Wired has an article about building a beanstalk. This means a structure reaching from someplace on the Equator right into geosynchronous orbit and beyond; depending on your point of view, this can be considered a tower, an elevator, or a cable. There are several proposals. Placing an asteroid into orbit and spinning out two cables – one inwards, one outwards – seems to be the most practical way. Elevators would go along the cable and lower the cost of getting stuff into and out of orbit to less than 1% of current levels.

The only material theoretically able to resist the enormous stresses and support its own weight is made of carbon nanotubes. Only in the past few weeks some researchers have been able to make longer strands of nanotubes (20 centimeters) in larger quantities, but a beanstalk would need billions of tons of meter-long nanotubes bound into a light but strong matrix. This kind of manufacturing may become possible over the next few decades.

As any SF reader knows, this sort of thing has been featured in several books. Arthur C. Clarke’s “The Fountains of Paradise” is the first I recall offhand. I’ll post a more comprehensive list later.

Kim Stanley Robinson’s “Red Mars”, “Green Mars” and “Blue Mars” go into much more detail about the technical and political aspects of building such a thing. But he also discusses the scary aspects: who will control the elevator? What if it’s brought down by terrorists?