Pedagoguery

Orbital dynamics is frequently counter-intuitive for those of us born and raised on a planet. For example, if you speed up, you end up slowing down. This is because your increased energy lifts you into a higher orbit, and therefore your orbital speed decreases. Tethers in orbit introduce many additional peculiarities, and electrodynamic tethers provide startling capabilities.

If you were to take two large masses, attach them together by a tether, and place them in orbit, an interesting thing happens. The masses will move apart from each other and eventually pull the tether taught. Since the lower mass is in a faster orbit, it pulls the upper mass along. Likewise, the upper mass will act as a drag on the lower mass, slowing it down. The end result is that the tether is perfectly aligned radially outwards from the orbited body. A small pseudo gravity is felt at both masses – inward at the inner mass and outward at the outer mass. The strength of this pseudo gravity depends on the orbital speed of the system and the length of the tether – the faster the orbit and the longer the tether, the stronger the force.

These principals work when you are in close orbit around a massive body, such as low Earth orbit. For an interplanetary mission, they would not be sufficient, but adjustments can be made. For example, the system can be spun around its center of gravity -- thus providing pseudo gravity and helping potential astronauts maintain bone density.

Effects get really interesting, however, when the tether is electrically conductive and the surrounding environment has a reasonably strong magnetic field. Movement of a conductive object in a magnetic field leads to induced currents. Those currents in turn interact with the magnetic field to apply a force on the object carrying the current. The direction of the force depends on the direction of the current. Normally, the current will be from the upper mass to the lower mass. This current can be tapped for energy, but there is a drawback. The energy has to come from somewhere, and that somewhere is the orbital energy of the system. So tapping this energy leads to a drag on the object's orbit, causing the orbit to decay. Not necessarily a desired outcome. However, if you pump energy into the tether, reversing the current, the interaction with the magnetic field leads to thrust, increasing your orbital energy and thus your orbit.

These effects lead to some pretty startling capabilities. Imagine a space probe that utilizes a tether to explore the moons of Jupiter. Jupiter has a very strong magnetic field, allowing extraordinary maneuverability of an electrodynamic tether without expending any rocket fuel. All they would need is a large battery system to store electricity while the satellite decelerates, and a medium to small generation capacity to provide extra electricity for acceleration along with whatever is stored in the battery. The battery and generator would also provide all the energy that the satellite would need to operate.

Other potential uses for tethers can be found closer to home. Picture, for example a space station composed of two pieces connected by an electrodynamic tether. If the station were equipped with solar panel arrays, it could easily adjust its orbit up or down as needed, never having to worry about orbital decay due to atmospheric drag. By the same token, low Earth orbit is getting quite crowded with dead satellites, expended boosters, and other space junk. It all eventually falls back to Earth, but a satellite in a 1000 km orbit typically takes 2000 years to fall back to Earth. A simple electrodynamic tether can accelerate that. If deployed at the end of the satellite's operational life, it can rapidly decelerate the satellite to allow it to burn up in the atmosphere, no longer cluttering up near Earth space.

Next issue, I will talk about the dynamics of accretion disks.