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NICMOS Cooling System (NCS): Astronauts
retrofitted an existing but dormant instrument called
the Near Infrared Camera and Multi-Object Spectrometer (NICMOS)
with a new, experimental cooling system to return it to
active duty. NICMOS was placed on Hubble in 1997, but 2
years later, after depleting the coolant needed to maintain
low temperatures for its infrared detectors, it became dormant.
By fitting NICMOS with the experimental cryogenic system,
the detectors were re-cooed lto below -315 °
F (-193 ° C or 80 K), reviving its infrared
vision, and extending its life by several years.
The NCS consists of separate components such as the NICMOS
Cryo-Cooler (NCC) and a radiator which is run along
the length outside of Hubble's body.
At the heart of the NCC is a turbo-machine, consisting
of a compressor and tiny turbines, turning at up to 400,000
rpm (about 100 times the operating speed of a typical car
engine). Hubble's engineering team successfully demonstrated
this technology in 1998 aboard STS-95 in the first on-orbit
test of a high-performance, high-efficiency, mechanical
cryocooler.
Reaction Wheel Assembly
(RWA): One of four reaction wheel assemblies (RWA),
which is part of Hubble's pointing control system, was replaced by a refurbished unit. The four RWAs onboard
Hubble are a very important component of the telescope's
Pointing Control Subsystem (PCS).
The spinning of the RWAs (3000 rpm) cause rotational, or
angular momentum to point the telescope from one target
to another (this motion is called "slewing"), and to keep
it stably pointed once the target is acquired.
Astronauts also performed the following
"get ahead" tasks. These were not urgent or primary
tasks, but accomplishing them on this mission freeed
up time and resources for the next visit to Hubble.
Gyroscopes:
The gyroscopes, or gyros, are part of Hubble's sophisticated
pointing system. They measure attitude when Hubble is changing
its pointing from one target (a star or planet, for example)
to another, and they help control the telescope's pointing
while scientists are observing targets.
To conduct science at the time, Hubble needed three working gyros.
The telescope carries six; three serve as backups. The gyros
are paired in boxes called Rate Sensor
Units (RSUs). It is the RSU that astronauts change
when they replace gyros, so gyros are always replaced two
at a time. The crew replaced one RSU.
New Outer Blanket
Layers (NOBLs): Astronauts fitted Hubble with
several, specially coated, stainless steel sheets to Hubble's
exterior. These new outer layers prevent damage from sunlight
and extreme temperature changes, and help maintain Hubble's
normal operating temperature. NASA tested these covers to
withstand at least 10 years of exposure to charged particles,
X-rays, ultraviolet radiation, and thermal cycling. Astronauts
installed several NOBL panels on Hubble in 1999.
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