Thursday, March 3, 2011

The Space Elevator: Past, Present, Future

There are two main reasons why, for the past sixty years, man has launched rockets from the surface of the Earth. 

The first reason is to place satellites into an orbit around our planet.  The moon is a Celestial body that orbits the Earth in an elliptical fashion at an average of about 380,000 km away from the Earth.  Other than this large body and other small space debris that orbit the Earth, all other bodies that do so were placed in their respective orbits by man to serve some purpose.  One can think of an orbit as a path that a falling object takes continuously around a larger body.  Much in the way that a baseball’s path curves to fall towards the Earth once it is thrown, an orbit is in a perpetual state of falling.  The difference is that the orbiting body is moving much faster than the baseball (think kilometres per second), and the baseball faces the resistance of air, whereas the drag forces in space are negligible. 

It boggles the mind, but we (and when I say we, I mean America and Russia) have transported thousands of satellites into space.  Most satellites are used for communication, be it internet, phone, or television, while some are used for Earth observation leading to weather prediction, the tracking of oil spills, and the marvel that is Google Earth.  Most satellites are located hundreds of kilometres above the surface of the Earth (note that an airplane’s altitude rarely exceeds 12 km) in an area known as LEO (Low Earth Orbit).  

Another popular zone is at the GEO (Geosynchronous) altitude of about 35,800 km altitude; this is a special altitude for which the orbital period of a satellite is the same as that of the Earth.  If one were to stand on the equator and stare up at an equatorial GEO satellite, one would observe no motion at all.  It would be as though the satellite were attached to the Earth by an invisible cord.  The typical operational life of a satellite is fifteen years.  As such, the majority of satellites currently in orbit are decommissioned and commonly referred to as space junk.

The other reason for which man sends payloads to space via rockets is for interplanetary travel.  Such ventures are done in the name of research, observation, exploration, and discovery.  Sometimes these payloads contain astronauts, as in some trips to the moon, but often these are unmanned missions, or probes, investigating, for example, Mars or Saturn’s moons.  The missions are very exciting, sometimes providing answers to important scientific questions, other times demonstrating what we can achieve when we put our minds to it; the 1969 moon landing was perhaps the defining moment, technologically speaking, of the twentieth century. 

An even more formidable task would be sending man on a return trip to Mars.  The greater distance to Mars as well as the greater Martian escape velocity means that the payload leaving the Earth would be much more massive than that for Apollo 11.  Also, the trip length (years instead of days) would make the challenge for the astronauts far greater.  However, it is the issue of the large mass which makes the Mars mission unrealistic today.  Transporting mass away from the Earth using rockets is prohibitively expensive.  The cost for satellite placement in GEO is in the area of $10,000 US per kilogram of payload; it is substantially greater for interplanetary travel. 

In today’s space industry, every gram is questioned.  In order for a payload to achieve the escape velocity required for interplanetary travel, over 90% of the mass leaving the Earth must consist of fuel; this is a chemical constraint associated with rocket travel.  For this reason and others, the rocket will probably not be the principal mode of travel used to escape the Earth’s gravity fifty years from now.  

The most promising upgrade from the conventional rocket appears to be the space elevator, which could reduce the cost to GEO to as low as one hundred or even ten dollars per kg (reducing satellite placement costs by at least a factor of one hundred).  An operational space elevator, which would carry no fuel, would open up space to mankind; it would bridge the gap between us and the vastness we see when we stare up at night, both figuratively and literally.

The idea of a space elevator came out of Russia in 1895, but was really just a figurative dream.  Yuri Artsutanov proposed a physical model for the structure in 1960.  Artsutanov showed how if an extremely long ribbon were extended from the surface of the Earth somewhere along the equator to a large satellite beyond GEO, the centripetal acceleration due to the spin of the planet would counteract its gravitational pull.  This ribbon would be in tension, providing a vertical railroad to be scaled by climbers containing payloads. 

In 1975, the feasibility of such a model was analyzed in depth by Jerome Pearson.  Pearson determined that while the setup would be physically possible, no known material could withstand the tension that would manifest in the ribbon due to the counteracting forces.  Indeed, a suitable material, we may call “Unobtanium”, would require a strength to density ratio fifty times greater than that of steel.  Steel is a hard material to lap fifty times over in a race.

In 1991, Sumio Iijima discovered Carbon nanotubes, which, if synthesized properly, could one day solve the material challenge that has long plagued the space elevator project.  Still in its infancy, carbon nanotube technology is evolving at a steady pace today, and may be able to meet the space elevator ribbon’s specifications in a couple of decades.  The current design for the space elevator, detailed by Bradley Edwards in 2003, includes climbers whose wheels grip the ribbon on both sides, and whose electric motor receives energy via laser from the Earth.

There are more than one hundred scientists and engineers in the world today doing research directly related to the space elevator; I am one of them.   My Master’s thesis in Mechanical Engineering was entitled: “Space Elevator Dynamics” (it is available at the McGill University library and in my parents’ living room).  I studied how the structure would move.  The ribbon itself would oscillate like a very long elastic band.  It would also sway back and forth due to external forces such as the wind in the Earth’s atmosphere and lunar gravitational fluctuations. 

Also, when climbers scale the ribbon to bring payloads to space, they would cause the ribbon to sway due to what is known as the Coriolis force.  Although understanding the dynamic behaviour of this amazing structure seems a bit premature today, it will be essential before construction, potentially some decades from now.  Other areas of research include laser-beaming technology and the avoidance of the orbiting space junk mentioned earlier.

Perhaps another hundred or so engineers and enthusiasts are participating in what is known as the space elevator games.  Two challenges with million-dollar prizes were issued by NASA.  The first contest is a material challenge.  The prize would be awarded to a team producing a ribbon that exceeds the strength per unit density of the top commercially available material by 50%.  No team has come close to attaining the mark thus far. 

However, the second challenge, which involves building laser-powered climbers and racing them up one kilometre of ribbon, at an average rate of at least 2 m/s, was met by a Seattle-based company in 2009.  Although the actual ribbon of the space elevator will likely span 100,000 km, we have to start somewhere.  The desired climbing rate for a climber will be in the neighbourhood of 60 m/s, making a trip to GEO a weeklong transit (with spectacular views).

The enormous material challenge facing the space elevator ribbon is one that will be met over time.  Carbon nanotube technology is being developed for many other uses, such as the aviation industry. 

The only other huge technological hurdle for the space elevator appears to be surviving the onslaught of orbital debris (both manmade and celestial) that will likely be on or near the path of the ribbon on a regular basis.  The debris breaks down into two categories.  The larger items, say twenty five centimetres in diameter, can be easily detected by radar.  If one were to fall in the path, the ribbon would need to be manoeuvred properly to avoid the collision.  The space elevator design calls for the ribbon to be fastened to an oil tanker that floats in the ocean.  On occasion, this mobile base will need to move some kilometres so as to bring the ribbon out of harm’s way. 

Smaller debris (the second category), which cannot be detected, will occasionally pierce the ribbon.  The ribbon must be designed in such a way that small holes weaken it locally, but do not sever it.  The details concerning space debris avoidance were recently tackled by a wonderful organization called ISEC (more on them soon).

There is a small shopping list of other engineering problems that must be addressed.  In addition to carrying payloads, climbers will have the task of repairing and even thickening the ribbon as they ascend it.  Other risks to the structure include lightning and hurricanes at the base; situating the ribbon surface connection in a calm climate somewhere along the equator will minimize the likelihood of such occurrences.

So, with a conceptual design seemingly devised, and a set of a few challenges mapped out, why might we have to wait until the year 2050 or later to see construction of the space elevator commence?  There are a few reasons. 

One deterrent is the large capital required now, for a return on investment that is very far away.  The only way that companies will invest in technologies related to the space elevator is if they also yield spin-off technologies that are relevant for an application today. 

Another reason why the project is moving decidedly slowly is that one hundred scientists and engineers are not nearly enough.  What is needed is a larger pool of researchers, but also a large infrastructure in which they can work.  ISEC (the International Space Elevator Consortium) is currently attempting to create this infrastructure.  ISEC organizes the annual space elevator conference in the United States, but also tracks the progress made at space elevator conferences held in Europe and Japan.  Incidentally, the 2011 Space Elevator Conference will be held August 12-14 at the Microsoft campus in Redmond, Washington.  

Finally, what is needed is greater public awareness for the project.  When I give seminars on the space elevator, be it to my physics class or to the general public, the majority of audience members have never heard of the concept.

Arthur C. Clarke’s Science Fiction, “Fountains of Paradise,” follows the building of the first space elevator, and the many challenges surrounding it.  The impact of the space elevator on society was depicted as dramatic, and, the impact of the real one will no doubt be so.  Without question, the space elevator would have the kind of effect on space exploration and exploitation that the internet has had on communication.  And, let us not forget that the internet was made possible by space exploitation. 

I will be paying very close attention to this project over the coming decades, and you may wish to also.  In my experience as a teacher, few ideas have captured the imagination of my students like this one.

8 comments:

Anonymous said...

Great post The Engineer! Santi.

The Engineer said...

Thanks Santi! Let's build this thing!

I.D. said...

Hi Engineer! I'm presently doing a research project on waste management and your paragraph describing how the space elevator could be used to bring down space junk gave me an idea. Could we use it instead to bring up some waste from the earth and then catapult it into space? Could that be feasible and cost effective, let's say, for radioactive waste (which are such a problem because they require to be buried in isolated areas and contaminate the environment)?
Merci d'avance pour votre opinion,
Isabelle :)

The Engineer said...

Isabelle,

Hmmm... Junk is junk no matter where you put it. The space elevator would bring down the cost of transporting it to Earth orbit or to elsewhere in the solar system. Ethically, I think it is wrong to litter space around our planet, and even more wrong to litter space elsewhere.
That being said, radioactive waste is a particular case where the volume is not that high, but the human risks of keeping it here are (certainly, people worry about it). I think placing radioactive waste beyond Geo altitude is sensible, though not ideal. Overall, it is better than polluting our atmosphere with greenhouse gases, which is the alternative to nuclear energy production for large scale in the short to medium term.

I.D. said...

Dear Engineer,
I understand your ethical point of view of such a solution to waste elimination. This might look like we would want to send our problems elsewhere in a careless manner to get rid of them, to refuse to face them. However, health of people/environment is at stake... in that conditions, don't you think space would be a big safe place for radioactive matter (and other matter) to continue its decay (which can last thousands of years) without harming anyone/anything? Or....could some complications arise: let's say, some still radioactive matter's trajectory be deflected by asteroids and come back to crash on earth (no, I don't watch to many sci-fi movies...)
Isabelle :)

The Engineer said...

Let me first say that a cost assessment and risk assessment for this endeavour is quite complex. There are certain unknown factors, like how crowded will Earth orbit get over time? My first instinct is that a major catastrophe is more likely when dangerous debris is orbiting at thousands of km/hr than when stationary on Earth... but, then again, we cannot predict when/where the next major earthquake will be.
As a final note, I suspect (hope) that by the time the 1st space elevator is operational (say, 2050), a sizable chunk of energy production will be via solar panels.

tonyon said...
This comment has been removed by a blog administrator.
wintow said...

Thanks for the interesting read!