Re: Why are polar orbits in two directions?

From: Gerhard HOLTKAMP (
Date: Tue Aug 08 2006 - 16:48:40 EDT

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    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
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