A Nanotube Space Elevator

The chemical element carbon is one the most abundant elements in the universe. The structure of carbon lends itself to make millions of different compounds; it has four valence electrons, allowing it to bond with up to four other atoms at a time. In one form of carbon—graphite—each carbon atom is bonded to three other carbon atoms in a plane of hexagonal rings; the bonds within the plane are extremely strong, even stronger than in diamond.

Starting in the late 20^th century, a great deal of research has been focused on fullerenes, a form of carbon that is similar in structure to the plane of hexagonal rings in a single layer of graphite, but formed into shapes such as hollow spheres (buckyballs) and cylinders (nanotubes). In particular, carbon nanotubes, which are essentially tiny tubes of carbon with a diameter of just about a nanometer (a billionth of a meter), have exceptional properties that seem to give them unlimited potential for use. They are incredibly strong and light; compared to steel, carbon nanotubes have a far greater tensile strength (a measure of how much a material can be stretched until it breaks) at a fraction of the density. In addition, carbon nanotubes can have a range of electrical and thermal properties, making them useful for a variety of applications, such as composite materials, computers, electronics, optics, and biotechnology.

One potential use for carbon nanotubes may be as part of an elevator from Earth to space. Although it sounds like a fantastical idea of fiction, such an elevator may very well be possible. The concept itself is relatively simple—imagine a long cable that is anchored to Earth and connects to an orbiting object in space; if the cable is taut, vehicles could climb it, transporting people and other objects into space.

In order for the cable to remain in place, it would be tethered to a location near the equator and would extend through geostationary orbit (about 22,000 miles above Earth's surface) to a counterweight in space. If the center of gravity of the structure is at geostationary orbit, where objects orbit the planet at exactly the same rate at which Earth rotates, the elevator system will stay in the same position relative to a fixed point on Earth's surface. The cable would be held taut similar to the way a ball on a string extends away from your hand as you rotate the string. If the upper end of the cable in space is moving faster than the velocity needed to maintain orbit, it moves outward away from the planet, pulling the cable taut; as Earth rotates, the tension of the cable provides the centripetal force to keep the counterweight in orbit. Because the cable would be extremely long, it would need to be made of a material that is both extraordinarily strong and light. No such material yet exists, but carbon nanotubes may provide a solution.

To learn more about the chemistry of carbon, check out Ringed-Carbon Compounds and Diamonds: The Science Behind the Sparkle.

To learn more about centripetal force, check out Circular Motion.

To learn more about orbits, check out Satellites Orbiting Earth and Earth System: Satellites.

To learn more about materials, check out The Impact of Technology: Nylon and Elements of Steel.

Find viewing ideas on NOVA Teachers for this video segment.

• What is nanotechnology?
• Describe several different arrangements of carbon molecules. What properties of carbon atoms explain their use in nanotechnology?
• In what other ways do you see nanotubes being used?
• Science fiction stories of the past have predicted technologies that we have today. Can you identify any stories where this has happened? Do you think Arthur C. Clarke's idea will come true?