Re: Why the HELL haven't we gone back to the moon yet?

From: Jeff Findley (jeff.findley_at_ugs.nojunk.com)
Date: 10/08/04 

Date: Fri, 8 Oct 2004 10:27:21 -0400

"Jay Windley" <webmaster@clavius.org> wrote in message
news:ck66cj$oa6$1@news.xmission.com...
>
> "Jeff Findley" <jeff.findley@ugs.nojunk.com> wrote in message
> news:amw9d.682$nu6.128@fe37.usenetserver.com...
> |
> | With little to no atmosphere on the moon, it's a better place for a
> | telescope than earth.
>
> Earth's surface, yes, but not better than orbiting the Earth. From Earth
> orbit you can see the entire sky once every 90 minutes. On the moon half
> the sky is obscured for two weeks.

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.

A better place than the moon or LEO might be a very high earth orbit
(outside of the van-Allen belts), or even a solar orbit. Unfortunately,
such an orbit isn't as accessable (for maintenance) as LEO or a the lunar
surface.

> It also complicates the servicing
> missions.

Admittedly, it is more complicated than servicing a LEO telescope.

> Plus with gravity you still have to deal with droop, and with
> dust there's the problem of contaminated optics.

Droop is an issue, but at 1/6 gravity, it isn't as bad an on the surface of
the earth.

You'd certainly want to cover the optics during any activity that stirs up
dust, just as Hubble covers its optics during any shuttle servicing mission.
I'm envisioning a man tended telescope on the lunar surface where dust is
only stirred up during infrequent servicing visits mounted from a nearby
lunar base.

> Further, you can't point
> the telescope within a certain angular distance of the horizon during the
> daytime for the same reason you can't point the Hubble within a certain
> angle of the sun.

As for pointing, I'd think the earth is a bigger obstacle than the sun for a
LEO satellite. In LEO, the Earth fills up quite a bit more "sky" than the
sun. But on the surface of the moon, you have this problem as well, since
you can't see through the moon itself. ;-)

Again, a better location for observations would be HEO or solar orbit.

> You're right in that Earth's surface is one of the worst places from which
> to observe the universe, but Earth orbit is probably better than the lunar
> surface.

The devil is in the details as both LEO and the lunar surface present their
own unique set of challenges.

Jeff 

-- 
Remove icky phrase from email address to get a valid address.

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