Orbital tethers: Overview

Introduction

‘There is a saying amongst the rocket scientists that goes along the lines of ‘there is no such thing as a free launch’ this is true, even modern technology has not got away from this problem entirely, you simply have to put energy into the object to get it out of a gravitational well, methods that try to subvert this by changing the craft’s mass, or bending space still consume energy, but the most efficient way to date is by using orbital tethers.

The principle of an orbital tether is quite clear, and can easily be though of by a short thought experiment which I will just describe. We know that different orbits above a celestial body take different times to complete, those closest take very little time, ones further away slower, so for any rotating body there must be an orbit at some distance from the body that takes exactly the same time to complete one orbit as the body completes one rotation. When this orbit is in the same direction as rotation it would be possible for a satellite to remain fixed over one spot on the planet below, this is what we come to understand by the term ‘geostationary orbit’.

Once we are happy with this idea it only takes a little more imagination to construct the idea of an orbital tether. If we simply extend in our mind’s eye a line from the satellite to the surface of the planet we have simply constructed the tether, as the ground has the same angular velocity as the satellite however high up, the line, or in reality cable can be secured firmly to the ground, and effectively provide a ladder into space. One or two extra thoughts needed to be added at this point, firstly the cable is heavy so unless it is counterweighted it will drag the satellite to the ground, secondly the centre of mass of the structure (if we consider the cable not attached to the planet) must also be in this geostationary orbit.’

Orbital tethers the reality

The real obstacle between the first thinkers of this concept, and them seeing the fruits of there thoughts is primarily material science, though the orbital tether requires astounding engineering feats in manufacture and orbital calculation, the main problem is getting a cable strong enough to support itself over thousands of kilometers and not to snap with the strain of the all the cable hanging below it. Conventional materials such as the best metal alloys simply are not upto the job, the taper required for some would mean a taper width at the top about the same diameter as the terrestrial planet you are try to connect to, so a new material is needed which has great tensile properties so that not so much material is needed, which reduces the overall strain on the structure as well as reducing the required taper. Fortunately such materials are available.

The nearly ubiquitous solution is the use of carbon nanotube materials, these can (if the individual nanotubes are long enough) create tethers of the 30 thousand km or so length required for such a project without diameters exceeding more than a few metres (both at top and bottom). Other materials such as diamond sheet and silicon carbon compounds also have appropriate strengths and are sometimes also weaved into tethers to refine the physical properties.

Even using these materials, and thinning the tether down to the minimal requirement the amount of material required is still in the gigatonnes kind of range. To supply a project of the magnitude new methods of manufacture are required, it is infeasible to send processed material up from the planet as the massive energy cost is prohibitive, and the rate would be slow, even using advanced flight technology.

Instead the usual methods for supplying the material are either to process them from an asteroid, and send the material out to the construction site, or more commonly push the asteroid into the correct orbit for construction and use it for the base of the project. Because of the specific composition of the required construction material, selection of the parent asteroid has to be made to contain sufficient amounts of carbon to firstly build the cable length required, and secondarily to have it with such abundance as to make its collection quicker, other factors such as water content are also important, as separation of the elements can provide simple chemical rocket motors to move the asteroid, or provide hydrogen for fusion reactors to drive the process.

Tether synthesis is effectively a non-stop process, with advanced robotic technology the construction plant can not only regulate the machines behind the production of the cable but also to coordinate the collection of carbon to adequately match the rate of production, or even to expand its processing ability by creating more machines from the regolith. Even with such powerful and self organizing machines behind the process the sheer enormity of the project mean that many years will pass before the cable is of length, and though this is the major engineering challenge it is by no means the conclusion of the project.

During the production of the cable, and whilst the tether is descending its way towards the planet’s surface, the asteroid’s and tether’s centre of gravity will start shifting away from synchronous orbit, when the cable is short this might not be such a great problem, but if the cable starts dragging in the atmosphere, then the whole structure will rapidly decay in orbit. Therefore the asteroid and cable must continuously adjust to maintain the correct centre of gravity, generally the shift required is so slow and steady that even technology such as ion thrusters would be enough to maintain the project’s stability. A simple and commonly used method to asses drift is simply to use a ground based laser pointing upwards towards some target on the asteroid, by using this beam as a reference point coordination to correct for drift can easily be made.

When the cable is of full length, it needs to be secured to the ground, not to maintain its orbital stability, but to stop the atmosphere from thrashing it about. Usually at the base complex a large gantry is erected to support the tether in the last few hundred meters of its descent and physically secure it. Additional features integral to the cable may also be used to damp the effects of the turbulent atmosphere, such as magneto damping, where large currents are passed through coils in the structure, and force the cable into line.

Once the whole structure is secured it is only a matter of development to turn it into an elevator into space.