417 Miguel Yagües Palazón Environmental Challenges in Outer Space in the... and equatorial orbit at 35,786 kilometres, but extends to ± 200 kilometres and to ± 15 degrees. One factor that plays an essential role in mitigating LEO waste is the effect of atmos-pheric drag. This element is “the resistive force of the molecules of the atmosphere”.97 Given this resistive force, the atmospheric drag exerts a negative and a positive influen-ce on satellites. The negative effect implies that the atmospheric resistance generates a disturbance in the trajectory of the satellites, slowing them down and forcing them to use fuel to maintain stable orbits. The atmospheric pressure becomes even denser if the pressures from solar radiation are added to the atmospheric drag.98 The positive effect, on the contrary, means that once the fuel is exhausted and therefore the speed that keeps it in orbit, the satellite progressively loses altitude and ends up re-entering the atmosphere and once inside it, disintegrates and vaporises after being exposed to high heat flows between 90 and 75 kilometres. The disintegration is not complete because it is estimated that between 20 and 40 percent of the waste ends up reaching the surface of the Earth.99 Atmospheric density decreases as altitude increases and the effects of the drag finish gradually fade to reach 965 kilometres.100 Thus, an object at an altitude between 350 and 400 kilometres would take a year and a half before it re-enters the atmosphere; several decades if it is located at 700 kilometres, and more than two centuries if it is above 800 kilometres.101 Unfortunately, satellites are neither distributed equidistantly nor remain close to the densest areas of atmospheric drag; the highest concentration is between 650 and 900 kilometres and between 1,400 and 1,500,102 regions where the North American constellations are located: Iridium (790 kilometres), Globalstar (1,410 kilometres) or the Russian Rodnik (1480 kilometres), that is, in areas where the drag is either very weak or simply non-existent. In fact, the number of satellites that orbit at altitudes below 500 kilometres is very small. In relation to LEO, there are two events that have significantly increased the amount of waste. The first was the aforementioned ASAT test conducted by China in January 2007 that collided with the obsolete Chinese satellite Fengyun-1C at 865 kilometres and at a speed of 32,400 km per hour. The impact caused a twenty percent increase in the 97 U.S. Army Training and Doctrine Command, Army Space Reference Text, chapter 5, Space Division, 2000, <http://fas.org/spp/military/docops/army/ref_text/> consulted: 24-1-2017. 98 Priester, W., Roemer, M., & Volland, H., “The Physical Behavior of the Upper Atmosphere deduced from Satellite Drag Data”, Space Science Reviews, 1967, vol. 6:6, pp. 708-710. 99 Роскосмос: Тгк “ Прогресс М-27м” прекратил существование (Roskosmos: Tgk “Progress M-27m” prekratil sushhestvovanie), 8 May 2015, <https://www.roscosmos.ru/21474/> consulted: 15-9-2018. 100 U.S. Army Training and Doctrine Command. óp. cit., note 97, chapter 5. 101 National Research Council, Orbital Debris. A Technical Assessment, Washington, D. C.: The National Academies Press, 1995, p. 29. 102 Yanofsky, D. & Fernholz, T. Quartz. This is every active satellite orbiting Earth. <http:// qz.com/296941/interactive-graphic-every-active-satellite-orbiting-Earth/> consulted: 15-9-2018. Revista del Instituto Español de Estudios Estratégicos n.º 12 - Año: 2018 - Págs.: 397 a 431
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