By far, the tallest hurdle we have to clear on the way to building the Space Elevator is the development of a suitable tether material. The main metric we are concerned about is the strength-to-weight ratio of the material, also known as its specific strength, or tenacity. Steel wire, for example, can be made very strong, but it is also very heavy, so is not a good candiate. Spectra 2000 fiber, in comparison, is a little bit stronger, but what's more important, is about 8 times lighter, making it a much more suitable material, though not quite good enough.

In the chart below, we track the state of the art in advanced materials. The diagonal line is a "reference curve" that depicts a 50% yearly improvement in the specific strength of materials, starting off at what is available today (2005). This reference curve is not a prediction - it is simply something to compare with. As far as material properties go, this is an extremely fast growth curve. Since Carbon Nanotubes are a new class of materials, we do not expect real-life results to follow this smooth gradual curve, but rather advance in larger steps. With a theoretical limit of about 200 GPa-cc/g, the potential of CNTs exceeds our goal, and just like early metals or plastics, the initial improvement in material properties will be very fast.

In the next few years, we will have a more complete picture of whether CNT materials were able to advance at a 50% yearly rate, and whether we encounter fundamental roadblocks that will prevent further advances.

In addition to the reference curve, we plot the following types of data points:

New!!! The latest result out of Cambridge, #10 below!

The units for measureing specific strength (or tenacity) are confusing - traditionally, people use either GPa-cc/g for the former, and N/Tex for the latter. These two units are the same in fact, and are equal to 1E6 N-m/kg, which is what the pure metric unit should be - force per linear density.

To end confusion once and for all, we propose to name the pure metric unit for both specific strength and tenacity as a Yuri (in honor of Yuri Aatsutanov), and so a tether with a linear tensity of 0.001 kg/m that breaks at 1000 N will have a breaking strength of 1 Mega Yuri.

When building a Space Elevator, the higher the load that the tether can take, the smaller the Taper Ratio, and the lighter the overall Elevator. Beyond a taper ratio of 3, the Space Elevator starts to get very bulky compared to its payloads. Based on this, in order to build the Space Elevator, we need a material we can load with at least 45 MYuri and with a safety factor of 50%, this means a Specific Strength of 65 MYuri. There's no "must have" requirement though - the stronger the material, the less the overall elevator will weigh. With a 100 MYuri material, the Space Elevator tether will weigh less than 1/3 what it would have weighed if constructed from a 50 MYuri material. With a only a 25 MYuri material, the elevator will be heavier by an additional factor of 5.6. Looking at it another way, a 100 MYuri elevator can lift its own weight to orbit in 6 months. A 25 MYuri Space Elevator needs almost 20 years to do the same.

In short - we need to reach the Green numbers on the right side of the graph using macroscopic tethers. To date, theoretical modeling and micro-scale measurements are promising, and the latest results out of Cambridge are very impressive.

We will keep watching!