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"Satellites which orbit under influence of gravity can be expressed as a set of orbital elements which can be regarded as time - dependent fixed entities. In general, two-body approach is enough, but in reality there are saome gradual shifts which can't be ignored. There are some disturbance which occur in satellites which orbitting the earth so the performances of these satellites are not as good as expected. The main cause of this perturbation are the existence of third body, imperfection of earth's symetry, atmospheric drag, and solar radiation pressure. The effect of perturbation to the satellite orbit can be catagorized according to the periodicity of this perturbation. Secular variation shows variations which change the orbit linierly in time, so in the long run it can affect the orbit of this satellite. The most dominant cause of secular variation is the aspherity of the earth. Short term variation is a periodic variation which occur in the orbit in which the time scale is shorter than the orbital period. Long term variation is a periodic variation which occur in the orbit in which the time scale is longer than the orbital period. Atmospheric drag depends linierly on the atmospheric density and High Earth Orbit Satellites are more influenced by solar radiation pressure."

"Tidak hanya di permukaan bumi, ternyata polusi juga sudah menjadi masalah di lingkungan antariksa. Polusi yang berupa sampah antariksa berasal dari sebaran benda-benda langit, yang kemudian ditambah benda-benda bekas buatan manusia seperti roket, satelit, pesawat antariksa yang sudah tidak berfungsi, tetapi tetap berada di orbit, mengakibatkan gangguan terhadap satelit yang beroperasi. Berdasarkan hasil studi pada tahun 1999 diperkirakan terdapat sekitar 2 juta kg sampah antariksa yang tersebar di orbit rendah dengan ukuran lebih besar dari 1 cm 3 . Benda dengan ukuran sebesar 1 cm 3 tersebut sudah dapat merusak kamera dan komponen satelit lainnya. Untuk memecahkan masalah tersebut, para ahli antariksa sibuk melakukan studi, seperti NASA yang terkenal dengan proyek ORION-nya, atau dikeluarkannya aturan-aturan khusus dalam perancangan satelit. (Jika satelit tidak lagi beroperasi tidak akan merupakan salah satu sumber penyebab terjadinya sampah antariksa)"