Electrostatic Tethers

Electrically conducting tethers can also be used for purposes other than changing orbits of satellites and electrical power production. One interesting concept is to use them to remove charged particles from the vicinity of Earth.

The Sun sends out a continuous stream of electrons and ions. Ions are atoms that have an electrical charge because they contain too many or too few negatively charged electrons to compensate for the number of positively charged protons in the atom's nucleus. Earth's magnetic field traps these charged particles like a magnet attracts iron dust, and keeps them locked up in the so-called Van Allen radiation belts. The belts were discovered by James

Van Allen, using data from the first American satellite, Explorer 1. The particles in the Van Allen belts pose a serious threat to satellites and people venturing into them. They can upset and even destroy sensitive onboard electronics, degrade spacecraft materials, and cause biological damage in the cells of astronauts' bodies.

The Van Allen belts vary in intensity with latitude; they are thickest above the equator and diminish in the direction of the North and South Pole. Moreover, the intensity varies with altitude, so that spacecraft in the most useful orbits relatively close to Earth and satellites in high orbits such as GEOs are fortunately little affected. However, satellites transferring from low orbits to higher orbits such as GEOs or on their way to other planets necessarily have to fly through the belts.

However, low Earth orbit satellites are not completely safe. Earth's inner Van Allen belts are symmetrically aligned with the planet's magnetic axis. However, this axis is tilted with respect to Earth's rotational axis by about 11 degrees. In addition, the magnetic axis is offset from the rotational axis by some 450 km (280 miles). Due to this offset and tilt, the inner Van Allen belt is closest to Earth's surface over the southern Atlantic Ocean. The consequence is an increase in radiation levels in region off the east coast of South America. This area is called the South Atlantic anomaly and can affect satellites in otherwise safe, low orbits between altitudes of about 500 and 1000 km (300 and 600 miles) (Fig. 1.16).

Outer Radiation Belt

South Atlantic Anomaly

Figure 1.16: A cross section of the Van Allen radiation belts and the location of the South Atlantic anomaly.

Outer Radiation Belt

South Atlantic Anomaly

Figure 1.16: A cross section of the Van Allen radiation belts and the location of the South Atlantic anomaly.

The explosion of an atomic bomb at high altitude could create an artificial and even more dangerous radiation belt. The highly energetic particles ejected by such a nuclear explosion would threaten satellites that otherwise orbit in low-radiation, low Earth orbits. Such a weapon could thus be used to destroy military observation and communication satellites. In the process it would also damage other satellites, nonmilitary and those of neutral countries, in orbits affected by the artificially created radiation. Experiments performed in the 1960s with nuclear bombs detonated in space showed that the lethal radiation belts can persist for many years (the testing of nuclear weapons in space has since been prohibited by the United Nations Outer Space Treaty of 1967).

It would be nice to be able to flush the Van Allen belts of charged particles, and it may be especially important to be able to get rid of any artificially created radiation belts as soon and as fast as possible. This could be done by employing electrically conducting tethers of several tens of kilometers in length into orbits that bring them into the radiation belts. When these are charged to very high voltage levels, the electromagnetic fields thus generated can scatter the energetic radiation particles, over time sending many of them out of the radiation belts (into the atmosphere or further into space) and thereby lowering the radiation levels.

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