Even though the space elevator has made several appearances in science fiction, few people are familiar with the concept. In the most basic description the space elevator is a cable with one end attached to the Earth and the other end roughly 60,000 miles out in space (see figure 1.1). Standing on the Earth at the base of this "beanstalk " it would look unusual but simple, a cable attached to the ground and going straight up out of sight. Now even the youngest of you reading this manuscript will know that a rope can not simply hang in mid-air, it will fall. This is true in all of our everyday situations; however, a 60,000-mile long cable sticking up into space is not an everyday occurrence. This particular cable will hang in space, stationary and tight. The difference between why a 10-ft piece of rope will fall and a 60,000-mile long cable will not has to do with the fact that the Earth is spinning. The cable for the space elevator is long enough that the spinning of the Earth will sling it outward, keeping it tight. The 10 ft. length of rope is too short to really feel this effect. To illustrate what I mean, let me give an example. If you take a string with a ball on the end and quickly swing it around your head the string sticks straight out and the ball doesn't fall. Now imagine that string is 60,000 miles long and your hand holding the string is the Earth. The two situations, the ball swinging around your head and the space elevator swinging around the Earth, are really quite similar. Okay, great, so we now have a cable pointing straight up into space, so what. The so what part is that it is possible to climb this cable from Earth to space, quickly, easily, and inexpensively. Travel to space and the other planets will become simple if not routine.
That all sounds straightforward, doesn't it? The 60,000 mile part may give some of you pause but trust me man has built much more massive and more complicated structures than what we will be discussing. This one is just in a particularly unique shape and location. With that said I hope you will also trust me and believe that building and using a space elevator is not nearly as simple as I have explained so far. I have left out a few details, thus the rest of this manuscript. I should also state right at the offset that this manuscript is an extension of a paper I put together that will be published any time now in Acta Astronautica [Edwards,2000]. The concept is the same but this study has modified many of the details found in the Acta Astronautica paper.
The concept of a space elevator first came from an inventive Russian at the dawn of the space age [Artsutanov, 1960] but the appearances of the space elevator I enjoy most came in several science fiction books. Arthur C. Clark put together an interesting tale of the construction of the first space elevator in Fountains of Paradise [Clarke, 1978]. Kim Stanley-Robinson had a different and well-thought out take on how the first space elevator might arise in Red Mars [Stanley- Robinson,1993]. These books point out many of the basic aspects and challenges of building a space elevator and keeping it operational. I highly recommend them for their entertainment value but remember they are fiction and I wouldn't suggest following their model for building a space elevator in reality, just follow their insights. Let me explain.
In both of the books I just cited a natural object, asteroid or moon, is moved into a proper orbit and mined for its carbon. This carbon is then used to build a very strong, very large cable extending both upward and downward (figure 1.2). This was and still is a reasonable conceptual suggestion for one possible construction method [Smitherman, 2000]. However, I would consider this method as too expensive and too difficult to be a viable option outside of science fiction. The capture and movement of an asteroid, though not impossible, would be extremely challenging. In addition, the operations that would be required at very high Earth orbit (mining and cable fabrication) are also beyond what I would consider economically feasible at this time. I may be wrong on both of these but... well, allow me to continue.
Outside of science fiction there was some work done on the space elevator during the first decades of the space age [Isaacs, 1966: Pearson, 1975: Clarke, 1979]. These early publications worked out the physics of the space elevator and discussed some of the components such as the optimal cable design being one of a tapered cable. But even in the past few years the space elevator concept has often been discarded out-of-hand as inconceivable or at least inconceivable for the next century. The reason for the general pessimism was that no material in existence was strong enough to build the cable. Steel, Kevlar, carbon whiskers, spider web or any other material known ten years ago simply would not work. That changed in 1991 with the discovery of carbon nanotubes [Iijima, 1991]. Carbon nanotubesB&D are extremely long molecular tubes of carbon where the atoms are arranged in a pattern similar to what is found in geodesic domes. Theoretically they are stronger per kilogram than any other material by a factor of 40. As an example, a fiber made of carbon nanotubes 1/8" (3mm) in diameter could support 45 tons (41,000 kg). For building the space elevator this strength is critically important. Using a material other than carbon nanotubes it was estimated that it would take 750,000 shuttles to place the space elevator in orbit [Pearson. 1975], not really something most people would seriously consider. This is the reason for the science fiction scenario of building the cable on-orbit using materials naturally existing in space. However, I believe there is a better way.
If we assume for the moment that we can get all the carbon nanotubes we need to build a space elevator, we can build it in a similar way to how difficult bridges were built in the past. In building a bridge, the first thing that was done was a small string was thrown or shot across a canyon. Then a larger string is attached to this first small string and pulled across. This process is repeated until many ropes and eventually structures are placed across the canyon. We have a serious canyon and the string is longer but the concept is the same. First, a satellite is sent up and it deploys a small "string" back down to Earth (see figure 1.3). To this string we attach a climber which ascends it to orbit. While the climber is ascending the "string" it is attaching a second string alongside the first to make it stronger. This process is repeated with progressively larger climbers until the "string" has been thickened to a cable, our space elevator. That's a pretty simple breakdown of what we are considering, allow me to add a few more details.
In considering the deployment of a space elevator we can break the problem into three largely independent stages: 1) Deploy a minimal cable, 2) Increase this minimal cable to a useful capability, and 3) Utilize the cable for accessing space.
The initial "string" we deploy from orbit is actually a ribbon about 1 micron (0.00004 inches) thick, tapering from 5 cm (2 inches) at the Earth to 11.5 cm (4.5 inches) wide near the middle and has a total length of 55,000 miles (91,000 km). This ribbon cable and a couple large upper stage rockets will be loaded on to a handful of shuttles (7) and placed in low-Earth orbit. Once assembled in orbit the upper stage rockets will be used to take the cable up to geosynchronous orbitB&D where it will be deployed. As the spacecraft deploys the cable downward the spacecraft will be moved outward to a higher orbit to keep it stationary above a point on Earth (a bit of physics we will explain later). Eventually the end of the cable will reach Earth where it will be retrieved and anchored to a movable platform. The spacecraft will deploy the remainder of the cable and drift outward to its final position as a counterweight on the end of the cable. This will complete deployment of a stable, small, initial cable under tension that can support 2724 pounds (1238 kg) before it breaks.
The next stage is to increase this ribbon we just deployed to a useful size. During this stage climbers will ascend the cable and epoxy additional ribbons to the first one as they climb. At the far end of the cable the climbers themselves will become counterweights for the space elevator. One problem is how to get power to these climbers. Gasoline engines don't work well in space where there is no air and wouldn't have the required range, solar cells are too inefficient for their mass to be feasible, nuclear reactors are too heavy, an extension cord just simply wouldn't work, etc. The best option is to beam up the required energy. By using a large laser directed at solar panels on the bottom of the climber, we can efficiently send up lots of power to the climbers. This power is easily converted to electricity for running an electric motor to climb the cable.
As each climber completes its ascent the cable would be 1.5% stronger. After 207 climbers (2.3 years), the cable would be capable of supporting a 22 ton (20,000 kg) climber with a 14 ton payload (13,000 kg). This cable will have a cross sectional area forty times the initial cable I mentioned above. Payloads can be taken up the elevator to any Earth orbit or if released from the end of the cable be thrown to Venus, Mars or Jupiter. These payloads (large satellites, cargo, supplies, etc.) can be launched every four days. Additional cables of comparable capacity could be produced every 170 days using this first cable and "shipped" to other sites along the equator by dragging the lower end of the cable. In 2.8 years the capacity of any individual 22 ton (20,000 kg) cable could be built up to 1100 tons (1x10 6 kg) or roughly the size of a shuttle orbiter. And again, payloads as large as the shuttle orbiter can be sent to Earth orbit, Venus, Mars or Jupiter every four days from one of these larger elevators.
The primary use of an initial 20,000 kg capacity cable may be to place spacecraft into low-Earth through geosynchronous orbits. The recurring costs of this system would be the cost of the climber to transport the payload. The uses for a larger cable as discussed above would probably be for manned activities such as building and supplying a station at high Earth orbit or on Mars. All of the launch costs for putting things in orbit from the space elevator would be a small fraction of what they currently are with rockets.
Now some of you have seen concepts for space elevators that entail grand designs, large futuristic transports, several-hour travel times, large city complexes at the cable anchor and complex systems with multiple tracks on single cables. The original science fiction concepts had these and it is a wonderful scenario. The design I am proposing probably sounds small, plain and boring when compared to these. However, keep in mind that the first automobile was not a Porsche 911 and if man had refused to build any automobile unless it was a Porsche 911 horses would still be our primary mode of transportation today.
We've all heard of Murphy's laws - what can go wrong, will go wrong. If we assume we can get the material to build the cable and that we can actually construct it as discussed above, are we home free? Not by a long shot. This is where Murphy has been working overtime. Getting the space elevator up is one thing, keeping it up there is something else.
The space environment is not a pleasant one; it's more like a burning and freezing, radioactive, corrosive, shooting gallery with no air. On top of that our own environment is not that pleasant at times with things like hurricanes and lightning. There is a whole set of environmental threats the space elevator will need to survive including:
Most of these are capable of destroying our space elevator on short order if we aren't careful. The first lightning storm or strong wind would destroy the bottom end of the cable, meteors would shred it before we even got the initial ribbon deployed, atomic oxygen will eat it in a month whereas a low-Earthorbit object would hit it every 250 days. Fortunately there are solutions to each of these problems.
What we will find and discuss in the following chapters is that each of the environmental problems will drive our design. We will also find that we are actually fairly lucky, there appear to be reasonable solutions too all of our problems. As you read through the initial chapters you may see design details that are driven by problems discussed later in the manuscript. I beg your indulgence and trust that I will address these completely later in the manuscript. Let me give you a few examples. The simplest cable design is round. Our cable design is a curved ribbon. The reason for not choosing the simplest design is that the round cable would be destroyed quickly by meteors where as the curved ribbon is more robust. Our anchor is also not a simple hook in the ground someplace in Kansas. Our anchor is located on a mobile, ocean-going platform in the Pacific 1000 miles west of the Galapagos Islands. The reason for this is severalfold. It turns out we can avoid the lightning and wind problems by locating our anchor at this specific point on Earth and by making the anchor mobile we can avoid collisions with satellites and debris in orbit. Each of the problems we may encounter including stuck climbers and a possible severed cable will force us to a specific design. In the end we find that we can solve all of the problems with a single and feasible design. We never found one killer problem that makes the design impossible.
Our society has changed dramatically in the last few decades from the first transistor to the internet, DVD's and supercomputer laptops, from propeller airplanes to men on the moon, from hybrid plants to mapping human DNA. Often great advances in our society take a single, seemingly small step as a catalyst to start a cascade of progress. And just as often the cascade of progress is barely imagined when that first small step is taken. The space elevator could be a catalytic step in our history. We can speculate on many of the things that will result from construction of a space elevator but the reality of it will probably be much more.
At the moment we can at best speculate on the near-term returns of a space elevator. To make a good estimate of the returns we can expect we need to know where we are now, how the situation will change if we have an operational space elevator and what new possibilities this change will cultivate. First, where we are now:
That's the current situation. The next thing we need to know is how the situation will change if we have an operational space elevator. The space elevator will be able to:
Having an operating space elevator would dramatically change our 'reality' picture of space operations as we described above. With this new set of parameters for space operations and the same economic reality we live in, we could reasonably expect the following in roughly chronological order:
These are some of the applications of the first space elevators and all but possibly the last two items would be feasible within the first fifteen years of operation including the manned exploration and colonization of Mars (see the section on destinations accessible with the space elevator). And again I believe these are feasible within the current economic environment when the commercial returns from the cable are factored in. Beyond fifteen years the best way to describe the impact of a space elevator is to say that we would have few limits in our solar system. For speculation of the possible long-term scenarios of space elevator operation I would suggest Kim-Stanley Robinson's Red Mars/Green Mars/Blue Mars series of science fiction novels or Arthur C. Clark's 3001, my guess would be no better than theirs would.
Again, it is hard to grasp the magnitude of impact the space elevator would have on our society but I hope it is clear from our discussion that it would dramatically advance our society both immediately and in the distant future.
This feasibility report on the design and construction of a space elevator addresses all technical aspects of the problem from the deployment of the elevator to its survivability. This is not a definitive study or the final say but a first cut at the concept. What we have found is interesting. As we will discuss, building a space elevator will be challenging but not impossible and the initial elevator could be built for approximately $40 billion, less than many of our larger national programs. Yet the long-term return we (humans) would receive on the construction of a space elevator is staggering, it would literally change our world.
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