QUESTION: 
How far can the Hubble Space Telescope see?

ANSWER from Joe Pesce on April 25,1995;
This is actually quite a complicated question and touches on a number 
of things that are hot topics in astronomy today. In fact, it is one 
of the reasons for which the Hubble was created. I won't go into 
cosmology, current theories, etc., but will refer you to the answers 
to "Hubble's Range of Vision", "Age and Speed of the Universe", and 
"HST See the Past", all of which give some background to this 
question.  The important point to remember is the finite speed of 
light, and that it takes light time to reach us from where it started:
i.e. we see things as they were at some point in their past.

Now I started by saying this is a complicated question. The exact 
answer depends on which model of the universe you use, and 
the numbers change depending you which "constants" (like the 
Hubble Constant -homework:  what is the Hubble Constant?) you plug 
into the equations, but the HST has seen objects that are probably 
some large fraction of the age of the universe, like 40-50% (these 
"objects" are Quasars - look for definition in the archive - the 
brightest, most distant things known). We don't know exactly how
old the universe is - this is a key question - but a good guess is 
something like 15 billion years.  In this case, the HST is seeing 
objects 6-8 billion light years away (that is, the  light we see today 
left the objects when the universe was about half it's present age).
Joe Pesce


ANSWER from Helen Hart on April 5, 1996:
I assume that the two questions are different phrasings of 
the same question.

The most distant galaxies that HST has now seen are about 11-12 billion
light years out there.  That represents looking back in time about 90% of
the time since the Big Bang.  Note that the nearest star (other than our
sun!) is Proxima Centauri, at a distance of 4.26 light years.  The Milky
Way galaxy is about 100 thousand light years (or 0.1 million light years)
across its disk.  The nearest large galaxy, Andromeda, is 2.15 million
light years distant (or 0.002 billion light years).