# The Space Elevator

Most of us have seen Star Trek, where journeys through space start with being 'beamed up' to a starship which is already in orbit around the planet. In reality, getting into space involves burning enormous quantities of rocket fuel. How can we ever conquer the universe if we have to expend such energy just getting into orbit?

The space elevator is probably the most innovative solution to this problem. First proposed by Russian scientist Konstantin Tsiolkovsky in 1895 after looking at the Eiffel Tower, it was introduced to the world by science fiction writer Arthur C Clarke, the man who gave us 2001: A Space Odyssey and the invention of the telecommunications satellite. In his book The Fountains of Paradise (1979), he explains the concepts of the space elevator in great detail. We'll take it one step at a time.

### Orbits

Orbit is the state in which you are whizzing around the planet fast enough that you stay up. In an elliptical orbit, your height will vary, while in the special case of a circular orbit, you will stay at the same height above the planet's surface. There are two ways of looking at this. The simpler explanation is to say that the centrifugal force caused by your circular motion cancels out the force of gravity pulling you downwards, so that you stay at the same height. The explanation favoured by physicists is that an object in orbit is actually falling towards the Earth, but due to its horizontal motion, the Earth's surface curves away at the same rate that the object is falling - it keeps 'missing' the planet.

Whichever explanation you prefer, it's a fact that at any height, there's a certain speed (the orbital velocity) that you have to go to stay at that height. If you go faster than this, you will start to go outwards, away from the Earth. Go more slowly, and you will start to fall back towards the Earth. The higher you go, the weaker the Earth's gravity becomes and the slower you have to go to maintain your orbit.

### Geostationary Satellites

Because the orbital velocity is different at different heights, there must be a height at which an object will orbit the Earth in exactly 24 hours. If a satellite is at this height, orbiting above the equator, it will orbit around the Earth at the same rate as the Earth is spinning, so the satellite will always be over exactly the same point on the Earth's surface. From the point of view of somebody on the ground, the satellite will appear to hang in the sky with no visible means of support.

Such a satellite is called geostationary. The critical distance is about 22,300 miles (35,900km) above the surface of the Earth. There are many geostationary satellites orbiting around the Earth, used for transmitting satellite TV channels to eager viewers. For example, the Sky television company operates a geostationary satellite known as Astra.

### A Fixed Point in Space

Since the geostationary satellite is a fixed point in the sky, we should be able to hang a long rope from it, all the way to the ground. Now we can attach a lift (elevator) to the rope and haul it up to the satellite. Hey presto! We've just built the space elevator. Getting into space is now no more difficult than riding an elevator up a tall building.

Unfortunately, the weight of the rope will pull the satellite downwards, but we can compensate by 'hanging' another rope upwards from the satellite into space. This second counter-weight rope will be travelling around the Earth faster than is necessary to stay in orbit at that height, so it will be flung outwards away from the Earth, compensating for the first rope pulling downwards.

### Using the Space Elevator

Once we have built the structure, we can start hauling up raw materials and build a proper space station at the geostationary point. This would be a perfect place to launch spaceships from, because they are already in orbit. All it would take is a little push to launch them into space. Returning spaceships could dock here and the crew could ride the elevator downwards to the surface, without all that messy business of re-entry, heat shields and splash down.

NASA1, the US space agency, reckons that a transportation cost of less than \$10 per kg for shipping things into orbit can be achieved, which compares well with the current cost of more than \$10,000 per kg using conventional rocket technology.

### Practical Considerations

Is this practical or just a dream? There are a few details that may prevent us from building the space elevator in the near future. However, these are engineering details - they don't appear to be insurmountable.

#### Strong Rope

The rope hanging out of the space station will be about 20,000 miles long and needs to be able to support its own weight. There is no known material of that strength at present. In his book, Clarke proposed using a type of 'carbon monofilament' which has a spiral crystalline structure around a central 'screw dislocation'. Such filaments have been constructed, and are incredibly strong, but as yet it has not been possible to make them more than a few centimetres in length.

Since the publication of Clarke's book, another carbon-based material has been developed: the carbon nanotube, which is a net of carbon atoms curled around to make a tube. These have strengths approaching that of diamond.

#### Where to Site the Elevator

Clarke positioned his elevator in the island of Sri Lanka, because of his love for the country. He glossed over the fact that Sri Lanka is not actually on the equator, where it would need to be for the elevator to work. There are suitable places on the equator, but perhaps not as many as you might think. The Earth is not a perfect sphere and this introduces a level of instability - only some sites are suitable.

#### The Lifts

Although the above description has compared the part that moves up and down to a normal lift/elevator, it would have to be much faster than that to cover the 20,000 mile journey in a reasonable amount of time. Of course, after the first 300 miles or so, there would be no atmosphere to slow it down. Something similar to a magnetic levitation train could be accelerated to high speed and would probably serve the purpose.

Ideally, a system that recovers energy from trains as they descend could be used to boost trains going up, so the net energy input would be small.

#### Protecting the Elevator from Collisions

The equator is a region of the planet that thankfully is not prone to high winds such as hurricanes. It would be necessary, however, to protect the cable from aeroplanes. A barrage of balloons could be hung around it at commercial cruising height to make it more visible, and an internationally-agreed no-fly-zone could be implemented.

### The Future

It appears that the space elevator is possible in principle. Although prohibitively expensive at the moment, the cost of technology is falling all the time, so it should eventually become affordable. Indeed, in 2012 Japan's Obayashi Corporation announced plans to build a space elevator by 2050, although they have no idea as yet how much it will cost. Other countries will surely follow this lead.

Once built, humankind can truly enter the Space Age.