are there, but a safety factor is necessary and also we need some way to make a cable rather than a single fiber. Still, this is looking very, very promising.

What would the tether be like?

It would probably be like a tape, of varying width. Near geosynchronous orbit it would be quite wide, several centimeters, at ground level it would be very thin, almost thread like.

Making it like a tape helps it pay out, and makes construction easier. Making it narrower towards the earth allows for the fact that you've got less cable below you, and hence you can slim it down. It turns out that an exponential taper would keep the stress constant at every point on the cable, minimising the cable weight at every point.

The tape might be made by sticking carbon nanotubes together sideways in parallel; but you can't afford to have 38000km of glue, the glue would weigh too much, so it would only be stuck every few centimeters in bands.

How can we deploy the cable?

This is fairly straightforward. First you take a spool with a thin cable up to geosynchronous orbit using a rocket, for example you could use the Space Shuttle. The spool may only weigh 20 tonnes- this is just a thin elevator to start us off. Then you lower one end of the cable down towards the earth. It basically self-deploys. The cable is naturally stretched into a line towards the Earth by 'tidal forces' (the combination of gravity and the rotation of the orbit) the spool gradually and naturally falls away from the earth, as it pays out the cable. The thin end moves down towards the Earth. Theoretically only a brake is required on the spool to do this, although in practice, the end you are lowering would probably have a small rocket motor that controls the position of the end during the deployment.

The end lowers down towards the earth, and enters the atmosphere. Near the ground somebody catches it and ties it off.

Once this light, thin cable is in place, another cable of similar mass is run up the cable from the earth, doubling its mass. The spool itself will stay at the top of the cable helping to act like a counterweight. Once this has reached the top, another cable is run up again, and so forth until the cable is much stronger. This will take a few months to bring it up to useful strength.

You would then be able to climb up the elevator cable from the ground, let go, and be in space!

Where can we build it?

The best place to build it would be at sea. Occasionally, space elevators are likely to fall down due to meteorite impacts. If it is at sea then nobody is likely to get hit by the falling parts.

This falling down bit...?

Yes, occasionally a natural meteorite (or so called 'space junk') will impact an elevator cable and sever it. Some designs of tethers can mostly survive this the hoytether is a good example; nevertherless this is not guaranteed.

The material that makes up the tether is probably not terribly polluting, although wildlife might have issues if they eat or inhale the fibers. Mostly though it's just glue and stringy stuff- nothing particularly toxic, and most of it will burn up in the atmosphere- only the lower hundred kilometer or so will survive.

Elevator cars will probably need to bear this in mind and have reentry survivability build in to them.

(One interesting feature of the elevator cable is that it can be moved around by quite a few miles by moving the payload around. Whilst a payload goes up the elevator the cable tends to swing west by a degree or so, as it leans to pull the payload up to speed. When a payload goes down it swings east as it slows. Additionally you can move the base around as it is kept at sea. The net upshot is that if a known object is going to collide with the tether, the tether can get out of the way!)

What would the elevator car look like?

This is hard to say. But there's a few things we can say.

  • They will be powered from the ground, probably by a laser or microwave link. (If you were to put electrical wires all the way up the elevator the extra weight would easily snap the cable, and the resistance of the wires would be too great for this to work anyway.) Semiconductor lasers can deliver 1 watt and cost about a dollar at todays prices. Ganging them up and pointing them at a vehicle through a telescope is fairly straightforward. Aiming is the hardest part, but it seems doable.

  • Radiation shielding is an issue- once you get above the atmosphere the Van Allen belts start, and a foot of shielding is needed for people to survive. That weighs a lot, and the tether is going to have weight limits. Roughly a 20 tonne tether can only lift 5 tonnes of weight. Of course the tether can be made wider, but that costs money. See the HiVolt node for a proposal to remove the Van Allen Belts.

How much to build it?

The price isn't nearly as bad as you might think. It's looking like the Space Elevator would cost about $10-20 billion. This is very comparable to other big construction projects. For example the Channel Tunnel cost about $15 billion.

One nice thing about it too, is that it may be self financing- as soon as you have the seed cable up, you can deliver cables to geosynchronous orbit for other countries for a profit- it costs more to put up the first one, so once it's up you have the other countries over a barrel as you can do it more cheaply than they can!

How long would it take to build?

It would take about 12 years start to finish. Most of that is designing and building the spool, the tether and the elevators, and the last 5 years or so would be installation.

How long would it take?

It would take a week or two to get to geosynchronous orbit.

How much would a trip cost?

Ok, disappointment time I'm afraid. At the moment the elevator is mainly aimed for freight. The tether goes right through the Van Allen belts around the Earth and out the other side, which has nasty levels of radiation. The radiation problems mean quite a bit of shielding (perhaps a foot or more) is necessary for people to use it, otherwise you'd raise your chance of death from cancer quite noticeably. However freight doesn't mind radiation so very much and so the early tethers are likely to be used for that instead. Brad Edwards is currently projecting a cost of ~$200 per pound, which is 50 times cheaper than the Space Shuttle. However the exact price will depend on how much the elevator is used, how often they are severed, how thick the elevator is built up to be and how much the cables actually cost to make. My suspicion is that this will get much cheaper over time, and atleast some of the Space Elevators would be made strong enough to carry people.

Any other advantages of this idea?

The cable will probably be more like 60-70,000km long, with the top part acting as a counterweight of the lower part. This means that the top is moving faster than escape velocity. In fact it is going fast enough for something released there to reach Mars, Venus or Jupiter if it were to be released at just the right time.

Also, dropping something from 1/2 way up to geosynchronous orbit puts something in an orbit that just grazes the atmosphere and can be aerobraked into LEO. This means that delivering equipment or fuel to say, the ISS becomes cheaper. This means that travel for people to the Moon from LEO in turn becomes very cheap- normally delivering sufficient fuel for the trip would be way too expensive.

Is it for real?

Yes. It's for real. Mankind can probably do this. The only bit we can't do right now is the tether; carbon nanotube of the right length and strength is fairy close to real technology. Finance can probably be done for this, the engineering seems plausible. Probably a big push of research now would have an elevator up in about 15 years. Brad Edwards himself has set up a company to work towards this goal called Highlift Systems.

(1) Ok, there was a prior concept called 'Jack and the Beanstalk'; but the technological concept left something to be desired :-)