Most space activities involving humans occur below about 600 km; there are currently few spacecraft in these low orbits because atmospheric drag at these altitudes causes fairly rapid orbital decay.
The more than 3,600 space missions since 1957 have left a legacy of thousands of large and perhaps tens of millions of medium-sized debris objects in near-Earth space. Unlike meteoroids, which pass through and leave the near-Earth area, artificial space debris orbit the Earth and may remain in orbit for long periods of time. Of the 23,000 space objects officially cataloged by the U.S. Space Surveillance Network (SSN) since the beginning of the space age, nearly one-third remain in orbit about the Earth; the majority of these are expected to stay in orbit for tens or hundreds of years. The increasing population of cataloged space objects is represented in Figure 1-2. It is imperative to note that this figure shows only the objects large enough to be repeatedly tracked by ground-based radar. The vast majority of debris is too small to be tracked and is not represented in the figure.
Objects in Earth orbit that are not functional spacecraft are consid-