QUESTION: What does NASA do if something malfunctions or breaks on the
Hubble Telescope?  Is it possible for another object in space to collide 
with the Hubble?

ANSWER from Chris Wilkinson on April 13: 
I will treat this as a two part question and answer with two responses.
First, with regard to malfunctions, the Space Telescope Operations 
Control Center (STOCC), located in Greenbelt Maryland, monitors 
the engineering telemetry from the satellite.  This telemetry will 
indicate that something is not as expected.  

There are 6000 different measurements which we receive.  Each parameter is
checked to see if it is too high or too low.  When a parameter exceeds these
limits we try to determine what has happened.

We have engineering teams which are experts in each subsystem.  The
subsystems are:
Electrical Power System
Thermal Control Systems
Structures and Mechanisms
Safing
Pointing Control System
Optical Telescope Assembly
Data Management
Science Instruments
Instrumentation and Communications

Sometimes a problem is isolated to one subsystem.  Sometimes it involves
several.  in each case, the experts study the data and try to analyze what
is going on.  This can be tricky.  We then have several meetings for
everyone to discuss the problem and decide on a course of action.

Sometimes we deside that it really isn't a problem.  It was a bad reading.
Sometimes we can modify the way we use the system in the future.  Sometimes
we realize that a part has really broken.

The telescope was built with redundant systems.  (Look up "redundant" in
the dictionary.)  It means we put in extra duplicate equipment just in case
something breaks.  We often label these parts the "A Side" and the "B
side".  So when something breaks we switch over to the other side.

We are not always in radio contact with HST.  If something really important
breaks, the HST has a sophisticated safing system which takes control.  The
safing system with protect the satellite from further harm until we regain
contact and figure out a solution.
  
Second, with regard to collisions, yes it is possible for object to collide 
with HST.  We hope this doesn't happen.  There are natural and artificial 
objects in space.  Natural micrometiorites have hit the HST. These can be 
the size of a grain of sand or smaller.  The smaller, the better.  

There are larger debris from man-made satellites which could cause problems.  
The large pieces are tracked by ground radar to establish their orbit.  
I don't know of any that will hit HST.