QUESTION:
Does the HST experience minor breakdowns, equipment failures, "crashes",
etc?  If it does, how do you fix them?  Can you give some examples?

Answer from Dick Shaw on April 23, 1996:
These are interesting and insightful questions about HST. HST is a very
sophisticated satellite with many complex systems, including
systems for power, pointing control, health and safety, communications,
and of course the science instruments themselves.  Like all complex
machines HST occasionally experiences problems with its hardware and
computer software.  Thankfully, these problems have been relatively few.

Since you asked this question, I'm guessing your class already realizes
that it can be very difficult and expensive to fix problems on a satellite
in space.  The engineers and scientists that designed and built HST have
a fairly elaborate strategy for dealing with the possibility of equipment
failures on-board HST.  This kind of effort is often called "risk
management" and it is applied to building almost any complex thing, from
satellites to aircraft to automobiles.  It goes something like this:

   1.   Use reliable equipment so that failures are unlikely in the first
        place.  This means doing extensive testing of the components (and
        later of the assembled telescope) on the ground until the engineers
        are satisfied that the equipment will work correctly once the
        satellite is launched.  The ground system testing was very
        extensive for HST.

   2.   Monitor the equipment to detect and correct small problems before
        they become big ones.  There are thousands of sensors on-board
        HST to report temperatures, voltages, power levels, etc. so that
        the operations support staff can monitor in detail the health and
        safety of HST 24 hours a day.

   3.   Include back-ups for critical components.  That way if something
        important fails, the back-up (or redundant) equipment can take over.
        This strategy is usually reserved for the most critical systems.
        HST has several redundant systems, including the gyroscopes that
        are used to maintain its precise pointing.

   4.   Here's the difficult one: design a system so that if it fails,
        that failure does not jeopardize the entire mission.  (It is not
        always possible to "compartmentalize" system failures so neatly,
        though.)  For instance, if one of the solar arrays were to be damaged
        or stop operating, the other array can provide enough power to
        operate the critical systems, and possibly to continue operating
        one or more science instruments.  If a gyroscope were to fail, the
        telescope would put itself into a "safe" pointing until it received
        instructions from the ground to activate one of the redundant
        gyros.

   5.   Send up a crew of astronauts to fix the problem.  This is the
        most expensive and dangerous option, and it works well only
        for satellites like HST that were designed from the beginning
        to be serviced in space.  You may recall that several pieces of
        equipment were repaired or replaced during the first HST servicing
        mission in late 1993, including some failed gyros.

If you are familiar with the saying: "an ounce of prevention is worth a
pound of cure" you can see how it applies to HST.  All but the last item
really amount to "prevention" rather than "cure."   That's because
prevention is much cheaper in time and money.

Here's an exercise for your class to try: Identify how each of the steps
above applies to manufacturing and maintaining today's automobiles.  Now
suppose you wanted to sell these automobiles to people on a Martian
colony, where there are no repair shops (yet!).  What types of repairs
or maintenance are most often needed on cars today, and how would you get
around them?