Into Earth Orbit and Beyond

Up to now, the use of space tethers has been experimental and mostly on a rather small scale. This chapter discusses the potential operational uses of tethers. Some proposed concepts apply what we have already seen but on a much grander scale, some combine multiple space tether types into one system, and some utilize them in completely new ways. There are plenty of ideas, some more feasible than others, but all fascinating and inspiring. The potential tether application closest to Earth is for use in de-orbiting old satellites at the end of their useful lives, making them fall back into the atmosphere to burn up. That would lower the collision risk for operational spacecraft and prevent the generation of clouds of space debris frequently caused by inactive, deteriorating spacecraft. Cleaning up the space around Earth is a serious issue. At the moment there are about 10,000 satellites brocket upper stages zooming around our planet, of which only 4j2rcent consists of active spacecraft. Objects with a low perigee altitude remain in orbit for years, but ultimately get sufficiently slowed down by the tiny aerodynamic drag to fall back to Earth and burn up in the atmosphere. However, satellites in orbits above about an altitude of 700 km (450 miles) generally stay there for hundreds or even thousands of years. Because of the breakup of old equipment, collisions between objects, explosions caused by leftover propellant, and constant erosion by micro-meteoroids, the amount of debris orbiting Earth is currently over a million identifiable pieces. With precise radar measurements from the ground, some 70,000 objects the size of postage stamps have been detected in orbits between altitudes of 850 and 1000 km (530 and 620 miles) alone—all bits of frozen nuclear reactor coolant leaking from a number of old Soviet satellites. In addition, more than 200 objects were thrown out of the Mir space station during its first 10 years of operation, most of them rubbish bags.