Hey guys! Thanks this week go out to Chris and Craig for the topic suggestion. Requests are always welcome, and can be sent to me at firstname.lastname@example.org!
As a physics major, I was required to attend weekly seminars on topics of contemporary physics every Friday afternoon . They were an hour long, and though the department tried their best, the challenge of finding a topic that would actually hold the attention of a group of exhausted undergrads sitting in darkness at the desperate end of a formidable school week was a daunting one. When they found someone to give us a lecture on space elevators, a record-setting number of us managed to stay alert. I did so not out of the shared excitement that my colleagues radiated, but out of a keen sense of skepticism. Following the promptings of many of my educated, astute, and discerning friends without scientific backgrounds, I have reopened my conflict with this topic in order to write about it here, and it has allowed me to reflect on why I was so pessimistic on that cold, caffeinated Friday afternoon amidst a sea of the engineering-inclined. I was too much of a chemist, and though my wistful heart will always belong to physics, my practical mind knows better.
Here’s the thing about the space elevator -- it’s tantalizingly technologically feasible. It’s composed of only a few simple parts that, compared to rockets, aren’t that expensive. We can construct almost all of it with just our current technology. In fact, we have constructed most of these pieces, in part because of the efforts of NASA and the Spaceward Foundation and their annual, funded competition known as the Space Elevator Games, in which very real scientists and engineers compete for very real money towards achieving very real goals. In the second part of this segment, which I will conclude next week, I’ll explain all the things that truly work about this ambitious plan to reach the stars. That way, I can leave you with your child-like senses of wonder intact. But because I am ever the pessimist, I initially crush you by starting my discussion with the one vital setback holding back space elevator development: carbon nanomaterials.
The Basic Design Idea
It’s our first intuition to understand that, in order to break orbit and get out into space, we need to push off from the earth fast enough. But what if instead we could just ride out on a railroad, or be pulled out like on a ski-lift? When we recall that the earth is constantly moving, and we understand the physics of rotational motion, we discover that such a thing is quite theoretically possible.
The elevator would consist of only a few parts, all of which I will go over in much more detail next week: a base on earth, a track that extends out into space, a counterweight on the other end of this line, cars that can move up and down (the elevator junkies call them “climbers,”), and a light-weight way to power them. There are engineering challenges to each element, but for this segment we will only consider the most defiant one: what can we build a ridiculously long cable out of?
In brief, elevators would take advantage of the fact that the earth is rotating. When we spin a ball tied to the end of a string in a circular motion around our hand, the string remains taut, so long as we keep it spinning. Imagine that our hand is the earth, the string is the track, and the ball on the end is the counterweight. If the circumference of the earth at the equator is about 24,800 miles, that means that anyone standing near the equator is moving at a speed of about 1,030 miles per hour. If we were to extend a string about 25,000 miles upward (the theoretical height above the earth’s surface required to make this work) the end would move about 7,600 miles per hour. That’s a lot of tension you could generate on a spinning string, and a lot of inertial (fictitious) centrifugal force. So the track wouldn’t necessarily suspend from the counterweight, the counterweight would pull it out as it was flung around the earth like a tetherball careening towards an elementary school child.
Not even steel can cut it
The demands placed on such a track are high. It must be strong enough to withstand not only the weight of a climber and the massive generated tension force, it must also support its own weight, which could be huge for something that long. And, cyclically, the tradeoff for strength is usually density, so the stronger the cable is, the heavier it usually is. Even steel, the most common material in the world for nearly all modern infrastructure due to its light weight and high strength (not to mention other advantages), doesn’t even come close to meeting the required parameters.
Tensile strength and specific strength
We discuss the strength of materials by putting stress on them and observing when they begin to deform, when that bend becomes irreversible, and when they completely buckle. Tensile strength is an intensive property (meaning it does not change with how much material you test) that quantifies the pressure put on a specimen at the point that it compresses irreversibly. It is the force applied per unit area, and the unit we use most commonly to measure this is called a pascal (Pa) or, if you’re in industry, a pound per square inch (psi). Specific strength is tensile strength divided by density, so something that is very dense will have a low specific strength, and something with a very low density will have a very high specific strength.
Let’s toss out a number – it is estimated that the space cable will require a specific strength of at least 100,000 kN/[kg/m] (ignore that unit if it scares you). Steel alloy has a specific strength of about 250. Kevlar, the lightweight material that we use to protect human life, is about 2,500. And this is where the snag has always been. For years, even our best efforts didn’t come close.
Enter Nanoscience, the hot new wave of the future
Early on in chemistry, one of the of the lessons they teach you is that structure defines function. A bunch of carbon, for example, becomes graphite when its arranged one way, diamond in another. But here’s another funny thing about the universe: it turns out that, when you make something small enough, it acts in a completely different way than its larger-sized sibling. When we started dabbling in the nano-scale world, we realized that tiny deviations in size and structure can dramatically affect chemical, electrical, thermodynamic, and mechanical properties.
In this century, a new class of structures for carbon was proposed, called fullerenes. Fullerenes are nets of carbon atoms (like the above graphite sheet-arrangement) wrapped up into single hollow spheres or tubes less than a nanometer in diameter. These tiny, independently existing structures are classified as nanoparticles, and the bonds between atoms in these arrangements are incredibly strong (stronger than the bonds in diamond). The cylindrical arrangement is called a nanotube, and there is no conceivable limit to how long they can get.
The thrill of discovery
Before nano-structured materials, the space elevator was a distant dream. It wasn’t until 1991 when carbon nanotubes were first synthesized that the dream came bursting forth into drool-inducing existence. This new stable structure of simple, abundant carbon exhibits extraordinary strength, unique electrical properties, and efficient heat conduction.
Remember our 100,000 specific-strength mark? Since they are mostly hollow, carbon nanotubes have an extremely low density, and they have crushed the records for the highest tensile strength of any material ever measured, weighing in at around 60 gigapascals (that’s 60 billion pascals), with a theoretical limit around 300. After all is said and done, that creates a specific-strength that has been experimentally measured at about 45,000 for a single nanotube rope, and a theoretical limit that is several times larger.
And now I will crush your dreams
Currently, the best length-to-diameter ratio of a synthesized carbon nanotube is 132,000,000:1. That’s pretty incredible. However, it only translates to 18.5 centimeters long. It would need to be about 530 million times longer than that to make an elevator cable. We could weave a cluster of them into fibers and make ropes out of them, but doing so significantly reduces the tensile strength, since now you’re relying on the weak fibrous bonds between materials wrapped together rather than the dominantly strong bonds between molecules; it takes much less energy to rip apart a piece of fabric than it does split a diamond in half.
So what’s the real timeline?
When the space elevator guest speaker brought up carbon nanotubes to my physics department on that cold Friday afternoon, I was almost swept away in his enthusiasm. It seemed like any day we could build an elevator to space because of this awesome new discovery. Surrounded as I was by builders and toy-lovers, I felt pulled to do my part. But when I caught a glimpse of that word – centimeters – I suddenly remembered that I’m not a builder, or an engineer, or even a pure physicist. Physicists have an innocence that allows them wrap themselves up in the long-term dream despite the short-term implausibility, and that’s a good thing. They are our most forward thinkers, and they push us to strive.
But I had been through the rigors of purifying compounds that took days of meticulous, expensive work. I knew that 18 centimeters was probably scratched and scraped for with more money, time, and effort than I had spent cumulatively in all my years as an undergrad. Miles are much, much larger than centimeters.
This is the prohibitive part of the space elevator timeline. Some people are saying they could be here in five to ten years (such as the Spaceward Foundation). Others are saying twenty (The LiftPort Group). I am fully aware of the power of scientific and technologic advancement -- I have seen it by observing smartphone development. But the chemist in me sees this as a real challenge. We’re still shy by several orders of magnitude. It’s a dramatic obstacle.
Tune in next time...
I promise you this – the physicist in me is not dead, and it still provides me with a decent amount of wonderment. I will be much more positive next time, and I will end this now on a high note.
The first man to seriously discuss the theory of a space elevator was Sir Arthur C. Clark in the middle of the 20th century. Towards the end of his life, he declared that it would be built, “about ten years after everyone stops laughing.” I hope to tantalize you into coming back next week by leaving you with this goofy video, just to re-direct your laughter.
Most of the space-elevator related information and the pictures contained in this article were obtained from www.spaceelevatorgames.org, supported by the Spaceward Foundation: “A non-profit organization dedicated to furthering space science and technology in education and in the public mindshare,” (www.spaceward.org). It’s a great organization with a very informative site. I encourage you to check it out, if you’re interested.