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Artificial satellites are rarely placed in retrograde orbit.〔http://www.wseas.us/e-library/conferences/2009/istanbul/TELE-INFO/TELE-INFO-08.pdf "Most satellites are launched in a prograde orbit because the Earth's rotational velocity provides part of the orbital velocity with a consequent saving "〕 This is partly due to the extra velocity (and fuel〔) required to go against the direction of the rotation of the Earth. Most commercial Earth observing satellites use retrograde orbit,〔http://www.ioccg.org/training/turkey/DrLynch_lectures2.pdf "Most Earth observing satellites are launched so as to have retrograde orbits."〕 and almost all communication satellites use prograde orbits.〔http://www.sac.gov.in/Satcom_Overview.doc "Orbits of almost all communication satellites are prograde orbits, as it takes less propellant to achieve the final velocity of the satellite in prograde orbit by taking advantage of the earth's rotational"〕 ==Examples== Israel has successfully launched seven Ofeq satellites in retrograde orbit aboard a Shavit launcher. These reconnaissance satellites complete one Earth orbit every 90 minutes and initially make a half-dozen or so daylight passes per day over Israel and the surrounding countries though this optimal sun synchronized orbit degrades after several months. They were launched in retrograde orbit so that launch debris would land in the Mediterranean Sea, and not over populated neighboring countries on an eastward flight path.〔(【引用サイトリンク】title=Shavit (Israeli launch vehicle) -- Encyclopedia Britannica )〕〔(【引用サイトリンク】title=Shavit )〕 The USA launched two Future Imagery Architecture (FIA) radar satellites into 122 degree inclined retrograde orbits in 2010 and 2012. The use of a retrograde orbit suggest these satellites employ SAR techniques. Earth-observing satellites may also be launched into a sun-synchronous orbit, which is slightly retrograde. This is typically done in order to keep a constant surface illumination angle, which is useful for observations in the visible or infrared spectrums. SEASAT and ERS-1 are examples of satellites launched into sun-synchronous orbits for this reason. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Artificial satellites in retrograde orbit」の詳細全文を読む スポンサード リンク
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