Some problems seem quite difficult to solve because of the way they are stated. Critical thinkers, when faced with an apparently insolvable problem, will think carefully about the way the problem is stated and what assumptions might underlie that statement.

For example, there’s no getting around the fact that a spacecraft must reach a high velocity in order to escape the Earth’s gravitational field. One could state the problem as, “How can we lift the extra fuel that is needed to power a vehicle’s rocket engine and so achieve the needed velocity?” But this statement is based on the assumption that rocket engines are the only way to give a vehicle greater velocity. By rephrasing the question more basically as, “How can we give a spacecraft enough velocity to escape earth’s pull?” the door is opened to consider a wider range of solutions.

Ask questions

1) What methods are there for accelerating (speeding up) a vehicle?

2) When an object’s velocity is increased, energy must be supplied. Where does the energy come from?

3) How efficient are these methods?

4) What’s required for actual implementation?

Attack the problem

1.What methods are there for accelerating a vehicle? If you consider your own experience, you can probably think of three common methods for causing an vehicle to speed up:

1) Gravity

2) Use of an engine within the vehicle

3) A push or a pull from an external source

Since our goal in this case is to accelerate the vehicle in the opposite direction of the pull of earth’s gravity, method #1 (gravity) can be eliminated.

2.Where does the energy come from?

In the case of an internal engine, the energy is stored within the vehicle, usually as chemical energy in the fuel or in batteries.

If the energy is supplied by a push or a pull, it comes from an object that is external to the vehicle.

3.How efficient are these methods?

With an internal engine, the vehicle must carry its own fuel (or batteries), as well as the engine itself. These will inevitably add mass and weight to the vehicle, which means that extra energy will be required to accelerate it.

Additionally, engines of any type are subject to the laws of thermodynamics, which limit their maximum theoretical efficiency.

A push or a pull, on the other hand, can result in 100% transfer of the energy of motion, if it can be accomplished without frictional effects.

This reasoning leads to a recognition of the possibly very great advantage of a space tether as a method of accelerating a vehicle once it has lifted itself up out of the Earth’s atmosphere. (Of course the vehicle would still have to carry enough fuel to get that far, but it would NOT have to carry the fuel that would otherwise be needed to enter a stable orbit or escape Earth’s gravity for a longer journey.)

4.What’s required for actual implementation? An approach to a practical space tether must take into account at least these factors:

- Required strength of the tether

- The ability of the vehicle and its contents to withstand the accelerations involved

- The loss of some energy of motion from the large orbiting satellite (as it is transferred to the spacecraft) will cause the satellite to fall into an orbit closer to the earth. Some means must be found of accelerating that satellite back into its original orbit, or of providing power during the tether operation so that it stays in that orbit. This cannot be rocket-based because in that case fuel would have to be shipped up from the surface of the earth, defeating the purpose of the tether.

Fortunately, there is a source of energy that is relatively abundant in space: sunlight. How this energy can be captured and efficiently converted into energy of motion for the satellite is an intriguing problem in itself.

Explore these topics further with these resources:

NASA Institute for Advanced Concepts: Tether Transport System

Tethers Unlimited, Inc.

A little physics and a lot of string