Tom Wagner asks: >Why are some polar orbit satellites moving in one direction and others in >the same part of the sky going the opposite way? Does it only have to do >with where they are launched? >Also is there a term used to describe these opposite directions? You probably mean why when watching these satellites in the night sky some of them go northbound and other are southbound. Of course if a satellite moves north and reaches the North Pole there is no other way to go but south (but it does so on the opposite side of the Earth). If a particular (polar orbiting) satellite can be seen northbound (this is called an ascending pass) or southbound (this is called a descending pass) depends on its orbit plane. The Iridium constellation for example uses six different orbit planes and you have always some satellites on ascending and some on descending passes over your sky (moving in various planes). Earth observing satellites are often in sun-synchronous orbits in which the orbit plane keeps its relation to the sun-direction. Such satellites will always pass in a certain direction at certain times of the day. Envisat for example is in a descending morning orbit. It will always pass a particular site southbound in mid-morning and northbound in mid-evening. So if you observe Envisat in the evening sky it will always move from North to South. Any polar orbit can be reached from any launch site. It just depends on which time of the day you launch (you simply wait until your launch site has turned so it is under the desired orbit plane). Gerhard HOLTKAMP Darmstadt, Germany ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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