Hitching a Ride: Space Elevators on Earth and Titan
Along with their research into self-driving cars and the wearable “Google Glass” computer, GoogleX is apparently researching a space elevator. Despite their success, many are skeptical that Google can succeed at this project. I'd like to discuss some of the issues involved.What is a space elevator?
A space elevator is a way to get to space without using rocket based propulsion. It’s main component is a tall, thin tower, called a tether, that goes from Earth into space. A vehicle attaches to the structure and can ride up into space. The main advantage of this arrangement is that, once constructed, it will be much cheaper to send objects up into space. Current estimates for the cost of putting a payload into space are well over $10,000 per pound. A viable space elevator can cut those costs by a factor of 10 or even 100. It will make it much easier to get into space and the first country to successfully deploy a space elevator can effectively control space travel.
As we will see, building a space elevator on Titan may well be easier than on Earth. From points on a space elevator, it is possible to easily launch a spaceship that will travel elsewhere in the Solar System or out of the Solar System altogether. It is a technology that Titan residents should consider.How does it work?
An illustration of a space elevator (Courtesy: Wikimedia Commons)
Many satellites are in Low Earth Orbit (LEO), or about 100-200 miles above the Earth’s surface. A space elevator cannot end in LEO, because the structure will be unstable to tipping over, as would any structure that high. A more common approach is to extend the space elevator above geosynchronous orbit (GEO). At GEO, an object stays in orbit by travelling as the Earth is rotating – it is where centripetal force equals the gravitational force. At points above GEO, centripetal force actually forces the object to move away from Earth. So the part of the space elevator and a well-placed “counterweight” above GEO and cancels out the weight of the elevator below GEO. So the entire structure just floats!
What are the obstacles?
The clearest obstacle to a space elevator is that GEO is much, much further than LEO. GEO is about 22,000 miles from Earth; this means that a space elevator would require a thin continuous ribbon of material that considerably greater than 22,000 miles long. Compounding this problem is that every section of the tether must support the weight of the tether that is below it. The point of the tether at GEO will be supporting the weight of 22,000 miles of cable below it! Even strong materials such as titanium or steel will break under the weight of about 10-20 miles of it hanging vertically. Super strong materials such as carbon fiber and Kevlar can extend for 100-200 miles before snapping under its own weight. This is really, really far from the 22,000 miles needed!
Recent studies have examined the possibility of using carbon nanotubes as the material comprising the space elevator tether. Carbon nanotubes are very strong and very light. Simple models show that it can support the weight of thousands of miles of tether. However, the longest carbon nanotubes are currently only a few inches long, which is a long way from how long a space elevator tether has to be!
Another important way to maximize the resiliency of a tether is to taper it. Make the tether the widest at the point it is under the greatest stress- which is GEO, as all the weight below it is pulling down and all the weight above is pulling up. As the taper gets closer to the ends, the tether should get narrower, as it has to support less weight. The thinner parts of the tether contribute less weight, and let the material be longer without snapping under is own weight.
An illustration of carbon nanotubes (Courtesy of NASA)Space Elevators on Titan
Because Titan is about 50 times less massive than the Earth, one might think that a space elevator would be closer to the surface and therefore easier to build. This is not true because Titan rotates very slowly. Titan is locked to Saturn and therefore only rotates once every 16 days. A slow rotation means a weaker centripetal force and therefore a much higher centripetal orbit. Additionally, the placement of a space elevator is complicated due to the interaction of Saturn’s gravity as well as the gravity of the other moons orbiting Saturn.
For these reasons, space elevators present even greater challenges on Titan.