http://saturn.jpl.nasa.gov/news/newsreleases/20090202statusreport/

Cassini-Huygens Mission Status Report
February 2, 2009

PASADENA, Calif. - The Cassini spacecraft will swap to a backup set of
propulsion thrusters in mid-March due to degradation in the
performance
of the current set of thrusters.

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 current set of eight thrusters, referred to as branch A, has been
in
use since Cassini's launch more than 11 years ago. The redundant set,
branch B, is an identical set of eight thrusters.

Propulsion engineers began to see a lower performance from one of the
thrusters on branch A in October. A second branch A thruster is also
now
showing some degraded performance.

An extensive review with the propulsion system contractor, Lockheed
Martin Space Systems, Denver, Colo., the thruster manufacturer,
Aerojet,
Sacramento, Calif., and propulsion experts at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., was completed last week. The
recommendation was made to swap to side B as soon as is practical.

Mid-March is the earliest practical opportunity to make the swap. This
allows time for the team to properly test and prepare the sequence of
commands that will be sent to the spacecraft. Science planners have
identified a period where no high-priority science will be lost during
the switch, which will be done over a seven-day window. It also is a
time when no navigation maneuvers are required to maintain the
spacecraft's trajectory.

The swap involves commanding a latch valve to open hydrazine flow to
the
B side, and powering on some thruster control electronics. No
pyrotechnic devices are involved in the swap, and the action is fully
reversible if necessary.

Almost all Cassini engineering subsystems have redundant backup
capability. This is only the second time in Cassini's 11 years of
flight
that the engineering teams have gone to a backup system. The backup
reaction wheel was brought online a few years ago and is currently
functioning as one of the three prime wheels.

Cassini successfully completed its four-year planned tour and is now
in
extended mission operations.

More information on the mission is available at:
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a division of
the California Institute of Technology in Pasadena, manages the
Cassini
mission for NASA's Science Mission Directorate, Washington, D.C. The
Cassini orbiter and its two onboard cameras were designed, developed
and
assembled at JPL.

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Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina.martinez{at}jpl.nasa.gov
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