Re: Why are polar orbits in two directions?

From: Roger (
Date: Tue Aug 08 2006 - 18:36:33 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).
    I just took a look at an Iridium at Heavens-above.  the orbit was listed
    as 86 degrees.  I'm assuming that I won't always see that exact satellite
    as ascending, sometimes it could be descending for my location, right?
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