Thread: Sat launches
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Old March 19th 06, 06:31 PM posted to sci.space.station
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Default Sat launches

On Sun, 19 Mar 2006 13:55:38 +1100, "BlagooBlanaa"
wrote:

Considering the number of research satellites that fail or partially fail
due to instrument or other presumably fixable problem, why aren't satellites
(whose mission profiles permit) launched to a near enough vicinity of the
ISS so that astronouts can go out and fix 'em


Because different satellites need to be in different orbits.
Observation satellites are in orbits that pass over (or nearly over)
the poles, so that they overfly all of the Earth's surface over a
given period of time as the Earth rotates beneath the orbit. They also
like to be down low to get as sharp a view as possible. But down low,
you need a lot of fuel to counteract the drag of the tenuous upper
atmosphere, so if you don't have to be down that low, you don't want
to be. Meanwhile, most relay satellites need to be perfectly aligned
with the equator and way up high, where one orbit of the Earth takes
24 hours and the satellite appears to be stationary in the sky, which
in turn makes pointing antenna on the ground easier. But it takes a
larger rocket and a more fuel to get a satellite up to that altitude
and change its inclination to match the equator. And for satellites
where the exact orbit is not critical, such as some astronomy
missions, the sponsoring agency tends to choose the most efficient
orbit its rocket can reach. For NASA, that's a 28.5 degree inclination
orbit due east out of Cape Canaveral. For Russia, its a 51 degree
orbit more or less due east out of Baikonour. Due east launches take
most advantage of Earth's rotational velocity, meaning a heavier
satellite can be launched on a given rocket, or a smaller, cheaper
rocket can be used. That results in satellites being launched in
numerous, incompatible orbits. Building one Space Station that can
service all orbits is more or less impossible.

Brian