In his last years, Sir Arthur C. Clarke predicted that the space elevator will be built "about 10 years after everyone stops laughing". We believe people will stop laughing once a proper tether is demonstrated, and we think that this will happen within 5 to 10 years.
The other point to note is that "will be built" should really be "can be built", because as always, technology is only a necessary condition -- not a sufficient one. It's a matter of political and popular will for mankind to venture into space. The space elevator offers us almost infinite promise, but it's up to us to take the initiative and benefit from this promise.
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 characteristic we are most concerned with is the strength-to-weight ratio of the material, also known as its specific strength, or tenacity. Steel wire, for example, can be very strong, but is also very heavy, so is not a good candidate. Spectra 2000 fiber, in comparison, is a little bit stronger than steel, but what's more important, is about 8 times lighter, making it a much more suitable candidate, though not quite good enough.
We measure strength-to-weight using a unit called a Yuri. (After Yuri Artsutanov, who proposed the Space Elevator in 1960). One Million Yuris, or a Mega-Yuri is somewhere between steel wire (0.5 MYuri) and Spectra (3.5 MYuri). To build a Space Elevator, we need a tether that is about 40 MYuri strong. In a pinch, we could perhaps do with a tether that is 30 MYuri strong. The best of today's materials is less than 4 MYuri strong.
(In other fields, it is more common to see the units of GPa-cc/g and N/Tex, both of which are equivalent ot a MYuri)
Our first goal is the production of a CNT-based tether that is at least twice as strong as today's best materials. We'd like to see this done in the coming years, perhaps as soon as 2010. This is also the target of the Spaceward tether challenge.
It is hard to predict how long it will subsequently take to get to a strong enough tether, but based on the measurements of real CNTs, this will be a relatively quick process, so we estimate people will finally stop laughing in or about the year 2015... Adding in a 5-year detailed design period, this puts the beginning of construction around the year 2020 - 2025.
While the Space Elevator is the world's longest structure, it is one of the world's simplest! All in all, we estimate the entire program to cost less the $10B - a small amount compared to the development and construction costs of programs like the Space Shuttle, the ISS, or even "regular" rockets.
1960 article by Yuri Artsutanov
proposing the Space Elevator.
(Thanks to Roger Gilbertson)
Space Elevator tether challenge, 2006
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:
- In Blue, we show numbers measured at the Elevator:2010 competition.
- In Red, we show spec-sheet data for commercial off-the-shelf products.
- In Purple, we show published measurements and analysis results.
- In Green, we show Space Elevator goals.
New!!! Two recent data points, #10 and #11!
The Tether Strength Table
Data Point References
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 5, 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 30 MYuri and with a safety factor of 50%, this means a Specific Strength of 45 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!