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From: DIETZ@slb-test.CSNET ("Paul F. Dietz")
Newsgroups: sci.space
Subject: TAU?
Message-ID: <8611210054.AA01077@s1-b.arpa>
Date: Thu, 20-Nov-86 16:35:00 EST
Article-I.D.: s1-b.8611210054.AA01077
Posted: Thu Nov 20 16:35:00 1986
Date-Received: Thu, 20-Nov-86 23:46:05 EST
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Additional comments on TAU and astrometric scopes...

There's an astrometric scope, Hipparchos, that will be able to
get down to 1 to 2 milliarcseconds accuracy (as will the HST).
That should be good enough to determine distances of stars out
to perhaps a thousand light years.  After that, the Astrometric
Telescope Facility (New Scientist, 11/13/86) will have an
accuracy approaching 1 microarcsecond (!), which should, for
bright objects, be able to measure distances out to perhaps
a million light years, detect "Jupiters" out to thousands
of light years and "Earths" about nearby stars.

[Short editorial: they plan to mount the ATF on the space station.
This seems to me to be an incredibly stupid idea.  Do they really
expect microarcsecond pointing accuracy with astronauts banging around
inside, shuttles docking, spacewalking astronauts firing nitrogen
gas all over the place, etc., even with a (no doubt expensive)
vibration isolation system?  Once again, valid scientific projects
are being perverted to help support needless human activity in space.
I hope the ATF is being designed so it can fly free also.]

It would seem that the ATF (or a larger version thereof) could perform
the major task being touted for TAU: calibration of the distance scale
used in computing the Hubble constant.  This should not be suprising.
The idea of fly-by interstellar probes has always struck me as
pretty silly.  The farther the probe has to go, the bigger the
advantage of stay-at-home space telescopes.  A solar-system-wide
microwave interferometer, for example, has the entire universe
in its near field, returns data almost immediately, is cheaper,
and can be used on more than one target.

Also, a comment was made that, even if TAU is passed by faster spacecraft,
it will give valuable experience with its propulsion system.  This is
true only if nuclear ion engines are not a dead-end technology.
I would think that radically different systems, such as nuclear pulse
rockets, would be the long term choice (Hyde would say that long term
= 30 years).