Cassini has fallen back on a reserve set of propulsion
thrusters during its extended tour of Saturn.
Lacking space tugs or
robotic repairmen, the Cassini
spacecraft activated its backup thrusters because of slow degradation in
the performance of its main thrusters. The primary thrusters have kept the
space probe going since its launch in 1997 until now.
The Cassini mission has uncovered many new findings about
Saturn, its rings and local moons. It sent its Huygens
probe down to peer beneath the orange shroud of methane around the moon
Titan, sampled icy particles spewing from cold
Enceladus and surveyed bizarre
weather patterns on Saturn.
The thrusters are used for making small corrections to
the spacecraft's course, for some attitude control functions, and for making
angular momentum adjustments in the reaction wheels, which also are used for
attitude control. The redundant set represents an identical set of eight
thrusters.
This marks only the second time during Cassini's 11 years
of flight that engineering teams have decided to go to a backup system. Almost
all Cassini engineering subsystems have redundant backup capability.
Cassini has already completed its original four-year
planned mission, and is currently in extended mission mode. Plans are underway
for a proposal to add even more years to the spacecraft's life by extending
its tour through 2017.
NASA's budget currently includes about $80 million a year
to continue the spacecraft's operations and science.
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