Here comes the hard part. Last week I gave a brief overview of the theoretical Space Elevator, focusing most of my attention on the massive obstacle presented by the tether, and the practical wall that stands between where we are now and the tantalizing future of carbon nanotubes. This week, despite a pessimistic tendency that I like to call “realism,” I will delve into the parts of this transporter that are technologically feasible with what is accessible today. It will be a stretch for me. While I normally enjoy images of bearded, lab-coat-wearing physicists in hard hats, building robots powered by lasers (I’m a huge nerd, so it’s a comforting picture), I just can’t overlook the silliness of the rampant enthusiasm:
“While the initial Space Elevator will be used mostly for cargo (to build the space hotels that will become the destinations of space tourism), second generation Space Elevators will most certainly carry passengers.” – Spaceward.org
I just can’t help it, phrases like “space hotels” and “space tourism” evoke images of overweight, costume-wearing Star Trek enthusiasts discussing the construction details of the newly upgrade warp core on the starship Enterprise. “If the tether were to be struck by space-debris or targeted by a space-terrorists, manned climbers will naturally have the ability to soft-land,” they would say, full of conviction.
In the current economic climate, interest in the space elevator projects is primarily held by hobbyists and tinkerers, and the last three years of the space elevator games have been met with plenty of failures and waning optimism. Nevertheless, I made you all a promise to be positive, and what kind of scientist would I be if I was dishonest?
Revisiting the scientific principles that make this idea feasible
Recall last week when I described the analogy of a ball spun around on a string to explain how a track could remain “suspended.” If we connect an anchor on earth to a weight out in orbit, the rotation of the earth will keep the connecting “string” taut. Conceptually, we have experienced how this fundamental concept of physics works, even though it’s often incorrectly referenced. It is the concept of centripetal and centrifugal force.
Centripetal (“towards the center”) force is responsible for causing objects to travel in circular motion. Imagine what would happen if you suddenly let go of the string as you were spinning the ball around – it would fly off in a horizontal direction, no longer following its circular path. That’s because it’s the string, anchored to your hand, that is providing an inward-pulling force that prevents the ball from flying off tangentially. The string is constantly changing the direction of the speed of the ball.
Centripetal force is a real force that is created by real interactions between bodies: the string and the ball. Centrifugal (“away from the center) force, however, is felt, but it is not real. When you turn a corner in your car, the friction between your tires and the road is providing the physical force that alters your path from a straight one to curved one. Losing that friction (a wet road) will cause you to slip. But you might notice as you turn a corner that objects in your passenger seat will tend to slide outward. This is not because there is an outward-pressing force generated by circular motion; it’s simply a matter of how you’re looking at it.
The frame of reference that you’re sitting in (your car) is moving. But someone hovering above your car in a helicopter wouldn’t see objects being pushed outwards, they would see those objects continuing along the straight path that they were originally traveling in, while your car changes direction. There is not enough friction between objects and the seat of your car to pull them in the same circle, so though it seems as if they are pulled outward, it is you that is being pulled inward. The force is real in the sense that, from where you’re sitting, you witness it. It is not real in the sense that it actually generated from a physical interaction between objects.
Sitting on a spinning earth, we are most certainly on a moving reference frame. An object that is traveling upward to break orbit, but firmly tied down to a rotating earth, would feel an upward pull eventually, just like water in a bucket being spun over your head. Using this understanding, we can design a lift that would actually feel a pull out into space, overcoming gravity at a certain height.
This tenuous setup implies some care. The spinning tether must be firmly attached to the ground, and it must maintain a constant height. In other words, the top of the elevator must be in geostationary orbit, meaning that it always stays in the same spot in the sky at all times. We have watched the moon orbit the earth, and we notice that it moves around in the sky quite a bit. An elevator like that would not do. It must have the same orbital period as the earth, meaning it would have to rotate back around to exactly the same spot in the sky every 24 hours. The would be accomplished by setting its base at the equator.
Ideas for base stations come in two categories: mobile and stationary. Mobile bases would likely be large ocean vessels that have the ability to withstand and maneuver through high winds, rough seas, and storms. Stationary bases would likely be placed at high-altitudes, such as on the tops of mountains or on manmade towers. This latter option, while losing the ability to maneuver away from danger, would have the advantage of having an easier access to reliable power sources, and the decrease in required cable length, which could decrease the weight limits and cost.
Some early ideas involved capturing an asteroid from space and using it as a counterweight, which evokes some strange visions of cowboy-wrangling-astronauts that is appealingly American. Other groups envision a space station on the other end, but that presents plenty of challenges in construction and transportation that are far more complicated than the elevator itself. Most people admit that, initially, some kind of man-made counterweight would have to be launched using conventional methods. Newer ideas suggest assembling it from the equipment used to build the tether, or positioning a spool of cable in exactly the position in the atmosphere where the string would fall towards earth, be caught and connected to the base, and then the spool would “fall upwards.”
The design wouldn’t be elevator-like in the sense that it would employ multiple moving cables to pull a non-mechanical carrier. Not only would extra cables add extra weight, but the varying force along the length of the cable (because gravity and the centrifugal force oppose each other) would necessitate that the cable be constructed with varying widths at different positions. That’s why scientists involved in these projects call them “climbers” instead of “cars” – they would be robotically engineered to actively climb up the cable with their own power.
Robotics is not a field that the world’s top scientists are shy about. Designs are rapidly advancing, and it is now a simple matter to design some kind of platform that would latch onto the cable with friction-gripping gears and roll its way upward. That kind of a design has been implemented for many an annual space elevator competition with success. The challenge lies in figuring out how to power it. Fuel adds weight. Electricity would require miles of wiring, which would dangerous, impractical, and essentially worthless. But what if we thought of a way to wireless transmit electricity to the car, or better yet, locally generate it? Prepare to add this to our long list of reasons why lasers are awesome.
The propelling power of lasers
Solar power is becoming more and more prevalent in modern society, and it’s currently possible (though still extremely inefficient) to design light-collecting panels that are as thin as strips of paper. It has been suggested that a climber could be covered in these light-weight, sun-to-electricity converting grids known as photovoltaic cells, and the sun would power the climber as it ascends. But solar energy conversion there is always the question of storage -- what happens when the sun isn’t out? So instead, other groups -- including a Seattle based company called LaserMotive -- have been honing a technology they call Power Beaming.
Solar cells are tuned to accept a broad spectrum of colors of visible light at a low intensity (because they come all the way from the sun, and they’re dispersed). But Power Beaming instead requires photovoltaic panels that are tuned to accept a very specific wavelength of light shone at them at a very high intensity, and then convert that into electricity. In essence, plug a laser into the ground and shoot it into the sky at a few downward facing photovoltaic cells that are strapped to the climber, and the electricity will be generated on-board. This works because lasers are specifically designed to create a focused, high-intensity beam that doesn’t disperse over long distances. Whereas copper wire is priced per unit distance, a laser beam will cost the same and contribute about the same amount of intensity output regardless of how far it has to travel. In this way, you have designed something that acts like wireless electricity transmission.
Because of this technology, LaserMotive has been dominating the competition at the annual Space Elevator Games, winning a level one prize of $900,000 for driving a vehicle using their power beaming to a height of one kilometer at a speed of more than two meters per second (their average speed was four, but it was still short of that year’s ambitious goal of five)
The exciting and the ridiculous conclusions of this design
I’m sorry, enthusiasts. I don’t mean to trash you. I believe you are an integral part of our society and its ability to advance technologically. The physicist in me wishes I could share the unblemished hope that you carry, but I still see this as something that we can’t even begin to take seriously until we advance significantly forward in nanotechnology, and unfortunately, the competition for nanomaterials is heavy in earth-bound industry.
Nevertheless, I will end with another Arthur C. Clarke quote, just to prove that the child inside me is not yet dead to skepticism:
“Every revolutionary idea — in science, politics, art, or whatever — seems to evoke three stages of reaction. They may be summed up by the phrases:
(1) "It's completely impossible — don't waste my time";
(2) "It's possible, but it's not worth doing";
(3) "I said it was a good idea all along."
Here’s to hoping we see the third phase sooner than I expect us to.
Most of the information and the pictures contained in this article were obtained from www.spaceelevatorgames.org, supported by the Spaceward Foundation (www.spaceward.org) as well as from www.spaceelevatorblog.com. See you back again next Thursday. Email any suggestions or requests to firstname.lastname@example.org!