QUESTION:
If the Hubble breaks how long does it take to fix it?

ANSWER from Andy Doherty on May 13, 1996:

This question has two answers.
    
The first answer is a couple of days for big items that break. The 
second answer is several years when an item that breaks needs an 
astronaut to replace it.
   
With the Hubble, there is great need to make sure that it is available 
to the astronomers as much of the time as is possible. The make sure 
that it is, the Hubble was designed with redundancy. Redundancy 
provides for a backup in case something breaks. For many items on 
Hubble we have 2 items that can do the same thing. That way, if the 
first one breaks then the second one can do the job. Switching from 
the primary unit to the backup unit can take anywhere from a couple of 
seconds (it is done automatically by the Hubble) to several days when 
the ground controllers must change the configuration.
     
If the item must be replaced by an astronaut, then several years of  
planning must be done to train and prepare for a Shuttle mission. The 
replacement item is carried into orbit in the Shuttle payload bay. The 
astronaut will take out the old item and then install the new one.
     
An example of this is the tape recorders we use to record science data 
on the Hubble until it can be sent to the ground. In January 1996, one 
of the three recorders we have jammed and quit working. We are using 
only two recorders right now and on the next mission in February 1997 
the one that broke will be replaced.