Sunsync orbits - was RE: Titan 4 launched

Philip Chien (kc4yer@amsat.org)
Sat, 21 Dec 1996 02:44:01 -0500

>Bill Krosney wrote:

>>The Keyholes are visible light imaging are they not?  Why else
>>would it be in a sun-synchronous orbit?

Ted Molczan replied:

>Well, COBE (Cosmic Background Explorer) went sun-synch, as
>I recall, so that its solar panels would almost always be
>illuminated.

It was much more than that.  Recall that COBE had a dewar filled with
liquid helium.  In a sunsync orbit you've got one side of the spacecraft
bathed in sunlight, and one side which is always in darkness.  Solar panels
be designed to rotate to follow the sun and it's not worth putting a
satellite in to a sun-sync orbit just for that reason.  But it is extremely
desirable to keep a cryogenic dewar in darkness to minimize exposure to any
heat sources.

The *primary* reason for sun-synchronous orbits is Earth observations
(whether in the white or black world) since you get a constant sun angle.
But there are other advantages for various applications.

>The main reason that I do not expect today's launch to
>turn out to be a Lacrosse, is that the first two did not
>go into sun-synch orbits. Of course, there is always a
>first time, but I doubt that will happen.

Agreed.  Since a sun-sync orbit is a retrograde orbit there's a penalty to
be paid in terms of launch vehicle performance, so you only go sun-sync if
it's necessary.  With Lacrosse the key requirement is to see up to polar
(or near polar) regions, The maximum look-range is typically dependent on
what you think is the maximum latitude of where you think your adversary
has installations of strategic importance.


Philip Chien, KC4YER
Earth News - space writer and consultant
note new E-mail address - pchien@digital.net