Rymdfysik 2009-10-26Institutionen för FysikUmeå Universitet

The Space Elevator

And how it is moving out of Sci-Fi and into Reality

Erik Sandström – ersa0005@student.umu.se

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Introduction:Space. A vast area beyond even our wildest imaginations, and something that mankind have been interesting in for probably as long as we've existed on this world. We've speculated about it, pondered upon it's meanings and secrets, and dreamed of reaching it. The last, was a dream that was realized in 1961 when the Soviet cosmonaut Yuri Alekseyevich Gagarin was sent into space in the rocket Vostok 1.

Now, as it was almost 50 years ago, rockets are the only possible way for us to leave earth, and it comes with a hefty price tag. During these last 50 years, the cost for a rocket launch has remained largely unchanged, and lies at around $22.000 (approximately 150.000 SEK) per kilogram of cargo, i.e. personell, materials and the spacecraft itself. One can easily see that with such a costly means of transportation, reaching the stars in large scale is not viable for us, but during the course of the last few years, a possible alternative mean of leaving earth has moved out of Science-Fiction and closer to reality. That, is the idea of a Space Elevator.

The idea of the space elevator was first penned by Konstantin Tsiolkovsky in 1895. His idea was to build a large tower reaching up into geostationary orbit, with cables to secure it and keep it upright. Ofcourse building such a large building that high, was just not feasible since mankind had yet to find a material strong enough to support its own weight such extremes. Since then several scientists have made their contributions to the idea of a space elevator, improving it slightly and bringning it closer to reality with more feasible methods or the invention of new materials or technologies. Yet even during most of the previous century, the space elevator was seen mostly as a science fiction idea. Not the feasible alternative to rockets we might almost see it as today. It was first in 1991 that the idea of a space elevator really took its first step out of science fiction and towards reality. That, was with the discovery of Carbon Nanotubes. Today, it looks closer than ever before to actually leaving the theoretical stages and moving into construction. When, rather than if, it is built, the cost of reaching orbit around earth will drop from the hefty $22.000 down to an estimated $2-4 per kilogram.

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The Space Elevator The space elevator is a massive structure reaching from the surface of the earth way up into space. The main purpose of the space elevator is to transport people and materials up towards geostationary orbit around earth at much lower costs than rockets. The concept for the space elevator is a cable that runs from an station, a so called anchor station, somewhere near the equator of the earth up to a counterweight as far out as 91.000km, for the purpose of keeping the cable straight and the center of mass of the whole structure firmly in geostationary orbit. Elevator climbers would then run up and down the cable, enabling transport of people and cargo at but a neglible fraction of the costs of todays space shuttle launches.

Now let’s take a closer look at each part of the massive design from the anchor station up to the counterweight.

Figure 1: The simplified design of the Space Elevator (http.martianchronicles.files.wordpress.com)

The anchor station would be the tether for the space elevator on the earth-side, being the point from which lifts could be received as well as sent space-ward. At the moment there are two different ideas for the anchor station – stationary and mobile. The stationary anchor-station can be thought of as a tall tower. Ideally it would be built as high as possible, preferably high up in the mountains or similar, and around the equator. The mobile anchor station however, would probably be a large vessel moving out at sea. Both possibilities have their own pros and cons. While a mobile station would be able to avoid potentially harmful things such as bad weather and space debris, it would not have the same access to a reliable power source, as is the case of a stationary station. Nor would it have the advantage of being built at higher altitudes – decreasing the required length of the cable, and thus the mass of the whole construction. Of course this doesn’t mean that there might not be future inventions and ideas that make the choice easier. For example it might not be required to have a power on earth if the space elevator can be made self-reliant by use of solar cells. At the same time we might not know if multiple cables and other shielding materials can be used to

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decrease the wear and tear of space debris and foul weather. Today the main design leans towards a mobile anchor station situatied somewhere around the Galapagos islands due to the low percentage of dangerous weather situations in that area.

The cable running between the two stations is the main part of the space elevator. It connects earth with space and enabled elevators to travel the approximate 91.000 km into space. So it is easy to see why it is the most important part of the space elevator’s design as well as why it is the part which gives the developers and scientists the greatest headache. It proven itself to be a most difficult part to develop, design and manufacture. It has been estimated that the current plausible designs of the elevator to work, the cable need to be strong enough to handle tensile pressures of around 120 GPa. Not only that, but it needs also be relatively lightweight and inexpensive to mass-produce. At present, only Carbon Nanotubes (CNT) has come anywhere close to those criteria’s, with an estimated tensile strength of around 60-150 GPa, and even now scientists have only managed to make short lengths of it, up to about 5 millimeters. At this year’s Space Elevator Conference, a Japanese group showed a CNT-ribbon of over 40 meters, but the tensile strength was only around 750 MPa, far below the required criteria. It did however come to show that progress was being made towards making increasingly longer CNT-ribbons. Perhaps next year a group of scientists will be even closer to reaching the required tensile strength and win the multimillion dollar prize promised by NASA for this, the so called ‘Tether Challenge’.

Figure 2: A cut-out of the proposed design for the CRT-cable. (Edwards - The Space Elevator)1

The climbers are almost as important as the cable. It is the lifters that are the whole purpose of the construction. The lightweight shuttles of different sizes would be capable of ferrying cargo such as material and people to and from the stations on earth and in space. The main problem with the climbers is how to power them. Since rocket propulsion is out of the question, another way to power the lifts needs to be found. At the moment the best and most probable idea is to use a laser beam to send what can afterwards be converted into energy for propulsion to the lift and use it to propel it up along the cable. As with the cable, this is a challenge proposed by NASA, and called the ‘The Power Beaming Challenge’, where the first group that can show a way of beaming enough energy over a long enough distance is entitled to the price. Apparently a group of scientists almost managed to win this price in 2007, and it is believed that the criteria for the challenge will be reached before 2010.

1 A link to the dokument written by Bradly C. Edwards can be found on the last page.

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Figure 3: A simple picture of the idea behind the climbers. (Edwards - The Space Elevator)

The counterweight can be seen as acting like an orbital station in geosynchronous orbit, its main purpose to keep the center of mass firmly in geostationary orbit, and keeping the cable be straight and taut. Without a counterweight the cable would have to be far longer or thicker for the same purpose, and that would be an ungainly design. The counterweight will probably be made up of the materials and vessles used in the early stages of the construction, before cables can be sent down to ferry more materials up. Afterwards it is possible that a space station or similar installation will act as the counterweight.

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Dangers and precautionsThe Space Elevator will most certainly become a massive thing, when (rather than if) it is built. Such a large building is therefore also subject to a multitude of threats and dangers , a number of which can cause major problems and probably even catastrophic failures. Below is a list of some of the more severe dangers the elevator might face.

Figure 4: Some of the possible threats to the Space Elevator (Edwards – The Space Elevator)

Collisions: Space is filled with a lot of debris and satellites, as well as meteoroids. It is without a doubt certain that a space elevator will come on collision course with something sooner or later. As mentioned earlier a mobile anchor-station might be able to avoid most of the larger threats, but even small objects can cause major issues to the cable or the climbers. One idea protect the structure from this kind of threats are to move the orbits of intervening satellites, and use lasers or some other kind of defensive equipment, to deal with the rigors of space. Also increasing the safety factor of the cable's tensile strenght and thickness is a simple and possible solution to this problem.

Corrosion: In the upper atmosphere, atomic oxygen are steadily attacking most materials, weakening the structure. This might also be the case for the cable unless it is made of materials resistant to the corrosion occurring up there. For example it might be possible to coat the cable in platinum or gold, which both can remain basically untouched by the corrosion of atomic oxygen. Also materials which are more common to us, like aluminum, have some resistance, at least slowing the process down to a pace which is easier to handle. That way the cable could easily be repaired before it would break apart.

Weather: Weather can easily add to the wear and tear of the cable, and should therefore be avoided if possible. It is therefore preferable to keep the construction close to the equator, where strong and violent storms are rare. Beyond that, making the cable thinner further down would decrease the effects of strong winds upon the cable, enabling it to better last under such duress.

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Sabotage: We must admit that the space elevator might just be seen as a large target and cornerstone of the capitalist world. Therefore it is foolish discard the possibility of sabotage and terrorist acts. Although this might not be the most prominent danger, it might certainly carry some weight upon a decision on where to build the elevator, as well as require security personnel both during construction as well as when it is finally up and running.

Vibrations: Vibrations are also a cause for great concern. As with most strings, the cable of the space elevator will have a resonance frequency. If for example climbers running up and down the cable can excite it to this frequency it might increase its vibrational energy, putting further strain on the cable’s tensile strength, not to forget that it might easily throw off the climbers moving along it. This problem can be decreased or perhaps even almost completely avoided by use of a suitable dampening system.

Radiation: Not only will the elevator itself be under attack by the forces of the universe, but also the people riding the climbers. Radiation is a major cause for concern. The protection given by the earth’s magnetosphere decreases drastically as you move further out toward geosynchronous orbit. This means that the cable, and climbers, will be under ionizing radiation as they near the station. To prevent this kind of issue structurally weakening the elevator, and causing problems for the travelers, some kind of shielding will most probably be in order. This will also cause an increase in weight of the climbers, reducing their effective speed or cargo capacity. There are those that believe that in the near future only cargo will travel up and down the elevator, while people still head up into space in rockets.

So as one can easily see, there are a multitude of possible threats to the Space Elevator, all more or less capable of causing a catastrophic failure to the construct itself, causing it to break in some way or other. So far all threats we've been able to find have had only simple and plausible solutions. If however, the cable would break, the end result is more or less the same regardless of where along the cable it breaks, or what caused it. But for simplicity we split it into two different situations: The cable breaking close to the anchor station, and the cable breaking closer to the counterweight.

A cut near the anchor point might be seen as the less catastrophic of the two since the whole construct would just drift away into space because of the rotational velocity inherent in this kind of construction. The damage to the globe would be limited if not negliable, whereas the Space Elevator itself would be lost, along with any and all materials and people currently situated in it.

A cut further up however, would cause much more severe damage to the surface. As the break occurs, the top part would, just as in the previous example, drift of into space, but the part of the cable still tied to earth would start to coil around the earth before smashing into the surface. It is however disputed as to how severe this might become. Some say that in this event, the cable itself will crash into earth along the equator, thousands of kilometers of cable causing massive destruction to the planets surface. Others say that it is highly unlikely since the cable will be designed so that the high-altitude parts of the cable will break and burn up in the atmosphere, whereas the rest of the cable will just drift to the ground with a force comparable to, if not less than, a sheet of paper, considering it's ribbon-like shape being slowed down by air resistance.

In the case of failure such as mentioned above, the climbers would most certainly be flung off, but depending on the height, it is possible that they, instead of falling to earth, are flung out into an orbit around the earth. In that case it should be relatively easy to have spacecrafts ready to rescure the now orbiting climbers.

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Building the Space ElevatorSo how will they build such a massive and seemingly complicated structure? Today there are two different ideas for how to construct the space elevator. Both start in the same way, with a rocket being launched carrying the materials required for the first part of the cable: A ribbon a few centimetres wide and a few microns thick. After the first cable has been lowered down to earth and anchored down at the base station, the two ideas differ. The simpler yet more expensive idea is to use multiple launches of rockets to ferry materials up into space for the construction, and for all parts of the cable to be dropped one after the other from space, and for the construction site to drift further out towards the altitude of 91.000km from earth. The other, seemingly less expensive idea, yet perhaps more difficult, is to use the intial ribbon to carry a first climber up into space with more material. At the same time the climber will bring with it the 2nd cable from the surface. This process will then progress until the number of ribbons is sufficient for the cable. At the same time ofcourse the construction site in space will drift outward, to keep the counterweight from drifting away from geostationary orbit. It is estimated that regardless of method it would take between two and three years to finish building the space elevator.

The first elevator would also be limited as to the size and number of climber it could carry up and down its cable. However the construction of the first space elevator would enable us to send materials much easily up into space and also enable space exploration and exploitation at a much grander scale, easily making the construction of more and larger space elevators a possibility. Not only at the equator around earth but also on the moon and perhaps even on other planets in time. There is no doubt that the Space Elevator would be yet another catalyst for the progress of the human race.

Today's progressSo when might we see one of these up and operational? That is a much discussed topic actually. There are some that firmly believe that we won't be able to build Space Elevators before somewhere around 2100. Others say 2050, 2030 and some even 2018. The reason behind the different answers is that we can't really say for sure how quickly scientific progress will make massproduction of the cable possible. With present technology we can already build all the other parts of the Space Elevator, so its really only a wait while the cable and the final design is brought forward. As I mentioned earlier, NASA has two contests up for claim by anyone that manages to produce a good energy transfer method, and the cable production technology, so we know by now that the US is sponsoring progress in science with the aim of enabling the construction of a Space Elevator. Also the government in Japan has put alot of money and effort into projects related to the Space Elevator. Especially into the development of stronger materials, specifically into research related to Carbon Nanotubes. Who knows, maybe in as little as 10-15 years we can travel into space at a decent cost and experience by ourselves all that space have to offer.

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Sources:•http://en.wikipedia.org/wiki/Space_elevator

•http://science.howstuffworks.com/space-elevator.htm

•Edwards – The Space Elevator http://www.niac.usra.edu/files/studies/final_report/521Edwards.pdf

•http://www.liftport.com/wiki/id,space_elevator/

•http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html

•http://wiki.spaceelevator.com/Open_Wiki/Documents/Research_Papers

•http://techlahore.wordpress.com/2009/05/22/ (The picture on the front page)

•http://www.hplusmagazine.com/editors-blog/notes-space-elevator-conference-august-13-16-2009

•http://science.nasa.gov/headlines/y2000/ast07sep_1.htm

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