QUESTION:
We do not understand how the Hubble moves in orbit.  Is it always
orbiting in the same position, like on an axis?  Or does it turn to
always face the Earth (or Space) in the same direction?

ANSWER from Lynn Foster Bassford on March 28, 1996:
Hello, Sheri!
You and your students have asked a very good question.
HST is a low earth orbit satellite constantly moving over different areas
of the ground.   Before and during an observation, Hubble's Fine Guidance
Sensors (FGSs) lock on to guide stars (stars of known position/brightness) 
near the target's location in space. Proposals often observe 
a target for more than one Hubble orbit (roughly 96min). 
Therefore, during some part of HST's orbit the Earth will be in the way
of the stars we are locked on.  When this happens, HST will stop viewing
those guide stars until the Earth passes out of our viewing zone. Then
the FGSs will reaquire the same guide stars to continue a proposal or
lock onto new ones for a new proposal.  

Another feature of HST's orbit (as well as some other satellites) that
should be mentioned is its slow orbital decay.  HST's orbit decays
because of air resistance (remember there is still a thin layer of
atmosphere at HST's altitude).  Air resistance creates drag on HST which
reduces the orbital velocity. Therefore, Hubble decends into a lower
orbit which creates a need for a reboost from the Shuttle Astronauts to a
higher orbit every so often.  

Hope this helps.
Lynn