# Re: Why are polar orbits in two directions?

From: Roger (roger@efn.org)
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?

Roger

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