In 1895, Konstantin Tsiolkovsky, a Russian rocket scientist, proposed using a free standing tower that would be placed on Earth’s equator and rise to the height of 35,800 km, or geosynchronous orbit. Once the tower was in place, items could be taken aloft by an elevator on the tower at very low cost. There were two problems with the concept. First, there is no known material that could support a tower that tall. Second, there would be enormous side to side stresses that would threaten to bend the tower in two – or three, or four.

In 1959, Yuri Artsutanov, another Russian scientist, proposed something a little more novel. A satellite above geosynchronous height (144,000 km out) would be tethered to a station on the ground with a very long cable, using the force generated as it rotates around the Earth to keep the cable tense and relatively straight. The problem with the cable concept is the opposite of that with the tower concept, namely that there was not a material strong enough in tension to hold together under the enormous loads generated. As anything gets long enough, it can’t hold itself together under its own weight if enough centripetal force is generated.

For a long time, the concept of a space elevator was laughed at because the materials simply did not exist to make it feasible.

Enter the nanotube.

In the 1990s, something called a carbon nanotube was developed. As small as 1/100th the size of a human hair, a carbon nanotube has a tensile strength as much as 100 times that of stainless steel and 30 times that of Kevlar while at the same time being extremely light. How strong a material is in relation to its weight is called specific strength.  Carbon steel is pretty strong, used in all manner of demanding applications. Carbon nanotubes have a specific strength roughly 300 times that of carbon steel, meaning they can be used to create very long structures without their own weight tearing them apart – long structures like a massively long cable connected to a satellite more than a 100,000 km up.

More recently, something called a diamond nanothread has been developed. It promises to be stronger than the carbon nanotube, offers superior flexibility, and has a very strong ability to return to its original shape if it’s stretched out. In other words, it’s a perfect candidate for building the cable for a space elevator.

Potential Issues:

In 1996, NASA launched the second Tethered Satellite System on Space Shuttle mission STS-75. The idea behind the mission was to dangle from the orbiter, a small satellite on a tether into the ionosphere and use the highly charged layer of Earth’s atmosphere to generate power for spacecraft. The experiment worked – a little too well. It was only 1 km short of its maximum deployment distance  of 20.5 km when the cable snapped. Later analysis would show that the voltage and current grew to values so high that it arced through a minute imperfection in the cable insulation to the deployment boom and basically melted it through on the spot. A friend of mine was working the science mission control console in Huntsville at the time. Normally a sleepy shift, he suddenly saw the satellite’s position and velocity data shoot off the charts. He knew something was wrong as the satellite plummeted into Earth’s atmosphere.

Now in the NASA mission, the idea was to generate power. For a space elevator it’s not. Yet, like Ben Franklin and his kite, the potential exists for a charge, possibly significant, to be generated. That has to be accounted for. No one wants the satellite flying away and a tens of thousands km long cable falling all over the Earth.

Another potential issue is an impact with the cable. An aircraft, spacecraft, even an unlikely meteor or space debris could sever the cable with disastrous consequences. Regardless of how unlikely any of these occurrences may be, there must be contingencies to account for them. One proposal is to not have a cable, but more of a ribbon, connecting a satellite to the earth. While that proposal adds redundancy for meteor/debris impact, it doesn’t eliminate the threat.

A Half Measure

thoth-technology-space-elevatorEarlier this year, a Canadian company, Thoth Technology, applied for, and was granted, a U.S. patent for an inflatable space elevator. The idea would be to inflate the structure, made up of Kevlar cells, to a height of about 20 km, or 12 miles, at which point spacecraft could take off and land from it in the upper reaches of the atmosphere. While it wouldn’t go high enough to take cargo to orbit, it would significantly reduce the fuel and launch capabilities to do so.

Will a space elevator ever be a reality? If so, when? In the early 1990s, Arthur C Clarke was asked the same question. “Probably about 50 years after everybody quits laughing,” he answered.  At the Space Elevator 2nd Annual Conference in 2003, he changed his answer to “ten years after everybody stops laughing… and I think they have stopped laughing.” Now ten years have come and gone since Clarke gave his updated answer, but certainly a lot of progress has been made.

Whether it’s with a space elevator or reusable space craft, one thing seems certain: the cost of getting to space will be dropping dramatically in the foreseeable future.

Inflatable elevator photo credit: Thoth Technology

 

Posted by Darren Beyer

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