Space Debris and Other Threats From Outer Space
next 200 years. This is because space debris collides and generates more debris of smaller and smaller size. Since Liou’s and Johnson’s analysis there have been over 500 additional launches, and many of these had multiple payloads. The main problem is thus not cleaning up after new launches (although this is certainly part of the equation) but rather dealing with the current debris that is slowly grinding out additional debris elements. Even here there is a need for “triage” to address the most crucial problem first and then seek solutions to the rest of the problem later. This most urgent part of the debris mitigation process would be to remove from the low Earth Sun synchronous polar orbits the largest pieces of debris first. This is because these derelicts in space could generate the largest amounts of major new debris elements if there would be a major collision. This we know directly from experience.
    There have been a number of studies conducted by various space agencies about space debris and its future potential increase. On one hand these studies are reassuring and on the other quite disturbing. At one level, these studies confirm there is a huge amount of open space around Earth relatively free of debris. Even in a so-called “congested area” such as the polar region in low Earth orbit, as depicted in Fig.  2.1 , the likelihood of a collision remains extremely small. Figure 2.1 seems so frightening in large part because the scale depicted in this graphic is about 90 million to 1. The worst news of all is that more debris is forming than is returning to Earth due to gravitational effects. In fact there are now well over 6,000 tons of debris in orbit.
    Fig. 2.1
    Charting the significant increase in space debris that has occurred since 1960 (Graphic courtesy of NASA)
    Space Debris in Orbit
    The creation of additional space debris comes from a great variety of sources such as explosions of fuel tanks, launch vehicle upper stages and fairings as well as active and defunct satellites being bombarded by debris, and so on. Further micro-meteorites from space are constantly raining down on the inner parts of the Solar System. These micro-meteorites are responsible for an estimated 12–15 % of the “hits” on spacecraft, based on the latest studies by various research institutes and researchers that monitor this activity.
    Twenty-five years ago the cascade effect of debris crashing into other orbital objects produced a modest amount of new debris elements as can be seen in Fig.  2.1 . But in time things began to change. Today this cascade effect is the largest source of new debris elements as the number of micro-debris elements that are less than 1 mm in size has climbed into the millions. There are perhaps enough of these various debris elements from the smallest chips of paint to the largest derelict satellites and upper stage rockets to increase the “number” of debris elements by a factor of four to six times over the next two centuries, even if there was to be a total moratorium on all future launches. This projection is based on the findings from the Liou and Johnson study in 2006 and factors in the number of new elements since that time including the Iridium-Kosmos collision and the Chinese anti-satellite missile test.
    Orbital debris is not evenly distributed around Earth’s orbit. There are particular bands where these orbital debris are currently concentrated. The worst congestion is in the LEO region and particularly the Sun-synchronous polar orbits. The depiction of the LEO region that is below the Van Allen Radiation Belt is clearly shown in Fig.  2.1 above. The other orbital region such as the MEO region above the Van Allen Belts and the GEO region still contain a number of satellites and debris elements, but relative speaking these are much less congested. This is because that not only are there far fewer debris elements, but also because the debris has a much larger volume in which to
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