: THe limit on space-walking is a function of suit supplies (MASS)
: and Orbiter Duration.   

: In order to perform the re-boost of the HST, the OMS engines
: will be fired for a long period.  Now the shuttle is a heavy
: thing.  THe HST isn't light either.  THe amount of OMS fuel
: needed to fly both up is substantial.   a small booster
: carried up and used to boost HST on it's own will weigh significantly
: less then the OMS fuel required to Boost  both HST and SHUttle,
: for a given orbital change.  

: From what i understand,  the mass margins on the HST missions are
: tight enough they can't even carry extra Suits or MMU's.

: pat

I haven't seen any specifics on the HST repair mission, but I can't see why
the mass margins are tight.  What are they carrying up?  Replacement components
(WFPC II, COSTAR, gyros, solar panels, and probably a few others), all sorts of
tools, EVA equipment, and as much OMS fuel and consumables as they can.  This
should be lighter than the original HST deployment mission, which achieved the
highest altitude for a shuttle mission to date.  And HST is now in a lower 
orbit.  

Seems like the limiting factors would be crew fatigue and mission complexity.
