Space Elevator Going Down

The scoop: An elevator that lifts payloads into space could supplant rocket missions into Earth orbit. But from an engineering standpoint, is it feasible? Nicola M. Pugno, professor of structural engineering and geotechnics at Politecnico di Torino, in Italy, says only if some initial assumptions are put aside and the resulting challenges overcome.

For the opposite viewpoint, read Ben Shelef's My Take, "Where's My Space Elevator," on our sister site Discovery Space.

Using a rocket to launch cargo into space is a tried and true method, having been around since the late 1960s. But it's expensive and time-consuming and so researchers have been investigating other means for breaking out of Earth's gravitational pull.

One new radical idea proposes building a megacable out of carbon nanotubes that would stretch 150,000 kilometers from the ground into the sky. Carbon nanotubes are five times lighter than steel and 100 times stronger. And at that length, the centrifugal force of our planet's rotation exceeds the gravitational tug that would otherwise pull the cable back to Earth. By building elevator cars that can accelerate up the cable, we could deploy payloads into space safely, quickly and cheaply.

But in order for the concept to work, one major design factor must be taken into consideration.

Current estimates, several millimeters in depth and centimeters in width for the size of the cable assume that the carbon nanotube material is free of defects. To date, it is not. Several researchers have shown that, statistically, carbon nanotubes contain small defects called vacancies. These are essentially tiny holes or deformities in the atomic structure that will be obviously larger in an Earth-to-orbit cable.

Think of a rubber band with a tiny pinprick in it. If you stretch the rubber band to a very long length, eventually the hole will render the band useless. It will break.

Current proposals also suggest building a cable that is nearly the same width and depth at Earth's surface as it is way out in space. But a constant size along the cable is not good design because the stress on the cable is not uniform. If that cable is made out of carbon nanotubes with defects, the stress variation could become huge enough to compromise the structure.

Thus, the space elevator is going down if the current design proposal is assumed.

On the other hand, a sufficiently strong cable can in principle be built by adopting a "flaw-tolerant" design.

The idea my team proposes is building a cable that is able to withstand the stress that will cause the most critical defect. This means building a cable two orders of magnitude larger than current proposals and also designing it with the proper taper ratio. In other words, building it so that it's several centimeters in width and several millimeters in depth at Earth's surface but 100 times bigger at the point of geosynchronous orbit.

Until we can engineer such a carbon nanotube cable, the space elevator, in my opinion is out of order. But, never say never.

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