.title
Support Systems Module

.synonym
solar arrays

.description
The structures and equipment which provide power, communications,
orientation, and control to the Space Telescope.
.contents
     The Support Systems Module .~ (fig. 7)~ will enclose 
the .~ Optical Telescope Assembly~ and .~ scientific instruments~
and will provide all interfaces with the Shuttle orbiter.

.para
     The module has four main sections:  the light shield, the forward
shell, the equipment section, and the aft shroud.  These four pieces fit
together like stacked cannisters to enclose the telescope assembly and
scientific instruments.

.para
     The .~ aperture door~, which also serves as a light shield, is
located at the front of the Support Systems Module.  The telescope must
be shielded from the Sun, Earth, and Moon so that the scientific 
instruments are not damaged by a flood of light.  While in operation, the
telescope will not be turned to within 50 degrees of the Sun,
precluding observations of the planet Mercury.  Internal .~ baffles~ in the
Support Systems Module door and just below the .~ secondary mirror~ prevent
scattered light from degrading the focal plane image.

.para
     Just behind the light shield and the forward shell is the equipment
section of the spacecraft.  This doughnut-shaped section is made up of 
10 compartments that house the electronic and control modules.  Access
to many of these modules will be easy as most compartments open from the
outside via a hatch or access door.  An astronaut can open a hatch,
disconnect a module from its plug-in compartment, and insert a new
module, all without disturbing the other instruments in the spacecraft.

.para
     The .~ aft~ shroud fits over the section containing the
 .~ scientific instruments~ and the .~ fine guidance sensors~.  Access
covers on this shroud enable astronauts to get at the instruments and
sensors for removal or maintenance.

.para
     Handrails and portable foot restraints are fixed on the external
surface to permit astronauts to perform their maintenance and
refurbishment tasks.  Four .~ magnetic torquers~, also located on the
shroud exterior, work with four reaction wheels within the Support
Systems Module equipment section to control the spacecraft's orientation
in space.  The magnetic torquers are magnetized metal rods controlled
by an onboard computer.  A change in their position causes the
spacecraft to re-align itself in relation to the Earth`s magnetic field.

.para
     Electrical power is supplied by onboard batteries, which are
charged by the two .~ solar arrays~ during the sunlit portion of the
telescope's orbit.  An average of 2.4 kw is needed to operate the
spacecraft and its instruments.