23 September 2008

Building a Space Elevator

Japanese scientists and engineers have set their sights on one of the most challenging tasks of a hard science fiction concept: the space elevator.

The space elevator, first popularised by Arthur C. Clarke in his novel Fountains of Paradise (an excellent read, if anyone's interested), appears to be a highly inexpensive way to travel to space, or at least to low-orbit space. It is a popular idea in science fiction and has also made it to scientific journals, being quite solidly based in proper physics.

The basic idea is that we connect a point on Earth to a geostationary satellite directly above it. This connection can be a tower (which is not feasible due to the weight) or merely cables that pulls a lift between these two points that are stationary in the reference frame of an Earth-bound observer. However, if we consider the Earth base as an anchor point and the cables plus satellite as a system, this system will start to swing sideways because its centre of mass is not in geostationary orbit, so we have to extend and attach some sort of mass beyond the satellite to counter this. In the novel, the construction started from the satellite and went both ways - up and down - simultaneously in such a way that the centre of mass stays in the orbit.

Of course, the cables will have a Coriolis force acting on it. On top of that, the length of the cable implies, even if it is of low density, a very strong tension throughout the cable. Therefore, this is the leap in this science fiction concept: a lack of such lightweight yet awesomely strong material. However, these Japanese scientists seem to have the solution: carbon nanotubes. But I think there is still a huge challenge since carbon nanotubes may not be strong enough yet, and mass production of large scale nanotubes are nowhere near a reality. Of course, this still does not take into account the multitude of engineering feats that has to be performed.

This space elevator, if ever built, will serve as a very cheap mode of space travel. Not only does it save the need to launch a space shuttle that gobbles fuel like an F1 race car, it also conserve energy just like a typical lift: as a lift comes down, it'll pull a weight of slightly smaller mass upwards, which will in turn act as a gravitational battery when the next lift goes up. It will also cut down the cost of space travel (to beyond geostationary orbits) by moving the launch site to the satellite, which bypasses a tremendous part of the energy consumption.

Personally, I'm sceptical about the plausibility of pulling this off. The material of the cable remains the greatest challenge, and I think carbon nanotubes are still way off from being a satisfactory material for such a construct. Moreover, as witnessed by the multiple failures in the Large Hadron Collider (LHC), new toys present new problems of their own, and this space elevator can be pretty disastrous if it fails in the wrong way. On top of that, such grand projects are bound to be costly, and I doubt Japan can pull it off by themselves. The LHC is funded by a collection of wealthy nations; I'm not sure if the space elevator will be any less.

Well, on the bright side, even if I'm wrong, at least there's a chance I can visit space in an environmentally friendly way. And what's more, the base station on Earth has to be on the equator (an off-equator geostationary orbit projects a sinusoidal curve on Earth's surface), so there's a chance the base station is in Singapore. If we reclaim southwards furiously enough, that is.


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Daniel said...

The base station wouldn't have to be on the equator. If the base is north or south of the equator, the cable would just need to be longer and hence heavier and more expensive. The cable would extend from the base station to a point on geostationary orbit, and would hence intersect the ground at an angle off-vertical. A space elevator could even be anchored at the North Pole, with a cable tied parallel with the ground. You would need a counterweight beyond geostationary orbit to balance the weight of the cable.

Pandemonium said...


What I meant was that a geostationary orbit that is at an angle to the equator will trace out a sinusoidal curve on Earth's surface.

I don't think an equatorial geostationary orbit and an off-equator base station will work. Think of it this way: the satellite is directly above the equator, and the cable is diagonal to an off-equator base station (and assuming that the centre of gravity is at the geostationary orbit). Then there is a horizontal force that pulls the satellite away from the equator.

Moreover, this will require the cable to be rigid, which is not necessary the case.

Daniel said...

Sure it will work, even with a non-rigid cable. If the cable were weightless, then it is easy -- we are just tethering a point on geostationary orbit with an arbitrary point on the ground, even if it is off-equator. If the cable has no weight, then no problem.

Now let's look at the case of a weighted cable. You mentioned that there would be a "horizontal force" that tries to pull the satellite off its geostationary orbit. That is correct - for the cable to be in tension, the geostationary satellite would have to pull back on the cable with equal and opposite force as the ground pulling on the cable. From your response, it seems that you are puzzled how the satellite can exert a horizontal force. This is because a geostationary satellite alone will not work with a weighted cable (not even with a cable tied to the equator) -- the weight of the cable would pull it down from its orbit. So you need a counterweight BEYOND geostationary orbit to pull the cable outwords by centrifugal force. If the base station is off equator to the North, then the counterweight will lie beyond geostationary orbit and towards the South (instead of directly above the geostationary point). The cable is pulled tight because of the counterweight, and the center of mass of the counterweight-satellite-cable system will not drift from the geostationary point as long as they initially coincide.

If we use the space elevator to launch mass out of earth orbit, we are actually getting the energy from the earth's rotational inertia. If we do it too often, the earth will stop spinning! Haha...

Pandemonium said...

Bleh! Of course, you're right! So long as the centre of mass of the entire structure (satellite + cable) is in an equatorial geostationary orbit, that'll do.

Oops! What a blunder! Thanks for correcting it!