Re: Why the HELL haven't we gone back to the moon yet?
From: Knud (astro2_at_amoebius.org)
Date: 10/08/04
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Date: Fri, 08 Oct 2004 19:56:43 +0200
Jeff Findley wrote:
>
> The thermal environment in LEO is much more challenging because it changes
> radically over the period of that 90 minute orbit. Remember the problems
> with the orignial Hubble solar arrays causing serious problems for the
> control system? You don't have those sorts of short term thermal cycling
> problems if you can avoid LEO.
>
> Also, the cyclic thermal environment of LEO creates "focus breathing"
> problems, also known as ?OTA breathing? or ?HST breathing?. This is a well
> documented problem, so searching for these terms will yeild PDF files like
> this:
>
> NICMOS Focus and HST Breathing
> http://www.stsci.edu/hst/nicmos/documents/isrs/isr_98_015.pdf
>
> Also, for very long exposure types of experiments, I believe the earth can
> get into the way of obervations if you're in LEO. On Hubble, these super
> long exposures have lasted literally days (one exposure at an area of space
> near the Big Dipper was 10 days in duration). I'd think that this would
> limit the sorts of targets available if you're making these observations
> from LEO.
Well, you can reach any point in the sky with HST, some only for 50
minutes per orbit, but that doesn't limit science. You need to read out
the detector once in a while anyway, to be able to remove cosmic ray
hits etc. The only trouble is that you need to do re-acquisition of
targets outside of the continuous viewing zone.
Breathing is not so much a problem on an orbit-to-orbit base, but with
time or days. Those same problems you will have with any telescope, may
it be on the moon, earth or LEO.
But (the big but), why would you want to go to the moon, which costs 100
times as much as to put a telescope into LEO or to L2 like the JWST? You
would need to land the telescope on the moon and fasten it there. You
will likely need men for that, which will make any telescope
hyper-expensive. Rather build a couple of NGSTs for a 10th of the price
and shoot them to L2, unmanned.
> Admittedly, it is more complicated than servicing a LEO telescope.
Make them non-reusable, it will be much much cheaper. Nowadays its clear
that the HST concept was very good with the will to use Shuttles in the
80es. But by now there is no need anymore for showing-off spaceflight,
except to boost the re-election of the president of a larger country, so
if you want to make it economic, make them one-way. Build a few of them
for the same price and when a space telescope is used up, launch another
one.
Knud.
- Next message: Benign Vanilla: "Re: Why the HELL haven't we gone back to the moon yet?"
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