From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Selecting Orbits
Date: Wed, 10 Jul 1996 21:25:33 GMT

In article <4rseu0$lld@peabody.colorado.edu> fcrary@rintintin.Colorado.EDU (Frank Crary) writes:
>I disagree with the reasons you give. I don't think radiation and
>ground station contact are significant issues for astronomical
>satellites...

This would surprise the operators of IUE, or the Hipparcos project, or the
ISO people, or the folks planning Integral; see below.  If memory serves,
even in LEO, Hubble has to shut some of its detectors down for South
Atlantic Anomaly passages.  And certainly all of the astronomers with
satellites in LEO regularly curse the need to use TDRS to get their data
back, because TDRS scheduling is a colossal pain.

>The main issues are the cost/mass/fuel required
>to reach higher aptitudes, the problem that a low orbit means
>half the sky (a half that changes on an hourly basis) is blocked
>by the Earth, and thermal control for IR and microwave telescopes.

Those are the issues, together with communications, that go into making
the choice between low orbit and very high orbit.  However, they don't
invalidate my main points:  the intermediate choices are out of the
question because of radiation, and communication is a reason why you might
choose geostationary orbit in particular if you're planning a high orbit. 

IUE is in geostationary orbit, for easy communication and a stable orbit
above (much of) the belts.

Hipparcos was meant to be there too, for the same reasons.  When its
apogee motor failed to fire, and it was stuck in GTO instead, the initial
reaction was that it was a writeoff.  ESA did what they could to salvage
the mission -- including hastily arranging for more tracking stations --
and in the end Hipparcos lasted long enough to do its full mapping job,
even though data taken near perigee had to be discarded because of
excessive radiation-induced noise.

ISO is in high elliptical orbit because that's the best that could be done
with a heavy payload and an Ariane 4 launch.  Since its life is limited to
18 months or so by its helium supply, radiation effects are a lesser
worry.  It does have to shut down observing for about 2 hours out of every
12, due to radiation noise in its detectors at perigee. 

Integral, with a Proton launch that can put it almost anywhere, has chosen
a very high elliptical orbit, with *perigee* out beyond geostationary.
This was done specifically and explicitly to keep its detectors out of the
belts.  The fallback plan, if political problems make Proton unavailable,
is to make the best of a bad situation with an Ariane 5 launch and an orbit
rather like ISO's (although with a higher perigee to avoid the worst of
the belts entirely); observing would be done only in the portion of the
orbit beyond 40,000km.

The general rule of thumb here is that very high orbits are preferred if
you can get there, with a slight preference for geostationary if your
hardware isn't too radiation-sensitive.  Some missions can make do with
a high elliptical orbit, at the cost of rapid subsystem degradation
and the need to shut down observations for perigee passages.  And people
use LEO because they have to, not because they want to.
-- 
If we feared danger, mankind would never           |       Henry Spencer
go to space.                  --Ellison S. Onizuka |   henry@zoo.toronto.edu